Bragg-Gray Cavity Theory

According to the Bragg-Gray cavity theory, the ionization produced within a gas-filled cavity inside a medium is related to the energy absorbed in that surrounding medium.

If the cavity is small enough that it does not change the number or distribution of the electrons that would exist in the medium in the absence of the cavity, then::$E\_\; u\; =\; J\_\; u\; cdot\; W\; cdot\; ho$where
:$E\_\; u$ is the energy absorbed per unit volume of the medium
:$J\_\; u$ is the ionization per unit volume produced in the gas
:$W$ is the average energy lost by the secondary electrons per pair of ions formed in the gas
:$ho$ is the ratio of the stopping power of the medium and the gas for the secondary electrons

References

# Khan, F. M. (2003). The physics of radiation therapy (3rd ed.). Lippincott Williams & Wilkins: Philadelphia. ISBN 978-0-7817-3065-5.
# Gray, L. H. (1936). An ionization method for the absolute measurement of $gamma$-ray energy. "Proceedings of the Royal Society A", 156, pp. 578-596