(a) The Bragg-Gray conditions state that a gas-filled cavity is small enough not to…

PHYSICS · Subjective · Fermi University Physics · Biophysics & Medical Physics · Nuclear medicine, dosimetry and radiotherapy · Nuclear medicine, dosimetry and radiotherapy

The Nucleus Resolved, Ch Dose: kerma, cavity theory, and what energy does to a cell

Problem

(a) The Bragg-Gray conditions state that a gas-filled cavity is small enough not to disturb the charged-particle fluence set up by the surrounding medium, and that photon interactions inside the gas itself are negligible. Starting from the statement that the same charged-particle fluence

\Phi

crosses both the gas and the medium, derive the relation between the absorbed dose in the medium,

D_{\rm med}

, and the absorbed dose in the gas,

D_{\rm gas}

, and state in words what the quantity

\bar s_{\rm med,gas}

that appears in it represents. (b) A worker's medical procedure delivers an effective dose

E = 15.0\,\mathrm{mSv}

. Using the linear no-threshold (LNT) model with the nominal risk coefficient

5.0\times10^{-2}\,\mathrm{Sv^{-1}}

, write the general expression for the nominal probability

p

of a stochastic effect from an effective dose

E

, then compute

p

for this worker. State whether this number is a measurement or an extrapolation, and why.

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