Gamma-gamma coincidence spectrometry

[mitch_1211]

hey everyone,

I know gamma-gamma coincidence detection is used to somehow determine the activity of a radioactive source, I was wondering if someone could explain to me in fairly simple terms how this is done?

Also, is determining activity generally the only application for gamma-gamma coincidence or are there others?

Mitch :)

 

[bp_psy]

The basic scheme for measuring activity is as follows:
You have some nuclide in some excited state. When it will decay it will drop to some other excited state and emit a photon with some set energy E1. You use a detector that can detect a photon at E1 to start a timer whenever it is activated . The newly formed excited state will also decay after some period of time and emit another photon of some energy E2. You now use another detector that will stop the timer whenever it sees a photon of E2. The difference between the start and the end of the timer, is the amount of time that nucleus was in the second excited state. (with some uncertainties and errors to be determined of course). When you do the experiment you measure many many such events and you get the distribution of the lifetime of an excited state. From this you get whatever you want to measure. In this kind of experiment the calibrations are very important since you want to have the start and end photon coming from the same nucleus.

 

[mitch_1211]

that makes sense, thanks for that :)

 

[mitch_1211]

any idea what 'coincidence summing effects' might be in relation to gamma-gamma coincidence?

 

[bp_psy]

any idea what 'coincidence summing effects' might be in relation to gamma-gamma coincidence?

If you have two photons reaching the detector in a very short time interval, it may be possible that your detector registers them as the same event.You will record them as single photon with energy equal to the sum of the two initial photons. This means that you get counts for energies that you should not see and less counts for the summed photons.