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Experiments on small-angle scattering of neutrons on liquid helium at temperatures of 1.0–5 K at the ISIS pulsed neutron source (England) are reported. Detailed measurements of the angular distribution of the scattered neutrons reveal a significant change in the temperature dependence of the second moment of the pairwise correlation function (the first derivative of the angular distribution for small scattering angles). At high temperatures the angular distribution of the scattered neutrons follows the classical description for small-angle scattering, but at temperatures below the λ-point, quantum mechanical behavior is observed (scattering of neutrons on quantum fluctuations). It is confirmed experimentally that over the entire temperature range the neutron scattering cross section at zero angle is determined by classical thermodynamic fluctuations in the density.

We have measured the inelastic neutron scattering spectrum of solid parahydrogen (at low pressure and T=13.3 K) using the thermal original spectrometer with cylindrical analyzers spectrometer at the ISIS pulsed neutron source (UK). From the experimental spectrum we have obtained the parahydrogen density of phonon states which has been compared with the estimates available in the literature. The present determination improves substantially the previous experimental scenario from the point of view of both statistics and accuracy. The comparison with the most recent estimate obtained from a quantum mechanical simulation of the molecular dynamics calls for an improvement of the computational methods..

We describe a method, based on the neutron spin-echo technique, to achieve good resolution in many neutron-scattering experiments, without sacrificing signal intensity. By “good resolution” we do not mean the ∼10−6 energy resolution usually associated with neutron spin-echo spectrometers, but rather the level of resolution traditionally obtained by collimation or monochromatization of neutron beams, often on specialized instruments, and almost always at some penalty in measured intensity. The method we discuss allows good resolution to be achieved in any chosen direction in the vector space defined by the neutron scattering wave vector, Q, and the energy transfer, E. Although the method has general applicability to many neutron scattering measurements, we discuss in detail its application to the problem of separating diffuse scattering from specular reflection in neutron reflectometry. The technological basis of the method is the availability of thin films of magnetic or easily magnetizable material that ...

The response of CR-39 track detectors to neutrons has been characterized and used to measure neutron yields from implosions of DD- and DT-filled targets at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], and the scaling of neutron fluence with R (the target-to-detector distance) has been used to characterize the fluence of backscattered neutrons in the target chamber. A Monte-Carlo code was developed to predict the CR-39 efficiency for detecting DD neutrons, and it agrees well with the measurements. Neutron detection efficiencies of (1.1±0.2)×10−4 and (6.0±0.7)×10−5 for the DD and DT cases, respectively, were determined for standard CR-39 etch conditions. In OMEGA experiments with both DD and DT targets, the neutron fluence was observed to decrease as R−2 up to about 45 cm; at larger distances, a significant backscattered neutron component was seen. The measured backscattered component appears to be spatially uniform, and agrees with predictions of a neutron-transport code. A...

A gas-pressure chamber has been designed, constructed, and tested at a moderate pressure (3.4 MPa, 500 psi) and has the capability of mechanical loading of steel specimens for neutron scattering measurements. The chamber will allow a variety of in situ neutron scattering measurements: in particular, diffraction, quasielastic scattering, inelastic scattering, and imaging. The chamber is compatible with load frames available at the user facilities at the NIST Center for Neutron Research and Oak Ridge National Laboratory Spallation Neutron Source. A demonstration of neutron Bragg edge imaging using the chamber is presented.

The authors have developed a 2.45 MeV neutron double crystal time-of-flight (DC–TOF) spectrometer for deuterium plasmas in JT-60U. The DC–TOF neutron spectrometer consists of two fast plastic scintillators (BC-408 made by Bicron, 50 cm2 and 1800 cm2, thickness 2 cm), located on constant TOF spheres. The constant TOF spheres have a radius of 1 m which gives a neutron flight length of 1.64 m and a TOF of 92 ns for 2.45 MeV neutrons. The calculated spectrometer efficiency and resolution are 2.8×10−2 cm2 and 100 keV, respectively. The energy resolution corresponds to a time resolution of 2.0 ns. The spectrometer has been installed in the basement of JT-60U, 10 m away from the plasma center with vertical line-of-sight. Neutron energy spectra have been obtained when the neutron yield from the plasma is ∼1015 neutrons/s.

Neutron time-of-flight (nToF) signals from current-mode neutron detectors are often used to determine burn-averaged ion temperature from ICF targets because the spread of the neutron energy distribution is a function of the temperature of the reacting ions. The measured signal will, however, be a convolution of the actual neutron signal, the detector response, and the response of the recording system. In addition, scattered neutrons will arrive at the detector later than unscattered neutrons, further broadening the signal. The ion temperature derived from nToF data depends strongly on the functions used to fit the data or the methods utilized to unfold the neutron energy spectrum. A functional form based on known and measured properties of the detectors is used to fit the integral of the time-of-flight signal, allowing ion temperature derivations from targets with lower neutron yield than previously possible.

We designed the radial neutron spectrometer using a new type DT neutron spectrometer base on a recoil proton counter-telescope technique aiming ion temperature measurement for the International Thermonuclear Experimental Reactor (ITER). The neutron spectrometer will be installed on the well collimated neutron beam line. A large area recoil proton emitter is placed parallel to the incident neutron beam and microchannel collimating plates are inserted between the radiator and the recoil proton detectors away from the neutron beam in order to limit the scattering angle of protons to the proton detectors. Here a very thin polyethylene film and a silicon surface barrier detector are employed as the radiator and proton detector, respectively. The energy resolution and detection efficiency are estimated to be 2.5% and 1×10−5 counts neutron−1 m2, respectively, for the DT neutron through Monte Carlo calculations. Five units of the spectrometers will be installed just outside the biological shield, will include a f...

At the National Ignition Facility, the diagnostic instrument manipulator-based neutron activation spectrometer is used as a diagnostic of implosion performance for inertial confinement fusion experiments. Additionally, it serves as a platform for independent neutronic experiments and may be connected to fast recording systems for neutron effect tests on active electronics. As an implosion diagnostic, the neutron activation spectrometers are used to quantify fluence of primary DT neutrons, downscattered neutrons, and neutrons above the primary DT neutron energy created by reactions of upscattered D and T in flight. At a primary neutron yield of 1015 and a downscattered fraction of neutrons in the 10-12 MeV energy range of 0.04, the downscattered neutron fraction can be measured to a relative uncertainty of 8%. Significant asymmetries in downscattered neutrons have been observed. Spectrometers have been designed and fielded to measure the tritium-tritium and deuterium-tritium neutron outputs simultaneously in experiments using DT/TT fusion ratio as a direct measure of mix of ablator into the gas.

A new radial neutron camera system has been developed and operated recently in the HL-2A tokamak to measure the spatial and time resolved 2.5 MeV D-D fusion neutron, enhancing the understanding of the energetic-ion physics. The camera mainly consists of a multichannel collimator, liquid-scintillation detectors, shielding systems, and a data acquisition system. Measurements of the D-D fusion neutrons using the camera have been successfully performed during the 2015 HL-2A experiment campaign. The measurements show that the distribution of the fusion neutrons in the HL-2A plasma has a peaked profile, suggesting that the neutral beam injection beam ions in the plasma have a peaked distribution. It also suggests that the neutrons are primarily produced from beam-target reactions in the plasma core region. The measurement results from the neutron camera are well consistent with the results of both a standard (235)U fission chamber and NUBEAM neutron calculations. In this paper, the new radial neutron camera system on HL-2A and the first experimental results are described.

Neutron scattering with polarization analysis is a powerful tool to determine magnetic structures and excitations. A common setup is to mount the sample at the center of a Helmholtz-type coil which can provide a magnetic field of any direction at the sample position and also a guide field along the neutron flight paths around the sample. We have extended the standard setup to a combination of a large vacuum tank surrounded by the electrical coils. The vacuum tank eliminates the air scattering, and we can use the polarization analysis down to the lowest accessible momentum transfers. The coils themselves also show some appealing features: In contrast to the classic (symmetric) coil distribution, we use an asymmetric setup which gives the advantage of a larger scattering window. Due to a more sophisticated current distribution, this modified Helmholtz coil does not need to be rotated for different scattering conditions. The whole setup is currently being constructed at the IN12 instrument, a three-axis spectrometer for cold neutrons, operated by FZ Julich in collaboration with CEA Grenoble as a CRG-B instrument at the Institut Laue Langevin in Grenoble. (C) 2003 American Institute of Physics.

A neutron detector capable of measuring the time‐dependent yield of 14‐MeV neutrons from a D–D plasma producing predominantly 2.5‐MeV neutrons has been developed. The detector consists of a thick polythene recoil proton radiator backed by a graphite foil attached to a large area totally depleted ion‐implanted diode. Protons scattered in the forward direction by 14‐MeV neutrons pass through the graphite foil and are registered in the diode. Recoil protons from 2.5‐MeV neutrons, however, are prevented from reaching the diode by the foil. When operated with a 1.5‐MeV energy bias, the measured neutron detection efficiency for 15‐MeV neutrons is 3.2×10−3 per neutron. The corresponding figure for 3.1‐MeV neutrons is a factor of 540 lower. The neutron detector and its laboratory calibration are described, as is its deployment at the Joint European Torus where it serves as a triton burn‐up monitor.

A novel experimental technique to observe the Bragg scattering of evanescent neutron waves is presented. The so‐called EVA diffractometer, installed at the high‐flux reactor of the Institut Laue‐Langevin, allows the identification of neutron scattering from single crystal surfaces. We discuss the experimental setup, intensity, and resolution considerations and some first Bragg scattering signals from evanescent neutron waves excited at CaF2, InP, and MnF2 single crystal surfaces. The experimental results are discussed within the framework of the so‐called distorted wave Born approximation and within a dynamical scattering theory. The limitations of both theoretical approaches to describe the Bragg scattering of evanescent neutron waves are indicated.

Absolutely calibrated measurements of the neutron yield are important for the evaluation of plasma performance such as the fusion gain Q in D–D operating tokamaks. The time‐resolved neutron yield is measured with 235U and 238U fission chambers and 3He proportional counters in the JT‐60U tokamak. The in situ calibration was performed by moving the 252Cf neutron source toroidally through the JT‐60 vacuum vessel. Detection efficiencies of three 235U and two 3He detectors were measured for 92 locations of the neutron point source in toroidal scans at two different major radii. The total detection efficiency for the torus neutron source was obtained by averaging the point efficiencies over the whole toroidal angle. The uncertainty of the resulting detection efficiency for the plasma neutrons is estimated to be ±11%.

The TFTR multichannel neutron collimator provides spatially localized information on nuclear reaction rates in fusion plasmas. Absolutely calibrated NE 451 ZnS scintillators detect chord‐integrated neutron emission along ten vertical sightlines, which are defined by a massive shielding structure in the facility basement. If such observations are to be of value, virgin neutrons along each sightline must be distinguishable from a background of gamma rays and scattered neutrons, and the measurements must be sufficiently precise, over a wide range of plasma conditions, to allow meaningful comparison with results of transport analysis computer codes. As a useful check, the volume‐integrated neutron emission must agree with independent measurements of the global neutron source strength. Although NE 451 detectors are insensitive to gamma rays and to neutrons with energy less than ∼2 MeV, the scintillation pulse height distribution depends on both the incident neutron energy spectrum and the physical parameters o...

A method is described to measure deformation textures in polycrystals by time‐of‐flight of thermal neutrons with a 2d position sensitive detector. The procedure is illustrated for a standard sample of deformed calcite limestone and results from the Los Alamos Neutron Scattering Center and the Intense Pulsed Neutron Source at Argonne National Laboratory are compared with those from more conventional neutron diffraction experiments.

At present the high flux research reactor FRM-II is under construction in Garching, Germany. As part of the first set of instruments, a triple axis spectrometer is under construction at the cold source in the reactor hall. This instrument will be mainly used to study inelastic magnetic phenomena in condensed matter physics. Besides an unpolarized beam setup, PANDA is equipped with devices allowing full polarization analysis. This instrument largely profits from its vicinity to the reactor, and uses state of the art neutron optics. Thanks to the large focusing Heusler monochromator and analyzer the neutron flux of the polarization setup will be of the same magnitude as that of an unpolarized beam at a high flux reactor. The accessible energy and Q ranges will be extremely large due to an advantageous flux distribution of the cold source and a particular construction of the beam tube and the biological shielding which contain supermirror neutron guides. A resonant spin echo device, used to perform line shape analysis of dispersive excitations, will complete the extensive array of equipment, available on PANDA.

We present the design, description, calibration procedure, and an analysis of systematic effects for an apparatus designed to measure the rotation of the plane of polarization of a transversely polarized slow neutron beam as it passes through unpolarized matter. This device is the neutron optical equivalent of a crossed polarizer/analyzer pair familiar from light optics. This apparatus has been used to search for parity violation in the interaction of polarized slow neutrons in matter. Given the brightness of existing slow neutron sources, this apparatus is capable of measuring a neutron rotary power of dϕ/dz = 1 × 10(-7) rad/m.

We present a new instrument for spin echo small angle neutron scattering (SESANS) developed at the Low Energy Neutron Source at Indiana University. A description of the various instrument components is given along with the performance of these components. At the heart of the instrument are a series of resistive coils to encode the neutron trajectory into the neutron polarisation. These are shown to work well over a broad range of neutron wavelengths. Neutron polarisation analysis is accomplished using a continuously operating neutron spin filter polarised by Rb spin-exchange optical pumping of (3)He. We describe the performance of the analyser along with a study of the (3)He polarisation stability and its implications for SESANS measurements. Scattering from silica Stöber particles is investigated and agrees with samples run on similar instruments.

Neutrons spectra from most of known sources require being collimated for numerous applications; among them one is the Neutron Activation Analysis. High energy neutrons are collimated through a mechanical procedure as one of the most promising methods. The output energy of the neutron beam depends on the velocity of the rotating Polyethylene disks. The collimated neutrons are then measured by an innovative detection technique with high accuracy.

We constructed a compact in situ polarized 3He neutron spin filter based on spin-exchange optical pumping which is capable of continuous pumping of the 3He gas while the system is in place in the neutron beam on an instrument. The compact size and light weight of the system simplifies its utilization on various neutron instruments. The system has been successfully tested as a neutron polarizer on the triple-axis spectrometer (HB3) and the hybrid spectrometer (HYSPEC) at Oak Ridge National Laboratory. Over 70% 3He polarization was achieved and maintained during the test experiments. Over 90% neutron polarization and an average of 25% transmission for neutrons of 14.7 meV and 15 meV was also obtained.

The Spallation Neutron Source at Oak Ridge National Laboratory now hosts four direct geometry time-of-flight chopper spectrometers. These instruments cover a range of wave-vector and energy transfer space with varying degrees of neutron flux and resolution. The regions of reciprocal and energy space available to measure at these instruments are not exclusive and overlap significantly. We present a direct comparison of the capabilities of this instrumentation, conducted by data mining the instrument usage histories, and specific scanning regimes. In addition, one of the common science missions for these instruments is the study of magnetic excitations in condensed matter systems. We have measured the powder averaged spin wave spectra in one particular sample using each of these instruments, and use these data in our comparisons.

We present a versatile high voltage sample stick that fits into all cryomagnets and standard cryostats at the Swiss Spallation Neutron Source, Paul Scherrer Institut, and which provides a low effort route to neutron scattering experiments that combine electric field with low temperature and magnetic field. The stick allows for voltages up to 5 kV and can be easily adapted for different scattering geometries. We discuss the design consideration and thermal behavior of the stick, and give one example to showcase the abilities of the device. (C) 2014 AIP Publishing LLC.

Neutron yields are measured at the National Ignition Facility (NIF) by an extensive suite of neutron activation diagnostics. Neutrons interact with materials whose reaction cross sections threshold just below the fusion neutron production energy, providing an accurate measure of primary unscattered neutrons without contribution from lower-energy scattered neutrons. Indium samples are mounted on diagnostic instrument manipulators in the NIF target chamber, 25-50 cm from the source, to measure 2.45 MeV deuterium-deuterium fusion neutrons through the (115)In(n,n')(115 m) In reaction. Outside the chamber, zirconium and copper are used to measure 14 MeV deuterium-tritium fusion neutrons via (90)Zr(n,2n), (63)Cu(n,2n), and (65)Cu(n,2n) reactions. An array of 16 zirconium samples are located on port covers around the chamber to measure relative yield anisotropies, providing a global map of fuel areal density variation. Neutron yields are routinely measured with activation to an accuracy of 7% and are in excellent agreement both with each other and with neutron time-of-flight and magnetic recoil spectrometer measurements. Relative areal density anisotropies can be measured to a precision of less than 3%. These measurements reveal apparent bulk fuel velocities as high as 200 km/s in addition to large areal density variations between the pole and equator of the compressed fuel.

We report on the in situ polarized (3)He neutron polarization analyzer developed for the time-of-flight Magnetism Reflectometer at the Spallation Neutron Source at Oak Ridge National Laboratory. Using the spin exchange optical pumping method, we achieved a (3)He polarization of 76% ± 1% and maintained it for the entire three-day duration of the test experiment. Based on transmission measurements with unpolarized neutrons, we show that the average analyzing efficiency of the (3)He system is 98% for the neutron wavelength band of 2-5 Å. Using a highly polarized incident neutron beam produced by a supermirror bender polarizer, we obtained a flipping ratio of100 with a transmission of 25% for polarized neutrons, averaged over the wavelength band of 2-5 Å. After the cell was depolarized for transmission measurements, it was reproducibly polarized and this performance was maintained for three weeks. A high quality polarization analysis experiment was performed on a reference sample of Fe/Cr multilayer with strong spin-flip off-specular scattering. Using a combination of the position sensitive detector, time-of-flight method, and the excellent parameters of the (3)He cell, the polarization analysis of the two-dimensional maps of reflected, refracted, and off-specular scattered intensity above and below the horizon were obtained, simultaneously.

The PF-1000 plasma focus was modified by adding the cathode disk 3 cm in front of the anode. This modification facilitated the evaluation of neutron energy spectra. Two neutron pulses were distinguishable. As regards the first neutron pulse, it lasted 40 ns during the plasma stagnation and it demonstrated high isotropy of neutron emission. A peak neutron energy detected upstream was 2.46±0.02 MeV. The full width of neutron energy spectra of 90±20 keV enabled to calculate an ion temperature of 1.2 keV. These parameters and a neutron yield of 109 corresponded to theoretical predictions for thermonuclear neutrons.

High-flux neutrons for imaging and materials analysis applications have typically been provided by accelerator- and reactor-based neutron sources. A novel approach is to use ultraintense (>1018 W/cm2) lasers to generate picosecond, collimated neutrons from a dual target configuration. In this article, the production capabilities of the present and upcoming laser facilities are estimated while independently maximizing neutron yields and minimizing beam divergence. A Monte Carlo code calculates angular and energy distributions of neutrons generated by D–D fusion events occurring within a deuterated target for a given incident beam of D+ ions. Tailoring of the incident distribution via laser parameters and microlens focusing modifies the emerging neutrons. Projected neutron yields and distributions are compared to conventional sources, yielding comparable on-target fluxes per discharge, shorter time resolution, larger neutron energies, and greater collimation.

We report on the demonstration experiment of the multiple small angle neutron scattering (MSANS) technique at a 5.6m long neutron beam line, leading to a q resolution of 3×10−4A−1. The MSANS technique is based on two two-dimensional multihole apertures placed at the front end of the collimator and close to the sample, respectively. By choosing the proper MSANS geometry, individual diffraction patterns are superimposed leading to a large gain in intensity. Using MSANS as an option for standard small angle neutron scattering beam lines, the q resolution could be increased to 10−5A−1 without dramatically sacrificing intensity.

We describe two spin-echo instruments for neutron small angle scattering, which have been installed at the reactor institute in Delft. The first setup is using a monochromatic beam and magnetized foils as spin flippers, while the second uses resonant spin flippers in a pulsed neutron beam. The components that play an essential role for operation are described in some detail. Each setup has specific advantages in its range of spin-echo lengths that covers the range of correlation lengths that could be measured. This is demonstrated in a comparative measurement, the setup with magnetized foils measuring at spin-echo-lengths up to 20 μm and the setup with resonant flippers measuring in the range up to 0.5 μm.

The vibration spectra of some hydrogeneous compounds have been studied over the range 300–4000 cm−1 by observing incoherent neutron scattering. The paper describes the use of a crystal analyzer technique with a pulsed neutron source based on the Harwell 45‐MeV electron linac. Time‐of‐flight spectra of neutrons downscattered to a final energy, fixed by the crystal spectrometer, were measured. Data for ZrH2, H2O, NaNH2, and C6H5NH2 are reported and compared with available results from optical spectroscopy and, in the first two cases, other neutron scattering data.

Observations are reported on the effect of specimen motion upon the neutron diffraction pattern of iron. Specimen speeds of 120 m/sec combined with a neutron speed of 3310 m/sec are found to produce easily measured angular shifts, integrated intensity changes, neutron wavelength alteration, and peak width changes all of which are dependent upon the direction of motion relative to the scattering vector. The effects can be conveniently interpreted with the reciprocal lattice construction and the calculated changes with motion agree satisfactorily with the observations.

A slow neutron spectrometer that uses a rotating single crystal to chop and monochromate the reactor neutron beam is described. A high transmission mechanical chopper is operated in phase with the rotating crystal to eliminate undesired crystal reflections, higher order contamination, and to suppress the background of time uncorrelated neutrons. Pulsed neutron beams of high quality can be obtained from the thermal energy range to greater than 0.5 eV. Energy widths as low as 1% with burst widths around 7 μsec can be attained at 0.3 eV. Intensities are adequate for inelastic scattering studies. The spectrometer is designed primarily for downscattering measurements. Scattering measurements are conducted in a completely shielded scattering chamber with eleven scattered neutron detectors arranged at scattering angles of 15° to 165°. Time of flight techniques are used to determine the scattered neutron spectra. The design, construction, and performance characteristics are given. Some representative measurements...

Application of the moment method of L. V. Spencer and U. Fano is made to the penetration and slowing down of neutrons in infinite media. General considerations are restricted mainly to those modifications of the method necessary for the neutron problem; in particular, a special technique is given for reducing the integral term of the neutron transport equation. Calculations are carried out for point isotropic monoenergic neutron sources of 1, 100, 500 kev, 1, 2, 5, and 14 Mev in an infinite atmosphere of air. Best available numerical values of the cross sections are used, and the effects of absorption, anisotropic angular distributions, and inelastic scattering are included. Distances out to 10 mean free paths are considered.

The neutron decay spectrometer depends on the spectral analysis of protons born from the decay of neutrons to determine the energy distribution of the neutrons. It comprises a containment magnet for confining the decay protons, a magnetic analyzer, and a detection system. It is shown that the average speed of the decay protons is equal to the speed of the parent neutron. The spread in the speed of the protons from the decay of monoenergetic neutrons determines the resolution possible with this method of spectrometry. The calculated energy spread varies from 30 to 2.7% over the neutron energy range 10 keV≤En≤5 MeV. The instrument was used to measure the neutron spectrum of the University of Florida Training Reactor, and the 1/E distribution was obtained and compared with the calculated spectrum.

A simplified variable speed, rotating mechanical beam chopper has been developed for reactor neutron spectrum measurements, and for use in conjunction with a crystal spectrometer for inelastic neutron scattering measurements. Neutron pulses are produced with a time jitter which is small compared to the time between pulses, the rotor speed during runs of several hours being constant to at least 5%. Rotor designs for beam sizes of 1.59×4.45 cm have been successfully used for neutrons with energies up to 1.0 eV and with a transmission of about 80%.

A high intensity neutron monochromator for the subthermal energy region is described. The instrument utilizes the velocity selectivity of a rotating helix. A complete analysis is made of the instrument resolution for several practical constructions. For wavelengths longer than one angstrom, a mechanical selector may yield resolutions of 1.0 to 0.1 and fractional transmissions of 60 percent. Design and tests of a selector are described and compared with the theory.

A surprisingly simple expression in ``closed form'' for the cross section d2σ/dΩde for the scattering of thermal neutrons (including polarized neutrons) from an ideal quantum gas is derived. This result extends the work of Van Hove on the quantum gas. An expansion is obtained for dσ/de. The case of elastic scattering is treated separately. From these expressions is obtained a criterion for ignoring the statistics of the scatterer in favor of classical (Boltzmann) statistics. This criterion should have some validity for weakly interacting systems. It is shown that the effects of statistics on the neutron cross section for a helium‐4 gas range from 5% or less for the noninteracting gas up to as much as 40% for the interacting system.

Neutron scattering spectra from polycrystalline benzene have been obtained at temperatures between 77 and 270°K. These measurements are compared to recent calculations of spectra using a lattice frequency distribution based on optically determined force constants. The results at different temperatures are discussed, and qualitative agreement between calculated and measured cross sections at lower temperatures is obtained.

The neutron transformation of helium‐3 has been applied to neutron spectroscopy in the medium energy range in order to overcome the disadvantages of previous methods which have usually been based on neutron scattering. By observing the pulse‐height distribution when a proportional counter filled with helium‐3, krypton, and carbon dioxide is irradiated with neutrons, the spectrum can be obtained. Details of the counter construction and vacuum equipment for filling the counter are described. In the range of energy from thermal to 1 Mev the spread (relative standard deviation of a peak) is about 3% and the wall effect agrees with theoretical predictions. The pulse height varies linearly with energy and some evidence suggesting that the mean energy required to produce an ion pair in krypton for helium‐3 nuclei is about 6% greater than that for protons is presented. Since the neutron reaction cross section of helium‐3, as a function of energy, is required to evaluate neutron spectra from observed distributions...

In many types of experiments, time‐of‐flight neutron spectrometers have the advantage of higher counting rates than crystal spectrometers, especially in view of the fact that multiple counters may be used. A method of magnetically pulsing a neutron beam has been developed that avoids the necessity of using high‐speed rotating equipment.The magnetic intensity in a Bragg reflection depends on the orientation of the atomic magnetic moments relative to the scattering vector, and it becomes zero when the moments are directed along the scattering vector. The direction of magnetization in ferrite crystals can be switched rapidly with an external magnetic field so that these crystals can serve both as a neutron monochromator and pulser. Furthermore, the neutron pulse‐width and repetition rate can easily be varied by changing the current pulse that produces the external field.The nuclear contribution for the (111) or (331) reflections can be made very small for 7Li‐Ti ferrite of the correct composition. Large magn...

The spatial distribution of thermal neutrons originating from a fast neutron line source, with slowing down described by age theory, and thermal percolation through an infinite, isotropic medium described by diffusion theory, is presented.