FWHM Energy Resolution Research Articles

X-ray, γ-ray and neutron detector development for future space instrumentation

An overview of the material developments currently being pursued by the European Space Agency (ESA) with respect to X-ray, γ-ray and neutron detection media is given. The benefits underlying the selection of specific materials are discussed. In the X-ray regime the basic microphysics behind degraded spectral performance caused by the polarization effect and Te inclusions, have been investigated. We have found that both effects degrade the spectra by producing a localised distortion of the electric field. The underlying cause for the distortion does however differ. For the γ-ray regime, a low-noise equivalent scintillator to LaBr3 has been developed in the form of CeBr3. Three inch crystals with a FWHM energy resolution of 4.4% at 662keV have successfully been produced. For neutron detection, a boron compound based solid state neutron detector has been developed that is sensitive to neutrons, alpha particles and 60keV X-rays. The measured detection efficiency in detection for thermal neutrons was found to be in the order of a few per cent.

Long Term Stability of Thallium Bromide Gamma-Ray Spectrometers

Thallium bromide (TlBr) has been under development for room temperature gamma ray spectroscopy. As a result of improvements in material processing and detector fabrication over the last several years, the electron and hole mobility-lifetime products of TlBr are close to those of CdTe. This has allowed us to fabricate 10 mm thick arrays. Without any correction, 4.1% FWHM energy resolution was measured from one pixel of 3×3 array at 662 keV at room temperature. To minimize the polarization effect, lowering the operation temperature was applied to 13 mm thick array and the stable operation beyond 8 months could be seen without any significant degradation of detector performance. Much improved energy resolution of 2.2% or less at 662 keV has been also obtained from one pixel with depth correction. In this paper improved charge collection with Cr contacts and our recent progress made in long term stability of TlBr detectors at room temperature by applying post annealing are presented.

Double Salts Iodide Scintillators: Cesium Barium Iodide, Cesium Calcium Iodide, and Barium Bromine Iodide

AbstractIn this study we review the state‐of‐the‐art for double salt iodide scintillators, in particular cesium barium iodide (CBI), cesium calcium iodide (CCI) and barium bromine iodide (BBI), as well as report on their scintillation and optical properties. Double salt iodides inherently have high density and atomic number which translates to good stopping power for energetic particles, in particular gamma rays. Light yields of 54,000 ph/MeV for CBI, 51,000 ph/MeV for CCI, and 46,000 ph/MeV for BBI were measured. A FWHM energy resolution for the 662 keV full absorption peak was observed at 5.7% for CBI, 16.3% for CCI and 3.56% for BBI. The principal scintillation decay timing for CBI was 840 ns, 462 ns for BBI, and two distinct time components of 9 ns and 1900 ns were observed for CCI.

Energy and coincidence time resolution measurements of CdTe detectors for PET

We report on the characterization of 2 mm thick CdTe diode detector with Schottky contacts to be employed in a novel conceptual design of PET scanner. Results at -8°C with an applied bias voltage of -1000 V/mm show a 1.2% FWHM energy resolution at 511 keV. Coincidence time resolution has been measured by triggering on the preamplifier output signal to improve the timing resolution of the detector. Results at the same bias and temperature conditions show a FWHM of 6 ns with a minimum acceptance energy of 500 keV.These results show that pixelated CdTe Schottky diode is an excellent candidate for the development of next generation nuclear medical imaging devices such as PET, Compton gamma cameras, and especially PET-MRI hybrid systems when used in a magnetic field immune configuration.

Performance of thin long scintillator strips of GSO:Ce, LGSO:Ce and LuAG:Pr for low energy γ-rays

Interest in fibers or strips of single crystalline scintillators is increasing for their possible applications in fine-segmented particle detectors for particle physics experiments as well as γ-ray detectors for medical diagnoses. We compared 2×2×100mm3 strips of Gd2SiO5(GSO):Ce, Lu1.9Gd0.1SiO5 (LGSO):Ce, and Lu3Al5O12(LuAG):Pr single crystals for 0.511MeV γ-rays with respect to the effective light attenuation length λa, light yield LY, FWHM energy resolution ΔE/E and rms position resolution σ(z) of the injection position z along the length. The obtained result was (λa, LY, ΔE/E, σ(z))=(128mm, 380phe/MeV, 17.7%, 8.7mm) in GSO:Ce, (509mm, 1760phe/MeV, 12.2%, 10.9mm) in LGSO:Ce, and (171mm, 690phe/MeV, 15.0%, 8.9mm) in LuAG:Pr. The z position was obtained independently from the pulse height ratio as well as the timing difference between both ends of the strip. The latter method gave comparable or even smaller σ(z) than the former only in LGSO:Ce having both large LY and fast rise time. The λa in the 100mm long GSO:Ce strip was found to be twice as large as that in the 200mm long one [1] with the same cross-section. To obtain good ΔE/E and σ(z), large light yield, fast rise time, and moderately large attenuation length are important for the scintillators.

Development of a High-Resolution Muon Tracking System Based on Micropattern Detectors

A muon tracking system consisting of four 9cm x 10cm sized bulk Micromegas detectors with 128um amplification-gap and two 10cm x 10cm triple GEM detectors is foreseen for high-precision tracking of 140GeV muons at the H8 beamline at CERN with a rate of up to 10kHz and an overall resolution below 40um. Larger detectors with an active area of 0.5m^2 and more are under development for detector studies in high neutron or gamma ray background environments at the Gamma Irradiation Facility at CERN and the Munich tandem accelerator. Signal studies of both detector types have been performed by recording cosmic muon and 5.9keV X-ray signals with a single charge-sensitive preamplifier using several gas-mixtures of Ar:CO_2. The signals were digitized using 1GHz VME based flashADCs with 2520 sampling points. The analysis of the complete signal-cycles allows for the determination of rise times, pulse heights, timing fluctuations and discrimination of background, resulting in a FWHM energy resolution of about 20% and detection efficiencies of 99% and more. Models for signal formation in both detector types will be presented. The single detector spatial resolution of 80um was measured using a fast Gassiplex based strip readout with readout strips of 150um width and a pitch of 250um. The Gassiplex readout, formerly used at the HERMES experiment, had to be substantially adapted. No more crosstalk or non-linearities were observed after reconfiguration of the multiplexing amplifier on the frontend boards. The observed spatial resolution is limited by multiple scattering of the cosmic muons used in the laboratory. We also report on the sensitivity to gamma- and neutron background and on the behaviour of spatial resolution as a function of background rates.

Impact of drift time variation on the Compton image from large-volume CdZnTe crystals

In pixelated CdZnTe detectors, multiple interaction gamma-ray events are critical because they permit Compton imaging. However, in large-volume CdZnTe crystals, the depth of multiple interaction events is often poorly reconstructed due to significant nonuniformity of electron drift velocity while the depth of single interaction events can be accurately reconstructed using signal ratios. The nonuniformity of the drift velocity is likely due to nonuniformity in the electric field within the detector. This causes variation in the drift time even amongst events that occur at the same depth of interaction in the same pixel. The degradation in the depth reconstruction results in poorer imaging performance. The electron drift velocity at each depth is measured using 241Am alpha particles incident on the entire cathode surface of each detector. Cathode waveforms are recorded for events collected by each element in the 11×11 array of anode pixels. To illustrate the impact of electron drift time variation, two CdZnTe crystals fabricated by Redlen Technologies with similar spectral performance, but significantly different Compton imaging performance are selected. The detector with poorer imaging performance demonstrated significant nonuniformity of the electron drift velocity. This work clearly shows that even CdZnTe crystals with 1% FWHM energy resolution at 662keV can have a nonuniform electron drift velocity, on the sub-pixel scale, which seriously degrades the Compton imaging performance of pixelated detectors.

ZnMoO4: A promising bolometer for neutrinoless double beta decay searches

We investigate the performances of two ZnMoO4 scintillating crystals operated as bolometers, in view of a next generation experiment to search the neutrinoless double beta decay of 100Mo. We present the results of the α vs β/γ discrimination, obtained through the scintillation light as well as through the study of the shape of the thermal signal alone. The separation obtained at the 2615keV line of 208Tl is 8σ, using the heat-light scatter plot, while it exceeds 20σ using the shape of the thermal pulse alone. The achieved FWHM energy resolution ranges from 2.4keV (at 238keV) to 5.7keV (at 2615keV). The internal radioactive contaminations of the ZnMoO4 crystals were evaluated through a 407h background measurement. The obtained limit is <32μBq/kg for 228Th and 226Ra. These values were used for a Monte Carlo simulation aimed at evaluating the achievable background level of a possible, future array of enriched Zn100MoO4 crystals.

Determination of 239Pu/240Pu isotopic ratio by high-resolution alpha-particle spectrometry using the ADAM program

A novel analysis program to unfold alpha-particle energy spectra was introduced, demonstrated and validated using radiochemically processed test sources, which contained different amounts of 239Pu and 240Pu. A high-resolution alpha spectrometer was used for data acquisition. The software known as ADAM unfolds the spectra using nuclide-specific decay data as a constraint. The peaks can have different shapes and the software can also cope with the coincidences between alpha particles and electrons/photons. In the present paper, the 239Pu/240Pu activity ratios from alpha spectrometry agreed, within the stated uncertainties, with the reference values. Number of counts in the 239,240Pu peak group must be larger than 100 to obtain reliable values when using semiconductor detector of energy resolution FWHM=10.6keV.

Pixellated Cd(Zn)Te high-energy X-ray instrument

We have developed a pixellated high energy X-ray detector instrument to be used in a variety of imaging applications. The instrument consists of either a Cadmium Zinc Telluride or Cadmium Telluride (Cd(Zn)Te) detector bump-bonded to a large area ASIC and packaged with a high performance data acquisition system. The 80 by 80 pixels each of 250 μm by 250 μm give better than 1 keV FWHM energy resolution at 59.5 keV and 1.5 keV FWHM at 141 keV, at the same time providing a high speed imaging performance. This system uses a relatively simple wire-bonded interconnection scheme but this is being upgraded to allow multiple modules to be used with very small dead space. The readout system and the novel interconnect technology is described and how the system is performing in several target applications.

Imaging X-ray detector front-end with high dynamic range: IDeF-X HD

Presented circuit, IDeF-X HD (Imaging Detector Front-end) is a member of the IDeF-X ASICs family for space applications. It has been optimized for a half millimeter pitch CdTe or CdZnTe pixelated detector arranged in 16×16 array. It is aimed to operate in the hard X-ray range from few keV up to 250keV or more. The ASIC has been realized in AMS 0.35μm CMOS process.The IDeF-X HD is a 32 channel analog front-end with self-triggering capability. The architecture of the analog channel includes a chain of charge sensitive amplifier with continuous reset system and non-stationary noise suppressor, adjustable gain stage, pole-zero cancellation stage, adjustable shaping time low pass filter, baseline holder and peak detector with discriminator. The power consumption of the IDeF-X HD is 800μW per channel. With the in-channel variable gain stage the nominal 250keV dynamic range of the ASIC can be extended up to 1MeV anticipating future applications using thick sensors. Measuring the noise performance without a detector at the input with minimized leakage current (programmable) at the input, we achieved ENC of 33 electrons rms at 10.7μs peak time. Measurements with CdTe detector show good energy resolution FWHM of 1.1keV at 60keV and 4.3keV at 662keV with detection threshold below 4keV. In addition, an absolute temperature sensor has been integrated with resolution of 1.5°C.

Influence of Ring Length on Electric Field Distribution in CdZnTe Frisch-Ring Detector

The CdZnTe (CZT) devices with various screening depth in dimensions 3×3×6mm3 were fabricated. The influence of Ring length (screening depth) and length of crystal on the electric field distribution in CZT devices has been explored by finite element method. The results indicated that ring length (screening depth) plays an important role on the detecting performance of CZT Frisch-ring devices. Longer screening depth gives rise to an electric field which is compressed more greatly. Measured spectra indicated that extreme compressed electric field could reduce detecting resolution. A FWHM energy resolution of approximately 3.71% at 662 keV was obtained for a device with dimensions 3×3×6mm3 and 4.8mm screening depth.

Study on the spectrum improvement of a capacitive Frisch grid structure for CdZnTe Detector

The design and fabrication of a capacitive Frisch grid structure for CdZnTe (CZT) detector have been investigated in this paper. The simulation results show that a bigger ratio of L/H (metal screen ring length/device height) with the ratio of ɛr/d at 20 mm−1 (relative dielectric constant/the thickness of insulation layer) helps to compress the weighting potential towards the anode, and therefore to improve the energy resolution as showed by the experimental results. The capacitive Frisch grid structures significantly improve spectroscopic performance of planar CZT detectors. In particular, a passivation layer obtained by two-step passivation processing, combined with a Teflon tape was used for an insulated layer of the capacitive Frisch grid detector which can be applied at high voltage. A typical FWHM energy resolution of 4.58% at 662 keV was obtained for a device with dimensions of 5 × 5 × 10 mm3.

Digital pulse processing and electronic noise analysis for improving energy resolutions in planar TlBr detectors

Digital pulse processing and electronic noise analysis are proposed for improving energy resolution in planar thallium bromide (TlBr) detectors. An energy resolution of 5.8% FWHM at 662 keV was obtained from a 0.5 mm thick planar TlBr detector at room temperature using a digitizer with a sampling rate of 100 MS/s and 8 bit resolution. The electronic noise in the detector–preamplifier system was measured as a function of pulse shaping time in order to investigate the optimum shaping time for the detector. The depth of interaction (DOI) in TlBr detectors for incident gamma-rays was determined by taking the ratio of pulse heights for fast-shaped to slow-shaped signals. FWHM energy resolution of the detector was improved from 5.8% to 4.2% by implementing depth correction and by using the obtained optimum shaping time.

Study of a high-resolution, 3D positioning cadmium zinc telluride detector for PET

This paper investigates the performance of 1 mm resolution cadmium zinc telluride (CZT) detectors for positron emission tomography (PET) capable of positioning the 3D coordinates of individual 511 keV photon interactions. The detectors comprise 40 mm × 40 mm × 5 mm monolithic CZT crystals that employ a novel cross-strip readout with interspersed steering electrodes to obtain high spatial and energy resolution. The study found a single anode FWHM energy resolution of 3.06 ± 0.39% at 511 keV throughout most of the detector volume. Improved resolution is expected with properly shielded front-end electronics. Measurements made using a collimated beam established the efficacy of the steering electrodes in facilitating enhanced charge collection across anodes, as well as a spatial resolution of 0.44 ± 0.07 mm in the direction orthogonal to the electrode planes. Finally, measurements based on coincidence electronic collimation yielded a point spread function with 0.78 ± 0.10 mm FWHM, demonstrating 1 mm spatial resolution capability transverse to the anodes—as expected from the 1 mm anode pitch. These findings indicate that the CZT-based detector concept has excellent performance and shows great promise for a high-resolution PET system.

The Thick-COBRA: a new gaseous electron multiplier for radiation detectors

The operation principle and preliminary results of a novel gas-avalanche patterned hole electron multiplier, the Thick-COBRA (THCOBRA), are presented. This micro-hole structure is derived from the THGEM and MHSP. Sub-millimeter diameter holes are mechanically drilled in a thin G10 plate, Cu-clad on both faces; on one of the faces the Cu is etched to produce additional anode strips winding between circular cathode strips. Primary avalanches occurring within the holes are followed by additional ones at the anode-strips vicinity. Gains in excess of 5*104were reached with 22.1 keV x-rays in Ar, Ne and Ar-10%CH4, with 12.2 % FWHM energy resolution in Ar-10%CH4. Higher gains were measured with single photoelectrons. This robust multiplier may have numerous potential applications.

Characterization of CMOS position sensitive solid-state photomultipliers

We have designed position sensitive solid-state photomultipliers (PS-SSPM) using a complementary metal-oxide–semiconductor (CMOS) process. While only needing four signal output channels to readout, the device provides spatial information on the micro-pixel level. Four variations of the PS-SSPM were fabricated, however, we only show the characterization results from two. These two PS-SSPMs were characterized for their energy and coincidence timing resolution, spatial resolution, and scintillator array imaging. Each PS-SSPM is 1.5×1.5 mm 2, however, each device has different micro-pixel geometries and a different micro-pixel electrical readout for event position sensing. The FWHM energy resolution at 511 keV was measured as a function of bias using a 1×1×20 mm 3 LYSO crystal. The energy resolution was ∼13–14% for both PS-SSPM designs. The LYSO scintillator coincidence timing resolution was measured with results ranging from 2.1 to 1.0 ns between the two PS-SSPMs. Spatial resolution studies were conducted using a focused (∼15 μm beam spot diameter) pulsed 635 nm diode laser. For each PS-SSPM, its X and Y FWHM spatial resolution was measured to be 70 μm. Lastly, we demonstrate the PS-SSPM imaging capabilities using a LYSO scintillator array having 500×500 μm 2 pixels.

Development of an 8×8 array of LaBr3(Ce) scintillator pixels for a gaseous Compton gamma-ray camera

We have developed LaBr3(Ce) pixel scintillator arrays for an electron-tracking Compton camera (ETCC) that consists of a gaseous time projection chamber (TPC) and the scintillators. The TPC measures the three-dimensional track and energy of a Compton recoil electron, while the scintillators measure the energy and position of the scattered gamma ray. Therefore, the ETCC is able to reconstruct Compton scattering event by event. We adopted LaBr3(Ce) scintillators because the angular resolution of the ETCC depends on the energy resolution of the scintillators, and LaBr3(Ce) has an FWHM energy resolution of about 3% at 662keV using a single-anode photomultiplier tube (PMT). We have developed an 8×8 array of LaBr3(Ce) scintillator pixels with a pixel size of 5.8×5.8×15.0mm3, which has an FWHM energy resolution of 5.8±0.9% at 662keV when coupled to a multi-anode PMT (Hamamatsu H8500). The ETCC with the LaBr3(Ce) arrays has an FWHM angular resolution of 4.2±0.3∘ at 662keV.

Assessment of the Radiation Tolerance of CdZnTe and HgI$_{2}$ to Solar Proton Events

Single crystals of CdZnTe of dimensions 10 times 10 times 3 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and HgI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> of dimensions 10 times 10 times 2 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> were fabricated into planar detectors. Four sets of detectors were then exposed to simulated solar proton events of varying magnitude at the Kernfysisch Versneller Instituut in the Netherlands. Events were simulated by generating protons over the energy range 60 MeV to 200 MeV in 10 energy bands which approximated the spectral shape of the August 1972 solar particle event. One pair of detectors (a CdZnTe and a HgI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) was exposed to an integral fluence of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> protons cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , a second to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> protons cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , a third to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> protons cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> and the fourth to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> protons cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . The latter corresponds to an absorbed dose in silicon of 100 krad or in SI units, 1 kGy. The detectors were characterized both before and after the irradiations in terms of background count rate and spectral performance using <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">55</sup> Fe, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">109</sup> Cd, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">241</sup> Am, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">57</sup> Co and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs radioactive sources. Typical pre-irradiation FWHM energy resolutions of 3 keV and 4 keV were recorded at 60 keV for the CdZnTe and HgI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> detectors, respectively. It was found that HgI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is very radiation tolerant showing little degradation in energy resolution or charge collection efficiency (CCE) for input fluences up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> protons cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . CdZnTe, on the other hand, showed a clear degradation in energy resolution and CCE for input fluences above 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> protons cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . For example, the energy resolution degraded by a factor of 3 between 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> protons cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> while the CCE decreased by a factor of 2.

A study of low resistivity, deep diffused, silicon avalanche photodiodes coupled to a LaBr 3:Ce scintillator

Radiation Monitoring Devices (RMD) has modified their production of deep diffused, planar silicon avalanche photodiodes (APDs), which resulted in significant performance improvements. This modification involves an alternative planar process to influence the p–n junction contour to create a planar bevel while using 4 Ω cm n-type neutron transmutation-doped silicon wafers, where previously 30 Ω cm silicon wafers were used. These new APDs still offer a high gain (∼10 3), but with an increased quantum efficiency and a reduced noise by a factor of 4–5, compared to our standard planar processed 30 Ω cm APDs with the same detection area. We have characterized these new devices for their intrinsic and spectroscopic properties. In our study a 14×14 mm 2 APD, made from 4 Ω cm silicon, was coupled to a 1 cm 3 LaBr 3:Ce scintillator. We measured a FWHM energy resolution at 662 keV to be 2.55% at room temperature (24 °C).