Worker Dose from Nitrogen-13 for the DARHT Second Axis

During the development of the hazard analysis for the Dual Axis Radiographic Hydrodynamic Test (DARHT) Facility and the writing of a Safety Assessment Document (SAD), a concern was raised regarding radiation dose to facility workers from photonuclear activation of the Accelerator Hall atmosphere. The primary radiation hazard is from N-13 that is produced by (?,n) reactions in N-14. with the x-rays produced from the 20-MeV electron beam impinging on the beam stop material (graphite and tungsten) or on stainless steel inadvertently left in the beam pipe. The N-13 (9.96-min half-life) decays by positron emission that results in annihilation photons posing an external dose hazard to entering personnel. A first estimate using handbook techniques for continuous beam accelerators resulted an upper bound of 13 mrem per beam pulse. Personnel entering the Accelerator Hall immediately after each pulse for up to 100 pulses per day could receive 1 rem per day. To provide a more realistic estimate of dose to personnel, N-13 production in all interior air of the Accelerator Hall was calculated with the Los Alamos MCNP code incorporating N-14 (?,n) cross sections. All the N-13 activity was artificially collapsed to a smaller volume of air near the beam stop (10more » m by 7 m by 7 m), and the N-13 activity concentration in this reduced volume was used to calculate external dose using a Derived Air Concentration value for N-13, considering accelerator pulsing rates, and personnel entry times immediately after cessation of pulsing. The resulting dose per pulse for the beam stop is 0.00021 mrem and 0.42 mrem for the stainless steel accident case. For immediate entry after 100 pulses spaced 1 minute apart, the doses are 0.0031 mrem and 6.3 mrem, respectively.« less