Shaping Time Constants Research Articles

Cs2ZrCl6 scintillation properties studied using γ-ray spectroscopy and Compton coincidence technique

The scintillation properties of cesium-zirconium hexachloride (Cs2ZrCl6) were investigated by γ-ray spectrometry and the Compton Coincidence Technique. The tested cylindrical sample had 20 mm diameter and 12 mm height. The crystal shows a high light output of 40 900 photons per MeV as measured by a spectroscopy type photomultiplier with 20 μs pulse integration time under γ-ray excitation at room temperature. A small deviation from linear response as a function of energy deposited in γ-ray absorption or Compton scattering processes, combined with a high light yield, results in good energy resolution equal to 4.3 % at 662 keV and 2.3 % at 2.6 MeV. Due to long lasting scintillation decay the sample properties were studied at different shaping time constants in an analogue spectroscopy chain or using different integration time gates when employing a digital analyzer.

LuAG:Pr, LuAG:Pr,Mo and LuYAG:Pr relative light yield measured at wide temperature range with MPPC readout

Relative changes of light yield were investigated for LuAG:Pr, LuAG:Pr,Mo and LuYAG:Pr as a function of sample temperature. The measurements were performed using a dedicated cryostat filled with liquid nitrogen allowing for controlling the temperature of the samples between 90 K and 425 K. Scintillation light was detected by an MPPC photodetector, which was installed on a copper frame separated from the crystal and the cooling rod. Light yields as a function of temperature were investigated by irradiation of the crystals with 662 keV gamma-rays from a 137Cs source. The temperature dependences were recorded using a short (1μs) and long (10μs) shaping time constants in the spectroscopy amplifier to study the changes in the intensity of fast and slow scintillation components. In addition, the ratio of light yield measured with short and long shaping time was calculated, providing some information on the balance between fast and slow decay modes in the tested scintillators.

Radiation effects on Silicon Drift Detector based X-ray spectrometer on-board Chandrayaan-2 mission

The space radiation damage effects on the Silicon Drift Detector (SDD) has been studied by measuring the leakage current and the energy resolution for various gamma (60Co) and X-ray (55Fe) doses. It is observed that there is no significant change in the leakage current and the energy resolution for the gamma ray dose up to 3 krad. The energy resolution is degraded from ∼ 160 eV to ∼ 210 eV at 5.89 keV for the gamma ray dose of ∼ 10 krad for the detector operating temperature of ∼ -40°C. This meets the requirement of Chandrayaan-2 payload performance for the mission life of 2 years. Irradiation tests were also carried out using 55Fe X-ray source for the doses up to 64 krad and it is observed that there is no significant change in the leakage current and the energy resolution. The radiation damage to the electronic components such as internal JFET (specifically transconductance-gm) and the change in the total input capacitance are quantified by measuring the energy resolution for various pulse shaping time constants, before and after irradiation. In this paper, we present a summary of the irradiation measurements and their effects on the SDD devices.

A novel acquisition method for nuclear spectra based on pulse area analysis**Supported by National Natural Science Foundation of China (11375077, 11027508)

A novel simple method based on pulse area analysis (PAA) is presented for acquisition of nuclear spectra by a digitizer. The PAA method can be used as a substitute for the traditional method of pulse height analysis (PHA). In the PAA method a commercial digitizer was employed to sample and sum in the pulse, and the area of the pulse is proportional to the energy of the detected radiation. The results of simulation and experiment indicate the great advantages of the PAA method, especially as the count rate is high and the shaping time constant is small. When the shaping time constant is 0.5 μs, the energy resolution of PAA is about 66% better than that of PHA.

Performance of resistive-charge position sensitive detectors for RBS/Channeling applications

The performance of two types of 1×1cm2 photodiode position sensitive detectors (PSDs) based on resistive charge division was evaluated for their use in Rutherford Backscattering/Channeling (RBS/C) experiments in blocking geometry. Their energy resolution was first determined for ~5.5MeV alpha particles from a radioactive sources, and values of full width half maximum (FWHM) of 22keV and 33keV were achieved using a shaping time constant of τ=2.0μs. Additional tests were performed using backscattered 4He particles from the 2.0MeV beam of a Van de Graaff accelerator. While the 22keV FWHM detector failed after exposure to less than 5×106cm−24He particles, the other did not show any noticeable deterioration due to radiation damage for a fluence of 4×108cm−2. For this type of PSD position resolution (τ=0.5μs) standard deviations of ΔL=0.072mm at ~5.5MeV and ΔL=0.247mm at 1.1MeV were achieved.RBS/Channeling experiments using PSD were performed on several crystalline samples, showing that this setup seems suitable for lattice location studies, particularly for heavy ions implantation (D≳1015at/cm2) on light substrates like Si, SiC, and AlN.

Multiplexing Approaches for a 12 x 4 Array of Silicon Photomultipliers

Two resistor network multiplexing circuits for a 12 × 4 array of SiPMs were constructed and tested. Both circuits encode the position and energy information from 48 SiPM pixels in only 4 analog channels. The two circuits differ in that one buffers each SiPM output with a non-inverting voltage-feedback operational amplifier before multiplexing, whereas the second one connects the output of the SiPMs directly to a charge division resistor network. The energy and timing resolution were measured with a 4 × 4 array of LYSO scintillator crystals with size matched to the SiPM pixel size. The measurement was done in 3 steps to cover all 12 × 4 SiPMs. Both circuits gave an energy resolution of 14%. The single sided timing resolution for the buffered output circuit was 2.86 ns, using a 350-650 keV energy window. In comparison, the timing for the circuit with direct connections between SiPMs and the resistor network was 3.54 ns, using the same energy window. Based on these results, the predicted coincidence timing resolutions are 4.0 ns and 5.0 ns, respectively. The coupling of the SiPM capacitance with the resistor network results in different signal shaping time constants for different SiPMs in the passive resistor network, causing a delay in trigger time for the inner SiPM signals. On the other hand, the circuit with buffer amplifiers does not suffer from this effect, and the pulse shape is more uniform across the SiPMs. We also demonstrate, using a 10 × 10 array of 1.5 mm LYSO crystals, that the inclusion of multiple SiPMs in both circuits reduces the detector's ability to resolve crystals in the flood histograms. The amount of noise increases with number of SiPMs in the multiplexing circuit.

The Effect of Photon Statistics and Pulse Shaping on the Performance of the Wiener Filter Crystal Identification Algorithm Applied to LabPET Phoswich Detectors

A fast Wiener filter-based crystal identification (WFCI) algorithm was recently developed to discriminate crystals with close scintillation decay times in phoswich detectors. Despite the promising performance of WFCI, the influence of various physical factors and electrical noise sources of the data acquisition chain (DAQ) on the crystal identification process was not fully investigated. This paper examines the effect of different noise sources, such as photon statistics, avalanche photodiode (APD) excess multiplication noise, and front-end electronic noise, as well as the influence of different shaping filters on the performance of the WFCI algorithm. To this end, a PET-like signal simulator based on a model of the LabPET DAQ, a small animal APD-based digital PET scanner, was developed. Simulated signals were generated under various noise conditions with CR-RC shapers of order 1, 3, and 5 having different time constants (τ). Applying the WFCI algorithm to these simulated signals showed that the non-stationary Poisson photon statistics is the main contributor to the identification error of WFCI algorithm. A shaping filter of order 1 with τ = 50 ns yielded the best WFCI performance (error ~1%), while a longer shaping time of τ = 100 ns slightly degraded the WFCI performance (error ~3%). Filters of higher orders with fast shaping time constants (10-33 ns) also produced good WFCI results (error 1.4% to 1.6%). This study shows the advantage of the pulse simulator in evaluating various DAQ conditions and confirms the influence of the detection chain on the WFCI performance.

Evaluations of pure and ytterbium doped transparent ceramic complex perovskite scintillators

Optical and scintillation properties of complex perovskite transparent ceramic scintillators of pure and Yb 3+ doped Ba(ZrMgTa)O 3 and (LaSr)(AlTa)O 3 ceramics are reported. The materials were produced by Murata Manufacturing. Their optical properties including transmittance and photoluminescence spectra were evaluated. The ceramics demonstrated high transparency (50–70%) in visible wavelength region. Additionally, strong emission peaks were observed at 470 nm in Ba(ZrMgTa)O 3 under 284 nm excitation and at 500 nm in (LaSr)(AlTa)O 3 under 324 nm excitation. The photoluminescence decay times of Ba(ZrMgTa)O 3 and (LaSr)(AlTa)O 3 samples were 14 and 16 μs, respectively. Judging from these optical properties, Yb 3+ emission was not observed in these materials. In radio luminescence spectra, all specimens exhibited the same emission peaks with relatively higher emission intensity. 241Am 5.5 MeV α-ray induced pulse height spectra of the samples were also measured, and (LaSr)(AlTa)O 3 demonstrated ∼500 ph/5.5 MeV α with 10 μs shaping time constant.

Energy resolution of CsI(Na) scintillators

Abstract The presented paper on CsI(Na) scintillators is a continuation of systematical studies of alkali halide crystals exhibiting more than one exponential decay time. The CsI(Na) crystals are known to have short (∼550 ns) and long (few microseconds) components in the scintillation pulse. Previous studies showed that integration of slow components of the light pulse improved the light output, non-proportionality and energy resolution of CsI(Tl), NaI(Tl) in lower temperatures and undoped NaI crystal at liquid nitrogen temperature. In this work, an influence of the shaping time in the spectroscopy amplifier on the light output, non-proportional response to γ-rays, and energy resolution of two different size CsI(Na) scintillators is examined. Each crystal was coupled to the Photonis XP5212 PMT with a photocathode blue sensitivity of 12.2 μA/l mF. The data analysis showed improved proportionality of the crystal response, higher number of photoelectrons/MeV-γ, and consequently, a better overall energy resolution obtained for 12 μs shaping time constant. Finally, the CsI(Na) characteristics are compared to that obtained previously for NaI(Tl).

Configuration of an alpha–gamma coincidence spectrometer for utilization of safeguards measurements

An alpha–gamma coincidence spectrometry (AGCS) system was developed at the International Atomic Energy Agency's (IAEA) Safeguards Analytical Laboratory (SAL) in order to improve the detection of alpha–gamma emitting radionuclides of relevance for safeguards, such as 239Pu and 241Am. The AGCS design was based upon a linear gate, whose resolving time is dependent upon the amplifiers’ shaping time constants (STC). A coincidence spectrum of a Pu/Am sample was acquired which showed that the AGCS system could be utilized in safeguards measurements.

A Study of CsI(Tl) Scintillator with Optimized Conditions of Large Area Avalanche Photodiode

Avalanche photodiode (APD) is a kind of photodiode that internally amplifies the photocurrent by an avalanche effect. The quantum efficiency of the APD is about 80% which is five times higher than that of typical photomultiplier tube (PMT). A 16 mm diameter of beveled-edge large area avalanche photodiode (LAAPD) made by Advanced Photonics was used for the CsI(Tl) scintillator characterization. A CsI(Tl) crystal was attached to LAAPD and energy spectra were measured with various γ-ray sources. Measurements of energy spectra of the CsI(Tl) crystal to optimize the energy resolutions were performed with different shaping time constants by shaping amplifier and with different gains by high voltage variation. The absolute light yield and energy resolution of CsI(Tl) crystals were measured with the LAAPD for 662 keV γ-rays from a 137Cs source. The LAAPD was calibrated with X-ray of 55Fe source for the calibration of the number of e-h pairs per channel. The absolute light yield of newly developed SrCl2 scintillation crystal is determined by this method. This study shows that it is possible to use the LAAPD for the characterization of newly developed scintillation crystal and for the photo-sensor of scintillation detector.

Non-Proportionality and Energy Resolution of CsI(Tl)

Experimental studies of CsI(Tl) crystals with different amounts of Tl activator are presented. Properties such as non-proportionality, energy resolution and the number of photoelectrons per 1 MeV-gamma were inspected at two different shaping time constants, i.e., 3 and 12 mus. A CsI(Tl) crystal was coupled to the XP5212 PMT with a photocathode blue sensitivity of 12.2 muA/lmF. Six CsI(Tl) crystals of different sizes were tested. Among them, three samples were of known Tl concentration, which amounted roughly 0.05, 0.1 and 0.25 mol%. The data analysis showed a much more proportional character of the crystal response, higher number of photoelectrons/MeV-gamma, and consequently, a better overall energy resolution obtained for 12 mus time constant for all tested samples. The calculated intrinsic resolution improved for longer shaping time constants as well, indicating that, for CsI(Tl) crystals, a more proportional response implies better intrinsic energy resolution of the host crystal. In contrast, the intrinsic resolution at the low energy region (below about 30 keV) deteriorated for the longer shaping time constant. An alpha/gamma ratio was also measured for each tested CsI(Tl) crystal in order to find any indication of an explanation of the intrinsic resolution deterioration for low energy photons at the 12 mus integration time.

Multiple gated-integrator as an effective tool for processing nuclear radiation detector signals

In a previous paper the double gated-integrator with exponential prefilter was investigated in detail [J. Gál, Nucl. Instr. and Meth. A 462 (2001) 506]. It was shown that it has very good noise characteristics. Its noise figure of demerit relating to the cusp shaping is 1.016. However, a more detailed analysis of the output of the double gated-integrator shows that the ballistic deficit of this signal, due to its sharp peaking, is not negligible, if the rise time constant of the detector pulse is comparable with the shaping time constant. In the present paper the theoretical study of the so-called multiple gated-integrator is presented. It will be shown that using multiple gated-integration and performing linear combination of the multiple integrated signals several pulse shapes can be realized. For example, by combining the outputs of the single and the double gated-integrators ramp or trapezoidal shaping can be implemented. The effect of the ballistic deficit on the spectral resolution is also discussed. The calculations confirm the well known fact that trapezoidal shaping is a good compromise when both the ballistic deficit and the system noise have comparable effect on the spectral resolution. At the same time, the multiple integrated signals can be used to derive such quantities which characterize the shape of the input signal. This makes it possible to realize pulse shape discrimination in order to identify particle type or to eliminate unwanted pulses appearing e.g., due to slow charge collection or due to pile-up. In the present paper the characteristics of a shape parameter deduced from the single and double gated-integrator outputs are investigated and the figure of merit of the pulse shape discrimination using this shape parameter is calculated.

Intrinsic energy resolution of pure NaI studied with large area avalanche photodiodes at liquid nitrogen temperatures

The energy resolution and non-proportionality of the light yield vs γ-ray energy of pure NaI scintillators directly coupled to Large Area Avalanche Photodiodes (LAAPDs) and cooled down to the liquid nitrogen temperatures have been studied. Large improvements of energy resolution and nearly proportional light yield response have been obtained at very long shaping time constants. Energy resolution of 3.8±0.1% for 662keV γ-rays from a 137Cs source was recorded at 50μs integration time. The intrinsic resolution of the crystal vs energy of γ-rays has been estimated by measurements of the number of primary electron–hole pairs generated in LAAPD by scintillation light.

High Count Rate Electron Probe Microanalysis.

Reducing the measurement uncertainty of quantitative analyses made using electron probe microanalyzers (EPMA) requires a careful study of the individual uncertainties from each definable step of the measurement. Those steps include measuring the incident electron beam current and voltage, knowing the angle between the electron beam and the sample (takeoff angle), collecting the emitted x rays from the sample, comparing the emitted x-ray flux to known standards (to determine the k-ratio) and transformation of the k-ratio to concentration using algorithms which includes, as a minimum, the atomic number, absorption, and fluorescence corrections.This paper discusses the collection and counting of the emitted x rays, which are diffracted into the gas flow or sealed proportional x-ray detectors. The representation of the uncertainty in the number of collected x rays collected reduces as the number of counts increase. The uncertainty of the collected signal is fully described by Poisson statistics. Increasing the number of x rays collected involves either counting longer or at a higher counting rate. Counting longer means the analysis time increases and may become excessive to get to the desired uncertainty. Instrument drift also becomes an issue. Counting at higher rates has its limitations, which are a function of the detector physics and the detecting electronics.Since the beginning of EPMA analysis, analog electronics have been used to amplify and discriminate the x-ray induced ionizations within the proportional counter. This paper will discuss the use of digital electronics for this purpose. These electronics are similar to that used for energy dispersive analysis of x rays with either Si(Li) or Ge(Li) detectors except that the shaping time constants are much smaller.

Design of a fast-shaping amplifier for PET/CT APD detectors with depth-of-interaction

An integrated 0.35-/spl mu/m CMOS fast-shaping amplifier has been designed for coincidence detection and zero-cross time pulse-shape discrimination (PSD) in positron emission tomography (PET) and for high-rate event counting in computerized tomography (CT) with avalanche photodiode (APD)-based detectors. Analytical simulations of CR-RC/sup n/ filters of various order and shaping time constant were carried out to optimize the timing performance, keeping in mind the stringent channel density requirements of the detector front-end electronics. The filter was implemented with the biquadratic bandpass architecture using a folded cascode transconductance amplifier. The electronic timing resolution of the coincidence circuit was 92 ps in the ideal case without an APD (C/sub in/ = 0 pF), and 243 ps with an APD (C/sub in/ = 30 pF). By comparison, the same system with the CMOS shaper replaced by an Ortec 579 Fast Filter Amplifier set to a shaping time of 10 ns yielded an electronic time resolution of 79 ps in the ideal case without capacitance and 236 ps with the APD. The 511-keV coincidence time resolution for a crystal scintillator-based lutetium oxyorthosilicate (LSO)-APD detector was measured to be 1.49 ns.

Single detectors and pixel arrays based on TlBr

TlBr crystals, grown by hydrothermal and "Bridgman-Stockbarger" methods, were investigated as potential single and pixel detectors of X- and gamma radiation. Technology of detectors manufactured from TlBr crystals is reported. The spectra of the radionuclides Fe-55, Am-241, Cd-109, Co-57, and Cs-137, measured by single TlBr detectors are presented. Full-width at half-maximum (FWHM) energy resolutions of 500 eV at 5.9 keV; 518 eV at 9.87 keV; 670 eV at 13.92 keV; 777 eV at 22.1 keV; 2.7 keV at 59.5 keV; 3.7 keV at 88 keV; 4.4 keV At 122 keV, and 29 keV at 662 keV have been achieved. Small format (3/spl times/3) pixel detectors with pads 0.35/spl times/0.35 mm and gap 0.1 mm were also fabricated on single TlBr crystals of dimensions 2.7/spl times/2.7/spl times/1.0 mm/sup 3/. The interpixel resistivities measured at a bias of 50 V were 400-600 GOhm. The pixel leakage currents at a bias of 250 V were typically less than 0.5 nA. The best spectra were obtained at a temperature of -30/spl deg/C, a shaping time constant of 6 /spl mu/s and high voltage of 400 V. Energy resolutions of 2.2, 3.0, 3.7 and 29 keV were measured at input energies of 59.5, 88, 122, and 662 keV, respectively. Values of /spl mu//spl tau/ have been evaluated for two different ingots produced by the "Bridgman-Stockbarger" method. They are /spl mu//sub e//spl tau//sub e/=2.5/spl times/10/sup -4/ cm/sup 2/ V/sup -1/ and /spl mu//sub h//spl tau//sub h//spl sim/10/sup -6/ cm/sup 2/ V/sup -1/ for one ingot at -30/spl deg/C and /spl mu//sub e//spl tau//sub e/=7/spl times/10/sup -5/ cm/sup 2/ V/sup -1/, and /spl mu//sub h//spl tau//sub h/=1.5/spl times/10/sup -5/ cm/sup 2/ V/sup -1/ for the other ingot at -10/spl deg/C.

Portable spectrometer based on segmented HPGe detector with background suppression

The results of development of a portable spectrometer based on a segmented HPGe detector with background suppression for radioactive waste monitoring are presented. The detector has a planar and a coaxial segments, 68 mm in diameter and, respectively, 20 and 50 mm in length. The energy resolution of the planar segment was 1.3 keV on the energy of 122 keV and of the coaxial segment-2.5 keV on the energy of 1.33 MeV. The relative efficiency of the whole crystal was 60% at the energy of 1.33 MeV. The background suppression system has independent channels for amplifying and shaping the spectrometric pulses at the planar, coaxial and common outputs of the segmented detector. The planar and coaxial channels have a nonswitchable shaping time constant of 2 /spl mu/s and the common channel-a switchable shaping time constant of 2 /spl mu/s or 4 /spl mu/s. The background suppression system is PC-controlled by the special software. Testing of the spectrometer with isotopes Cs-137 and Cs-137 + Co-57 has shown that the relative improvement in peak-Compton ratio for registration with and without suppression is 1.4 for the energy of 662 keV and exceeds 1.5 for the energy of 136 keV.

Selection of the Shaping Circuits of a Multilayer Semiconductor Spectrometer of Charged Particles

The influence of the charge collection time in silicon detectors on the accuracy of charged particle energy measurements is considered. Calculation and experimental data show that this parameter exerts a significant effect on the selection of the shaping time constants τ of multilayer silicon spectrometers. This is especially important for systems based on Si(Li) detectors, which are designed for detecting long-range charged particles (p, d, t) with energies E ∼ 100 MeV. It is shown that the time constant τ = 1.5 μs ensures a high accuracy in the absolute energy calibration.

Pulse shape recognition for CdZnTe semiconductor detector by using multi-shaping amplifiers method with neural network algorithm

A new pulse shape recognition method with multi-shaping amplifiers, combined with a neural network algorithm, has been developed, where four pulse heights are sampled from one signal pulse through four linear amplifiers with different shaping time constants. The four pulse heights are used as characteristic parameters to recognize the pulse shape with a neural network. This method has been applied to signal processing for a CdZnTe semiconductor detector to improve the deteriorated energy spectra caused by pulse height deficits due to the different mobilities of electrons and holes in the detector. The neural network recognizes the pulse shape patterns and provides the corrective magnification factors of the pulse heights. After the corrective procedure, the energy spectrum for /sup 137/Cs gamma-rays is improved from 9.3 keV to 7.4 keV in the energy resolution (FWHM) of the 662 keV gamma rays photopeak. The photopeak becomes a considerably symmetrical shape without a low-energy tail. It has been verified that this method is simple and useful for pulse shape analyses, which can be used for many other applications.