Geiger counters and activity

  • #1
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I'm doing high school physics and if a question says something like:


A radioactive source gives count rate of 110 counts per second

Can you say that the ACTIVITY is also 110 bq?


My second question is how would a gieger counter detect gamma radiation? It is the least ionising of all three radiation so not many particles inside the G-M tube would be ionised giving a low count rate? where as alpha particles would ionise a significant amount more particles then gamma radiation giving a higher count rate?
How would a geiger counter compensate for these different ionising powers?
 

Answers and Replies

  • #2
gleem
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Can you say that the ACTIVITY is also 110 bq?
No. Not for any radiation. If you desire activity in Bq then the count rate must be corrected for the efficiency of the GM counter. GM counter are not very efficient for gamma and the efficiency depends on the energy. Also geometric corrections must be made since the detector only is exposed to some of the disintigrating radiation. This is called a solid angle correction.

For beta and alpha radiation the efficiency is much higher but there are still corrections that have to be made . The window of the detector must be thin to allow the particles into the sensitive volume. Even thin mylar or mica window do absorb some of these particles. Also depending on how close the detector is to the sample you need to consider radiation that is emitted away from the detector but is backscattered into it.

So using a GM counter is not a direct reading instrument for activity. It is better for radiation exposure measurement but still must be calibrated for different energies.

The amount of ionization per particle results in one count. It just has to be large enough to initiate the avalanche process by which the electric pulse are produced.
 
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  • #3
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Can you say that the ACTIVITY is also 110 bq?
What happens if the source and the counter are in different rooms, as extreme example?

My second question is how would a gieger counter detect gamma radiation?
Hope for a conversion to a high-energetic electron (Compton effect) or pair creation, both produce charged particles that lead to ionization. A gamma photon that does not get converted in or before the detector does not get detected.
 
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gleem
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Hope for a conversion to a high-energetic electron (Compton effect) or pair creation, both produce charged particles that lead to ionization. A gamma photon that does not get converted does not get detected.
Gamma radiation interacts via other less energetic processes as multiple atomic ionizations, Photo electric effect and Compton effects releasing more less energetic electrons and low energy x-rays.
 
  • #5
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And nuclear photoeffect, photodisintegration and so on.
The concept of all those processes is the same - the photon transfers energy to a very small number of charged particles (typically electrons and/or positrons), and those are detected in the Geiger counter.
 

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