E. Jericha

Test of a perfect crystal neutron storage device

Abstract A system of two perfect silicon crystal plates (1.07 m apart) both placed in back reflection position ((1, 1, 1)-planes) was used to capture highly monochromatic (6.27 A) neutrons. We have been able to show that a pulsed magnetic field is a proper means to fill the system with neutrons and to release the neutrons after a period of up to 30 ms. Here we present our first results obtained from measurements at the ISIS pulsed neutron source at Rutherford Appleton Laboratory.

Neutron capture cross section of unstable 63Ni: implications for stellar nucleosynthesis.

The 63Ni(n,γ) cross section has been measured for the first time at the neutron time-of-flight facility n_TOF at CERN from thermal neutron energies up to 200 keV. In total, capture kernels of 12 (new) resonances were determined. Maxwellian averaged cross sections were calculated for thermal energies from   kT=5-100  keV with uncertainties around 20%. Stellar model calculations for a 25M⊙ star show that the new data have a significant effect on the s-process production of 63Cu, 64Ni, and 64Zn in massive stars, allowing stronger constraints on the Cu yields from explosive nucleosynthesis in the subsequent supernova.

Phase-object approximation in small-angle neutron scattering experiments on silicon gratings

The phase-object approximation for neutron scattering based on a one-dimensional dynamic forward scattering theory is discussed and used to calculate the differential cross section of an object. It is shown that this approximation is valid in ultra-small-angle neutron scattering (USANS) and spin-echo small-angle neutron scattering (SESANS) experiments on silicon gratings. In the weak scattering limit, the phase-object approximation reduces to the kinematic or first Born approximation. The spatial coherence function is used to describe instrumental resolution effects. Measurements on three different instruments are in good agreement with calculation results. In the experiment with a time-of-flight SESANS instrument, the effect of Pendellosung with object size is observed.

Noncyclic Pancharatnam phase for mixed state SU(2) evolution in neutron polarimetry

Abstract We have measured the Pancharatnam relative phase for spin- 1 / 2 states. In a neutron polarimetry experiment the minima and maxima of intensity modulations, giving the Pancharatnam phase, were determined. We also considered general SU ( 2 ) evolution for mixed states. The results are in good agreement with theory.

Ultra-small-angle neutron scattering studies of artificial lattices

Ultra-small-angle neutron scattering (USANS) with the use of perfect silicon crystals provides a resolution of the order of 10-5 A-1 in reciprocal space, which corresponds to rad in scattering angles and m structures in real space. From small-angle scattering by artificial lattices follows a unique test procedure for the related devices and techniques. Corresponding measurements were performed at the USANS facilities of the Atominstitut in Vienna and of the S18 instrument at the ILL. We observed diffraction patterns from samples being periodically structured in one and two dimensions. These measurements take advantage of the extended coherence function of the set-up and the high quality of the manufactured silicon sample lattices. Due to these characteristics up to 50 interference orders were obtained at the S18 instrument. Scattering from two-dimensional periodic structures was observed for different orientations of the sample which shows characteristic diffraction maps in reciprocal space.

GEANT4 simulation of the neutron background of the C6D6 set-up for capture studies at n_TOF

The neutron sensitivity of the C6D6 detector setup used at n TOF for capture measurements has been studied by means of detailed GEANT4 simulations. A realistic software replica of the entire n TOF experimental hall, including the neutron beam line, sample, detector supports and the walls of the experimental area has been implemented in the simulations. The simulations have been analyzed in the same manner as experimental data, in particular by applying the Pulse Height Weighting Technique. The simulations have been validated against a measurement of the neutron background performed with a nat C sample, showing an excellent agreement above 1 keV. At lower energies, an additional component in the measured nat C yield has been discovered, which prevents the use of nat C data for neutron background estimates at neutron energies below a few hundred eV. The origin and time structure of the neutron background have been derived from the simulations. Examples of the neutron background for two di erent samples are demonstrating the important role of accurate simulations of the neutron background in capture cross section measurements.

Optimisation of a crystal design for a Bonse- Hart camera

Bonse-Hart double-crystal diffractometers (DCDs) with multi-bounce channel-cut crystals show rocking curves that depart dramatically from dynamical diffraction theory in their wings. The intrinsic background is many orders of magnitude higher than the predictions of dynamical diffraction theory. This effect was studied at the ultra-small-angle neutron scattering facility at the Atominstitut in Wien and at facilities in Grenoble, Julich and Villigen. The scattering intensity contains Bragg reflections from the front and the back faces, and thermal diffuse scattering from the internal volume. The aim of this study was to eliminate this contamination and develop a new crystal design which provides optimal resolution. Therefore different ways were tested. In the first step the contamination was eliminated by cutting a groove in the middle of the back plate of the channel-cut crystals and inserting a cadmium absorber in this groove. With this modification an additional suppression of the wings of the rocking curve of about one order of magnitude was achieved. After this, we developed a new design for a DCD. The concept for this new crystal design was to avoid the back reflection and the thermal diffuse scattering. The different steps on the way to produce these crystals are presented in a detailed way. The crystal preparation and the different instruments where these crystals have been tested are also described.

Neutron Decay with PERC: a Progress Report

The PERC collaboration will perform high-precision measurements of angular correlations in neutron beta decay at the beam facility MEPHISTO of the Forschungs-Neutronenquelle Heinz Maier-Leibnitz in Munich, Germany. The new beam station PERC, a clean, bright, and versatile source of neutron decay products, is designed to improve the sensitivity of neutron decay studies by one order of magnitude. The charged decay products are collected by a strong longitudinal magnetic field directly from inside a neutron guide. This combination provides the highest phase space density of decay products. A magnetic mirror serves to perform precise cuts in phase space, reducing related systematic errors. The new instrument PERC is under development by an international collaboration. The physics motivation, sensitivity, and applications of PERC as well as the status of the design and preliminary results on uncertainties in proton spectroscopy are presented in this paper.

Upon the versatility of spatial neutron magnetic spin resonance

Abstract It is demonstrated that a magnetic neutron spin resonator placed between a pair of high-quality polarizers and driven in a ‘travelling-wave’ mode of operation allows to produce neutron pulses whose spectral distribution and temporal width can be varied over a broad range just by purely electronic means.

PERFORMANCE OF AN IMPROVED PERFECT CRYSTAL NEUTRON STORAGE CAVITY

Abstract An improved version of a cold neutron storage apparatus, (VESTA), consisting of two parallel perfect silicon-crystal mirror plates in precise back reflection, has been installed and tested at the ISIS pulsed neutron source. The first measurements on VESTA and its performance compared to the prototype setup are reported. The neutron storage times were increased by one order of magnitude up to about 2 s. Characteristic features of the storage process and future perspectives are discussed.