Algorithms for pulse shape analysis using silicon detectors

Gamma-ray imaging techniques, such as Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT), are becoming increasingly important for clinical diagnosis. As the power of these methods becomes more evident, the use of these techniques is becoming more widespread However, the performance of existing systems is restricted by the inherent properties of the scintillator detectors they are based on (e.g. poor energy resolution). The development of High Purity Germanium detectors has now reached the stage where they can be used as effective imaging devices. The electrical segmentation of the Germanium crystal coupled with digital signal processing electronics allows the measurement of energy, time, and position following the interaction of a gamma-ray. We propose to build a PET imaging system composed of two segmented planar HPGe detectors. The position and energy information are extracted following the digitisation of the detector charge signal, analysed using Pulse Shape Analysis (PSA) algorithms. Such a technique can potentially give a position resolution of about 1 mm/sup 3/ inside the detector volume. A prototype of a Compton camera has been built in order to test our pulse processing techniques and develop some gamma-ray tracking algorithms. The system is composed of a planar detector with 5mm strips in a 24/spl times/12 configuration, and a 16 segment coaxial Clover detector. Before being able to reconstruct the path of gamma-rays with the camera, it is necessary to calibrate the response of each detector as a function of the position. Several characterisation measurements have been made using an automated detector scanning systems. Preliminary results from the characterisation of the Clover will are presented in this paper.

A pulse shape analysis algorithm and a method for suppressing the noise component of signals from a large copper proportional counter in the experiment aimed at searching for 2K capture of Kr-78 are described. These signals correspond to a compound event with different numbers of charge clusters due to from primary ionization is formed by these signals. A technique for separating single- and multipoint events and determining the charge in individual clusters is presented. Using the Daubechies wavelets in multiresolutional signal analysis, it is possible to increase the sensitivity and the resolution in extraction of multipoint events in the detector by a factor of 3-4.

Exploring hit positions of recorded events can help to understand and suppress backgrounds in rare event searches. We propose a pulse shape analysis method to discriminate single-site events (SSEs) in the inner and outer layer of a small contact P-type germanium detector (HPGe). SSEs in the inner and outer layer have different pulse shape features, of which the rise time of the (TQ)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(T_{Q})$$\\end{document} and current pulse (TI)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(T_{I})$$\\end{document} are selected for discrimination. A 500 Bq Thorium-228 (Th-228) source is used to determine the boundaries between the two layers. The double escape peak events from 2614.5 keV γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gamma $$\\end{document}-ray are selected as typical SSEs, their numbers in the two layers are used to calculate the volumes and shapes of those layers. Considering the statistical and systematic uncertainties, the inner layer volume is evaluated to be 47.2% ± 0.26%(stat.) ± 0.18%(sys.) ± 0.22%(sys.) of the total sensitive volume. Selecting the inner layer as the analysis volume can reduce the external background in the signal region of Ge-76 neutrinoless double beta (0νββ)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ u \\beta \\beta )$$\\end{document} decay. We use the Th-228 data to validate the inner layer model and evaluate the background suppression power in the 0νββ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ u \\beta \\beta $$\\end{document} signal region (Qββ=2039\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(Q_{\\beta \\beta }=2039$$\\end{document} keV). The virtual segmentation further reduces the background from the external Th-228 source by about 10%. The virtual segmentation could be used to efficiently suppress surface background like electrons from Ar-42 decay in 0νββ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ u \\beta \\beta $$\\end{document} experiments using germanium detectors immersed in liquid argon.

A detector of O(1 kg) modular mass with O(100 eV) threshold at O(1 kg-1keV-1day-1) background level finds tremendous application in the field of neutrino and dark matter physics. This novel detector demands overcoming several challenges at both hardware and software levels. The collaboration is exploring Germanium detection technology and highlights of the R & D program are presented. The salient features of various detector configuration and the applied analysis methodologies are discussed. In particular the differentiation of surface and bulk events by pulse shape analysis in point contact Germanium detector is described. These advances pave the way for new detector technique to be fully exploited.

Algorithms for shape analysis are reviewed and classified under various criteria, whether they examine the boundary only or the whole area and whether they describe the original picture in terms of scalar measurements or through structural descriptions. The emphasis is on methodologies which have been popular during the last five years and among them, those which are information preserving.

The response of a 19 kg active mass liquid argonscintillation detector to gamma, alpha and neutron radiation and itscharacteristic scintillation photon time distribution has been studiedin the framework of the Gerda double beta decay research anddevelopment program. The achieved photo-electron yield of 1.24 pe/keVwas stable during the two years of operation. The detector exhibitsexcellent energy resolution of 8.7/17.2 keV (σ) for gammaenergies of 60/239 keV. Radon was loaded into the liquid argon tostudy alpha energy quenching and decay time correlation of itsprogenies. A robust pulse shape analysis method was used to identifyand discriminate amongst the different radiation types. 60 keV gammasignals could be discriminated against neutron recoils of the samevisible energy with a miss-identification probability of < 5 · 10−4 limited by statistics and ambient backgrounds. Xenon dopingof liquid argon increased the photo-electron yield and improved thespectroscopic performance of the detector leading to an energyresolution of 7.2/15.4 keV (σ) for 60/239 keV. Thediscrimination power improved slightly with the addition of xenon up to concentrations of 300 ppm. Applications for background identification anddiscrimination in double beta decay search with 76Ge crystals, aswell as for Dark Matter search with liquid argon are discussed.

The heat capacity calibration ‘constants’ of a commercial MTDSC system (TA 3100) were determined in a variety of experimental conditions. For a given modulation frequency, the calibration constants are the same within a few percents for different temperatures, and over a wide range of modulation amplitudes and scan rates. This variation decreases below 1% if hidden instrumental constraints are taken into account, which are related with the capability of the control system to achieve the desired temperature program. On the other hand, the calibration constant changes substantially with the period, and takes anomalously high values for the short modulation periods (20+40 s). Rules to optimize the accuracy of the system are given.

When studying short-lived nuclei via $ \alpha$ spectroscopic methods at recoil separators, the decay events tend to happen immediately after the recoil implantations or the decays of parent nuclei and therefore pile-up pulses are often encountered. Digital acquisition systems now allow the recording of waveforms of pile-up events rather than rejecting them as in traditional analog systems. Therefore, the extraction of the energy and time information stored in pile-up events with suitable precision is of great significance to the measurements of short-lived nuclei. In this paper, a pulse shape fitting method for processing pile-up events from silicon detectors are proposed. The method is developed on the basis of an improved analytical function for describing the shapes of preamplifier signals. The validity of the method is demonstrated by the experimental data of some short-lived nuclei produced in the reaction 40Ca + 184W.

Due to their excellent spectroscopic capabilities and sensitivity to the position of γ-ray interactions, segmented High-Purity Germanium (HPGe) detectors are frequently used in γ-ray imaging applications. The quality of produced images is heavily dependent on the precision of the identification of interaction position within the detector that can be achieved through segmentation and the application of pulse shape analysis (PSA) techniques. In this work, a simulated database of signals was produced for a HPGe planar detector used in a multi-tiered Compton camera system. The impurity concentration and temperature-dependent mobility parameters of the simulation were optimised and validated by evaluating the pulse-shape response at select positions using experimentally-measured collimated γ-beam data. A grid-search algorithm was developed for analysis of single and double-site γ-ray interactions using signal comparison PSA and is shown to improve the position resolution within the detector.

Utilizing times of flight on the order of a nanosecond for heavy particle identification with silicon detectors, it is desirable to reliably distinguish time differences of approximately 100 picoseconds or less. In addition, the required timing circuitry must not substantially deteriorate amplitude resolution capability. In the system designed for this purpose the detector charge is first collected on the capacitance of the detector and fast, low noise, preamplifier and is later transferred to the charge sensitive preamplifier. The fast signal is processed to obtain good time resolution and low walk. The system has a FWHM time resolution of 40 picoseconds, referred to 5.8 MeV deposited in a 100 pF silicon detector. An actual peak width from 3 MeV deposited in an 80 pF silicon by He3 particles was 165 psec FWHM, which is about twice that expected if more optimum pulse shaping had been employed. The instrumental walk for a 100 to 1 amplitude range is less than 150 picoseconds, and is approximately 100 picoseconds for a 30 to 1 range. The noise added to the slow system is equal to that caused by an additional 17 pF of capacitance at its input, plus an overall increase of 5% or less for commonly used slow pulse shaping.

Human fibroblasts have exhibited enhanced DNA synthesis when exposed to sinusoidally varying magnetic fields for a wide range of frequencies (15 hertz to 4 kilohertz) and amplitudes (2.3 X 10(-6) to 5.6 X 10(-4) tesla). This effect, which is at maximum during the middle of the S phase of the cell cycle, appears to be independent of the time derivative of the magnetic field, suggesting an underlying mechanism other than Faraday's law. The threshold is estimated to be between 0.5 X 10(-5) and 2.5 X 10(-5) tesla per second. These results bring into question the allegedly specific magnetic wave shapes now used in therapeutic devices for bone nonunion. The range of magnetic field amplitudes tested encompass the geomagnetic field, suggesting the possibility of mutagenic interactions directly arising from short-term changes in the earth's field.

Monte Carlo optimization of sample dimensions of an 241Am–Be source-based PGNAA setup for water rejects analysis

XRF analysis allows for quick and accurate evaluation of chemical composition of wide range of materials, and is actively used in different fields of science and industry. The accuracy of a measurement depends on number of counts recorded to spectrum, and can be improved either by extending the measurement time, which is unwelcome especially when analyzing a large amount of samples, or by increasing the count rate of spectrometer. The latter however leads to parasitic effects due to pulse pile-up caused by detectors finite resolving time. When acquiring XRF spectrum at high count rates with semiconductor detectors pulse pile-up leads to distortion of recorded spectrum, in particular to appearance of so-called pile-up peaks. Pile-up peaks appear due to detector being unable to separate two or more incident quanta arriving in time interval less than detectors resolving time, and recording them as a single quantum with energy that equals to sum of incident quanta energies. If not accounted for pile-up peaks may lead to quantitative and qualitative errors while making composition calculations based on the recorded spectrum. In this article the statistic of pile-up peaks in XRF spectra, their dependence on spectrum acquisition parameters and ways of improvement of pulse pile-up rejection algorithms are considered. Experimental data was acquired using Elvatech Prospector 3 spectrometer. A high dependency of spectrometers resolving time on quanta energies was observed. Based off experimental data an improvement to pile-up rejection algorithm was proposed.

Single line of sight frame camera based on the RadOptic effect of ultrafast semiconductor detector

Rare earth nuclei are probable candidates for exhibiting cluster radioactivity. In the present work, a theoretical study of alpha decay and heavy cluster emission from Dy, Er, and Yb nuclei in the mass range 150 < A < 190 has been carried out in a fission model approach. Some of the proton-rich isotopes of these nuclei are found to have long half-lives, but within the measurable range. Many of the neutron-rich isotopes under consideration are far from beta stability and the most probable light clusters predicted are highly exotic in nature and the corresponding half-lives are extremely short. However, the decays of most probable heavy clusters predicted have half-lives in the measurable range. The calculations on neutron-rich isotopes indicate the emergence of new magic numbers for protons and neutrons. The centrifugal barrier effect on decay rate of some proton-rich Er isotopes is also investigated.