Alpha particle deflection by 180 degree in Rutherford's gold foil experiment

Did some of the aplha particles back trace their path after hitting the gold foil ? (Turn by 180 degrees.)

If so, how were they detected ?

• Sure, some are retroreflected. There are two possible ways to deal with this (I'm not claiming any particular experimenter did so). First, place detectors at $180 \pm\epsilon$ and interpolate; second, put a 50-50 "beamsplitter" in the outgoing path and collect $\alpha$ particles off the splitter path. Commented Apr 25, 2014 at 12:29
• @CarlWitthoft Thank you. But I have very less background in physics and am unable to understand what you are saying. Could you simplify that ? Commented Apr 25, 2014 at 12:44
• @biogirl What he said is that some particles are reflected back (retro) and there is detector placed everywhere around foil. It is like they were going around foil with "microphone" and recording how much signal (particles) they get. Commented Apr 25, 2014 at 14:37

Rutherford's experiment looked much like this:

As you can see, the incoming alpha particles hit the gold foil and could scatter in multiple directions, but the detector went around the whole foil (sparing some small region so that the alpha particles could enter the experiment) so even back scattered particles would be detected.

• Thanks. I can understand that the particles which are deflected by angles greater than 90 can be detected by this but I am still not getting how particles deflected at exactly 180 can be detected... Commented Apr 25, 2014 at 12:52
• If there's no detection on the detector, then it must have gone back through the hole it entered. Commented Apr 25, 2014 at 12:53
• @biogirl Your thinking is correct. In fact it is rarely possible to prove something 100 percent. Especially in something to do with geometry. And your question is how do we know it is exactly 180??? We don't. We just extrapolate it from the fact that there are particles reflected on angles 170, 177 degrees and so on, Approaching 180 degrees as close as perfect you are with your experimentation. Another thing to help you understand this matter is that there is no IDEA which forbids this to happen. Commented Apr 25, 2014 at 14:41
• @biogirl What i mean is that there is no visible theoretical reason why this should not happen. No physical law forbids reflection on 180 degrees. Commented Apr 25, 2014 at 15:10
• BTW, the "detector" in the diagram above was originally a grad student sitting in the dark for a few hours, watching the screen through a microscope and counting flashes... Then move a few degrees and repeat... Commented Apr 25, 2014 at 17:44

Rutherford's law is that the number of scintillations would depend on the scattering angle $\phi$, measured with respect to the incoming trajectory, according to $\sin^{-4}{\phi/2}$. If $\phi=$ 0° (180° according to your way of measuring it), $\sin^{-4}{\phi/2}$ is undefined.

• It is undefined at 0. That implies that it is not possible to have a deflection of 180 , right ?? Commented May 4, 2014 at 2:47