Development of the fast neutron standard using a Be( ,n) reaction at the National Metrology Institute of Japan

Abstract

This paper describes the 8-MeV neutron field where the neutrons are generated in the (9)Be(alpha,n)(12)C reaction by bombardment of a beryllium target with a 2.4-MeV (4)He(+) beam from a Van de Graaff accelerator. The neutron field is being prepared for a new national standard on neutron fluence in Japan. Absolute measurement of the neutron fluence was taken using a proton recoil neutron detector, consisting of a silicon surface barrier detector with a polyethylene radiator. Neutron spectra were measured using a newly developed recoil proton spectrometer and a liquid organic scintillation detector. The gamma rays existing in the field were also characterised using a liquid organic scintillation detector. The ambient dose equivalents of the gamma rays were estimated to be <100 microSv at the neutron fluence of 10(7) neutrons cm(-2).

Monoenergetic fast neutron reference fields: I. Neutron production

“…Other more exotic neutron production reactions were occasionally used at metrology institutes for special applications. For example, the 9 Be(α, n) 12 C reaction was applied to produce quasi-monoenergetic 8 MeV neutrons with alpha particle beams from a small-scale electrostatic accelerator [34]. Another example is the 15 N(p, n) 15 O reaction which offers the possibility of producing monoenergetic neutrons in the energy region below 5.742 MeV using the salient features of the gas-target technique with respect to subtraction of target-related background [35].…”

Recent activities on neutron standardization in Japan

“…Thick radiator detector 8.0 MeV 9 Be( ,n) 12 C neutrons using the Van de Graaff accelerator 10,11) .…”

“…241 Am-Be standard source the transition to the ground and first excited states of their residual 12 C nuclei and the contribution of the 3.5 MeV neutrons will be subtracted in actual calibrations 10) . The neutron fluence is determined using the TR detector with a dE-E configuration, incorporating a thin transmission-type silicon detector (dE) with a 50 mm depletion depth sandwiched between a 1 mm thick polyethylene radiator disk and a silicon detector (E) with a 2 mm depletion depth.…”

Response measurements of a neutron dosimeter for epi-thermal region using a pulsed white beam

“…The responses of neutron dosimeters are usually evaluated using calibration with mono-energetic neutrons and calculations. In the National Institute of Advanced Industrial Science and Technology (AIST), it is possible to calibrate responses of neutron dosimeters or neutron detectors for thermal neutrons and monoenergetic neutrons with energies of 24 keV, 144 keV, 565 keV, 2.5 MeV, 5.0 MeV, 8.0 MeV and 14.8 MeV as national standards [1][2][3]. However, it is not easy to produce mono-energetic neutrons for the epi-thermal neutron region.…”