Gamma-ray Discrimination Research Articles

Discrimination of neutrons and gamma rays by a neutron detector comprising a superconducting tunnel junction on a single crystal of Li 2B 4O 7

We demonstrate that neutrons and gamma rays can be discriminated by a neutron detector comprising a superconducting tunnel junction on a single crystal of Li 2B 4O 7. The neutron detector was irradiated with X-rays, gamma rays and neutrons, and the signal pulses were analyzed in terms of pulse heights and rise times. It was found that most of the signal pulses for neutron events exhibited slower rise times than those for X-rays and gamma rays, indicating that neutrons and gamma rays could be discriminated by the detector on the basis of the rise times of the signal pulses.

An approach to multiplicity counting for a versatile new sensor for plutonium assay with a very short die-away time, and independent measurements of neutrons and gamma rays

A unique detector design incorporating a 6Li-based capture medium, ZnS scintillator, and wavelength shifting optical fibers is the basis of a new neutron coincidence counter for measurements of plutonium in highly impure residues. The sensor elements have a high efficiency for detecting neutrons and exhibit excellent gamma-ray discrimination based on pulse-shape analysis. The short die-away time of the counter that is based on these detector elements allows coincidence-gate settings shorter than 10μs. This qualifies the technology for measurements of materials with high yields of uncorrelated neutrons from 241Am(α,n) reactions. The characteristics of the new neutron counter will be illustrated with test data from measurements of plutonium, 252Cf, and gamma-ray sources. The integrated electronics design of the new detector also permits the simultaneous but independent measurement of both neutrons and gamma rays. Recent test results that illustrate some unique applications of the sensor's versatility will also be presented.

Scintillating-Glass-Fiber Neutron Sensors, their Application and Performance for Plutonium Detection and Monitoring

Most neutron detection sensors presently employ 3He gas-filled detectors. Despite their excellent performance and widespread use, there are significant limitations to this technology. A significant alternative neutron sensor utilizing neutron-active material incorporated into a glass scintillator is presented that offers novel commercial sensors not possible or practical with gas tube technology. The scintillating optical fiber permits sensors with a multitude of sizes ranging from devices of a single fiber of 150µm to sensors with tens of thousands of fibers with areas as large as 5 m2 depending on the neutron flux to be measured. A second significant advantage is the use of high-speed electronics that allow a greater dynamic range, not possible with gas detectors. These sensors are flexible, conformable and less sensitive to vibration that optimizes the source-to-detector geometry and provides robust performance in field applications. The glass-fibers are sensitive to both gamma-rays and neutrons. However, the coincidence electronics are optimized for neutron to gamma-ray discrimination allowing very sensitive measurements with a low false-alarm rate. Applications include SNM surveillance, material control and accountability (MC&A), safeguard inspections, Pu health physics/bioassay and environmental characterization.

A triple-crystal phoswich detector with digital pulse shape discrimination for alpha/beta/gamma spectroscopy

Researchers at the University of Missouri – Columbia have developed a three-crystal phoswich detector coupled to a digital pulse shape discrimination system for use in alpha/beta/gamma spectroscopy. Phoswich detectors use a sandwich of scintillators viewed by a single photomultiplier tube to simultaneously detect multiple types of radiation. Separation of radiation types is based upon pulse shape difference among the phosphors, which has historically been performed with analog circuitry. The system uses a GaGe CompuScope 1012, 12 bit, 10 MHz computer-based oscilloscope that digitally captures the pulses from a phoswich detector and subsequently performs pulse shape discrimination with cross-correlation analysis. The detector, based partially on previous phoswich designs by Usuda et al., uses a 10 mg/cm 2 thick layer of ZnS(Ag) for alpha detection, followed by a 0.254 cm CaF 2(Eu) crystal for beta detection, all backed by a 2.54 cm NaI(Tl) crystal for gamma detection. Individual energy spectra and count rate information for all three radiation types are displayed and updated periodically. The system shows excellent charged particle discrimination with an accuracy of greater than 99%. Future development will include a large area beta probe with gamma-ray discrimination, systems for low-energy photon detection (e.g. Bremsstrahlung or keV-range photon emissions), and other health physics instrumentation.

Tritium Discrimination in a Mixed Radiation Field Using an Ionization Chamber

A new method of tritium gamma-ray discrimination based on the statistical analysis is proposed.

An integrated pulse-analysis system for neutron spectroscopy

A new dual-parameter pulse-shape-discrimination circuit for the acquisition of proton-recoil pulses (and elimination of gamma-ray-induced pulses) from fast-neutron spectrometer systems has been built and tested. Except for the detector, its bias supply and preamplifier, the system requires only the dual-amplifier—ADC board described in this paper and a personal computer. The basis for this system is a dual-amplifier—ADC board that utilizes dual-amplifier sections for shaping and amplification, and dual Wilkinson-type, 10-bit ADCs from The Nucleus, Inc., all resident on a 20 cm × 28 cm (8 in. × 11 in.) double-sided, printed circuit board. Digital signals from the two ADCs and the associated coincidence circuit are fed to an IBM PC/XT clone through a triple, 8-bit digital interface board. Data-acquisition software has been written in assembler language to reduce data acquisition time, resulting in a 25% deadtime at 5000 counts per second using the 4 MHz processor of the IBM PC/XT. The system has been tested using both spherical and cylindrical gas-filled proton-recoil detectors and an NE-213 liquid scintillator. Quasi-monoenergetic spectra from filtered neutron beams at 24 keV and a broad energy spectrum from an aluminum/sulfur filter have been measured. In these cases the measured spectra agreed well with expected results, indicating that gamma-ray discrimination has been accurately achieved using the system.

Beta-particle spectroscopy with active gamma-ray discrimination

A spectrometer was developed which was capable of measuring a beta-particle energy distribution while simultaneously (actively) rejecting the system's response to gamma rays. A two-detector configuration was used, where the first detector was a thin, pancake-type, gas-flow counter, positioned in front of the entrance window to a BC-400 plastic scintillator. The gas-flow counter was designed to be insensitive to gamma rays so that it could act as a sensor which would gate the spectrometer to accept only those signals induced by beta-particle interactions in the scintillator. The gamma-ray rejection capability of the spectrometer was a function of gamma-ray energy. The system's discrimination ratio, the number of gamma-ray-induced scintillator pulses which are rejected to those which are recorded, has a value of 279:1 for gamma rays with an energy of 278.3 keV, 43:1 for an energy of 661.6 keV and 9:1 for a gamma-ray energy of 1778.9 keV.

Neutron and gamma ray measurements carried out around a Varian Clinac 2500 linear accelerator

Auckland Hospital, New Zealand, has recently installed a Varian Clinac2500 linear accelerator. This unit is capable of producing 24 MV peak photon energies. It is well known (Fox et al, 1977; Ing et al, 1982; Palta et al, 1984) that such photons are capable of producing neutrons via a photonuclear reaction. The sources of such neutrons from this accelerator have been described (LaRiviere, 1985). In this study a survey was carried out around this new installation to observe some of the characteristics of the neutrons produced, and to assess likely radiological hazards to radiotherapy staff. Equipment and methods Equivalent neutron dose was measured using an Alnor neutron dose rate meter (rem-meter) model 2202D. Remote pulse counting was carried out using an Elscint model INS-30 nuclear spectrometer. The manufacturer of this unit claims that the detector exhibits 200 R/h gamma ray discrimination. Initially this would appear satisfactory for carrying out a survey, but owing to the fact that linear accelerators are operatedin pulsed mode, peak leakage and scatter photon dose rates within the treatment room could well exceed 200 R/h. This was confirmed by operating the accelerator in 6 MV photon mode (i.e. producing no expected neutrons) and showing that the neutron meter did in fact respond. However, in the entrance maze to the treatment room the scattered X-ray dose rate was attenuated sufficiently so that it was not contributing to detector response.

Neutron spectroscopy in the 1–30 keV energy range

Abstract A spherical, hydrogen-filled, proton recoil neutron spectrometer (Benjamin detector) has been implemented for spectral measurements down to 1 keV in energy (with primary emphasis in the 1–30 keV range). It is generally acknowledged that gas-filled proton recoil spectrometers are capable of detecting neutron spectra down to an energy of about 1 keV, but few data are available which experimentally verify this claim. To confirm the capability of gas-filled proton recoil spectrometers to accurately measure neutron energies in this energy range, a system has been designed and tested. In the development of this system, careful attention has been given to gamma-ray discrimination techniques, fill-gas and quench-gas partial pressure, detector high voltage, and noise reduction techniques to obtain accurate spectra. Experimental measurements have been made of neutron beams and scattered neutron spectra in the 1–30 keV range to verify the accuracy of the detector system.

4504737 Tritium monitor with improved gamma-ray discrimination: Samson A Cox, Edgar F Bennett, Thomas Yule assigned to the United States of America as represented by the United States Department of Energy

4504737 Tritium monitor with improved gamma-ray discrimination: Samson A Cox, Edgar F Bennett, Thomas Yule assigned to the United States of America as represented by the United States Department of Energy

Pulse-shape discrimination of high-energy neutrons and gamma rays in NaI(Tl)

Pulse-shape discrimination can be used to separate neutron and gamma-ray interactions depositing energies up to in excess of 50 MeV in NaI(Tl) crystals. The secondary alpha particles, deuterons and protons produced in the neutron interactions are also resolvable.

Fast-neutron spectrum measurement by a stilbene scintillation spectrometer

A stilbene scintillation spectrometer for continuous fast-neutron spectra measurement is described in this paper. Discrimination of gamma rays is achieved by utilizing differences in the shapes of scintillation pulses produced by recoiled protons and electrons. In addition, results of measurements of fast-neutron spectra of the reactor beam are given.

Measurement of the Neutron Total Cross Sections of Zircaloy-2, Zirconium-90, and Carbon between 0.4 and 2.4 MeV

The total neutron cross sections of Zircaloy-2, zirconium-90, and carbon have been measured in the energy range 0.4 to 2.4 MeV, using the Bettis 3-MeV Van de Graaff pulsed-beam time-of-flight system in conjunction with a NE213 pulse-shape discrimination transmission detector for gamma-ray discrimination. Results obtained far Zircaloy-2 are significantly lower than prior evaluated data near 1 MeV. In addition, the zirconium-90 results average 6 to 7% below the Zircaloy-2 results.

Fast-neutron flux measurement with a lithium-drifted germanium detector

The use of lithium-drifted germanium detectors for fast-neutron flux measurement has been investigated. The proposed detection mechanism is observation of the internal conversion electrons emitted during the de-excitation of the 691 KeV 0 + state in 72Ge following excitation via fast-neutron scattering. A 4.0 cm 3 (approx.) planar Ge(Li) detector has been calibrated for neutron flux measurements over the energy range E n = 700−5000 keV. The absolute efficiency for this detector (ratio of neutrons detected to neutrons incident on the detector) has been determined to be (8.2±0.5) × 10 −4 at E n = 1500 keV which is about 10% of the full-energy peak efficiency for 1332 keV 60Co gamma rays. Cross-sections for excitation of the 691 keV 0 + state in 72Ge via neutron scattering have been extracted from the measured efficiency function. These cross-sections can be used to calculate efficiency functions for other detectors. Insensitivity to thermal neutrons, gamma-ray discrimination capability and stability are among the desirable features of this type of fast-neutron detector. Submicrosecond timing is not possible because of the isomeric nature of the 691 keV 0 + state in 72Ge. While Ge(Li) detectors suffer radiation damage from fast-neutron bombardment, the detector used for these measurements withstood an integrated dose exceeding 10 10 neutrons/cm 2 without serious deterioration of its performance.

Gamma-ray discrimination in a proton-recoil spectrometer for a fast reactor spectrum measurement

Studies have been made on gamma-ray discrimination for a proton-recoil neutron spectrometer with pulse-shape discrimination electronics for in-core measurements. A small proportional counter filled with hydrogen, in conjunction with a pulse-shape discrimination system, was used in experiments to examine the characteristics of gamma-ray discrimination in the energy region below 100 keV. Characteristics of the discrimination by two types of systems, using the “dividing” and the “rise time” methods respectively, were compared with each other under the same conditions. Distributions of relative specific ionization or rate of rise in various energy bands and gamma elimination ratios in the energy region from 5 to 30 keV are obtained. The results of the experiments indicate that the two methods are able to distinguish completely between neutron and gamma events above 40 keV and the gamma elimination ratios decrease rapidly at energies below 10 keV. The dividing method is observed to be slightly better than the rise time method with respect to the pulse-shape discrimination.

Pulse shape discrimination: An investigation of n-γ discrimination with respect to size of liquid scintillator

Neutron and gamma ray discrimination for liquid organic scintillators of 4″ to 8″ in dia. has been studied with a “true zero crossover” type pulse shape discriminator. The effectiveness of various configurations of scintillators, light pipers and 5″ dia. photomultipliers in n-γ discrimination for meutrons of 100 keV to 3 MeV has been investigated and data are presented. The poor performance of psd systems with 8″ dia. scintillators has been examined with reference to self absorption effects, increased optical path lengths and transit time spreads and photocathode sensitivity of photomultipliers. Variations in the transit time and sensitivity across a 5 t ld photomultiplier were studied with a light emitting diode. A technique for optimizing the performance of photomultipliers to obtain more efficient pulse shape discrimination is discussed and results are presented after such optimization.

Total Cross Sections of Carbon, Oxygen, Fluorine and Thorium for Fast Neutrons

The neutron total cross sections of carbon, oxygen, fluorine and thorium have been measured in the energy range of 3.3 to 5.0 MeV. Measurements were made with a good-geometry arrangement and an energy resolution of about 20 keV, using a neutron detector which had a high efficiency and a good characteristic for gamma-ray discrimination. For carbon, oxygen and fluorine, several resonances in the cross sections were analyzed to give spin assignments. For thorium, a giant resonance was observed at 4.2±0.2 MeV.

AN ILLUSTRATION OF A TYPE OF TRIPLE CORRELATION MEASUREMENT WHICH CAN BE EASILY INTERPRETED

The Mg26(He4, nγ)Si29 reaction has been used to illustrate the simplifications introduced in the interpretation of triple angular correlations by choosing a target and bombarding particles of zero spin and by observing the emitted particles, in this case neutrons, in a counter fixed at 0° to the beam. The angular correlations of the gamma rays with respect to the incident beam then depend only upon the properties of the final states in the residual nucleus. The angular correlation of the electric quadrupole 2.03-Mev gamma ray is predicted uniquely by theory and this prediction has been verified experimentally. The angular correlations of the 1.28-Mev and 2.43-Mev gamma rays have yielded for the E2/M1 amplitude mixing ratios +0.25 ± 0.05 or −3.4 ± 0.5 and −0.26 ± 0.08 or −1.10 ± 0.16 respectively. In addition, the experiment provides an illustration of the value of the recently discovered technique of neutron – gamma-ray discrimination in an organic scintillator.