NEUTRON STORAGE IN A LONGITUDINALLY VIBRATING SILICON CRYSTAL

The time structure and integrated diffraction profile of cold neutrons of wavelength lambda = 6.27 Angstrom transmitted through a longitudin ally vibrating silicon crystal were calculated by Monte Carlo simulati ons and measured on the backscattering spectrometer IN10 at the Instit ut Laue-Langevin. Neutrons of velocity v(N) = 630 ms(-1) require t(T) = 158.5 mu s for direct transit through the 100 mm-long silicon resona tor. This time is long compared with the vibration period T-P = 22.3 m u s and most of the neutrons experience multiple Bragg reflections in the oscillating Doppler-strain field. Monte Carlo calculations predict that neutrons will be stored in the crystal and released with a time structure determined by the vibration period and by the energy width o f the incident beam. For a continuous beam, the usual time modulation of the diffracted neutrons with twice the vibration frequency is expec ted. If a neutron pulse much shorter than the vibration period impinge s on the crystal, the transmitted signal should be a decaying sequence of pulses separated by the vibration period. For pulses which are lon g compared with the vibration period, the effect of neutron storage sh ould be manifest as a delayed staircase-like intensity variation on bo th pulse edges. A silicon crystal was set with the (111) lattice plane s in backreflection and excited in a lambda/2 resonance at 44.78 kHz. A typical deformation amplitude was u(0) = 1 mu m corresponding to a s cattering range of Delta E = +/-1.9 mu eV. The response of the lambda/ 2 resonator to a quasicontinuous beam and to neutron pulses of lengths Delta t(P) = 3 ms and 33 mu s FWHM was measured. Experiments were per formed with neutron beams of two energy widths Delta E = +/-0.35 and /-1.23 mu eV. In agreement with the Monte Carlo simulations, neutron s torage in the silicon crystal was observed. The storage time was 250 m u s for both long and short incident pulses. This time equals about 11 vibration periods of the crystal resonator and is in good agreement w ith the calculations for the Si 111 reflection and chosen vibration pa rameters. A first indication of the dependence of the time structure o n the energy width of the neutron beam was seen in the experiments. Th e predicted pulsed structure of the transmitted signal in response to neutron pulses shorter than the vibration period could not be resolved . With a shortest incident pulse width of Delta t(P) = 33 mu s, the co ndition Delta t(P) much less than T-P was not fulfilled. The measured transmission profile is in good agreement with the calculations for th e lambda/2 resonator. Compared with vibrating crystals excited into hi gher harmonics at the same applied strain, the lambda/2 resonator has a lower reflectivity.