NEUTRON DEPTH PROFILING WITH THE NEW NIST COLD NEUTRON SOURCE

Neutron depth profiling (NDP) is a method of near-surface analysis for isotopes that undergo neutron-induced positive Q-value charged partic le reactions, e.g. (n, alpha) and (n, p). The method is based on measu ring the energy loss of the charged particles as they exit the specime n. Depending on the material under study, depths of up to 10 mu m can be profiled and depth resolutions of the order of 15 nm can be obtaine d The most studied analytes via NDP at the National Institute of Stand ards and Technology (NIST) are boron, lithium and nitrogen. These anal ytes have been determined in a variety of matrices. The NIST research reactor has recently undergone a major upgrade, including the addition of a new liquid-hydrogen cold source, which reaches a lower effective temperature and more fully illuminates the neutron guides. The cold n eutron depth profiling (CNDP) facility has been placed on a curved sup er-mirror guide. Because of the guide curvature, the NDP chamber does not directly view the reactor and its associated fast neutrons and gam ma rays, and therefore a 135 cm thick sapphire filter used previously in front of the beam port is no longer necessary. The increase in the neutron fluence rate at the end of the guide leads to an improvement i n detection limit of more than a factor of four over the previous dept h profiling instrument. The reduction in the gamma-ray background in t he chamber will improve oar ability to measure elements such as nitrog en. (C) 1997 by John Wiley & Sons, Ltd.