INTERNATIONAL ACCELERATOR RADIOLOGICAL PROTECTION E-MAIL (IARPE) NEWSLETTER "The Official Publication of the Accelerator Section of the Health Physics Society" (with Contributions from International Correspondents) ====================================================================== November 1992 Vol.1 #10 ====================================================================== OFFICERS ====================================================================== President: Ralph Thomas, LLNL Past President: Wade Patterson, LLNL President-Elect: Geoff Stapleton, SSCL Secretary/Treasurer:Nisy Ipe, SLAC Directors (1 Year): Frank Masse', MIT BATES Paul Neeson, D.O.E. Directors (2 Year): Gerald Fallon, MIT BATES Paula Trinoskey, LLNL Directors (3 Year): Carter Ficklen, CEBAF Jerry Miller, LANL FROM THE EDITOR'S TERMINAL (Nisy Ipe ) ====================================================================== This is the week of Thanksgiving, a commemoration of the Pilgrims' celebration of the good harvest of 1621 in the U.S. It is a time for family, a time to feast, but above all a time to give thanks. And as I look back over the year, I have a lot to be thankful for, the newsletter has made it for almost an entire year. So this is an appropriate time to say THANKS to all our correspondents, for faithfully submitting their contributions every month, THANKS to all those who were gracious enough to provide me with feature articles on time (and didn't turn me down), THANKS to Wade Patterson and Ralph Thomas for their constant encouragement, and above all THANKS to you dear readers (for what is a newsletter without any readers!). Here's wishing all of you in the U.S. HAPPY THANKSGIVING! ANNOUNCEMENTS ===================================================================== Call for Papers - Accelerator Section, HPS Annual Meeting ---------------------------------------------------------------------- (Geoff Stapleton ) ---------------------------------------------------------------------- This is a call for submission of abstracts of papers to be considered for presentation at the Accelerator Section Meeting of the HPS Annual Meeting at Atlanta, Georgia, USA from July 11 - 15, 1993. The Health Physics Society has included in the HPS Newsletter (November issue) a form for members to make an application to submit an abstract for either an oral or poster presentation at the Annual Meeting of the Health Physics Society. Now it is very important that we in the accelerator section use the meeting to the full and we must try to make the meeting as useful and as interesting as possible. To that end we must try to build a program that takes advantage of our wide international membership and includes information on a wide range of topics. I would urge everyone to review their recent work with a view to making a presentation at the meeting. Please submit an abstract as soon as possible so that we can decide on the amount of time that must be scheduled. It is extremely difficult to reschedule presentations to include papers submitted at the last minute. One suggestion about the program is to include a number of keynote papers from around the world. Of course this would rely on the generosity of overseas presenters paying their own way to the conference and relying on the benefits resulting from collecting together so many international experts with a wealth of knowledge of radiation control at a wide variety of accelerator centers. Members of the section who are not HPS members and who reside overseas may obtain copies of the application/abstract form from Nisy Ipe (). Non-HPS members who reside in the U.S. may obtain forms from the HPS Secretariat, 8000 Westpark Drive Suite 130, McLean, VA 22102, phone no. 703-790-1745, fax no. 703-790-9063. All non-HPS members have to be sponsored (on the application form) by a HPS Member. Those submitting contributed papers should select `A' for LETTER CODE and `ACCELERATORS' for DESCRIPTION under SESSION SELECTION on the abstract form. Those who have been specially invited to deliver a keynote address should select `AA" for the LETTER code. Executive Board Meeting (Nisy Ipe ) --------------------------------------------------------------------- The Accelerator Section will be holding an executive board meeting at the midyear HPS meeting at Coeur d'Alene (40 miles east of the Spokane Washington International Airport). Coeur d'Alene is in Idaho. The meeting is scheduled to take place on Tuesday 26 January, 1993 from 5.00 to 6.30 pm in BOARD 5. This meeting will also be open to the members of the section. Agenda Introductory Remarks .......Geoff Stapleton Status of Initiatives Proposed by Past President (Wade Patterson) a) Section Directory/ List of E-Mail Addresses b) Long Range Planning Committee c) Special Event at HPS Meeting, San Francisco, 1994 d) Cosmetic Changes to IARPE Newsletter e) Letterhead for E-Mail Correspondence Status of Non HPS Members/Due Collection Accelerator Section Papers/ Review Committee Agenda for Annual Meeting a) Executive Board b) Accelerator Section Nominations for New Officers - Executive Board Inclusion of Editor on Executive Board New Editor/ Rotating Panel of Editors for IARPE Newsletter E-Mail Package (information) for Prospective Members of Section Accelerator Section Session at IRPA, Vienna Any Other Business NEWS FROM INTERNATIONAL CORRESPONDENTS ====================================================================== News from Frascati (Maurizio Pelliccioni ) ---------------------------------------------------------------------- ADONE, the LNF storage ring, will be closed at the end of April 1993. Then the machine will be decommissioned and dismantled. Inside the existing buildings will be installed a new complex of accelerators, the Phi-factory DAFNE. The new complex will consist of three main sections: a linear accelerator, a Damping Ring accumulator and the Main Rings. The aim is to accumulate, into the Main Rings, two electron/positron beams with a maximum intensity of about 10E+13 particles/beam (average current about 5 A) at 510 MeV. The Linac will be supplied by Titan Beta (USA). The first electron beam is expected by the end of 1994. Designs for Damping Ring and transfer lines have been completed and the procurement procedures are in progress: the commissioning would be completed during 1995. Parameters for the Main Rings have been definitively fixed. The main radiation protection problems of the new complex have been studied. An internal report about shielding is already available [A. Esposito and M. Pelliccioni, DAFNE Shielding, LNF-92/044]. Other reports dedicated to the skyshine effect and to the gas-bremsstrahlung are in preparation. News from LBL (Tony Greenhouse ) ---------------------------------------------------------------------- The Lahet Code System (LCS) from Los Alamos is available via anonymous ftp from academic.lbl.gov (128.3.12.48). All cross section data is included. Takers are requested to contact Rick Donahue () ---------------------------------------------------------------------- LINAC COHERENT LIGHT SOURCE (LCLS) AT SLAC. Since about March 1992 a collaboration group with members from SLAC, SSRL (Stanford Synchrotron Radiation Laboratory - now part of SLAC), UCLA, LLNL and LBL, has been meeting regularly to study the possi- bility of building a high power Free Electron Laser using SLAC's Linac. Here is a short list of possible design specifications: - tunable, short wavelength 0.1-10 nm (and longer, at lower E) - short pulse (<0.5 picosecond) - high peak power (gigawats) In the most likely scenario, a 7 GeV electron beam produced by the last 10 sectors of the Linac would be injected into a 50 m long undulator, located in the FFTB (Final Focus Test Beam) enclosure. The LCLS beamline and undulator would run in parallel with the FFTB beamline, sharing the same beam dump. The undulator itself would contain 2000 pairs of permanent magnets. This design could produce about 10E+14 coherent 4 nm photons within a 0.5 ps pulse at a rate of 120 Hz. The use of the whole Linac length, i.e. energies of 50 GeV, would produce an x-ray laser with wavelengths down to 0.1 nm. The interest in using only the last third of the Linac (7 GeV run) is the possible simultaneous operation with the B-factory, since the latter would use only the first 20 sectors. A workshop on scientific applications of short wavelength coherent light sources took place at SSRL/SLAC last month. The main purpose of this workshop was to get feedback from the scientific community on possible uses of such a unique tool. Originaly, the authors of the LCLS project saw its main application in biology, such as X-ray microscopy and holographic imaging. 1.0E+14 coherent photons would enable one to take a single hologram image during 0.5 ps. However, there are limitations due to radiation damage of the sample and other problems in detector technology yet needed to be overcome. As a result, biology is no longer perceived the primary client for LCLS. The project raised, however, a lot of enthusiasm in other fields, such as chemistry, atomic physics and science of materials. The current "most likely" parameters were chosen with regard to the (sometimes contradictory) requirements from these various fields of application. A Technical Review on November 20-21 was held to further assess the merits and feasibility of the project and help better define or choose from various options. Since there are several competing projects for free electron lasers with short wavelenghths currently being prepared, the DOE has asked the National Academy of Sciences to study their feasibility. The outcome of this study will most likely determine whether the LCLS project has a chance to be realized. NEW PUBLICATIONS/ INTERNAL REPORTS ====================================================================== Tromba et al, " Gas Bremsstrahlung from Bending Magnets", Internal Report, Trieste, Italy. May be requested from G.Tromba (). R. Donahue et. al., "Design of a Superconducting Linear Accelerator for an Infrared Free Electron Laser of the Proposed Chemical Dynamics Research Laboratory at LBL", LBL-32182/UC-406, August (1992). May be requested from R. Donahue () A. Esposito and M. Pelliccioni, "DAFNE Shielding", LNF-92/044. May be requested from A. Esposito (). ====================================================================== FEATURE ARTICLE Synchrotron Light Sources by John Arthur () ====================================================================== For just about 100 years, short-wavelength electromagnetic radiation has been used to probe the structures of materials. During the first 80 of those years, this radiation was generated by using high- voltage electron collisions to produce atomic excitations in a solid or gas target, which emitted the desired radiation as the excitations relaxed. The amount of radiation generated in this way by discharge tubes and x-ray tubes was limited by the power that the target material could safely absorb, typically no more than a few kilowatts. This produced no more than a few tens of watts of useful radiation, and often very little of it was emitted in the direction of the sample. Nevertheless, x-ray diffraction developed into the single most powerful tool for analyzing the atomic structures of materials. About 20 years ago, a new method for producing short-wavelength radiation appeared as a by-product of high-energy physics research. The brightness of the most intense x-ray sources, which had risen by only about a factor of 10 since the turn of the century, took a sudden turn upwards. (Brightness depends not only on total intensity, but also on how well it is collimated. It is measured as intensity per solid angle.) Since then x-ray source brightness has been increasing exponentially with a doubling time of under one year -- a factor of more than one million during 20 years. This has of course led to great increases in the precision and sensitivity of x-ray diffraction measurements, and has also brought about the extension of optical absorption spectroscopy into the vacuum ultraviolet and x-ray regions of the spectrum as a standard analytical tool. The physical process that has led to this spectacular rise in brightness is known as synchrotron radiation, after the electron accelerators where it was first noticed. These machines were designed to accelerate electrons to very high speeds while the electrons orbited repeatedly around closed loop paths. The looping orbit was established by using magnetic fields to bend the path of the electrons, and the speed was increased by pushing with a microwave-frequency electromagnetic field each time the electrons passed a certain point in the orbit. The orbiting motion of the electrons gives rise to emitted electromagnetic radiation, as in a radio antenna, but at relativistic electron speeds the radiation pattern is rather different from that of a radio antenna. A simple dipole radiation pattern in the moving frame of an electron becomes relativistically transformed into a narrow cone of radiation opening around the forward direction of travel, as if the speeding electron possessed a headlight. The opening angle of this cone is given to good approximation by the ratio of the electron rest mass energy (0.5 MeV) to its total energy (typically several thousand MeV in modern accelerators). This is therefore a small angle, less than a hundredth of a degree. The relativistic transformation is one of the key contributing factors to the high brightness of synchrotron radiation sources. The other major contributing factor is the large amount of radiation produced in large accelerators. The intensity of the synchrotron radiation is proportional to the square of the total electron energy, and can reach many megawatts in a large machine. This is what ultimately limits the capability of electron synchrotron accelerators for high-energy physics experiments, since the synchrotron radiation energy must be constantly replaced in order to keep the electrons up to speed. (Note that the amount of radiated power also depends inversely on the square of the rest mass of the speeding particle -- synchrotron radiation is negligible at proton accelerators.) To use synchrotron radiation, one drills a hole in the shielding surrounding an electron (or positron) accelerator and observes the light produced at a particular point in the particles' orbit. Since radiation is only produced at points where the orbit is curving, and since the curving is produced by magnetic fields, the source point for each synchrotron radiation beamline is associated with a particular magnetic field. The spectral range of the synchrotron radiation is proportional to this magnetic field, and to the square of the particles' energy. This spectral range is typically many keV, and its broad width is another desirable feature of synchrotron radiation. A user nearly always employs some form of monochromator to select a small fraction of the total bandwidth at one time, but the broad source spectrum allows for quick changes in the energy of interest. This tunability has been particularly exploited by experimenters who measure the absorption characteristics of materials as a function of energy, with a view toward understanding their chemical and structural properties. A variety of acronyms, such as XPS (x-ray photoemission spectroscopy), EXAFS (extended x-ray absorption fine structure), and even SEXAFS (surface EXAFS), have come to be used for the various measurements of the absorption characteristics of a material as a function of energy. Only the simplest experiments of this type were attempted before the advent of synchrotron light sources, and were viewed as demonstrations of principle rather than the serious tools they have become. As techniques for using synchrotron radiation have become more sophisticated, there has been a push for even more intensity and brightness, and for more synchrotron light facilities. Whereas the first synchrotron light centers were built around accelerators designed for high-energy physics (at SLAC, Wisconsin, Cornell, and DESY), within the last 10 years more than a dozen large machines have been built (or are under construction) around the world for the express purpose of providing synchrotron light. The most recent of these machines provide for enhancement of the synchrotron light through special magnetic devices called wigglers and undulators. Both are periodic arrays of magnets which deflect the high-energy electrons along a nearly sinusoidal path with a period of a few centimeters. This multiple bending action produces more radiation than the simple bending that guides the electrons around the accelerator. The subtle distinction between a wiggler and an undulator involves the amount of bending which the electron beam undergoes. The bending is smaller in an undulator device, which allows radiation emitted from one bend to overlap that emitted from successive bends. Interference within the radiation field leads to highly enhanced output at certain wavelengths, and greatly reduced output at all others. This is a great advantage for fixed- energy work such as x-ray diffraction(undulators are the brightest x-ray sources available today, other than nuclear bombs), but is generally not a good idea if rapid energy tunability is required. Undulators and wigglers are collectively referred to as insertion devices, being inserted into an accelerator as additional elements which play no part in the machine's accelerating function. It is likely that the brightness and intensity of sources of short-wavelength radiation will continue to grow at a rapid rate over at least the next 20 years. Initial studies are now in progress using sources incorporating very long (sometimes more than 10 m) undulator insertion devices. With such undulators, it is possible to create a radiation field strong enough to stimulate the electrons to radiate even more, creating what is called a free electron laser. Concentrating this self-enhanced synchrotron radiation power into a well-collimated beam with very narrow bandwidth, future devices of this type should make today's high-power wiggler sources seem as comparatively weak as an old fashioned x-ray tube. John Arthur is a physicist working at the Stanford Synchrotron Radiation Laboratory, a division of SLAC. His research interests lie in the field of x-ray diffraction, which he uses to study the structures of magnetic materials. CLOSING THOUGHTS ====================================================================== Two kinds of gratitude: the sudden kind We feel for what we take, the larger kind We feel for what we give. E. A. ROBINSON, Captain Craig