Alpha-gamma Counting for High Accuracy Fluence Measurements

Neutron fluence is measured by counting gamma-rays from the reaction n+¹⁰B®⁴He+⁷Li + g(478KeV) with a calibrated gamma detector. The gamma detector is calibrated in a multi-step procedure that uses a precisely calibrated Pu alpha source (re-calibrated in 2006), an integrated alpha particle detector (the alpha-gamma counter was restored to operation in 2006), a neutron beam, and a thin ¹⁰B target.

In regular operation, the thin target is replaced with a thick one and the detector operates as a black detector counting the number of neutrons impinging on the target per second. The detector is currently installed on our monochromatic beamline. The apparatus is functioning and the observed relative statistical precision of the gamma detector calibration is less than 0.1%. Several systematic effects are being characterized at present including the effects of beam size, dead time, and neutron scattering. It is believed that the combined final uncertainties on these effects will be less than 0.1%. In order to use this device to calibrate a thin foil "1/v" neutron detector, such as the one used in our beam-type neutron lifetime measurement, it is necessary to know additionally the wavelength of the beam. A wavelength measuring device was installed upstream of the detector and several wavelength measurements made during the past two years exhibit a relative statistical precision at the sub 0.1% level. We are close to our goal of having a black neutron detector capable of counting a beam of neutrons with an absolute relative uncertainty of 0.1%. This is a new primary calibration method. It will be used to recalibrate the fluence monitor that was used in our beam-type neutron lifetime measurement, thereby simultaneously measuring the ⁶Li(n,t) and ¹⁰B(n,a) thermal neutron cross sections, and to recalibrate the USA national neutron standard NBS-I. The marked disagreement of new neutron lifetime experimental results with previous measurements has created serious uncertainty in the value of this important quantity at the 1% level, which is a factor of 10 larger than the relative uncertainty quoted by the Particle Data Group. In ours, the most accurate cold neutron beam determination of the neutron lifetime based on the absolute counting of decay protons, the largest uncertainty was attributed to the uncertainty of the fluence monitor efficiency. The black detector has the potential to reduce the uncertainty in the monitor efficiency by more than a factor of three. This would reduce the uncertainty on our beam-type lifetime measurement by 32% (to 0.25%).

The ⁶Li(n,t) and ¹⁰B(n, a) 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. The recalibration exercise will yield a direct absolute measurement of these cross sections at near-thermal energies. Finally, the USA national neutron standard NBS-I, 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.

Image Description: (Top) Photograph of Alpha-Gamma detector setup with the 1/v detector shown to the left. (Bottom) A typical alpha and gamma spectrum.