Characteristics of Prompt Radiation Field and Shielding Design for Taiwan Photon Source

Determining and Comparing Neutron Spectra at Taiwan Photon Source before and after the Installation of Local Injection Shielding

Accurate neutron spectrum measurements at light source facilities are difficult to perform because of relatively low and time-varying neutron intensities. A homemade Bonner cylinder spectrometer was used to determine the energy spectra of neutrons outside the lateral shielding wall of the Taiwan Photon Source before and after the installation of local injection shielding. The spectrometer, similar to the design of conventional Bonner spheres, features (1) highly sensitive neutron detection and (2) a wide-range response to neutrons with energies up to the GeV range. Neutron measurements were conducted by intentionally parking the injected 3-GeV electrons at the septum of the storage ring. On the basis of high-fidelity FLUKA simulations, neutron spectra at the measurement location under the experimental conditions were obtained and adopted as an initial guess for spectrum unfolding. The neutron spectra determined before and after the local shielding were comprehensively compared in terms of their intensities and characteristics. The local shielding resulted in overall reductions of approximately 44% and 38% in total neutron flux and dose rate, respectively. Both before and after the local shielding, high-energy neutrons (>10 MeV) were the dominant component of the radiation field, which contributed approximately 30% to 35% of the total neutron flux and 55% to 59% of the total neutron dose rate.

Spectral correction factors for conventional neutron dosemeters used in high-energy neutron environments

High-energy neutrons (>10 MeV) contribute substantially to the dose fraction but result in only a small or negligible response in most conventional moderated-type neutron detectors. Neutron dosemeters used for radiation protection purpose are commonly calibrated with (252)Cf neutron sources and are used in various workplace. A workplace-specific correction factor is suggested. In this study, the effect of the neutron spectrum on the accuracy of dose measurements was investigated. A set of neutron spectra representing various neutron environments was selected to study the dose responses of a series of Bonner spheres, including standard and extended-range spheres. By comparing (252)Cf-calibrated dose responses with reference values based on fluence-to-dose conversion coefficients, this paper presents recommendations for neutron field characterisation and appropriate correction factors for responses of conventional neutron dosemeters used in environments with high-energy neutrons. The correction depends on the estimated percentage of high-energy neutrons in the spectrum or the ratio between the measured responses of two Bonner spheres (the 4P6_8 extended-range sphere versus the 6″ standard sphere).

Predicting induced radioactivity for the accelerator operations at the Taiwan Photon Source

This study investigates the characteristics of induced radioactivity due to the operations of a 3-GeV electron accelerator at the Taiwan Photon Source (TPS). According to the beam loss analysis, the authors set two representative irradiation conditions for the activation analysis. The FLUKA Monte Carlo code has been used to predict the isotope inventories, residual activities, and remanent dose rates as a function of time. The calculation model itself is simple but conservative for the evaluation of induced radioactivity in a light source facility. This study highlights the importance of beam loss scenarios and demonstrates the great advantage of using FLUKA in comparing the predicted radioactivity with corresponding regulatory limits. The calculated results lead to the conclusion that, due to fairly low electron consumption, the radioactivity induced in the accelerator components and surrounding concrete walls of the TPS is rather moderate and manageable, while the possible activation of air and cooling water in the tunnel and their environmental releases are negligible.

Investigation of radiation streaming and maze design for the Taiwan Photon Source

This study investigates the radiation streaming through the personnel access maze designed for the Taiwan Photon Source, with special interest in the characteristics of radiation fields along the labyrinth and the comparison of different estimation methods. The effect of maze orientation with respect to the beam direction has also been examined in detail. The FLUKA Monte Carlo code was used to simulate the radiation production and transport for a beam loss occurring near a typical three-legged maze. In addition, we have also tested three factorized approximation formulae for the neutron and gamma-ray attenuation in our maze design. It was found that Cossairt's expression fits better and can predict satisfactory results for the neutron attenuation along the maze. We accordingly proposed a set of fitting parameters used with Cossairt's formulae for estimating the gamma-ray attenuation in labyrinths for high-energy electron accelerators. The information presented here will be valuable for our further design revisions and may be useful to those performing similar studies.