Author's note: The tenth course organized by the School of Radiation Damage and Protection of the Ettore Majorana Centre for Scientific Culture in Erice, Sicily, is to be held October 2-9, 2001. The topic will be accelerator radiation protection. Dr. Alessandro Rindi, a former director, was a catalyst for the course by suggesting that the 25th anniversary of the first course organized by the School of Radiation Damage and Protection on High-Energy Radiation Dosimetry and Protection, held in October 1975, be celebrated in some way. It was hoped that the tenth course would be held in October 2000, but formidable difficulties had to be surmounted. I am pleased and proud that the hard work and determination of several members of the Accelerator Section of the Health Physics Society, latterly in collaboration with CERN, played a crucial role in bringing this course to fruition. In particular, the efforts of Vashek Vylet and Lutz Moritz have been outstanding.
The recent evil events in New York and Washington have made it uncertain whether the entire faculty from North America will be able to deliver their lectures in person, but their impact will nevertheless be significant. The lectures that were (or that were to be) delivered at Erice will be published in December in a special issue of Radiation Protection Dosimetry, Volume 96 (#4).
The first paper at the course was to have been presented by the author but events made it impossible for him to attend. Linnea Wahl invited the author to provide a shortened version of the paper, entitled "The History and Future of Accelerator Radiological Protection," for Accelerator Section Newsletter readers. The task of reducing the original some 13,500 words to less than 4,500 words required some compromises. The original takes a global view; this shortened version has selected several of the passages describing work done at Lawrence Berkeley National Lab.
The development of accelerator radiological protection from the mid-1930s, just after the invention of the cyclotron, to the present day is described in the context of three major themes: physics, personalities, and politics.
This paper is a sequel to the first review given by Professor Mario Ladu at the 1975 School of Radiation Damage and Protection course entitled "High Energy Dosimetry and Protection" and indicates the changes that have occurred in the intervening period up to the present day. Among other topics discussed are
In the final section, which discusses the future of accelerator radiological protection, some emphasis is given to the social and political aspects that must be faced in the years ahead.
At Berkeley, "the birthplace of the cyclotron," there was an immediate interest in the application of accelerators to medicine, and the radiation hazards around the new cyclotrons began to be identified. This was in no small measure stimulated by John Lawrence, Ernest's brother, who was a physician by education. As early as 1935, a neutron protection standard, based upon the results of studies of the changes in the peripheral blood of rodents, of 0.01 r/day for n-rays was in effect at the Crocker Lab on the Berkeley campus. This standard was an order of magnitude lower than the then-contemporary standard for x-rays of 0.1 r/day.
In the post-second world war period, two events were to generate a sense of urgency to accelerator radiation studies. The first was the observation of lens opacification in accelerator workers whose eyes had been exposed to particle beams. The second was new operational experience with the high-intensity and high-energy proton synchrotrons. Both the Cosmotron and the Bevatron were more successful than anticipated. For example, the Bevatron was designed to accelerate protons to a kinetic energy of 6.2 GeV with an initial beam intensity of 10^9 protons per pulse, but within a year an intensity of 10^10 protons per pulse had been achieved. Its operation was limited to intensities of about 10^11 protons per pulse until a shielding roof was added in 1962.
There is a delicious irony in the fact that the "imprudence" of the designers of the 184-inch cyclotron, the Cosmotron, and the Bevatron paid a double dividend for accelerator radiation protection. First, the design of the accelerators above-ground facilitated shielding studies. Second, radiation specialists became valued members of the experimental team, making efficient utilization of accelerators for their intended purpose, which was nuclear physics research. "Health physicists" were perceived not as part of the problem, as is unfortunately nowadays sometimes the case, but as part of the solution.
In the beginning, there were no health physicists at accelerator laboratories - just accelerator physicists. During the early post-war period, several distinguished scientists became interested in radiation studies; for example, the Lawrences and Moyer at Berkeley, Lindenbaum at Brookhaven, Ramsey at Harvard, and Pief Panofsky at Stanford. Panofsky performed calculations of the attenuation of neutrons in concrete, electromagnetic cascades, and skyshine. Tobias at Berkeley was interested in neutron relative biological effectiveness (RBE) and Rossi at Columbia was establishing his concepts of dual radiation action and the relationship between RBE (later, quality factor) and linear energy transfer (LET).
In 1947, Ernest Lawrence placed in Professor Burton Moyer's hands the task of establishing at Berkeley one of the first, if not the first, teams of professional health physicists capable of supporting research workers and accelerator designers. Moyer was a distinguished physicist held in high esteem, both as a man and as a physicist, by many eminent scientists.
After its experience with cyclotrons throughout the Manhattan Project, Lawrence Berkeley National Lab was particularly well-placed to take a leading role, and Moyer had many knowledgeable colleagues to consult. Moyer and Tobias were members of ICRP Committee IV, which made the first internationally accepted recommendations for neutron fluence-to-dose-equivalent conversion coefficients and neutron quality factors. With such talent available, it was inevitable that "the birthplace of the cyclotron" should also become "the birthplace of accelerator health physics."
Most importantly, Moyer wrote some of the seminal papers that set the direction for many in accelerator radiological protection. He is today often remembered for the "Moyer Model." First used to design the shielding for the improved Bevatron, the model has been "fine-tuned" in some respects and has served in the design of many, many accelerator shields.
One of Moyer's most important decisions was to recruit Wade Patterson, the first professional accelerator health physicist. During his tenure through five decades at Lawrence Berkeley National Lab, Wade turned his hand to the solution of a great number of diverse accelerator radiation problems. In 1966, Wade and his team helped to design and execute the most ambitious accelerator-shielding experiment even to this day. The measurements were made at the 28-GeV proton synchrotron at the European Organization for Nuclear Research (CERN), and the database obtained was invaluable for the design of the third generation proton synchrotrons: the 300-GeV Super Proton Synchrotron (SPS) at CERN and the 200-GeV Fermilab synchrotron.
By his establishment of an independent professional 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.
The development of the accelerator health physics profession from the 1950s to the present day - from Lawrence Berkeley National Lab to the Superconducting Super Collider (SSC) - has been documented in a collection of personal recollections by many of the early pioneers in the field. More than thirty physicists from more than 24 high-energy laboratories have described their work in detail and from their own perspectives in the book, "The History of Accelerator Radiation Protection."
The operational problems of the first generation weak-focusing proton synchrotrons discussed at a meeting held in New York in 1957 stimulated radiation studies at all those laboratories planning, designing, or constructing new accelerators. Laboratories cooperating in these experiments included Brookhaven National Lab, Deutsches Electronen-Synchrotron (DESY), CERN, Lawrence Berkeley National Lab, Oak Ridge National Lab, Rutherford Lab, and the Stanford Linear Accelerator Center.
Throughout the entire decade of the sixties, many shielding experiments were performed to determine empirically the data needed to design economic accelerator shielding with practicable shielding materials (earth, concrete, and steel). These were usually collaborative efforts by several high-energy laboratories such as those listed above.
Shielding experiments were treated in the same fashion as other nuclear physics or high-energy physics experiments when competing for the principal use of accelerator time. Proposals were submitted and approved, and accelerator time was scheduled. Data-gathering occupied weeks to months. Analysis of the data took months to years. Such experiments were costly and often beyond the resources of one laboratory. It is indeed a tribute to the accelerator community that these opportunities were afforded to the radiation physicists for shielding studies, particularly at the front rank accelerators, when there was fierce competition for accelerator time for fundamental studies.
These experiments were of two types. Both involved probing the radiation field in shielding, either in the form of a "blockhouse" specially constructed for the purpose, or in the existing shield of a working accelerator.
The typical beam-stop experiment consisted of a large array of blocks assembled so that radiation detectors could be inserted to explore the spatial distribution and composition of the radiation field in the assembly. These experiments were large: for example, a concrete beam-stop assembly used at Lawrence Berkeley National Lab measured 8.5 m (28 ft) long by 6.7 m (22 ft) wide by 5.5 m (18 ft) high and weighed 755 tons.
Laboratories cooperating in these experiments included Brookhaven National Lab, DESY, CERN, Lawrence Berkeley National Lab, Oak Ridge National Lab, Rutherford Lab, and the Stanford Linear Accelerator Center. Although many data were obtained from these experiments and a great deal of understanding of cascade mechanisms was achieved in consequence, it was found difficult to interpret the data in a simple manner.
A definitive shielding experiment important for the understanding of shield design was performed by probing the earth shield of the 28-GeV CPS. Teams from CERN, Lawrence Berkeley National Lab, and Rutherford Lab collaborated to provide information for the shield-design of the Fermilab proton synchrotron and the SPS at CERN. A breakthrough was made in data interpretation by adapting the point-kernel model developed by Moyer to take account of the finite dimensions of the radiation source inside the accelerator. With this adaptation, it was possible to reproduce measured data in the earth shield to an accuracy of (20% over regions extending spatially over nearly 100 m and over a range of five orders of magnitude in fluence, with as few as five parameters. Following this achievement, significant advances in radiation transport calculations and comparison between theory and experiment made shielding design much more accurate.
Over the past forty years, an enormously powerful weapon has been added to the armory of accelerator health physicists with the development of computer codes that may simulate the physical interactions in matter for complicated geometries. This resulted from a happy congruence of many factors, including among others, the compilation of extensive databases of cross sections, the development of nuclear models that describe particle production, the ability to encode complicated three-dimensional geometries, and, not least, fast computing facilities. Indeed, modern computing techniques are a formidable cocktail of sophisticated technology and the application of the academic pursuit of the theory of random numbers by Von Neumann, Turing, and others. The development of modern radiation transport codes will be discussed during the Erice Course by Ferrari and has been described in several reviews, for example by Nelson and Jenkins and by Fasso et al.
Among the many transport codes in use by accelerator designers, HETC, developed at Oak Ridge National Lab, has such great flexibility that it is regarded as a "bench-mark code" to which other codes are compared.
Other important codes in use include LAHET developed by Los Alamos National Lab; FLUKA, largely due to the consistent hard work and enthusiasm of J. Ranft; and MARS, developed by Mokhov and his colleagues. Comparisons with calculations by CASIM and FLUKA show good agreement. MARS has now largely replaced CASIM, a code developed at Fermilab by Van Ginneken. MARS has included an extremely versatile geometry module. Electron Gamma Shower (EGS) is a powerful code for calculating electromagnetic cascades. The energy range of the code is from 10 KeV to at least 1 TeV. The most recent version of the code is described by Nelson et al.
"Skyshine" was the first "environmental" impact observed from high-energy particle accelerators. The lack of roof shielding at both the Cosmotron and Bevatron led to the observation of neutrons resulting from accelerator operation, even out of sight. Relatively low-energy neutrons resulting from nuclear interactions in the air were scattered back down to earth. This inadvertent and unintentional source of radiation exposure to workers and members of the general public was to be eventually eliminated by shielding and efficient accelerator operation. In the short term, however, empirical and theoretical studies were undertaken to understand the mechanisms and reduce radiation exposures. Most of the data available were provided by direct measurement. Theoretical studies of skyshine are limited. A comprehensive bibliography describing these studies from the late 1950s up to 1994 is given in the original paper.
It is no longer possible to make experimental studies of skyshine around most operating accelerators because the problem has essentially been solved by shielding it away - the annual dose equivalent rates at the site boundaries of most accelerator laboratories are now very low (typically less then 100 microSv per year in the United States). With this removal of the practical problem, there has been little enthusiasm for deploying the resources necessary to carry out a comprehensive theoretical study despite our ability now to calculate skyshine rather precisely. Such a study would be of important intellectual and academic interest but also has utility. For example, very recently a computer simulation has revealed that the estimates of dose equivalent reported at the site boundary of Lawrence Berkeley National Lab during the late 1950s to mid-1960s may now be reduced by a factor of at least five. This reduction is due to three factors: first, better evaluation of neutron spectra; second, the application of appropriate fluence-to-dose-equivalent conversion coefficients; and, finally, some degree of conservatism in calibration of the neutron detectors.
A second potential impact discussed in the original paper is the activation of ground and ground water around high-energy accelerators. An extensive bibliography of the development of the history of environmental protection at accelerators is provided.
Radiation protection standards and radiation protection dosimetry will be discussed in several papers given at the course in Erice. Most importantly, Moyer emphasized that the physical characterization of the accelerator radiation environment in terms of the type of radiations and their angular and energy distribution was to be preferred in radiation protection at particle accelerators in that it provided an unchanging basis that could be used to interpret the subsequently improving, and thus ever-changing, biological information. Conversion coefficients are a vital part of this technique and accelerator radiation physicists have played an important role in their development. These coefficients have been remarkably stable with time despite many detailed changes in their definition and in the methods of their calculation.
The accelerator radiological protection community owes an enormous debt to those who have taken the time to assemble and analyze their data, submitting them to the rigors of peer review and publication so that an historical record of the progress in our understanding of accelerator-produced radiation has been developed.
One early book by Barbier entitled "Induced Radioactivity" (1969) has proved to be of inestimable value to those working at accelerators in controlling exposures to radioactive materials and contamination and is in use to this day.
Three volumes not published in English deserve special mention. Monographs by Zaitsev et al. (1971), Eberhard Freytag (1972), and Lucci et al. (1974) were important in drawing together the strands of information widely spread throughout the scientific literature to form a coherent fabric.
The monograph written by members of the team that evaluated the radiation safety of the CERN SPS entitled "Shielding against High-Energy Radiations" compiles the data needed to design shielding, particularly in the energy region between 1 GeV and 10 TeV and is a most valuable desk reference.
Beyond the standard monographs, much information is to be found in the publications of quasigovernmental organizations and in the published proceedings of workshops and review panels. The International Atomic Energy Agency (IAEA) has done valuable service in assembling information on accelerator radiation safety not generally available to small institutions or university departments. Operational aspects of accelerator radiation safety are described in detail in three technical reports of the IAEA dealing with neutron generators, electron linear accelerators, and proton accelerators. All three reports include extensive bibliographies. The proceedings of meetings organized by the Nuclear Energy Agency of the Organisation for Economic Cooperation and Development (OECD) provide much material on the shielding aspects of accelerator targets and irradiation facilities. The summaries of a series of workshops held at the Continuous Electron Beam Accelerator Facility (CEBAF), organized by Geoffrey Stapleton, provide much information on topical interests including shielding design, skyshine, ground-water contamination, environmental monitoring, and safety interlocks. The probability and consequences of serious accidents with high-energy, high-intensity accelerators is explored in the proceedings of a workshop held in Los Alamos in 1991.
Educators should not be forgotten and worthy of special attention are the notes prepared by Cossairt for the series of lectures that he has presented at many venues within the United States for the past several years. This lecture series follows in the tradition of the Accelerator Health Physics School held annually in Berkeley from 1967-1970 that trained many of the second-generation accelerator health physicists in the United States.
Finally, with respect to teaching, special mention must be made of the monograph entitled "A Guide to Radiation and Radioactivity Levels near High-Energy Particle Accelerators" by Sullivan, published in 1992. This brilliant little book is a "good read," excellent for students and busy scientific-administrators alike.
One thing does seem certain: there will be accelerators in our future. Professor Rindi will discuss the steady increase in the number of accelerators in the world and the increase in the number of their applications. Silari will touch on a special example of these increases in medicine at this conference. For the foreseeable future the application of accelerators to diagnosis, radiopharmaceutical production, and therapy is likely to increase.
Many physicists and engineers believe that some form of safe nuclear energy offers the most viable solution to the world's energy shortages. The choices are largely political but if the nations of the world opt for nuclear energy - whether it be "conventional" nuclear reactors, breeder reactors, or fusion devices - then accelerators or accelerator-like devices will be involved.
More than 20 years ago, at the time of the world oil crisis, Van Atta studied the economic feasibility of producing plutonium by a high-energy, high-intensity (100-mA) proton linear accelerator. The plutonium produced could serve as fuel for fast breeder reactors.
One of the major objections to power generation by nuclear means has been the issue of waste handling and disposal. There have been many suggestions that high-intensity proton linear accelerators could effectively be used to "incinerate" this waste and transmute the troublesome long-lived fission products to species of shorter halflife. For example, about ten years ago Venneri et al. suggested that an accelerator similar to that proposed by Van Atta would produce neutron fluence rates about 100 times higher than those in the cores of nuclear reactors and that energy-efficient schemes could be constructed. Significant reduction in the inventory of radioactinides on-site could be achieved at a nuclear power station operating an accelerator-driven transmutation of waste (ATW) system.
Neither does there seem to be any lack of demand for new accelerators for application to fundamental research. The Large Hadron Collider is scheduled to operate in 2006. The competition to construct the next generation linear collider (to operate about 2010) has just heated up with the recent TESLA proposal from DESY, to vie with the collaborative designs for NLC/JLC from Stanford and Japan. CERN has a proposal for a linear-collider known as CLIC, which could operate around the middle of the next decade if funded promptly.
What does all this mean for our profession? Some things are not in our control. We can take care of the physics. The personalities are difficult to control but we must not let the politics take care of us. Some of the topics that are worth discussing along these lines are
Leadership of the radiological protection profession: There are many who now feel that the leaders of our profession have put themselves beyond the scope of the scientific method in their discussion of low-level radiation effects in humans, the linear no-threshold hypothesis (LNT), and hormesis.
In the view of Karl Popper, one of the necessary criteria for an hypothesis to be within the scope of the scientific method must be that it is testable against falsification. LNT is not testable against falsification and thus, some have argued, cannot be within the purview of science.
The two methods of determining the truth developed by society, the law and science, are today in serious conflict. The well-known aphorism says that whereas scientists seek to discover and assemble facts to discover theories, attorneys, on the other hand, first establish a theory and then search for those facts that buttress it.
The neutron RBE problem: If accelerators are to make a significant contribution to word-wide energy production, high-LET radiations will make up a significant contribution to the radiation exposure of workers and to the general public. It is a sign of our times that there is now concern expressed by aircrew and their representatives about exposure to cosmic radiation that is "accelerator-like." In this regard, it seems to the author that as a profession we have snatched defeat from the jaws of victory. After a wonderfully successful thirty years in which radiation exposures to workers have, in general, steadily declined, we are left with a situation in which airline personnel are among those workers experiencing the highest estimated radiation exposures - and that due to natural radiation to boot. When I began my career in 1948 such an outcome would have been regarded as a triumph! Menzel will discuss this topic in depth at the course in Erice.
At the present time, the data available to make judgments about appropriate protection limits is woefully inadequate. There is conflict between some who interpret the Hiroshima epidemiological studies as showing that the neutron RBE is surprisingly small (i.e., much lower than 10) and those that insist that the much larger values (100 or greater) found from single-cell in vitro studies are more applicable to standards setting. Better neutron RBE data are needed to enable a scientifically based decision that will command the respect of workers and the general public. We probably cannot wait until our understanding of the implications of the structure and function of the human genome for the mechanisms of radiation-induced cancer provides the answer.
Decommissioning of particle accelerators: If accelerators will be increasingly used by society to meet its needs, then society must ultimately face its own "not-in-my-backyard (NIMBY) crisis." The disposal of radioactive waste has become a politically charged problem and its solution must be political. The disposal of radioactive accelerator components, even those of low-specific activity, is extremely difficult and costly. There are two papers on this topic on decommissioning low-energy proton and heavy ion accelerators by Moritz and Musolino, respectively, to be presented at the conference in Erice that will emphasize this point.
Heavy ion accelerators and spallation neutron sources: The bombardment of high mass number targets by energetic protons or heavy ions results in the production of fission fragments, many of which may be alpha-emitters and some of which may be volatile (e.g., radon). This will move accelerators such as SNS or the Isotope Separator and Accelerator (ISAC) Facility of the Tri-University Meson Facility (TRIUMF) into new operational regions where contamination control becomes extremely important.
Information overload and the decline in scholarship: The body of information that makes up accelerator radiation safety has not been accumulated in a continuous and systematic manner. Rather, efforts have been applied to the solution of specific problems, often in crisis, or to the design of new accelerators. Much of the information obtained in this way was consigned to institutional internal reports such as design studies and not published in the scientific literature and, after some years, forgotten.
There is an irony in that now, even though data are much more easily accessible than in former times, there is some evidence that research workers are unaware of the early literature and this has led, in some cases, to the unnecessary and wasteful rediscovery or reinvention of facts already known. As has been suggested previously, the accelerator radiological protection community thus owes an enormous debt to those who have taken the time to assemble their data, analyze them, and record the progress made by submitting them to the rigors of peer review and publication.
Intellectual surprises: Partly out of intellectual curiosity and partly to allay any public apprehension, Cossairt et al. have studied the dose equivalent produced by neutrinos produced at Fermilab and at the NuMI project in Minnesota. Comparison was made with the dose equivalent from stellar neutrinos. The dose equivalent was found to be insignificant. However, Mokhov and Van Ginneken, in considering the possibilities of building muon colliders, have shown that the dose resulting from neutrino scattering is, indeed, a significant radiation hazard at readily measurable levels if the energy of the circulating beam is above about 200 GeV.
The moral of this cautionary tale is to keep on thinking!