Answer to Question #11024 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

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

Q

I need some advice evaluating the radiation dose rate measurement and the significance of the radiation health concern for a situation where there is a narrow gap of about 0.127 cm by 10.2 cm long in a radiation shield. The peak soft-tissue dose rate is calculated to be 1 to 2 mGy h-1 on the surface at the gap.

  1. Can this dose rate be quantified using a portable Geiger Mueller (GM) detector or ionization detection chamber on contact with the gap opening? If not, how do I obtain an accurate radiation dose rate under these conditions?
  2. From a regulatory perspective, how does this narrow radiation beam relate to the occupational exposure limit to the whole body or to the extremities? It seems that the standard of performance is designed for significant portions of the body irradiation conditions.
  3. Are there International Commission on Radiological Protection (ICRP) or National Council on Radiation Protection and Measurements (NCRP) guidelines that apply to this situation?
A

The emergence of radiation beams from cracks or other small openings in a radiation shield is a problem that comes up from time to time and can entail a number of concerns, as you note. I shall attempt to address each of your numbered questions/concerns.

1. Regarding measurement of the emergent radiation field, the most confounding aspect is that the beam width often is less than the areal dimensions of the portion of the detector on which the radiation is incident. The obvious result of this effect if the instrument is used without any corrections is that the entire facial area of the detector will not be irradiated, resulting in less than the whole volume of the detector being irradiated. Since calibrations of most instruments are done with the detector volumes completely irradiated in a uniform field, the narrow beam measurements may appreciably underestimate the actual dose rate in the beam. As long as the dose rate is low enough that significant dead time losses do not occur, a measurement with an instrument with an active volume facial area, AD, may be corrected by determining the area of the emergent beam incident on the detector face, AB, and multiplying the reading obtained by the area ratio, AD/AB.

For example, if a value of 0.1 mGy h-1 was measured with a pancake GM detector with an active diameter of 4.44 cm and a facial area of 15.5 cm2, and the beam area incident on the detector face (centered on the emergent beam) was (4.44 cm)(0.127 cm) = 0.56 cm2, the estimated dose rate in the beam would be (15.5 cm2/0.56 cm2)(0.1 mGy h-1) = 2.75 mGy h-1. One caution in such a case in which a GM detector is used is that dead-time losses may be a problem even at what appear to be relatively modest dose rates (before the area correction). When possible, I would recommend using an ionization chamber rather than a GM detector to make the measurement at the surface of the gap. You would have to know the dimensions of the active volume, but this should not be a problem. The ionization chambers used in portable instruments are generally cylindrical in shape, and if the chamber is oriented so that the radiation from the gap is incident on a flat surface rather than on the curved surface, the same type of area ratio as used above may be applied. If the chamber is oriented with its curved surface facing the gap and has a radius R and a depth H, you should use the ratio of the volume of the chamber (πR2H) to the volume irradiated by the beam (πR2W), where W is the beam width, as the multiplying factor; this assumes that the depth of the beam, 10.2 cm in your case, is greater than the diameter of the chamber.

2. If an individual worker were to walk through the emergent radiation beam, the worker's personal dosimeter that assesses deep dose and possibly eye dose may well not be exposed to the beam radiation, depending on the position of the dosimeter on the individual's body relative to the beam location. As you are likely aware, the U.S. Nuclear Regulatory Commission (NRC) in 10 CFR 20.1201(c) generally requires that the deep dose to the most highly irradiated portion of the body, as measured with an appropriate external dosimeter, be used in place of the effective dose. (The annual deep dose or effective dose limit is 0.05 Sv.) In such a case as of concern here, in which the individual may be exposed but that individual's dosimeter is not, the health physics staff may have to attempt to reconstruct the worker's activities and to identify the portions of the body irradiated and the likely exposure times. An individual moving across the beam may have exposed a relatively small volume of tissue across a limited cross-sectional portion of the body, the dimensions of such tissue volume being constrained by the beam dimensions. Attention would have to be paid to physical spreading of the beam as distance from the gap surface increased. The information may then be used to attempt to determine the effective dose equivalent to the individual; such determination requires knowledge of the portions of the body irradiated, the respective equivalent doses to the irradiated tissues, and the respective tissue weighting factors that apply. If the projected dose is low, you may be able to make life simpler by simply calculating the surface dose or deep dose and using that as a conservative estimate of the effective dose. Such a method would likely have to be approved by the regulating agency (e.g., NRC or the state radiation control office) before the determined effective dose could be used as the dose of record.

There is also a requirement to assess skin dose, independent of extremity dose. The 10 CFR 20.1201(c) requirement is that the skin dose be determined for the contiguous 10 cm2 of area that receive the highest dose. The annual dose limit is 0.5 Sv. Again, this may require appropriate dose reconstruction when the dosimeter has not been in the radiation beam.

If the individual had been wearing an extremity dosimeter, any significant extremity dose may have been recorded by the extremity dosimeter, if the extremity dosimeter was in the beam. Such a measurement would be recorded as the best estimate of the extremity dose if it is known that no other extremity was exposed. Extremity dosimeters are generally small in areal dimensions and it is likely that any measured extremity dose would be the result of irradiation of the entire active element in the beam or the result of the extremity moving through the beam, sweeping the element through the beam. Naturally, if is unknown whether other extremities or other parts of the extremity that bore the dosimeter may have been exposed, you would again likely have to include extremity dose estimates in your attempt at dose reconstruction. The extremity annual dose limit is 0.5 Sv (evaluated at a depth of 7 mg cm-2).

3. The NCRP and the ICRP have discussed the concept of effective dose and described its determination based on multiplying the equivalent dose (absorbed dose times radiation weighting factor) to each significantly irradiated organ or tissue by its respective tissue weighting factor (a factor that reflects the probability of radiation-induced serious damage, primarily cancer) and summing up all such products for the significantly irradiated tissues to obtain the effective dose. By implication, when only a portion of the body is irradiated, the effective dose would be calculated by determining the tissue-weighted equivalent dose to the limited number of tissues that were irradiated and summing up such doses to estimate effective dose. This is not a simple task when an external source irradiates the body since it requires knowing not only the field intensity and spatial variations in intensity but also its direction, the type and energy distribution of the incident radiation, the orientation of the body in the field, and the effects of attenuation and buildup of the radiation that delivers dose to each of the irradiated tissues/organs. Because of these complexities and the fact that the required information may not be available, such calculations are often impractical. Beyond this discussion, to my knowledge the NCRP and ICRP have not made more specific detailed recommendations regarding approaches to adopt in the cases of partial-body irradiations that would be applicable. The NCRP has made some recommendations regarding the use of multiple dosimetry in medical x-ray diagnostics when a lead apron is worn and the body is not irradiated uniformly.

Other groups and agencies have addressed the general topic of determining effective dose when the body is not irradiated in a uniform fashion. Particular emphasis has been on the use of multiple dosimeters positioned at different locations on the body and assigning specific weighting factors to the measured dose results, the factors varying with the portion of the body affected. Examples in this regard can be found in U.S. NRC Regulatory Guide 8.40, Methods for Measuring Effective Dose Equivalent From External Exposure, 2010 and the ANSI/HPS standard 13.41-1997, "Criteria for Performing Multiple Dosimetry." The latter standard is available free to Health Physics Society members or it can be purchased through the Society's distributor, IHS. The use of multiple dosimeters would probably not be of very much practical use for the case you describe where a very narrow and limited depth beam of radiation is involved.

As we have noted, for cases such as yours, if it is known that one or more individuals was exposed to the beam you may have to attempt to reconstruct the dose by doing a time and motion study to determine the portions of the body irradiated and the time profiles of the exposures. Uncertainties may have to be accommodated by making conservative assumptions. Evaluation of the dose at the surface of the body would provide a conservative estimate of deep or effective dose. If an exposure consisted of a single pass though the beam, it would probably not be difficult to estimate the dose, but if a worker were exposed multiple times and/or in multiple orientations to the beam the estimations would likely be more complex and subject to greater uncertainty.

For the single-pass case, given your maximum dose rate of 2.0 mGy h-1 and a beam width of 0.127 cm, if we used an arbitrary speed of passage of 1 cm s-1  through the beam, the dose at the body surface for the 10.2 cm long portion of the body irradiated in the single pass would be (2.0 mGy h-1)(1 h/3,600 s))((0.127 cm)/(1 cm s-1)) = 7.1 x 10-5 mGy, which would be equivalent to a body surface equivalent dose of about 7.1 x 10-5 mSv, not a real dose concern. This assumes that the 2.0 mGy h-1 is the actual maximum dose rate in the beam, not the direct and uncorrected value measured with the GM detector or ion chamber. Of course, if the exposed individual were to remain stationary in the beam, the localized dose would accrue to a greater extent.

I hope this is helpful to you.

George Chabot, PhD

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