Thank you. At the time he carried out the experiment, the atom was thought to be a blob of positive charge with negative electrons embedded inside it. Now Rutherford set up an experiment to direct a beam of alpha particles, which he knew to be heavy positively charged particles, and we now know...
with the nuclei of helium atoms through a very thin gold foil. The brass can in the centre of the picture contains a small americium-241 source producing alpha particles. Most of the particles absorbed in the brass, there is a small opening producing a...
beam of alpha particles. They pass through a gold foil, which is about 1.5 microns in thickness, and they are then detected by one of the two detectors set up behind the foil. One of the detectors counts particles going straight ahead, which is the majority, and we count a rate of between 1 and 2,000 per second. The second counter we can move to measure the scattering rate as a function of angle. The experiment that he and his colleagues carried out was very laborious.
It involved them sitting for hours in a dark room, holding a tiny fluorescent screen, and looking just by eye for flashes of light on the screen. We've replicated that with modern detector technology. In its current position, behind the foil, it will measure one count every few seconds.
As Rutherford deduced, most of the gold atoms empty space. So the alpha particle just goes straight through, doesn't see anything, and goes through in a straight line. Occasionally, one of the particles will come close enough to the nucleus of the gold atom that it will be deflected by the electric field from the positive nucleus.
If it comes at a glancing blow, it will be deflected through a small angle. It hits it head on, it'll just bounce straight back. It's fascinating, I think, to them that this was telling us something very important that we didn't know about the very deep structure of matter.
It showed that the atom had a heavy nucleus with mostly empty space with electrons distributed around it. which is exactly the picture we now have, it was also the start of particle physics. What Rutherford was doing, what we're doing here, is taking a beam of particles, firing at a target, looking at the way the particles are scattered, and from that deducing what was going on inside the target. A hundred years ahead we now have the Large Hadron Collider where we do the same thing. We fire a beam of particles at a target.
In that case the target is another beam coming the other way. We look for the particles being scattered out of that collision process and try to deduce what was going on in the collision from the behavior of the scattered particles. And all that comes from Rutherford's original idea of firing a beam at a target.
So in a century we've come from an apparatus that would fit on the desk, that was the cutting edge of physics, to the Large Hadron Collider. which is 27 kilometres in circumference. What was happening was that the alpha particles were going through and missing the nucleus because the nucleus is extremely small and the atom by comparison is very big so it's almost all empty space and we had to wait a long time before one of those alpha particles hit the nucleus almost head-on and bounce back into our detector.