Scintillation Detector Principle

Jan 2020
2
0
Vienna
Hello everyone, I'm studying for a radiochemistry exam (MSc. in Chemistry) and I have a probably silly question to the principle of detection of ionizing radiation (for example in a scintillation detector, but same applies for an ionization chamber etc.).

If I understand it correctly, the way a scintillation detector can distinguish between for example a 100 keV gamma and 200 keV gamma is based on the different pulse heights of the electric potential pulse generated by the photomultiplier (PMT). As a trivial (numerically incorrect) example - a single 100 keV γ-photon generates 1000 photons of visible light inside the scintillator, which lead to a 1 mV pulse in PMT whereas a single 200 keV γ-photon generates 2000 photons of visible light which lead to a 2 mV pulse in PMT. Now if we measure over a longer time period (with many 100 and 200 keV photons striking the scintillator) and we count the corresponding pulses, we obtain a spectrum, in which the x-axis is the pulse height = radiation energy and y-axis is the pulse count = radiation intensity - so we get two signals, at 100 and 200 keV with corresponding intensities.
What intrigues me about this is the following - what if a 100 and a 200 keV photon enter the scintillator at the same time? I know, "same time" with radioactive decay is nearly impossible, but let's say within 1 nanosecond. This is quite realistic, since in 10ˆ21 particles (a few hundred miligrams) of a moderately radioactive nuclide with a half-life of 1 thousand years we have 22 bilion particles decomposing per second (in the beginning). 22 bilion per second means 22 per nanosecond. Let's assume that we have two such nuclides (similar half-life) in one sample, but one decays to an excited state which emits 100 keV gamma and the other radiates 200 keV gamma after decay. My question - how does the scintillation detector distinguish between them? The "light flashes" occur so fast it cannot attribute each one of them a proper energy. Or is it just the instrumental setup, that we always dilute the probe so much?

Many, many thanks for any answers to this.
 
Oct 2017
597
305
Glasgow
I don't really know anything about PMTs, but it seems to me like a PMT isn't designed to cope with many incident photons... it's designed to be able to detect single photons. If you have multiple incident photons, you'll probably find that the scintillation chambers will be filled with many more lower energy photons, causing more electrons to be released from the photocathode and consequently create a bigger spike in the output response, causing a degeneracy between two 100keV photons and one 200 keV photon.

Can you use a filter to restrict the wavelength (and hence the energy) of the incident photons? Failing that option, there might be some techniques from spectroscopy that you can apply to analyze the PMT outputs in more detail, but I haven't done this kind of thing myself, so I'm not sure.
 
Jan 2020
2
0
Vienna
I don't really know anything about PMTs, but it seems to me like a PMT isn't designed to cope with many incident photons... it's designed to be able to detect single photons. If you have multiple incident photons, you'll probably find that the scintillation chambers will be filled with many more lower energy photons, causing more electrons to be released from the photocathode and consequently create a bigger spike in the output response, causing a degeneracy between two 100keV photons and one 200 keV photon.

Can you use a filter to restrict the wavelength (and hence the energy) of the incident photons? Failing that option, there might be some techniques from spectroscopy that you can apply to analyze the PMT outputs in more detail, but I haven't done this kind of thing myself, so I'm not sure.
Yes, that's exactly my track of thought, this degeneracy is the same kind of a problem....Also if there are 150 and 200 keV gammas emitted from a sample, then we should occasionally observe 300 and 400 keV signals...But that's not the case in any gamma spectrum I have seen.
I could theoretically use a filter (some kind of a monochromator), but that's not how these detectors work according to textbooks. There must be some key-fact I'm missing since beginning...
 

Attachments