Rastgo Hawrami

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.

Results for aliovalent doping of CeBr3 with Ca2

Despite the outstanding scintillation performance characteristics of cerium tribromide (CeBr3) and cerium-activated lanthanum tribromide, their commercial availability and application are limited due to the difficulties of growing large, crack-free single crystals from these fragile materials. This investigation employed aliovalent doping to increase crystal strength while maintaining the optical properties of the crystal. One divalent dopant (Ca2+) was used as a dopant to strengthen CeBr3 without negatively impacting scintillation performance. Ingots containing nominal concentrations of 1.9% of the Ca2+ dopant were grown. Preliminary scintillation measurements are presented for this aliovalently doped scintillator. Ca2+-doped CeBr3 exhibited little or no change in the peak fluorescence emission for 371 nm optical excitation for CeBr3. The structural, electronic, and optical properties of CeBr3 crystals were studied using the density functional theory within the generalized gradient approximation. The calculated lattice parameters are in good agreement with the experimental data. The energy band structures and density of states were obtained. The optical properties of CeBr3, including the dielectric function, were calculated.Despite the outstanding scintillation performance characteristics of cerium tribromide (CeBr3) and cerium-activated lanthanum tribromide, their commercial availability and application are limited due to the difficulties of growing large, crack-free single crystals from these fragile materials. This investigation employed aliovalent doping to increase crystal strength while maintaining the optical properties of the crystal. One divalent dopant (Ca2+) was used as a dopant to strengthen CeBr3 without negatively impacting scintillation performance. Ingots containing nominal concentrations of 1.9% of the Ca2+ dopant were grown, i.e., 1.9% of the CeBr3 molecules were replaced by CaBr2 molecules, to match our target replacement of 1 out of 54 cerium atoms be replaced by a calcium atom. Precisely the mixture was composed of 2.26 g of CaBr2 added to 222.14 g of CeBr3. Preliminary scintillation measurements are presented for this aliovalently doped scintillator. Ca2+-doped CeBr3 exhibited little or no change in the...

SrI2 scintillator for gamma ray spectroscopy

We are working to perfect the growth of divalent Eu-doped strontium iodide single crystals and to optimize the design of SrI2(Eu)-based gamma ray spectrometers. SrI2(Eu) offers a light yield in excess of 100,000 photons/MeV and light yield proportionality surpassing that of Ce-doped lanthanum bromide. Thermal and x-ray diffraction analyses of SrI2 and EuI2 indicate an excellent match in melting and crystallographic parameters, and very modest thermal expansion anisotropy. We have demonstrated energy resolution with SrI2(4-6%Eu) of 2.6% at 662 keV and 7.6% at 60 keV with small crystals, while the resolution degrades somewhat for larger sizes. Our experiments suggest that digital techniques may be useful in improving the energy resolution in large crystals impaired by light-trapping, in which scintillation light is re-absorbed and re-emitted in large and/or highly Eu2+ -doped crystals. The light yield proportionality of SrI2(Eu) is found to be superior to that of other known scintillator materials, such as LaBr3(Ce) and NaI(Tl).

SrI2, a Novel Scintillator Crystal for Nuclear Isotope Identifiers

The growth and scintillating properties of undoped and Eu2+ doped Strontium Iodide indicate excellent potential for gamma ray spectroscopy. Energy resolution at 662 keV was found to be as good as 2.7% at 662 keV. The effect of purification by zone refining was also studied and crystal growth of SrI2 by the Bridgman technique was found to be less subject to cracking compared to the growth of lanthanum halide scintillators.

Dual gamma neutron detection with Cs2LiLaCl6

Some applications, particularly in homeland security, require detection of both neutron and gamma radiation. Typically, this is accomplished with a combination of two detectors registering neutrons and gammas separately. Recently, a new scintillator, Ce doped Cs2LiLaCl6 (CLLC) that can provide detection of both has been investigated for gamma and neutron detection. This material is capable of providing very high energy resolution, as good as 3.4% at 662 keV (FWHM), which is better than that of NaI(Tl). Since it contains 6Li, it can also detect thermal neutrons. In the energy spectra, the full energy thermal neutron peak appears near 3 GEE MeV. Thus very effective pulse height discrimination can be achieved with this material. The CLLC emission consists of two main components: Core-to-Valence Luminescence (CVL) spanning from 220 nm to 320 nm and Ce emission found in the range of 350 to 500 nm. The former emission is of particular interest since it appears only under gamma excitation. It is also very fast, decaying with a 2 ns time constant. This provides CLLC with different temporal responses under gamma and neutron excitation and it can be used for effective pulse shape discrimination.

Instrument Development and Gamma Spectroscopy With Strontium Iodide

Development of the Europium-doped Strontium Iodide scintillator, SrI2(Eu), involves advances in crystal growth, optics and readout methodology for prototype detectors. We have demonstrated energy resolution of 3% at 662 keV for a 26 cm3 SrI2(Eu) crystal, which is equivalent to the performance obtained with Cerium-doped Lanthanum Bromide of equivalent size. Compared to standard analog readout, use of a digital readout method allows improved energy resolution to be obtained with large volume SrI2(Eu) crystals. Comparative gamma spectra acquired with LaBr3(Ce) and NaI(Tl) quantitatively depict the value of the high resolution of SrI2(Eu) in discriminating closely spaced gamma lines for radioisotope identification applications.

Spectral responses of virtual Frisch-grid CdZnTe detectors and their relation to IR microscopy and x-ray diffraction topography data

Virtual Frisch-grid CdZnTe detectors potentially can provide energy resolution close to the statistical limit. However, in real detectors, the quality of the crystals used to fabricate the devices primarily determines energy resolution. In this paper, we report our findings on the spectral response of devices and their relation to material-characterization data obtained using IR microscopy and X-ray diffraction topography.

Characteristics of undoped and europium-doped SrI 2 scintillator detectors

High energy resolution gamma-ray detectors that can be formed into relatively large sizes while operating at room temperature offer many advantages for national security applications. We are working toward that goal through the development of SrI2(Eu) scintillator detectors, which routinely provide <3.0% energy resolution at 662 keV with volumes >10 cm3. In this study, we have tested pure, undoped SrI2 to gain a better understanding of the scintillation properties and spectroscopic performance achievable without activation. An undoped crystal grown from 99.999% pure SrI2 pellets was tested for its spectroscopic performance, its light yield, and uniformity of scintillation light collection as a function of gamma-ray interaction position relative to the crystal growth direction. Undoped SrI2 was found to provide energy resolution of 5.3% at 662 keV, and the light collection nonuniformity varied by only 0.72% over the length of the crystal. Measurements of both a 3% Eu-doped and the undoped SrI2 crystal were carried out in the SLYNCI facility and indicate differences in their light yield non-proportionality. The surprisingly good scintillation properties of the pure SrI2 crystal suggests that with high-purity feedstock, further reduction of the Eu concentration can be made to grow larger crystals while not adversely impacting the spectroscopic performance.

Prospects for High Energy Resolution Gamma Ray Spectroscopy with Europium-Doped Strontium Iodide

Europium-doped strontium iodide scintillators offer a light yield exceeding 100,000 photons/MeV and excellent light yield proportionality, while at the same time, SrI{sub 2} is readily grown in single crystal form. Thus far, our collaboration has demonstrated an energy resolution with strontium iodide of 2.6% at 662 keV and 7.6% at 60 keV, and we have grown single crystals surpassing 30 cm{sup 3} in size (with lower resolution). Our analysis indicates that SrI{sub 2}(Eu) has the potential to offer 2% energy resolution at 662 keV with optimized material, optics, and read-out. In particular, improvements in feedstock purity may result in crystal structural and chemical homogeneity, leading to improved light yield uniformity throughout the crystal volume, and consequently, better energy resolution. Uniform, efficient light collection and detection, is also required to achieve the best energy resolution with a SrI{sub 2}(Eu) scintillator device.

Strontium Iodide Instrument Development for Gamma Spectroscopy and Radioisotope Identification

Development of the Europium-doped Strontium Iodide scintillator, SrI2(Eu2+), has progressed significantly in recent years. SrI2(Eu2+) has excellent material properties for gamma ray spectroscopy: high light yield (<80,000 ph/MeV), excellent light yield proportionality, and high effective atomic number (Z = 49) for high photoelectric cross-section. High quality 1.5” and 2” diameter boules are now available due to rapid advances in SrI2(Eu) crystal growth. In these large SrI2(Eu) crystals, optical self-absorption by Eu2+ degrades the energy resolution as measured by analog electronics, but we mitigate this effect through on-the-fly correction of the scintillation pulses by digital readout electronics. Using this digital correction technique we have demonstrated energy resolution of 2.9% FWHM at 662 keV for a 4 in3 SrI2(Eu) crystal, over 2.6 inches long. Based on this digital readout technology, we have developed a detector prototype with greatly improved radioisotope identification capability compared to Sodium Iodide, NaI(Tl). The higher resolution of SrI2(Eu) yields a factor of 2 to 5 improvement in radioisotope identification (RIID) error rate compared to NaI(Tl).