McMaster University Neutron Crystal Spectrometers

Three crystal spectrometers of unusual and differing designs have been constructed over the past five years. The spectrometers feature new design principles, which at once greatly decrease cost and improve instrument flexibility. In particular, the angular ranges of most variables extend upwards of 145°. Extensive use is made of commercial machinists' rotary tables; these are available with accuracy of a few hundredths of a degree and a backlash of a tenth, at modest cost. Analysing spectrometers employ two side-irradiated helium three counters mounted on arms ∼ 15 cm in length which move within cylindrical shielding. One counter records signal, the other background. Crystals are copper slabs, normally used in reflection from (200) or (220) planes. The mosaic spreads were increased by mechanical deformation. Extensive studies of different methods of deformation have been made; the best appears to involve repeated bending and straightening of the crystal round a vertical radius (∼ 90 cm) at room temperature. Improvements in reflectivity by a factor of four, and integrated reflectivities of ∼ 10 minutes of arc have been obtained. The theoretical limit has not been reached, however. At the McMaster University reactor two instruments use the same original beam, one of them an unconventional multiple-axis spectrometer which offers the possibility of use of multiple analysers. Borated water in iron tanks (protected by magnesium ingots) is used for shielding. An instrument at the NRU reactor at Chalk River employs a dual monochromator deep within the experimental hole. After double reflection from {220} planes of two parallel crystals, the now monoenergetic beam emerges along its original direction. The specimen is at a position fixed in space, thus making spectrometer construction simple. Because of the double reflection the beam should be more than usually monochromatic and the background low. (Double monochromators using two polarizing crystals (e.g. Fe) may provide the long-sought improved (over cobalt) monochromator for polarized beam work.) Both spectrometers are fully automatic with all the usual capabilities. The control units employ IBM (0-24) card punches as card readers. Extremely simple logic permits complete non-linear control of six independent variables, in one or two analysing spectrometers. (author)