An ion chamber is placed in a photon field and collects an induced charge from of air

PHYSICS · Multi-part · Fermi University Physics · Nuclear Physics · Biophysics & Medical Physics · Radiation interactions, detection and dosimetry

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

Problem

An ion chamber is placed in a photon field and collects an induced charge Qind=0.25 nCQ_{\rm ind} = 0.25\,\mathrm{nC} from m=5.0×10−7 kgm = 5.0\times10^{-7}\,\mathrm{kg} of air, with Wion/e=33.97 J C−1W_{\rm ion}/e = 33.97\,\mathrm{J\,C^{-1}} for dry air. The water-to-air mass stopping-power ratio for this beam is sˉwater,air=1.130\bar s_{\rm water,air} = 1.130. Assume the two Bragg-Gray conditions hold. ((a)) Use Dgas=(Qind/m)(Wion/e)D_{\rm gas} = (Q_{\rm ind}/m)(W_{\rm ion}/e) and the Bragg-Gray relation Dmed=Dgassˉmed,gasD_{\rm med} = D_{\rm gas}\bar s_{\rm med,gas} to find the dose to water. ((b)) Suppose this dose is delivered uniformly to the whole body of a person as a single photon exposure, so that the effective dose (in sievert) is numerically equal to the dose to water (in gray). Using the nominal risk coefficient of 5×10−2 Sv−15\times10^{-2}\,\mathrm{Sv^{-1}}, estimate the nominal probability of a fatal cancer this exposure is assigned, and state whether this number is a measurement or an extrapolation.

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