G. William Morgan Lecture Summary: Accelerator Radiologial Protection--A Personal and Privileged Odyssey
Note: This summary is based on the complete text, which, together with the slides used during the lecture, is available on the web.
The dying years of the nineteenth century saw Roentgen’s discovery of x-rays with the aid of a primitive electron accelerator, foreshadowing the important role that particle accelerators were to play in laying the foundations of the health physics profession.
X-rays immediately found application to medicine, and for the next 30 years, the primary concern of radiological protection was the prevention of harm that might result from overexposure to photons (low linear energy transfer [LET] radiations).
In 1932 the invention of ion accelerators at Berkeley and Cambridge led to the first concerns for protection from external exposure to neutrons (high-LET radiations). By 1934 John Lawrence suggested that the relative biological effect (RBE) for neutrons was between five and ten and appropriate protection standards were set at Berkeley.
From 1932 a succession of discoveries followed-–the neutron, induced radioactivity, and radioactivity of tritium (1938) and of neptunium and plutonium (1940-42). These fundamental discoveries directly led to the birth of nuclear medicine (1934); studies of radionuclide metabolism, neutron radiobiology, and neutron radiotherapy (1936); and initial studies of the medical effects of transuranics (1942).
Thus by the end of 1942 the fundamental aspects of the tasks that have occupied health physics for the past 60 years were defined by the work of accelerator laboratories. On December 2, 1942, the first self-sustaining neutron chain reaction was achieved by Fermi and his colleagues at the University of Chicago.
In 1947 Ernest Lawrence requested that Professor Burton Moyer establish a professional health physics group at the Radlab (now Lawrence Berkeley National Laboratory). With his creation of an independent health physics group, with which accelerator designers could consult on matters of accelerator radiation safety, Moyer set a pattern followed by accelerator laboratories around the world.
Moyer is best remembered for the "Moyer Model," first used to design the shielding for the Bevatron, and he subsequently assisted in the design of many accelerator shields. Moyer’s publications on the philosophy of radiation dosimetry have had a lasting impact around the world. It is a philosophy that seems all the wiser with the passage of time.
In the mid-1950s the early proton synchrotrons, the Cosmotron and the Bevatron, were at the center of a radiation crisis that needed prompt attention. Ironically it was a crisis caused neither by failure nor by any sin of omission but rather by overperformance of the machines.
In 1962 Moyer successfully installed shielding, designed to reduce radiation intensities by a factor of 100, at the Bevatron. During the 1960s studies of the hadronic cascade in matter provided data that facilitated efficient accelerator-shield design and assisted in the development of modern radiation transport codes. Laboratories cooperating in these experiments included Brookhaven National Laboratory, DESY, CERN, Lawrence Berkeley National Laboratory, Oak Ridge National Laboratory, Rutherford Laboratory, and Stanford Linear Accelerator Center.
Radiation fields outside the shielding of high-energy accelerators are "mixed" in character, consisting of muons, neutrons, and photons. Neutrons, with widely distributed energies, are usually the most important component. The physical parameters of the radiation field (integrated particle fluence, energy spectrum, and the angular distributions of particles in the field) are clearly defined and measurable and are the basis for the dosimetry of accelerator radiation fields.
Any desired modified adsorbed dose quantity, Dm (a radiation protection quantity), may be related to the total fluence, phi, by
Dm = <g> x phi
where <g> is the appropriate spectrum-weighted conversion coefficient. Modern transport calculational methods now make it possible to determine neutron spectra and conversion coefficients for typical irradiation geometries of anthropomorphic phantoms.
The application of accelerator technologies will lead to an increasing potential for exposure to neutrons. This trend suggests that the dosimetry and radiation protection quantities for high-LET radiations deserve prompt attention.
Over the past 30 years the International Commission on Radiological Protection (ICRP) has paid insufficient attention to these topics, and the advice it has given has often been confused and ephemeral. Consequently the logical and theoretical basis for ICRP protection standards for high-LET radiations has been much criticized in the scientific literature. ICRP is aware of these difficulties and has announced that it is in the process of revising its comprehensive recommendations in 2005. It is to be hoped that this revision will adequately address these important topics.