Speaker
Description
A 7Li(p,n) reaction and 235U system will replace the 252Cf based radioactive ion beam source at the Argonne Tandem Linac Accelerator System (ATLAS) accelerator. Re-purposing a room with no shield walls that housed a 252Cf source based RI source with a 30 kW intense n-γ producing source posed challenges from the radiation physics and operational radiation protection perspectives. FLUKA and PHITS Monte Carlo codes were used to study the radiation environments, with identical geometries and material compositions. Both codes predicted almost identical solid angle-integrated yields and directional doses from the 7Li(p,n) reaction for photons and neutrons. The number of fissions in the 235U foil predicted by PHITS was about 2 times higher than FLUKA, at 109 fissions per second. Comparison of photon spectra, due to the transport of neutrons and photons across the complex geometry, showed reasonable agreement in the shape and differential yield, but PHITS overpredicted the annihilation peak by a factor of a few and did not predict some of the peaks present in the FLUKA output, particularly from the Mo and Zr layers. The neutron spectrum and dose predicted by the codes were in good agreement. While the polyethylene/Masonite shielding showed good efficacy in reducing the neutron doses to less than 5 μSv/h, the prompt gamma dose required additional shielding. Weight considerations ruled out lead as the additional shield, and concrete was chosen to reduce the dose in the general public area. Induced activity was the highest in the target from 7Be, at 1012 Bq after one month of operation, followed by the Mo window. PHITS predicted about 2 orders of magnitude lower 7Be activity. Residual dose was estimated after shutdown without disturbing the shielding, and also from the target holder when it was removed after shutdown. For the latter scenario, the effect of a steel cask in reducing the dose was studied. Argonne’s radiation protection practices will be relied upon to keep the prompt and residual doses in other locations As Low As Reasonably Achievable (ALARA).
| Topics | T4. Shielding and dosimetry |
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