Abstract
ABSTRACT In pulsed neutron sources, a neutron absorber called decoupler, which is characterized by a decoupling energy (Ed), is attached to the moderator to sharpen the neutron pulses for achieving good neutron energy resolutions. Cadmium and boron carbide (B4C) are widely used as the decoupler materials. However, it is difficult to use B4C in MW-class spallation neutron sources owing to high burn-up, which decreases Ed and increases helium gas swelling due to neutron absorption. To solve these issues concerning a B4C decoupler, we introduce the concept of pre-decoupler to reduce neutron absorption in the B4C decoupler, which is sandwiched by appropriate neutron absorption materials. Then, we study impacts of the pre-decouplers on B4C decoupler in terms of burn-up by performing simplified model calculations. It is shown that neutron absorption in B4C is reduced by 60% by using a Cd pre-decoupler without neutron intensity penalty. Moreover, helium gas swelling in B4C is restrained to be one-third of the value when not using the pre-decoupler.
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