# Homework Help: Determine the solid angle subtended by a detector

1. Sep 13, 2010

### jeremytrack

1. . A cylindrical detector, similar to the NaI detectors you use in lab, has a diameter of 5 inches and a length of 5 inches. A 60Co source is placed on the cylindrical axis, 20 cm away from the front face of the detector.

a. Determine the solid angle subtended by the detector. Calculate the uncertainty in that solid angle by considering the difference between the solid angles subtended by the front face and back plane of the detector.
b. Calculate the probability of interaction for the 1.17 and 1.31 MeV gamma rays through 5 inches of NaI.
c. Determine the overall efficiency of the NaI detector, assuming that every time a 60Co gamma ray interacts in the NaI detector, you are able to identify that event (i.e., assume that every interaction results in a full-energy peak).

I would assume that the solid angle would be $$\Omega$$ = A/(4pi(r^2))

but i don't know how to calculate the probability of interaction or the overall efficiency.