Answer to Question #6886 Submitted to "Ask the Experts"

To say that a dosimeter element is body- or tissue-equivalent implies that irradiation of the element by a given radiation field for a fixed time period will produce about the same dose to the element as that same field would deliver to tissue during an equal exposure time. Commonly used TLD elements, such as LiF or Li₂B₄O₇, have atomic numbers very close to that of soft tissue, and if the dosimeters are irradiated by ionizing photons, for example, they undergo interactions of the same types and at about the same relative frequencies as would occur in soft tissue, a necessary characteristic for a tissue-equivalent dosimeter.

If we compare CR-39, a polycarbonate plastic, with soft tissue we find there are significant differences in elemental composition between the two. The CR-39 has an approximate composition given by C₁₂H₁₈O₇, which structure implies a hydrogen mass fraction 0.066. Soft tissue has a composition of the four major elements of approximately C₁₁H₅₄O₂₀N (inferred from ICRU Report 44, International Commission on Radiation Units and Measurements, 1989), implying a hydrogen mass fraction of about 0.104. In its normal configuration CR-39 is a fast neutron detector that responds to recoil nuclei produced within it (or in some cases from an external recoil nucleus radiator). If we allow that recoil protons are the major source of soft tissue dose from fast neutrons and consider the production of such protons in equal masses of soft tissue and CR-39, we would conclude, on the basis of the relative hydrogen contents, that the CR-39 would underrespond by about 40 percent with respect to the fast neutron soft-tissue dose.

In actuality, when we are interested in assessing dose equivalent, other interactions subsequent to initial fast neutron scattering, which produces initial recoil nuclei, may also contribute to the dose equivalent. For example, fast neutrons may undergo multiple scattering events with associated recoil nuclei in the body, eventually slow down, and be captured, often yielding capture gamma rays that may also contribute to dose equivalent. Even if the CR-39 were exactly equivalent in atomic composition to soft tissue, it would not properly record the entire dose because these secondary events would seldom occur in the very thin piece of film that comprises the CR-39 element; at best such an element might provide an estimate of the fast neutron kerma to the dosimeter, and this would be similar to the soft tissue kerma.

We would conclude that, from the point of view of measuring dose equivalent, the CR-39 element is not soft -tissue equivalent. It is only through the calibration process, in which the areal track density is correlated with dose equivalent, that the dose equivalent response of the CR-39 element is established.

Hope this answers your question.

George Chabot, PhD, CHP