Monoenergetic Neutron Sources Research Articles

High-Resolution Fast-Neutron Gamma-Ray Production Cross Sections for Iron up to 2100 keV

High-resolution gamma-ray production cross sections for the 846-keV gamma ray of iron have been measured up to an incident neutron energy of 2100 keV. The measurements were performed using the Oak Ridge Electron Linear Accelerator as the neutron source, and they were obtained by a ratio measurement to the 7Li 477-keV gamma-ray cross sections. Three NE-213 detectors were used at 30, 90, and 125 deg to derive the total inelastic cross sections and the angular distributions. The 1250 angular distributions measured with ∼0.1 ns/m resolution show considerable fluctuations as a function of energy over the resonances seen in the inelastic cross sections. The results are compared to the ENDF/B-IV evaluation, high-resolution data at 125 deg and, after suitable averaging, with recent monoenergetic neutron source data that average over the structure experimentally. The general consistency of the data with recent measurements, using different techniques and normalization procedures, indicates that our knowledge of this important cross section for fission reactor applications may now be known to an accuracy better than 10%. This is a significant achievement in view of the wide scatter of earlier data on such a fluctuating cross section.

Geometrical Efficiency Equation for Proton Recoil Telescopes

The geometrical efficiency equation for a proton recoil telescope exposed to a monoenergetic point neutron source is solved by a power series expansion that permits simple and accurate reduction of raw telescope data to absolute neutron fluxes or source strengths. Two parameters are allotted for description of source anisotropy and three parameters are allotted for description of hydrogen scattering anisotropy in the center-of-mass system. The relativistic transformation of the hydrogen differential scattering cross section from center-of-mass to laboratory system is used.

Dosimetry Related to Controlled Thermonuclear Research

Abstract Controlled thermonuclear research (CTR) dosimetry is presently concerned with measurements of neutron flux, fluence, and neutron spectra. This information is being used to (1) correlate radiation effects data, (2) evaluate and describe the irradiation environment of currently available neutron sources, and (3) provide diagnostics for plasma physics studies. Current radiation effects studies are emphasising surface phenonena such as sputtering, and are utilising (d,t) and Be(d,n) reactions as the sources of neutrons. The energy region of 8 to 15 MeV is of most interest for CTR purposes. Experimental constraints necessitate use of the foil activation technique for dosimetry measurements. For the cases of monoenergetic neutron sources (RTNS), accuracies of about 7.5% are being obtained. Cross section uncertainties are limiting the accuracy for broad spectrum measurements, such as from Be-d sources. Because no present neutron source adequately represents the type of radiation environment expected to be encountered in future CTR devices, emphasis must be placed on carefully documenting and standardizing the dosimetry measurements so that the radiation effects data can ultimately be extrapolated to anticipated CTR conditions. Future efforts in CTR dosimetry must develop methods for characterizing charged and neutral particle radiation.

Neutron propagation and 2.2 MeV gamma-ray line production in the solar atmosphere

We have calculated the 2.2 MeV gamma-ray line intensity from the Sun using a Monte Carlo method for neutron propagation in the solar atmosphere. We provide detailed results on the total gamma-ray yield per neutron and on the time profile of the 2.2 MeV line from an instantaneous and monoenergetic neutron source. The parameters which have the most significant effects on the line intensity are the energies of the neutrons, the position of the neutron source on the Sun, and the abundance of He3 in the photosphere. For an isotropic neutron source which is not too close to the limb of the Sun, the gamma-ray yield is between about 0.02 to 0.2 photons per neutron, provided that the neutron energies are in the range 1 to 100 MeV and the ratio He3/H is less than about 5 × 10−5.

A high-precision neutron time-of-flight facility

A high-precision neutron time-of-flight facility has been constructed. The facility couples the advantages of an intense monoenergetic neutron source produced by a pulsed and bunched ion beam with those of a high-energy resolution, high mechanical precision, heavily shielded neutron time-of-flight spectrometer-goniometer. The system provides the necessary incident neutron energy stability and resolution, scattered neutron energy and angular resolution, and unusual freedom from background radiations to achieve measurements of differential scattering cross sections with magnitudes as small as 1 mb/sr to better than 5% absolute accuracy. The measures required to achieve this sensitivity and accuracy include optimization of the mechanical, shielding, and electronic systems, as well as the procedures for obtaining and analyzing the data.

Spectra and dose fractions of monoenergetic neutrons passed through different shields and their application for dose-meter evaluation.

Leakage neutron spectra for monoenergetic sources and for slab shields of different compositions and thicknesses are calculated. S ource energies ranging from 0.34 keV to 10.5 MeV are considered, for the following shield materials: water, concrete, boron-loaded polythene and iron, at thicknesses ranging from 5 to 60 cm. Corrections to readings of several dose-meters are determined, making use of the spectra. The measured percentages of the dose-equivalent are evaluated for the following devices: thermal neutron detectors, the Harvey albedo dose-meter, Yoshida-Dennis proportional counter, Dennis-Loosemore counter, nuclear emulsion and proton-recoil scintillation counter. Dose-meter reading calculations are extended up to 15 MeV energy, which is of great practical importance at (d, t) neutron generator monitoring. Finally recommendations are given concerning the selection of dose-meters for different monoenergetic neutron sources with different shieldings.

Fast forward neutron production in the 7Li(p,n) 7Be reaction for 41- and 64-MeV protons

Measurements have been made by the time-of-flight technique of the neutron energy spectra observed at zero degrees from the 7Li(p,n) 7Be reaction using 41- and 64-MeV incident protons. The neutron production cross sections for the fast forward peak (corresponding to excitation of the ground and first excited states of 7Be) has been found to be 33±6 mb/sr at a mean energy of 39 MeV and 36±4 mb/sr at 60 MeV. These results are compared with those reported elsewhere, and the suitability of such 7Li targets as a source of monoenergetic neutrons is discussed.

Certifying and utilizing monoenergetic neutron sources of accelerators

At present the problems of certifying monoenergetic neutron sources by means of an accelerator and their utilization for certifying various measurement means has not as yet been fully solved, since the metrological organizations have encountered this problem in relatively recent times. It was noted at the 1st All-Union Conference on the Metrology of Neutron Radiations, which was held in 1971 in Moscow, that reference and standard monoenergetic neutron sources are being produced in the USSR on the basis of EG-2M and EG-2.5 accelerators, and that the standard techniques in direct measurements in pulsed neutron fields are being developed [2]. Experience gained in carrying out this work has shown that the successful solution of the certification problems requires joint efforts of metrologists, measuring equipment developers, and customers.

Coincidence spectra from d + d → d + p + n and p + d → p + p + n reactions with neutron detection at zero degrees

Proton-neutron coincidence spectra from the p + d → p+p+n reaction were measured for θ n = 0° and θ p = 15°–40° at E p = 10.0 MeV. Deuteron-neutron coincidence spectra from the d + d → d + n + p reaction were measured for θ n = 0° and θ d = 15°–35° at E d = 10.0 MeV. Coincidence spectra were analyzed in terms of a quasi-free scattering model. The possibility of using these reactions as a source of monoenergetic neutrons is discussed.

Polarization studies using the neutron spin-precession method with a continuous energy spectrum of neutrons

Precise measurements of the polarization of neutrons in the MeV region have frequently made use of the spin-precession method. These measurements have been carried out with nearly mono-energetic neutron sources. In this paper, we present a development of the method that makes it suitable for the more general class of experiments involving a continuous energy spectrum of neutrons. For a fixed value of the solenoidal field, the angles of precession for neutrons of all energies are determined using a high resolution time-of-flight spectrometer. The power of the technique is demonstrated by measuring the polarization of photoneutrons from the reaction 16 O(γ, n ) 15 O at an angle of 45°. The neutron energy range covered in a single experiment is from 5 to 15 MeV.

Considerations in Establishing a Standard for Neutron Displacement Energy Effects in Semiconductors

Over the past several years a number of semiempirical or theoretical curves have been generated to represent the neutron-induced displacement effects in silicon as a function of neutron energy. Since the mode of failure for most silicon semiconductor devices is a loss of gain and this is, in turn, proportional to the reduction in carrier lifetime the same curves are used to express both the neutron energy damage effectiveness for devices as well as the degradation of lifetime. A useful feature of such a curve is that the damage produced in a semiconductor by a neutron fluence of a given energy spectrum may be predicted by folding the spectral information together with the damage curve. Once this is done, the damage may be expressed as that resulting from an equivalent monoenergetic neutron source (the monoenergetic equivalent). This procedure enables the results of an exposure to a source of one spectrum to be used to predict the effects of a different one. In order for this technique to be useful as a means of communication there must be some standardization of the damage curve, the neutron energy to which the damage is referred and the description of the spectrum. Recent interest, throughout the radiation effects community, in establishing such a standard has prompted several proposals each of which contains some shortcomings. This paper compares the various deficiencies and merits and arrives at what constitutes a minimum set of requirements for standardization at this time.

Zur Verwendung von Rechenprogrammen bei der Bestimmung von Wirkungsquerschnitten schneller Neutronen nach dem Aktivierungsverfahren

Special computer-programs are discussed which may be used in the analysis of measurements of fast neutron cross sections applying the activation method. For various shapes of the activated samples the average neutron energy and the average neutron flux density are calculated for the monoenergetic fast neutron sources usually used in experiments. Moreover, a method is described for deducing the absolute activities from the measured total decay curve which works sufficiently well also if several activities with different half-lives and intensities are present. All programs are written in ALGOL and are available on request.

La reaction 51V(p,n) 51Cr comme source de neutrons monoenergetiques

The reaction 51V(p,n)51Cr has been studied between 2.2 and 4 MeV in order to determine the possibility of using this reaction as a source of monoenergetic neutrons; angular distributions have been measured and found to be quasi-isotropic. It is shown that for some applications this source is better than the conventional ones because of the slow variation of intensity and energy with angle and also because of the ease with which the targets can be made and utilised. Simple empirical relations between the energy of the protons and the absolute cross sections are deduced. Cross sections and coefficients of Legendre polynomial expansions have been tabulated.

Operation of a 4 ma Dynamitron as a Monoenergetic Neutron Source

The Cornell Dynamitron is being used as a source of monoenergetic neutrons in the range 30-500 keV, with fluxes up to 109 n/sr/s. Problems of generating and handling the 2 Mev, 4 ma beam are discussed. Analyzing magnet slit feedback is essential for accurate control of neutron energy. Contrary to other published data for these machines, we observe up to 3% drift in the energy when it is controlled by the voltage feedback system. Discrepancies between this and previously published data are discussed.

Nuclear design parameters for a monoenergetic neutron source

The nuclear design parameters in the construction of a monoenergetic laboratory neutron source have been evaluated. The most promising reaction was 13C( α, n) 160, employing 210P or 242Cm, that is expected, because of strong emission anisotropy, to yield about 13,000 neutrons/ s-Ci in forward and backward 7% FWHM peaks centered at 6·45- and 4·25 MeV. The peak positions and energy spreads were obtained by use of a graphic-numerical analysis of thin target data. The efficiencies and emission properties of two source configurations are presented and discussed.

The LET Distribution of the Recoil Proton Dose from Poly-energetic Neutron Sources

Previous investigations on the linear energy transfer (LET) distribution of the recoil proton dose from monoenergetic neutron sources have been extended to include cyclotron produced neutrons and Pu-Be neutrons as examples of poly-energetic sources. The recoil proton absorbed dose, measured at depth in a tissue-equivalent phantom using a Bragg-Gray type detector, is expressed as a function of the LET and is compared with the distributions predicted by theory. Excellent agreement is found between the experimental and theoretical curves.

Anisotropy of the scattering in space-time neutron transport theory

The ℒ-transformed space-angle-time solution of the Boltzmann equation in the case of infinite plane symmetry, anisotropic scattering, constant cross-sections and unit-strength infinite plane isotropic pulsed source of monoenergetic neutrons is here worked out in terms of the eigenfunctions, which correspond to the eigenvalues associated with the non-Hermitian operator of the given balance transport equation. These eigenvalues are seen to separate into a discrete and continuous spectrum. It is then shown that, among the eigenvalues belonging to the discrete spectrum, at least one can be in general extracted and expressed as a power series of the complex parameter associated with the Laplace integral. This turns out to be very useful when the ℒ-transform of the sought solution is to be inverted. It is thus possible to recognize that the transport solution here obtained is asymptotic to the result of the simple diffusion theory.

Modulation of Diffracted Neutrons with a Piezoelectric Crystal

THE production of pulses of neutrons from a reactor beam by means of a rotating scatter is a well-known technique1. We have investigated the possibility of using a quartz piezoelectric crystal as a means of modulating a beam of neutrons. It was believed that in this way it might be possible to obtain a variable frequency pulsed or sine-modulated source of mono-energetic neutrons.

Study of the spectra and doses created in the iron-water shielding of a monoenergetic neutron source

Many-group calculations were made for the penetration of neutrons, emitted from monoenergetic sources, through water, iron, andwater-iron systems of finite dimensions; the results of these calculations are presented. The neutron spectra resulting from the passage of such neutrons through water and iron shielding layers were calculated on the twenty-group diffusion-transport approximation, Detailed attention was paid to the high-energy part of the spectrum; certain peculiarities in neutron migration and moderation processes in shielding of the type in question were elucidated. Dose curves D(r) were plotted for neutrons of various energies.

A measurement of v̄ for neutron-induced fission of 240Pu

Measurements have been made of v̄, the number of prompt neutrons per fission, following neutron-induced fission of 240Pu. The measurements were made using a pulsed source of monoenergetic neutrons. The 240Pu was contained in a fission ion chamber placed close to the neutron source, and the neutrons detected in a 10 cm diameter plastic scintillator 60 cm from the fission chamber. A number of geometries were investigated experimentally verifying calculated corrections for neutron-fragment correlations etc. Values of v̄ obtained were as follows: Neutron v ̄ energy 0·1 MeV 2·89±0·19 1·6 MeV 3·26±0·12