Steven L. Hunter

Cesium hafnium chloride: A high light yield, non-hygroscopic cubic crystal scintillator for gamma spectroscopy

We report on the scintillation properties of Cs2HfCl6 (cesium hafnium chloride or CHC) as an example of a little-known class of non-hygroscopic compounds having the generic cubic crystal structure of K2PtCl6. The crystals are easily growable from the melt using the Bridgman method with minimal precursor treatments or purification. CHC scintillation is centered at 400 nm, with a principal decay time of 4.37 μs and a light yield of up to 54 000 photons/MeV when measured using a silicon CCD photodetector. The light yield is the highest ever reported for an undoped crystal, and CHC also exhibits excellent light yield nonproportionality. These desirable properties allowed us to build and test CHC gamma-ray spectrometers providing energy resolution of 3.3% at 662 keV.

Development of Transparent Ceramic Ce-Doped Gadolinium Garnet Gamma Spectrometers

Transparent polycrystalline ceramic scintillators based on the garnet structure and incorporating gadolinium for high stopping power are being developed for use in gamma spectrometers. Optimization of energy resolution for gamma spectroscopy involves refining the material composition for high stopping and high light yield, developing ceramics fabrication methodology for material homogeneity, as well as selecting the size and geometry of the scintillator to match the photodetector characteristics and readout electronics. We have demonstrated energy resolution of 4% at 662 keV for 0.05 cm3 GYGAG(Ce) ceramics with photodiode readout, and 4.9% resolution at 662 keV for 18 cm 3 GYGAG(Ce) ceramics and PMT readout. Comparative gamma spectra acquired with GYGAG(Ce) and NaI(Tl) depict the higher resolution of GYGAG(Ce) for radioisotope identification applications. Light yield non-proportionality of garnets fabricated following different methods reveal that the fundamental shapes of the light yield dependence on energy are not intrinsic to the crystal structure, but may instead depend on trap state distributions. With exposure to 9 MeV Brehmsstrahlung radiation, we also find that GYGAG(Ce) ceramics exhibit excellent radiation hardness.

Deep hydraulic fracture imaging: Recent advances in tiltmeter technologies

Abstract Significant advancements in tiltmeter technologies has improved our ability to map the geometry of hydraulic fractures at great depth. This Department of Energy sponsored effort with widespread industry collaboration has extended the capability of near-surface tiltmeters to image details of the hydraulic fracture process at depths approaching 10,000 feet. Improvements in the analog-digital circuitry, the data-logger, and the remote mechanical leveling device has enabled us to install tiltmeters in relatively deep holes where the degradation affects due to the natural-cultural noise sources on the tilt-signal are minimized. Another benefit of utilizing this technology in the oil and gas industry is that it provides valuable information on the geometry of these hydraulic fractures, which is important for production field design. Modeling tilmeter data with algorithms that take into account details of the fracture growth process and the local geology are currently being developed, especially since recent results indicate that signal strength from deeper hydraulic fractures may be stronger than previous models had suggested.

Nonproportionality of Scintillator Detectors. III. Temperature Dependence Studies

This paper is the third in a series of articles on the basic physics of nonproportionality in scintillators. Here, we focus on the temperature dependence of six scintillators, NaI(Tl), CsI(Tl), CsI(Na), CeBr₃, LaBr₃(Ce), and undoped SrI₂, and report their nonproportionality curves at -40^°C, 0^°C and + 40^°C. We fit the data to a modified form of our previously employed model, including the competition of carrier trapping with the Onsager-mediated attraction between electrons and holes.

Development of a real-time radiological area monitoring network for emergency response at Lawrence Livermore National Laboratory

A real-time radiological sensor network for emergency response was developed and deployed at the Lawrence Livermore National Laboratory (LLNL). The real-time radiological area monitoring (RTRAM) network comprises 16 Geiger-Mueller sensors positioned on the LLNL Livermore site perimeter to continuously monitor for a radiological condition resulting from a terrorist threat to site security and the health and safety of LLNL personnel. The RTRAM network sensor locations coincide with wind sector directions to provide thorough coverage of the one-square-mile site. These low-power sensors are supported by a central command center (CCC) and transmit measurement data back to the CCC computer through the LLNL telecommunications infrastructure. Alarm conditions are identified by comparing current data with predetermined threshold parameters and are validated by comparison with plausible dispersion modeling scenarios and prevailing meteorological conditions. Emergency response personnel are notified of alarm conditions by automatic radio- and computer-based notifications. A secure intranet provides emergency response personnel with current condition assessment data that enable them to direct field response efforts remotely. The RTRAM network has proven to be a reliable system since initial deployment in August 2001, and it maintains stability during inclement weather conditions.

Nonproportionality of Scintillator Detectors. V. Comparing the Gamma and Electron Response

This paper is the fifth in a series of articles on the basic physics of light yield nonproportionality in scintillators. Here, we compare and contrast the nonproportionality as registered by gamma rays and high-energy electrons. As has been noted in the past, these two types of data have different curve shapes (for plots of the light yield against electron or gamma energy). Herein, we show how the experimental gamma nonproportionality curve can be calculated from the electron response by accounting for the distribution of high energy electrons created by the gamma photon via the photoelectric interaction. Similarly, we measure and model the gamma-induced resolution as a function of energy and compare this data to predictions from our model. The utility of the model is explored using data acquired with the scintillators SrI2(Eu), GYGAG(Ce) and CsI(Na).

Bismuth-loaded plastic scintillators for gamma spectroscopy and neutron active interrogation

Bismuth-loaded plastic scintillators capable of gamma spectroscopy are being scaled up to multiple cubic inch sizes. High light yields of >38,888 Ph/MeV are obtained with Iridium-complex fluors due to efficient harvesting of both singlet and triplet excitons, providing energy resolution of ~10% at 662 keV for 3 in3 scintillators. Although singlet fluors provide a poorer light yield of ~12,888 Ph/MeV and resolution at 662 keV of 14% in the Bismuth-loaded plastics, the fast decay time of <;100 ns and the low neutron capture cross-sections of the constituent elements lend themselves to applications in neutron active interrogation. Measurements of the scintillation light yield non-proportionality reveal that exciton-exciton annihilation is suppressed in Iridium-complex activated plastic, in contrast with the singlet fluor activated plastic.

History and current status of strontium iodide scintillators

Eu-doped strontium iodide single crystal growth has reached maturity and prototype SrI2(Eu)-based gamma ray spectrometers provide detection performance advantages over standard detectors. SrI2(Eu) offers a high, proportional light yield of >80,000 photons/MeV. Energy resolution of <3% at 662 keV with 1.5” x 1.5” SrI2(Eu) crystals is routinely achieved, by employing either a small taper at the top of the crystal or a digital readout technique. These methods overcome light-trapping, in which scintillation light is re-absorbed and re-emitted in Eu2+-doped crystals. Its excellent energy resolution, lack of intrinsic radioactivity or toxicity, and commercial availability make SrI2(Eu) the ideal scintillator for use in handheld radioisotope identification devices. A 6-lb SrI2(Eu) radioisotope identifier is described.

Solution-Grown Rubrene Crystals as Radiation Detecting Devices

There has been increased interest in organic semiconductors over the last decade because of their unique properties. Of these, 5, 6, 11, 12-tetraphenylnaphthacene (rubrene) has generated the most interest because of its high charge carrier mobility. In this work, large single crystals with a volume of ~1 cm3 were grown from solution by a temperature reduction technique. The faceted crystals had flat surfaces and cm-scale, visually defect-free areas suitable for physical characterization. X-ray diffraction analysis indicates that solvent does not incorporate into the crystals and photoluminescence spectra are consistent with pristine, high-crystallinity rubrene. Furthermore, the response curve to pulsed optical illumination indicates that the solution grown crystals are of similar quality to those grown by physical vapor transport, albeit larger. The good quality of these crystals in combination with the improvement of electrical contacts by application of conductive polymer on the graphite electrodes have led to the clear observation of alpha particles with these rubrene detectors. Preliminary results with a 252Cf source generate a small signal with the rubrene detector and may demonstrate that rubrene can also be used for detecting high-energy neutrons.

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.