# Homework Help: How to get number of photons?

1. Jan 17, 2012

### amrian

1. The problem statement, all variables and given/known data
A collimated monochromatic beam of photons with intensity 10^20/s and energy
10 MeV is directed to a iron absorber with thickness 3 cm. What kind of interactions are expected? Suppose, each interaction removes one particle from the beam. I need to know, what amount of energy does each type of interaction remove from the beam?
2. Relevant equations
N=N0(1-e^(-Δx μ))
equations of cross sections

3. The attempt at a solution

I think, that here may be expected photoelectric, compton, pair production interactions. To get amount of energy removed by each interaction I need to know number of interactions and then multiply it by the energy, but I also need to know number of photons to get the number of interactions, or I am very wrong?

2. Jan 17, 2012

### Simon Bridge

10MeV photons beam usually means each photon has 10MeV energy (gamma rays)
https://netfiles.uiuc.edu/mragheb/www/NPRE%20402%20ME%20405%20Nuclear%20Power%20Engineering/Gamma%20Rays%20Interactions%20with%20Matter.pdf [Broken]
The main interactions will be photoelectric-effect, compton scattering, and pair production.

This means that 1021MeV/s is arriving at the Fe target (10^20 photons per second arriving multiplied by 10MeV per photon).
1MeV is 1.6022x10-13J so this rate is something like 160MW.

Each photon removed from the beam thus transfers 10MeV to the target.

You need to know the cross-sections for the interactions - these will give you the probability of the interaction for one photon, so multiply this by the photon flux to get the number of photons involved in the interaction (per second).

Last edited by a moderator: May 5, 2017