| Neutrons undergo beta-decay to produce a proton, an electron and an anti-neutrino. The decay rate plays an important role in particle physics and in cosmology, and since the late 1940s, numerous attempts have been made at the most precise measurement of this rate. The difficulty in detecting neutrons has kept this a challenging experimental problem. This work, based at the NIST reactor, is concerned with a decay rate measurement method in which simultaneous measurements are made of the decay protons and the neutrons in a well-defined volume of neutron beam. From the two, the lifetime can be determined by employing the differential form of the radioactive decay law, dN/dt = -N/tau n. The precision goal of the NIST measurement is a part in a thousand; at this time, the largest source of uncertainty is in the determination of the neutron density in the beam. In order to improve on this, we compare the device at the 0.1% level against an absolute detector with unit efficiency for neutron detection. This absolute neutron detector operates by measuring the thermal power produced by neutron capture reactions in a neutron absorber. The primary challenges to this technique are: (1) the accurate detection of very small amounts of power (less than a muWatt) produced in the particular beam used for this measurement, and (2) the demonstration that all of the kinetic energy of the reaction products appear as heat in the target. Such small energy deposits are detectable with a cryogenic radiometer operating at liquid helium temperatures and we have achieved the required precision goal for the instrument: the measurement uncertainty in the neutron rate over a period of a day is below 0.1% for a neutron rate of 3 x 10 5 s-1. The verification of the absolute accuracy of the radiometer is in progress. |
