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.
An aliovalently calcium-doped cerium tribromide (CeBr3:Ca2+) crystal was prepared with a gamma-energy resolution (FWHM) of 3.2% at the 137Cs 662 keV gamma energy. We completed a crystal assessment and calculated the predictive performance and physical characteristics using density functional theory (DFT) formalism. Detector performance, characteristics, calcium doping concentration, and crystal strength are reported. The structural, electronic, and optical properties of CeBr3 crystals were investigated using the DFT within generalized gradient approximation. Specifically, we see excellent linearity of photons per unit energy with the aliovalent CeBr3:Ca2+ crystal. Proportionality of light yield is one area of performance in which Ce-doped and Ce-based lanthanide halides excel. Maintaining proportionality is the key to producing a strong, high-performance scintillator. Relative light yield proportionality was measured for both doped and undoped samples of CeBr3 to ensure no loss in performance was incurred by doping. The light output and proportionality for doped CeBr3, however, appears to be similar to that of undoped CeBr3. The new crystal was subjected to additional testing and evaluation, including a benchmark spectroscopy assessment. Results, which present energy resolution as a function of energy, are summarized. Typical spectroscopy results using a 137Cs radiation source are shown for our crystallites with diameters <1 cm. We obtain energy resolution of 3.2% before packing the crystallite in a sealed detector container and 4.5% after packing. Spectra were also obtained for 241Am, 60Co, 228Th, and background to illustrate the spectrosocopic quality of CeBr3:Ca2+ over a broader energy range.
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).
He-3 tubes are the most popular thermal neutron detectors. They are easy to use, have good sensitivity for neutron
detection, and are insensitive to gamma radiation. Due to low stockpiles of the He-3 gas, alternatives are being sought to
replace these devices in many applications. One of the possible alternatives to these devices are scintillators
incorporating isotopes with high cross-section for neutron capture (e.g. Li-6 or B-10). Cs2LiYCl6:Ce (CLYC) is one of the scintillators that recently has been considered for neutron detection. This material offers good detection efficiency
(~80%), bright response (70,000 photons/neutron), high gamma ray equivalent energy of the neutron signal (>3MeV),
and excellent separation between gamma and neutron radiation with pulse shape discrimination. A He-3 tube alternative
based on a CLYC scintillator was constructed using a silicon photomultiplier (SiPM) for the optical readout. SiPMs are
very compact optical detectors that are an alternative to usually bulky photomultiplier tubes. Constructed detector was
characterized for its behavior across a temperature range of -20°C to 50°C.
Despite the outstanding scintillation performance characteristics of cerium tribromide (CeBr3) and cerium-activated
lanthanum tribromide (LaBr3:Ce), their commercial availability and application is limited due to the difficulties of
growing large, crack-free single crystals from these fragile materials. The objective of this investigation was to employ
aliovalent doping to increase crystal strength while maintaining the optical properties of the crystal. One divalent dopant
(Ca2+ ) was investigated 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 investigated using the density functional theory within 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.
Some long-term, remote applications do not have access to conventional harvestable energy in the form of solar radiation (or other ambient light), wind, environmental vibration, or wave motion. Radiation Monitoring Devices, Inc. (RMD) is carrying out research to address the most challenging applications that need power for many months or years and which have undependable or no access to environmental energy. Radioisotopes are an attractive candidate for this energy source, as they can offer a very high energy density combined with a long lifetime. Both large scale nuclear power plants and radiothermal generators are based on converting nuclear energy to heat, but do not scale well to small sizes. Furthermore, thermo-mechanical power plants depend on moving parts, and RTG’s suffer from low efficiency. To address the need for compact nuclear power devices, RMD is developing a novel beta battery, in which the beta emissions from a radioisotope are converted to visible light in a scintillator and then the visible light is converted to electrical power in a photodiode. By incorporating 90Sr into the scintillator SrI2 and coupling the material to a wavelength-matched solar cell, we will create a scalable, compact power source capable of supplying milliwatts to several watts of power over a period of up to 30 years. We will present the latest results of radiation damage studies and materials processing development efforts, and discuss how these factors interact to set the operating life and energy density of the device.
Scintillator crystal detectors form the basis for many radiation detection devices. Therefore,
a search for high light yield single crystal scintillators with improved energy resolution, large
volume, and the potential for low cost, is an ongoing process that has increased in recent years due to
a large demand in the area of nuclear isotope identification. Alkaline earth halides, elpasolites and
rare earth halides are very interesting because many compositions from these crystal families
provide efficient Ce3+/ Eu2+ luminescence, good proportionality and good energy resolution. They
also have small band-gap leading to higher light yields. Ce3+and Eu2+ are efficient, and the emission
wavelengths in the 350-500 nm region matches well with PMTs and a new generation of Siphotodiodes.
In this presentation, we will the present progress made in the crystal growth of these
compositions, and scintillator properties of large diameter SrI2:Eu2+ single transparent crystals. The
crystals were grown successfully using the vertical Bridgeman technique. Crystals with different
diameters of 1”, 1.3”, and 1.5” will be discussed. SrI2:Eu was discovered a half century ago, and
was recently found to be an outstanding material for gamma ray-spectroscopy with high light yield,
very good non-proportionality, and excellent energy resolution.
We will also discuss growth and properties of larger Cs2LiYCl6 (CLYC) crystals. Recently,
it has been shown that crystals from the elpasolite family, including CLYC, can be successfully
employed for a dual gamma ray and neutron detection, which is possible with the help of pulse shape
discrimination (PSD). PSD allows for recognition of an incident particle’s nature based on the shape
of the corresponding scintillation pulse. CLYC has the potential to minimize the cost and
complexity of dual sensing gamma ray and neutron spectrometers. We also address progress in
growth of CLYC crystals with large diameters (1” and 2”) that are transparent and crack free.
Recently discovered scintillators for gamma ray spectroscopy - single-crystal SrI2(Eu), GYGAG(Ce)
transparent ceramic and Bismuth-loaded plastics - offer resolution and fabrication advantages compared to
commercial scintillators, such as NaI(Tl) and standard PVT plastic. Energy resolution at 662 keV of 2.7% is
obtained with SrI2(Eu), while 4.5% is obtained with GYGAG(Ce). A new transparent ceramic scintillator for
radiographic imaging systems, GLO(Eu), offers high light yield of 70,000 Photons/MeV, high stopping, and
low radiation damage. Implementation of single-crystal SrI2(Eu), Gd-based transparent ceramics, and Bi-loaded
plastic scintillators can advance the state-of-the art in ionizing radiation detection systems.
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).
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.
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.
Barium titanate crystals were grown by top seeded solution growth technique, nominally pure and also 0.05% and 1% Cr3+ impurity. We have conducted electron paramagnetic resonance (EPR) and photo-EPR studies at room temperature to investigate the role of Cr3+ impurity in photoinduced electron transfer. Nominally pure crystals contained Fe3+ as impurity, and its EPR is consistent with work reported by previous investigators. The Cr3+ doped crystals also contained Fe3+ impurities. It was observed that the site symmetry and the strength of the axial field parameter for Fe3+ complex were significantly different in Cr3+ doped crystals compared to nominally pure BaTiO3. The EPR spectra of Cr3+ were distinguished using the hyperfine structure of odd isotope 53Cr (I=3/2). By Photo-EPR technique we observe that in the presence of Cr3+, Fe3+ is not significantly photosensitive. In contrast Cr3+ exhibited higher photosensitivity in the presence of Fe3+. This was monitored by locking the magnetic field to 1/2↔1/2 transition of Cr3+, and recording intensity as a function of time, under insitu laser illumination. In lightly doped crystals the intensity of Cr3+ signal is sharply reduced immediately after switching the laser OFF showing non-exponential decay. In heavily doped crystals photo-EPR signal clearly shows that the fast decay of Cr3+ was followed by slow and steady build up of Cr3+ signal. The growth of Cr3+ signal was attributed to photoinduced decoupling of Cr3+ dimers. Thus, by doping BaTiO3 with Cr3+ more efficient grating formation can be achieved and time dependent phenomena are observed.