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Description
Introduction
NanoTerasu is a fourth-generation synchrotron light source consisting of a 3-GeV linear accelerator and a storage ring with a circumference of 349 m. The spatial distribution of secondary neutrons generated by electron beam losses contains information on the locations and intensities of beam losses, and is therefore important for shielding design and radiation protection. In this study, the neutron dose distribution in the storage-ring tunnel was measured to identify beam-loss locations. Based on the measured distribution, the beam lifetime was evaluated, and the relationship between neutron doses inside and outside the tunnel was investigated.
Methods
To measure the neutron dose distribution around the entire 349-m circumference of the storage ring, approximately 100 passive dosimeters (Nagase Landauer Luminescence Badges) were installed on the inner wall of the storage-ring tunnel at intervals of approximately 3 m. The dosimeters measured neutron doses during an operating period of 290 h with an average stored beam current of approximately 190 mA. Localized beam-loss locations were identified from peaks in the measured neutron dose distribution.
Results
Localized neutron dose-rate peaks of 4.5 μSv/h and 5.9 μSv/h were observed near the injection section and in one of the storage-ring cells, respectively. Residual radioactivity was also detected at the latter location, suggesting the presence of localized beam loss. Outside the tunnel, neutron doses were below the detection limit at most measurement points. However, at locations corresponding to relatively high neutron dose rates inside the tunnel, neutron doses exceeding the detection limit were also observed outside the tunnel. These results indicate that localized beam losses contribute to the radiation field outside the tunnel after transmission through the shielding structure.
Furthermore, the beam lifetime determined by the identified localized beam loss was estimated to be approximately 64 h, while that associated with Touschek scattering and residual-gas scattering was approximately 13 h. Combining these loss mechanisms yielded an overall beam lifetime of approximately 11 h, which was in good agreement with the beam lifetime determined from the measured decay of the stored beam current.
Conclusions
Neutron dose mapping enabled the identification of localized beam-loss locations and their relative intensities in the NanoTerasu storage ring. Increased neutron doses outside the tunnel were observed at positions where relatively high dose rates were observed inside the tunnel, demonstrating a correlation between localized beam losses and the external radiation field. These results are useful for identifying beam-loss hotspots and for assessing shielding measures. In addition, the beam lifetime evaluated from the neutron dose distribution was consistent with that derived from operational data, demonstrating that this method is effective for both beam-loss evaluation and radiation-field assessment.
| Topics | T4. Shielding and dosimetry |
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