# Rutherford Scattering Spectra

I am using SimNRA to simulate the classical Rutherford Scattering. Playing around with it, I came across some spectra that I cannot explain...

First of all, if someone plots the spectrum of scattering angle $\theta=0$ will get one! I expected that I won't get any spectrum at all. Are there any higher order terms apart from $1/\sin^4\theta$?

Secondly I get three different kind of spectra. For angles $0<\theta<60$ I get a perfectly gaussian distribution

If I am around $90^\circ (80<\theta<120)$ I get a rhather confussing spectrum which looks like that

For scattering angles $120<\theta<180$ I get a weird spectrum as well

Why am I getting so different a spectrum for those scattering angles ranges? I've read that the last one is used in RBS to define the thickness of a leyer but why does it have that specific behaviour?

And what is this small peak around $90^\circ$? Could that be a recoil gold atom? If so, I cannot understand how a light $4.7\;MeV$ particle can move away a heavy atom...

Any help or hint will be more than welcome!!!

EDIT All the above are for a target of $2\mu m$ thickness and a beam with $500keV$ spread. If I turn off the energy spread the backscattering specrtum has the same flat distribution.

In addition if the target is $5\mu m$, with no energy spread the spectrum is

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Looking at the vertical axis of the last plot it becomes instantly clear that this piece of software is not plotting histograms of counts (which is the way you should be looking at this kind of data, simulated or not). The smooth curves you're seeing are lying to you. –  dmckee May 3 '13 at 16:32
@dmckee: Thank you very much for your comment! Indeed, it's not plotting histos, but why is it lying to me? The shape is-let's say- something that I've been expecting. I've seen similar from RBS spectra. What I am trying to get is why there is such a flat distribution in the last plot and what is the peak in low energies in the second plot. –  Thanos May 3 '13 at 16:45
Is it simulating energy loss as well as Rutherford scattering? –  Ben Crowell May 3 '13 at 16:52
@BenCrowell: Thank you very muach for your comment. It's simulating the spectrum that should occur for different scattering angles. In general SimNRA can "simulate" scattering! –  Thanos May 3 '13 at 17:10
A vacuum chamber doesn't help when the target is of finite thickness. Worse, for alphas very, very low areal densities are still significant so even thin foils are significant. Ben's questions is important; it goes to the question of how sharp the energy distribution of the alphas was at the time of the interaction and how much energy they may have lost in leaving the target as well. –  dmckee May 3 '13 at 18:27