Comparison of long counter measurements of monoenergetic neutron fluences

Project Description

Monoenergetic neutron fluence measurements are usually performed using one of two approaches. One involves the use of the n-p scattering reaction in devices such as proton recoil counters or telescopes. Even when automated these measurements tend to be long and complicated. Another approach is to use a moderator type of device such as a long counter (LC). In contract to n-p scattering based devices, the efficiency of LCs cannot be derived from first principles, and the instrument has to be calibrated. This can be done using radionuclide sources together with neutron transport calculations. The advantages of LCs are their high efficiency, which makes measurements quick, the simplicity of the data acquisition, and the fact they can be used for low energy neutrons below the range of n-p scattering based devices, i.e. below ∼50 keV. They also provide a viable alternative to n-p scattering based devices so that international neutron fluence comparisons are not all based on the same type of device. Experience of using long counters is, however, limited.

To date no comparison of LC neutron fluence measurements has been performed, and IRSN, PTB, and NPL propose to hold one. Not all LCs are identical. The three labs mentioned have between them 4 LCs, two near identical commercial instruments, and two home-made instruments. The intention is to compare their performance in measuring monoenergetic fluences in one or more laboratory. Instruments will be transported to a host laboratory so that they all measure the same field.

The outcomes are expected to be improved understanding of the measurements and better knowledge of the efficiencies and effective centres. This in turn will lead to reduced uncertainties and greater confidence in long counter results.

Preliminary plans have been drawn up for measurements to be made at NPL in the late autumn of 2007 or spring of 2008 using initially some or all of the available ISO recommended monoenergetic series of energies (144, 250, 565, 1200, 2500 and 5000 keV) and possibly lower energies, down to 8 keV, produced by the Sc(p,n) reaction.

Final Report 2016-06-22

Results published in Radiation Measurements

Ionising Radiation (IR)
Neil Roberts, NPL (United Kingdom)
Phone: +44 20 8943 6869
Coordinating Institute
NPL (United Kingdom)