Nondestructive evaluation of jades by means of 14 MeV neutron activation

Fourteen MeV neutron activation of jades was used to test the authenticity of jades: to ascertain whether they are genuine jadeite and whether their greenish colour is genuine, both of which are important criteria for the high market value of jadeite. For given activation and measurement conditions of the jades, the gamma-ray spectrum was observed to change as a function of the type of the jades; the counting ratio of the photopeaks from the two most prominent constituent elements varies greatly as a function of the type of the jades. The ratio of counts for the Compton edges of the 1.779 MeV gamma-ray of²⁸Al, from²⁸Si(n,p)²⁸Al reaction, and the 1.434 MeV gamma-ray of⁵²V, from⁵²Cr(n,p)⁵²V reaction, varied as a function of the type of the jades and also as a function of the color. These results can be applied for a fast and nondestructive evaluation of jades.     

A multiwire chamber system for measurements of neutron spectra

A new type of detector for measuring neutron flux and energy over a wide range of angles and energies is being developed. Measurements of neutron elastic and inelastic scattering as well as neutron energy continua are possible. Time-of-flight is not used for measuring outgoing neutron energy, and so for continuum measurements this system has some distinct advantages over conventional neutron detectors. Neutron energy measurement is carried out by measuring the energy and angle of the recoil proton produced by the neutron in a CH₂ converter. Spectra from ⁷Li(p, n)⁷Be at 62 MeV and ⁴⁰Ca(n, n′χ) at 65 MeV are presented.     

NIST Calibration of a Neutron Spectrometer ROSPEC

A neutron spectrometer was acquired for use in the measurement of National Institute of Standards and Technology neutron fields. The spectrometer included options for the measurement of low and high energy neutrons, for a total measurement range from 0.01 eV up to 17 MeV. The spectrometer was evaluated in calibration fields and was used to determine the neutron spectrum of an Americium-Beryllium neutron source. The calibration fields used included bare and moderated ²⁵²Cf, monoenergetic neutron fields of 2.5 MeV and 14 MeV, and a thermal-neutron beam. Using the calibration values determined in this exercise, the spectrometer gives a good approximation of the neutron spectrum, and excellent values for neutron fluence, for all NIST calibration fields. The spectrometer also measured an Americium-Beryllium neutron field in a NIST exposure facility and determined the field quite well. The spectrometer measured scattering effects in neutron spectra which previously could be determined only by calculation or integral measurements.     

Differential absorbed dose distributions in lineal energy for neutrons and gamma rays at the mono-energetic neutron calibration facility

Absorbed dose distributions in lineal energy for neutrons and gamma rays of mono-energetic neutron sources from 140 keV to 15 MeV were measured in the Fast Neutron Laboratory at Tohoku University. By using both a tissue-equivalent plastic walled counter and a graphite-walled low-pressure proportional counter, absorbed dose distributions in lineal energy for neutrons were obtained separately from those for gamma rays. This method needs no knowledge of energy spectra and dose distributions for gamma rays. The gamma-ray contribution in this neutron calibration field >1 MeV neutron was <3%, while for <550 keV it was >40%. The measured neutron absolute absorbed doses per unit neutron fluence agreed with the LA150 evaluated kerma factors. By using this method, absorbed dose distributions in lineal energy for neutrons and gamma rays in an unknown neutron field can be obtained separately.     

Measurement of neutron excitation functions using wide energy neutron beams

A technique for measuring neutron excitation functions using wide energy neutron beams is explored. Samples are activated with a set of neutron fields, each covering a relatively wide energy interval and created using an ion accelerator and conventional nuclear reactions. Measured activities are determined using gamma-ray spectrometry and reduced to excitation curves using spectrum unfolding. The technique is demonstrated on the measurement of the excitation function curve up to 5.6 MeV for ¹¹³In(n,n′)¹¹³In^m using the ¹¹⁵In(n,n′)¹¹⁵In^m reaction as an internal standard.     

Two types of multi-moderator neutron spectrometers: Gamma-ray insensitive type and high-efficiency type

Two types of multi-moderator neutron spectrometers were developed; one is a gamma-ray insensitive type, and the other is a high-efficiency type. An indium activation detector is loaded in the former spectrometer, which can measure the photon-dominant pulsed neutron field such as in the primary photon beam of a high-energy medical electron accelerator. The latter, in which a ³He counter is loaded, is so sensitive that it can measure leakage neutrons from a well shielded facility or even the skyshine neutrons. The response functions of the spectrometers were measured by thermal and mono-energetic neutron standard fields, and were also calculated by the one-dimensional discrete ordinates transport code, ANISN. The measured and calculated responses showed generally good agreement. A benchmark measurement of ²⁵²Cf fission neutrons by using these two spectrometers agreed well with the calculated spectrum. The spectrometers were used in the measurements of neutrons produced by a medical electron accelerator and of skyshine neutrons from an intense 14 MeV neutron source facility.     

In-flight energy calibration for the X-ray and gamma-ray spectrometers on the solar-A satellite

The electronically gated in-flight energy calibration is applied to the X-ray and gamma-ray spectrometers on a SOLAR-A satellite. The NaI X-ray spectrometer covering the 20–400 keV energy band is calibrated by an ²⁴¹Am radioactive source which decays by the simultaneous emission of 60 keV X-rays and 5.48 MeV alpha-particles. The X-ray calibration spectrum is accumulated in coincidence with an event tag pulse generated by the simultaneous detection of an X-ray and an alpha-particle. The BGO gamma-ray spectrometer covering the 0.2–10 MeV energy band is calibrated by a ⁶⁰Co radioactive source which decays by the simultaneous emission of 1.17 and 1.33 MeV gamma-rays and a beta-ray (maximum energy is 313 keV). The gamma-ray calibration spectrum is accumulated in a manner similar to the X-ray spectrometer. Since the present method enables to select the calibration pulse without the disadvantage of introducing extra pulses, it is suitable for a space experiment where external conditions and background counting rates can significantly change.     

A 6130 keV gamma-ray source using the 13C(α, n)16O reaction

A high energy gamma-ray source has been produced by mixing the alpha emitter ²³⁸Pu with ¹³C to produce the 6130 keV gamma-ray from the de-excitation of the second excited state ¹⁶O. The gamma-ray spectrum and emission rate have been measured and the neutron emission rate and mean energy have been determined.     

Establishment of a simple neutron calibration field using a moderated 252Cf source: Part II. Experimental characterization of simple neutron calibration field

This paper describes the development and experimental standardization of neutron fields simply arranged for detector calibrations used for radiation control and environmental measurement. These fields are the following: (1) bare ²⁵²Cf fission field, (2) iron-moderated ²⁵²Cf field, (3) carbone-moderated Cf field, and (4) polyethylene-moderated ²⁵²Cf field. These fields are most suitable for calibrating the detectors used in and around nuclear and radiation facilities, since the fields are designed to simulate the typical neutron fields in and around the facilities.The direct neutron components of these fields have been standardized by the following two methods: (1) calculation by the ANISN code, and (2) measurements with and without a shadow shield by detectors standardized in the national standard field at the Electrotechnical Laboratory (ETL). The neutron emission rates of the ²⁵²Cf source have been calibrated also at ETL. We have standardized only direct components because of their independence of room size and peripheral structures. The standardized values are energy spectra and dose equivalent rates of the direct neutron components; the accuracies have also been evaluated to be 20% below 100 keV, 15% at 1 MeV, and 50% above 5 MeV. These fields including room scattered components have also been characterized especially to calibrate neutron detectors having sensitivity to low energy room scattered neutrons, because of large errors caused by shadow shield subtraction.     

A spectrometer for double-differential neutron-emission cross section measurements in the energy range 1.6 to 16 MeV

A neutron spectrometer for the measurement of double-differential neutron-emission cross sections has been set up.An electron linac (GELINA) is used as a pulsed white neutron source. The energy of the incident neutrons is determined by the time-of-flight method. The secondary neutron spectra are determined by unfolding the pulse-height distributions observed in eight NE213-scintillators surrounding the sample.The measured spectra are normalised to the shape of the incident neutron flux measured with a ²³⁵U-fission chamber, and afterwards converted to absolute cross sections using as standard the carbon differential elastic scattering cross section below 2 MeV.     

A combined neutron and gamma-ray multiplicity counter based on liquid scintillation detectors

Multiplicity counters for neutron assay have been extensively used in materials control and accountability for nonproliferation and nuclear safeguards. Typically, neutron coincidence counters are utilized in these fields. In this work, we present a measurement system that makes use not only of neutron (n) multiplicity counting but also of gamma-ray (γ) multiplicity counting and the combined higher-order multiples containing both neutrons and gamma rays. The benefit of this approach is in using both particle types available from the sample, leading to a reduction in measurement times compared with single-particle measurements. We present measurement results of n, γ, nn, nγ, γγ, nnn, nnγ, nγγ and γγγ multiples emitted by Mixed-Oxide (MOX) samples measured at Idaho National Laboratory (INL). The MOX measurement is compared to initial validation of the detection system done using a ²⁵²Cf source. The dual radiation measuring system proposed here uses extra measurables to improve the statistics when compared to a neutron-only system and allows for extended analysis and interpretation of sample parameters. New challenges such as the effect of very high intrinsic gamma-ray sources in the case of MOX samples are discussed. Successful measurements of multiple rates can be performed also when using high-Z shielding.     

The 9Be(d,n)10B-reaction as intense neutron source with continuous energy spectrum

Neutron energy spectra produced by deuterons of 3 to 8 MeV in a thick ⁹Be-target were measured at various scattering angles. Significant angle dependences were observed. Angular distributions of the most energetic neutrons produced in thin ⁹Be targets can be described quantitatively in DWBA, which is an indication for a direct reaction mechanism. As a consequence all but 0°-neutrons are polarized to a certain extent. Also presented is the neutron energy spectrum of ⁷Li(d,n)⁸Be at 0° produced in a thick ⁷Li-target. The potential of these intense 0°-neutron beams with continuous energy distributions is demonstrated by a measurement of the neutron absorption cross section of natural carbon.     

Instrumental background in balloon-borne gamma-ray spectrometers and techniques for its reduction

A study of the instrumental background in balloon-borne gamma-ray spectrometers is presented. The calculations are based on newly available interaction cross sections and new analytic techniques, and are the most detailed and accurate published to date. Results compare well with measurements made in the 20 keV to 10 MeV energy range by the Goddard Low Energy Gamma-ray Spectrometer (LEGS). The principal components of the continuum background in spectrometers with Ge detectors and thick active shields are (1) elastic neutron scattering of atmospheric neutrons on the Ge nuclei, (2) aperture flux of atmospheric and cosmic gamma rays, (3)β^? decays of unstable nuclides produced by nuclear interactions of atmospheric protons and neutrons with Ge nuclei, and (4) shield leakage of atmospheric gamma rays. The improved understanding of these components leads to several recommended techniques for reducing the background. These include minimizing the passive material inside the shield and reducing the level of the shield threshold. A new type of coaxial n-type Ge detector with its outer contact segmented into horizontal rings can be used in various modes to reduce background in the 20 keV to 1 MeV energy range. The resulting improvement in instrument sensitivity to spectral lines is a factor of ～ 2 in this energy range.     

The Chandigarh variable energy cyclotron

The characteristics and performance of the variable energy cyclotron at Chandigarh have been described. The machine operates for protons at 1 to 5 MeV, for deuterons at 4 MeV, for alphas from 1 to 2 MeV and 7 to 8 MeV, and He³⁺⁺ upto 11 MeV. The resolved beams of different particles from 30 nA to 1 μA have been obtained at the target with a resolution of about 30 keV. The magnetic field and the beam profile in the chamber are discussed. Various gamma-ray and charged particle spectra are given to indicate the performance of the accelerator.     

Evaluation of large deuterated scintillators for fast neutron detection (E=0.5-20 MeV) using the D(d,n)He, C(d,n) and Al(d,n) reactions

The response of large deuterated liquid scintillators (up to 10 cm diameter by 15 cm) to neutrons 0.5-20 MeV has been studied using the 2.5 MeV neutron generator at the University of Michigan, and the d(d,n), ¹³C(d,n), ²⁷Al(d,n) and other reactions at the University of Notre Dame FN tandem accelerator. The latter utilize 9 and 16 MeV deuteron beams including a pulsed beam, which permitted time-of-flight measurements. Combining pulse-shape discrimination and time-of-flight allows gating on specific neutron energy groups to determine the detector response to specific neutron energies. This will permit accurate simulation of the detector response functions for applications of these detectors in nuclear research and homeland security applications.     

Efficiency measurements of deuterated liquid scintillators using d(d,n)He coincidence events

Deuterated liquid scintillators are promising from the perspective of n/γ separation and neutron energy measurement without time-of-flight. In order to turn these into precision instruments for nuclear physics, however, the absolute efficiency must be known. The efficiency is typically a function of energy and is strongly dependent on the electronics thresholds used with the photomultiplier detectors. This study involves the extraction of this efficiency using the d(d,n)³He reaction (E_d=16.0 MeV) and n-³He coincidence events. The detection and identification of the ions are very efficient and allows one to project pure monoenergetic neutron spectra all the way down to the electronics threshold. Timing information was also recorded in these measurements to provide further constraints and a consistency check.     

A gamma-ray spectrometer system for fusion applications

A NaI scintillator spectrometer system for the measurement of gamma-ray spectra in tokamak discharges has been developed and installed on the Frascati Tokamak Upgrade. Two NaI scintillators are viewing the plasma at two different angles with respect to the equatorial plane. The main features of the spectrometer system (energy range: 0.3–23 MeV) and of the unfolding technique used to restore physical spectra from the pulse-height distributions are described: a method of solution with regularisation for matrix equations of large size, allowing to process count distributions with significant statistical noise, has been developed. A dedicated software, portable to any platform, has been written both for the acquisition and the analysis of the spectra. The typical gamma-ray spectra recorded in hydrogen and deuterium discharges, also with additional heating, are presented and discussed; two components have been observed: (a) thick-target bremsstrahlung gamma-rays produced by runaway electrons hitting the inconel poloidal limiter and/or the vessel; and (b) neutron capture gamma-rays generated in the detector shielding and tokamak structures. The maximum energy resulting from the bremsstrahlung spectra is in agreement with the runaway energy predicted by a test particle model of runaway electron dynamics.     

Beryllium neutron activation detector for pulsed DD fusion sources

A compact fast neutron detector based on beryllium activation has been developed to perform accurate neutron fluence measurements on pulsed DD fusion sources. It is especially well suited to moderate repetition-rate (<0.2 Hz) devices, such as the plasma focus or Z-pinch. The detector comprises a beryllium metal sheet sandwiched between two large-area xenon-filled proportional counters. A methodology for calculating the absolute response function of the detector using a “first principles” approach is described. This calibration methodology is based on the ⁹Be(n,α)⁶He cross-section, energy calibration of the proportional counters, and numerical simulations of neutron interactions and beta-particle paths using MCNP5. The response function R(E_n) is determined over the neutron energy range 2-4 MeV. The count rate capability of the detector has been studied and the corrections required for high neutron fluence measurements are discussed. For pulsed DD neutron fluencies >3×10⁴ cm⁻², the statistical uncertainty in the fluence measurement is better than 1%. A small plasma focus device has been employed as a pulsed neutron source to test two of these new detectors, and their responses are found to be practically identical. Also the level of interfering activation is found to be sufficiently low as to be negligible.     

The neutron spectra of nuclear facilities as the superposition of physically validated spectra

Neutron energy spectra in the energy range 10⁻¹⁰–18 MeV for 100 neutron fields of nuclear reactors and neutron generators are re-established. A method of forming the a priori spectrum in the form of the superposition of physically validated neutron spectra is used in the calculation. __________ Translated from Izmeritel’naya Tekhnika, No. 6, pp. 60–66, June, 2006.     

Detection of SNM by delayed gamma rays from induced fission

The Pulsed Neutron Interrogation Test Assembly (PUNITA) is an experimental device for research in NDA methods and field applicable instrumentation for nuclear safeguards and security applications. PUNITA incorporates a standard 14-MeV (D-T) pulsed neutron generator inside a large graphite mantle. The generator target is surrounded by a thick tungsten filter with the purpose to increase the neutron output and to tailor the neutron energy spectrum. In this configuration a sample may be exposed to a relatively high average thermal neutron flux of about (2.2±0.1)×10³ s⁻¹ cm⁻² at only 10% of the maximum target neutron emission. The sample cavity is large enough to allow variation of the experimental setup including the fissile sample, neutron and gamma detectors, and shielding materials.The response from SNM samples of different fissile material content was investigated with various field-applicable scintillation gamma detectors such as the 3×2 in. LaBr₃ detector. Shielding in the form of tungsten and cadmium was applied to the detector to improve the signal to background ratio. Gamma and neutron shields surrounding the samples were also tested for the purpose of simulating clandestine conduct. The energy spectra of delayed gamma rays were recorded in the range 100 keV-9 MeV. In addition time spectra of delayed gamma rays in the range 3.3-8 MeV were recorded in the time period of 10 ms-120 s after the 14-MeV neutron burst. The goal of the experiment was to optimize the sample/detector configuration including the energy range and time period for SNM detection. The results show, for example, that a 170 g sample of depleted uranium can be detected with the given setup in less than 3 min of investigation. Samples of higher enrichment or higher mass are detected in much shorter time.