# Photon numbers

The entrance exposure during a mammography examination is 100mR. (a) Suppose the photon bean consists of photons each of energy 20 keV. How many photons are incident on a 1 cm^2 surface of breast tissue? (b) Now suppose the energy of each photon is 50 keV. How many photons are incident on a 1 cm^2 surface of tissue?

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#### Solution Preview

So here we are looking for the number of photons that are incident on the breast tissue. There are two main concepts that deal with the number of photons:

1. Energy Fluence = dN*hv/da

where dN = number of photons, hv = photon energy, da = cross-sectional area

2. Fluence = dN/da

where dN = number of photons, da = cross-sectional area

Now in this problem we are trying to solve for dN. In this problem we are going to concentrate on using Energy Fluence represented by psi. So to do this we have to find a relation that links energy fluence to exposure.

The relationship that we are looking for ...

#### Solution Summary

This solution explains how to calculate the number of photons on a 1 cm^2 surface of breast tissue during a mammography examination, based on the energy in keV of the photons.