Preview of Accelerator Session Presentations
For those readers who will not be able to attend the 48th Annual Meeting of the Health Physics Society in San Diego, California, in July, we’ve provided a sampling of abstracts of the presentations. For those who will be attending, let this serve to whet your appetite! All presentations will be on Wednesday, July 23, at the time noted. If you’d like more information about the presentation, please contact the author noted. If you'd like to see the full listing of presentations, please consult the preliminary meeting program.
Presentation WAM-B.2: 9:15 to 9:45 AM)
by J. D. Cossairt and N. V. Mokhov
Investigators at several laboratories are seriously considering the long-term storage and even distant transport of very low-energy antiprotons as a part of basic physics research programs and perhaps even for practical applications. To do this successfully will require proper attention to the prompt radiation hazards resulting from the release of energy in the annihilation of antiprotons with nuclei under either planned or accidental circumstances. In this presentation, the broad scientific motivations for storing and using low-energy antiprotons, which extend across the breadth of science and technology from basic physics research to potential applications in medicine and perhaps even spaceflight propulsion, are discussed. The basic physical mechanisms of storing low energy antiprotons in so-called “Penning traps” are surveyed and recent progress in this area of technology is reviewed. The major features of the somewhat novel radiation fields associated with annihilating low-energy antiprotons are described starting from first principles and a detailed Monte Carlo-based design of possible shielding configurations is provided. It is concluded that these radiation fields are readily understood and that the hazards can be mitigated using conventional techniques. However, for large numbers of stored antiprotons, the necessary shields will be quite massive.
Note: Work supported by the U. S. Department of Energy under contract DE-AC02-76CHO3000.
(Presentation WAM-B.4: 10:45 to 11:15 AM)
by S. H. Rokni, J. D. Liu, K. R. Kase, S. Roesler, S. Taniguchi, and S. Nakamura
Energy and time-of-flight spectra of high-energy neutrons (5 MeV to 800 MeV) were measured behind the lateral shield of the electron beam dump at the Final Focus Test Beam (FFTB) Facility at the Stanford Linear Accelerator Center (SLAC). Neutrons were generated in the interaction of a 28.7-Gev, pulsed electron beam with the aluminum beam dump of the FFTB, which is housed inside thick, steel-and-concrete shielding. The measurements were performed using an NE213 organic liquid scintillator placed behind various thicknesses of concrete shield that were added to the existing shielding. The neutron energy spectra were obtained by unfolding the measured pulse height spectra with the detector response function using the FORIST code. The attenuation length of neutrons in concrete was then derived. The neutron energy and time-of-flight spectra were also calculated using the FLUKA interaction and transport code. The experimental results show good agreement with the simulated results, adding confidence to the use of FLUKA for the design of shielding for high-energy electron accelerators.
Note: This work was supported by the Department of Energy under contract DE-AC03-76SF00515.
(Presentation WAM-B.7: 11:45 AM to 12:00)
by X. S. Mao, A. A. Prinz, H. Y. Khater, and R. Seefred
The experiment E158 was a precision measurement of left-right asymmetry (ALR) in Møller scattering. A 48-GeV, polarized electron beam was scattered off unpolarized electrons in a 150-cm (0.173 radiation length) liquid hydrogen target in Stanford Linear Accelerator Center’s (SLAC’s) End Station A (ESA). The E158 spectrometer consisted of three dipoles followed by four quadrupole magnets and the Møller detector. After passing through the target, scattered electrons and photons generated in the target struck collimators, magnets, and other components in the spectrometer, detector, and the beam pipe. The total beam losses for Experiment E158 were 15 kW inside the ESA. This paper presents the radiation protection policy and the tools used to calculate the shielding requirements. Extensive measurements by active devices were performed around the ESA to check the calculations. Radiation from muons were studied. Dosimeters reported the cumulative doses during the run and were compared with active measurements.
Note: Work supported by the Department of Energy under contract DE-AC03-76SF00515.
"Radiation Protection Scheme for 48-GeV and 500-kW E158 Experiment"
(Presentation WPM-C.5: 4:00 to 4:15 PM)
by P. Demopoulos, G. Andrews, P. Kotrappa
Electret ion chambers (EICs) are commercially available, passive integrating devices. Typical uses include measurement of indoor air and water radon concentrations as well as environmental gamma radiation measurements. An electret is a Teflon disk that is electrostatically charged. The disk is mounted in an electrically conductive chamber. The charge present on the electret is measured using a portable voltmeter (reader) designed for that purpose. Ionizing radiation penetrating the chamber results in a current flow in the chamber, thereby discharging the electret. The reduction in charge, or voltage drop, is measured using the reader. The voltage drop is compared to a calibration factor specific to the chamber used. A total exposure for the measurement period is then calculated. An average exposure rate can then determine an exposure rate by dividing the total exposure by the measurement time period. The dose rate present in prototype accelerator applications may not be consistent with time. Several linear accelerator configurations were monitored for gamma radiation field levels. The MIFELA (Microwave Inverse Free Electron Linear Accelerator) is a 6-to-8-MeV accelerator with 2.5-microsecond pulse width, a 1-pulse-per-second pulse repetition frequency, and a 0.05-amp instantaneous current. The MICA (Microwave Inverse Cherenkov Accelerator) is a 6-MeV accelerator with a 2.5-microsecond pulse width, a 2-pulse-per-second pulse repetition frequency, and a 0.05-amp instantaneous current. The Magnecon is 0.5-MeV linear accelerator with a 1.5-microsecond pulse width, a 2-pulse-per-second pulse repetition frequency, and a 200-amp instantaneous current. Surveys were taken at different locations inside of the exclusion area of the accelerators and in locations in the unrestricted areas. H chambers for low exposures and L chambers for high exposures were used. The exposure data could be immediately assessed, which enabled the experiments to proceed without very much interruption. Qualitatively the electrets compared favorably with other radiation detection equipment including a portable pressurized ionization chamber used in the integrate mode, an area pressurized ion chamber, electronic CdTe integrating personnel dosimeters, and thermoluminescent dosimetry (TLD) badges. The electrets proved to be a versatile tool for gamma monitoring in the accelerator exclusion areas as well as the unrestricted areas.
(Presentation WPM-C.6: 4:15 to 4:30 PM)
by J. Shanahan, A. Arndt, C. Campbell, R. Brey, M. Rudin, and M. Patton
A research consortium has been established to assess the radiological health risks to workers associated with the operation of high-intensity proton accelerators. The primary objective of this research is to calculate internal and external dose coefficients for anthropogenic radionuclides which are not currently presented in Federal Guidance Reports No. 11, 12, and 13 or Publications 68 and 72 of the International Commission on Radiological Protection (ICRP). Information obtained from this study will be used to support the siting and licensing of future accelerator-driven nuclear initiatives within the U.S. Department of Energy complex, including the Spallation Neutron Source (SNS) and Accelerator Production of Tritium (APT) projects. Dose coefficients, the committed dose equivalent to an organ or tissue per unit intake or the committed effective dose equivalent per unit intake, allow simple determination of dose to an exposed individual when multiplied by a known intake. The consortium developed the methodology needed to perform internal and external dose coefficient calculations. Furthermore, radionuclides that could be produced from the spallation of a liquid mercury target were analyzed and categorized by half-lives. Eighty-six radionuclides with half-lives greater than 2 min and less than 10 min were identified as lacking calculated dose coefficients. Thirty of these 86 radionuclides have agreeing nuclear decay data in the Evaluated Nuclear Structure Data File (ENSDF) and NUBASE, the database for nuclear and decay properties of nuclides in their ground and isomeric states. Dose coefficients have been calculated for these radionuclides.
(Presentation WPM-C.7: 4:30 to 4:45 PM)
by G. Kharashvili, R. R. Brey, D. P. Wells, J. F. Harmon, and T. F. Gesell
129I is an isotope of iodine that is produced naturally and as a byproduct of uranium fission in nuclear reactors. The half-life of 129I is 1.57E7 y. During the decay of 129I, a 48.8-keV beta and a 39.58-keV photon with a yield of 7.51E-4 are produced. These low-energy, low-yield radiation emissions are difficult to detect when 129I is present in low activities. Because 129I is produced in fission, it is of interest as an environmental tracer to evaluate the environmental distribution of other, shorter-lived isotopes of iodine. Common methods of measuring 129I in the environment are accelerator mass spectrometry, neutron activation analysis, liquid scintillation analysis, and various insensitivity chemical methods including laser spectrometry. Previous work at Idaho State University (ISU) has established the 129I can be efficiently converted into 128I through a gamma-neutron reaction that has an 8.8-MeV threshold requirement. The half-life of 128I is 24.99 min, and it produces an easily detected 442.9-keV photon with a yield of 16%. This gamma activation technique for the detection and quantification of 129I has great enough sensitivity to perform environmental analysis of 129I under some circumstances. However, the efficacy of this analysis method is hard to quantify for all situations without first knowing the cross-section of the 129I(gamma,n)128I reaction. Our goal has been to measure the cross-section of this photonuclear reaction as a function of incident photon energy. Samples of 129I and 127I (the stable isotope of iodine) in solution were irradiated using a Bremsstrahlung spectra generated from 30-MeV, 24-MeV, and 18-MeV variable energy electron linear accelerators. Using this suite of equipment and appropriately operated bending magnets, Bremsstrahlung spectra with maximum photon energies from 6 MeV through 30 MeV were obtained. By employing knowledge of the 127I(gamma,n)126I reaction cross-section, the Bremsstrahlung spectral characteristics and other known parameters, the values of 129I(gamma,n)128I reaction cross-sections were obtained as a function of energy.