Investigating Possible Impurities on RBS Spectra of a Rhodium-Silicon System

[atha]

I was given a sample, that it is suppossed to be a medium thickness Rhodium on top of an infinite silicon waffer. The experimental RBS spectrum along with the simulation looks like that

Apparently, there is another element(or maybe 2) that appear on the experimental spectrum.

I am relatively new to RBS, so I find rather confusing to see two narrow peaks, so close to each other on the substrate part of the spectrum.

If the waffer wasn't pure, I would expect some steps, rather than peaks. The way I see it, it's an indication of a possible thin and light layer of unknown elements. How to specify which elements could that be?

Is there also a way to find out if it is really another layer and its position?

On a second look, it seems that the real spectrum has less events that the simulated, which could be an indication of a non pure Rhodium sample. The question is, which element(s) is responsible for the possible impurity?

 

[mfb]

Do you see the same peaks without your sample?
Could that be background from some radioactive material?

 

[atha]

Do you see the same peaks without your sample?

Unfortunately, all I've got is the attached experimental data;nothing else...

Could that be background from some radioactive material?

It's a possible scenario, but I doubt if I can prove it with the simulation.

One thaught, was that the vacuum wasn't well built, so I thought to add a layer of oxygen and nitrus. Unfortunately I couldn't adapt this scenarion to the simulation...