J.M. Carpenter

Wide-energy-range, high-resolution measurements of neutron pulse shapes of polyethylene moderators

We report measurements of the emission time distributions (pulse shapes) as functions of energy for neutrons emerging from two heavily irradiated, ambient-temperature polyethylene moderators in IPNS. A time-focused crystal spectrometer arrangement provided resolution such that instrumental broadening was insignificant; cooling the Ge monochromator to 10 K provided adequate reflectivity in the high energy region. Measurements covered the range 2.5 < E < 1000 meV. We introduce a novel set of functions which fit the pulse shapes over the entire range of energies with four wavelength-independent parameters.

Pulsed spallation neutron sources for slow neutron scattering

Abstract The role of pulsed spallation neutron sources for slow neutron spectroscopy is sketched. Various methods of neutron production, and some aspects of the spallation neutron production process are outlined. Accelerators for pulsed spallation neutron sources are discussed. Materials and coolants for neutron-producing targets and moderators are surveyed. The performance of moderators and the effects of varying size, composition, and temperature are summarized. Expressions for estimating moderator performance are given. The effect of moderator-reflectors, which enhance the slow neutron beam current at low energies, are examined. The ZING-P prototype pulsed neutron source and the proposed Intense Pulsed Neutron Source are described. Pulsed spallation sources are a new generation of sources for slow neutron spectroscopy which offer an order of magnitude improvements over currently available thermal neutron flux and several orders of magnitude greater epithermal flux.

The Time-of-Flight Small-Angle Neutron Diffractometer (SAD) at IPNS, Argonne National Laboratory

The design, development and performance of the time-of-flight (TOF) small-angle diffractometer (SAD) at the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory are described. Similar TOF-SANS instruments are in operation at the pulsed neutron sources at Los Alamos National Laboratory, USA, at Rutherford Appleton Laboratory, England, and at KEK, Japan. These instruments have an advantage by comparison with their steady-state counterparts in that a relatively wide range of momentum transfer (q) can be monitored in a single experiment without the need to alter the collimation or the sample-to-detector distance. This feature makes SANS experiments easy and very effective for studying systems such as those undergoing phase transitions under different conditions, samples that cannot be easily reproduced for repetitive experiments, and systems under high temperature, pressure or shear. Three standard samples are used to demonstrate that the quality of the SANS data from SAD is comparable with those from other established steady-state SANS facilities. Two examples are given to illustrate that the wide q region accessible in a single measurement at SAD is very effective for following the time-dependent phase transitions in paraffins and temperature- and pressure-dependent phase transitions in model biomembranes.

On the short range atomic structure of non-crystalline carbon

Abstract Neutron diffraction data show there isllittle tetrahedral bonding in glassy carbon, and correspond to the Stenhouse-Grout structure factor model for amorphous carbon taking the degenerate case in which the amount of tetrahedral bonding is negligible. This is in contrast to the X-ray results of Noda and Inagaki. Further analysis of the radial distribution function from the neutron diffraction data is presented. A brief summary of other X-ray diffraction measurements on a variety of amorphous carbons is given, all of which confirm the predominantly trigonal coordination. The disagreement of the earlier X-ray and electron diffraction measurements is due to the poor resolution and normalization of the data, and also perhaps the method of preparation. The amount of tetrahedral bonding present in amorphous carbon requires careful diffraction measurements at large scattering vectors to enable better resolution of the peaks in the radial distribution function.

Scattering function of vitreous silica

The scattering function S(Q, E) of vitreous silica a-SiO 2 was measured with inelastic neutron scattering over a wide range of variables (1 Q −1 , 15 E 170 meV), using a chopper spectrometer at a pulsed source. The data are analyzed in terms of the Q dependence of S(Q, E) at different energies, and in terms of the E dependence of the effective one-phonon density of states G(E) . Implications for the structure and dynamics of a-SiO 2 are discussed.

The resolution function of a pulsed-source neutron chopper spectrometer

Abstract We introduce a formulation for the evaluation of the incident neutron intensity distribution for a chopper time-of-flight spectrometer at a pulsed neutron source. This treatment, incorporated with an assumed scattering function of the sample and the detector geometry, enables calculations of the shape of the time-of-flight intensity profiles of the incident and the scattered neutrons sensed by a neutron detector, thus providing direct comparison with experimental results. The resolution function, R ( Q , E ), is calculated for a nondispersive scatterer at a resonant energy E . The results of the calculations on the basis of this theory are substantiated by measured spectra obtained by the two chopper spectrometers, HRMECS and LRMECS, at the Argonne Intense Pulsed Neutron Source under a variety of experimental conditions. In all cases we find excellent agreement between calculations and experiments. Using these results we present a procedure for the determination of the mean incident neutron energy and the calibration of the energy-transfer scale for pulsed-source chopper spectrometers. These latter do not follow accurately from simple analysis, and are the main objects of this paper.

2. Neutron Sources

Publisher Summary This chapter discusses the neutron sources. All neutron sources are based on nuclear reactions that free neutrons bound in nuclei. These are categorized according to the mechanisms by which this is done: fission chain reactions, fusion reactions, e - -bremsstrahlung-induced photoneutron and photofission reactions, charged-particle nuclear reactions, and spallation. The last three together termed as accelerator-based reactions. Muon-catalyzed fusion and the neutron focus represent other categories of neutron-producing reactions. Electron-bremsstrahlung sources produce evaporation neutrons with an average energy of a few mega-electron-volts, whether due to photofission or photonuclear reaction. Since the neutron production mechanisms involve first producing bremsstrahlung photons, which then interact to produce neutrons, these sources produce very large gamma-ray fluxes as well, which sometimes represent a troublesome background. Neutron shielding is like that for a reactor. Spallation sources also produce predominantly evaporation neutrons, which boil off in the process of “cooling off” of highly excited nuclei. The types of accelerators that provide the highest intensities are resonant accelerators, which all have inherent pulse structure. In cyclotrons, synchrotrons, and storage rings, bunches of particles circulate at frequencies of a few megahertz.

Neutron guide tube gain for a remote finite source

Abstract Neutron guide tubes are essential features of many neutron beam instruments. Even though detailed analysis of practical systems including guide curvature and reflectivity losses may require numerical calculation, analytical methods such as the present one are useful for understanding idealized systems and for assessing the accuracy of numerical methods. We use a technique which is an adaptation of one used in the analysis of particle transport in accelerators, describing trajectories in the transverse (i.e. position and angle) phase space of the neutrons. Similar methods are well known in neutron optics which have been used to calculate transmissions of neutron choppers and guides, and which rest upon representations in either two momentum or two spatial dimensions. We derive expressions for the gain (ratio of the total neutron current through the guide to the total current without internal reflections) of parallel-sided one-dimensional neutron guide tubes, explicitly including the effects of a remote finite source. We identify several distinct cases. For the realistic situation of a two-dimensional guide of rectangular cross section, the gain is the product of gains computed for the one-dimensional systems in each dimension.

Production of ultra-cold neutrons using Doppler-shifted Bragg scattering and an intense pulsed neutron spallation source

We present an analytic and a computer generated simulation of the production of ultra-cold neutrons (UCN) using Bragg scattering from a moving crystal to Doppler-shift higher velocity neutrons into the UCN region. The calculation was carried out with a view toward its application at the Intense Pulsed Neutron Source (IPNS) now under construction at Argonne National Laboratory. This method for the production of UCN appears well matched to a pulsed source, and we show that the UCN can be stored in a neutron bottle at the peak flux which can potentially be much higher than at the present high flux reactors. The predicted density of stored UCN indicates that a highly precise measurement of the neutron electric dipole moment (EDM) will be possible within the next few years.

A resonance detector spectrometer at KENS

Abstract We have tested a resonance detector spectrometer at the KENS neutron source, using 181Ta, 121Sb and 149Sm resonances and bismuth germanate (BGO) scintillators. In the process we encountered and solved numerous background problems, and discovered a time-focussing principle. We measured the scattering from a number of materials and have so far analyzed results for bismuth, vanadium and graphite, which we present. Tests of cooled absorbers indicate that resolution of 70 meV is possible with 181Ta.