Y. Danon

Neutron capture and total cross-section measurements and resonance parameters of gadolinium

Abstract Neutron capture and transmission measurements were performed by the time-of-flight technique at the Rensselaer Polytechnic Institute linac facility using metallic and liquid Gd samples. The liquid samples were isotopically enriched in either 155Gd or 157Gd. The capture measurements were made at the 25-m flight station with a multiplicity-type capture detector, and the transmission measurements were performed at 15- and 25-m flight stations with 6Li glass scintillation detectors. The multilevel R-matrix Bayesian code SAMMY was used to extract resonance parameters. Among the significant findings are the following. The neutron width of the largest resonance in Gd, at 0.032 eV in 157Gd, has been measured to be (9 ± 1)% smaller than that given in ENDF/B-VI updated through release 8. The thermal (2200 m/s) capture cross section of 157Gd has been measured to be 11% smaller than that calculated from ENDF. The other major thermal resonance, at 0.025 eV in 155Gd, did not display a significant deviation from the thermal capture cross section given by ENDF. In the epithermal region, the analysis provided here represents the most extensive to date. Twenty-eight new resonances are proposed, and other resonances previously identified in the literature have been revisited. The assignment of resonances within regions of complicated structure incorporated the observations of other researchers, particularly on the six occasions where ENDF resonances are recommended to be removed. The poor match of the ENDF parameters to the current data is significant, and substantial improvement to the understanding of gadolinium cross sections is presented, particularly above 180 eV where the ENDF resolved region for 155Gd ends.

Electron and positive ion acceleration with pyroelectric crystals

The phenomenon of pyroelectric electron emission has been employed to develop miniature x-ray sources, such as the Cool-X by Amptek (www.amptek.com/coolx.html). The source strength of a pyroelectric x-ray generator is dependent on the emitted electron energy and current. Similarly, the source strength of a pyroelectric neutron generator will be dependent on the energy and production rate of deuterium ions in the fill gas. This paper summarizes our results in experiments directed toward creating high-energy electrons and positive ions with a pyroelectric source. Single-crystal sources are shown to produce positive ions with energies of up to 98keV and electron energies of up to 143keV. X-ray spectra are presented as proof that a paired-crystal source can increase electron energy to at least 215keV. In addition, we offer independent verification of the “bunched” electron emission effect observed by [Brownridge et al., Appl. Phys. Lett. 78, 1158 (2001)].

High-energy x-ray production with pyroelectric crystals

The invention of pyroelectric x-ray generator technology has enabled researchers to develop ultraportable, low-power x-ray sources for use in imaging, materials analysis, and other applications. For many applications, the usefulness of an x-ray source is determined by its yield and endpoint energy. In x-ray fluorescence, for example, high-energy sources enable the excitation of the K-shell x-ray peaks for high-Z materials as well as the lower-energy L-shell peaks, allowing more positive sample identification. This report shows how a paired-crystal pyroelectric source can be used to approximately double the endpoint x-ray energy, in addition to doubling the x-ray yield, versus a single-crystal source. As an example of the advantage of a paired-crystal system, we present a spectrum showing the fluorescence of the K shell of thorium using a pyroelectric source, as well as a spectrum showing the fluorescence of the K shell of lead. Also shown is an x-ray spectrum with an endpoint energy of 215 keV.

Application of Monte Carlo Chord-Length Sampling Algorithms to Transport Through a Two-Dimensional Binary Stochastic Mixture

Abstract Monte Carlo algorithms are developed to calculate the ensemble-average particle leakage through the boundaries of a two-dimensional binary stochastic material. The mixture is specified within a rectangular area and consists of a fixed number of disks of constant radius randomly embedded in a matrix material. The algorithms are extensions of the proposal of Zimmerman et al., using chord-length sampling (CLS) to eliminate the need to explicitly model the geometry of the mixture. Two variations are considered. The first algorithm uses CLS for both material regions. The second algorithm employs limited CLS (LCLS), using only CLS in the matrix material. Ensemble-average leakage results are computed for a range of material interaction coefficients and compared against benchmark results for both accuracy and efficiency. Both algorithms are exact for purely absorbing materials and provide decreasing accuracy as scattering is increased in the matrix material. The LCLS algorithm shows a better accuracy than the CLS algorithm for all cases while maintaining an equivalent or better efficiency. Accuracy and efficiency problems with the CLS algorithm are due principally to assumptions made in determining the chord-length distribution within the disks.

Self-powered micro-structured solid state neutron detector with very low leakage current and high efficiency

We report on the design, fabrication, and performance of solid-state neutron detector based on three-dimensional honeycomb-like silicon micro-structures. The fabricated detectors use boron filled deep holes with aspect ratio of over 12 and showed a very low leakage current density of ∼7 × 10−7 A/cm2 at −1 V for device sizes varying from 2 × 2 to 5 × 5 mm2. A thermal neutron detection efficiency of 4.5% ± 0.5% with discrimination setting of 500 keV and gamma to neutron sensitivity of (1.1 ± 0.1) × 10−5 for single layer was measured without external bias for these devices. Monte-Carlo simulation predicts a maximum efficiency of 45% for such devices filled with 95% enriched 10boron.

Characterizing tantalum sputtered coatings on steel by using eddy currents

With the goal of building a system for fast inspection of coatings, we have developed a method that uses induced eddy currents to characterize tantalum alpha and beta phases in a layer of thin sputtered tantalum on steel. The detection of the tantalum phases is based on the large difference in electrical conductivity between them. Measurements based on the method agree well with values based on theoretical calculations. We applied the method in a two-probe differential system having higher sensitivity and less noise than a one-probe system. The probe uses pulsed eddy currents with a pulsewidth of 1 /spl mu/s, allowing us to scan at rates of up to 10/sup 5/ pulses per second on a computer-controlled XY table for fast data acquisition. When the system was used to scan steel samples coated with 12.5-30 /spl mu/m of tantalum, a clear difference between alpha and beta phases was observed. The system was also used to measure the conductivity of the alpha and beta phases. We present here a conductivity map of the sample.

Design and construction of a thermal neutron target for the RPI linac

Abstract To perform thermal cross section measurements the low energy neutron intensity from the RPI linac facility was increased. A new Enhanced Thermal Target (ETT) was designed, constructed and tested. The thermal flux of the new target was up to six times greater than the flux from the previous RPI Bounce Target (BT). This additional gain allows transmission measurements to be performed in the energy range of 0.001 to 15 eV with high statistical accuracy in a short time (∼ 40 h). The ETT was also designed to be coupled to a cold moderator that will give an additional flux increase factor of about 9 below 3 meV. Design calculations for the cold moderator including neutronics and cryogenics are also given.

IMPLEMENTATION OF CHORD LENGTH SAMPLING FOR TRANSPORT THROUGH A BINARY STOCHASTIC MIXTURE

Neutron transport through a special case stochastic mixture is examined, in which spheres of constant radius are uniformly mixed in a matrix material. A Monte Carlo algorithm previously proposed and examined in 2-D has been implemented in a test version of MCNP. The Limited Chord Length Sampling (LCLS) technique provides a means for modeling a binary stochastic mixture as a cell in MCNP. When inside a matrix cell, LCLS uses chord-length sampling to sample the distance to the next stochastic sphere. After a surface crossing into a stochastic sphere, transport is treated explicitly until the particle exits or is killed. Results were computed for a simple model with two different fixed neutron source distributions and three sets of material number densities. Stochastic spheres were modeled as black absorbers and varying degrees of scattering were introduced in the matrix material. Tallies were computed using the LCLS capability and by averaging results obtained from multiple realizations of the random geometry. Results were compared for accuracy and figures of merit were compared to indicate the efficiency gain of the LCLS method over the benchmark method. Results show that LCLS provides very good accuracy if the scattering optical thickness of the matrix is small (≤1). Comparisons of figures of merit show an advantage to LCLS varying between factors of 141 and 5. LCLS efficiency and accuracy relative to the benchmark both decrease as scattering is increased in the matrix.

Electrostatics of pyroelectric accelerators

Derivations for equations for calculating the potential and field strength in both single-crystal and two-crystal pyroelectric accelerators are presented. Such expressions for the single-crystal system are well established in the literature, but with cursory derivations. We provide a rigorous derivation of the single-crystal system and expand upon this physical understanding to derive expressions for the potential and field in a two-crystal system. The expressions are verified with finite element modeling and compared with experimental results. This allows for better understanding of pyroelectric accelerators.

Passive electrical model of silicon photomultipliers

An electrical model is developed to simulate, characterize, and predict the response of SSPM detectors for different device geometries and measurement circuit configurations. In particular, the model allows investigation of the effects of increasing parasitic capacitance with increasing diode area on the timing and magnitude of the readout signal. Passive components in the model are extracted from measurements and then used in the model to understand and predict device performance. The avalanche is represented with a switch in series with a voltage source and diode resistor, instead of a current source. This approach allows the change in potential, current through the diode, and timing of the avalanche to be simulated. Experimental and modeled pulses are compared for two different size devices. The model is first developed and validated using the 1×1 mm2 device. Predictive capability is demonstrated with the 3×3 mm2 device; in the scaling-up of the devices, only expected model parameters are changed, and the experimental and modeled pulses are in good agreement. The current through the diode and voltage change across the diode as functions of time agree with expectations.