The double focusing iron-core electron-spectrometer “BILL” for high resolution (n, e −) measurements at the high flux reactor in Grenoble

Abstract

A new high resolution iron-core electron spectrometer (BILL) has been constructed at the High Flux Reactor in Grenoble. It is currently being used for measurements of conversion electrons after thermal neutron capture. An internal target (maximal size 40 cm 2) is irradiated in a thermal neutron flux of 3 × 10 14 n cm −2 sec −1. The electrons pass through a vertical beam tube, 14 m long and 10 cm in diameter, defining the solid angle of 3.4 × 10 −6 / 4π. The spectrometer itself consists of two independent flat electromagnets. Both magnets act as double focusing spectrometers, where the double focusing is achieved by the combination of homogeneous and 1 r fields. The first, a 58° deflecting magnet, forms a 10:1 scaled down intermediate image of the large, 14 m distant target in the reactor hall. The dispersion of this magnet is 94 cm. The second magnet views this reduced image with a very high dispersion of 540 cm. The radius of the optic circle of both magnets is 50 cm. The energy range of the spectrometer is 15 keV < E < 10 000 keV. Multi-wire proportional counters are used as detectors. The best measured momentum resolution has been 8 × 10 −5 at 300 keV with a target of 2 × 100 mm 2. Routine measurements have been carried out with targets of 30 × 100 mm 2 achieving a resolution of Δp/ p = 4 × 10 −4. The momentum precision of the instrument is 1 part in 10 5 and the sensitivity for neutron capture conversion electrons 300 μb at 400 keV.