So let's talk very quickly about this part of it and then we can go to the documentation. So in the middle, and in fact, well first of all this is a safety type apparatus so that people can't open it up. Now opening it up means pulling out these two plugs here and or pulling this out.
So this is a rod which at the end has a little gripper and it's holding on to the radioactive source. The physical size... Okay, so anyway, so suspended from this rod is the radioactive source. So it's sodium-22, and sodium-22 emits a positron.
The positron comes out with exceedingly low kinetic energy. Hardly enough to penetrate the walls of the housing that it's constrained within. So this is what you call a sealed source.
So short of sawing it in two, you're not going to get the source out of there. So that's a safety. It's a signal to the radiation control people.
that this source is in a different category than, say, a little vial of radioactive material that you could spread all over the place. So it means that the safety measures applying to it are not as stringent as they are to, or say, a box of radioactive material that you can open up and spread around. That's why it's in here instead of in that cabinet.
Well, no, the real reason here is because it's constrained within this little vial, this new housing, which is welded shut. So that's the key. Right, what I mean is- The source itself is, well, the thing that makes it a sealed source Yeah.
Is that it's within the housing. But the reason that you can have it here, that's outside instead of- Yes, that's correct. I see.
So this is all lead here. So there's a through hole that goes all the way through this one and then this hole just goes halfway down and then intersects that hole that goes through. So the source then is dangling within, it's not loose, I mean it's held firmly, within the middle of the shaft and it goes all the way through.
So what we do then is, maybe we should, it emits a beta, a positron. The positron... is that there's a spectrum of kinetic energies and only the very highest energy of this spectrum the tails of it you know what a beta spectrum looks like so because neutrinos are produced at the same time the neutrino carries away some of the kinetic energy and it's shared between that and the the beta or the positron And so you don't know ahead of time what the energy is going to be because they share it in a way that varies from one decay to the next. So, but you can take the extreme case where the neutrino carries away almost none of the energy and so the positron has all of the, or nearly all of the energy. So that would be the case where the positron has the highest kinetic energy.
So some decays have more than the average, some have less. It's a statistical thing. So those betas at the very highest energy can escape the housing if they're going in the right direction because the housing isn't a sphere, it's more of a cylinder.
So depending on what direction it goes, it has to pass through more or less, I believe it's stainless steel. I think it's the housing. Okay, so basically all the betas.
lose energy and stop within the housing itself. And then they, what happens next now is kind of interesting and there's a lot of atomic physics to this now. So the positron doesn't typically annihilate immediately. It will... Orbit some electron one of the probably available It's from the stainless steel So so now we got a positron and an electron bound to each other in some high what you call a Rydberg Orbit.
Oh, that's what the positronium is. Yeah, that's what positronium is. But this would be an excited state.
And rather than annihilating in that state, because it's pretty far apart, it's going to radiate. Roughly optical gammas, optical photons, until it gets down to the ground state. This is the typical thing that happens. So the electron and positron, forming positronium, will end up in the ground state before annihilation. There he is.
Is it tricky? It's on the battery. Let me check.
I think it's on. It should be battery checked. The battery is good. There should be a source. Cool.
Yeah, it's working. Right. So let's go down now to the level and it takes a little bit to settle down.
So I can close the door. Yeah. Yeah. OK.
So so what we're measuring right now is cosmic ray background and we're on this scale. Let's see right here. And the number is something like point.
Let's make it 0.2, okay? And this is on the 0.1 scale, so the number is actually 0.02 millirem per hour. So that's cosmic, well that's the background which includes cosmic rays and radioactivity from the bricks and things like that.
It does not include radon, it doesn't include your chest x-rays or tooth x-rays. So it's some fraction of the total dose that you get every year. Up to a half. It's in the ballpark of a half of the radiation you get every year.
So it's of order 0.02 millirem per hour. So we can compare that now. Let's take this over here closer.
You can tell now it's higher, but this is still... Let's go down and find something. Yeah.
I'm changing the scale now. So it's about... Let's say it's 0.4 now. So this is 20 times higher.
But 20 times higher for... Actually, this would be allowed not for the general public, but for someone doing experiments like us. We would want then to, if we were going to sit here for some period of time.
Right next to it. Yeah, we would probably want to wear a film badge. But Radiation Safety has come and examined this in great detail.
done all sorts of measurements and deemed it not necessary for students because they're not going to be sitting right here with their head there hugging that stuff right but now that's not the worst of it let's leave it on for a moment um so we got these This is a tube like this one, but it's been, lead was poured into it, in open lead, and now it's solidified. So now we're opening it up. Let's see, I forgot where we put these, I think here.
So now... We're opening it up and now if we put that there we're going to get a lot. Oh that's enough. Right, yeah.
So now we're getting those. Primarily along the beam. Yeah, yeah, right.
So that's why it's positioned this way with the idea that nobody's going to come sit on the counter with their head in this. So again, the radiation safety control people deemed that an unacceptable arrangement. That's why we have these things to stick in there.
Now, the way we actually run this most of the time is with these in place. Now this is aluminum, this is aluminum. Aluminum on aluminum galls. Do you know what galling is?
It's when one metal surface doesn't rub easily across another metal surface. And if you try to force it, you're gonna rip up the surface. Now, rip it up on a microscopic scale.
But nevertheless, you can. can get it stuck in there if you do that. So you do not want to force this in.
You want to work it in easily, carefully, without forcing at all. So being very sensitive to the idea that you can call it. See if it goes in the other one in this ear.
So it's like the hole was in. Okay, there. So it went in, okay. But be very careful.
Don't force it because it'll make it only harder to get out. Yeah. Okay, now what's going on?
Why are we doing this? So you'll see that there's a hole in here. Yes. This hole, the diameter of this hole matches the diameter of the hole in...
This hole right here is bigger than the hole in the lid. So this matches the hole in the lid. Okay, so now what's going on?
Why don't I put this here while we're talking. The positron is now captured by an electron in the housing of the source. They're orbiting each other in some excited state and they radiate protons. and then fall to the ground state. Typically, not always, but most of the time.
So they collapse down to the ground state. Now the ground state is an S state. And so in an S state, it means their orbit has no zero angular momentum. So the classical picture would be that they're doing like this, which means that they are, or to put it another way, the radial wave function at the origin, the origin being the place they overlap. So when the two of them overlap, the wave function is not going to zero, which it is for a P state and a D state.
The radial wave function is going to zero at the origin, but not for an S. So that's where it's most likely to annihilate. And so it goes to an S state.
But now in the S state, you can have two possible spins, orientations. So now one orientation would be like this, which now can be either an S equals a... The sum of the two can be either 0 or 1. And of course, this way, they're always 1. So there's two total spins.
spin angular momentum, which can be, let's call it S. So it's, well, I guess I shouldn't call it S. But this would be, so this is the multiplicity of the, when it's one, the orientation can be either like this or like this or like this. All of those can add up to a total spin of one.
So there are three possibilities there. So that's the number you put here, how many possibilities. To be spin zero it has to be like this. That's the only choice.
So that there's only one possibility there. So this is the multiplicity of possibilities. So in the one s and zero now is the total angular momentum overall.
It's the s is telling you the orbital and then this is telling you the multiplicity of the spin angular momentum. total spin angular momentum and this is the total. Okay so and of course this is consistent because the multiplicity of one means that the spin angular momentum is zero, the s tells you the orbital angular momentum is zero and therefore the total has to be zero.
Okay so both of those will annihilate. The 3s1, which we're not considering, must, it cannot decay to two gammas. So that you're violating some quantum numbers. If you, if you were to claim that the 3s1 decays to, so that's another ground state.
The 3s1 and the 1s0 are nearly degenerate, not exactly, but very close to degenerate. So, um, So it'll end up in either one of those two states, but when it's the 3s1, it's going to decay to three gammas, and the chance that two of those are going to be back to back is nearly zero. So the only ones we're going to see are the ones from the 1s0 where it's two gammas, they're exactly back to back, and so now there's a better chance of both of them coming out through the holes.
Okay, so they come out through the hole and then travel some distance until it gets to the aluminum. Okay, so the aluminum is something like this, which roughly matches this. This distance right here.
So what's happening now is the gamma coming this way comes out, hits the aluminum somewhere between here and here roughly, and will scatter, comp and scatter now. So this will be a Compton scattering, and it'll scatter at any angle whatsoever. But we're interested in the angles close to 90 degrees. So that's the geometry that's set up here.
So gamma comes along, scatters in the aluminum, and scatters at roughly 90 degrees. And of course, that will go in any direction in phi, but occasionally they go in this direction. The other gamma does the same thing.
Okay, but it's more if one is catched on this direction, then the other one is more likely to be catched on the perpendicular. Right, because now let's talk about the polarization. Yeah, the polarization in Compton scattering. And there's a little section in here on that. Under theory.
Let's see, so here's a really short, this is perhaps the shortest Fisrev article, because it goes from here to here, very short. And then there's another article here somewhat longer. And then here's something out of Jackson, which talks about Plain electromagnetic waves, so in other words, polarized electromagnetic waves, which is very useful. And then here is the quantum mechanics of the whole thing.
And then here is a discussion of the actual quantum scattering calculation right here. which I find to be particularly good. And then here's a lot of the data on They've just taken the formula and made plots, which is very useful. So gamma comes in here, Compton scatters at this point, and now this shows you probabilities of what direction it's scattering in. So these curves are equal probability lines?
I believe so, yeah. So let's see what alpha is. No, alpha is a measure of the energy of the gamma relative to the rest mass energy of the electron. So, and we, oh yeah, so these two gammas coming out now, we know their energy with great precision.
It's got to be the electron mass, rest mass energy. Because you've got electron, positron, the kinetic energy here is completely negligible compared to the half of MEV of rest mass energy. This energy is...
Let's see, we should know. So we know hydrogen is 13.6. The grounds say the hydrogen is 13.6. What we really want is the kinetic energy, I think. Is it equal to that?
The source is 70? I think it's half that. Pardon? I think the...
The kinetic energy is half that? Half the potential. Remember the potential is negative and the kinetic energy is positive. And the total is what we're talking about.
Yeah, and you get a half canceling when you do the subtraction. Yeah, that sounds... Well, doesn't that make the kinetic energy twice? No, because the potential makes the total energy negative when it subtracts.
Right. So the potential is twice the kinetic energy. Yeah, okay. The total is negative kinetic energy.
The total is negative. Negative kinetic energy. Right. The kinetic energy, of course, is always positive. Mm.
So, um... Right, so that sounds right. So the kinetic energy is half the 13.6. Okay, so, but then now we're replacing the proton in hydrogen with a positron.
And we've got another, so that makes another factor too. So which way does that go? So it's going to decrease it because you have mass in the denominator. It decreases it, that's right. So it decreases another half.
So that's another half. Okay, so the, anyway, the atomic energy levels that are involved here are tiny on the scale of the rest mass energy. You can neglect them as far as... A million times smaller. Yeah, right.
So, maybe two million times smaller. Actually, 200 million because it's 511 MeV, right, versus... And you made out...
It's .5 MeV. Oh, it's .5 MeV. Okay, so it's order one MeV versus, yeah, right.
Okay. Okay, anyway, so the gammas are coming out with pretty much monochromatic. Okay, now, so that would be an alpha, the case of alpha being one, which is this one right here. So this tells us now the angle distribution, the probability of scattering at a certain angle.
So most of them are going to go forward, but we're interested in the ones that go at 90 degrees. It seems like there's an extremely small probability of scattering at 90 degrees. For alpha 1, then you're looking at this small.
number right here right so the so the cross section is dropping steeply as the angle gets bigger mm-hmm so that means if you actually wanted to do a little calculation what you're gonna find is the ones most of them are actually gonna be the case where the ones that get into the scintillator are going to be the ones that interact early here and then come off at an angle which just hits over here. So if you wanted to do this very carefully, you'd find that the average angle is not 90 degrees. It's a little bit less because the spectrum is falling so steeply with angle. Okay, but let's pretend now that it is 90 degrees.
Okay. The attempt, at least, is made to be 90 degrees. And the reason for 90 degrees is that the polarization now, the gamma.
So let's assume, let us consider linear or transverse polarization states. We could either do circular or linear. So let's do linear.
So if this... gamma comes out with this polarization so it's going this direction this polarization it's more likely to scatter here and then here and that's you can sort of see that classically because the gamma that's oscillating this way is going to excite the electron that it hits classically it'll get it oscillating this way and if you know how a dipole oscillation works there's no radiation along this direction and then it starts increasing the further you get away from that. Okay now this particular state if you work through the quantum mechanics of this you'll see that if this state is polarized this way this must be polarized this way.
And so the whole point of the experiment is to prove that. Okay, and so what happens is you got exactly what you said. You orient these first this way, and so the only way they would hit, gamma would hit here and hit here at the same time, the only way that would happen is if they both come out this way, which they don't. One comes this way, then the other one must be this way. So you're not going to see many counts this way.
So you take one of these, either this one or this one, you rotate it off, and you could rotate it this way too, but for safety reasons so that people don't catch on the cables. Rotate it this way. So considering it's not perfect 90 degrees away are still going to get some signal, but not too many Yeah, right. Well Yes, and of course, it's not exactly One angle of fire either.
Yeah, so and then you'll see that the Well, there are lots of other confounding effects that come in too, so. But still, the effect is reasonably strong that you can fairly easily see it just by eye. But then the students should really work it out quantitatively. Is it something that is easy for you to show us the equipment actually measuring it? Yeah so yeah so we're just getting the theory behind this first.
I just want to make sure that we didn't get to the end and now we don't have time to do the actual experiment. We have 25 minutes. Oh, because you have something...
We have two o'clock class, yeah. Oh, yes, yes, okay. So let me move along then. Okay, so this right here is a scintillator. Okay.
It's sodium iodide. Iodine because it's... heavier and so the gammas interact more readily the higher the Z of the material so the iodine is important rather than sodium chloride let's say and likewise over here.
So it's just this part right here and it's completely encased here partly to keep the light out but also because sodium iodide just like sodium chloride is hygroscopic it means it absorbs water and turns into a goopy mess. So it's sealed even on the side where the light comes out and hits the photomultiplier tube. It's going through I think a quartz window I think. So it's sealed from humidity as well. light.
Okay, so the light, so when the gamma goes through and hits the sodium iodide, it excites the the sodium iodide. Why is it higher probability of interaction for a heavier, for a higher z? So what's happening is the gamma is coming fairly close to the nucleus.
So it's interacting in the electric field of the nucleus. That's what we're interested in. So, it's, and there are three possible processes.
that happen. One is called photoelectric absorption, which has nothing to do with a photoelectric effect. It's an entirely different process where an inner shell electron is knocked out, usually completely out. of the atom. This is how x-rays are made for instance.
So you but usually x-rays are done with an electron rather than a photon but otherwise it's making x-rays okay which are pretty high energy. So it's not the electron being knocked out that we care about it's the fact that it makes a vacancy and the rest of the electrons then collapse you know falling down to fill that vacancy. see. So one from the next shell falls down and then from the next shell that fills the one that was left by the one that dropped down etc. So it's like a cascade.
Emitting a lot of light which now some of that is even in the optical range but much of it is still not in the optical range. So there's a doping in here. I think it's thallium. Okay, so I believe it's TL, which converts the higher energy gammas into optical, higher energy photons into optical photons.
So a fair amount of light is produced. The time constant is quite long on the scale that I'm used to. It's like a quarter of a microsecond. So, optical electrons are produced, they hit the photocathode of the photovoltaic tube, and that is now the photoelectric effect. Optical electrons are produced.
So, optical... Sorry, optical photons hit the photocathode. and why the photoelectric effect knocked out electron electrons. Yeah, because now it's optical, right?
Yeah. Just a simple photoelectric. Right.
And so that coating, which is on the inside of this... evacuated tube is so thin you can see through it. It looks like somebody's dark glasses if you look through it, because you can see all the workings of the photovoltaic tube through that window at the top of the tube.
Okay, so those electrons then are attracted to what we call a dinode, because a dinode is at a higher voltage than the photocathode, and so the electrons knocked out of the photocathode. are attracted to the dinode. They speed up as they approach the dinode. When they hit the dinode, they'll knock out maybe two or three additional electrons. Those then are attracted to the next dinode and the same thing happens.
So every time it goes to a dinode, you get two or three times as many. And there may be 10 to 12 of those dinodes in here. And then finally, the anode is at the bottom. And this is the base. for the multiplier to base and so you feed into it just one high voltage and this is basically just a whole bunch of string of resistors.
which are dividing the voltage supplied here into various different voltages. So the resistor chain is just one resistor after another. from the high voltage supply is running down that chain.
And so now you just pick off each dyno is connected to the junction between two resistors. And so you can pick off whatever voltage you want from that. And so that's how all the dynos get a different voltage.
And it's arranged in a way that each dyno is higher than the next. So the highest voltage drop from photocathode to anode is whatever is supplied. by the car support, which is here. So these change with time, but I think we're using this in this now.
And it says what voltage is to apply. And we need to turn on the crate. Let's see if this is, yeah, the crate is on. Let's see now. Oh, I see.
Each one of these individually has its plugged into the wall. So these are just stand-alone. They're just, this crate here is just a place to put them.
Okay. So there's, they're not using the dock plane. Okay, so adjusting the high voltage is one of the adjustments you have, but presumably these are good numbers.
So that's probably where you want to start. So, the first thing I recommend doing when you're doing the experiment, the first thing is to keep these things in place and get from the cabinet. one of these button sources whose activity is so low that they're not regulated. We have a couple over here already in the Geiger-Mueller, just sitting there.
They shouldn't be there. We forgot to put them back. Not to us. We didn't use it, but we saw it. We should have put it back while we had a chance.
We can't put it back because we don't have the access to it. Right. We had three minutes to think about it.
We're going to have to wait. and do it well. Yeah, but you don't want to carry it away in your pocket. Right, right.
But I'm just saying, if you wanted to use that real quick to demonstrate how to make the experiment work. Oh, no, well, I don't know if we have time to actually power up the scope and look at pulses. Okay. But what you would do is take the source.
I'm going to take the source. I would recommend is probably the CZM137. Right.
The one we used for the Compton. Yeah, yeah, but that's a hot one. Yeah, yeah. But we also use this one to test.
Yes, like in the micro... So is figuring out how to make the experiment run with all the equipment straightforward because we haven't even we haven't touched it and we don't really have much time to Spend eight hours figuring it out But there's a little table of a lot of radioactive sources. Maybe let's just put them all there.
Isotope information? Yeah, maybe. There we are. Yeah, we're very far away.
Right next to that. Ah, where's CZM? I should have that.
Here we are. So, .662. Okay, see this is the photons, this is electrons.
So we're not interested in the electrons in that button source because they don't have enough kinetic energy to go very far. So what we're interested in is the gammas. which are going to then penetrate and get into the seas.
of sodium iodide crystal and make light. So those, but now this 0.662, that's much higher energy because remember now the gamma is coming out with 0.511 MeV. It's scattering at 90 degrees and therefore if you do the constant scattering kinematics now, you'll see that it loses half its energy if it's 90 degrees.
So it'll be 0.25 something. Whereas this is 0.66. So another possible source is the barium, which has, here we are.
So barium has two gammas of roughly the same probability. So you always wanna check the probability because some probabilities are so rare you never see them. So one of these is 0.356, which is only a little bit higher than the gammas we expect.
But you can also see the point 081. So that gives you a range now, because you never know when you're looking at an instrument like this, you don't know where zero is. So you can't rely on just one energy to calibrate. You need at least two different ones.
But then you can throw in the cesium if you want it also and try to draw a straight line through it. So you get a calibration then. So what you know is that this is linear, but you don't know...
where the zero is until you actually calibrate it. Let's see, by the way, so there are three processes here. The gamma comes in.
It can either do this photoelectric absorption where it knocks out the inner shell electron. or another is constant scattering, but the higher the Z, the less important that is, and the other process is pair production. So the gamma comes in, and in the electric field of the nucleus, This is why it matters what Z is in the electric field of the nucleus.
It'll make an electron positron pair. So gamma comes in interacts with a gamma of the electric field. This is how you would do a Feynman diagram and outcomes of electron and the positron. Those then ionize in all cases what comes out is going to be what you're sensitive to in the sodium iodide is. the charged particles.
So the charged particles now are going to ionize and that's what makes the light. Okay, so you get light. So in the case of continent scattering, you're not going to get light out proportional to the energy in because in continent scattering there's a gamma going out as well.
gamma comes in, hits an electron, and goes out again. So what you see in the case of content scattering is a broad plateau, and we actually see that as well. But then you see the other processes where all of the energy goes into light. And so you see a peak.
So what you'll see on the pulsate spectrum is a peak due to the pair production and the photoelectric absorption. And then below that, you see a somewhat broad plateau, which is due to the complex scattering. OK, and you see that both here and here. And you would see that on the scope.
And you're looking now at the output here. So what's coming out of here is a pulse. So it's and it's.
of order a quarter of a microsecond long. The pulse is, or is it two microseconds? Anyway, for me that's a very long pulse.
I like shorter pulses. Okay, so then it comes into this device. So I forgot what TC-SCA is.
It's a pulse height. This is a device with... puts out so so the the pulse comes in from here so this is it so it's amplified and here are the so this is the course and the fine gain.
Okay, and then now you can set both this and this. So this will set the upper limit. If the pulse is bigger than this, it won't put an output out. This is the lower limit, and if the pulse height is below this, it won't put a pulse out either. If the pulse is between below the upper limit and above the lower limit, then it puts a pulse out.
Okay, so we get a pulse out here and now one thing it goes to this, which is a scalar. So this is a scalar is scaling, that is it counts, it's a counter. So and then here's the other side.
So this is one side, this is the other side. So that's what's coming out here and here. So and then these go to what we call a coincidence circuit. Let's see. Quick question.
Yeah. Is this amplifier here, this is just a standard peak detector, like similar to the one that we use on the Geiger-Muller? It's not. It's simply amplifying the pulse. It's just making it bigger, so it's not detecting.
Oh, but I thought you said if it's within the range of this. So this is peak. Yes. So this is sensing the peak.
And not the area underneath. We use it for triggering. So now over here is the amplified signal. So it's putting out, this we would call a digital signal because the height of the signal is always the same.
But it happens only if the input, which is the analog, is between this and this. So we get a digital signal out, that's what these are here, but also we get the amplified signal which is going then over to the screen. Oh wait a second, do I have this backwards?
No you got it right, amplifier is that in your hand. Oh yes, okay, so right, so this is, yes, so let's track the digital part first. So this is what's called a track and hold, and what it's going to do is the output, which is here, will always equal the input until a hold signal comes along. When the hold signal comes along, it'll freeze the output.
The input keeps changing with time, but it'll hold the output at whatever level it was at when the hold signal comes. So tracking is when before the signal comes along, before the whole signal comes along, the device is tracking. That is, whatever comes in comes out.
When the whole signal comes, whatever comes in is ignored, and it's just holding the output. Okay, so we'll come back and look at that again in a moment, but let's see what makes the whole signal. So here, now this device is what we call...
TTL, transistor-transistor logic, and the hold in the track take digital pulses. So we're feeding digital pulses into these, and these two down here are analog. So digital pulses, but there are two different standards of digital pulses, which there are many standards, but there are only two that we're concerned with.
So this is taking TTL, but this other stuff is putting out NIM, I think. N-I-M. So, yeah, so this is putting out... Let me check and make sure. No, no, let's see.
No, it looks like it is TTL. Okay, so TTL is coming in and NIM is coming out. The NIM now is going to a discriminator.
So here we have, so here we have a amplified signal. Let's see, why is it going to a, I guess this thing is. Okay, a discriminator normally takes an analog signal in and puts out a digital signal.
But here, we're just using it to change the width. So we're actually feeding it with a digital signal, which is not the way you normally do it. But that digital signal is big enough that it's above the threshold. So we're using this not in the way that you would ordinarily use a discriminator. Using the discriminator as a pulse width modulator.
Well, not modulate. Yeah, we just Just adjust it to one width, which is long enough for our purposes, or short enough for our purposes. Okay, so the digital signal comes in, the digital signal comes out of a different length, and then goes back into the level adapter again, which is now going from... from NIM now.
So this is a NIM standard. So it's going from NIM back to TTL and it's inverting as well, which you can see by this little switch right here. So it's changing true to false and false to true. Okay now, so this is interesting where this goes now.
This now goes to this coincidence unit. So And the other side is also going to this coincidence unit right here. And so what this device does when it's on 2, so it's on 2 right now, which means that any 2 of these can fire and produce an output on this cable right here, which then goes to this scaler. So we're counting how many coincidences we get as well. So this is how many times we had a pulse, which satisfies this.
This is how many times we get a pulse which satisfies this, and we usually set these the same. And then this one is counting how many times do we get those two pulses coming at the same time. So that's why it's called a coincidence. Only two of these five channels are activated by these switches right here.
The others are turned off. This is set to two, which means that any two can fire to give you an output if they fire at the same time. But there are only two which could possibly fire because the others are turned off.
So in this case it looks a little redundant, but there are other cases where that's useful. Why, what is this, we were trying to figure out what this does, does this accept another like small pin? No, I think this is just a monitor.
Because it looks like the adjustment is here. You can take a little screwdriver and adjust the time. Yeah, what's called the resolving time?
So the question is how much overlap these these it's possible that one comes earlier and one comes later So they don't have a lot of Right, so is this to actually watch the is this like I think you put a scope Okay. And Jimmie can help the other. Okay, good. Okay, so if you get two at the same time, a pulse comes out on here, it gets counted here, and then it goes to this device right here.
So this is called a gate generator, and this is important. So pulse comes in, and then a pulse comes out here later, and you can adjust how much later. by these two knobs right here.
Well, let's see now. This knob right here determines how much later it comes. And these are adjusting the properties of the gate signal.
Wait, so this this guy is counting how many coincidences we have. Right. And it's being sent to two different... No, this is in... No, these, yeah, it's...
Oh, they're both receiving. They're both receiving the... Okay. Right.
So this is the number of pulses and it's not only read by the scaler, but it also goes over to here and triggers this gate or delay generator. We're using it as a delay generator. So this is for the readout and this is for feedback?
Well, this is for the student to record. So at the end of a run, the student would record how many singles. did you get in this channel? How many singles did you get in this channel?
This channel? This channel right here? So there's two inputs here and you can go from one to the other by...
By right here, this A, B. Yes, that's right. So you just cycle through this so it goes to A to preset to B. So this is just telling you which readout. appears here. So by pushing this multiple times you can get this to light or this to light.
But they won't have to read these because this is hooked up to the The computer. No, you have to read it. Oh, I thought it was all hooked up to the computer.
They just read out the... No, this is important because the computer can't always keep up with this. So there is what we call dead time.
So here we're generating coincidences, which is going to tell the computer to read out the pulse heights. Basically, this device right here is going to read these outputs right here. See, remember now, this is an analog signal, which is now DC because the whole signal comes along and holds the pulse. This device then reads how big the pulse is. Well, it's no longer a pulse.
It's a DC level now because it's being held. But it's a DC that the amplitude is determined by what the count is? It's determined by the amplitude of the pulse at the time that the hold signal came along.
And the hold signal is coming along at a time determined by this. So I don't understand what is the hold signal doing? I mean why does it hold? Okay, so here there's a pulse coming along. It's going...
I think it goes positive and then negative, as I remember. It's set up because we've got these negatives over here. So the signal that comes in here, I think, is negative.
This turns it to positive, if I remember this correctly. So you've got a pulse that's coming in, which is first. Oh, this also does a lot of shaping.
So the pulse coming in is monopolar. This does a lot of shaping of the pulse, and it puts out a bipolar pulse, which first goes positive and then goes negative. But this is inverting it, so it's going to first go negative and then positive, because it's being inverted.
That's what comes in here. So most of the time, the pulse coming in here is zero. There's no pulse coming in. But when you get a gamma hitting the...
The photomultiplier hitting the scintillator, then it sees some light, makes a pulse, and you'll see the pulse. And the pulse you'll see here will be this monopolar positive pulse. That's what you would see at the end of this cable.
And if you were to look here, you would see a bipolar pulse, which first goes negative and then positive. So the bipolar pulse... pulse.
I thought that before you get here, this has already been conditioned. This is just a digital signal. That's right.
So the bipolar pulse is coming out of the discriminator. Or it goes into the discriminator. No, no, this is coming right from here. Okay.
This is the pulse. That's the input. Okay, and then the output is... No, no, this is the output. So this is analog.
Analog in, analog out. Right. Digital in and digital out. Signal coming in here, digital signal coming in here.
Okay. Okay. So at the beginning, it's set into the track state until a pulse, until two pulses come along which satisfy the coincidence.
Then a hold comes along. Because of this delay. Yep. And it's delayed such that it catches, remember the pulse goes first negative then positive?
This is delayed enough that it gets the positive. Oh, so the negative is saying, okay, look out for the positive now. No, no, the negative isn't doing anything.
Well, the negative that comes along here is just ignored, okay, because the hold hasn't come along yet. Where does the hold come from? What causes the hold to?
That's this coincidence unit here, but then delayed by this. So this generates the hold. but then the hole doesn't actually get to here until it passes through here.
So this, a coincidence now, triggers this unit. It puts out a pulse which is counted first, then goes to this, and then from here it goes to the... Here it's being level converted and then it's going to here and here. Oh, sorry, here. So it's going to here and here.
This signal here goes here and here, after being converted from... NIM to TTL. Okay, so what is the point of having the hold after the signal has already been counted? We want to catch the signal, so the signal is long.
So it first goes negative and then goes positive. We want to catch it at the peak of the positive. That's when the hold signal comes along where the pulse reaches its maximum. So the hold comes along and holds that.
And so now what's read out here is that level. So the trap and hold now is putting out a level, which is whatever was at the input at the time the hold came along. Okay, so I think what I was saying was because you're taking a positive pulse and you're shaping it so that it has negative and then positive.
Yeah. The negative part is first. Yeah, but we ignored that. Oh, you completely ignored it.
So it's not heralding this measurement. Yeah, that's right. It's not playing any role. Oh, okay. Why do it then?
Why do the shaping and do the... Partly because it was not... We couldn't find a way to get a pulse.
We could have put this in positive and so the first load with the pulse would have been positive. So we could just flip this to positive. But it's coming too early. We couldn't get the pulse, we couldn't get the logic to go fast enough and this thing short enough.
We didn't even need this. I mean we could get rid of it and still we couldn't get the pulse, we couldn't get the coincidence to come fast enough to catch the first, let me call it, load. Because this is a bipolar pulse coming out.
It's first positive and then negative if this is this way. Or it's first negative and then positive if it's this way. Oh, I thought you said it was a monopolar coming in.
What comes in is monopolar. Oh, right, right, right. Coming out is bipolar, right. This is going to affect the output. Right.
Okay, so we couldn't get the trigger to go fast enough. So what we're doing is then catching the second lobe instead. And that...
required a fair amount of delay. So that's why we're using this device. But how do you, since you couldn't capture the the first part fast enough, what, why, why did, why can you now catch the second part if the first part, first part is ignored? What does the first part do if it's ignored? If you, if something isn't fast enough then it seems like you're at least using the first part to initiate to capture the second part.
Well we're not using the first part. I mean it's true that you're always going to get a first part, but we aren't using it anywhere in this. So it's completely being ignored, but then you're saying that... But it does go through here, if you like. So first it's negative, then it goes positive.
But all the time that it's negative, it's just, this is in the track state. So what's coming out is exactly what's going in. So what comes out is just whatever it might be. But we're not going to read it. This is not going to read that level until it goes into the whole state.
And I haven't shown you how that works yet. So I still have to show you how that works. All right.
I'm ready. Okay. So here's the output now of this, which goes up and is now level converted. Let's see now. Oh, sorry.
That's the trigger. So let's, let's see now. The computer, at this point, the computer gets triggered. And here, let me get this right now.
Yeah, so the track and hold when it sees a hold either on this one or on this one, either one, it puts a signal out the back here. That's this, which then goes to the lap jack. When you put the computer now, so you run the computer.
Let's see if I can find the program. It might take 20 minutes just to log in. Oh, is the internet back up? I got an email.
Yeah, yeah. A while ago. But this is wired, isn't it? Oh, is it just wireless that was out?
Yeah. Nice. But wireless is up also. I'm going to pause this. Whatever the problem was has been solved.
Do you have more you want to say while we're waiting for that or should I pause this? Well, let's go through the rest of this part. So this part right here now is being monitored by the computer. When you put the computer into the run state, or the program I should say, into the run state, what it does is it simply continuously reads this in a little program loop until it sees that this digital state now has You see that some of these are called A for analog. Analog out.
And then I for, I don't know what I is for, but these are just digital signals. And these are all analog. So these, for instance, are the two things, the two levels that are coming out of the track and hold.
Yeah, so these are going in because it says AI, analog input. They're coming from the equipment. Right.
It's coming from the hole. When this goes into the hole state, it's holding whatever the input was at the time this came along. So now this is an analog level, which now can be read by the computer, but the computer doesn't read it until it gets a signal.
And where are these two guys coming from? These are also the... These two right here. You said it's waiting for these two.
Yeah, so we've done this one right here. This one is coming from the track and hold. Right. To say that at least one of these channels has gotten a hold.
Okay. Now the way we've wired it, if this gets a hold, this gets a hold too, because they're coming from the same place. Why have that redundancy? No, it's not redundancy. This is for one channel.
This is for the other channel. Oh, I see. And we've determined that two pulses... A pulse has been here and here at the same time by this coincidence unit.
Oh, so now we want to look at those two pulses that we've decided. Okay, that's right. So, but the computer will only do that when it sees a signal on this line here.
This line here is the line that goes back and resets this, puts this into the track state. When you're finished reading it out, so the program now, It's going to see, it first sees that a coincidence occurred that is coming into the computer on this line. When the computer has now read these two, it's ready for the next event. So it puts the signal out on this one, which puts this back, this and this back into the track state. Right.
Ready for the next event. Okay. Okay, now let's see.
Okay, this is the type you might get. Yeah. So I think this is just calibration data. So here we see a radioactive source. And let's see.
And here now is probably that same radioactive source, but it's the peak is in a different place because the gains are not exactly the same, which is probably because the high voltage on the photomultiplier tubes needs to be adjusted a little bit. So you want, well, it depends what this radioactive source is. I'm going to guess it's the barium source. So you want these pulses to be fairly high, but you don't want them to be up in this range because this is the upper level.
What is the x-axis? the pulse height in volts. This is in volts.
So this is the height of the pulse which is being read here in volts. So that's rather nice that it's actually calibrated. But it's not perfect.
The calibration isn't perfect because this should be putting our pulses as big as 10 volts. But this is not. It's 8.2 or something. So you don't want your pulse to be so high that it gets to this region, which is very nonlinear, because a pulse bigger than this... It's going to add up.
The reason this is big high right here is that it's actually all pulses greater than 8.2, which is not being output by this device here. So bigger pulses are coming out as 8.2. Okay, so, but this is, this peak here is well below that.
How do you know it's coming out at 8.2? I'm just reading it off the scale. Because you know, you know because there's nothing else bigger than there. Yeah, that makes sense It just it just doesn't make sense that there would be a peak here.
It makes more sense that it's flat That's that's what it would be right the peak is only an instrumental effect Right, right because the device can't handle a pulse bigger than that Okay, but here in if I were to see this I would say I believe it's calibration data. So in a single run what? The student did was put the radioactive source first on one scintillator and then on the other, or they could get two identical radioactive sources and put one on one side, one on the other. But you wouldn't want coincidences in this case.
You'd want this to be on singles. So all you need to do to calibrate is put this on here. Okay.
Okay. And then you would take a data, whether there's a pulse here or here, either way. You would get a trigger.
Is it set up where we can just put cesium on there real quick and actually demonstrate the counting? Yeah, we might be able to. Let's see now, I'm just looking at the plot.
The program which runs this. Gamma, gamma, right there? Yeah.
I'm not even sure LabVIEW is licensed on this. I think we had a problem last time we tried to... Oh dear.
You mean it's run out of... Oh yeah, see. Login to activate. I don't know if there's like a... Okay, so let's ask Mike Eklund to do something about this.
Let's leave it just like this. Okay. Let's have a smite at that.
I don't know how busy. Well at least if you can, we can at least see the counts on here. And we can see that they're going to be, they should be about the same if we have similar sources on each detector, right? With the same activity, yeah. But one might have been bought before the other.
Right, right. What's the half-life of cesium-127? Is it 137 or 127? 137. And it's a half-life of what? 30 years.
So as long as they're within a few years of each other, they should be pretty similar, right? Yeah, that's right. But they may not be.
Do they have dates on them? I've been here, I don't know. But I've been here 33 years.
Oh, did he put them away already? Oh, he put them over by the thing. I saw them putting it somewhere.
Oh yeah, it does have dates. December 2006. April 20... Not sure I can read that date. April 2018. April 2018. Oh good, since we have two of the same date, then they should have the same...
These are cobalt 60. And this is cesium 137. Oh, they're two different. Doesn't turn good. This one looks like a 2017 or 18. 28. Oh, it might be 13. April 2013. Yeah, it could be.
Okay, anyway, it's not terribly important that they be identical. What you can do in the middle of the run is just change them rapidly. Right.
Okay, but we're stuck here on the software license. And I have a feeling that the login has to... ...be done with somebody who has...
And if Mike Antwin doesn't have... If he's unavailable, somebody else might be. I know...
Do you think you can explain just a quick rundown of the typical sorts of tuning that you're gonna have to do on these dials or whatever? Yeah, the only thing I would do is adjust the high voltage a little bit to get the... So first of all, I would take the barium. That's the one I usually do. Yeah.
So barium source. And then remember that's two energies. One is 0.356, I think.
Yeah, 0.356. And I would get that to be the same on both sides by adjusting the high voltage. And the high voltage is very non-linear, so a small change in high voltage makes a big change in the amplitude.
Oh, I thought you said it was linear. No, no, the high voltage is not linear. amount of light making a pulse that's linear.
But if you want to change the calibration constant, you can change the high voltage and it doesn't take, it's a very small change makes a big difference. So that's a disadvantage of photomultiplier 2s, they're rather sensitive to the high voltage. So you need a high voltage supply that's going to be rock stable.
Okay, so let's just run through the main ideas. When we get them, so what does it mean to get them to equal, to the same amplitude? We're talking about the analog pulse shape, right? And that's, so this is shaping the pulse. This is thresholding it in between the high and the low.
This is the raw data coming in. So we should be seeing the raw data. So you can change the pulse height two different ways.
You can either change the gain here or you can change the high voltage here. And either one will do the same thing within some range. So if you have different pulse heights, whatever is the most convenient, get them to the same pulse height. The way that you know they're the same pulse height is that you've set the thresholds the same. Yeah, but you want these knobs to be roughly where they are now.
So the reason you have this knotted, you want this at around one, which is where it is. So you don't want to accept voltage which pulses whose height is less than one volt. The reason for that is the noise levels are such that you get swamped.
Yeah so this is very similar to the Geiger-Muller. You're setting it high enough to where you're ignoring the noise. And these you want at 10. So these are set at 10. So you don't want to fiddle with these. Oh okay. Don't stay that way.
Okay that's good to know. But these you can fiddle with and over the years we fiddle with these a lot. but I think it's better to adjust the high voltage by a little bit. So then how do I know which the pulse shape? I'm getting in the raw data here, the raw pulse, the unipolar pulse.
I'm getting the bipolar pulse out of this. This is the amplified. This SCA, I forget what SCA stands for. This is just a digital signal which fires whenever the pulse height is within this range. Here you'll get a pulse out regress.
So then how do I see what my pulse looks like so that I know how to adjust the voltage? Right, so well one way is just to take the data which is what we usually do. Oh because you're counting how many pulses come in within a range?
So you're not actually looking at the pulse height? Well this looks at the This device here is actually measuring the pulse height of the second level. Yeah. Oh, so that's, okay, so you don't really care what the raw pulse looks like.
Well, you do, I mean, you do care what it looks like. And so we usually get it into the right range by simply taking, well, there are two ways to do this. We could either, and I'd probably recommend it.
You first take this and plug it into the scope. I see. So this already has an in... oh this already has... oh this has the...
So this is looking at the amplified output. The amplified and... Shaped.
So this will be looking at the bipolar pulse. Right. Which is also fine. Yeah, well, I mean it's still going to have the same amplitude characteristic, right?
If it's too high, if it's too low. Yeah, that's right. And it'll be even in volts.
This will be calibrated. You'll see it calibrated. So you'll be seeing here in volts what this is going to pick up. Why don't we just, can't we just turn this on and look at it?
Sure. I mean because since that doesn't work, but this does, then might as well look at something. Let's see, once this rack is turned on, I don't see an on-off, positive, uni, bipolar. Oh, so right, this is doing unipolar, bipolar. Right.
Amplifier, this... Oh, then there's a... This is the window it's talking about here, but what is this?
INT... It's integrate. So I think that's going to change the shaping.
So I don't see an on-off. So it's just essentially... No, there's no on-off.
Whenever you've got power here, it's on. Okay. So these are all...
I assume we're looking at a bunch of pulses here that I... Let me see. I'll see what the settings are.
100 milliseconds, so that's a really much shorter. Okay, now let's position this. You can just do a single trigger event. So we're probably triggering on channel 2. Yeah, so we're triggering on channel 2, and that's this one.
Okay, and let's... But it looks like it just triggered on one event, and it's holding that event, right? No, we're not looking at any holding right now. We're looking at just what's coming out of the... Oh, so this is the same.
This is hundreds of pulses that are just over and over again. Right, that's right. It's a very similar looking pulse, but they're different enough. You can, it's obvious that they're different. Now one thing you can see right here is that this is saturating.
Right. So it's, let's see what the scale is now. It's 20 volts per... division here. Oh, so we're way amplified.
Yeah, let's see that can't be right though. Oh 20, oh I want channel 2. Channel 2 is 50 volts. So let's um let's take channel 2 down too. 20 volts.
Let's see. I don't know. Oh, I see. No, this is the amplified pulse.
Let's see though. It's the amplified pulse Why is it so big? This is 50 volts per division Usually it says per division though.
Why is it just say 50 volts? Well because oh, that's just the way it reads it out Right, they would say per division, but they don't have enough space So it's always been this way for lifetimes. Oh, okay.
Okay, but now let's see. One thing, I'm using the cesium source, which gives off a 0.662, which is almost a factor three bigger than we want. But you're getting like a 200 volt. Yeah, I still don't understand the scale. Oh.
Is it the amplification is so big? No, I bet. Let's see now.
Let's see if this is 50 ohm. This should be... This is something I have to look up, I guess. There are things being done here which are not exactly... proper for transmission lines because you want all the this is 50 ohm cable so you want everything to be 50 ohm and oh because you're saying that this is high impedance input here and well i that's what i want to see what is the input because you can sometimes you can select that you want 50 or right um well i would say just probe 10x but um like i know ah there it is so that's right so we're now looking at Let's see, attenuation 20 times.
Let's see. No, I don't understand this. Wow, what a range.
But nothing's changing. We're not attenuating the signal with the probe though. Right, I know what you're saying and it's what...
Voltage sources have this high impedance versus transmission line. Right, yes, you're right. So some of them will allow you to select 50 ohms, and I haven't seen how to do that.
I don't think these cheap ones have that capability. I think it's just always high impedance so it doesn't ruin the instrument. Okay, well that might might be why we see so He's appearing to be so big Are we expect we're not expecting a current here though.
Aren't we just measuring a voltage? This device assumes that you're terminating in 50 ohms, so let's see if you don't oh I see So, and the transmission line, what comes down the transmission line, neglecting reflections for the moment, what comes down the transmission line will be, whatever it is in voltage, you use 50 ohms to get the current and vice versa. However, if there's a reflection here, then that completely goes out the window. And so then I'd have to...
Take a pencil and paper and figure out what's happening. My guess is that's what's going on right now. Yeah, but the reflection shouldn't be a big deal if it's a DC signal. Yeah, but these are not DC signals.
We're wondering why this is appearing to be 40-some volts. Well, what I mean is the signal that's coming here is just a single pulse. It's not like a...
a radio wave that is trying to send power in through here. Well, it is in a sense though. So you think that this is pulsing fast enough that it thinks it's a an RF signal and so it's got a standing wave, it's got this high reflection coefficient because of that?
No, even for individual pulses it'll behave the same way. Which you can see by just imagining that you Fourier analyze the pulse. Right, because the frequency components, not to mention it's 5, 10, 15 microseconds.
Right, and if you count this it's even longer. That's high kilohertz. Right, so the Fourier components would be quite high frequency. So then are you thinking that there's... Horse, what does that mean?
So it doesn't say anything about 50 ohm termination, so it's not 50 ohm terminated. So let's try the following. Well, it's not a 50 ohm termination.
Huh, no change. Why is that? Because if this were 50 ohm terminated, now I've got 50 ohms and 50 ohms in parallel, which would be 25 ohms, and the signal would change. Well, not to mention, the oscilloscope has very high impedance, input impedance, so if you're putting a 50 ohm there, then it would change it dramatically.
Okay, I don't know what's happening right now. All right, demonstrate that again while I have it on video. So now we're seeing no source, no source.
We're seeing a lot of different pulse heights, right? Now let me put this on. Now this is cesium-137, which emits a single energy gamma ray.
of 0.662 MeV. I see. So now we're seeing 0.662 MeV gammas most of the time. Now that background is still there, it's just that it's being overwhelmed by the source. And the divisions here is five volts, so after amplification this is 10 volts peak to peak.
Right. And you have the amplification set to 50 it looks like. Right. 50 here and then the fine adjust is... I usually write a little arrow in the run sheets to indicate where this is.
They're slightly different from each other? They're slightly different, right. And it's not a big... watch, it's not going to be a big difference now. See, I changed it up to roughly here now.
You can see it changing by a little bit. Now one thing we can see is that this is 0.662 MeV. And it's not saturating. See, this is only 6 volts maybe.
We want this to be something of order of 5 volts maybe at 0.25 MeV. But we're now using 0.662, which is almost a factor of 3. Well, 2.5, let's say, times bigger. So what we see now is that we've got too low a gain. for the signal that we're interested in.
But if we're going to use the barium source which is 0.356 MeV, we want that to be on scale also. So it's really we want the 0.356 to be somewhere six to six and a half volts somewhere in that range because we don't want to get too close to the 0.8 volts 8 volts we don't want to get too close to that because that's where it saturates and the other reason that we know that's where it's saturated is because we looked at the histogram I mean what I mean is what what is determining the 8 volt saturation it's it's this guy right here So the amplifier in here saturates at around 8 volts. Oh I see that doesn't output any more than 8 volts.
Plus or minus. But now it's actually supposed to be 10 volts and if you look on the plus side it looks like see I'm raising the gain here so we can see it easily. Let's go one more. Okay, so here I can see it's saturating at somewhere between 11 and 12 volts.
Yeah, so it's just offset. Right. Well, for some reason the positive side has a higher range than the negative side.
Right. But if you look at the literature, it says, as I remember, it says 10 volts. So I was a little surprised to see that this is less than 10 volts. It's close to 8 volts.
And that's where we saw this peak in the histogram. 8.2 volts. Okay, now I'm going to turn the gain back to where we were.
Okay. Now if we were using this source and we wanted to have a nice pulsate spectrum and make use of most of the range, then this would be a good choice. We're around a little four or four and a half volts or so. So that would give a very nice peak at around four volts.
But that's not what we want to do. We want to have the barium source peak be around six volts let's say. Right so we'll just have to replace the cesium with barium and then amp up to or increase the gain so that it's about the same.
Right, that's right. But why? And that's not so hard to do because we can see here, we can look at the pulse and see it.
What's magical about six volts as opposed to like seven or eight? I know eight is where it saturates but... Right, so you don't want to be close to eight and you can see that it jitters. around a little bit.
You just want some play above six and yeah because you're going to get a peak which has a width to it and you want you don't want part of your peak contributing to that overflow. Okay so essentially once we know we're getting the the the the uh the indication of the pulse that we're expecting, then we can be pretty confident that once we turn all this on, everything below here, like we can adjust the gain, but everything down here, there should be not much adjustment. Right, that's right.
Let's see, so one thing we should be putting this on one, I guess. So when you calibrate you want to put this on one. Assuming we have a sample on each one. Right.
But even if you don't... So can we just test it to see what it's... Well, for looking on the scope it doesn't matter.
Right, right, right. But if we're going to report it... Because we can't, both of them.
If there's a way to trigger both at the same time that would be nice, but the trigger menu only allows triggering on one source, channel one, that's it. Right, well you can trigger on one and look at the other, but even in the Gamma Gamma experiment, most of the pulses you're going to get... are not coming from the gammas due to the annihilation. It's coming from things like cosmic rays. But then there's a lot of gammas which penetrate through the lead.
And yet, you know, they might go right through the lead and come to this also. So in other words, trying to trigger on anything here, you're just going to get mostly noise you're going to be looking at. So you won't be able to see the coincidence. That's right.
You'll never see that gamma-gamma coincidence. So that's why all this down here you have to have to see coincidence. Exactly.
But you'll at least be able to see one of the channels triggering due to the gammas. Well, right now we're not using any trigger. We're just looking exactly what comes out of this device. Right here. Well, what I mean is the scope is triggering on the signal on channel one.
Yeah, that's right, which has nothing to do with any of this down here. Right, right. So, okay, so now we know we have a good looking pulse.
Let's count these two guys on the on the photo multipliers and see the count. accumulate and it seems like it seems like this is something that we should be able to have the computer account for us because these are going to be going so fast how are the students going to be able to collect the data on the on the display fast enough let's see um why are we not So we're counting coincidences. Or is that the timer? Is it on? Oh, it's counter.
Yeah, it's on count. Um, maybe this is gated. Yeah, there's a gate signal.
Yeah, it's being gated. Let's see. Thank you.
Now I'm going to turn this to 2 and this will stop. So if it doesn't stop, I mean we do occasionally get coincidences, but rarely. So what we typically see over here, when we actually have the thing opened up and we have the aluminum things in, we'll see maybe two events per second, something in that ballpark.
So in other words, two coincidences per second. And this thing is going, always going very fast. Because, let's see, let's change this.
Did it slow up? Let's watch this number. Oh yeah, way faster. Well, way faster, maybe a factor of four or something.
Yeah, not way faster, but... Yeah, I would expect it to be hundreds of times faster. Okay, now watch this and let me change the high voltage. Let's make sure, yeah.
So I'm going to raise the high voltage. You see the pulse getting any bigger? Yeah. You're almost saturated.
Oh, okay. So, and I hardly changed it. I went up by 100 volts out of 2000. And yet you see a big effect. And I went back now.
Alright, well it seems like... We screwed around with this enough that I think we figured out how to make it work and we just have to get the lab view working. But let me make the following suggestion that when it comes time to actually do this with a student, let me know.
And I can come down and get it. It's really not. So this is the peak that we're interested in.
And this really, if we had really, really good resolution, this would be just a spike at one energy. And then this would start at zero, rise steeply, and be more or less flat all the way down to zero. That would be the Compton continuum. So this would be the photoelectric effect and pair production if the energy is high enough, but our gammas are not high enough to pair produce.
So we don't get any pair production in the sodium iodide crystal because our gammas are not high enough energy. So this is all photoelectrical absorption. and then it should be a steep rise and then flat.
But for various reasons, not all of which I understand, it looks more peak-like. But it's this and this that we're interested in. So when you say this and this, you just explained this and this. Right.
This is the If I trace this out right here, this is the unfilled histogram, and then this is the filled histogram. So there are two histograms here overlaid one on top of the other. What do you mean unfilled and filled?
Well, filling means that you've got a color gray. case underneath all of these. So it's this right here.
Now the unfilled one is this one which is laid on top of that one. So if since it's unfilled it has it isn't filling it with anything. So when it overlaps this one which is filled you see this color.
Had I filled this with white you would see all white underneath this one. Right. So then what what what are the two So this is pulsite 1 and this is pulsite 2. So this unfilled is pulsite 1, this filled is pulsite 2. Oh, I see.
So... Why are they so different? Only, they're not so different actually, it's only that the gain is slightly different. So if we raise the gain on this one, this would move, this whole structure would expand a little bit and move outward. And the counts?
And the counts, uh, no, why are the counts more here than here? Um... somebody started counting different times? Well let's see, so one possibility which is probably not explaining all of it, but if I were to take this and expand it, it would leave fewer counts in each bin. So it would, as I expand it, the area stays the same, so the peaks would have to come down.
Oh I see. But I'm not sure they'd come down by enough to match this. So what you're saying is whatever's cut off here is being Alias back over it's kind of like aliasing on the histogram I mean, it's not literally aliasing but it's all the all the counts had to go somewhere And instead of being counted down here where they're thresholded out. They got tacked on over here somehow Is that I mean, you know, that's the thresholding would only happen up here. Yeah, that's what this is for That's what that's why we have this that's where the right.
I mean the saturation not in thresholding. That's the saturation Right. And if I increase the gain here, this would only get bigger.
So it looks like there's less area here than here, but not sure. I mean, this this comes from data and for every data, for every event. There's one pulsite one and one pulsite two.
There's... There should be just as many in this histogram as in this histogram. And it doesn't look like it to the eye. So maybe somehow... I'm not sure.
Maybe it's because this guy is wider and so it counts more and this guy is narrower and taller. Yeah. This guy might be a little more narrow and well, this guy is about the same width. If we were to count up all the... All the hits, all the contributions, they would certainly add up to the same.
Or at least they should. Well, they have to because that's the way the data is organized. So that's another thing we need to do is actually look at the data and then look at how that data then gets into a plot like this.
This plot is basically the same as this, but it shows the correlations where this doesn't because here you could say, okay, This bin right here has a contribution from some event. Where is that contribution for that same event for pulsite number 2? Here it shows it. So this bin right here is all those pulsites, which were roughly 6. It should be down to three or four. Right, yes.
I should go down to the base and then out. Right. So I go down to the base, which is… I'm following this one down.
Let's see. So it's going to be there. That's hard to do. But anyway, this particular bin right here represents all those events in which pulsite… Number one was a particular value and pulsite number two was a particular value. And you were saying the pulse shape wasn't something but it was more of a Compton Continuum?
What is that again? Yeah, so this would represent a photoelectric effect for this pulsite, a photoelectric absorption for this pulsite, but a Compton Continuum for this one. Right, right, right. This one right here is Compton Continuum for both. Oh, I see what you're saying.
Right, right, right. So this would be a case where both the given event would contribute here for the unfilled one and here for the filled one. And an event here would correspond to pulsate one here and pulsate two here. So both of them in the...
photoelectric absorption. But in both cases, on the other one, in all peaks, all four peaks here, they're all due to entangled pair gamma-gamas. It's just the left side might be decomposing into one process or it might be terminating in one process and the right side might be terminating another process.
Yes. But either way it's correct. What you say is correct.
Except of course there's some background under here. Right, right. And we have a procedure for subtracting that back. Which we should discuss at some point also. Because there are what we call chance coincidences.
Thank you. We've been here a while. Is it still going on or? Taking a tour. Oh, okay.
Who does the tour? Is it Rogan? Jingwen is doing it.
Normally it's the two of us, but he doesn't need me this time because I've been in the last year and a half We've been a year. We've been filming it because it's doing the tour. Yeah Won't need anybody anymore well Maybe once once I get this lab view up and running then You can just run us through really quickly how to yeah yes so We'll run through from the... Say that again, sorry.
I forget that I want to be filming this for the... So the time of day here is important because you're trying to get the coincidence times. Right. Okay.
Now, all you really care about is not these individual times. All you really care about is the time at the very end of the file. So this will take a lot of scrolling, I guess. That would have taken you months.
Right. So there. So now, that's how many seconds this run took. Now this column right here is not used, so ignore that. So this is the time, this is pulsite 1 in volts, this is pulsite 2 in volts.
So here you see an example where pulsite 1 was tiny, negligible, and pulsite 2 was substantial. Now this looks like it might be a calibration run. because all of them appear to have one pulsite big and the other one small.
So I'm going to guess that was the case for this particular calibration run. So let's see what we see that's any different. Well, here's one where they're both, well, let's see, one's negative though.
So yeah, so this... Yeah, it looks like almost always one is a... Remember our one is our threshold, one volt.
is our threshold. So one is above threshold and the other seems to be below threshold every time. How's that?
Well that would be a calibration run. But how would it give you one side is always the opposite of the other side? Well, so that's the case where we turn this to one. Right. And then put a radioactive source on both.
Oh, right. And it's very rare that you'd be getting two pulses at the same time. Right. I mean, out of...
So one is noise and one is signal from the... Calibration source. I mean this might this might be This is a total of like six.
Is this in seconds? Is this time in seconds? Yeah, so this this time went up to like less than 700 seconds, right? Which is awfully short, right? So in 700 seconds, you might might expect one or two Just random poison at coincidences Yeah, so in fact that's something you really want to calculate.
How many random coincidences do you expect within a certain time? And that you can calculate from the data. So you've got...
Remember that this is counting... there are two scalars in this device here and you're counting how many times did this one fire and how many times did this one fire. And if you assume that all of... those are random, which is true. It's because the signals are a tiny, tiny fraction of the total singles counts.
So assume they're all random. Now all you need to know is what is the coincidence overlap. So there's a single parameter that you need which is sort of related to this number we were looking at.
What was it? ...it's like a resolving time. It's not that... well, I think that resolving time right there is not the one that we're interested in. This device is putting out pulses of a certain length.
And once you know what that length is, and you can determine that by simply looking on the scope, then you know, then you can calculate... what the random coincidence fraction is. And you also want to make this dead time correction.
So there you're going to compare the number of events read in by the computer to the number of coincidences sent to the computer. And they're not the same because the computer can't always keep up with if you get two events too close in time, the computer hasn't had time to read out the first one and then set the track, set these tracking holes back into the track state before the next event comes along. So it loses events and it's a small fraction of loss that are lost but it's a correction.
that one should make. Because the rate when these are parallel and the rates where they're perpendicular are different rates. When they're perpendicular, the rate is higher, the loss is larger.
So you want to make a correction to the fact that in the perpendicular case, you're losing more events, and you want to correct for that. Why would you be losing events in the perpendicular? Because the rates are higher. Because this gamma comes out, if this gamma comes out this way, this gamma is coming out this way, and so this one prefers to scatter down and this one prefers to scatter to the side. So you're going to get higher count rates when they're perpendicular versus parallel.
Right. So, and the computer can't keep up with… Oh, I see what you're saying. You're getting a higher count rate, but the computer can't keep up, so that's why you're losing more.
So it loses more. Right. Now it's a small correction.
And how do you know how many are being lost because you know what to expect? Because you record separately from the computer, you've got to record these by hand. Oh, okay. That's what I was... You count this signal's rate, this signal's rate, and the coincidence rate.
But it's going so fast. How do you... This is at the end of the run. So at the end of the run...
So at the beginning of the run... Oh, I see. The beginning of the run, the way I do it is I put the cursor on the run button, and then I put my fingers over here on the count buttons, and I think I've got it wired so that you only have to hit this count. That's something we'll have to test. So I hit them both at the same time.
And then at the end of the run, when we're going to stop it, I do the same thing. I put this on the stop, I put my finger on the stop here, and I do them both at the same time. So there's no GPIB connection here where we can just have lab view sync? Right.
Correct. Okay. No GPIB.
It's just ancient, ancient stuff. But we've got it. But not here. All right.
Is that all you think? Thank additional for now until we do the experiment?