Answer to Question #10897 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
I know that the dose to the hand is limited by equivalent dose (a) to the skin and (b) to the "extremities."
The equivalent dose to the skin is determined to a depth of 0.07 mm (shallow depth, weakly penetrating). Is the depth for the extremities the same as "whole body," i.e., 10 mm (deep, strongly penetrating)?
What does a finger thermoluminescent dosimeter (TLD) actually measure? Skin dose or extremity dose?
If there is significant neutron emission from a source and persons are in close proximity to handle such sources (using tools or tongs), can the skin dose be dismissed on the basis that the depth is too shallow for significant neutron dose to deposit?
In the same circumstances, would extremity dose (deep finger/hand dose) be the more important factor, which could still result in effects to the skin nevertheless?
Are there any practical solutions to actually measure neutron dose to the skin or extremities? It seems that all finger TLDs are only for gamma and beta radiation.
The requirements in the United States for measuring extremity dose specify that the dose should be assessed at the accepted shallow depth, the 0.07 mm that you cite. Thus shallow dose, which may be different in magnitude from the 10 mm dose, is interpreted as the dose to the extremity, with no corrections for attenuation (or buildup) with depth. The 10 mm depth used in specifying deep dose has no direct application in assessment of extremity dose. In principle, the dosimeters used to measure extremity dose should be designed to measure the dose at the shallow depth, 0.07 mm, associated with usual skin dose evaluation. In actuality many of them, the TLD rings being among the most common, do not meet the desired specifications. Coverings over the TLD are sometimes greater than the 7 mg cm-2, and the significant thickness of the TLD often results in nonuniform energy deposition through the TLD thickness, especially when dealing with low-penetrating radiations. The consequence is that if low-penetrating radiations (e.g., low-to-moderate-energy beta radiation) are a concern, the dosimeters may have to be calibrated with radiations that simulate what is to be measured in the field.
Your points and questions regarding possible extremity doses from neutrons are well taken and may be relevant in some situations, especially in incidents/accidents in which a portable neutron source might have been picked up by hand or if an extremity is brought close to a fixed neutron source. While we presently have a standard that describes requirements for testing/evaluation of extremity dosimeters (ANSI/HPS N13.32-1995, Performance Testing of Extremity Dosimeters), the standard does not include neutrons as a test radiation or evaluation of neutron dosimeters for extremity dose measurements. The typical whole-body badges used for assessing personnel neutron doses are designed to allow interpretation of (whole body) dose equivalent using conversion factors for limited sources that are contained in ISO 8529; they do not address neutron extremity/shallow dose.
If neutrons are a concern in a particular situation, the assessment of the extremity dose may have to be carried out from knowledge about the specific characteristics of the neutron field. In particular, the neutron fluence and the energy spectrum at the extremity location are particularly helpful. If these data are known, one may carry out calculations of the soft-tissue kerma delivered by the neutrons near the surface of the extremity. One may use selected instrument measurements to assess the neutron radiation field in order to determine the kerma or dose. Instruments such as some tissue equivalent proportional counters can be used to obtain information about neutron fluence, energy, and absorbed dose.
The kerma may be used as an estimator for the absorbed dose, and the latter is the most useful quantity for purposes of extremity dose estimation. The quantity equivalent dose (earlier dose equivalent), in Sv, should not really be used for extremity dose since the quality factors (or radiation weighting factors) that apply to the radiation were based on consideration of specific stochastic effects, most notably cancer. For extremity irradiation, there are no specific such factors that apply, and the major concern should be with possible nonstochastic effects. There are groups and individuals who insist on reporting equivalent doses, rather than absorbed doses, as the dose of record for extremities. It is my opinion that such specification is inappropriate, since the quality factors (the International Commission on Radiological Protection currently uses radiation weighting factors) do not apply. Naturally, if one is dealing with only beta and gamma/x radiations, the quality factor is unity and the absorbed dose and equivalent dose are numerically the same, but this is not so for neutrons. Some regulators may resist this recommendation to report only absorbed dose, but I believe it is the most sensible approach.
You are correct in your inference that finger TLDs are not of any use for neutron extremity dose assessment. The TLDs used in whole-body neutron dose assessment rely on the albedo principle in which the trunk of the body acts as a neutron scatterer/moderator to direct low-energy neutrons into the TLD, and this system has no application to an extremity. Specialized dosimeters, such as bubble detectors, can be useful if they have been properly calibrated to provide response versus neutron absorbed dose for the energies of interest. There has also been use made of track etch detectors, most notably CR-39 (polyallyldiglycolcarbonate), a thin plastic film material that can provide a reasonable measurement of fast neutron doses to the extremities.
Thanks for the question. I hope the discussion is useful.
George Chabot, PhD