I can't quite get problem #14 on the GR0177 ETS exam (I'm paraphrasing):
An 8cm diameter by 8cm long gamma ray detector dectects specific energy from a point source. When the source is placed just next to the detector at the center of the circular face, 50% of all emitted gamma rays at that energy are detected. If the detector is moved to 1m away, the fraction of detected gamma rays drops to...
D) 8pi x10^-4
E) 16pi x10^-4
The answer is C. Here's how I see it;
If the point source is up against a circular face of the cylinder it makes sense that 50% is detected as long as all radiation that hits (and/or enters) the cylinder gets detected. So, when the source is at 1m away, can't you compare the surface area of the circular face of the cylinder to the surface area of a 1m radius sphere and get the fraction of radiation that gets detected? Am I misinterpreting the setup of the detector? Is the detector hollow?
Thanks for any help.