Arnold Burger

Infrared absorption and fluorescence properties of Holmium doped Potassium Lanthanum Chloride (Conference Presentation)

The development of new solid-state laser materials for mid-infrared (mid-IR) laser sources continues to be interest for potential applications in remote sensing of bio-chemical agents, IR countermeasures, and IR spectroscopy. Fluorescent materials based on Ho3+ doped crystals and glasses with narrow phonon spectra cover a wide wavelength range between ~1-4 µm. In this work, spectroscopic characterization on infrared emission properties of trivalent holmium (Ho3+) doped potassium lanthanum chloride (K2LaCl5) were explored. K2LaCl5 is slightly hygroscopic but possesses a maximum phonon energy of 235 cm-1. The low maximum phonon energy of K2LaCl5 leads to low non-radiative decay rates and efficient IR fluorescence. The studied Ho3+ doped K2LaCl5 material was grown by Bridgman technique. Using ~900 nm excitation, IR emissions centered at ~1.66, ~1.995, and ~3.90 µm were observed from Ho:K2LaCl5 corresponding to the 5I5-->5I7, 5I7-->5I8, and 5I5-->5I6 transitions of Ho3+ ions. Spectroscopic results and data modeling including the Stark level energies, Judd-Ofelt analysis, transitions cross-sections, and fluorescence dynamics will be presented at the conference.

Arrays of position sensitive virtual Frisch-grid detectors (Conference Presentation)

We present new results from testing a small array of position-sensitive virtual Frisch-grid gamma-ray detectors. Such arrays provide high-detection efficiency, excellent energy and position resolution. They can be used in compact hand-held instruments or in large-area gamma ray imaging cameras. The high granularity position sensing enables these detectors to correct the response non-uniformity caused by crystal defects. This important feature allows one to achieve high detection performance while using standard-grade (unselected) CZT crystals, which is expected to reduce the overall cost of field deployable high-resolution CZT gamma ray detection instruments. Here, we report the results of testing several array prototypes with configurations designed for different applications.

Neutron detector development for microsatellites

We present a preliminary design for a novel neutron detection system that is compact, lightweight, and low power consuming, utilizing the CubeSat platform making it suitable for space-based applications. This is made possible using the scintillating crystal lithium indium diselenide (6LiInSe2), the first crystal to include 6Li in the crystalline structure, and a silicon avalanche photodiode (Si-APD). The schematics of this instrument are presented as well as the response of the instrument to initial testing under alpha, gamma and neutron radiation. A principal aim of this work is to demonstrate the feasibility of such a neutron detection system within a CubeSat platform. The entire end-to-end system presented here is 10 cm x 10 cm x 15 cm, weighs 670 grams and requires 5 V direct current at 3 Watts.

Plastic scintillators for gamma spectroscopy and neutron radiography (Conference Presentation)

Plastic scintillators are widely deployed for ionizing radiation detection, as they can be fabricated in large sizes, for high detection efficiency. However, commercial plastics are limited in use for gamma spectroscopy, since their photopeak is very weak, due to low Z, and they are also limited in use for neutron detection, since proton recoils are indistinguishable from other ionizing radiation absorption events in standard plastics. We are working on scale up and production of transparent plastic scintillators based on polyvinyltoluene (PVT) loaded bismuth metallorganics for gamma spectroscopy. When activated with standard organic fluors, PVT scintillators containing 8 wt% bismuth provide energy resolution of 11% at 662 keV. When Iridium complex fluors are used, we can load plastics up to 20 wt% bismuth, while obtaining energy resolution of 10% at 662 keV. Another formulation, activated with Ir fluors for use as neutron radiography scintillator may be used for high energy neutron radiography.nAcknowledgementsnThis work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and has been supported by the US DOE National Nuclear Security Administration, Defense Nuclear Nonproliferation Research and Development under Contract No. DE-AC03-76SF00098

Cs2LiCeCl6: An intrinsic scintillator for dual gamma and neutron detector applications (Conference Presentation)

Intrinsic materials can offer advantages over doped materials for some important applications. The doped material might suffer from non-uniform distribution of the dopant, such as fine-scale striations and larger scale segregation, which might affect the overall device response, especially for large-volume detectors such as those in demand for homeland security applications for gamma spectroscopy. Cs2LiCeCl6 (CLCC), being an intrinsic scintillator, can be grown in large volume to produce large detectors with good performance, provided the crystals are free from unwanted scattering centers. CLCC belongs to the elpasolite family and the structure is cubic, so large-volume ingots can be grown without the strains resulting from anisotropic thermal expansion coefficients. In this presentation, we will discuss extensive material characterization and device response of CLCC for gamma and thermal neutron detector applications.

Sub-pixel resolution in pixelated CdZnTe gamma ray detectors with different pixel sizes (0.5 mm to 1.72 mm) using a focused laser beam (Conference Presentation)

High-resolution position-sensing has been proposed to correct response non-uniformities in Cadmium Zinc Telluride (CZT) gamma ray detectors by virtually subdividing the area into small voxels and equalizing responses from each voxel. 3D pixelated detectors coupled with multichannel readout electronics are the most advanced type of CZT devices offering many options in signal processing and enhancing detector performance. The main hurdle in achieving high sub-pixel position resolution is the relatively low signal induced on the neighboring pixels because of the electrostatic shielding effect caused by the collecting pixel. In addition, to achieve high position sensitivity one should rely on time-correlated transient signals, which means that digitized output signals must be used. Previous results have shown the benefit of using a focused laser beam to study position resolution in 3D pixelated detectors. We present the results of our studies to measure the amplitude of the pixel signals so that these can be used to measure positions of the interaction points. This is done with the processing of digitized correlated time signals measured from several adjacent pixels taking into account rise-time and charge-sharing effects. In these measurements we used a focused pulsed laser to generate a 10-micron beam at one milliwatt (650-nm wavelength) over the detector surface while the collecting pixel was moved in cardinal directions. The results include measurements that present the benefits of combining conventional pixel geometry with digital pulse processing for the best approach in achieving sub-pixel position resolution with different pixel dimensions ranging from 0.5 mm to 1.72 mm.

Recent advances in garnet scintillator gamma spectrometers (Conference Presentation)

Gadolinium Garnet transparent ceramics doped with Ce, ((Gd,Y,Ce)3(Ga,Al)5O12), for gamma-ray spectroscopy provide high density, high light yield, high energy resolution , high Z, mechanical robustness, and they are unreactive to air and water. Gadolinium garnet single crystals are costly to grow, due to their high melting points, and suffer from non-uniform light yield, due to Ce segregation. In contrast, transparent polycrystalline ceramic Garnets are never melted, and therefore are less costly to produce and provide the uniform light yield required to achieve high energy resolution with a scintillator. nnGYGAG(Ce) transparent ceramics offer energy resolution as good as R(662 keV) = 3.5%, in a pixelated detector utilizing Silicon photodiode array readout. We have developed a modular handheld detector based on pixelated GYGAG(Ce) on a photodiode array, that offers directional detection for point source detection as well as gamma spectroscopy. Individual modules can be assembled into detectors ranging from pocket-size to large panels, for a range of applications.nnLarge GYGAG(Ce) transparent ceramics in the 2-5 in3 size range have been fabricated at LLNL. Instrumentation of these ceramics with Silicon photomultipliers (SiPMs) and super bi-alkali PMTs has been explored and energy resolution as good as R(662 keV) = 5% has been obtained. Further improvements with SiPM readout will leverage their high quantum efficiency in the 500-650 nm range where GYGAG(Ce) emits, and implement electronics that minimize the effect of SiPM dark current and capacitance on the pulse height spectra. nnnThis work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and has been supported by the US Department of Homeland Security, Domestic Nuclear Detection Office, under competitively awarded IAA HSHQDC-12-X-00149 under Contract No. DE-AC03-76SF00098. LLNL-ABS-724480.

Crystal growth of CdZnTeSe (CZTS) gamma detectors: a promising alternative to CdZnTe (Conference Presentation)

Alloying of CdZnTe (CZT) with selenium has been found to be very promising and effective in reducing the overall concentration of secondary phases (Te precipitates/inclusions) and sub-grain boundary networks in the crystals. These two types of defects are the main causes for incomplete charge collection, and hence they affect the yield of high-quality CZT, resulting in a very high cost for large-volume, high-quality detector-grade CZT detectors. The addition of selenium was also found to very effective in increasing the compositional homogeneity along the growth direction of the CdZnTeSe (CZTS) ingots grown by the traveling heater method (THM) technique. The compositional homogeneity along the growth direction can enhance the overall yield of detector-grade CZTS, which should therefore be possible to produce at a lower cost compared to CZT. The electrical properties and detector performance of the CZTS crystals will be presented and discussed.