Argonne 7.7-Meter Bent-Crystal Gamma-Ray Spectrometer

Abstract

A bent‐crystal gamma‐ray spectrometer of the transmission type having a focal length of 7.7 meters has been built for studying neutron capture radiation. The source is placed inside the reactor CP‐5 in a horizontal straight‐through hole. The instrument is designed to utilize a 30 cm×30 cm quartz plate cut normal to the (310) planes. Undiffracted radiation is absorbed by a baffle consisting of 108 Pb plates each 40×11×0.040 inches mounted between tapered spacers so as to focus on the virtual image of the source. The detector is an array of 16 NaI (Tl) crystals comprising a volume 10½×10½×2 in. The drive system rotates the crystal one second of arc in 20 seconds. The 2:1 motion required between the crystal and the detector is maintained by an optical lever system. The Bragg angle is measured with an interferometer. The instrument now operates with a 10×15×0.6 cm crystal.