# Probability for detection

1. Nov 21, 2012

### india

Probability for detection !!

I have to calculate the probability of alpha particle or photon being detected by Si detector from the 241Am source. How to know no of alpha that can be detected from the 241Am source ?? source have acticity of 3.7kBq and distance between source and detector is 35mm. as well the area of square detector is 8*8mm2.

2. Nov 21, 2012

### Staff: Mentor

Re: Probability for detection !!

Consider a sphere with a radius of 35mm - which fraction of the surface area does your detector cover?
If you have air or some other material between source and detector, alpha particles might lose their energy before they reach the detector.

3. Nov 21, 2012

### india

Re: Probability for detection !!

So there will be the vacuum inbetween source and detector. So there will be very less chance for energy loss of alpha, That can be detected at 35mm distance. Also there is no material in between.
Area of detector is 64mm^2, and distance is 35mm. So i think fraction of surface are covers detector (or fraction of total radiation) can be given by, A/(4pi*R^2)=64/4*pi*34*35 = 0.0042
Am I right ?
and If so, then How to get the probability out of it ?

4. Nov 23, 2012

### india

Re: Probability for detection !!

So there will be the vacuum inbetween source and detector. So there will be very less chance for energy loss of alpha, That can be detected at 35mm distance. Also there is no material in between.
Area of detector is 64mm^2, and distance is 35mm. So i think fraction of surface are covers detector (or fraction of total radiation) can be given by, A/(4pi*R^2)=64/4*pi*34*35 = 0.0042
Am I right ?
and If so, then How to get the probability out of it ?

5. Nov 24, 2012

### Staff: Mentor

Re: Probability for detection !!

If your detector is perfect, that ratio is the probability simply by geometry (and assuming your source emits radiation in all directions with the same intensity, e.g. there is no shielding in the source itself). If it is not perfect, you need additional data about the detector.