T. Niedermayr

Scintillators With Potential to Supersede Lanthanum Bromide

New scintillators for high-resolution gamma ray spectroscopy have been identified, grown and characterized. Our development efforts have focused on two classes of high-light-yield materials: europium-doped alkaline earth halides and cerium-doped garnets. Of the halide single crystals we have grown by the Bridgman method-SrI2, CaI2, SrBr2, BaI2 and BaBr2-SrI2 is the most promising. SrI2(Eu) emits into the Eu2+ band, centered at 435 nm, with a decay time of 1.2 mus and a light yield of up to 115,000 photons/MeV. It offers energy resolution better than 3% FWHM at 662 keV, and exhibits excellent light yield proportionality. Transparent ceramic fabrication allows the production of gadolinium- and terbium-based garnets which are not growable by melt techniques due to phase instabilities. The scintillation light yields of cerium-doped ceramic garnets are high, 20,000-100,000 photons/MeV. We are developing an understanding of the mechanisms underlying energy dependent scintillation light yield non-proportionality and how it affects energy resolution. We have also identified aspects of optical design that can be optimized to enhance the energy resolution.

Design of a multichannel ultra-high-resolution superconducting gamma-ray spectrometer

Superconducting Gamma-ray microcalorimeters operated at temperatures around ~0.1 K offer an order of magnitude improvement in energy resolution over conventional high-purity Germanium spectrometers. The calorimeters consist of a ~1 mm3 superconducting or insulating absorber and a sensitive thermistor, which are weakly coupled to a cold bath. Gamma-ray capture increases the absorber temperature in proportion to the Gamma-ray energy, this is measured by the thermistor, and both subsequently cool back down to the base temperature through the weak link. We are developing ultra-high-resolution Gamma-ray spectrometers based on Sn absorbers and superconducting Mo/Cu multilayer thermistors for nuclear non-proliferation applications. They have achieved an energy resolution between 60 and 90 eV for Gamma-rays up to 100 keV. We also build two-stage adiabatic demagnetization refrigerators for user-friendly detector operation at 0.1 K. We present recent results on the performance of single pixel Gamma-ray spectrometers, and discuss the design of a large detector array for increased sensitivity.

Electronic sputtering of solids by slow, highly charged ions: Fundamentals and applications

Electronic sputtering in the interaction of slow (v < vBohr), highly charged ions (SHCI) with solid surfaces has been subject of controversial discussions for almost 20 years. We review results from recent studies of total sputtering yields and discuss distinct microscopic mechanisms (such as defect mediated desorption, Coulomb explosions and eAects of intense electronic excitation) in the response of insulators and semiconductors to the impact of SHCI. We then describe an application of ions like Xe 44a and Au 69a as projectiles in time-of-flight secondary ion mass spectrometry for surface characterization of semiconductors. ” 2000 Elsevier Science B.V. All rights reserved.

First-generation hybrid compact Compton imager

At Lawrence Livermore National Laboratory, we are pursuing the development of a gamma-ray imaging system using the Compton effect. We have built our first generation hybrid Compton imaging system, and we have conducted initial calibration and image measurements using this system. In this paper, we present the details of the hybrid Compton imaging system and initial calibration and image measurements

Barium iodide single-crystal scintillator detectors

We find that the high-Z crystal Barium Iodide is readily growable by the Bridgman growth technique and is less prone to crack compared to Lanthanum Halides. We have grown Barium Iodide crystals: undoped, doped with Ce3+, and doped with Eu2+. Radioluminescence spectra and time-resolved decay were measured. BaI2(Eu) exhibits luminescence from both Eu2+ at 420 nm (~450 ns decay), and a broad band at 550 nm (~3 μs decay) that we assign to a trapped exciton. The 550 nm luminescence decreases relative to the Eu2+ luminescence when the Barium Iodide is zone refined prior to crystal growth. We also describe the performance of BaI2(Eu) crystals in experimental scintillator detectors.

The effects of radiation on (1,3,5 - triamino - 2,4,6 - trinitrobenzene) TATB studied by time-of-flight secondary ion mass spectrometry

Abstract Highly Charged Ion (HCI) Time-of-Flight (TOF) Secondary Ion Mass Spectrometry (SIMS) has been employed to analyze the changes in the surface composition of TATB caused by low energy electron, ultraviolet, and Gamma ray irradiation. Comparisons are made between canary yellow (not irradiated) TATB and TATB that has been “greened” by exposure to radiation. We ascribe the color change from yellow to green to a loss of oxygen. Another striking aspect of this study is the presence of a feature at m/z = 30 (NO+) for highly charged ion SIMS, which does not occur in singly charged ion TOF SIMS.

A multichannel superconducting tunnel junction detector for high-resolution X-ray spectroscopy of magnesium diboride films

We have built a high-resolution soft X-ray spectrometer using a 3 /spl times/ 3 array of superconducting Nb-Al-AlO/sub x/-Al-Nb tunnel junctions. The detector is cooled to /spl sim/0.1 K by a two-stage adiabatic demagnetization refrigerator while being held at the end of a 40-cm-long detector snout that can be inserted into an experimental chamber. The 0.6 mm /spl times/ 0.6 mm array has an energy resolution of 10-15 eV FWHM for X-ray energies below 1 keV and total count rate capabilities above 100 000 counts/s. We demonstrate the capabilities of the instrument in a study of anisotropy in c-axis oriented MgB/sub 2/ films by near edge X-ray absorption spectroscopy. The absorption fine structure at the B K-edge differs for excitation with X-rays polarized along the c-axis and in the ab-plane of the MgB/sub 2/ film. We discuss the implications of the anisotropy for the density of states in MgB/sub 2/ at the Fermi surface.

Imaging performance of the Si/Ge hybrid Compton imager

The point spread function (PSF) of a fully-instrumented silicon/germanium Compton telescope has been measured as a function of energy and angle. Overall, the resolution was 3deg to 4deg FWHM over most of the energy range and field of view. The various contributions to the resolution have been quantified. These contributions include the energy and position uncertainty of the detector; source energy; Doppler broadening; and the 1/r broadening characteristic of Compton back-projection. Furthermore, a distortion of the PSF is observed for sources imaged off-axis from the detector. These contributions are discussed and compared to theory and simulations

A multichannel cryogenic detector system for synchrotron-based x-ray spectroscopy

Fluorescence-detected x-ray absorption spectroscopy probes the fine structure of electronic energy levels with sub-eV resolution by scanning a monochromatic synchrotron beam through the corresponding absorption edge and measuring the intensity of the resulting x-ray fluorescence. For dilute samples, grating spectrometers lack the detection efficiency and conventional Si(Li) or Ge detectors often lack the energy resolution to separate the weak fluorescence signal from strong nearby emission lines. We have built a high-resolution, high-efficiency cryogenic detector system for synchrotron-based soft x-ray spectroscopy. The sensor is a 3×3 array of 200 μm×200 μm superconducting Nb-Al-AlOx-Al-Nb tunnel junctions with an energy resolution of ≈15 eV below 1 keV and a total count rate capability of ≈100,000 counts/second. This sensor array is cooled to below 0.4 K by a two-stage adiabatic demagnetization refrigerator while held at the end of a 40-cm-long cold finger that can be inserted into a UHV sample chamber ...