Detector Pulse Research Articles

Pile-up corrections for high-precision superallowed [formula omitted] decay half-life measurements via [formula omitted]-ray photopeak counting

A general technique that corrects γ -ray gated β decay-curve data for detector pulse pile-up is presented. The method includes corrections for non-zero time-resolution and energy-threshold effects in addition to a special treatment of saturating events due to cosmic rays. This technique is verified through a Monte Carlo simulation and experimental data using radioactive beams of Na 26 implanted at the center of the 8 π γ -ray spectrometer at the ISAC facility at TRIUMF in Vancouver, Canada. The β -decay half-life of Na 26 obtained from counting 1809-keV γ -ray photopeaks emitted by the daughter Mg 26 was determined to be T 1 / 2 = 1.07167 ± 0.00055 s following a 27 σ correction for detector pulse pile-up. This result is in excellent agreement with the result of a previous measurement that employed direct β counting and demonstrates the feasibility of high-precision β -decay half-life measurements through the use of high-purity germanium γ -ray detectors. The technique presented here, while motivated by superallowed-Fermi β decay studies, is general and can be used for all half-life determinations (e.g. α -, β -, X-ray, fission) in which a γ -ray photopeak is used to select the decays of a particular isotope.

Asymmetry of characteristic X-ray peaks obtained by a Si(Li) detector

The asymmetry of the characteristic X-ray peaks obtained using a Si(Li) detector is mainly due to incomplete charge collection. Impurities and defects in the crystalline structure of Si can act as “traps” for holes and electrons in their trip toward the detector electrodes. Therefore, the collected charge, and consequently the detected energy, is smaller than the expected one. The global effect is that peaks may present a “tail” toward the low energy side. This phenomenon is more important for low energies (lower than 2.3 keV, in the case of the detector characterized). In this work, the parameters related to peak asymmetry were studied, allowing a better understanding of the trapping process mentioned above. For this purpose, spectra from mono- and multi-element samples were collected for elements with atomic number between 7 and 20. In order to describe the shape of the characteristic K peaks as a function of its energy, an asymmetric correction to a Gaussian function was proposed. Spectra were obtained by electron probe microanalysis for incidence energies between 5 and 25 keV using an energy dispersive spectrometer equipped with an ultra-thin window Si(Li) detector. It was observed that the area corresponding to the asymmetric correction exhibits an energy dependence similar to that of the mass absorption coefficient of the detector material. In addition, other two spectrometers were used to investigate the dependence of tailing on the detection system. When two spectrometers with the same kind of detector and different pulse processors were compared, peaks were more asymmetric for lower peaking time values. When two different detectors were used, differences were even more important.

On-Board Digital Signal Processing for 4 $\pi$-Detector Large-Area Telescopes

The digital pulse shape data acquisition (DAQ) system used by the large area telescopes of the 4pi-multidetector CHIMERA requires transferring large amount of data to the computer. This is necessary so that the same information can be extracted from the detector pulses as in previous analog based systems. To overcome this problem, we have used a sampling ADC-board equipped with two TigerSHARC digital signal processors. The board receives the data and reconstructs the event parameters online. The data volume is significantly reduced by transmitting to the DAQ only the reconstructed event parameters

The ATLAS liquid argon calorimeter read-out system

The Liquid Argon calorimeters play a central role in the ATLAS experiment. The environment at the LHC collider imposes challenging tasks to their read-out system. To achieve measurements of particles and trigger signals at high precision, the detector signals are processed at various stages before reaching the Data Acquisition system (DAQ). Signals from the calorimeter cells are received by front-end boards, which digitize and sample the incoming pulse. Read-out Driver (ROD) boards further process the data at a trigger rate of up to 75 kHz. An optimal filtering procedure is applied to optimize the signal-to-noise ratio. The ROD boards calculate precise energy, time and quality of the detector pulse, which are then sent to the DAQ. In addition, the RODs perform a monitoring of the data. The architecture of the ATLAS Liquid Argon detector read-out is discussed, in particular the design and functionality of the ROD board. Performance results obtained with ROD prototypes as well as experience from complete test setups with final production boards are reported.

Properties of a neutron detector based on ionization chamber with /sup 6/Li converters

The ICC neutron detector is an ionization chamber with three neutron absorbing converter electrodes of 6Li metal foils. On both sides the foils face collector electrodes that are positively biased. The neutron detection was investigated at the pulsed neutron source of the Argonne National Laboratory, Argonne, IL. The induced charge due to drifting electrons that are generated by the energetic 3H recoil nucleus in the C2F6 gas between the electrodes was measured as function of gas pressure, gas contamination, and applied drift voltage. The neutron detection efficiency was determined and the gamma insensitivity was estimated. The pulse heights were calibrated by the induced charge from test pulses. The timing characteristics of the detector pulses were assessed, and time-of-flight measurements demonstrated the usefulness of the detector. First, neutron diffraction experiments were made with structured collection electrodes for position-sensitive detection and for estimating the achievable spatial resolution

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.

Performance analysis of a digital positron lifetime spectrometer

A digital positron lifetime spectrometer has been set up and tested comprehensively. The system consists of a fast commercial digitizer connected to a computer, a simple coincidence circuit and software to extract the timing from the collected detector pulses. The digital system has the same time resolution as a conventional analog apparatus using the same detectors. The pulse processing part of the spectrometer is able to analyze and store in real-time several thousands of events per second, which is an order of magnitude more than the count rates in typical positron lifetime experiments. The data acquisition can handle small pulses, down to a few tens of millivolts, and its time-scale linearity and stability are very good. We discuss the advantages of timing with software, e.g. simple setup, use of different timing algorithms and possibility of an offline analysis of lifetime events. The idea is generally applicable to direct measurement of time intervals with picosecond accuracy.

Optimal Parameter Ratios Guaranteeing Safety During Reactor Startup

A method and relations are proposed for determining the optimal ratio of the parameters for putting a reactor into a critical state safely: the initial subcriticality before reactor startup, the reactivity variation step, the initial pulse frequency, the initial counting rate of the number of pulses, the frequency of the spurious signal pulse, and the sensitivity of the pulsed startup channel, the neutron-flux density at the detector site, the neutron source strength, and the frequency of the pulses in the startup channel that gives the required speed of formation of the emergency signal according to the time interval as the reactor is put into a critical state. The methods and formulas for determining the optimal parameters are obtained taking into account the statistical nature of the frequency of the detector pulses and of the spurious output signal.

The X-ray performance of a driftless gas proportional scintillation counter using short shaping-time constants for pulse analysis

Performance characteristics are evaluated for a xenon driftless gas proportional scintillation counter (GPSC), for which detector pulses are formatted using very short shaping-time constants (∼50 ns), and directly analyzed by the MCA without previous pulse time duration analysis and amplitude correction. The present detector and method allow the achievement of detector energy linearity and energy resolution similar to those of conventional GPSCs, reducing background levels and maximizing the detector count-rate capability. However, the obtained peak tails can be somewhat larger than those obtained with conventional GPSCs for X-ray energies above ∼30 keV. Energy resolutions of 7.7%, 4.5% and 3.8% can be achieved for 5.9, 22.1 and 59.6 keV X-rays, respectively.

SPI-specific analysis method and software overview

The SPI spectrometer on INTEGRAL features a camera system with 19 Ge detector modules, imaging photons through a tungsten coded mask. Background is reduced by an anticoincidence detector system surrounding these. The specifics of this instrument lead to data correction and analysis methods which are described here. Raw data for science analysis are detector event messages and spectra for different categories of detector hits and pulse shapes. Preprocessing combines calibrated spectra from these, which are then interpreted using the imaging and spectral response function for measured spectra where parts of the detector plane are occulted by the mask. Background dominates the overall signal, tailored background estimates and models are based on instrument-specific signatures, their correlations, and trends.

Counting statistics of X-ray detectors at high counting rates.

Modern synchrotron radiation sources with insertion devices and focusing optics produce high fluxes of X-rays at the sample, which leads to a requirement for photon-counting detectors to operate at high counting rates. With high counting rates there can be significant non-linearity in the response of the detector to incident X-ray flux, where this non-linearity is caused by the overlap of the electronic pulses that are produced by each X-ray. A model that describes the overlap of detector pulses is developed in this paper. This model predicts that the correction to the counting rate for pulse overlap is the same as a conventional dead-time correction. The model is also used to calculate the statistical uncertainty of a measurement and predicts that the error associated with a measurement can be increased significantly over that predicted by Poisson (N(-1/2)) statistics. The error differs from that predicted by a conventional dead-time treatment.

CdZnTe spectra improvement through digital pulse amplitude correction using the linear sliding method

This paper presents innovative algorithms for optimum filtering, rise-time discrimination and proper correction of solid-state detector pulses based on digital pulse processing (DPP) techniques and on a Linear Sliding Method. These techniques allow a significant improvement on the spectral resolution by taking into account both the electronic noise and the pulse rise time fluctuations mainly due to the incomplete charge collection in the CZT crystal. In order to develop these algorithms pre-amplified pulses coming from a planar XR100-T detector (Amptek) were digitized by a 12-bit PCI acquisition card (125MHz) hosted by a PC platform. The presented results show that, due to the possibility of fine adjustments on the discrimination and correction parameters during the processing stage, this flexible DPP implementation leads to significantly better results than the analog hardware discrimination solution. Comparative analysis of Am-241 spectra show a significant reduction of the low-energy “tail” on the 59.5keV photo-peak.

Digital measurement of positron lifetime

A prototype of a digital positron lifetime apparatus is presented. We demonstrate that direct digitizing of detector pulses and subsequent simple analysis with software, can be used to replace the conventional analog electronics chain (constant-fraction discriminators, time-to-amplitude converter and multichannel analyzer). In this work, we use a fast digital oscilloscope. The quality of the lifetime data is shown to be as good as with a usual apparatus. For a pulsed positron lifetime beam the digital system is particularly suitable as no coincidence detection is needed and only one analog pulse has to be analyzed.

An educational tool for demonstrating the TOF-PET technique

A detector system for positron emission tomography with time-of-flight capability has been built to serve as an educational tool for undergraduate students. The set-up consists of 48 BaF 2 scintillator crystals, each coupled to a fast photo-multiplier tube, mounted in a circular geometry. The analogue detector pulses are handled by fast constant fraction discriminators. A dedicated unit reduces the 48 channels to eight channels via delay-line encoding, and the signals are then fed to an eight channel fast time-to-digital converter. A VME processor sorts the events and sends them to a workstation where the coincident events are extracted. The time resolution of the detectors together with fast VME based electronics allows for time-of-flight measurements to improve on the signal-to-noise ratio in the reconstructed images. The system can be used for different types of exercises for the students, varying from the fundamentals of scintillator detectors to advanced image reconstruction. The set-up is described and some results are presented.

Analog fiber optic transmission link

An analog fiber optic transmission link (FOTL) has been designed for relaying signals from equipment held at high voltage potentials to equipment at ground potential. The need for a fiber link arises from the requirements of low energy, light ion experiments conducted at the Triangle Universities Nuclear Laboratory. In these experiments, ions are accelerated and transported to a high voltage scattering chamber and the FOTL is used to transmit signals from the scattering chamber to remote data acquisition systems. The performance of the link is mainly driven by the stringent requirements needed for successful transmission of analog pulses from radiation detectors located in the scattering chamber. The FOTL consists of a transmitter, receiver, and baseline restorer that are capable of transmitting radiation detector pulses without pulse height distortion, thus allowing high resolution energy measurements. The FOTL is also suitable for use with a wide variety of other analog signal types.

Method for effective dead time measurement in counting systems

The counting losses introduced by the dead time of a counting system are a limiting factor in counting measurements. The purpose of this work is to report an efficient method for the measurement of the effective dead time of a counting system and to characterize its dead time behavior, providing a way to investigate each experimental situation. The method, which we designate as Delayed and Mixed Pulses method, is based on the artificial piling-up of detector pulses with electronic pulses delayed by a specific time interval. It is applicable to the measurement of the effective dead time of a counting system, including both pile-up effects and the dead time characteristics of the elements of the counting chain. With counting systems relying on gaseous radiation detectors, we achieved a standard uncertainty of about 5–10% in the dead times measured.

A driftless gas proportional scintillation counter for muonic hydrogen X-ray spectroscopy under strong magnetic fields

An experiment involving muonic hydrogen requires an X-ray detector having large area and working under strong magnetic fields (5 T) with good energy and timing resolution. A compact, driftless gas proportional scintillation counter (GPSC) capable of operating under such magnetic fields is investigated. This GPSC uses a CsI photocathode deposited onto a microstrip plate as the UV scintillation readout photosensor. This photocathode has the advantage of operating in direct contact with the scintillation gas. The detector is filled with pure xenon and is designed to have a high detection efficiency for 2 keV X-rays. Energy resolutions of 23% and 22% were obtained for 1.74 and 2.3 keV X-rays, respectively. The low-energy detector limit due to the electronic noise is 300 eV. Its performance in the presence of strong magnetic fields was tested. At magnetic field of 5 T the detector pulse amplitudes are reduced by less than 25%, while the detector energy resolution and pulse rise time present a relative increase of less than 10%.

Measuring the arrival times of overlapped photo-events

We have developed and tested experimentally and by computer simulations a novel method for measuring the individual arrival times of temporally non-resolved (overlapped) single-photon detector pulses in high-intensity streams. The method is based on a set of linear transformations of photo-event pulses into a deconvolution, a transformation to standardized functions, a non-linear Chi-square fitting, etc. The retrieving accuracy varies within the range of 2–4 ns at an ADC sampling interval of 50 ns. The simultaneous measurement without dead time effects of the main photon statistics distributions as the distribution of intraevent intervals and the charge distributions has been demonstrated. The method covers the intermediate scale of intensities, where other well-known techniques for processing of photo-events are ineffective. It can be effectively applied in nuclear experiments, time-resolved spectroscopy, optical remote sensing, etc.

Investigating ion-surface collisions with a niobium superconducting tunnel junction detector in a time-of-flight mass spectrometer

The performance of an energy sensitive, niobium superconducting tunnel junction (STJ) detector is investigated by measuring the pulse height produced by impacting molecular and atomic ions at different kinetic energies. Ions are produced by laser desorption and matrix-assisted laser desorption in a time-of-flight mass spectrometer. Our results show that the STJ detector pulse height decreases for increasing molecular ion mass, passes through a minimum at around 2000 Da, and then increases with increasing mass of molecular ions above 2000 Da. The detector does not show a decline in sensitivity for high mass ions as is observed with microchannel plate ion detectors. These detector plus height measurements are discussed in terms of several physical mechanisms involved in an ion-surface collision.

Pulse loss and counting statistics with a digital spectrometer

A commercially available digital γ-ray spectrometer is tested at high count rates using a germanium detector and appropriate pulse processing parameters. Correction for pulse loss due to dead time and pileup is done by extending the live time according to the Gedcke–Hale method. The accuracy of this count-loss correction technique is tested experimentally up to saturation count rates. Also the statistical uncertainty applicable to the total number of counts in arbitrary regions of interest in γ-ray spectra is studied experimentally, for measurements at fixed real time as well as at fixed live time. The results are similar to those found with traditional spectrometers using analog technology.