KEYWORDS: Sensors, Weapons of mass destruction, Homeland security, System identification, Polarization, Platinum, Ions, Electric field sensors, Detector arrays, Defect detection
We present the results from testing over 100 5x5x12 mm3 TlBr detectors configured as 3D position-sensitive virtual Frisch-grid (VFG) detectors with platinum contacts. The primary objective was to comprehensively understand factors limiting performance and long-term response variations in these detectors. The incorporation of 3D position sensitivity allowed us to monitor internal changes in charge collection efficiency after applying voltage, and to correlate them with device performance changes. The biased detectors underwent defect distribution alterations due to electric field-enhanced ion migration. Our results are based on an extensive dataset obtained from TlBr crystals produced by Radiation Monitoring Devices (RMD). These measurements were part of our development of a handheld isotope identifier based on an array of position-sensitive TlBr detectors, supported by the Department of Homeland Security, Countering Weapons of Mass Destruction Office. The majority of the detectors exhibited a common trend of performance improvement within 1-2 weeks, stabilization for some period of time, then a slow degradation; however, some detectors deviated from this pattern.
Li-based semiconductor materials represent a promising alternative to 3-He and scintillation materials for thermal neutron detection and imaging instruments. Semiconductor crystals of LiInSe2, LiInP2Se6, and LiGaInSe2 (LiGa0.5In0.5Se2) were grown using natural and enriched lithium (6Li). The materials were characterized for electronic and optical properties including optical transmission, current-voltage (I-V) characteristic for resistivity, and bandgap. Thermal neutron detectors were fabricated and characterized for neutron and gamma-ray response. Pulse height spectra were collected from a moderated custom-designed 241AmBe neutron source and a 60Co gamma-ray source. The LiInSe2 samples exhibited a 2.8 eV cutoff in the optical spectrum and a resistivity of ~8×1011 Ω·cm. LiInSe2 devices exhibit a noise floor of <30 keV which operated at a field of 630 V/mm, for the 0.8-mm thick device. The Vertical Gradient Freeze (VGF) grown LiInP2Se6 samples exhibited a 2.2 eV cutoff in the optical spectrum and resistivity of ~4×1012 Ω·cm. The Chemical Vapor Transport (CVT) grown LiInP2Se6 devices exhibit a noise floor of <60 keV which operated at a field of 8,000 V/mm, for the 0.05- mm thick device. Furthermore, the long-term stability of LiInSe2 devices during multiple weeks under continuous bias was investigated.
Thallium bromide (TlBr) is a promising material for room temperature gamma radiation detection due to its high density, high atomic number, and wide bandgap. Additionally, TlBr has a cubic crystal structure and melts congruently at a relatively low temperature. Advances in material purification, crystal growth and device processing have led to improved material quality including a significant increase in the mobility-lifetime product of electrons in TlBr. This has enabled single carrier collection devices with thicknesses of 1 cm and beyond. The arrays have been flip-chip bonded to carrier boards using a low temperature curing conductive polymer. In this paper we report on results from planar and pixelated devices. Planar TlBr devices with dimensions of 12 mm × 12 mm × 7 mm exhibit an energy resolution ranging from 3% to 5% FWHM at 662 keV when using a shaping time of 2 s. The energy resolution in planar devices improves with a reduction of the shaping timing consistent with the expected amelioration of the depth dependence. The 1-cm thick pixelated arrays, with a pitch of 1.72 mm, produce an energy resolution in the anode spectrum ranging from 1.8% to 4.4%, without applying depth corrections. This work presents spectra from a selected pixel for 133Ba and 57Co irradiation. Measurements of the room-temperature stability of the planar and pixelated detectors show that the position of the 662-keV photopeak is stable over a period of ~200 days, but the shape of the photopeak in the anode spectra exhibits small changes. These detectors show promise for applications in radio-isotope identification devices and for medical imaging.
TlBr is a promising material for room-temperature semiconductor gamma-ray detectors currently under development by several groups around the world. TlBr has the optimal combination of properties: high atomic number, high density, high mu-tau product, low Fano factor, and lower fabrication cost compared to other materials. The presence of crystal defects and ionic drift-diffusion enchained by the electric field affects the performance of today’s TlBr detectors. As a bias is applied across a detector, a defect distribution inside starts changing due to ion migration. The changes appear to be most pronounced in the first weeks of applying a bias to newly-manufactured crystals during the “conditioning” period. The 3-D position-sensitive detectors provide an opportunity to investigate these processes and their effects on the device performance and on corrections applied to the spectrum. Here, we present results from analyzing response changes in TlBr crystals under applied biases using position-sensitive capacitive Frisch-grid detectors.
This work has been supported by the U.S. Department of Homeland Security, Countering Weapons of Mass Destruction Office, under competitively awarded contract 70RDND18C00000024. This support does not constitute an express or implied endorsement on the part of the Government.
We have previously made improvements to the longevity of TlBr semiconductor gamma ray detectors by applying electrodes having the mixed semiconductor composition Tl(Cl,Br) via surface treatments in HCl, leading to a significant enhancement to the lifetime of the detectors. In order to examine the electron transport properties more closely, we have monitored the first-derivative of the cathode waveform (being proportional to velocity and number of carriers) as a function of time and the point of the gamma-interaction. The observed decay in this signal, especially at lower voltage, would naturally be interpreted as the usual trapping phenomenon. However, this phenomenon alone is not able to account for the observed waveforms, most dramatically for the case of increasing signal as the electrons approach the anode, for waveforms originating at the cathode. After detailed consideration of alternative explanations, the cathode waveform data has been interpreted in terms of a non-uniform field owing to variation in the resistivity as a function of position. We have interpreted the shape of the decay as a “built-in” resistivity profile and have further verified this interpretation by reversing the sense of the field (which as expected reverses the “sense” of the waveform). We modeled this effect in order to quantitatively deduce the resistivity profile and are currently working to relate the waveform observations to the relative orientation of the crystal growth direction and the applied electrodes.
High purity, CVD grown diamond is a good candidate material for beta particle detection at elevated temperature in the
presence of a gamma ray background. Due to its wide band gap, low noise detector operation is possible at temperatures
in excess of 200 °C. Its low atomic number limits its gamma-ray interaction probability. Stacked diamond detectors
operated in coincidence can further reduce background due to gamma-ray interactions. In addition, high charge carrier
mobility and high breakdown voltage enable high count rate operation. In this paper we report on gamma-ray and beta
particle detection of CVD diamond detectors with thickness ranging from ~ 0.1 mm to 0.5 mm. CVD grown diamond
materials were acquired from Element Six. Planar devices were fabricated by depositing Au/Cr contacts by thermal
evaporation. Measurements of single diamond detectors and stacked detectors operated in coincidence as well as
measurements at elevated temperatures are presented in this paper.
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.
Lithium Indium Selenide (LiInSe2) has been under development in RMD Inc. and Fisk University for
room temperature thermal neutron detection due to a number of promising properties. The recent advances
of the crystal growth, material processing, and detector fabrication technologies allowed us to fabricate
large detectors with 100 mm2 active area. The thermal neutron detection sensitivity and gamma rejection
ratio (GRR) were comparable to 3He tube with 10 atm gas pressure at comparable dimensions. The
synthesis, crystal growth, detector fabrication, and characterization are reported in this paper.
In this paper we present results on a novel tin-loaded plastic scintillator. We will show that this particular plastic scintillator has a light output similar to that of BGO, a fast scintillation decay (< 10 ns), exhibits good neutron/gamma PSD with a Figure-of-Merit of 1.3 at 2.5 MeVee cut-off energy, and excellent energy resolution of about 12% (FWHM) at 662 keV. Under X-ray excitation, the radioluminescence spectrum exhibits a broad band between 350 and 500 nm peaking at 420 nm which is well-matched to bialkali photomultiplier tubes and UV-enhanced photodiodes.
Degradation of room temperature operation of TlBr radiation detectors with time is thought to be due to electromigration of Tl and Br vacancies within the crystal as well as the metal contacts migrating into the TlBr crystal itself due to electrochemical reactions at the metal/TlBr interface. Scanning Auger electron spectroscopy (AES) in combination with sputter depth profiling was used to investigate the metal contact surface/interfacial structure on TlBr devices. Device-grade TlBr was polished and subjected to a 32% HCl etch to remove surface damage and create a TlBr1-xClx surface layer prior to metal contact deposition. Auger compositional depth profiling results reveal non-equilibrium interfacial diffusion after device operation in both air and N2 at ambient temperature. These results improve our understanding of contact/device degradation versus operating environment for further enhancing radiation detector performance.
Scintillation light is widely believed to be Poisson or super-Poisson. We tested this hypothesis by measuring the temporal correlation between two detectors detecting scintillation light resulting from the same gamma-ray event in SrI2:Eu . Poisson light is expected to yield zero temporal correlations, while super-Poisson light is expected to yield positive, and sub-Poisson light is expected to yield negative temporal correlation. Scintillation light in SrI2:Eu was found to be negatively correlated. Therefore, we conclude that the scintillation light in SrI2:Eu is sub-Poisson.
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.
A review is presented of some recent work in the field of inorganic scintillator research for medical imaging applications, in particular scintillation detectors for Single-Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET).
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).
In this paper we report on plastic scintillators that contain organometallic iridium compounds as triplet harvesting complexes for neutron-gamma pulse shape discrimination (PSD). Our results show that these plastic scintillators have a relatively high light output (higher than BGO) and exhibit very good neutron-gamma PSD with a Figure-of-Merit of ≥ 2.0 at 2.5 MeVee cut-off energy. Under X-ray excitation, the radioluminescence spectrum exhibits a broad band between 400 and 650 nm peaking at 470 nm which is well-matched to bialkali photomultiplier tubes and UV-enhanced photodiodes. The scintillation decay due to Ir3+ luminescence is of the order of 1 us.
TlBr radiation detector operation degrades with time at room temperature and is thought to be due to electromigration of Tl and Br vacancies within the crystal as well as the metal contacts migrating into the TlBr crystal itself due to electrochemical reactions at the metal/TlBr interface. X-ray photoemission spectroscopy (XPS) was used to investigate the metal contact surface/interfacial structure on TlBr devices. Device-grade TlBr was polished and subjected to a 32% HCl etch to remove surface damage prior to Mo or Pt contact deposition. High-resolution photoemission measurements on the Tl 4f, Br 3d, Cl 2p, Mo 3d and Pt 4f core lines were used to evaluate surface chemistry and non-equilibrium interfacial diffusion. Results indicate that anion substitution at the TlBr surface due to the HCl etch forms TlBr1-xClx with consequent formation of a shallow heterojunction. In addition, a reduction of Tl1+ to Tl0 is observed at the metal contacts after device operation in both air and N2 at ambient temperature. Understanding contact/device degradation versus operating environment is useful for improving radiation detector performance.
Four thallium bromide planar detectors were fabricated from materials grown at RMD Inc. The TlBr samples were prepared to investigate the effect of guard ring on device gamma-ray spectroscopy performance, and to investigate the leakage current through surface and bulk. The devices’ active area in planar configuration were 4.4 × 4.4 × 1.0 mm3. In this report, the detector fabrication process is described and the resulting energy spectra are discussed. It is shown that the guard ring improves device spectroscopic performance by shielding the sensing electrode from the surface leakage current, and by making the electric filed more uniform in the active region of the device.
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.
Thermal neutron detectors in planar configuration were fabricated from B2Se3 (Boron Selenide) crystals grown at RMD Inc. All fabricated semiconductor devices were characterized for the current-voltage (I-V) characteristic and neutron
counting measurement. In this study, the resistivity of crystals is reported and the collected pulse height spectra are
presented for devices irradiated with the 241AmBe neutron source. Long-term stability of the B2Se3 devices for neutron detection under continuous bias and without being under continuous bias was investigated and the results are reported. The B2Se3 devices showed response to thermal neutrons of the 241AmBe source.
Device-grade TlBr was subjected to various chemical treatments used in room temperature radiation detector fabrication
to determine the resulting surface composition and electronic structure. Samples of as polished TlBr were treated
separately with 2%Br:MeOH, 10%HF, 10%HCl and 96%SOCl2 solutions. High-resolution photoemission measurements
on the valence band electronic structure and Tl 4f, Br 3d, Cl 2p and S 2p core lines were used to evaluate surface
chemistry. Results suggest anion substitution at the surface with subsequent shallow heterojunction formation. Surface
chemistry and valence band electronic structure were further correlated with the goal of optimizing the long-term
stability and radiation response.
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.
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.
Lanthanide gallium/aluminum-based garnets have a great potential as host structures for scintillation materials for
medical imaging. Particularly attractive features are their high density, chemical radiation stability and more importantly,
their cubic structure and isotropic optical properties, which allow them to be fabricated into fully transparent, highperformance
polycrystalline optical ceramics. Lutetium/gadolinium aluminum/gallium garnets (described by formulas
((Gd,Lu)3(Al,Ga)5O12:Ce, Gd3(Al,Ga)5O12:Ce and Lu3Al5O12:Pr)) feature high effective atomic number and good
scintillation properties, which make them particularly attractive for Positron Emission Tomography (PET) and other γ-
ray detection applications. The ceramic processing route offers an attractive alternative to single crystal growth for
obtaining scintillator materials at relatively low temperatures and at a reasonable cost, with flexibility in dimension
control as well as activator concentration adjustment.
In this study, optically transparent polycrystalline ceramics mentioned above were prepared by the sintering-HIP
approach, employing nano-sized starting powders. The properties and microstructures of the ceramics were controlled by
varying the processing parameters during consolidation. Single-phase, high-density, transparent specimens were
obtained after sintering followed by a pressure-assisted densification process, i.e. hot-isostatic-pressing. The transparent
ceramics displayed high contact and distance transparency as well as high light yield as high as 60,000-65,000 ph/MeV
under gamma-ray excitation, which is about 2 times that of a LSO:Ce single crystal. The excellent scintillation and
optical properties make these materials promising candidates for medical imaging and γ-ray detection applications.
Results are presented of investigations into the composition, uniformity and gamma-ray imaging performance of new ceramic scintillators with synthetic garnet structure. The ceramic scintillators were produced by a process that uses flame pyrolysis to make nanoparticles which are sintered into a ceramic and then compacted by hot isostatic compression into a transparent material. There is concern that the resulting ceramic scintillator might not have the uniformity of composition necessary for use in gamma-ray spectroscopy and gamma-ray imaging. The compositional uniformity of four samples of three ceramic scintillator types (GYGAG:Ce, GLuGAG:Ce and LuAG:Pr) was tested using an electron microprobe. It was found that all samples were uniform in elemental composition to the limit of sensitivity of the microprobe (few tenths of a percent atomic) over distance scales from ~ 1 cm to ~ 1 um. The light yield and energy resolution of all ceramic scintillator samples were mapped with a highly collimated 57Co source (122 keV) and performance was uniform at mapping scale of 0.25 mm. Good imaging performance with single gamma-ray photon detection was demonstrated for all samples using a BazookaSPECT system, and the imaging spatial resolution, measured as the FWHM of a LSF was 150 um.
Thallium bromide (TlBr) has been under development for room temperature gamma ray spectroscopy due to high density, high Z and wide bandgap of the material. Furthermore, its low melting point (460 °C), cubic crystal structure and congruent melting with no solid-solid phase transitions between the melting point and room temperature, TlBr can be grown by relatively simple melt based methods. As a result of improvements in material processing and detector fabrication over the last several years, TlBr with electron mobility-lifetime products (μeτe) in the mid 10-3 cm2/V range has been obtained. In this paper we are going to report on our unipolar charging TlBr results for the application as a small animal imaging. For SPECT application, about 5 mm thick pixellated detectors were fabricated and tested. About 1 % FWHM at 662 keV energy resolution was estimated at room temperature. By applying the depth correction technique, less than 1 % energy resolution was estimated. We are going to report the results from orthogonal strip TlBr detector for PET application. In this paper we also present our latest detector highlights and recent progress made in long term stability of TlBr detectors at or near room temperature. This work is being supported by the Domestic Nuclear Detection Office (DNDO) and the Department of Energy (DOE).
TlBr is a material of interest for use in room temperature gamma ray detector applications due to is wide bandgap 2.7 eV
and high average atomic number (Tl 81, Br 35). Researchers have achieved energy resolutions of 1.3 % at 662 keV,
demonstrating the potential of this material system. However, these detectors are known to polarize using conventional
configurations, limiting their use. Continued improvement of room temperature, high-resolution gamma ray detectors
based on TlBr requires further understanding of the degradation mechanisms. While high quality material is a critical
starting point for excellent detector performance, we show that the room temperature stability of planar TlBr gamma
spectrometers can be significantly enhanced by treatment with both hydrofluoric and hydrochloric acid. By
incorporating F or Cl into the surface of TlBr, current instabilities are eliminated and the longer term current of the
detectors remains unchanged. 241Am spectra are also shown to be more stable for extended periods; detectors have been
held at 2000 V/cm for 52 days with less than 10% degradation in peak centroid position. In addition, evidence for the
long term degradation mechanism being related to the contact metal is presented.
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.
Thermal neutron detectors in planar configuration were fabricated from LiInSe2 and B2Se3 crystals grown at RMD Inc.
All fabricated semiconductor devices were characterized for the current-voltage (I-V) characteristic and neutron
counting measurement. Pulse height spectra were collected from 241AmBe (neutron source on all samples), as well as
137Cs and 60Co gamma ray sources. In this study, the resistivity of all crystals is reported and the collected pulse height
spectra are presented for fabricated devices. Note that, the 241AmBe neutron source was custom designed with
polyethylene around the source as the neutron moderator, mainly to thermalize the fast neutrons before reaching the
detectors. Both LiInSe2 and B2Se3 devices showed response to thermal neutrons of the 241AmBe source.
For detecting neutrons, 3-He tubes provide sensitivity and a unique capability for detecting and discriminating
neutron signals from background gamma-ray signals. A solid-state scintillation-based detector provides an alternative to
3-He for neutron detection. A real-time, portable, and low cost thermal neutron detector has been constructed from a
6Li-enriched Cs2LiYCl6:Ce (CLYC) scintillator crystal coupled with a CMOS solid-state photomultiplier (SSPM).
These components are fully integrated with a miniaturized multi-channel analyzer (MCA) unit for calculation and
readout of the counts and count rates.
CLYC crystals and several other elpasolites including Cs2LiLaCl6:Ce (CLLC) and Cs2LiLaBr6:Ce (CLLB) have
been considered for their unique properties in detecting neutrons and discriminating gamma ray events along with
providing excellent energy resolution comparable to NaI(Tl) scintillators. CLYC's slower rise and decay time for
neutrons (70ns and 900ns respectively) relative to a faster rise and decay time for gamma ray events (6ns and 55ns
respectively) allows for pulse shape discrimination in mixed radiation fields.
Light emissions from CLYC crystals are detected using an array of avalanche photodiodes referred to as solid-state
photomultipliers. SSPMs are binary photon counting devices where the number of pixels activated is directly
proportional to the light output of the CLYC scintillator which is proportional to the energy deposited from the radiation
field. SSPMs can be fabricated using standard CMOS processes and inherently contain the low noise performance
associated with ordinary photomultiplier tubes (PMT) while providing a light and compact solution for portable neutron
detectors.
Thallium bromide (TlBr) and related ternary compounds, TlBrI and TlBrCl, have been under development for room
temperature gamma ray spectroscopy due to several promising properties. Due to recent advances in material
processing, electron mobility-lifetime product of TlBr is close to Cd(Zn)Te's value which allowed us to fabricate large
working detectors. We were also able to fabricate and obtain spectroscopic results from TlBr Capacitive Frisch Grid
detector and orthogonal strip detectors. In this paper we report on our recent TlBr and related ternary detector results
and preliminary results from Cinnabar (HgS) detectors.
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.
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).
Many materials used in radiation detectors are environmentally unstable and/or fragile. These properties are frustrating
to researchers and add significantly to the time and cost of developing new detectors as well as to the cost of
manufacturing products. The work presented here investigates the properties of HgS. This material was selected for
study based partly on its inherent stability and ruggedness, high density, high atomic number, and bandgap. HgS is found
in nature as the mineral cinnabar. A discussion of the physical properties of HgS, experimental characterization of
natural cinnabar, and initial radiation detection results are presented along with a discussion of potential crystal growth
techniques for producing crystals of HgS in the laboratory.
Lutetium oxyorthosilicate (Lu2SiO5:Ce3+, commonly known as LSO) is a scintillator of choice for medical imaging
applications such as Positron Emission Tomography (PET) because of its high light output, high gamma ray stopping
power and fast response. In the current study, phase-pure LSO ceramics were obtained with a high degree of optical
transparency and excellent scintillation properties. These LSO optical ceramics were prepared by combining
nanotechnology with a sinter-HIP approach. We found that the densities of the LSO ceramics increased with
increasing sintering temperature, which corresponds to a systematic decrease in porosity as found by SEM
examination. The residual pores were found to segregate at grain boundaries after sintering, and were essentially
removed by subsequent hot isostatic pressing (HIPing), which raised the density to essentially the value characteristic
of the single crystal and produced polycrystalline LSO ceramics with a high degree of transparency. Under
excitation a 22Na source such specimens displayed a light output as high as 30,100 ph/MeV. The LSO ceramics
showed an energy resolution of 15% (FWHM) at 662 keV (137Cs source) and a fast scintillation decay of 40 ns due to
the 5d → 4f transition of Ce3+. The excellent scintillation and optical properties make LSO ceramic a promising
candidate for future gamma-ray spectroscopy as well as medical imaging applications.
Recently SrI2, a scintillator patented by Hofstadter in 1968, has been rediscovered and shown to possess remarkable
scintillation properties. The light output of SrI2:Eu2+ has been measured to be even higher than previously observed and
exceeds 120,000 photons/MeV, making it one of the brightest scintillators in existence. The crystal also has excellent
energy resolution of less than 3% at 662 keV. The response is highly linear over a wide range of gamma ray energies.
The emission of SrI2:Eu2+ and SrI2:Ce3+/Na+ is well-matched to both photomultiplier tubes and blue-enhanced silicon
photodiodes. While SrI2:Eu2+ is relatively slow, SrI2:Ce3+/Na+ has a fast response. SrI2 crystals with many different
dopant concentrations have been grown and characterized. In this presentation, crystal growth techniques as well as the
effects of dopant concentration on the scintillation properties of SrI2, over the range 0.5% to 8% Eu2+ and 0.5% to 2%
Ce3+/Na+, will be discussed in detail.
TlBr is a promising semiconductor for gamma-ray detection at room temperature, but it has to be extremely pure to
become useful. We investigated the purification and crystal growth of TlBr to improve the mobility and lifetime of
charge carriers, and produce TlBr detectors for radioisotopic detection. Custom equipment was built for purification and
crystal growth of TlBr. The zone refining and crystal growth were done in a horizontal configuration. The process
parameters were optimized and detector grade material with an electron mobility-lifetime product of up to 3x10-3 cm2/V
has been produced. The material analysis and detector characterization results are included.
Illicit nuclear materials represent a threat for the safety of the American citizens, and the detection and interdiction of a
nuclear weapon is a national problem that has not been yet solved. Alleviating this threat represents an enormous
challenge to current detection methods that have to be substantially improved to identify and discriminate threatening
from benign incidents. Rugged, low-power and less-expensive radiation detectors and imagers are needed for large-scale
wireless deployment.
Detecting the gamma rays emitted by nuclear and fissionable materials, particularly special nuclear materials (SNM), is
the most convenient way to identify and locate them. While there are detectors that have the necessary sensitivity, none
are suitable to meet the present need, primarily because of the high occurrence of false alarms.
The exploitation of neutron signatures represents a promising solution to detecting illicit nuclear materials. This work
presents the development of several detector configurations such as a mobile active interrogation system based on a
compact RF-Plasma neutron generator developed at LBNL and a fast neutron telescope that uses plastic scintillating-fibers
developed at the University of New Hampshire. A human-portable improved Solid-State Neutron Detector
(SSND) intended to replace pressurized 3He-tubes will be also presented. The SSND uses an ultra-compact CMOS-SSPM
(Solid-State Photomultiplier) detector, developed at Radiation Monitoring devices Inc., coupled to a neutron
sensitive scintillator. The detector is very fast and can provide time and spectroscopy information over a wide energy
range including fast neutrons.
Single crystals of LaBr3:1% Pr and CeBr3:1% Pr have been grown by the vertical Bridgman technique. Crystals of
these scintillators can be used in the fabrication of gamma-ray spectrometers. The LaBr3:1% Pr and CeBr3:1% Pr
crystals we have grown had light outputs of ~73,000 and ~50,000 photons/MeV, respectively, and principal decay
constants of 11μs and 26 ns, respectively. There were a number of emission peaks observed for these compounds. The
emission wavelength range for the LaBr3:1% Pr and CeBr3:1% Pr scintillators were from about 400 to 800 nm. The
CeBr3:1% Pr scintillator had a dominating emission peak due to CeBr3 at 390 nm. These two materials had energy
resolutions of 9 and 7% FWHM, respectively, for 662 keV photons at room temperature. In this paper, we will report on
our results to date for vertical Bridgman crystal growth and characterization of Pr-doped LaBr3 and Pr-doped CeBr3
crystals. We will also describe the special handling and processing procedures developed for these scintillator
compositions.
Thallium bromide (TlBr) is a compound semiconductor with high density, high atomic numbers and wide
bandgap. In addition, recent results indicate that the mobility-lifetime product of electrons can be quite high,
approaching the values for CdTe and CZT. These properties make TlBr a very promising material for nuclear radiation
detector at room temperature. In this paper we report on our investigation of the performance of planar TlBr detector
under high flux x-rays irradiation. This study proposes an alternate contact method that reduces the polarization effects,
and the afterglow for a wide range of high flux applications.
Single photon emission computed tomography (SPECT) is an important technology for molecular imaging studies of small animals with an increasing demand for high performance imaging systems. We have designed a small animal imaging system based on position sensitive avalanche photodiodes (PSAPDs) detectors with the goal of submillimeter spatial resolution and high detection efficiency, which will allow us to minimize the radiation dose to the animal and to shorten the time needed for the imaging study. Our design will use fourteen 80×80 mm2 PSAPD detectors, which can achieve an intrinsic spatial resolution of 0.5 mm. These detectors are arranged in two rings around the object and are equipped with pinhole collimators to produce magnified projection data. A mouse bed is positioned in the center of the detector rings and can be rocked about the central axis to increase angular sampling of the object. The performance of this imaging system and of a dual headed SPECT system has been simulated using a ray tracing program taking into account appropriate point spread functions. Projection data of a hot rod phantom with 84 angular samples have been simulated. Appropriate Poisson noise has been added to the data to simulate an acquisition time of 15 min and an activity of 18.5 MBq distributed in the phantom. Both sets of data were reconstructed with an ML-EM reconstruction algorithm. We also derived spatial resolution and detection efficiency from analytical equations and compared the performance of our system to a variety of other small animal SPECT imaging systems. Simulations show that our proposed system produces a spatial resolution of 0.9 mm which is in good agreement with the resolution derived from analytical equations. In contrast, simulations of the dual headed SPECT system produce a spatial resolution of 1.1 mm. In comparison to other small animal SPECT systems, our design will offer a detection efficiency which is at least 2-fold higher at better or comparable spatial resolution. These results suggest that detectors based on PSAPD technology can be used to improve the design of small animal SPECT imaging systems considerably. Our small animal system design is very compact and can achieve high resolution and detection efficiency.
Vapor deposited lead iodide films show a wide range of physical attributes dependant upon fabrication conditions. High density is most readily achieved with films less than 100 μm. Thicker films, with lessening density, often show lower response (gain) as charge collection becomes less efficient. Lack of consistency in density throughout a deposition invariably leads to non-uniform electronic properties, which is challenging to both model and predict. To overcome this, tighter control of deposition parameters is required during the slow growth process (<10 μm/hour). Lead iodide films are characterized in forms of planar devices deposited onto conductive glass and active pixel arrays deposited onto a-Si TFT arrays1. Electronic properties (e.g. leakage current, gain) show little variation that can be traced to substrate choice. Films generally provide less than 100 pA/mm2 leakage current as they show saturation in gain (at approximate fields of 1 V/μm). We recently modified our readout electronics to accept positive bias. Using positive bias on the top electrode provides better charge collection for the lower mobility electrons and (despite process variability) better quality films can provide sensitivities greater than 6 μC/R*cm2, with only partial x-ray absorption, and show less than 20 pA/mm2 dark current.
Recent results have shown that capacitive Frisch grid structures significantly improve spectroscopic performance of planar CZT detectors especially at higher energies. This paper presents results obtained with larger detectors than those previously reported on. Devices with various aspect ratios and grid length-to-device thickness ratios were fabricated and evaluated. A FWHM energy resolution of approximately 2% at 662 keV was obtained for a device with dimensions of 5 mm x 5 mm x 9.2 mm.
Recent progress has been made in the development of the Modified Vertical Bridgman (MVB) technique for the growths of 3-inch diameter CZT crystals for fabrication of x-ray and gamma-ray detectors to operate at room temperature. 40% and 80% of the ingots have the single crystal volumes over 300cm3 and 100 cm3 per ingot respectively. Defects (such as Cd-vacancies, Indium dopants and purity) in CZT have been systematically studied. Detectors fabricated from these CZT ingots showed sharp energy resolution and good uniformity.
The Avalanche Photodiode (APD) is a unique device that combines the advantages of solid state photodetectors with those of high gain devices such as photomultiplier tubes (PMTs). APDs have internal gain that provides a high signal-to-noise ratio. APDs have high quantum efficiency, are fast, compact, and rugged. These properties make them suitable detectors for important applications such as LADAR, detection and identification toxic chemicals and bio-warfare agents, LIDAR fluorescence detection, stand-off laser induced breakdown spectroscopy (LIBS), and nuclear detectors and imagers.
Recently there have been significant technical breakthroughs in fabricating very large APDs, APD arrays, and position sensitive APD arrays (PSAPD). Signal gain of over 10,000 has been achieved, single element APDs have been fabricated with active area greater than 40 cm2, monolithic pixelated arrays with up to 28 x 28 elements have been fabricated, and position sensitive APDs have been developed and tested. Additionally, significant progress has been made in improving the fabrication process to provide better uniformity and high yield, permitting cost effective manufacturing of APDs for reduced cost.
Mercuric iodide (HgI2) and lead iodide (PbI2) have been under development for several years as direct converter layers in digital x-ray imaging. Previous reports have covered the basic electrical and physical characteristics of these and several other materials. We earlier reported on 5cm x 5cm and 10cm x 10cm size imagers, direct digital radiography X-ray detectors, based on photoconductive polycrystalline mercuric iodide deposited on a flat panel thin film transistor (TFT) array, as having great potential for use in medical imaging, NDT, and security applications. This paper, presents results and comparison of both lead iodide and mercuric iodide imagers scaled up to 20cm x 25cm sizes.
Both the mercuric iodide and lead iodide direct conversion layers are vacuum deposited onto TFT array by Physical Vapor Deposition (PVD). This process has been successfully scaled up to 20cm x 25cm -- the size required in common medical imaging applications. A TFT array with a pixel pitch of 127 microns was used for this imager.
In addition to increasing detector size, more sophisticated, non-TFT based small area detectors were developed in order to improve analysis methods of the mercuric and lead iodide photoconductors. These small area detectors were evaluated in radiographic mode, continuous fluoroscopic mode and pulsed fluoroscopic mode. Mercuric iodide coating thickness ranging between 140 microns and 300 microns and lead iodide coating thickness ranging between 100 microns and 180 microns were tested using beams with energies between 40 kVp and 100 kVp, utilizing exposure ranges typical for both fluoroscopic and radiographic imaging.
Diagnostic quality radiographic and fluoroscopic images have been generated at up to 15 frames per second. Mercuric iodide image lag appears adequate for fluoroscopic imaging. The longer image lag characteristics of lead iodide make it only suitable for radiographic imaging. For both material the MTF is determined primarily by the aperture and pitch of the TFT array (Nyquist frequency of ~3.93 mm-1 (127 micron pixel pitch).
Further progress has been made in the development of the Modified Vertical Bridgman method for the growth of CdZnTe crystals for fabrication of x-ray and gamma-ray detectors to operate at room temperature. Specifically, the diameter of the grown ingots has been increased from 2 to 3 inches. High quality, large volume (up to 6 in3) twin-free single crystals have been produced. Detectors fabricated with this material show sharp energy resolution and good uniformity.
We have developed a simultaneous CT/SPECT imager using a single Cd0.9Zn0.1Te detector in pulse counting mode. Pulse height energy discrimination is used to separate the CT and SPECT data. In pulse counting mode the x-ray flux into the detector must remain below 3 105cps for a linear count rate response. Our SPECT images compare to current clinical systems but our CT images have poor contrast resolution due to the low x-ray flux used to reduce the total photon flux in the detector and pulse pile-up. We are investigating a current mode operation of the detector to obtain high-resolution CT images. Count rate linearity measurements show three orders of magnitude increase in the maximum count rate of the detector and promises to improve the quality of our CT images from the system. Ultimately, interlaced CT/SPECT data could be obtained with no loss in the quality of the SPECT images, and with a significant improvement in the CT images.
Robert Street, Marcelo Mulato, Michael Schieber, Haim Hermon, Kanai Shah, Paul Bennett, Yuri Dmitryev, Jackson Ho, Rachel Lau, Evgenie Meerson, Steve Ready, Benjamin Reisman, Y. Sado, Koenraad Van Schuylenbergh, Alexander Vilensky, Asaf Zuck
X-ray imaging properties are reported for HgI2 and PbI2, as candidate materials for future direct detection x- ray image sensors, including the first results from screen- printed HgI2 arrays. The leakage current of PbI2 is reduced by using new deposition conditions, but is still larger than HgI2. Both HgI2 and PbI2 have high spatial resolution but new data shows that the residual image spreading of PbI2 is not due to k-edge fluorescence and its possible origin is discussed. HgI2 has substantially higher sensitivity than PbI2 at comparable bias voltages, and we discuss the various loss mechanisms. Unlike PbI2, HgI2 shows a substantial spatially non-uniform response that is believed to originate from the large grain size, which is comparable to the pixel size. We obtain zero spatial frequency DQE values of 0.7 - 0.8 with PbI(subscript 24/ under low energy exposure conditions. A model for signal generation in terms of the semiconducting properties of the materials is presented.
We report on device fabrication and testing of CZT grown by the Modified Vertical Bridgman (MVB) method. Several samples of single-crystal MVB grown CZT were obtained from Yinnel Tech. Both single element devices and 2-dimensional arrays were fabricated. Resistivity and electron mobility-lifetime product were measured, and pulse height spectra were recorded for various isotopic sources. Arrays 5 mm thick and an array 1.13-cm thick were evaluated.
Robert Street, Steve Ready, Jeffrey Rahn, Marcelo Mulato, Kanai Shah, Paul Bennett, Ping Mei, Jeng-Ping Lu, Raj Apte, Jackson Ho, Koenraad Van Schuylenbergh, Francesco Lemmi, James Boyce, Per Nylen, Michael Schieber, Haim Hermon
We report on a-Si direct detection x-ray image sensors with polycrystalline PbI2, and more recently with HgI2. The arrays have 100 micron pixel size and, we study those aspects of the detectors that mainly determine the DQE, such as sensitivity, effective fill factor, dark current noise, noise power spectrum, and x-ray absorption. Line spread function data show that in the PbI2 arrays, most of the signal in the gap between pixels is collected, which is important for high,DQE. The leakage current noise agrees with the expected shot noise value with only a small enhancement at high bias voltages. The noise power spectrum under x-ray exposure is reported and compared to the spatial resolution information. The MTF is close to the ideal sinc function, but is reduced by the contribution of K-fluorescence in the PbI2 film for which we provide new experimental evidence. The role of noise power aliasing in the DQE and the effect of slight image spreading are discussed. Initial studies of HgI2 as the photoconductor material show very promising results with high x-ray sensitivity and low leakage current.
This paper describes the preliminary results obtained from our study of optical and electrical properties of BiI3 crystals. The bismuth iodine polycrystals were grown using commercial starting material by vertical Bridgman method. For our measurements we used only single crystal samples that were cut out from grown crystals perpendicular to C6-axis.
Titanium dioxide (TiO2) films have been deposited on SnO2 coated glass substrates by screen-printing. Film morphology and structure have been characterized by scanning electron microscopy, x-ray diffraction and BET analysis. Dye-sensitized TiO2 photoelectrochemical cells have been assembled and characterized. Cells sensitized with anthocyanin and a ruthenium complex have been investigated. A 0.77 cm2 ruthenium dye sensitized cell with 6.1% power conversion efficiency under Air Mass (AM1.5) conditions was obtained. Results obtained with a pure anthocyanin dye and dye extracted from blackberries were compared. Finally, a natural gel was found to improve the stability of anthocyanin sensitized cells.
KEYWORDS: Sensors, X-rays, Electronics, X-ray detectors, Single photon emission computed tomography, Gamma radiation, Digital electronics, Digital signal processing, X-ray computed tomography, Imaging systems
A medical imaging system providing both x-ray transmission and radionuclide measurements would allow correlation of structural and functional information. We therefore are evaluating a pixellated CdZnTe detector for combined x-ray CT and SPECT imaging with various readout electronics. Gamma-ray spectra of 57Co measured using NIM electronics (2-microsecond(s) shaping time) and multichannel fast photon-counting electronics (50-ns shaping time) produced energy resolutions of 6.5 keV FWHM and 17 keV FWHM respectively at 122 keV. Fast photon-counting electronics achieved linear x-ray count-rate response up to 4 X 105 cps. Dual-mode digital readout electronics are described, which promise to improve SPECT and x-ray CT performance in comparison to the fast-counting electronics. The leakage current and x-ray response with the dual-mode electronics are studied. The leakage current as small as tens of pA is measured, while detector current over 5 orders of magnitude is measured with linearity over 4 orders of magnitude. Results suggest that the CdZnTe detector is capable of performing both x-ray CT and SPECT with the fast photon-counting electronics, and the digital readout electronics can improve the x-ray CT performance.
In this paper, we discuss recent progress that has been made in the development of high resolution X-ray imaging detectors using photoconducting films of lead iodide (PbI2). PbI2 is a wide bandgap semiconductor with high X- ray stopping efficiency. We have been investigating thick films of lead iodide which can be prepared in large areas in a cost effective manner. These films can be coupled to readout technologies such as amorphous silicon flat panel arrays and vidicon tubes to produce X-ray imaging detectors for applications such as mammography, fluoroscopy, X-ray diffraction and non-destructive evaluation. Recent results obtained when these PbI2 films are coupled to 512 X 512 flat panel a-Si:H array are reported. This includes dark current, signal and resolution measurements. Properties of lead iodide films which are relevant to imager performance are also discussed.
In this paper, characterization of new, planar silicon avalanche photodiode arrays for high-resolution PET applications is discussed. High gain, monolithic 4 X 4 element APD arrays (2 mm pixels) have been fabricated using planar processes. These devices were characterized by measuring their gain (> 103), quantum efficiency (60% at LSO emission) and noise (200 eV FWHM). Energy and timing resolution of these APDs were also measured by coupling them to LSO scintillators (2 X 2 X 10 mm) and were found to be 12% and 4 ns, respectively. An APD array was also coupled to a matching LSO array and successful experiments were conducted to identify the crystal which scintillated. Finally, initial experiments to measure depth of interaction have also been performed.
Robert Street, Jeffrey Rahn, Steve Ready, Kanai Shah, Paul Bennett, Yuriy Dmitriyev, Ping Mei, Jeng-Ping Lu, Raj Apte, Jackson Ho, Koenraad Van Schuylenbergh, Francesco Lemmi, James Boyce, Per Nylen
The x-ray imaging performance is reported using polycrystalline lead iodide as a thick semiconductor detector on an active matrix flat panel array. We have developed a test image sensor with 100 micron pixel size in a 512 X 512 format, using amorphous silicon TFTs for matrix addressing. The new 14 bit electronic system allows radiographic and fluoroscopic x-ray imaging. PbI2 has larger x-ray absorption and higher charge generation efficiency than selenium, and has the potential for higher sensitivity imaging. The films are deposited by vacuum sublimation and have been grown thicker than 100 micrometer. Measurements of the carrier transport and charge collection, together with modeling studies show how the x-ray sensitivity depends on the material properties. Imaging measurements find excellent spatial resolution and confirm models of the x-ray sensitivity. Both radiographic and fluoroscopic imaging are demonstrated. While good overall imaging is obtained, the dark leakage current and image lag need further improvement.
We report the fabrication and evaluation of a Pbl2 imager using large area amorphous silicon technology. This approach uses a thick Pbl2 x-ray photoconductor to absorb x-rays and collect ionization charge under the action of an applied field, while amorphous silicon thin film transistors (TFT) provide a matrix-addressed read out of the signal to external electronics. The x-ray sensitivity of Pbl2 is high, and mobility-lifetime product is large enough to yield a high charge collection at low applied fields. The test arrays used to evaluate Pbl2 have 256 X 256 pixels of size 200 microns. Each pixel contains an amorphous silicon switching transistor, gate and data addressing lines, a charge storage capacitor and a metal pad to contact the Pbl2 layer. Early evaluation of the image sensor indicates the promise of Pbl2 but indicates that reduction of the leakage current is important.
In this paper, a novel approach for developing a large area, high spatial resolution x-ray imaging detector is discussed. This approach integrates the flat panel amorphous silicon readout technology with the polycrystalline lead iodide photoconductive x-ray detection technology. This Pbl2 detector design is promising because it provides high x-ray stopping efficiency, high efficiency conversion of x-ray energy into electronic change, high signal amplitude due to efficient collection of these changes, and high spatial resolution due to electro-static focusing of these changes. We have designed and fabricated prototype 2' X 2' imagers with 200 micrometers pixels (256 X 256 elements) using this approach. The performance of these imagers is characterized by measuring their dark current, x-ray induced signal amplitude, spatial resolution, and uniformity of response. Some basic properties of lead iodide films are also evaluated and presented.
This paper discusses the x-ray detection and imaging characteristics of anew semiconductor material, lead iodide, when prepared in form of a vapor deposited film for use in digital imaging. Lead iodide is a wide bandgap semiconductor and provides direct conversion of x-ray energy into electrical charges. This provides higher signal amplitude than conventional systems using scintillation or phosphor screens since only about 5 eV is required to form a charge pair in lead iodide as opposed to more than 30 eV in case of phosphors to produce optical photons. FUrthermore due to very little lateral diffusion of charge pairs, high spatial resolution can be obtained with such direct conversion films. Finally, due to low dark current in these films, the electronic noise in the films is also very low. In this paper we discuss the lead iodide film preparation procedure, its electronic properties such as resistivity and charge transport, its signal amplitude, and its x-ray imaging performance.
We report for the first time, fabrication of photoconducting UV detectors made from GaN films grown by molecular beam epitaxy. Semi-insulating GaN films were grown by the method of electron cyclotron resonance microwave plasma-assisted molecular beam epitaxy. Photoconductive devices with interdigitated electrodes were fabricated and their photoconducting properties were investigated. In this paper we report on the performance of the detectors in terms of UV responsivity, gain-quantum efficiency product, spectral response, and response time. We have measured responsivity of 125A/W and gain-quantum efficiency product of 600 at 254nm and 25V. The response time was measured to be on the order of 20ns for our detectors, corresponding to a bandwidth of 25Mhz. The spectral response showed a sharp long-wavelength cutoff at 265nm, and remained constant in the 200nm to 365nm range. The response of the detectors to low-energy x-rays was measured and found to be linear for x- rays with energies ranging from 60kVp to 90kVp.
KEYWORDS: Sensors, Monte Carlo methods, Gamma radiation, Computer simulations, Semiconductors, Sensor performance, Electric field sensors, Convolution, Analytical research, Numerical simulations
Traditionally, researchers developing improved gamma ray detectors have used analytical techniques or, rarely, computer simulations to predict the performance of new detectors. However, with the advent of inexpensive personal computers, it is now possible for virtually all detector researchers to perform some form of numerical computation to predict detector performance. Although general purpose code systems for semiconductor detector performance do not yet exist, it is possible to perform many useful calculations using commercially available, general purpose numerical software packages (such as `spreadsheet' programs intended for business use). With a knowledge of the rudimentary mechanics of detector simulation most researchers, including those with no programming skills, can effectively use numerical simulation methods to predict gamma ray detector performance. In this paper we discuss the details of the numerical simulation of gamma ray detectors with the hope of communicating the simplicity and effectiveness of these methods. In particular, we discuss the steps involved in simulating the pulse height spectrum produced by a semiconductor detector.
Photoconductive detectors have been developed from a new wide bandgap (Eg equals 2.01 eV) semiconductor, InI, and these photodetectors are intended for use in scintillation spectroscopy of nuclear radiation. InI single crystals were produced using the Bridgman process and these crystals were characterized by measuring their optical transmission spectrum, micro-hardness, electrical resistivity, and charge transport properties. Photodetectors were fabricated from InI crystal slices by evaporating thin (< 100 angstrom) Pd front contacts, and applying graphite back contacts. These photodetectors showed considerable promise due to their high quantum efficiency (> 60%) in the 300 nm to 600 nm wavelength region and their uniform photo-response over the active detector area. Finally, these photodetectors were coupled to CsI(Tl) scintillator and were successfully tested as spectrometers at room temperature by irradiating the scintillator with 5.5 MeV (alpha) particles (241Am source) and 662 keV (gamma) rays (137Cs source).
Silicon drift photodiodes have been developed over the last five years and are a derivative of silicon drift chambers. These devices, while lacking the position sensitivity of the silicon drift chamber, retain the qualities of low capacitance and large area. These properties make them attractive for use in applications requiring low noise high efficiency photodetection, such as for scintillation light detectors in nuclear spectroscopy. These devices might also find other uses in photonics; as replacements for silicon p-i-n photodiodes in other applications demanding low noise operation. We report on our progress in fabricating silicon drift photodiodes for use as scintillator photodetectors, specifically optimized for detecting the 550 nm emission from CsI(Tl). The design we chose to build was a square photodiode 1 cm2 in area, having the general features of that described by Avset et al. Although some technical problems have temporarily delayed us from producing working drift photodiodes, we have made some diagnostic measurements on our devices and have made observations that may be of general interest.
The requirements of a semiconductor material intended to operate in a gamma-ray detector at room temperatures are discussed, and the status of the search for alternative materials is reviewed. The important material characteristics of a semiconductor gamma-ray detector material are high average atomic number, material's uniformity, resistivity, and electron and holes transport properties. Materials under investigation include GaAs, InP, TlBr, and PbI2. Theoretically, it is considered to be feasible to built a large volume semiconductor gamma-ray detector capable of good energy resolution at room temperature. But it is very unlikely that a semiconductor detector with germanium-like performance will be available in the next five years.
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