I get it - Applied Quantum Physics works

A project log for Gamma Ray Spectroscopy - small, cheap

The goal of this project is to design a light weight gamma ray scintillator in a 1u CubeSat frame

sparks.ronsparks.ron 12/07/2016 at 16:210 Comments

I did a bunch on the Project Management (PM) yesterday and continued reading on the physics of the detectors. My thinking had been to much influenced by Radio Frequency (RF) in my past. What I missed until last night is that the Gamma wave is actually a high energy quanta (forgive me if I do not have the nomenclature correct). Because of that there is no loss or attenuation of the energy in them until they hit something and release their energy into photons.

What that means to this project is that the shape and amplitude of the PMT pulse important. The thing that matters is the integral of the pulse to obtain the energy it contains. That eliminates the need for very high speed A/D. The problem then becomes one of accurate integration of the fast pulse, with all the associated issues of an integrator -- such as baseline offset, noise, accurate accounting of integrator losses, and such. There is a wealth of history on that due to work done on radar pulse reception.

So while all the losses in the detector affect the overall sensitivity of the device, they do not directly influence the accuracy. For example in the PVT scintillator I have, the sensitivity is specified as about 25,000 photons per MeV. That is about 2,500 for a low end gamma at 100 KeV. If you lose all but 10% in your detector arrangement you will only have, maybe, 250 photons hitting the PMT cathode. However, those 250 photons will all generate the same 100 KeV each when they hit the cathode.

So the pulse height may be very low compared to the baseline noise, but it will always have the same total energy. I am visualizing it like a mountain in the ocean (Hawaii - ahhh - oops, drifted off for a second). The total size of the mountain would be unchanged regardless of the sea level, only the amount sticking out above the water would appear to change.

That means if we integrate the amount of pulse above the noise base for a known emitter (Cs137), it will allow you to calculate the baseline value to subtract from the pulse energy of any pulse to get the actual pulse energy. This seems to be roughly how the professional devices use the calibration sources.

I want to thank anyone who is reading these blog entries for being patient and allowing me to "think out loud".

Hopefully any insight as to how I work through a problem and project will be a help (it could be either as a good example or negative example - readers choice).