Answer to Question #12211 Submitted to "Ask the Experts"

Category: Radiation Basics — Radiation Quantities and Units

The following question was answered by an expert in the appropriate field:

Q

Do you know if there is an algorithm or relationship between personal equivalent dose evaluated at 10 millimeters (mm), Hp(10), and dose to the lens of eyes at 3 mm, Hp(3), that could be used to estimate the dose to the lens of eyes for an occupationally exposed worker using cobalt-60 (60Co) or other gamma radiation source? Normally we have one dosimeter that gives us only the Hp(10) reading.

A

In most cases when dealing with photon exposures, the use of Hp(10), the personal equivalent dose evaluated at the 10 mm depth, is an acceptable estimate of (and generally somewhat conservative for) the equivalent dose to the lens of the eye at the 3 mm depth. By "most cases" I mean when photon energies are greater than about 30 kiloelectronvolts (keV) and radiation is incident in a more-or-less broad parallel beam on the front of the body with the dosimeter also being worn on the front of the body.

While I have not seen an algorithm for relating the two dose quantities, Figure 5.10 from International Commission on Radiation Units and Measurements (ICRU) Report 43, Determination of Dose Equivalents from External Radiation Sources—Part 2, shows a plot of the ratio of the eye lens dose equivalent to Hp(10) for front-to-back irradiation with the badge on the front of the body (as well as other radiation field and badge orientations). This plot illustrates that for photons between about 50 keV and 6 megaelectronvolts (MeV), the eye dose-to-Hp(10) ratio is not greater than about 1.25 (at 50 keV) and not less than about 0.9 (at 1 MeV).

You may also want to review a draft paper that validated the use of Hp(10) as a general surrogate for Hp(3) (bottom of p. 17 to top of p. 18). At low photon energies and for charged-particle irradiations, such as beta radiation, the value of Hp(0.07), normally used for skin dose, is often recommended as a conservative estimate for Hp(3).

George Chabot, PhD

Reference
Burlin TE, Bengtsson G, Chilton AB, Drexler G, Harvey JR, Portal G, Thomas RH, Wagner SR. Determination of dose equivalents from external radiation sources—Part 2. Journal of the International Commission on Radiation Units and Measurements 22(2); Report 43; 15 December 1988.

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