TlBr can surpass CZT as the leading semiconductor for γ- and X-radiation detection. Unfortunately, the optimum properties of TlBr quickly decay when an operating electrical field is applied. Quantum ...mechanical studies indicated that if this property degradation comes from the conventional mechanism of ionic migration of vacancies, then an unrealistically high vacancy concentration is required to account for the rapid aging of TlBr seen in experiments. In this work, we have applied large scale molecular dynamics simulations to study the effects of dislocations on ionic migration of TlBr crystals under electrical fields. We found that electrical fields can drive the motion of edge dislocations in both slip and climb directions. These combined motions eject enormous vacancies in the dislocation trail. Both dislocation motion and a high vacancy concentration can account for the rapid aging of the TlBr detectors. These findings suggest that strengthening methods to pin dislocations should be explored to increase the lifetimes of TlBr crystals.
MD simulation of dislocation migration under an electrical field.
Thallium bromide (TlBr) is a high atomic number (81, 35), dense (7.56 g/cm 3 ) wide band gap (2.68 eV) semiconductor. In addition, TlBr has a cubic crystal structure and melts congruently at a ...relatively low temperature (~460 C). Recently, mobility-lifetime product of electrons in TlBr has been reported to be greater than 0.001 cm 2 /V. These properties make TlBr a promising material for room temperature gamma radiation detection. Employing device designs such as small pixel arrays that depend primarily on the motion of a single carrier type allows fabrication of thicker devices with better energy resolution than planar devices of the same thickness. We report on our recent progress in developing larger TlBr detectors. Over the past several months we have increased the electron mobility-lifetime product of our TlBr by more than one order of magnitude. Electron mobility-lifetime values as high as 3.0 times 10 -3 cm 2 /V have been measured. Devices with small pixel design have been built with 3, 5, and 10 mm thickness and pixel pitch of 1 mm, 1.5, and 2.0 mm respectively. Pulse height spectra have been recorded over a range of energies from 60 keV to 662 keV. Energy resolution (FWHM) as high as approximately 5% at 122 keV and 1.7% at 662 keV has been obtained without any 3-D corrections. Such arrays are well suited for 3-D correction techniques similar to those applied to CZT devices, indicating that further improvement in energy resolution should be achievable. These latest results demonstrate promise for TlBr as a room temperature semiconductor gamma ray detector.
Thallium bromide (TlBr) and related ternary compounds, TlBrI and TlBrCl, have been under development for room temperature gamma-ray spectroscopy due to high density, high
Z and wide bandgap of the ...material. Low melting point and cubic crystal structure of selected compositions of these compounds facilitate crystal growth by melt techniques. Recent advances in material purification, crystal growth, and device processing have led to mobility–lifetime products of electrons in the mid
10
−3
cm
2/V range enabling working detectors of greater than 15
mm thickness to be fabricated.
In this paper we report on our recent progress on TlBr detector development and first results from TlBr
x
Cl
1−
x
devices. Pulse height spectra will be presented from TlBr arrays as thick as 18
mm. Depth corrected spectra will also be presented. For a 5
mm thick TlBr array, energy resolution of less than 1% (FWHM at 662
keV) was obtained after depth correction.
Abstract
Arrays of 3D position-sensitive detectors (3DPSD), operating at room temperature and using cadmium zinc telluride (CZT) and thallium bromide (TIBr) sensors, are suitable for gamma-ray ...spectrometry in many applications. One detector configuration, the 3D position-sensitive Virtual Frisch-Grid detector (VFG), is particularly advantageous for integrating into large area arrays. The signals generated inside each detector of the array are captured with the anode, cathode and four pads that enable the reconstruction of the position and energy of the ionizing interaction by measurements of amplitude and timing of the signals. For these applications, a low-noise front-end ASIC has been developed, capable of processing bipolar signals (needed because of AC-coupling of certain electrodes). The ASIC can be coupled to an ADC in order to form a compound “waveform digitizer” capable of post-processing the analog signals and determining amplitude and timing information. This paper describes a 32-channel front-end ASIC that is suitable for reading out a 3 × 3 or 4 × 4 element matrix in the VFG configuration. Each channel is composed of a low-noise charge amplifier with an adaptive continuous reset feedback circuit suitable for both positive and negative charge, a first order shaper and a single-to-differential converter output stage. Voltage and current references are all internally generated by 10-bit DACs and the chip is fully controllable with the I
2
C communication protocol. The readout channel response has been verified using the implemented injection circuit. Linear behavior up to ∼75 ke
±
with the gain of ∼80 mV/fC, and up to ∼200 ke
±
with the gain of ∼30 mV/fC was demonstrated. In conclusion, the first test result waveforms using a
137
Cs radioactive source on a 5 × 5 × 12 mm
3
TIBr crystal are reported.
Thallium bromide (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. 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. In addition the choice of electrode metal is shown to have a dramatic effect on the long term stability of TlBr detector performance 241 Am spectra are also shown to be more stable for extended periods; detectors have been held at 4000 V/cm for 50 days with less than 10% degradation in peak centroid position.
We are developing a compact positron emission tomography (PET) detector module with a depth of interaction capability (DOI) based on a lutetium oxyorthosilicate (LSO) scintillator array coupled at ...both ends by avalanche photodiode (APD) arrays. This leads to a detector with high sensitivity that can provide high and uniform image resolution. We report studies on improving the DOI resolution by optimizing the crystal surface treatment. Six 2/spl times/2/spl times/20 mm LSO crystals were treated with different surface finishes along their length: raw saw-cut, polished optical finish, and chemically etched by hot anhydrous phosphoric acid (H/sub 3/PO/sub 4/) with etching times varying from 1 to 5 min. The ratio of the signals from the two APD arrays was used to measure DOI, and the sum of the signals to measure the total light output. Crystals finished by chemical etching for 2-3 min gave the best overall detector performance, with DOI resolutions ranging from 3.1 to 3.9 mm for events above a 150-keV threshold and uniform light output for different DOI positions. The energy resolution ranged between 14% and 18%. This detector design appears promising for PET applications requiring very high resolution and high sensitivity, for example, in small animal imaging and human breast imaging.
LaCl3:Ce scintillator for γ-ray detection Shah, K.S.; Glodo, J.; Klugerman, M. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
06/2003, Letnik:
505, Številka:
1-2
Journal Article
Recenzirano
In this paper, we report on a relatively new cerium-doped scintillator—LaCl3 for γ-ray spectroscopy. Crystals of this scintillator have been grown using Bridgman method. This material when doped with ...10% cerium has high light output (∼50,000 photons/MeV) and fast principal decay time constant (∼20ns). Furthermore, it shows excellent energy resolution for γ-ray detection. For example, energy resolution as low as 3.2% (FWHM) has been achieved with 662keV photons (137Cs source) at room temperature. Also, high timing resolution (264ps—FWHM) has been recorded with LaCl3-PMT and BaF2-PMT detectors operating in coincidence using 511keV positron annihilation γ-ray pairs. Details of crystal growth, scintillation properties, and variation of these properties with cerium concentration are also reported.
The authors evaluated a compact, high resolution PET detector module using avalanche photodiode (APD) arrays to replace bulky position sensitive PMTs. The newly developed APD array is a planar ...processed 4/spl times/4 array which has a 2/spl times/2 mm/sup 2/ pixel size with 0.4 mm gaps between pixels, about 60% quantum efficiency at 420 nm wavelength, and uniform high gain (>1000) across all channels. A 4/spl times/4 array of 2/spl times/2/spl times/10 mm/sup 3/ LSO crystals was coupled to an APD array. Different readout electronics and signal multiplexing schemes were explored. All crystals in the detector array were clearly identified in the flood source histogram, with average peak-to-valley ratios of about 12:1 using a charge sharing resistor network. The energy resolution was measured to be /spl sim/14% at 511 keV in the detector array. The measured timing resolution was 2.6 ns in coincidence with a LSO/PMT detector. By optimizing the readout electronics currently being used, it is likely that detector performance can be further improved. The authors have also determined depth-of-interaction (DOI) by reading out two APD arrays connected to the ends of a 2/spl times/2/spl times/22 mm/sup 3/ LSO crystal. Preliminary measurements show good DOI measurement capability with DOI positioning uncertainty between 4 and 6.5 mm.
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