Gamma Spectrometry Forum

I was working with foot probes with three GM tubes, and the tube efficiencies were inconsistent. There was plastic over the grids to keep sand and grime away from the tubes, and sand and grime was building up in there anyway. But I thought, 660 KeV gammas, what's a layer of tape going to do to that? But location was the only variable that seemed to matter. So I cleaned it all out, put on a new layer of tape, and everything was good.

Turns out Cs-137 is a beta emitter, something like 510 KeV electrons. It decays to Ba-137m, which decays with a half-life of about 2.5 minutes to its ground state, and that's where the gammas come from. The point being that Cs-137 is spitting out electrons, and a layer of tape, or a hole plugged with grime, seems to matter a lot.

That makes me wonder, what is the range of 510 KeV electrons in plastic? How many are actually getting out of the check source and through the mylar windows of the GM tubes?

There are online calculators for the stopping power of electrons in various materials. For example this one https://physics.nist.gov/PhysRefData/St ... ESTAR.html
It says that 500 keV electrons have a range of 0.1667 g/cm^2 in Polyethylene. The density of Polyethylene is 0.94 g/cm^3. The range in millimeters (divide range by density) is ~1.77 mm.

Interesting. My check source is 3mm thick. I don't know what the internal geometry looks like. The tube's window is 1.7 mg/cm^2. I think betas explains what I saw.

I agree that betas explain the observed behavior. Looking at the decay radiation of 137Cs (https://www.nndc.bnl.gov/nudat3/decayse ... Cs&unc=NDS), there are betas with even higher energy than 510 keV emitted. There is a 5% chance for a decay from 137Csdirectly into the ground state of 137Ba. The betas emitted in this process have a maximum energy of 1175 keV (energy of these betas is continuous, and only very few will have the maximum energy). In addition the decay of 137Ba is not 100% via gamma emission. It can also decay via internal conversion, where an electron of the 137Ba atom takes the energy of the excited nucleus and is ejected. Most of these betas have 624 keV.
The measures shape of the beta spectrum on this page (https://maximus.energy/index.php/2020/1 ... -spectrum/) suggests, that most of the betas are actually from this internal conversion process (the sharp peak at the high end of the spectrum).

Thanks, Michael. That fills out the picture for me. I wasn't thinking of the complete decay chain, just the part that caught my interest. At 1175 keV electrons have around 8 times the range in polyethylene.