Shaping Time Constants Research Articles

Signal shaping optimization with /sup 3/He tubes in high rate neutron counting

A high rate neutron counting is achieved while minimizing the dead-time of the counting chain. The paper shows that too short a shaping time constant gives rise to double pulses at the output of the discriminator for a single neutron. Therefore, the shaping time must be optimized in order to minimize the dead-time while also avoiding counting errors. Simulations and experimental results for a /sup 3/He chain at different shaping time constants are presented and discussed.

Two-dimensional position sensitive silicon photodiode as a charged particle detector

A two-dimensional position sensitive silicon photodiode has been tested for measurement of position and energy of charged particles. Position nonlinearity and resolution, as well as energy resolution and ballistic deficit were measured for 5.486 MeV α-particles. The results obtained for different pulse shaping time constants are presented.

New pulse-shape analysis method with multi-shaping amplifiers

A novel pulse-shape analysis method that uses similarity to recognize an individual pulse shape is presented in this paper. We obtain four pulse heights by using four linear amplifiers with different shaping time constants. We treat a combination of the four pulse heights as a pattern vector. A similarity of the pulse shape can be obtained by comparison between the pattern vector and a discriminant vector which was given in advance. Each pulse shape is analyzed by using the similarity. The method has been applied for the improvement of characteristics of a CdZnTe semiconductor detector. The characteristics of the energy spectrum of the CdZnTe detector such as the photopeak efficiency or the peak-to-valley ratio are improved after the correction procedure with the similarity.

Investigation of GSO, LSO and YSO scintillators using reverse avalanche photodiodes

The performance characteristics of GSO, LSO and YSO scintillators coupled to new "reverse" avalanche photodiodes (ReAPD) were investigated and compared to those of BGO. The energy and timing resolutions were measured as a function of photodiode bias and amplifier shaping time constant. While the performance of BGO and GSO is critically dependent on these operating parameters because the signal-to-noise ratio is degraded by the dark and excess noise from the ReAPD, that of the high-luminosity LSO and YSO scintillators is not. The energy resolution in these crystals is demonstrated to be limited by the scintillator resolution. Further analysis indicates that excess variance in the number of photons generated in the crystal is mostly responsible for their disappointing energy resolution. This feature appears to be related to the important non-linearity in scintillation response as a function of incident gamma-ray energy observed for these high-luminosity scintillators.

Blue enhanced large area avalanche photodiodes in scintillation detection with LSO, YAP and LuAP crystals

Scintillation detectors consisting of LSO, LuAP and YAP crystals fitted to bevelled-edge large area avalanche photodiodes of 10 mm in diameter characterised by a high quantum efficiency up to 68% at 350 nm wavelength were studied. Among the properties measured were the number of electron-hole pairs, energy resolution and noise contribution of the LAAPDs at shaping time constants down to 20 ns at different gains. High electron-hole pair numbers of 11000/spl plusmn/550 e-h/MeV and 19500/spl plusmn/980 e-h/MeV for the YAP and LSO crystals were measured, respectively. The energy resolution for /sup 137/Cs /spl gamma/-rays of 5.9% in FWHM for the YAP crystal, much better than that of 9.9% for the LSO crystal were measured at 0.5 /spl mu/s shaping time constant and the APD gain of 40. The same study done with a fast shaping of 20 ns and the APD gain of 160 showed the energy resolution of 6.7%.

A novel electronic chain for high level neutron coincidence counters

This paper describes a counting chain (charge preamplifier, shaping amplifier and threshold discriminator) devoted to /sup 3/He neutron detectors. The preamplifier adopts an innovative circuit with the gate of the input JFET floating and a DC feedback loop that stabilizes the output voltage acting on the input cascade second transistor. Static and dynamic analysis, including the effects of the detector bias network, is developed and reported. The shaping amplifier is a fifth-order quasi-gaussian OPAMP active filter that minimizes the number of components. Shaping time constants ranging from 1 /spl mu/s down to 100 ns are explored in order to maximize the count rate capability, maintaining nevertheless neutron discrimination. Experimental signals and spectra show that 100 ns shaping time is a lower limit for the usual /sup 3/He proportional detector. Since this counting chain is characterized by single 5V supply operation, high counting rate, small size and low cost, it is well suited for high efficiency neutron well detectors where a large number (10-100) of counting tubes are used (High Level Neutron Coincidence Counter HLNCC).

Subnanosecond timing with large area avalanche photodiodes and LSO scintillator

The detection properties of 10 mm and 16 mm diameter bevelled-edge avalanche photodiodes fitted with LSO crystal were measured using /spl psi/-sources. Among the properties measured were the electron-hole pair yield, time resolution and energy resolution. These detectors provide excellent time resolution of 570/spl plusmn/17 ps for /sup 60/Co /spl gamma/-rays with the energy threshold set at 1 MeV. The energy resolution of the 662 keV /spl gamma/-peak from a /sup 137/Cs source was measured to be 11% FWHM with a shaping time constant of 0.1 /spl mu/s. A comparative discussion of the observed time resolution of these large area APDs emphasizes that APD gain, its quantum efficiency, and capacitance are important in timing applications.

Ballistic deficit correction methods for large Ge detectors High counting rate study

A comparative study is carried out for different ballistic deficit correction methods versus input count rate (from 3 to 50 kcounts/s) using four large Ge detectors of about 70% relative efficiency. The present work shows that, at shaping time constants shorter or equal to 2 μs, the Tennelec TC245 linear amplifier in the BDC mode (Hinshaw method) is the best compromise for energy resolution and throughput. At low and moderate input count rates (≤10 kcounts/s) the Ortec 675 Ge resolution enhancer based on the Goulding/Landis method offers excellent energy resolution and throughput at 2 μs shaping time. The energy resolution obtained with the Intertechnique 7201 gated integrator is very sensitive to count rate which lowers the interest in this module. At 3 kcounts/s it works very satisfactorily. All correction methods lead to narrow sum-peaks indistinguishable from single γ lines. More than the classical throughput, the full energy peak throughput is found representative of the pile-up inspection dead time of the corrector circuits. A new synthetic representation, plotting simultaneously energy resolution and throughput versus input count rate, is proposed.

Evaluation of CdZnTe detectors for soft X-Ray applications

High resistivity CdZnTe is a very promising material for X-ray detection at room temperature or slightly below. Cooling a Cd/sub 0.8/Zn/sub 0.2/Te crystal down to -30/spl deg/C reduces the leakage current to the picoamp level, which enables the use of low-noise pulsed optical feedback instead of noisier resistive feedback preamplifiers. Also, longer shaping time constants at the linear amplifier can be used for the optimum resolution. We have obtained resolutions of 240 eV (FWHM) for the 5.9-keV /sup 55/Fe line at -40/spl deg/C and of 282 eV at -30/spl deg/C. The Fano factor for the material at -40/spl deg/C was calculated to be 0.14. These results compare well with the results obtained with peltier-cooled HgI/sub 2/ detectors. >

A ratio pulse-shape discriminator

Abstract A simple pulse-shape discriminator has been designed to separate signals from a CsI(Na)/GSO phoswich detector. It can provide information on both the total energy deposited and the contribution of each crystal to that total, allowing a software veto to be used according to the distribution of the energy deposited. Test results show that even for photons having an energy of 17 keV, the location of most of the interactions can be found unambiguously. The system has also been tested on the most commonly used NaI(T1)/CsI(T1) phoswich detector without particular optimisation of the shaping time-constants. These results show that it can work well for gamma-rays below 17 keV. This technique could be very useful in a system which produces more than two decay components, such as a triple phoswich detector, or in a system where pulse-shape discrimination is used for particle identification.

Digital pulse processor using moving average technique

A digital pulse processor with improved differential linearity and reduced dead time has been designed. The circuit uses an 8-b flash ADC (analog-to-digital converter) running at 36 MHz and continually sampling the signal from the preamplifier or shaping amplifier. The digitized signal is then processed by a digital moving averager. A digital peak detector is used for measuring the amplitude of the shaped pulses. A novel, threshold-free circuit has been designed that combines both the moving average and peak detection functions. The circuit also provides a timing signal with an uncertainty of one sampling period. The number of the averaged samples (equivalent to the shaping time constant) is digitally controlled. The resolution of the processor is limited by the finite ADC resolution and the finite sampling frequency. Increasing the sampling frequency should improve the resolution in pulse height analysis and the timing precision. >

Silicon Detectors Highly Compensated by Neutron Induced Deep Levels for Low Energy X-RAY Detection

ABSTRACTFast neutron radiation damage in silicon results in defect levels which are predominantly acceptor-like at low fluences and may be used to compensate high resistivity ntype material to create very high effective resisitivity material. Compensated material to the order of Neff below 1011/cm3 enables depletion of diode thicknesses ≥ 1mm at reasonable biases (<100V), yielding diodes of reasonable area and capacitance<1 pF which are suitable for low noise applications such as X-ray spectrometry. Although exposure to fluences of this order will greatly increase the generation current and require cooling, most high resolution X-ray spectrometry systems are routinely operated at reduced temperature to achieve low noise operation of the front end electronics. Silicon p+ /n− /n+ implanted devices (area ≤0.25 cm2) made on high resistivity FZ silicon have been irradiated by 1 MeV neutrons to fluences of a few times 1012 n/cm2. Thick n− substrates (d=630 μm and 1000 μm) were used to achieve detector capacitances εεo/d in the range of 1 pF. After a neutron fluence of ϕn=2.9×1012 n/cm2, the total depletion of a p+/n−/n+ detector, 1040 μm thick and an area of 0.1 cm2, is reached at about V=50 Volts, with a Cd of 1 pF and a neutron induced leakage current of about 300 nA at room temperature. A total depletion of an 680 μm thick detector was reached after the fluence of 2-5×1012 n/cm2 at a voltage of 20 volts, and the capacitance of a 0.25 cm2 diode is 4.5 pF The resistivities of the compensated detector substrates are in the range of 100 K Ω-cm, and are not inverted to “p” type. The trapping of collected charge by neutron induced deep levels is modeled and simulated, and is found to be less than a few percent; with no obvious effect on peak shape. Using a resistive feedback preamplifier of modest noise contribution (225 eV), resolution of the Mn K∝ X-ray was 255 eV (FWHM) with 3 μsec shaping time constants. Other effects of uncollected charge will be discussed and comparisons between this type of detectors and Li-drift silicon detectors will be made.

Position Linearity and Resolution of Large Position-Sensitive Silicon Detector with a Highly Uniform Thickness

Large two-dimensional position-sensitive silicon detectors (PSDs) with good position linearity, resolution and thickness uniformity have been developed. The PSDs have an effective area of 62 mm×62 mm and a thickness of 500 µm. The non-linearity and position resolution for the whole sensitive area are found to be less than 0.26 mm in rms and 0.44 mm in fwhm, respectively, when an energy of 157 MeV is deposited in the detector and a shaping time constant of 12 µs is used in the pulse processing. The non-uniformity of detector thickness is ≤0.1% in rms. The PSDs recently developed are good enough to separate iron isotopes in a telescope aboard the GEOTAIL spacecraft scheduled to be launched at July in 1992.

Ballistic deficit correction methods for large Ge detectors

A comparative study of different ballistic deficit correction methods is carried out using three large volume Ge detectors of about 70% efficiency. The present work shows that for spectroscopy systems working with short shaping time constants the gated integrator is superior as it ensures the best energy resolution performance for both high and low γ-ray energies. For longer shaping time constants ( ≥ 2 μs) the Ortec 675 Ge resolution enhancer (the Ortec extension of the Goulding/Landis method) gives the best energy resolution due to a partial compensation of carrier trapping. It is shown that the gated integrator reduces the noise level in comparison to common linear amplifiers with Gaussian shaping. No evidence for its special sensitivity to low frequency noise, at least at short shaping time constants, has been observed. The compensation of ballistic deficit allowed us to determine the intrinsic energy resolution of large volume Ge detectors and to deduce their effective Fano factor.

A Sealed High Pressure Xenon-Carbon Dioxide Filled Position Sensitive Proportional Counter

A sealed position sensitive proportional counter, used in previously reported work with xenon-methane and intended for applications in X-ray astronomy, has been operated with xenon-carbon dioxide at 2 atmospheres pressure. As expected, the improved UV absorption of carbon dioxide limits the effect of secondary avalanches so that a position resolution of 230 microns FWHM is obtained compared with 360 microns in xenon-methane. The modulation of the position signal at the anode wire pitch, observed in xenon-methane is greater in xenon-carbon dioxide for the same amplifier shaping time constant. However, it does diminish for longer time constants and the position resolution improves, contrary to its behaviour in xenon-methane. Thus considerably improved position resolution with marginally degraded linearity are achieved with xenon-carbon dioxide.

Signal Coupling from a Semiconductor Detector to a Cold Resistance Preamplifier Through a Cable or a Delay - Line Transformer

The behavior of a charge-sensitive preamplifier exhibiting a stabilized cold resistance at the input port is analyzed in view of its application as a low-noise termination for a cable or a delay-line transformer. Its use in conjuction with a remote semiconductor detector of large capacitance is discussed. Results obtained with different values of cable length, detector capacitance and shaping time constants are given. It is concluded that such a preamplifier can be successfully employed in several circumstances associated with silicon live targets in high-energy experiments.

On a linear amplifier for high resolution nuclear pulse analysis

Investigations on a high performance linear amplifier for use in nuclear spectroscopy are presented. The amplifier is characterized by a large gain range, a high gain stability, and a wide range of shaping time constants in order to suit different detector systems. A prompt and a delayed pulse are simultaneously available at two independent outputs. The application of temperature stabilized transistor pairs as input active elements results in a highly stable output dc level. The most important circuit features of linear amplifiers are discussed.

Fast neutron spectrum measurements in the MTR scandium and iron filtered beams

The energy response of proton-recoil proportional counters was studied by making fast neutron spectrum measurements in the MTR scandium and iron filtered beams over the energy range of 200 eV to 1 MeV. Results of the measurements were compared with time-of-flight data. It was found that the calibration technique suggested by Bennett is adequate for accurate measurements if the hydrogen counter data is corrected for calibration pulse height losses due to shaping time constants in the linear amplifier. Further evidence of the energy loss per ion pair for hydrogen ( W) being constant with energy down to 2 keV was found. Cutoff models suggested for the hydrogen W, which account for an energy variation near 2 keV, can cause errors in the measured neutron spectrum. The results also showed that W for methane is energy dependent in the region around 100 keV.

Noise Studies of Ceramic Encapsulated Junction Field Effect Transistors (JFETs)

In low noise preamplifiers for nuclear and X-ray spectroscopy, a major source of electronic noise is the lossy glass insulator in the header of the input junction field effect transistor. Substitution of low loss ceramic headers for glass headers and encapsulation with unselected Union Carbide and Texas Instruments 2N 4416 JFET chips decreases total preamplifier electronic noise from 330 eV to 200 eV (FWHM)Si at room temperature. Further reduction in noise is accomplished by cooling the JFET. Preamplifier noise decreases to 125 eV for the TI JFET and to 160 eV for the UC JFET at their optimum temperatures. These noise values were obtained with a feedback resistor and with 4 μsec RC main amplifier pulse shaping time constants. The TI JFET exhibits a narrow minimum in its noise vs temperature curve, therefore it is essential that temperature be controlled quite precisely. A JFET temperature control system is described. Additional results concerning chemical contamination of JFET silicon chips by glass headers are presented. Finally, a method of decreasing noise still further by eliminating the feedback resistor and by using a pulsed light feedback system is discussed.

Compton Suppression Using Pulse-Shape Discrimination and Anticoincidence Techniques

The suppression of the background continuum in solid-state detectors by anticoincidence and/or pulse-shape discrimination aids in the interpretation of spectra and enhances the peak to Compton ratio. Studies were performed with both 137Cs and 59Fe sources. For the 59Fe source, an average reduction of 85% in the continuum resulted using a planar diode 110 mm2 by 2.4 mm deep in an anticoincidence annulus of NaI(Tl). Both the distribution and number of events rejected varies with the amplifier shaping time constants. Results are compared with those obtained by other investigators.