CINCH NucWik

Here an exercise is presented that uses a computer program to simulate how a detector absorbs and measures γ rays.
Using the program enables you to investigate the absorption and detection processes in detail.
It will help you to understand how a γ detector works and thus use it more efficiently.

Absorption of γ radiation occurs by three different effects:

• The photoelectric effect
• the Compton effe ct
• Pair formation

(for more, visit the NucWik page about Gamma-ray Interactions).

A NaI crystal will emit light when γ radiation is absorbed. If we optically couple the crystal to a sensitive light-detector (a photo multiplier tube), we can measure the amount of light emitted and therefore also the γ radiation. In this way we can construct a γ detector and we call it a NaI-scintillator detector (to scintillate means emits light).

The amount of light is proportional to the amount of energy deposited in the detector. The light detector is so fast that it's able to measure the light from each individually absorbed γ ray.
By analysing the pulse height from the detector (which is proportional to the amount of light) we can make a picture of the distribution of energy deposited in the detector, i.e. we make a histogram where each column represent the number of events within a small energy range - we will get a plot of event intensity as a function of energy deposited in the detector.
Such a histogram we call a spectrum.

If we have a source which only emits γ rays with a single energy, its spectrum should look like in Fig.1. However, if we measure such a source with a NaI scintillation detector, the spectrum we get will look like Fig. 2.
The reason is of course the manner γ rays are absorbed in the detector. A large fraction of the γ's will be absorbed by the Compton effect and this severely distorts the spectrum. If the energy is high enough we will also get pair formation that further distort the spectrum.

Other types of γ detectors work not by light emission (scintillation), but are based on semi-conductor technology. Here the γ-sensitive element is actually part of an electronic circuit and acts like an electrical diode. Radiation hitting the diode will generate a current pulse which is amplified and measured by the electronic circuit. Such detectors are usually named Ge-detectors since the Ge-detector crystal which constitute the larges part of the diode is made of this element. Ge-detectors have much better energy resolution than NaI detectors, but are much more expensive.

For any kind of detecter, the measurement of the generated signal (wheater it is an electric signal or a light pulse) will never be 100% accurate: the error in the measurement will widen the peaks and generally smooth the spectrum. Thus, details will be more difficult to distinguish.
The detector error is generally discussed in therms of the detectors response function.
A Ge-detector has a better response function than a NaI detector. The simulation program presented here will enable you to change the response function to see how it affects the measured spectrum.
This is also something which generally can not be done with a real detector.