# Where are the beta particles?

• I
This may be a pretty basic question ,but i'll through it out any way. As a project i built a muon detector from 2 matched PMTs and 2 4x6x1 plastic PVT scintillators. And pulse processing NIM modules. I was getting pretty good coincidence reading,some what less than predicted.ie ( 1 count/sqcm/min) However ,when I wanted to calibrate by using a small sample on strontium 90 (.5Mev beta ) i get no signal anywhere. I made sure there was no obstruction between it and the scintillator. Now from what i understand the average muon
energy at earth surface is around 2Gev. Im assuming the beta fully is stopped by the scintillator. Im not sure the bethe bloch formula is valid for the speed of the beta, So where should i be looking for this signal???
Should the gain be turned way down to see the beta? Im trying to get an idea of where the strontium signal is compared to the muons. Thanks j

Hmm, it's interesting. At 0.5 MeV of kinetic energy the electron will stop within the material. Remember that 0.5 MeV is the maximum kinetic energy for the electron, but the energy spectrum is continuous and the most probable energy is less.

If you have a cosmic ray background with an average energy of 2-3 GeV, the beta signal can be very low in comparison; or not, I don't know what fraction of its energy will deposit the cosmic ray inside your detector, maybe you should compute the stopping powers for both and compare (will the cosmic ray lose more than 0.5 MeV?). Anyway, don't forget to put the high voltage of the PMT in the region of the plateau.

Then, you can measure the background due to environmental radiation (including cosmic rays) to characterize it; next, place the source near the detector and start measuring. You must remove the background to extract the signal from the beta.

To get good statistics, you must take measures for a long time both to measure the background and the signal.

Thanks. Well ,on my multichannel analyzer,I saw no spikes,but I do register the muons. Also I used a single channel analyzer with a discriminator,to weed out the dark counts and saw nothing over that level. My Geiger counter shows I’ve got plenty of activity from the strontium, way over the dark count rate from the tubes. I adjusted the gains on both tubes so that they both drop off the dark counts at the same voltage on my amplifier chain. From what I can gather,the velocity of the beta is an order of magnitude lower than the muons. Next I’ll try dropping below the nominal voltage of the tubes to see if it shows up. Any other ideas? Thanks

bobob
Gold Member
Hard to say without actually looking at your equipment, but here's a few possibilities. How is the scintillator coupled to the phototubes? You have scintillator that is 4x6x1. How do you match it to the shape of the phototube so that you are collecting all of the light? Is the scintillator well polished and everything very light tight? Your betas could be under the background.