The absolute fluence of a monochromatic neutron beam has been measured with a totally absorbing alpha-gamma counter. The counter absorbs the entire beam in an enriched 10B4C target and infers the incident neutron rate by counting gamma-rays from the reaction n + 10B → 7Li + α + γ (478KeV) with calibrated gamma detectors. The detection efficiency for the 478 keV capture gamma is measured in a calibration process that successively transfers the well-known activity of a Pu alpha source. The alpha source is used to measure the detection efficiency of an integrated charged particle detector. A thin 10B target replaces the alpha source, and the beam is turned on. The observed rate of alpha particles is used to determine the neutron absorption rate in the target and the observed gamma rate per neutron absorbed is established. The thin target is then replaced with the totally absorbing 10B4C target to measure the absolute neutron rate.
The primary use for the alpha-gamma counter has been to calibrate a "1/v" neutron monitor based on neutron absorption in a thin 6Li target. The monitor detects alphas and tritons from the reaction n + 6Li → α + t in charged particle detectors masked by precision apertures. Prior to calibration with the alpha-gamma counter, the detection efficiency of the monitor was determined to 0.3% uncertainty from knowledge of the density of 6Li in the target (0.25% uncertainty), the 6Li thermal neutron cross section (0.15% uncertainty), and the detection solid angle (0.1% uncertainty). This was the limiting source of uncertainty in the NIST neutron lifetime measurement using the beam technique.
To calibrate the neutron monitor, it is operated with the alpha-gamma counter on a monochromatic neutron beam. The detection efficiency of the monitor is determined to a precision of 0.06 % by measuring the rate of alphas and tritons in the monitor, the neutron rate with the alpha-gamma counter, and the de Broglie wavelength of the beam with a Si crystal analyzer. The five-fold improvement in the neutron monitor calibration allows for immediate improvement of the beam lifetime result. It also presents opportunities to recalibrate the national neutron standard and to perform direct measurements of important reference cross sections.
Individual measurements of the neutron lifetime continue to improve in precision, quoting uncertainties as low as 0.8 s. Yet, the discrepancy between results using different measurement techniques calls into question the accuracy of this important quantity at the level of several seconds. This must be addressed by careful examination of systematic effects and improved measurement precision. By calibrating the neutron monitor, we have investigated and reduced the largest systematic uncertainty in the NIST beam lifetime measurement. The improved monitor calibration leads to a 33% reduction in reported lifetime uncertainty. When the improved monitor calibration is combined with improvements to what were previously next-to-leading-order systematic effects, it is reasonable to expect that a new beam measurement using the existing NIST apparatus could achieve a precision of 1 s.
The 6Li(n,t) and 10B(n, α) cross sections are important neutron cross section standards. Precise knowledge of these cross sections is essential because they are often used as reference standards for obtaining the neutron fluence in investigations of the properties of neutron-induced reactions and for accurate determinations of neutron cross sections. They are also used for fluence determinations in neutron dosimetry as well as fundamental physics experiments. Calibration of additional thin 6Li and 10B deposits will allow for a direct absolute measurement of these cross sections at near-thermal energies.
Finally, the national neutron standard NBS-1, a RaBe photoneutron neutron source, is an artifact standard that was most recently calibrated more than 40 years ago. It should be recalibrated using an updated technique. Its current relative uncertainty is 0.85% and this could be reduced using the black detector and a 252Cf transfer standard.
Photo caption: (Top) Photograph of Alpha-Gamma detector setup with the 1/v detector shown to the left. (Middle) Schematic of the neutron monitor calibration setup. (Bottom) A typical alpha and gamma spectrum from the thin 10B target.