Absolute Neutron Measurements in Neutron Decay

Zema Chowdhuri
Ionizing Radiation Division

Neutrons undergo beta-decay to produce a proton, an electron and an anti-neutrino. The decay process is part of the electroweak family of interactions, and the decay rate plays an important role in particle physics and in cosmology. Since the late 1940s, many attempts have been made to determine this rate precisely, but because of the difficulty of detecting neutrons, it has remained a challenging experimental problem.

There are two main approaches to the measurement of the lifetime. This work, based at the NIST CNRF, is concerned with the 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. Specifically, the uncertainty in the efficiency of the neutron monitor used in the experiment as obtained by conventional means is 0.4%. In order to improve on this, we are in the process of calibrating the device at the 0.1% level by comparing it 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 micro-Watt) 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 the 0.1% level for a neutron flux of 3 X 10^(5) n/s.