A method for measuring
gamma radiation. Analysis of the results can be used to determine not only how much
radiation is emitted, but also which
substance that's emitting it.
Below I will briefly explain a method of gammaspectrometry using a NaI-crystal detector and a multi-channel analyzer.
/\ Radioactive
\/ substance
/-----------------\
| NaI-crystal |
| detector |
| |
|-----------------|
| ___________ | Photo cathode
| |
| |
| Photo |
| multiplier | | |
| | r----| |----> to amplifier
| ___________ | | | |
| | | |
\--------+--------/ | |-----|
L----------------| |-----o +
|-----|
Every time the NaI-detector is hit by a gamma particle, i.e. a photon, it will emit an amount of light proportional to the absorbed photon energy. The photo cathode will then be hit by the light from the detector, and emit a number of electrons proportional to the strength of that light. The photo multiplier will increase the number of electrons, and eventually send an electric pulse to the amplifier.
Below is the flow of this process:
Radioactive substance -> detector -> amplifier -> multi-channel analyzer -> computer
The analyzer will sort each pulse into a channel, sorted by energy. The computer can then display a graph showing the number of pulses received in each channel. Here is a rough sketch of how this graph can look for Cesium-137 and Potassium-40:
/|\ Pulses
|
|
| |
| / \ Cesium-137
| | |
| | |
| | |
| | |
| | /-\ | |
| / \-/ \ | |
| | \ | |
| / \- | |
| | \-/ | Potassium-40
| | \ _
| | \_______/ \___ Channels
|------------------------------------------>
Note that the noise in the beginning is due to the Compton effect.
Using a substance that radiates gamma rays of known energy, the graphs can be calibrated to a set amount of energy for each channel (electron volts / channel). When this calibration constant is known, a comparison can be made between energy levels at the peaks in the graph and table-values over the energy of gamma radiation from various isotopes. Most computer programs have these tables stored, and will automatically look up possible matches within a certain region.