Beam loss estimation is very important for accelerator shielding design and accurate dose evaluation. The 150-MeV LINAC system of the Taiwan Photon Source (TPS) was assembled and installed in a temporary bunker for acceptance test. Gamma-ray and neutron dose rates around the area were repeatedly measured during beam commissioning. Monitoring results indicated that the beam loss pattern could change drastically over time, especially during the first few weeks of the test, which has caused radiation safety concerns in the area. To facilitate shielding analysis and beam loss diagnosis, FLUKA was used to estimate dose distributions around the area resulting from a series of ideal point beam losses. A full-scale geometry model including main components of the accelerator, structure of the bunker, and additional local shielding has been established to make the simulations as realistic as possible. The resultant dose distributions could be regarded as useful response functions in evaluating combined beam loss scenarios. Taking the beam test on September 23, 2011 as an example, this study has demonstrated that a reasonable beam loss pattern and a detailed dose distribution could be obtained through a synthetic analysis of the calculated response functions and on-site dose rate measurements.
Three high-sensitivity neutron detectors have been set up for monitoring neutrons from the commissioning of a 150-MeV LINAC: the first one is a bare cylindrical He-3 proportional counter with a high sensitivity of ~54 cps/nv to thermal neutrons, the second one uses the same neutron probe but wrapped with 6.5 cm thick polyethylene as a moderator optimized for neutrons in MeV range, the third one is an extended version of the second detector with a layer of lead embedded in the moderator aiming to increase the sensitivity of high-energy neutrons. Their response functions to both neutrons and high-energy gamma rays were calculated using MCNPX. Absolute neutron and gamma-ray spectra at the location of measurement were estimated by FLUKA simulations with a high-fidelity geometry model. Detector responses obtained by folding the spectra with response functions were analyzed and compared with measurements. This paper presents the design and characteristics of these detectors as well as a data analysis of the measured results taken during a full-power LINAC operation.
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