Kiril Dimitrov Ianakiev

Development of high efficiency, multi-element CdZnTe detectors for portable measurement applications

We describe the development of detector arrays and electronics for large-volume, hand-held CdZnTe detectors with the same counting efficiency as portable NaI(Tl) detectors presently used for nuclear material measurement applications. The pulse-height resolution of the multi-element detectors is at least three times better than NaI(Tl) over a wide energy range (from 100 keV to several MeV), enabling more accurate measurements of gamma-rays emitted by special nuclear material. Arrays of up to eight coplanar grid detectors can be combined to make detectors ranging in size from 4 to 14 cm3. Because the number of spectroscopy channels is small, low-power, hand-held detectors can be manufactured with conventional printed circuit board technology, thus keeping the cost of multi-element detectors to a minimum. The design and performance of an 8-element detector is presented.

Effect of differential bias on the transport of electrons in coplanar grid CdZnTe detectors

Segmented and pixilated electrode structures are used to compensate for poor hole transport in CdZnTe devices used for gamma-ray spectrometry and imaging. Efforts to model these structures have focused primarily on geometric effects; however, device performance also depends on the physical properties of the bulk and surface material, as well as the electrodes. In this paper, we describe experiments to characterize the electric field near the anode of a coplanar grid detector. The experiment is contrasted with a calculation that is based on an assumption commonly used to reduce the computational effort required to determine internal electric fields. Explanations for differences between the calculation and the experiment are proposed.

Characterization of a large-volume multi-element CdZnTe detector

In this paper, we present results of experiments to characterize a large-volume (4 cm3), multi-element CdZnTe detector for high-efficiency, gamma-ray spectroscopy. The module includes an array of eight 0.5 cm3 coplanar grid detectors manufactured by eV Products. An eight-channel data acquisition system with list mode output is used to record gamma-ray events for each detector in the array. The list mode data are analyzed to determine the efficiency for coincidence events and to demonstrate different modes of operation (e.g., Compton suppression). The total efficiency of the array is found to match Monte Carlo calculations to within a few percent; however, the full-energy (photopeak) efficiency is significantly lower than predicted by Monte Carlo. The observed difference is probably caused by a combination of electrode design and charge transport properties. Approaches to improve full-energy efficiency are proposed.

Branching transport model of NaI(Tl) alkali-halide scintillator

We measure the time dependence of the scintillator light-emission pulses in NaI(Tl) crystals at different temperatures, after activation by gamma rays. We confirm that there are two main nonexponential components to the time decay and find that their amplitude ratio shows Arrhenius temperature dependence. We explain these nonexponential components as arising from two competing mechanisms of carrier transport to the Tl activation levels. The total light output of the NaI(Tl) detectors shows a linear temperature dependence explained by our model.

Laser-plasmas in the relativistic-transparency regime: Science and applications

Laser-plasma interactions in the novel regime of relativistically induced transparency (RIT) have been harnessed to generate intense ion beams efficiently with average energies exceeding 10 MeV/nucleon (>100 MeV for protons) at “table-top” scales in experiments at the LANL Trident Laser. By further optimization of the laser and target, the RIT regime has been extended into a self-organized plasma mode. This mode yields an ion beam with much narrower energy spread while maintaining high ion energy and conversion efficiency. This mode involves self-generation of persistent high magnetic fields (∼104 T, according to particle-in-cell simulations of the experiments) at the rear-side of the plasma. These magnetic fields trap the laser-heated multi-MeV electrons, which generate a high localized electrostatic field (∼0.1 T V/m). After the laser exits the plasma, this electric field acts on a highly structured ion-beam distribution in phase space to reduce the energy spread, thus separating acceleration and energy-spread reduction. Thus, ion beams with narrow energy peaks at up to 18 MeV/nucleon are generated reproducibly with high efficiency (≈5%). The experimental demonstration has been done with 0.12 PW, high-contrast, 0.6 ps Gaussian 1.053 μm laser pulses irradiating planar foils up to 250 nm thick at 2–8 × 1020 W/cm2. These ion beams with co-propagating electrons have been used on Trident for uniform volumetric isochoric heating to generate and study warm-dense matter at high densities. These beam plasmas have been directed also at a thick Ta disk to generate a directed, intense point-like Bremsstrahlung source of photons peaked at ∼2 MeV and used it for point projection radiography of thick high density objects. In addition, prior work on the intense neutron beam driven by an intense deuterium beam generated in the RIT regime has been extended. Neutron spectral control by means of a flexible converter-disk design has been demonstrated, and the neutron beam has been used for point-projection imaging of thick objects. The plans and prospects for further improvements and applications are also discussed.

Pulse Shape Discrimination Properties of Neutron-Sensitive Organic Scintillators

The new plastic scintillators with n/γ pulse shape discrimination (PSD) properties being developed by the Lawrence Livermore National Laboratory (LLNL) and commercialized by Eljen Technology are addressing the toxicity and flammability issues of liquid scintillators, thus enabling a much wider range of practical applications for the detection of neutrons. These scintillation materials use multiple dyes, the concentration of which can vary, and therefore the light output and PSD properties of these new materials are expected to vary as well. In this paper, we compare the light signal time profiles of a liquid scintillator and two samples (one from LLNL and one from Eljen Technology) of new plastic scintillators with PSD properties. We acquired the light signal time profiles using both γ sources (60Co, 137Cs, 241Am) and neutrons calibrated in electron-equivalent by the gamma sources. The n/γ PSD properties for time profiles collected are analyzed and discussed with respect to charge integration time.

Detection of Special Nuclear Material by means of promptly emitted radiation following photonuclear stimulation

Techniques have been developed to exploit abundant prompt emissions from photonuclear reactions for the identification of special nuclear material (SNM). These enhancements are designed to reduce inspections times and delivered dose in systems which have, historically, relied solely on delayed emissions. Experimental evidence is presented for prompt neutron time-of-flight measurements, neutron/photon correlations in multiple detectors, and novel detector development, specifically LaBr3 scintillators with new gating and buffering circuits to identify prompt gamma signatures. Significant and specific signatures indicative of the presence of SNM can be distinguished for the prompt neutron time-of-flight experiment and the neutron/photon correlations in multiple detectors.

New generation enrichment monitoring technology for gas centrifuge enrichment plants

We report our progress toward development of new generation on-line enrichment monitoring technology for UF6 gas centrifuge plants based on a transmission source and a NaI spectrometer. We use an X-ray tube with transmission filters instead of a decaying isotopic transmission source to eliminate the costly replacement of this source. The UF6 gas density measurement is based on the energy dependency of the mass attenuation for two characteristic X-ray lines generated by the transmission filters. An analytical expression for the UF6 density is derived and criteria for the selection of transmission energies are discussed. Because of the differential method of measurement, the UF6 gas density does not depend on the intensity of the X-ray source. We describe a design of a sealed UF6 gas test stand for development testing and calibration of various on-line enrichment monitoring instruments. The sealed source is intended to replace a UF6 gaseous loop currently used for calibration.

Large area neutron detector based on /sup 6/Li ionization chamber with integrated body-moderator of high density polyethylene

We describe the development of a large area neutron detector based on a pulse-mode ionization chamber lined with /sup 6/Li foil in a low-cost housing of HDPE (high density polyethylene) that serves as the moderator. The use of inexpensive materials, technology for mass production and simple construction, which integrates the basic components of the sensor together, results in a substantial cost reduction. For a given sensor size, detection efficiency, or counting time, the proposed detector is expected to provide an order-of magnitude cost reduction compared to current neutron detection technology. This paper describes the detection concept including the optimization of the design using neutron transport calculations. It also gives experimental results on some prototype units using neutron and gamma sources. The properties affecting long term counting stability are discussed. An approach to reduce the effect of outgassing from the HDPE body is suggested.

Development of a model of an X-ray tube transmission source

In support of the development of an x-ray tube based source for transmission measurements of UF6 gas, we have developed a one-dimensional, spreadsheet-based model of the source. Starting with the spectrum produced by an x-ray tube we apply the linear attenuation coefficients for various notch filters, the aluminum pipe, and UF6 gas. This model allows calculation of the transmitted spectrum based on the type of filter, the thickness of the filter, the x-ray tube high voltage, the Al pipe thickness, and the UF6 gas pressure. The sensitivity of the magnitude of the transmission peak produced by the notch filter to any of these variables can be explored quickly and easily to narrow the choices for experimental measurements. To validate the spreadsheet based model, comparisons have been made to various experimental data.