Formulae derived by Rossi and Ellis (1950) for the calculation of radiation dose from distributed sources of beta-emitters have been made applicable to the case of tritium by a consideration of the appropriate value for the effective absorption coefficient employed in these expressions. An example is given of their application to the calculation of dose to a cell from tritium incorporated in the nucleus.The widespread use of tritiated thymidine in the study of DNA-synthesis lends importance to the problem of calculating the resulting radiation dose at points within and near the cell nucleus. This is a particular case of the general problem of the dosimetry of beta-emitting sources distributed in an absorbing medium. The first stage in the solution is the establishment, for the source and medium concerned, of the so-called 'point source function'. This is an expression giving the radiation dose D(,) as a function of the distance r in the absorbing medium from a point source of the emitter. Secondly, for a uniformly-distributed source of specified shape, the function D(,) is multiplied by the expression for the differential volume element of the source which is at a distance r from the point where the dose is required, and integration is performed over the volume of the source. The case of tritiated thymidine incorporated in a cell nucleus may be treated as a uniformly-distributed spherical source of tritium in a unit-density medium.Numerical values of the point source function for tritium have been computed by Robertson and Hughes (1959), who give the following expression for the energy spectrum of the beta-rays:(1) Using this spectrum in conjunction with an experimental curve of range versus energy (Brown 1941) and calculating the energy absorbed in concentric shells of 0.5 microns radial thickness, values are obtained for the dose D(r) in rads per disintegration at a distance of r microns. These values are indicated by the open circles in figure 1. For the solution of distributed-source problems, an algebraic representation of this point source function is now required. Robertson, Bond and Cronkite (1959) propose the following expression, shown as curve A in figure 1: D(r) = 185 exp (-5.55r) + 15 exp (-1.92r).(2) Goodheart (1961) found this expression not to be amenable to integration, and adopted instead the following equation, which is given as curve B in figure 1: 3'00 3 (0.01 +r 2 )(1 + 0.1r 2 )(1 +0.2r 2 )2 Q R.B. Int J Radiat Biol Downloaded from informahealthcare.com by Nyu Medical Center on 06/03/15For personal use only.
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