This paper presents the study related to the production of TlBr thin films. Films produced by thermal evaporation present better structural properties than those produced by spray pyrolysis. The main XRD peak of the evaporated films correspond to the (100) plane, whose structure is columnar as revealed by cross section SEM. The thickness decreases with increasing deposition height. Optical band gap of 3.0 eV and electrical resistivities about 109 Ωcm were obtained. EDS reveals a reduction in the amount of Br in the final films. One order of magnitude was obtained for the photo-to-dark current ratio when irradiation in the medical diagnosis X-ray mammography energy range was used.

Large-volume cadmium zinc telluride (CZT) radiation detectors would greatly improve radiation detection capabilities and, therefore, attract extensive scientific and commercial interests. CZT crystals with volumes as large as hundreds of centimeters can be achieved today due to improvements in the crystal growth technology. However, the poor performance of large-volume CZT detectors is still a challenging problem affecting the commercialization of CZT detectors and imaging arrays. We have employed Pockels effect measurements and synchrotron X-ray mapping techniques to investigate the performance-limiting factors for large-volume CZT detectors. Experimental results with the above characterization methods reveal the non-uniform distribution of internal electric field of large-volume CZT detectors, which help us to better understand the responsible mechanism for the insufficient carrier collection in large-volume CZT detectors.

Boron carbide crystals ranging in size from 50 microns to several millimeters have been grown from a copper-boron carbide flux at temperatures from 1500°C to 1750°C. The crystal size increased with growth temperature although copper evaporation limited growth at the higher temperatures. Synchrotron X-ray Laue patterns were indexed according to (001) orientation boron carbide structure, indicating the bulk crystals were single crystalline with {001} growth facets. Raman spectrum of boron carbide indicates an improved crystal quality compared to the source powder, but peaks of crystals grown from 11B -enriched source shifted to the lower energy by 1-4 cm−1 from literature values, possibly due to the boron isotope dependency. Five fold symmetry defects and twin planes were common as observed by optical microscope and scanning electron microscope. Raindrop shape etch pits were formed after defect selective etching in molten potassium hydroxide at 600°C for 6 minutes. Typically, the etch pit density was on the order of 106/cm2.

Low-cost, high-performance gamma-ray spectrometers are urgently needed for nonproliferation and homeland security applications. Available scintillation materials fall short of the requirements for energy resolution and sensitivity at room temperature. The emerging lanthanide halide based materials, while having the desired luminosity and proportionality, have proven difficult to produce in the large sizes and low cost required due to highly anisotropic properties caused by the non-cubic crystal structure. New cubic materials, such as the recently discovered elpasolite family (A2BLnX6; Ln-lanthanide and X-halogen), hold promise for scintillator materials due to their high light output, proportionality, and toughness. The isotropic nature of the cubic elpasolites leads to minimal thermomechanical stresses during single-crystal solidification, and eliminates the problematic light scattering at the grain boundaries. Therefore, it may be possible to produce these materials in large sizes as either single crystals or transparent ceramics with high production yield and reduced costs. In this study, we investigated the “cubic” elpasolite halide synthesis and studied the structural variations of four different compounds, including Cs2NaLaBr6, Cs2LiLaBr6, Cs2NaLaI6, and Cs2LiLaI6. Attempts to produce a large-area detector by a hot forging technique were explored.

We discuss defect engineering strategies in radiation detector materials. The goal is to increase resistivity by defect-induced Fermi level pinning without causing defect-induced reductions in the carrier drifting length. We show calculated properties of various intrinsic defects and impurities in CdTe. We suggest that the defect complex of a hydrogen atom and an isovalent impurity on an anion site may be an excellent candidate in many semiconductors for Fermi level pinning without carrier trapping.

Two aluminosilicate oxyfluoride glass systems, a lead-cadmium-aluminosilicate oxyfluoride and a lithium-lanthanum-aluminosilicate oxyfluoride, doped with different TbF3 concentrations, have been fabricated and investigated. By appropriate heat treatment of the as-prepared glasses above, transparent glass-ceramics (TGC) were obtained. The glass-ceramics contain Tb:Pb(Cd)F2 or Tb:LaF3 nano-crystals in the glass-matrix. Differential scanning calorimetry, Raman scattering, and luminescence under both UV and β-particle excitation have been investigated on as-prepared glasses and glass-ceramics. It has been found that the terbium-doped lithium-lanthanum-aluminosilicate oxyfluoride glass exhibits good UV excited luminescence and β-induced luminescence. The luminescence yield increases for glass-ceramic compared to that of the as-prepared glass. The including of LaF3 in the glass-matrix is beneficial for a higher Tb-doping concentration and a high light yield. The light yield of lithium-lanthanum-aluminosilicate oxyfluoride glass and glass-ceramic is comparable to that of Schott IQI-301 product. However, the terbium-doped lead-cadmium-aluminosilicate oxyfluoride glass and glass-ceramic have a detrimental luminescence performance. The lead cations in the glass-matrix may create non-bridging oxygen defects, which are a strong source of charge traps, and correlated to a strong Raman “Boson” peak.

Considering the desirable effects of doping CdTe with heavy elements like Bi, we have grown a Cadmium Zinc Telluride (Zn=10%) ingot with Bi (doping levels ∼1014 to 1015 at/cm3) as the heavy element dopant for use as a room temperature radiation detector, using the Bridgman method. In-spite of a high bulk resitivity (∼1010?cm), and the ability to hold high electric field (>2000 V/cm), these lightly doped crystals had a poor spectral resolution for the Co-57 photo peaks and ??e measurements were so low that these measurement were not reliable. Thermo electric effect spectroscopy (TEES) and thermally stimulated current (TSC) experiments on samples C and F (single crystals close to the tip and the heel of the ingot respectively) have revealed various defect levels in the band gap. Among these defect levels, we have identified and characterized two Bi-related deep levels namely a deep donor level L5 (thermal ionization energy: 0.33[5] to 0.39[5] eV and trap cross-section: 7.1[5] × 10-17 to 2.54 [5] × 10-16 cm2), and a deep acceptor level L8 (thermal ionization energy of 0.82 [5] eV and trap cross-section of 2.59 [5] × 10-12 cm2). These levels were responsible for the observed high electrical resistivity (∼1010 ?*cm) in the CdZnTe samples. From a comparison to studies on Bi doped CdTe samples, level L8 was tentatively associated with the (0/-) transition of (BiCd- - OTe) complex, however is still under study. Since these defect levels also act as trapping centers for charge carriers, in spite of the semi-insulating behavior the samples are poor radiation detectors.

From angle resolved photoemission, the (100) surface termination of Li2B4O7 is significantly more polar than the (110) surface termination although the accepted dipole orientation of this pyroelectric crystal is along (001). Consistent with the surface termination, the surface charging at the surface of (100) is significantly greater than observed at (110) and plays a role in the surface photovoltage effects. Because of the different interfaces formed, device properties likely depend upon crystal faces of lithium borate.

We report on experimental investigations of CexLa1-xF3 colloidal nanocrystals (CNCs) and their properties in function of Ce content. The CNCs were characterized by TEM, energy-dispersive X-ray spectroscopy (EDS), steady-state UV-VIS optical absorption and photoluminescence (PL) spectroscopy, and by PL lifetime measurements. We also report on observations of scintillation from the cerium-doped lanthanum fluoride CNC material in experiments on radiation detection.

The availability of large, single crystals of cadmium zinc telluride (CZT) with uniform properties would lead to improved performance of gamma radiation detectors fabricated from them. However, even though CZT crystals are the central element of these systems, there remains relatively little fundamental understanding about how these crystals grow and, especially, how crystal growth conditions affect the properties of grown crystals. This paper discusses the many challenges of growing better CZT crystals and how modeling may favorably impact these challenges. Our thesis is that crystal growth modeling is a powerful tool to complement experiments and characterization. It provides an important approach to close the loop between materials discovery, device research, systems performance, and producibility. Specifically, we discuss our efforts to model gradient freeze furnaces used to grow large CZT crystals at Pacific Northwest National Laboratories and Washington State University. Model results are compared with experimental measurements, and the insight gained from modeling is discussed.

The detection and identification of subatomic particles is an important scientific problem with implications for medical devices, radiography, biochemical analysis, particle physics, and astrophysics. In addition, the development of efficient detectors of neutrons generated by fissile material is a pressing need for nuclear nonproliferation and counterterrorism efforts. A critical objective in the field of radiation detection is to develop the physical insight necessary to rationally design new scintillation materials for specific applications. However, none of the material types currently used in has sufficient synthetic versatility to exert systematic control over the factors controlling the light output and its dynamics. Here we describe a spectroscopic investigation of two stilbene-based metal-organic frameworks (MOFs) we synthesized, demonstrating that they emit light in response to ionizing radiation, creating the first completely new class of scintillation materials since the advent of plastic scintillators in 1950. This highly novel and unexpected property of MOFs opens a new route to rational design of radiation detection materials, since the spectroscopy shows that both the luminescence spectrum and its timing can be varied by altering the local environment of the chromophore within the MOF. Therefore, the inherent synthetic flexibility of MOFs, which enables both the chromophore structure and its local environment to be systematically varied, suggests that this class of materials can serve as a controlled “nanolaboratory” for probing a broad range of photophysical and radiation detection phenomena. In this presentation we report on the time-dependent fluorescence and radioluminescence of these MOFs and related structures. Multiple decay characteristics have been observed for some materials under study, including fast (ns) exponential and slow (microsecond) non-exponential components. We interpret the results in terms of the electronic states, crystal structures, intermolecular interactions, and transport effects mediating the luminescence. The potential for particle discrimination schemes and large scale production of MOFs and will be discussed.

Real time imaging of the electric field distribution in CZT at low temperature has been carried out using the Pockels electro-optical effect. CZT detectors have been observed to show degraded spectroscopic resolution at low temperature due to so-called ‘polarization’ phenomena. By mounting a CZT device in a custom optical cryostat, we have used Pockels imaging to observe the distortion of the electric field distribution in the temperature range 240K - 300K. At 240K the electric field has a severely non-uniform depth distribution, with a high field region occupying ∼10% of the depth of the device under the cathode electrode and a low field in the remainder of the device. Using an alpha particle source positioned inside the vacuum chamber we have performed simultaneous alpha particle transient current (TCT) measurements. At low temperatures the alpha particle current pulses become significantly shorter, consistent with the reduced electron drift time due to a non-uniform electric field. These data provide useful insights into the mechanisms which limit the spectroscopic performance of CZT devices at reduced temperature.

Scintillating nanoparticles with a SiO2 core and a Gd2O3 shell doped with Eu3+ were synthesized with a sol-gel process. Based on transmission electron microscopy (TEM) data, a ∼13 nm Gd2O3 shell was successfully coated onto ∼220 nm mono-dispersed SiO2 nanocores. Eu3+ ions at concentrations of nominally 5 at% exhibited photoluminescent (PL) emission from the SiO2/Gd2O3 nanoparticles after being calcined at 800 0C for 2 h. The SiO2 remained amorphous after calcining, while the Gd2O3 crystallized to a cubic structure. The PL emission was from the 5D0-7F2 transitions of Eu3+ at 609 and 622 nm. Photoluminescence excitation (PLE) data showed that emission from Eu3+ could result from direct excitation, but was dominated by the oxygen to europium charge-transfer band (CTB) between 250 and 280 nm for Eu3+ doped in Gd2O3. The quantum yield (QY) from thin films drop cast from a mixture of 20 mg of calcined nanoparticles in 500 μL of polymethylmethacrylate (PMMA) and excited in the CTB was 20% for SiO2/Gd2O3:Eu3+ core/shell scintillation nanoparticles. Finally, the above core/shell nanoparticles were passivated with a shell of SiO2 to create e.g. SiO2/Gd2O3:Eu3+/SiO2 nanoparticles. The QYs for this nanostructure were lower than unpassivated nanoparticles which was attributed to a weak CTB for the amorphous SiO2 shell and a higher density of interface quenching sites.

Microstructured semiconductor neutron detectors have superior efficiency performance over thin-film coated planar semiconductor detectors. The microstructured detectors have patterns deeply etched into the semiconductor substrates subsequently backfilled with neutron reactive materials. The detectors operate as pn junction diodes. Two variations of the diodes have been fabricated, which either have a rectifying pn junction selectively formed around the etched microstructures or have pn junctions conformally diffused inside the microstructures. The devices with the pn junctions formed in the perforations have lower leakage currents and better signal formation than the devices with selective pn junctions around the etched patterns. Further, pulse height spectra from conformally diffused detectors have the main features predicted by theoretical models, whereas pulse height spectra from the selectively diffused detectors generally do not show these features. The improved performance of the conformal devices is attributed to stronger and more uniform electric fields in the detector active region. Also, system noise, which is directly related to leakage current, has been dramatically reduced as a result of the conformal diffusion fabrication technique. A sinusoidal patterned device with 100 μm deep perforations backfilled with 6LiF was determined to have 11.9 ± 0.078% intrinsic detection efficiency for 0.0253 eV neutrons, as calibrated with thin-film planar semiconductor devices and a 3He proportional counter.

4 Nines (99.99%) Cd and Te were purified to the semiconductor grade 6 Nines ∼ 7 Nines purity materials by the distillation and the zone melting processes, in order to be used for the growth of CdTe single crystal. The CdTe single crystal of 100 mm in diameter and 18kg in weight was successfully grown by the traveling heater method (THM). The shape of the growth interface had the key role for the single crystal growth. The distribution of the Te inclusion size was measured by IR microscopy. The uniformity of mobility-lifetime products and energy resolution in the wafer were also evaluated. The CdTe X-ray flat panel detector (FPD) was developed using the THM grown CdTe single crystal wafer. The CdTe pixel detectors with 100 mm pixel pitch were flip-chip bonded with the C-MOS readout ASIC and lined up on the print circuit board to cover the active area of 77 mm × 39 mm. The evaluation results showed that the CdTe X-ray FPD is promising as the imager for the non-destructive testing.

Recently a collaboration of LLNL and LBNL has constructed a second generation Compton coincidence instrument to study the non-proportionality of scintillators [1]. This device, known as SLYNCI (Scintillator Light-Yield Non-proportionality Characterization Instrument), has an over 30 time higher data collection rate than previous devices enabling complete characterization of a sample with less 24 hours of running time. Thus, SLYNCI enables a number of systematic studies of scintillators as many samples can be processed in a reasonable length of time. The studies include difference in non-proportionality between different types of scintillators, different members of the same family of scintillators, and impact of different doping levels. The results of such recent studies are presented here, including a study on of various alkali halides, and the impact of europium doping level in strontium iodide. Directions of future work will also be discussed.

The introduction of large single crystal and high performance CdZnTe (CZT) grown by the traveling heater method (THM) in 2006 has defied conventional myths about the capability of this crystal growth method with respect to the production of spectroscopic grade CZT and its commercialization prospect in medical imaging application. Since then, a lot of progresses have been made, both in the crystal growth and the devices sides. This paper focuses on the development of THM CZT in recent years. Crystalline defects which challenge the thickness scalability of large volume CZT detectors along with efforts and achievements in overcoming these challenges are discussed. Advances in THM CZT crystal growth include 100mm diameter ingot and state-of-the-art device fabrication will also be presented.

In this paper, we review our efforts in the spectroscopic detector development using epitaxially grown thick single crystal films of CdTe. The films were grown on GaAs and Si substrates using metalorganic vapor phase epitaxy growth technique. High crystalline quality thick single crystal CdTe films (>260 μm) were obtained where the growth rates could be varied from 10 to 70 μm/h by adjusting the precursor's flow rates, ratios and the substrate temperatures. Spectroscopic detectors were fabricated in a p-CdTe/n-CdTe/n+-GaAs or p-CdTe/n-CdTe/n+-Si heterojunction diode structure. Both types of detector were capable of detecting and resolving energy peaks from a gamma ray source. However, the spectroscopic performance of p-CdTe/n-CdTe/n+-Si detectors was better than that of the p-CdTe/n-CdTe/n+-GaAs detectors. Details on the growth characteristics, detector fabrication and the detector performance are reported. Furthermore, current challenges in this detector fabrication technique are discussed.

An excellent material for thermal neutron detectors is α-rhombohedral boron, due to the large neutron capture cross section of 10B, high hole mobility and ability to self-heal from radiation damage, to date, little work has been done on the crystal growth of α-rhombohedral boron. In this investigation, we attempt to grow α-rhombohedral boron by the solution growth method, employing copper as solvent. Well-faceted transparent red crystals several hundreds of microns in size have been made. Elemental analysis of the crystals detected boron, with negligible amounts of copper, suggesting that copper is a promising solvent for the crystal growth of α-rhombohedral boron crystal.

The defect structure in B12As2 epitaxial layers grown at two different temperatures on (0001) 6H-SiC by chemical vapor deposition (CVD) was studied using synchrotron white beam x-ray topography (SWBXT) and high resolution transmission electron microscopy (HRTEM). The observed differences in microstructures were correlated with the differences in nucleation at the two growth temperatures. The effect of the difference in microstructure on macroscopic properties of the B12As2 was illustrated using the example of thermal conductivity which was measured using the 3-ω technique. The relationship between the measured thermal conductivity and observed microstructures is discussed.