A cryogenic radiometer for absolute neutron rate measurement

We detail the design and operation of an electrical substitution radiometer, which measures the absolute fluence of thermal neutron beams based on neutron absorption in a Li-6-rich metallic alloy. We demonstrate that the device possesses a linear response to electrical power in the 50-500 nW range with better than 0.1% absolute accuracy and negligible zero offset. We show the response of the radiometer to a 3 meV neutron beam. We compare the noise of the radiometer to the noise observed upstream in a thin rate monitor and show that the radiometer noise is understood quantitatively. We establish that the radiometer measures neutron rates above 10(5) s(-1) to an absolute accuracy of 0.1%. (C) 2003 American Institute of Physics.