Traveling Molten Zone Research Articles

Advanced crystal growth techniques for thallium bromide semiconductor radiation detectors

Thallium Bromide (TlBr) is a promising room-temperature radiation detector candidate with excellent charge transport properties. Currently, Travelling Molten Zone (TMZ) technique is widely used for growth of semiconductor-grade TlBr crystals. However, there are several challenges associated with this type of crystal growth process including lower yield, high thermal stress, and low crystal uniformity. To overcome these shortcomings of the current technique, several different crystal growth techniques have been implemented in this study. These include: Vertical Bridgman (VB), Physical Vapor Transport (PVT), Edge-defined Film-fed Growth (EFG), and Czochralski Growth (Cz). Techniques based on melt pulling (EFG and Cz) were demonstrated for the first time for semiconductor grade TlBr material. The viability of each process along with the associated challenges for TlBr growth has been discussed. The purity of the TlBr crystals along with its crystalline and electronic properties were analyzed and correlated with the growth techniques. Uncorrected 662 keV energy resolutions around 2% were obtained from 5 mm x 5 mm x 10 mm TlBr devices with virtual Frisch-grid configuration.

Development and melt growth of novel scintillating halide crystals

Melt growth of scintillating halide crystals is reviewed. The vertical Bridgman growth technique is still considered as very popular method that enables production of relatively large and commercially attractive crystals. On the other hand, the micro-pulling-down method is preferable when fabrication of small samples, sufficient for preliminary characterization of their optical and/or scintillation performance, is required. Moreover, bulk crystal growth is also available using the micro-pulling-down furnace. The examples of growths of various halide crystals by industrially friendly melt growth techniques including Czochralski and edge-defined film-fed growth methods are also discussed. Finally, traveling molten zone growth that in some degree corresponds to horizontal zone melting is briefly overviewed.

Advances in crystal growth, device fabrication and characterization of thallium bromide detectors for room temperature applications

Thallium bromide (TlBr) is a promising room-temperature radiation detector candidate with excellent charge transport properties. However, several critical issues need to be addressed before deployment of this material for long-term field applications can be realized. In this paper, progress made towards solving some of these challenges is discussed. The most significant factors for achieving long-term performance stability for TlBr devices include residual stress as generated during crystal growth and fabrication processes, surface conditions, and the choice of contact metal. Modifications to the commonly used traveling molten zone growth technique for TlBr crystals can significantly minimize the stresses generated by large temperature gradients near the melt–solid interface of the growing crystal. Plasma processing techniques were introduced for the first time to modify the Br-etched TlBr surfaces, which resulted in improvements to the surface conditions, and consequently the spectroscopic response of the detectors. Palladium electrodes resulted a 20-fold improvement in the room-temperature device lifetime when compared to its Br-etched Pt counterpart.

Characterization of pixelated TlBr detectors with Tl electrodes

A 4.36-mm-thick pixelated thallium bromide (TlBr) detector with Tl electrodes was fabricated from a crystal grown by the traveling molten zone method using zone-purified material. The detector had four 1×1mm2 pixelated anodes. The detector performance was characterized at room temperature. The mobility-lifetime products of electrons for each pixel of the TlBr detector were measured to be >2.8×10−3cm2/V. The four pixelated anodes of the detector exhibited energy resolutions of 1.5–1.8% full width at half maximum (FWHM) for 662-keV gamma rays for single-pixel events with the depth correction method. An energy resolution of 4.5% FWHM for 662-keV gamma rays was obtained from a reconstructed energy spectrum using two-pixel events from the two pixelated anodes on the detector.

Characterization of Thallium Bromide Detectors Made From Material Purified by the Filter Method

Thallium bromide (TlBr) has been regarded as candidate detector materials for the gamma-ray spectrometers operating at room temperature. In this study, a simple and rapid method, the filter method, was performed to purify a raw TlBr material used for fabrication of TlBr detectors. The material was loaded on shards of crashed quartz and installed in a Pyrex tube, and was melted using a furnace. A purified material passing through interspaces of the shards of quartz was collected in a quartz ampoule located at the outlet of the Pyrex tube. After the purification, impurities colored black extracted from the raw material remained. TlBr crystals were then grown by the travelling molten zone method both from the raw material and the purified material. TlBr detectors were fabricated from the grown crystals, and were characterized by measuring mobility-lifetime products (μτ) for carriers and gamma-ray spectra ( <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs) at room temperature. μτ for electrons of a TlBr detector fabricated from the purified material was around 5 times higher than that of a detector fabricated from the raw material.

Timing Performance of TlBr Detectors

The timing performance of TlBr detectors was evaluated at room temperature (22 °C). 0.5-mm-thick planar TlBr detectors with Tl circular electrodes with a diameter of 3 mm were fabricated from TlBr crystals grown by the traveling molten zone method using a zone-purified material. The pulse rise time of the TlBr detector was measured using a digital oscilloscope as the cathode surface of the device was irradiated with a 22Na gamma-ray source. Coincidence timing spectra were obtained between the TlBr detector and a BaF2 scintillation detector when both detectors were irradiated with 511 keV positron annihilation gamma-rays. The timing resolution of the TlBr detector was found to be inversely proportional to the applied bias voltage. The TlBr detector, in coincidence with the BaF2 detector, exhibited timing resolutions characterized by a 6.5 ns full width at half maximum (FWHM) and an 8.5 ns FWHM with and without an energy window of 350 keV-560 keV, respectively.

TlBr Capacitive Frisch Grid Detectors

Capacitive Frisch grid detectors with the dimensions of 2 mm × 2 mm × 4.4 mm were fabricated from TlBr crystals grown by the travelling molten zone method using zone-purified materials. Spectroscopic performance of the detectors was evaluated at room temperature (24°C), - 5°C and - 20°C. The TlBr detector exhibited an energy resolution of 1.6% FWHM at 662 keV with a peak-to-Compton ratio of 8.7 with the depth correction at room temperature. Improvement of the detector performance was achieved by cooling the detector-preamplifier system. The energy resolutions of 3.4%, 1.5%, 1.0% and 0.9% FWHM at 122 keV, 356 keV, 662 keV and 1332 keV, respectively, were recorded with the TlBr detector without the depth correction at - 5°C. An energy resolution of 0.9% FWHM at 662 keV with a peak-to-Compton ratio of 18.2 was obtained from the TlBr detector with the depth correction at -20°C. An upper limit for the Fano factor of TlBr was estimated to be 0.43 at -20°C.

Characterization of ${\rm TlBr}_{\rm x}{\rm Cl}_{1\mathchar"702D {\rm x}}$ Crystals for Radiation Detectors

Thallium bromide chloride $({\rm TlBr}_{\rm x}{\rm Cl}_{1\mathchar702D {\rm x}})$ crystals have been evaluated as a material used for fabrication of room temperature radiation detectors. In this study, ${\rm TlBr}_{\rm x}{\rm Cl}_{1\mathchar702D {\rm x}}$ crystals with various chlorine (Cl) concentrations were grown by the travelling molten zone method and the detectors were fabricated from the crystals. The optical properties of the crystals were evaluated by measuring the transmittances. The charge transport properties were characterized by the Hecht analysis. The band gap energy of the crystals proportionally increased with Cl concentration. Mobility-lifetime products $(\mu\tau)$ of the crystals decreased with increasing Cl concentration.

Recent Development of TlBr Gamma-Ray Detectors

Planar detectors, strip detectors, and double-sided strip detectors were fabricated from TlBr crystals grown by the traveling molten zone method using zone-purified material. The detector performance including the leakage current, energy resolutions, and timing performance were evaluated in order to assess the capability of the detectors for PET and SPECT applications. The TlBr detectors exhibited excellent spectroscopic performance at room temperature. An energy resolution of 3.4% FWHM at 511 keV was obtained from a TlBr planar detector 1 mm thick. A TlBr strip detector 1 mm thick with four anode strip electrodes exhibited almost uniform detector performance over the strips with the average energy resolution of 4.4% FWHM at 511 keV. A TlBr double-sided strip detector exhibited an energy resolution of 6.3% FWHM for 122 keV gamma-rays. Coincidence timing spectra between a TlBr planar detector and a BaF2 scintillation detector were recorded at room temperature. Timing resolutions of 14 ns and 24 ns were obtained from TlBr detectors 0.5 mm and 1 mm thick, respectively. By cooling the detector to 0° C, an improved timing resolution of 12 ns was obtained from a TlBr detector 1 mm thick.

TlBr purification and single crystal growth for the detector applications

The combination of distillation, Bridgman–Stockbarger, hydrothermal recrystallisation and travelling molten zone (TMZ) methods were used for TlBr purification. Grown crystals were characterised by XRD rocking curve and FTIR spectroscopy methods, and by electrical measurements made from 200 to 300 K.

Improved process for the TlBr single-crystal detector

The combination of Bridgman, recrystallization by dissolving in pure water under hydrothermal conditions and travelling molten zone (TMZ) methods were used for TlBr purification. Detectors of 0.5 and 3 mm thickness were produced from a single crystal grown by the TMZ method. The samples had stable spectrometric characteristics from −40 °C to room temperature. A resolution of 3.7% at 60 keV ( 241Am source) was attained for 3-mm sample at 0 °C. Electrical and spectroscopic properties of the detectors are reported and discussed.

Temperature Dependence of Spectroscopic Performance of Thallium Bromide X- and Gamma-Ray Detectors

Thallium bromide (TlBr) is a compound semiconductor characterized by wide band-gap energy (2.68 eV), high atomic numbers (Tl: 81, Br: 35) and high density (7.56 g/cm3). Mobility-lifetime products for electrons and holes in TlBr crystals are 1times103 cm2/V and 3times10-4 cm2/V, respectively, which are comparable to for carriers in CdTe crystals. Therefore, TlBr is a suitable material for fabrication of X- and gamma-ray detectors. In this study, TlBr detectors have been fabricated from crystals grown by the traveling molten zone method. The detectors have thicknesses of approximately 0.4 mm, and have gold electrodes (3 mm in diameter) on both surfaces of the crystals. In order to evaluate the temperature dependence of the charge transport properties of the detectors, (mutau)e and (mutau)h in the detectors have been estimated by the Hecht analysis at various temperatures of the detectors (-40, 20, 0, 20 and 40degC). The (mutau)h in the detectors was of the order of 10-5 cm2/V at -40degC, and increased with increasing temperatures. The maximum value of (mutau)h the was obtained at 20degC, and was about ten times larger than the value at -40degC. On the other hand, the (mutau)e was approximately 1times10-3 cm2/V between -40degC and 20degC. The temperature dependence of spectral response of the detectors has been evaluated by acquiring energy spectra of gamma-rays from 241Am at -40, 20, 0, 20 and 40degC. At -40degC, the full-energy peak corresponding to 59.5 keV gamma-rays has been obtained from the detectors, but the energy resolution of the peak was degraded due to the hole tailing on the low-energy side of the peak. The energy resolution of the detectors gradually improved with increasing temperatures of the detectors. The best FWHM energy resolution was measured to be 5.4 keV (9.1%) at 20degC.

Fabrication of Indium Iodide X- and Gamma-Ray Detectors

Indium iodide (InI) is a compound semiconductor with a wide band gap. Due to its high atomic number (ZIn:49 and ZI:53) and high density (5.31 g/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ), InI exhibits high photon stopping power similar to that of CdTe. Since InI has wide band gap energy (2.0 eV), radiation detectors fabricated from InI are expected to realize low-noise operation at and above room temperatures. These physical properties indicate that InI is a very promising material for fabrication of room temperature X- and gamma-ray detectors. In this study, radiation detectors were fabricated from InI crystals. InI materials were purified by the multi-pass zone-refining method up to 80 times. InI crystals were grown by the traveling molten zone (TMZ) method with the zone-purified materials. The resultant InI radiation detectors were evaluated by measuring their electrical property, spectral responses and long-term stability. The resistivity of the InI detectors were found to be approximately 3times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> (Omegacm). The InI detector exhibited a clear peak corresponding to 22 keV X-rays from a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">109 </sup> Cd radioactive source at room temperature. To evaluate the long-term stability of the InI detectors, temporal change of the energy spectra of the detectors was measured for a period of time at 20degC and at -20degC. At 20degC, the InI detectors exhibited some degradation in spectral response. On the other hand, the InI detectors operated stably for more than 32 hours at -20degC

Spectroscopic performance and long-term stability of thallium bromide radiation detectors

Thallium bromide (TlBr) is a very promising detector material used for X- and gamma-ray spectroscopy due to its high atomic number (Tl: 81 and Br: 35), high density (7.56 g/cm 3) and wide band gap (2.68 eV). This paper reports results of research on spectroscopic performance and long-term stability of TlBr radiation detectors. TlBr detectors with simple planar structure have been fabricated from crystals grown by the traveling molten zone method. The TlBr detectors have thickness of around 0.4 mm. Gold electrodes (7 mm 2) were formed on the opposite two surfaces of the crystals by vacuum evaporation. Spectroscopic performance of the TlBr detectors was evaluated by measuring gamma-ray spectra ( 60Co). The TlBr detector operated at −20 °C exhibited an FWHM energy resolution of 57 keV (4.3%) for 1.33 MeV gamma-rays. The influence of long-term operation on spectral response of the TlBr detectors has been observed by acquiring energy spectra ( 137Cs) for a period of time at room temperature and at −20 °C. Although the TlBr detectors exhibited the polarization phenomena at room temperature, they exhibited no significant degradation in spectral response for 100 h at −20 °C.

Spectroscopic performance of pixellated thallium bromide detectors

The purpose of this study is to evaluate spectroscopic performance of pixellated thallium bromide (TlBr) radiation detectors. Pixellated TlBr detectors have been fabricated from TlBr crystals grown by the traveling molten zone (TMZ) method with zone-refined materials. Charge transport properties of the crystals were measured by performing the Hecht analysis. The TlBr crystals have exhibited the mobility-lifetime product of around 10/sup -3/ cm/sup 2//V for electrons, which is three times larger than the previously reported value. The pixellated TlBr detectors with /spl sim/2 mm thickness have 4 pixellated electrodes (0.57 mm /spl times/ 0.57 mm each) on the anode surface. In order to evaluate the spectroscopic performance, the detectors have been tested with /sup 57/Co, /sup 137/Cs, and /sup 60/Co gamma-ray sources at room temperature. The TlBr detector 1.9 mm thick has demonstrated energy resolutions of 6.7 keV (5.5%) and 22.3 keV (3.4%) for 122 and 662 keV gamma-rays, respectively. Full-energy peaks of 1.17 and 1.33 MeV gamma-rays from the /sup 60/Co source have also been detected with the detector.

Growth of bismuth tri-iodide platelets for room temperature X-ray detection

Bismuth tri-iodide synthesized from bismuth subcarbonate and potassium iodide, and then purified by zone refining followed by one sublimation, was used as starting material. Platelets were grown by physical vapor deposition (PVD) under argon (500 mmHg), with a source temperature of 300 /spl deg/C and during 5-7 days. Growth temperature was at about 260 /spl deg/C. Other set of platelets was grown by traveling molten zone (TMZ), at 460 /spl deg/C and with a traveling speed of 5 mm/h. Platelets are up to 20 /spl times/ 10 mm/sup 2/ in size, and from 50 to 80 /spl mu/m in thickness. Dendritic-like defects turned out to be the main surface defect for the PVD crystals. Some samples exhibit hexagonal microcrystals on their surface, which indicates that they grow from layers of hexagonal grains. Detectors were made with representative platelets by Pd or Au thermal deposition (contact areas of 0.03, 0.09, 0.12 cm/sup 2/), Pd wire attachment with aquadag and acrylic encapsulation. TMZ crystals show a better surface crystal quality, but this fact does not seem to particularly influence the detector electrical properties. Nevertheless, the metal used for contact deposition determines the resistivity, and Au emerges as better than Pd for this purpose. Detector X-ray response was measured by irradiation with a /sup 241/Am source of 3.5 mR/h. Resistivities up to 2.0 /spl times/ 10/sup 12/ /spl Omega/-cm were obtained, the best reported for detectors made with monocrystals of this material, which correlates with the fact that these detectors are the first BiI/sub 3/ ones that responds to an X-ray moderate radiation exposure. Platelets obtained in the present work give detector grade but not spectrometric grade material, for which energy resolution can not be calculated nor compared with previous results.

Pixellated thallium bromide detectors for gamma-ray spectroscopy and imaging

Recently, pixellated semiconductor detectors exhibit high-energy resolution, which have been studied actively and fabricated from CdTe, CZT and HgI 2. Thallium bromide (TlBr) is a compound semiconductor characterized with its high atomic numbers (Tl=81, Br=35) and high density (7.56 g/cm 3). Thus, TlBr exhibits higher photon stopping power than other semiconductor materials used for radiation detector fabrication such as CdTe, CZT and HgI 2. The wide band gap of TlBr (2.68 eV) permits the detectors low-noise operation at around room temperature. Our studies made an effort to fabricate pixellated TlBr detectors had sufficient detection efficiency and good charge collection efficiency. In this study, pixellated TlBr detectors were fabricated from the crystals purified by the multipass zone-refining method and grown by the horizontal traveling molten zone (TMZ) method. The TlBr detector has a continuous cathode over one crystal surface and 3×3 pixellated anodes (0.57×0.57 mm 2 each) surrounded by a guard ring on the opposite surface. The electrodes were realized by vacuum evaporation of palladium through a shadow mask. Typical thickness of the detector was 2 mm. Spectrometric performance of the TlBr detectors was tested by irradiating them with 241Am (59.5 keV), 57Co (122 keV) and 137Cs (662 keV) gamma-ray sources at temperature of –20°C. Energy resolutions (FWHM) were measured to be 4.0, 6.0 and 9.7 keV for 59.5, 122 and 662 keV gamma-rays, respectively.

A comparison of the X-ray performance of TlBr crystals grown by the Bridgeman–Stockbarger and travelling molten zone methods

We have investigated at optimal temperature the X-ray detection characteristics of two TlBr crystals by the Traveling Molten Zone (TMZ) technique. The resistivities were typically 1.5×1010Ωcm at room temperature, increasing to (1.1–1.7)×1012Ωcm at −15°C. In the temperature range −0°C to −50°C, both crystals exhibited mobility-lifetime products of ∼8×10−5cm2V−1 and ∼1.5×10−5cm2V−1, for electrons and holes respectively. From these crystals, two detectors were packaged and X-ray metrology carried out. For the best detector, the measured energy resolutions at an operating temperature of −15°C and 500V bias were 1.0keV at 5.9keV; 1.1 at 13.9keV; 2.5 at 59.54keV; 3.3keV at 88keV; 4keV at 122keV and 27.7keV at 662keV.A comparative analysis of the characteristics of detectors grown by TMZ to those grown by the Bridgeman–Stockbarger method is given.

Thallium bromide optical and radiation detectors for X-ray and gamma-ray spectroscopy

Compound semiconductor, thallium bromide (TlBr), has been investigated as an optical and radiation detector material for use in X- and /spl gamma/-ray spectroscopy. Single crystals of TlBr have been grown by the traveling molten zone method using zone-purified materials. Direct X- and /spl gamma/-ray detectors have been fabricated from the TlBr crystals. The direct TlBr detectors have exhibited good spectrometric performances at room temperature. Polarization in TlBr detectors has been observed to deteriorate detector performance. Optical detectors for scintillation spectroscopy have been fabricated from the crystals by depositing an optically transparent electrode of indium-tin-oxide (ITO) on the front surface of the crystals. Quantum efficiency of the TlBr optical detectors has been high in wavelength region below /spl sim/460 nm where scintillation emissions of LSO and GSO occur.

Large-volume thallium bromide detectors for gamma-ray spectroscopy

Thallium bromide (TlBr) is a wide-bandgap compound semiconductor characterized with high photon stopping power. In this study, large-volume /spl gamma/-ray detectors (approximately 3.8 /spl times/ 3.8 /spl times/ 3.8 mm/sup 3/) have been fabricated from TlBr crystals grown by the horizontal travelling molten zone (TMZ) method using zone-purified material. In order to extract the energy information of the incident radiation from the thick detectors, the short charge-collection time technique has been applied to the detectors. By this technique, the induced charge on the electrode is measured for a period of time that is short compared to the carrier transit time in order to minimize the deviation of the measured induced charge. The large-volume TlBr detectors irradiated with /sup 22/Na and /sup 137/Cs /spl gamma/-ray sources at room temperature have exhibited energy resolutions of 18.7% and 17.4% (FWHM) for the 511- and 662-keV peaks, respectively. To our knowledge, this is the first time that full-energy peaks have been obtained from TlBr detectors several millimeters thick. The time stability of the detector operation is also studied.