Difference between revisions of "Basics about Efficiency Calibration of Gamma Detectors"

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and is given as the <math>\gamma</math> intensity, usually symbolised with I<sub><math>\gamma</math></sub>. The formula we use to calculate the
 
and is given as the <math>\gamma</math> intensity, usually symbolised with I<sub><math>\gamma</math></sub>. The formula we use to calculate the
 
efficiency then looks like this:<br>
 
efficiency then looks like this:<br>
<math>\epsilon=\frac{R_{E}}{A\cdotI_{\gamma}}</math><br>
+
<math>\epsilon=\frac{R_{E}}{A\cdot I_{\gamma}}</math><br>
 
Notice that we have indexed the <math>\epsilon</math> and R with an E to indicate that these are energy dependant. Since
 
Notice that we have indexed the <math>\epsilon</math> and R with an E to indicate that these are energy dependant. Since
 
all the absorbation processes for <math>\gamma</math> radiation is strongly dependent on the <math>\gamma</math>-ray energy, the detector
 
all the absorbation processes for <math>\gamma</math> radiation is strongly dependent on the <math>\gamma</math>-ray energy, the detector

Revision as of 08:54, 27 June 2012

In this exercise you are to do an efficiency calibration of a Ge-detector. The efficiency to any detector is dependent on quantities such as the [math]\gamma[/math]-rays energy, the geometrical shape and size of the source, and the distance between the source and detector. In order to make an efficiency calibration we use sources with known amounts of activites. The sources are measured and then we can calculate the counting efficiency: If the source activity is A, the number of counts obtained with a given detector is R, then the counting efficiency is given by:
[math]\epsilon=\frac{R}{A}[/math]
To calibrate a [math]\gamma[/math] detector we must also take into consideration that a [math]\gamma[/math] ray will not necessarily be emitted every time the source disintegrate. The frequency [math]\gamma[/math] rays are emitted can be found in tables and is given as the [math]\gamma[/math] intensity, usually symbolised with I[math]\gamma[/math]. The formula we use to calculate the efficiency then looks like this:
[math]\epsilon=\frac{R_{E}}{A\cdot I_{\gamma}}[/math]
Notice that we have indexed the [math]\epsilon[/math] and R with an E to indicate that these are energy dependant. Since all the absorbation processes for [math]\gamma[/math] radiation is strongly dependent on the [math]\gamma[/math]-ray energy, the detector efficiency will also be dependent on energy. Thus, to calibrate a [math]\gamma[/math] detector we must use a range of sources with energies that cover the range of energy we are planing to measure in. The efficiency calibration will then look like something in Figure 1. The most common range to measure [math]\gamma[/math] radiation in is from about 50 keV and up to about 1-2 MeV. From Figure 1 we can se that the range between about 200 keV and up to 2 MeV is nearly linear in double-logarithmic scale. Below 200-250 keV this is not true.