Filtered reactor beams for fast neutron capture γ-ray experiments

Abstract

A flexible filtered neutron beam facility has been installed at the High Flux Beam Reactor at Brookhaven. In this facility, filters, of which the primary components are iron and scandium, are used to provide external beams of 2 and 24 keV neutrons, respectively, with low γ-ray and thermal-neutron contamination. The installation of these filters in a rotary collimator arrangement external to the biological shielding of the reactor allows for rapid interchange between filters and also for changes in the individual filter configurations with the reactor operating at full power. The collimator also includes a bismuth single crystal filter for producing a thermal neutron beam. Measurements with hydrogen-filled proton recoil detectors have been use to optimize the filter configurations and to deduce the neutron energy distributions in the filtered beams. These measurements have established the critical importance of the Al in the Fe/Al filter in obtaining the highest beam purity with minimum attenuation of the 24 keV neutron group. The average neutron flux over a beam area of ∼7.27 cm 2 at the collimator exit is 1.28 × 10 6 neutrons/cm 2·s for a filter consisting of 22.86 cm Fe, 36.20 cm Al and 6.35 S. With this filter arrangement, 98.1% of the transmitted neutrons are in the 24 keV group, and the energy distribution in the group is calculated to have a fwhm ≈ 2.0 keV. For the 2 keV filter we studied the effect on beam intensity and beam purity of filter components of Mg, Al, S, Ti, V, Cr, Fe, Co, and Ni added to the basic Sc filter. The results of this study establish that a filter composed of Sc, Ti, and Co has good beam purity with minimum attenuation of the 2 keV neutron group. The average 2 keV neutron flux over a beam area of 7.27 cm 2 is 6.54 × 10 6 neutrons/cm 2·s for a filter consisting of 188.5 g/cm 2 Sc. The fwhm of the 2 keV neutron group is observed to be ∼0.9 keV. With this filter arrangement ∼70% of the transmitted neutrons are in the 2 keV group, while for a filter consisting of 188.5 g/cm 2 Sc, 8.497 g/cm 2 Ti and 26.92 g/cm 2 Co we estimate that ∼91% of the transmitted neutrons are in the 2 keV group. The average 2 keV neutron flux with this latter filter arrangement is estimated as ∼2.0 × 10 6 neutrons/cm 2·s. Features of these filtered neutron beams which make them useful for (n, γ) spectroscopy are discussed.