Spectrometer crystal, neutron

Figure 1. Schematic of a triple-axis inelastic neutron spectrometer. The thermal-ized beam from the reactor is monochromated by Bragg reflection from crystal M. Neutrons that scatter from the sample S are energy analyzed by Bragg reflection from crystal A and enter detector D.

Neutron capture as well as charged particle reactions produce in general very dense y-ray spectra. The high resolution electron [MAM78] and curved crystal spectrometers [K0C80] at ILL in Grenoble present excellent... 

G. Caglioti, A. Paoletti, and F.P. Ricci, Choice of collimators for a crystal spectrometer for neutron diffraction, Nucl. Instrum. Methods 3, 223 (1958). 

This chapter discusses in detail all the neutron detection methods mentioned above, as well as the Bragg crystal spectrometer, the time-of-flight method, compensated ion chambers, and self-powered neutron detectors (SPND). Other specialized neutron detectors, such as fission track recorders and thermoluminescent dosimeters, are described in Chap. 16. 

NEUTRON ENERGY MEASUREMENT WITH A CRYSTAL SPECTROMETER... 

The measurement of neutron energy with a crystal spectrometer is based on the same principle of Bragg diffraction as is the measurement of X-ray energy (see Sec. 12.10). 

Monoenergetic neutron sources at the energy range provided by crystal spectrometers are necessary for the study of low-energy neutron cross sections with resonances. Consider, as an example, the total neutron cross section of iridium shown in Fig. 14.20. To be able to measure the resonances of this cross section, one needs neutron energy resolution less than 0.1 eV, resolution that can be achieved only with crystal spectrometers or time-of-flight measurements (see Sec. 14.8). 

Most of the discussion presented in Sec. 12.10 for X-ray crystal spectrometers is also valid for neutron spectrometers (i.e., rocking curve, alignment, higher order reflections, types of spectrometers). There are some differences, however, which are discussed next. 

The resolving power of a neutron crystal spectrometer is given (based on Eqs. 14.49 and 14.50) by... 

Figure 14 1 Resolving power and energy resolution of a neutron crystal spectrometer (n = 1, d = 0.2 nm, A0 = 0.3°).

The change of resolution with neutron energy is the same for TOF and crystal spectrometers. In both systems, the energy spread AE changes, essen-... 

The analysis of the complex cascade transitions in the [p y) reaction by scintillation spectrometry is simplified by the use of a three-crystal spectrometer (Sect. 14) as in the work of Hird et al.. These authors have also established one particular cascade by coincidence counting. The energy of the main ground state transition has been determined by Carver and Wilkinson by pulse height analysis of the photoprotons from deuterium in a high pressure ionisation chamber. The (pn) reaction with has a high threshold and the neutron resonances lie at a much greater excitation in N than the levels just discussed they have been observed by Bair et al. 

In the heavy elements, such radiation widths as have been measured vary little from one nucleus to another they lie in a narrow range, from roughly 25 to 200 millivolts. They are difficult to measure at present very little accurate data is available. Such small widths can be measured directly only in heavy elements where the radiation width constitutes the greater part of the total width and then only for resonances of very low energies and with neutron spectrometers of high resolution (e.g. crystal spectrometers). Even then the energy variation of the total cross section through the resonance will depart from the... 

Kristallspektrometer, spectrometer for slow neutrons, crystal spectrometer 375, 378. 

The problem of determining T thus becomes one of measuring Vg. This velocity could be obtained by measuring the relative number of neutrons as a function of velocity with a crystal spectrometer or chopper. The maximum of the distribution thus measured would give Vg and therefore T from Eq. (4). 

In most cases, the resonance integrals have been found by direct measurement using a material with a well-known o(E) as a standard detector. The values of the resonance activation integrals will be taken from Reference 3, where gold was taken as the standard. The major resonance at 5 eV was measured with a crystal spectrometer, and the small contribution from the next six resonance were taken from neutron time-of-flight spectrometer studies. The cadmium ratio of gold was compared with other resonance detectors, and from this comparison the relative ratio of the resonance integral was determined ... 

In the first part of this experiment the diffraction of neutrons by a single crystal is demonstrated and neutron wave properties experimentally verified. To do this, a crystal spectrometer is aligned in a beam of neutrons and a rocking curve is measured. The resolution of the spectrometer is calculated. The energy spectrum of neutrons coming out of the reactor beam hole is then measured and compared with the calculated theoretical distribution. The approximately Maxwellian shape of the beam spectrum is thus shown. The use of the experimental spectrum plot as a direct method for obtaining the effective neutron temperature is demonstrated. 

In order to determine the dynamics of atoms we have to carry out an inelastic neutron scattering measurement. With a reactor source this can be done with a triple-axis spectrometer, which has an analyzer crystal. Tripleaxis refers to the three axes for the monochromator, sample, and analyzer, all moving independently and controlled by a computer. With a pulsed source we use a mechanical chopper, which is a rotating cylinder with a hole perpendicular to the rotating axis that allows neutrons with a chosen range of velocity to go through. The neutrons scattered by the sample are detected... 

Some of the alternative TOF instrument designs involve replacing the beryllium filter with either a crystal or a mechanical chopper to monochromate the incident beam. With this change, the spectrometer can be used with a higher incident neutron energy (typically E 50 meV) so that a smaller momentum transfer Q is possible for 5 the same energy transfer (21,22). With a monochromatic incident beam, a beryllium filter is sometimes substituted for the chopper after the sample in order to increase the scattered intensity but with a sacrifice in the,minimum Q attainable. Energy transfers up to 100 meV (800 cm" ) can be achieved with TOF spectrometers at steady state reactors before the incident neutron flux is limited by the thermal spectrum of the reactor. (With hot moderators such as at the Institut Laue-... 

Elastic neutron scattering experiments performed on single crystals with the MARI spectrometer at the ISIS pulsed neutron source (Rutherford Appleton Laboratory, Chilton UK)3 have evidenced quantum interference in accordance with Eq. (20) [Ikeda 1999], For example, the best fit to the cut of S (Qx, Qy, 0) along Qy is presented in Fig. 16. 

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