Study on low activation decoupler material for MW-class spallation neutron sources

The Japan Spallation Neutron Source (JSNS) at the Japan Proton Accelerator Research Complex (J-PARC) has started its operation on May 30, 2008. The Ag–In–Cd (AIC) alloy was adopted as a decoupler material for two decoupled moderators. A high decoupling energy at 1 eV was for the first time achieved in MW-class spallation neutron sources due to the adoption of the AIC alloy. Although the AIC decoupler is superior in the neutronic performance, it has a demerit in high residual radioactivity due to production of Ag-110 m (half life: 250 days) and Ag-108 m (half life: 418 years). To overcome this demerit, we studied on possibilities of a low activation decoupler material with high decoupling energy as the AIC alloy, that is, Au–In–Cd (AuIC) alloy. Neutronic performance of this material was investigated by using neutronics calculations. As a result, it was found that the AuIC decoupler could provide neutron pulses with almost the same characteristics as those for the AIC decoupler even when the burn-up effects were considered. Excellent low activation property of the AuIC alloy to the AIC alloy was demonstrated by residual radioactivity calculations. On viewpoint of neutronics performance, it was concluded that the AuIC decoupler was available as the substitute of the AIC decoupler.