An associated particle neutron time of flight system

Abstract

A fast 3Hen timing system having significantly improved timing resolution has been developed for the D(d, n) 3He reaction at incident deuteron energies below 400 keV. The associated particle technique is used to define a neutron beam at an angle in the laboratory between 40° and 120°, determined by the angle at which the associated 3He particles are detected. Unambiguous 3He particle detection, resulting from the use of a magnetic analyser to shield the surface barrier 3He particle detector from scattered deuterons, allows the absolute neutron flux in the associated neutron beam to be precisely determined. The derivation of a fast timing signal from the detector voltage pulse, enables a constant fraction pulse height discriminator to be triggered on 3He pulses of equivalent energy less than 500 keV without spurious triggering on noise. The 3He-n timing resolution obtained after compensation for the energy dependent 3He flight times is 2.8 ns (fwhm) in the particular case of a 2.40 MeV neutron beam defined at 102°, and 3.2 ns (fwhm) in the case of a 2.80 MeV neutron beam at 50°. The effect of instrumental timing jitter and walk in the system is virtually negligible. Stable neutron beams having intensities of over 120 neutrons sec −1 msterad −1 averaged over the beam profile are maintained for long periods. The random background count rates outside the neutron beam typically correspond to a neutron flux of less than 2 × 10 −5 that of the direct beam.