Practitioners of health physics who design or review radiation protection around particle accelerators are currently troubled by not just one, but two dilemmas. Expert witnesses on the subject of accelerator radiation safety will of course eventually have to deal with the results of changing definitions and regulations, and these will come from two quite different directions.
Health physicists who do not deal with particle accelerators (cyclotrons, synchrotrons, linear accelerators and other similar devices) are already familiar with one of these dilemmas. There are changes anticipated in acceptable (no longer permissible) legal limit of dose-equivalent that can be received by radiation workers ("occupational dose"). A change in the legal limit for the general public ("non-occupational dose") has already occurred.
The Nuclear Regulatory Commission (NRC) is the primary US agency charged with setting both the annual non-occupational and occupational doses. There are subcategories of exposure, such as dose-equivalent to extremities. However, the values are 100 millirems per year for a non-occupational dose and 5 rem per year for the occupational dose. But, only a few years ago, the non- occupational dose was 500 millirems: 10% of the occupational value. The origin of this reduction is instructive.
The NRC is guided by an entirely different governmental organization, the National Council on Radiation Protection and Measurements (NCRP), which is only entitled to make recommendations. The NCRP tends to follow the recommendations of the International Council on Radiation Protection (ICRP), supported by the major industrialized nations. Within the last five years, the ICRP recommended reduction of the non-occupational dose to its present 100 millirem value; the NCRP concurred, and the NRC accepted this recommendation. At the same time, the ICRP (also echoed by the NCRP) recommended reduction of the occupational dose to 2 rem, but the NRC did not buy it. This change was also resisted in Europe, although within the last year, Italy has in effect accepted the 2 rem annual occupational dose, and several US installations (e.g., Fermilab) accept it. It is expected to become 2 rem everywhere, eventually. This is one "moving target" for the radiation protection specialist.
Must the NRC regulations always be followed? No, because the states need not accept them, as written. All states in fact have regulations very similar to those of the NRC. However, the majority of states with any nuclear industry have become agreement states, and such states are bound to the NRC requirements. Moreover, the NRC has authority over (civilian) nuclear reactors and projects in nuclear medicine wherever they may be. The jurisdictional complexities, however, become imposing when accelerators-of any size- are considered: in the US, they are always regulated by the state (and in Canada, by the province). The NRC has no jurisdiction over accelerators; it cannot even regulate shipment of an accelerator-produced radioisotope! State control apparently grew out of their historical responsibility for regulation of x-ray machines.
Now, particle accelerators are the primary source of neutrons commonly encountered in scientific laboratories, although a few radioactive materials like Californium-252 also produce them. And neutrons have come under recent scrutiny by the ICRP, resulting in a new set of recommendations, relating only to neutrons, which will impact almost every existing particle accelerator. A very small number are used for nuclear research, but many are used for medical and industrial purposes.
The unit of dose-equivalent, the rem, stands for r"ntgen- equivalent-man. It intended to include biological effects, and is obtained from measurement of a physical dose, D (basically, the density of energy deposition per unit mass, something that can be measured in a laboratory) and then multiplying D by a quality factor, Q, to get the dose-equivalent, H. That is: H = Q*D.
For many years, the Q-value for all x-rays, gamma rays, and electrons has been 1.0, while that for neutrons has depended on their energy. It varies from 2 at very low energies to about 11 for neutrons from the fission process, then drops to 5 at still higher energies. But, as a result of reinterpretation of old data from the Hiroshima explosion, fission neutrons near have been assigned a Q of 20 by the ICRP, and indeed, the Q has been (essentially) doubled all across the neutron spectrum. The preceding equation indicates that this will have the effect of approximately doubling the dose- equivalent wherever the views of the ICRP (and also, of the NCRP) prevail, even if the physical dose, D, is unchanged.
The prospective doubling of the legal dose-equivalent for neutrons is another contingency against which the conscientious health physicist must plan if he works on accelerator safety and this comprises a second (if rather specialized) moving target. It should be stated that this increase of Q has not yet been officially accepted anywhere. Indeed, it has been vociferously opposed by many experts in radiological safety, but there is some sense of its inevitability: in the past, the recommendations of the ICRP have always eventually prevailed.
Taken together with the proposed reduction in the acceptable occupational dose rate, adoption of the recommended increase in Q would abruptly cause a number of installations, now considered to be within regulations, to be producing radiation dose-equivalents at five times the legal annual rate. Accelerator operators are not the only occupation to be adversely affected: pilots and crews of commercial and military jet aircraft receive most of their high- altitude radiation dose from energetic cosmic-ray produced neutrons. The present dose-equivalent rate is about one millirem per hour: if the ICRP reecommendations were accepted, it could suddenly become two millirems per hour without any change in the actual physical dose rate, D. Taken together with a 2 rem annual occupational dose, this would limit air crew to a maximum of 1000 hours annual flying time. Passenger travel might also be affected.
Looking ahead, it seems probable that administrative changes recommended by the ICRP may become the basis for a number of legal actions in the not-too-distant future. At least in the U.S., past changes either in acceptable annual dose or in quality factor, Q, have not allowed a "grandfather" exemption for radiation shielding which has become inadequate. Either it was modified to meet the new requirement, or the accelerator had to be turned off. In the case of radiation exposure in jet aircraft, no effective radiation shielding can be provided. The only option is to limit time spent aloft, with concomitant economic impact. Expert witnesses will wish to follow the responses of the NRC to the ICRP recommendations for a number of years.
* Dr. Knowles is an expert on radiation health and safety as well as particle accelerator theory, design and operation.