Answer to Question #13068 Submitted to "Ask the Experts"
Category: Radiation Basics — Radiation Shielding
The following question was answered by an expert in the appropriate field:
Has any research been conducted to determine the beta radiation shielding properties of eye protection (i.e., thickness of PERSPEX® safety glasses) used in industries that need to store, transport, and handle ß- radioactive isotopes that may pose a safety problem arising from the potential exposure of workers and public to the directly ionizing radiation emitted?
There have been various papers that have been published over the years that relate to the attenuation of beta radiation in various media, and some have been specific to the effectiveness of plastic lens and glass lens safety glasses. I should perhaps first note that the interest in potential and real exposures from beta radiation got considerable attention, especially during the late 1970s, the 1980s and the 1990s. I believe this interest related to increased development of and availability of new types of personnel dosimetry systems; the increased need that arose for measuring beta doses as a consequence of significant surface contamination associated with some notable nuclear power reactor accidents, and the findings at even normally operating plants that hot particles, commonly containing significant quantities of beta emitters were a concern from the point of view of dose to the skin and possibly the eye. At that time, a number of researchers sought out work from earlier years by such individuals as Loevinger (1956), who had developed a point source dose distribution function for beta emitters that allowed reasonable predictions of beta dose transmission through air or tissue-equivalent material. The earlier information was revisited and built upon. Consequently, some of the papers that may be of interest may be several decades old.
If you are a member of the Health Physics Society you can gain access to past publications of the Health Physics Journal through the Members Only section of the website. A good example of an older, but pertinent, paper is Dunlap et al. (1977). The paper provides results and discussion for attenuation of beta radiation from a couple of common higher energy beta emitters (90S-90Y, and 32P) by typical protective eyewear that used either plastic or glass safety lenses. The composition of the plastic in the safety glasses was not given, but it is unlikely that there would be any appreciable differences in attenuation among plastics that might be used in safety glasses. Calculations in the cited paper were done using a simple formula for the mass attenuation coefficient for the beta radiation. The calculations were based on a simple exponential transmission estimation of the form
I = I0 e-(u/ρ)x,
where I is the transmitted beta intensity, I0 is the intensity incident on the aluminum attenuator of thickness x, and u/ρ is a beta mass attenuation coefficient that actually goes back to some old data describing aluminum attenuation of beta radiation that was fit by Robley Evans (Evans 1955). The fit expression was
μ/ρ = 17/Em1.34,
where Em is the maximum beta energy, in MeV, for the beta emitter of interest, and u/ρ has units of cm2 g-1. The fact that these data were for aluminum would produce some expected variation between calculated and measured results that the authors report.
The calculations and the measurement results showed some appreciable differences, but the trends were similar, with the most notable result likely being that for the high energy beta emitters, the plastic safety lenses, which were about 3.5 mm thick were not as effective as safety glass (3.65 mm thick), the plastic allowing (based on measurements) about 10% beta transmission for 90Sr-90Y beta radiation (Em = 2.28.MeV) and 1% for the 32P beta radiation (Em = 1.7 MeV); the safety glass results were about 0.4% transmission (somewhat uncertain) for the 90Sr-90Y and 0.1% for the 32P.
Depending on your needs, there are certainly other approaches to evaluate the effectiveness of selected materials in attenuating beta radiation. An easy-to-use computer code that has been popular for more than twenty years (now in its third revision) is the VARSKIN 6 code, which was developed primarily for determining beta skin doses from skin or clothing contamination, but it allows other types of calculations that can be easily implemented to evaluate dose transmission through more-or-less tissue equivalent materials, as would be many common plastics, at least from a beta transmission point of view. The VARSKIN 4 version is available (but at a cost) from the Radiation Safety Informational Computational Center at Oak Ridge National Laboratory. This is a deterministic code that runs very quickly and is easy to learn to use, unlike typical probabilistic codes, such as Monte Carlo simulation codes that are very effective but require a considerable learning curve for the uninitiated and sometimes long run-times.
I hope the above is sufficient to lead you in the right direction to address your needs.
George Chabot, PhD, CHP
References
Dunlap JH, Harvey PW, Schwing RL. Comparison of the effectiveness of contemporary ophthalmic lenses against beta radiation. Health Phys 32:555–559; 1977.
Evans R. The atomic nucleus. McGraw-Hill; 1955.
Loevinger R. The dosimetry of beta sources in tissue. The point source function. Radiology 66:55; 1956.