- #1
lamadwp
- 3
- 0
Hi!
In my lab class, we are using a photomultiplier to examine gamma ray emission lines and determine absorption coefficients of different radioactive materials.
As we test for absorption (by layering on metals, incrementally increasing the thickness), we consider spectral graphs. We compare each subsequent graph (one from each thickness increment) to that of the nucleus without any absorption material and notice that, while the peaks we are following (they are the decay energies, eg. for Barium, we followed energy 0.356 MeV) are decreasing (the height of the peak is determined by the number of counts recorded by the photomultiplier) with increased metal thickness, other peaks are increasing in count.
We want to know what is causing the increase in lower-energy readings.
We also want to understand all the spectral features of the graphs as we are very unsure of any peaks outside the given decay energies..
{see attachment for graph} Each Series in the legend denotes an increase in absorber thickness of about 6mm.
Not indicated on the graph is that we followed the peak at about 100 on the x-axis (it's not actually 100 MeV or anything--the program we used filtered energies into channel bins, which we calibrated. The indicated energy here is 0.356 MeV). We attribute its decrease to absorption of the gamma rays as we added more material between our source and the detector. However, the peaks before channel 50 increase with material thickness. This is what we are trying to figure out.
We are considering Compton scattering, elastic scattering, and photoelectric absorption, but are unsure as to where these are applicable.
In my lab class, we are using a photomultiplier to examine gamma ray emission lines and determine absorption coefficients of different radioactive materials.
Homework Statement
As we test for absorption (by layering on metals, incrementally increasing the thickness), we consider spectral graphs. We compare each subsequent graph (one from each thickness increment) to that of the nucleus without any absorption material and notice that, while the peaks we are following (they are the decay energies, eg. for Barium, we followed energy 0.356 MeV) are decreasing (the height of the peak is determined by the number of counts recorded by the photomultiplier) with increased metal thickness, other peaks are increasing in count.
We want to know what is causing the increase in lower-energy readings.
We also want to understand all the spectral features of the graphs as we are very unsure of any peaks outside the given decay energies..
Homework Equations
{see attachment for graph} Each Series in the legend denotes an increase in absorber thickness of about 6mm.
The Attempt at a Solution
Not indicated on the graph is that we followed the peak at about 100 on the x-axis (it's not actually 100 MeV or anything--the program we used filtered energies into channel bins, which we calibrated. The indicated energy here is 0.356 MeV). We attribute its decrease to absorption of the gamma rays as we added more material between our source and the detector. However, the peaks before channel 50 increase with material thickness. This is what we are trying to figure out.
We are considering Compton scattering, elastic scattering, and photoelectric absorption, but are unsure as to where these are applicable.
Attachments
Last edited: