Preamplifier Signals Research Articles

Implementation of a machine learning technique for estimating gamma direction using a coaxial High Purity Germanium detector

We demonstrate the ability to obtain the direction of the gamma rays using a standard coaxial high purity germanium (HPGe) detector using the direction-sensitive information embedded in the shape of the pre-amplified HPGe signals. We deduced the complex relationship between the shape of the signal and the direction from which the gamma-ray enters the detector active volume using a two-step machine learning technique. In the first step, we collected pulses from the HPGe detector due to a 133Ba radioactive source placed in four distinct positions around the detector while keeping the distance from the center of the detector crystal constant. A subset of the pulses collected with radioactive source kept at the four positions was used to train an artificial neural network (ANN) called a self-organizing map (SOM) to cluster the HPGe waveforms based on their shape. The trained SOM network was then utilized to produce direction-specific maps corresponding to pulses generated when the 133Ba source is at a specific location with respect to the detector. In the second step, we used the SOM-generated direction-specific maps to train another network composed of a single feedforward layer for predicting the direction of the gamma ray from the pulses produced by the HPGe detector due to the gamma energy deposition. Our results show that even without employing complex methodologies, a standard coaxial HPGe detector can estimate the direction of incoming gamma rays and thus, provide initial guidance on the gamma-emitting radioactive source direction with reference to the detector.

Radio frequency over fiber technology for SKA-low receiver

The signal reception chain is an essential element for achieving the square kilometer array-low (SKA-low) system requirements in terms of high sensitivity and dynamic range. The balance between gain, linearity, and low power consumption, as well as the cost, are fundamental parameters that influence the selection of the most suitable technology for SKA-low. Further factors, such as low self-generated radio frequency (RF) interference, high reliability, robustness under extreme environment, and last but not least, the distance between the antennas and the acquisition systems, have impacts on the selection for both architecture and receiver system design. The selected technology for the SKA-low RF signal transportation is RF-over-fiber systems, where the preamplified RF signal picked up by the antennas is carried via analogue modulation over optical fiber. The rationales behind the selection are reported, along with descriptions on the development of the receiver prototypes. The prototypes were deployed and installed on the demonstrator arrays at the selected SKA-low site in Western Australian. Particular attention has been put on the thermal characterization of the receiver system under the actual operating temperature on site, especially when both transmitting part and the optical medium are subjected to external ambient temperature variations. Performance issues encountered in the demonstrator arrays are also discussed along with some proposals for future activities.

Development of a modular high-sensitivity high-uniformity gamma camera for radiation monitoring applications

One direct way to improve the gamma camera’s sensitivity is using large-area gamma detectors. During construction of a large-area gamma camera, achieving high uniformity (including both spatial uniformity and events count map uniformity) across the entire active area is a major challenge. Another fabrication concern is that large-scale gamma sensors (mainly with reference to scintillators) and photoelectric converters such as position-sensitive photomultiplier tubes (PSPMTs) and silicon photomultipliers (SiPMs) are required. In this work, to design a large-area gamma camera with high uniformity, a 22×22 CsI(Na) array with a pitch size of 4.6×4.6 mm 2 is divided into four optically isolated 11×11 modules, where each module is coupled independently to an 8×8 SiPM array (6.13×6.13 mm 2 per channel) and is then read out using separate preamplifier electronics. The preamplified signals for each module are sampled independently via their respective analog-to-digital converters (ADCs) and the digitized signals are then processed in a single field-programmable gate array (FPGA). Extra dead zones and optical losses are avoided between the modules as a result of the optimal design used in the junction area for the four modules. This modular concept leads to a high counting rate capability. 2D flood field images of the 22×22 CsI(Na) array are obtained under uniform irradiation using 241Am. The counting rate measurements are performed using 137Cs under various attenuation conditions. The sensitivity is tested and compared with existing devices. The results show that high uniformity (1.43) and high counting rate capability (∼72% improvement) are achieved. Thanks to the large-area gamma sensor, the gamma camera developed in this paper achieves highest sensitivity among the listed devices (Locating 137Cs producing 0.6±0.02μSv/h in < 10 s). The principles developed in this paper can be applied to the construction of large-area radiation detectors.

Evaluation of a Digital Shaping Amplifier for Double-Sided Silicon Strip Detectors (DSSD)

This paper reports the digital implementation of analog shaping amplifiers by digital pulse processing in gamma-ray spectroscopy. For digital implementation, a numerical algorithm was developed by referring to the semi-Gaussian filter of the analog shaping amplifier. In order to verify the developed algorithm, we evaluated the obtained energy spectrum and compared the energy resolution with the existing analog processing results. The gamma-ray detector used a double-sided strip detector, and the preamplifier signal of the detector was digitized directly by using a commercial digitizer. Using the standard gamma-ray sources Am-241 (59 keV), Ba-133 (81 keV), and Co-57 (122 keV), we were able to detect gamma rays in a relatively low energy region. The energy resolution was improved at energies below 100 keV compared to the result of treatment with an analog shaping amplifier.

A reconfigurable CMOS sensor for tracking, pre-shower and digital electromagnetic calorimetry

Recent advances in CMOS imaging sensor technology have led to designs able to withstand much higher radiation levels, up to those required at hadron colliders for all but the innermost layers. Also, with stitching, wafer scale devices have been fabricated on the same process as used for the prototypes described in this submission, for applications such as, for example, direct electron detection in transmission electron microscopy. A small sensor prototype has been designed and fabricated in the TowerJazz 180 nm CMOS imaging process. The prototype has a pixel matrix of 64x64 pixels with a pitch of 55 x 55μm and reads out using fast logic at 40 MHz. Each pixel contains four collection electrodes, trimming logic, pre-amplifier, shaper and discriminator with digital output. It can be reconfigured to function as either a binary short strip sensor, for particle tracking including as a pre-shower, or as a pad sensor, counting the number of pixels above threshold for digital calorimetry. As well as providing a seamless transition from outer tracking to EM calorimetry, for optimal use of particle flow algorithms, digital calorimetry also can give excellent energy resolution. This concept is proposed as a possible option for future digital calorimeters able to read-out at LHC collision rates with good radiation hardness and realisable in low cost commercial technologies for equipping large areas. Results on physics performance simulation in a FCC-hh context and the characterisation of the prototype sensor are presented. The summing logic is demonstrated and the analogue pixel performance is validated by illuminating a test pixel within the matrix with a laser of wavelength of 1064 nm. The absolute laser intensity is calibrated such that the injected charge is similar to that expected for a minimum ionising particle. The measured pre-amplifier and shaper signals are compared to Cadence simulations. Laser illuminations in the digital pixel area and the response measured using a threshold scan confirm successful digital functionality in strip and pad operation modes.

High throughput rate digital processor based on real-time current pulse analysis

Charge sensitive type preamplifiers are usually used in scintillators, such as NaI (TI) and BGO, to preprocess the current pulse output of detectors. Under high-intensity gamma radioactive field, the wide falling edge of a charge sensitive preamplifier signal will cause a serious pulse pile-up, and the pile-up pulse will be discarded. Consequently, the pulse throughput rate declines significantly. Meanwhile, the charge sensitive amplifier can easily integrate two pulses with an approximate time of arrival into a single pulse to generate an accidental coincidence effect. After using a current-sensitive amplifier on the current pulse output from the detector in the system designed in this work, such output is digitalized by using a waveform digitizer with 125MHz sampling rate. The baseline is estimated statistically via a histogram, a real-time fitting and peeling algorithm is designed for the falling edge pile-up of the current pulse signal to recover the pile-up signal, and the digital charge of the current pulse signal is integrated to obtain particle energy. A 4L NaI(TI) scintillator detector is used in the experiment to measure the 137Cs γ radioactive source. Under a 165kcps counting rate, the pulse throughput rate is up to 99 %, the accidental coincidence probability is 0.387%, and the energy resolution reaches 7.54%, Even under a 3Mcps counting rate, the pulse throughput rate is still 85.43%. Therefore, the overall performance is superior to conventional voltage-type digital spectrometers under high counting rate application.

Fast Sampling and Read-Out for Ground-Based Imaging Cherenkov Telescopes

The SCT telescope has been proposed as a medium-sized telescope for the Cherenkov Telescope Array. One of the major upgrades is the design of the new Front End Electronics Module (FEEM). New FEEMs aim to read-out and digitize the SiPM pre-amplified signals down to the single photoelectron (phe). This phe signal is assumed equivalent to a signal with 2 mV peak amplitude and 500 MHz maximum bandwidth. FEEM should have a linear response up to 2 V for a required dynamic range of 1000 phe. FEEM noise equivalent of 0.5 phe - 1 mV of RMS noise - is an acceptable value. Due to the severe mechanical constraints, and to have a very compact electronics and low noise performance, FEEM consists of two stacked-up submodules, one dedicated to the power supplies and the other to house the FPGA, which reads out and sends digitized data to the main backplane. An FEEM is capable of digitizing 64 analog channels with a sampling frequency of 1 GSamples/s.

Pile-up correction algorithm for high count rate gamma ray spectroscopy

In nuclear applications related to radiation energy measurement, digital pulse processing for fast sampling facility has great advantages compared to the traditional analog pulse processing. A distortion in energy spectrum and degradation in timing resolution take place when pulse pile-up occurs. This in turn leads to degradations of the signals used in applications such as nuclear medicine. Pile-up pulses are commonly removed in most algorithms if they are detected, but in this case, getting a clean spectrum becomes a challenge due to data loss. A proposed pile-up detection and recovery algorithm based on fitting the preamplifier data and signal extrapolation is presented. The extrapolated pulse is used for recovering (extracting) the piled-up pulse to estimate its parameters. A smoothed first derivative algorithm is used for peak tracking inside the detected events. To evaluate the proposed pile-up recovery algorithm theoretically and experimentally, it is applied on both simulated input signals generated with different time displacements between the piled-up events, and experimental preamplifier radiation signals. The obtained results prove that the proposed algorithm achieves better utilization of power and can be used for high count rate (high activity elements) spectroscopy systems.

Development of a 2D digital Coincidence Doppler Broadening Spectrometer

A 2D digital spectrometer for coincidence measurement of Doppler broadening of positron annihilation radiation is developed and tested. The spectrometer is based on a 14-bit waveform digitizer which gives the possibility of fast and direct sampling of the preamplifier signals of the HPGe detectors and preserves the information required by the physics of Doppler broadening spectroscopy. Some features of the digital spectrometer are compared with its analog counterpart. Superior energy resolution, better peak symmetry, effective elimination of ballistic deficit and better stabilization of peak position are the advantages of the digital spectrometer. The performance of the digital spectrometer to support the CDBS experiment is tested by investigation on the electronic structure of some elements at 1st series of transition metals. The results show that the spectrometer is sensitive enough to distinguish the electronic structure of 3d metals differing a little in their valance band configuration.

A digital pulse fitting method for the $ \alpha$α decay studies of short-lived nuclei

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.

Field-Effect Transistor-Integration with TiO2 Nanoparticles for Sensing of Cardiac Troponin I Biomarker.

The development of electrical biosensor towards device miniaturization in order to achieve better sensitivity with enhanced electrical signal has certain limitations especially complexity in fabrication process and costs. In this paper, an alternative technique with minor modification in the device structure is presented for signal amplification by implementing ambipolar conduction in the biosensor itself. We demonstrated the field-effect transistor (FET)-based biosensor coupled back-gate for attaining a higher sensitivity with the detection of lower target abundance. To utilize the coupled back-gate as a pre-amplifier, silicon-on-insulator wafer with thicknesses of top-silicon and buried oxide (BOX) layers of 70 nm and 145 nm, respectively were desired. Titanium dioxide (TiO2) nanomaterial was deposited using sol-gel method on the channel which acts as a transducer. Surface functionalization on TiO2 thin film allowed an effective immobilization of anti-cardiac troponin I antibody to interact cardiac troponin I (cTnI). Binding events at each step was validated by X-ray photoelectron spectroscopy (XPS) analysis. Further, electrical characterization (Id-Vd) confirms the potentiality of FET-based biosensor to detect cTnI (represents acute myocardial infarction disease) with the concentration ranges from 10 μg/ml down to 1 fg/ml. The sensitivity of 459.2 nA (g/ml)-1 and lower detection limit of 1 fg/ml were achieved at Vbg = -5 V and Vd = 5 V. The designed device demonstrates its ability to detect lower level of cTnI with pre-amplified electrical signal by back-gate biasing.

Development of a position-sensitive CZT detector with coplanar grid electrode

The finite–element analysis was done to optimize position-sensing coplanar grid electrode geometry. CdZnTe crystals manufactured by PAM-XIAMEN with customized electrode geometry have been tested. After digitizing pre-amplifier signals, trapezoidal digital filter was adopted to enhance the signal to noise ratio. Energy resolution of 2.7% FWHM at 662 keV γ-ray energy was obtained. The measured results show the dramatic improvement with position-sensing technique.

Radiation Measurements without Posing the Inversion Problem

Purpose: Radiation measurement is an exponential analysis, which is an “illposed” problem, where the solution is not unique. Therefore, the accuracy of data points is critical. If the radiation detector is sensitive during the dead time, usually an extended dead time signal processing method is necessary and the calculation of the dead time and pile up rate invoke the inversion problem, which is difficult to solve, and only approximated. We have used two new approaches, where the inversion problem is not posed, and there is no need to assume the Poisson distribution or constant source strength. It goes beyond all previous techniques by handling the presence of discriminators. The preamplifier-signal processing chain has dead times and applies discriminators, as a consequence disturbs the randomness, and other distributions may also contribute. Although an undisturbed nuclear decay should be Poisson in nature, the measured spectrum usually does not have a Poisson distribution. The robustness of the two methods was investigated, focusing on the traceable derivation of statistical uncertainty. The two new methods were applied in gamma spectra measurements of 152Eu calibration sources, and nuclear half-life of 68Ga PET isotope, and compared with measurements using previous techniques. Methods: CSX (Cambridge Scientific) digital signal processors were used with five HPGe detectors in two operation modes in two measurement series. In quality assurance mode, the CSX processes all events, both the accepted and rejected ones, placing each event into one or more spectra based on the applied discriminators. Based on the accepted spectrum, the rejected spectra were analyzed to determine the single, double and triple events, the rate of unrelated, and noise events all to obtain the true input counts. The second and independent approach was a time interval histogram analysis for the measurement of the gamma ray intensities. In time interval histogram mode, the CSX creates an energy spectrum as well as an interval histogram of arrival times between successive events. This operation mode does not apply discriminators and has no dead time. The CSX signal processor was selected, as it offers a dead time free and less than 1% pile up rate up to about a million counts per second input rate, as well as quality assurance at the signal processing level. Results: The CSX processors employ a non-extended dead time approach and in quality assurance mode the true input rate is readily determined. As a consequence, the inversion problem was avoided. The uncertainties and uncertainty propagation are clearly justified. For the half-life measurement of 68Ga, the time interval histogram analysis gave six times smaller standard uncertainty than the square root of the counts. The inter-arrival time histogram was sensitive to non-deterministic behaviour of the detectors, and electronic disturbances, discrediting several measurement series. This additional ability is significant for quality measurements. Conclusion: Spectroscopy with HPGe detectors has lacked a method to credibly determine the uncertainties when discrimination is applied. In addition, a calibration procedure was required to establish the output rate versus input rate relations for each input rate preferably with a similar spectral distribution as the spectrum of interest. The preamplifier signal shape varied as a function of the input rate for each of the HPGe detectors studied. In such cases both the accepted and rejected event spectra are necessary for a proper evaluation. Accounting for all the events allows the determination of the uncertainties. The time interval histogram analysis is simple and straightforward, and offers an elegant way to determine the true input rate and uncertainty. The measurement can be applied over large input counting ranges, and does not require calibration.

A digital CDS technique and its performance testing**Supported by National Natural Science Foundation of China (10978002)

Readout noise is a critical parameter for characterizing the performance of charge-coupled devices (CCDs), which can be greatly reduced by the correlated double sampling (CDS) circuit. However, a conventional CDS circuit inevitably introduces new noise since it consists of several active analog components such as operational amplifiers. This paper proposes a digital CDS circuit technique, which transforms the pre-amplified CCD signal into a train of digital presentations by a high-speed data acquisition card directly without the noisy CDS circuit, then implements the digital CDS algorithm through a numerical method. A readout noise of 3.3 e− and an energy resolution of 121 eV@5.9 keV can be achieved via the digital CDS technique.

An experimental characterisation of a Broad Energy Germanium detector

The spectroscopic and charge collection performance of a BE2825 Broad Energy Germanium (BEGe) detector has been experimentally investigated. The efficiency and energy resolution of the detector have been measured as a function of energy and the noise contributions to the preamplifier signal have been determined. Collimated gamma-ray sources mounted on an automated 3-axis scanning table have been used to study the variation in preamplifier signal shape with gamma-ray interaction position in the detector, so that the position-dependent charge collection process could be characterised. A suite of experimental measurements have also been undertaken to investigate the performance of the detector as a function of bias voltage and we report on anomalous behaviour observed when the detector was operating close to the depletion voltage.

Pulse shape analysis of a two fold clover detector with an EMD based new algorithm: A comparison

An investigation of Empirical Mode Decomposition (EMD) based noise filtering algorithm has been carried out on a mirror signal from a two fold germanium clover detector. EMD technique can decompose linear as well as nonlinear and chaotic signals with a precise frequency resolution. It allows to decompose the preamplifier signal (charge pulse) on an event-by-event basis. The filtering algorithm provides the information about the Intrinsic Mode Functions (IMFs) mainly dominated by the noise. It preserves the signal information and separates the overriding noise oscillations from the signals. The identification of noise structure is based on the frequency distributions of different IMFs. The preamplifier noise components which distort the azimuthal co-ordinates information have been extracted on the basis of the correlation between the different IMFs and the mirror signal. The correlation studies have been carried out both in frequency and time domain. The extracted correlation coefficient provides an important information regarding the pulse shape of the γ-ray interaction in the detector. A comparison between the EMD based and state-of-the-art wavelet based denoising techniques has also been made and discussed. It has been observed that the fractional noise strength distribution varies with the position of the collimated gamma-ray source. Above trend has been reproduced by both the denoising techniques.

Spectroscopic Tools Applied to Element Z = 115 Decay Chains

Nuclides that are considered to be isotopes of element Z = 115 were produced in the reaction 48 Ca + 243 Am at the GSI Helmholtzzentrum fur Schwerionenforschung Darmstadt. The detector setup TASISpec was used. It was mounted behind the gas-filled separator TASCA. Thirty correlated -decay chains were found, and the energies of the particles were determined with high precision. Two important spectroscopic aspects of the o line data analysis are discussed in detail: the handling of digitized preamplified signals from the silicon strip detectors, and the energy reconstruction of particles escaping to upstream detectors relying on pixel-by-pixel dead-layer thicknesses.

Event identification in 3He proportional counters using risetime discrimination

We present a straightforward method for particle identification and background rejection in 3He proportional counters for use in neutron detection. By measuring the risetime and pulse height of the preamplifier signals, one may define a region in the risetime versus pulse height space where the events are predominately from neutron interactions. For six proportional counters surveyed in a low-background environment, we demonstrate the ability to reject alpha-particle events with an efficiency of 99%. By applying the same method, we also show an effective rejection of microdischarge noise events that, when passed through a shaping amplifier, are indistinguishable from physical events in the counters. The primary application of this method is in measurements where the signal-to-background for counting neutrons is very low, such as in underground laboratories.

A high speed digital data acquisition system for the Indian National Gamma Array at Tata Institute of Fundamental Research

A digital data acquisition system for the Compton suppressed clover detector array has been implemented at the TIFR-BARC accelerator facility for the high resolution gamma ray spectroscopy using the Pixie-16 Digital Gamma Finder modules by XIA LLC. This system has a provision for simultaneous digitization of 96 preamplifier signals of high purity germanium crystals. The energy and timing characteristics of the clover detectors have been investigated in detail. In-beam data has been collected both in singles and in the coincidence mode. The system has been tested with 64 channels with each of the 64 crystals having an event rate up to 5kHz and 2-fold clover coincidence rate up to 15kHz. The use of the digital data acquisition system has improved the high counting rate handling capabilities for the clover array. Conventional systems with analog shaping are being replaced by digital system that provides higher throughput, better energy resolution and better stability for the multi-detector Compton suppressed clover array.

Development of brain PET using GAPD arrays

In recent times, there has been great interest in the use of Geiger-mode avalanche photodiodes (GAPDs) as scintillator readout in positron emission tomography (PET) detectors because of their advantages, such as high gain, compact size, low power consumption, and magnetic field insensitivity. The purpose of this study was to develop a novel PET system based on GAPD arrays for brain imaging. The PET consisted of 72 detector modules arranged in a ring of 330 mm diameter. Each PET module was composed of a 4 × 4 matrix of 3 × 3 × 20 mm(3) cerium-doped lutetium yttrium orthosilicate (LYSO) crystals coupled with a 4 × 4 array three-side tileable GAPD. The signals from each PET module were fed into preamplifiers using a 3 m long flat cable and then sent to a position decoder circuit (PDC), which output a digital address and an analog pulse of the interacted channel among 64 preamplifier signals transmitted from four PET detector modules. The PDC outputs were fed into field programmable gate array (FPGA)-embedded data acquisition (DAQ) boards. The analog signal was then digitized, and arrival time and energy of the signal were calculated and stored. The energy and coincidence timing resolutions measured for 511 keV gamma rays were 18.4 ± 3.1% and 2.6 ns, respectively. The transaxial spatial resolution and sensitivity in the center of field of view (FOV) were 3.1 mm and 0.32% cps/Bq, respectively. The rods down to a diameter of 2.5 mm were resolved in a hot-rod phantom image, and activity distribution patterns between the white and gray matters in the Hoffman brain phantom were well imaged. Experimental results indicate that a PET system can be developed using GAPD arrays and the GAPD-based PET system can provide high-quality PET imaging.