A perfect crystal as neutron storage device

A system of two perfect silicon crystal plates (1.07 m apart) both placed in back reflection position ((1,1,1)-planes) has been constructed. This system can be used to capture highly monochromatic (6.27 A) neutrons. Thereby, the neutrons are reflected back and forth. To reduce the lateral losses, a neutron guide was placed between the crystal plates. A pulsed magnetic field (1.3 T at the site of the crystal plates) fills in and releases the neutrons. Storage times up to 30 ms have been achieved and computer simulations show that they could be extended up to more than 100 ms. During the measurement period, the losses of the neutrons have been reduced to 9% per back and forth reflection (3.4 ms) by alignment of the neutron guide. Computer simulations show that the achieved rate should be lowered significantly by further alignment. There is also a probability for the neutrons that they are captured without a magnetic pulse. These neutrons get lost after several back and forth reflections and are (partly) detected as equally spaced peaks in the time-of-flight spectrum. Due to the very low background at the IRIS facility (in between the direct neutron pulses), these neutrons showed up in the spectrum without magnetic pulsing. (Author)