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12. FY06 SCIENCE AND TECHNOLOGY PROGRESS IN THE RADIATION PROTECTION DEPARTMENT
by Sayed Rokni
Appendix B Self-Evaluation FY2006

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The Radiation Protection Department (RPD) performs applied research in areas related to radiation safety and shielding analyses. Results of these research efforts are published in refereed journals, SLAC internal technical notes and are presented at various national and international workshops and conferences. In FY06, RPD’s research was focused on production, attenuation and interactions of radiation with matter and on the development, maintenance and benchmarking of radiation production, interaction and transport computer codes.

Measurement of neutron energy spectra; benchmarking with different Monte Carlo radiation transport codes

Measurements of high-energy neutron spectra are an important research goal for the RPD since neutrons dominate the requirements for lateral shielding at high-energy accelerator facilities. Analysis of experimental data obtained in 2004 at the CERN-EU High Energy Reference Field (CERF) facility was completed last year. In that experiment, a 120 GeV/c mixed hadron beam interacted with a copper target creating a stray radiation field which was attenuated by a lateral shield of either 80 or 160 cm thick concrete or 40 cm thick iron. High-energy neutron spectra were measured with a NE213 organic liquid scintillator located outside of the shielding. The measurement locations covered an angular range with respect to the beam axis between 13 and 110 degrees. Energy spectra in the energy range from 32 to 380 MeV were obtained by an unfolding method using a newly upgraded response matrix.

Energy spectra of neutrons were then calculated with the Monte Carlo radiation generation and transport codes FLUKA, MARS and PHITS for different measurement locations for benchmarking purposes. For many locations, the measured neutron fluence was within the results obtained from the different codes within the experimental uncertainties. Comparing the predictions of the three codes to each other, it was observed that FLUKA and PHITS predicted similar fluences, while the energy spectra calculated with MARS were slightly lower. In order to investigate differences, simulations were also performed for a simplified cylindrical geometry. Results demonstrated that the differences partially can be explained by differences in the high energy hadron production in the copper target. Results of these studies were documented and presented at two international workshops.

Proposed measurements of induced radioactivity and residual dose rates at the End Station A (ESA)

The first FLUKA benchmark measurements of induced radioactivity at electron accelerators were performed at SLAC in the year 2000 with samples of different materials irradiated laterally to a copper target in the Beam Dump East facility. Results were compared to predictions from detailed FLUKA simulations showing that some of the measured special activities were underestimated by the calculations by factors of 2.5. Possible reasons for the discrepancies were investigated but a conclusive answer could not be given due to the large uncertainties, both with regards to the measurements and to the simulations. In FY06, staff from the Radiation Protection groups at CERN and SLAC prepared and submitted a new proposal for an experiment at ESA to avoid many of these uncertainties. Additionally, by extending the benchmark to residual dose rates, the propose experiment allows for a far more thorough check of the FLUKA predictions for activation at electron accelerators. This proposal also takes into account the important increase in predictivity brought about in this field by recent improvements in the code.

Development of the FLUKA Computer Code

RPD staff contributed to a major new revision of the FLUKA Monte Carlo code which has been released in 2006 (version 2006.3). In addition to many new features and improvements, both in the user interface (input by name, use of parentheses) and especially in the physics (in particular better prediction capability concerning activation), photoproduction of muon pairs - already tested in previous versions with many approximations - is now fully implemented. The User Manual, available as Report SLAC-R-773, has been updated.