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Network Time Synchronization of the Readout Electronics for a New Radioactive Gas Detection System
Wolfgang Hennig, Vincent Thomas, Shawn Hoover, Olivier Delaune
IEEE Transactions on Nuclear Science (Early Access, December 2018)
ABSTRACT: In systems with multiple radiation detectors, time synchronization of the data collected from different detectors is essential to reconstruct multi-detector events such as scattering and coincidences. In cases where the number of detectors exceeds the readout channels in a single data acquisition electronics module, multiple modules have to be synchronized, which is traditionally accomplished by distributing clocks and triggers via dedicated connections. To eliminate this added cabling complexity in the case of a new radioactive gas detection system prototype under development at the French Atomic Energy Commission, we implemented time synchronization between multiple XIA Pixie-Net detector readout modules through the existing Ethernet network, based on the IEEE 1588 precision time protocol. The detector system is dedicated to the measurement of radioactive gases at low activity and consists of eight large silicon pixels and two NaI(Tl) detectors, instrumented with a total of three 4-channel Pixie-Net modules. Detecting NaI(Tl)/silicon coincidences will make it possible to identify each radioisotope present in the sample. To allow these identifications at low activities, the Pixie-Net modules must be synchronized to a precision well below the targeted coincidence window of 500-1000 ns. Being equipped with an Ethernet PHY compatible with IEEE 1588 and synchronous Ethernet that outputs a locally generated but system-wide synchronized clock, the Pixie-Net can operate its analog to digital converters and digital processing circuitry with that clock and match time stamps for captured data across the three modules. Depending on the network configuration and synchronization method, the implementation is capable to achieve timing precisions between 300 ns and 200 ps.

Data acquisition and analysis software for gamma coincidence spectrometry
Martin Shetty, Dagistan Sahin
Journal of Radioanalytical and Nuclear Chemistry (2016)
ABSTRACT: Coincidence counting in neutron activation analysis has well-known advantages, most importantly improvement of detection limits. One obstacle to the wider use of this technique is the complexity of the data acquisition and reduction systems that it requires. The usual approaches to multi-detector data acquisition incur significant dead-time, involve redundant work in repeatedly developing limited tools and risk potential errors in low-level code. The paper describes progress made in developing a software framework to address these shortcomings.

A 24-element Silicon PIN diode detector for high resolution radioxenon measurements using simultaneous X-ray and electron spectroscopy
Christopher E. Cox, Wolfgang Hennig, Alan C. Huber, William K. Warburton, Peter M. Grudberg, Stephen J. Asztalos, Hui Tan, Steven Biegalski
IEEE Nuclear Science Symposium Conference Record (2013)
ABSTRACT: The measurement of atmospheric radioxenon is an important tool for monitoring nuclear weapons testing. The development of new and improved xenon detection methods supports the monitoring program of the Comprehensive Test Ban Treaty Organization (CTBTO). In the current work we have developed a 24-element Si PIN (P-type, Intrinsic, N-type) diode detector to measure both the characteristic X-rays and the high energy mono-energetic conversion electrons emitted by the xenon radioisotopes. The low noise properties and ultra-thin entrance window of the PIN diodes are well suited for resolving the relatively low energy X-ray lines while simultaneously measuring the high energy conversion electrons with high collection efficiency and near-Gaussian peak shapes. The use of coincidence gating between the X-rays and conversion electrons can further improve the detection sensitivity, which we show to rival the current HPGe and scintillator based xenon detection systems that rely mostly on gamma-ray and beta/gamma coincidence detection, respectively. A significant benefit of the high resolution Si PIN system is the detection of metastable xenon isotopes in the presence of large quantities of 133Xe produced by the fission process, which is an important tool for distinguishing nuclear explosions from civilian sources. The Si PIN detector arrangement offers others advantages compared to current xenon detection methods, such as compact construction, intrinsically low background, and the lack of any memory effect from previous measurements. We discuss the construction of the detector and present measurements performed with 131mXe, 133Xe, 133mXe, and 135Xe. Finally, we make an estimate of the minimum detectable concentration (MDC) for each isotope and compare with the CTBTO requirements.

Development of a Phoswich Detector for Radioxenon Field Measurements
W. Hennig , S.J. Asztalos, W. K. Warburton, A. Fallu-Labruyere, A. Samie, P. Mekarski
IEEE Transactions on Applied Superconductivity, Volume: 61 Issue 5 (2014), p 2778-2785
ABSTRACT: The Comprehensive Nuclear-Test-Ban Treaty Organization deploys a variety of radioxenon detection systems as part of its International Monitoring System to detect nuclear explosions. To achieve the high sensitivity required, the systems extract xenon from several cubic meters of air and look for characteristic radioactive emissions, using either high resolution high purity germanium gamma detectors or multiple scintillators for high efficiency beta/gamma coincidence detection. The high sensitivity comes at the expense of heavy lead shielding, and for the latter, calibration and gain matching of multiple photomultiplier tubes as well as a memory effect of the plastic scintillator used for beta detection which absorbs Xe. Existing systems are also stationary by design, though in some applications, for example on-site inspections, a portable detector is required. In this work, we therefore redesigned a previously developed phoswich detector to reduce size, weight, cost, complexity and memory effect with only minor impact on the sensitivity. The phoswich design requires only a single photomultiplier tube with beta/gamma coincidences being detected by digital pulse shape analysis. Additional gain stabilization addresses varying environmental field conditions, such as temperature changes. Three phoswich geometries were modeled through Monte Carlo simulations, the most promising was built and tested. In this paper we describe each of the initial designs, their simulated performances and the factors that lead us to the chosen design. Preliminary results from testing of the prototype detector are presented and compared with simulation.

A Method to Correct Differential Nonlinearities in Analog-to-Digital Converters Used for Digital gamma-ray Spectroscopy
A. Hennig, C. Fransen, W. Hennig, G. Pascovici, N. Warr, M. Weinert, A. Zilges
Nuclear Instruments and Methods in Physics Research A 758 (2014) 69-76
ABSTRACT: The influence on gamma-ray spectra of differential nonlinearities (DNL) in subranging, pipelined analog-to- digital converters (ADCs) used for digital gamma-ray spectroscopy was investigated. The influence of the DNL error on the gamma-ray spectra, depending on the input count-rate and the dynamic range has been investigated systematically. It turned out that the DNL becomes more significant in gamma-ray spectra with larger dynamic range of the spectroscopy system. An event-by-event offline correction algorithm was developed and tested extensively. This correction algorithm works especially well for high dynamic ranges.

Digital Data Acquisition System for experiments with segmented detectors at National Superconducting Cyclotron Laboratory
K. Starosta , C.Vaman , D.Miller , P.Voss , D.Bazin , T.Glasmacher , H.Crawford , P.Mantica , H.Tan, W.Hennig , M.Walby , A.Fallu-Labruyere, J.Harris, D. Breus , P.Grudberg , W.K.Warburton
Nuclear Instruments and Methods in Physics Research A 610 (2009) 700-709
ABSTRACT: A 624-channel Digital Data Acquisition System capable of instrumenting the Segmented Germanium Array at National Superconducting Cyclotron Laboratory has been implemented using Pixie-16 Digital Gamma Finder modules by XIA LLC. The system opens an opportunity for determination of the first interaction position of a gamma-ray in a SeGA detector from implementation of gamma-ray tracking. This will translate into a significantly improved determination of angle of emission, and in consequence much better Doppler corrections for experiments with fast beams. For stopped-beam experiments the systemp rovides means for zero dead time measurements of rare decays, which occuron timescales of microseconds.

Evaluation of Multi-Channel ADCs for Gamma-ray Spectroscopy
Hui Tan, Wolfgang Hennig, Mark D. Walby, Dimitry Breus, Jackson Harris
Nuclear Instruments and Methods in Physics Research A 758 (2014) 69-76
ABSTRACT: As nuclear physicists increasingly design large scale experiments with hundreds or thousands of detector channels, there are growing needs for high density readout electronics with good timing and energy resolution that at the same time offer lower cost per channel compared to existing commercial solutions. Recent improvements in the design of commercial analog to digital converters (ADCs) have resulted in a variety of multi-channel ADCs that are natural choice for designing such high density readout modules. However, multi-channel ADCs typically are designed for medical imaging/ultrasound applications and therefore are not rated for their spectroscopic characteristics. In this work, we evaluated the gamma-ray spectroscopic performance of several multi-channel ADCs, including their energy resolution, nonlinearity, and timing resolution. Some of these ADCs demonstrated excellent energy resolution, 2.66% FWHM at 662 keV with a LaBr3 or 1.78 keV FWHM at 1332.5 keV with a high purity germanium (HPGe) detector, and sub-nanosecond timing resolution with LaBr3. We present results from these measurements to illustrate their suitability for gamma-ray spectroscopy.

Wolfgang Hennig, Christopher E. Cox, Stephen J. Asztalos, Hui Tan, Patrick J. Franz, Peter M. Grudberg, and William K. Warburton
2011 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies
ABSTRACT: The radioxenon detectors currently used in applications related to nuclear explosion monitoring have a very high sensitivity to detect small amounts of radioxenon, but an improved discrimination of Xe isotopes is desirable to better distinguish the radioxenon background created by nuclear power plants and medical isotope facilities from a nuclear explosion. In the work reported here, silicon-based detectors (silicon drift detectors, PIN diodes, and Si(Li) detectors) have been explored as a possible alternative to existing systems. Silicon detectors are sensitive to X-rays, gamma rays, beta radiation, and mono-energetic conversion electrons; have very high resolution for X-ray lines that the existing systems cannot clearly separate; and as small solid state devices, can be assembled into compact systems. On the other hand, their detection efficiency is low for higher-energy photons, and available detector sizes are small, so a careful study of the improvements and drawbacks is required. Resolutions at key energies were measured with a variety of sources and Si detectors. For example, using a small PIN diode at room temperature, we measured a full width at half maximum (FWHM) resolution of ~1 keV for 200 keV conversion electrons and 420-600 eV for 30 keV X-rays (~300 eV cooled). Detector backgrounds are extremely low, on the order of 0.005 counts/s, and practically zero if beta/photon coincidence is applied. Interactions of characteristic electrons and photons were modeled with Monte Carlo simulations to determine detection efficiencies. For electrons, the probability to deposit the full energy in an idealized Si detector is ~72% for all energies studied. For photons, it depends strongly on the energy, ranging from 86.4% at 4.1 keV to 12.0% at 30 keV and ≤ 0.6% for 80 keV and higher. The results from measurements and simulations were used to compute the minimum detectable concentration for Xe isotopes in several possible detector geometries. We found minimum detectable concentrations of 1.5-1.9 mBq/m3 for most isotopes with a simple 1 cm3 cube detector with two active sides and ~0.1 mBq/m3 for a cube with six active sides. The exception is 135Xe, although the addition of a 1 inch diameter CsI detector element brings its minimum detectable concentration below 1 mBq/m3 as well. As specifications for existing applications are 1.0 mBq/m3, we conclude that Si detectors are indeed viable alternatives to existing detectors. Furthermore, the low background allows for fast screening of samples to determine if radioxenon is present or not, potentially useful in onsite inspections with large number of samples.

W. K. Warburton, Stephen J. Asztalos, and Wolfgang Hennig
2011 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies
ABSTRACT: Well designed detectors, including such examples as ARSA, SAUNA, and XIA’s PhosWatch, can readily achieve the state of the art radioxenon detection limits required for nuclear explosion monitoring. They are also reliable, robust detectors that do not require cryogenic cooling for operation. All three employ the principle of beta-gamma coincidence detection to reduce background counting rates, using a BC404 plastic scintillator to detect the betas and a CsI(Tl) or NaI scintillator to detect the gamma-rays. As a consequence of this commonality of design, all three also display a “memory effect” arising from the diffusion of Xe into BC404. Thus, when one sample is pumped out of the detector, a fraction remains behind, embedded in the BC404, where it artificially raises the signal counting rate for the next sample. While this is not a fatal flaw in phoswich detectors, developing a method to eliminate the memory effect would significantly enhance their utility. In this work we proposed to develop thin, amorphous Al2O3films to act as diffusion barriers on the BC404 surfaces exposed to radioxenon. The amorphous property is critical, since it eliminates the high diffusivity grain boundaries that occur with high densities in thin crystalline films, and Al was selected since it is known to naturally form high density amorphous films in thin layers. While we initially proposed to grow the films by depositing Al in a low pressure oxygen environment, our subsequent research revealed that ALD (atomic layer deposition) was a superior alternative, particularly because it could deposit uniform layers on arbitrarily curved surfaces (e.g. the spherical interior of the PhosWatch’s BC404 detector). We therefore established a collaboration with Prof. Steven George and his group at the University of Colorado, Boulder, who are internationally known for their work on ALD and ALD deposited Al2O3 films in particular. Prof. George’s group has, for example, developed methods to grow Al2O3 films that are impenetrable by H20, a molecule that is significantly smaller in diameter than Xe. Our experimental approach is therefore as follows. We have designed and built a “doser” that uses an old PhosWatch detector to deliver a calibrated dose to thin cylindrical BC404 samples. We have also built a custom counting jig to reproducibly count the BC404 samples in a phoswich geometry using a 2″ CsI(Tl) crystal. Prof. George’s group has created one set of BC404 samples coated with 4 thicknesses of Al2O3 at one temperature and another set of the same thickness coated at 3 different temperatures so that we can explore the parameters that create the highest quality films. We are currently developing our dosing and counting techniques using radon, which is readily available and also shows the memory effect, and then will repeat our measurements with radioxenon, which is harder to obtain and handle. Initial results with radon show a rapid initial decay in the memory effect (time constant about 1 hour) followed by a slower decay rate typical of the normal radon half life. Our present interpretation is that the shorter half life reflects diffusion of radon out of the test samples, while the slower component is a combination of radon remaining in the sample and detector contamination. The latter is most probably from exposure to atmospheric radon, since we also see it in undosed background samples. This suggests that the background effect might be substantially reduced by changing the standard measurement protocol to replace the present eight hour background measurement with an eight hour continuous pumpout.

Results from an investigation of the physical origins of nonproportionality in CsI(Tl)
S. Asztalos, W. Hennig, W.K. Warburton
Nuclear Instruments and Methods in Physics Research A, Volume 652 (2011) pp. 216-220
ABSTRACT: The relative scintillation response per energy deposited by Compton electrons, or nonproportionality, has traditionally been considered an intrinsic scintillator property. However, such an interpretation is inconsistent with recent results that show nonproportionality to depend on external factors such as shaping time, temperature and supplier. Apparently, at least some of the overall nonproportionality has an extrinsic origin. In this work we describe the results from a suite of measurements designed to test the hypothesis that nonproportionality in CsI(Tl) material has an extrinsic component that correlates with impurity levels. Our choice of material was motivated by the excellent energy resolution observed in one bulk crystal (6.4%) marked departure from that measured with conventional CsI(Tl) stock (8 – 8.5%). Six bulk CsI(Tl) crystals were procured and diced into 44 wafers. Using X-ray fluorescence techniques no conclusive evidence for impurities was found in any of the wafers at the 1-50 ppm level. One crystal exhibited a distinct correlation among energy resolution, decay lifetimes, nonproportionality and a very low level of Tl doping.

T.D. McLean, R.H. Olsher, L.L. Romero, L.H. Miles, R.T.Devine, A. Fallu-Labruyere, P. Grudberg
Radiat Prot Dosimetry (2007) 126 (1-4): 223-228
ABSTRACT: CHELSI is a CsI-based portable spectrometer being developed at Los Alamos National Laboratory for use in high energy neutron fields. Based on the inherent pulse shape discrimination properties of CsI(Tl), the instrument flags charged particle events produced via neutron-induced spallation events. Scintillation events are processed in real time using digital signal processing and a conservative estimate of neutron dose rate is made based on the charged particle energy distribution. A more accurate dose estimate can be made by unfolding the 2D charged particle versus pulse height distribution to reveal the incident neutron spectrum from which dose is readily obtained. A prototype probe has been assembled and data collected in quasi-monoenergetic fields at the The Svedberg Laboratory (TSL) in Uppsala as well as at the Los Alamos Neutron Science Center (LANSCE). Preliminary efforts at deconvoluting the shape/energy data using empirical response functions derived from time-of-flight measurements are described.

Extending the Operation of a Position-Sensitive Photomultiplier Tube to 1 Million Counts Per Seconds
A. Fallu-Labruyere, H. Tan, W. Hennig, Y.X. Chu, M. Momayezi W.K. Warburton
Nuclear Instruments and Methods in Physics Research A, Volume 563 (2006) pp. 54-57
ABSTRACT: While position sensitive photomultiplier tubes (PSPMTs), coupled to fast scintillators, are widely used as photon detectors in applications such as medical imaging systems (PET, gamma camera, etc.) where it is desirable to combine good time resolution with the capability of locating the point of photon interaction, their count rate limitations (of order of tens of thousands of cps) have precluded their use in more demanding applications. Recently, in a neutron imaging application, we found that, by using custom designed fast anode and dynode readout circuits, coupled to a fast digital pulse processing board, we could operate a PSPMT at rates approaching 1 million cps while retaining good position resolution, linearity and time resolution. These developments therefore significantly extend the range of PSPMT application.

Position resolution in a Ge-strip detector
M.Momayezi, W.K. Warburton, R.Kroeger
SPIE Vol. 3768 (1999) pp 530-537
ABSTRACT: Digital Gamma Finder has been applied to reconstruct in three dimensions the interaction positions for gamma-rays penetrating into a double-sided planar Ge cross strip detector. It has been shown both theoretically and experimentally that the 3D-reconstruction problem can be reduced to three one dimensional ones, which greatly simplifies the task of position reconstruction. Measurements performed on a 10 mm thick detector with 2 mm strip pitch show that 2 mm position resolution can easily be achieved perpendicular to the detector plane. In-plane resolution is presently limited to the strip pitch, i.e., 2 mm. Work is in progress to develop algorithms to improve the in-plane resolution using captured ADC waveforms. Captured waveforms are presented that indicate the possibility of reconstructing more complex events such as Compton scattering.

A Versatile Multichannel Digital Signal Processing Module for Microcalorimeter Arrays
H. Tan, J. W. Collins, M. Walby, W. Hennig, W. K. Warburton, P. Grudberg
J. of Low Temp. Phys., June 2012, Vol 167, Iss 5-6, pp 609-619
ABSTRACT: Different techniques have been developed for reading out microcalorimeter sensor arrays: individual outputs for small arrays, and time-division or frequency-division or code-division multiplexing for large arrays. Typically, raw waveform data are first read out from the arrays using one of these techniques and then stored on computer hard drives for offline optimum filtering, leading not only to requirements for large storage space but also limitations on achievable count rate. Thus, a read-out module that is capable of processing microcalorimeter signals in real time will be highly desirable. We have developed a multichannel digital signal processing electronics that is capable of on-board, real time processing microcalorimeter sensor signals from multiplexed or individual pixel arrays. It is a 3U PXI module consisting of a standardized core processor board and a set of daughter boards. Each daughter board is designed to interface a specific type of microcalorimeter array to the core processor. The combination of the standardized core plus this set of easily designed and modified daughter boards results in a versatile data acquisition module that not only can easily expand to future detector systems, but is also low cost. In this paper, we first present the core processor/daughter board architecture, and then report the performance of an 8-channel daughter board, which digitizes individual pixel outputs at 1 MSPS with 16-bit precision. We will also introduce a time-division multiplexing type daughter board, which takes in time-division multiplexing signals through fiber optic cables and then processes the digital signals to generate energy spectra in real time.

Development of a Real-time Pulse Processing Algorithm for TES-based X-ray Microcalorimeters
Hui Tan, Wolfgang Hennig, William K. Warburton, W. Bertrand Doriese, Caroline A. Kilbourne
IEEE Transactions on Applied Superconductivity, Volume: 21 (2011), pp. 276 – 280
ABSTRACT: We report here a real-time pulse processing algorithm for superconducting transition-edge sensor (TES) based x-ray microcalorimeters. TES-based microcalorimeters offer ultra-high energy resolutions, but the small volume of each pixel requires that large arrays of identical microcalorimeter pixels be built to achieve sufficient detection efficiency. That in turn requires as much pulse processing as possible must be performed at the front end of readout electronics to avoid transferring large amounts of data to a host computer for post- processing. Therefore, a real-time pulse processing algorithm that not only can be implemented in the readout electronics but also achieve satisfactory energy resolutions is desired. We have developed an algorithm that can be easily implemented in hardware. We then tested the algorithm offline using several data sets acquired with an 8×8 Goddard TES x-ray calorimeter array and 2×16 NIST time-division SQUID multiplexer. We obtained an average energy resolution of close to 3.0 eV at 6 keV for the multiplexed pixels while preserving over 99% of the events in the data sets.

Development of 500 MHz Multi-Channel Readout Electronics for Fast Radiation Detectors
Wolfgang Hennig, Stephen J. Asztalos, Dimitry Breus, Konstantin Sabourov and William K. Warburton
IEEE Nuclear Science Symposium Conference Record (2009) pp.613-615
ABSTRACT: We describe the development of readout electronics for fast radiation detectors that digitize signals at a rate of 500 MHz, process the digital data stream to measure pulse heights, bin the results in on-board MCA spectra, and optionally capture waveforms for pulse shape analysis. The electronics are targeted for applications requiring good energy resolution and precise timing, for example life time measurements on exotic nuclei, timing measurements with fast scintillators such as LaBr3 or BaF2, or pulse shape analysis with liquid scintillators or phoswich detectors. Upgrading the existing XIA Pixie-4 spectrometer design with a 12-bit, 500 MHz analog to digital converter, we built a prototype of a 4-channel electronics module and evaluated its performance in terms of energy resolution, timing resolution, and improvements in pulse shape analysis.

High Rate Pulse Processing Algorithms for Microcalorimeters
Hui Tan, Dimitry Breus, Wolfgang Hennig, Konstantin Sabourov, Jeffrey W. Collins, William K. Warburton, W. Bertrand Doriese, Joel N. Ullom, Minesh K. Bacrania, Andrew S. Hoover, Michael W. Rabin
Nuclear Science Symposium Conference Record, 2008. NSS ’08. IEEE (2008) pp 1130-1133
ABSTRACT: It has been demonstrated that microcalorimeter spectrometers based on superconducting transition-edge- sensors can readily achieve sub-100 eV energy resolution near 100 keV. However, the active volume of a single microcalorimeter has to be small in order to maintain good energy resolution, and pulse decay times are normally on the order of milliseconds due to slow thermal relaxation. Therefore, spectrometers are typically built with an array of microcalorimeters to increase detection efficiency and count rate. For large arrays, however, as much pulse processing as possible must be performed at the front end of readout electronics to avoid transferring large amounts of waveform data to a host computer for post-processing. In this paper, we present digital filtering algorithms for processing microcalorimeter pulses in real time at high count rates. The goal for these algorithms, which are being implemented in readout electronics that we are also currently developing, is to achieve sufficiently good energy resolution for most applications while being: a) simple enough to be implemented in the readout electronics; and, b) capable of processing overlapping pulses, and thus achieving much higher output count rates than those achieved by existing algorithms. Details of our algorithms are presented, and their performance is compared to that of the “optimal filter” that is currently the predominantly used pulse processing algorithm in the cryogenic-detector community.

The DGF Pixie-4 spectrometer – Compact Digital Readout Electronics for HPGe Clover Detectors
W. Hennig, Y.X. Chu, H. Tan, A. Fallu-Labruyere, W.K. Warburton and R. Grzywacz
Nuclear Instruments and Methods in Physics Research B: Volume 26 (2007), pp. 175-178
ABSTACT: Large volume HPGe detectors are commonly used in applications that require good energy resolution and high detection efficiency, but are expensive and difficult to grow. Clover detectors consisting of 4 smaller crystals in a common cryostat are a possible alternative, but traditionally require complex readout electronics. In contrast, the DGF Pixie-4 is a compact, digital spectrometer providing on a single 3U CompactPCI/PXI card all the electronics required for clover detectors, including computation of addback spectra.. This paper describes the DGF Pixie-4 system architecture, characterizes its energy resolution and throughput, and presents results of test measurements with a clover detector.

A Digital Spectrometer Approach to Obtaining Multiple Time-Resolved Gamma-Ray Spectra for Pulsed Spectroscopy
H. Tan, A. Fallu-Labruyere, W. Hennig, Y.X. Chu, L. Wielopolski and W.K. Warburton
Nuclear Instruments and Methods in Physics Research B: Volume 263 (2007), pp. 63-66
ABSTRACT: Neutron-induced gamma emission and its detection using a pulsed neutron generator system is a recognized analytical technique for quantitative multi-elemental analysis. Traditional gamma-ray spectrometers used for this type of analysis are normally operated in either coincidence mode by counting prompt gamma-rays from inelastic scattering when the neutron generator is ON, or anti-coincidence mode by counting prompt or delayed gamma-rays from thermal neutron capture or delayed activation when the neutron generator is OFF. We have developed a digital gamma-ray spectrometer for concurrently measuring both the inelastic and capture gamma-rays emitted from a sample when activated by 14 MeV neutrons from a pulsed neutron generator. The spectrometer separates the gamma-ray counts into two independent spectra together with two separate sets of counting statistics based on the external gate level. Occasionally there might be a need for multiple time gates to acquire gamma-ray spectra at different time intervals. For that purpose we are developing a multi-gating system that will allow gamma-ray spectra to be acquired concurrently in real time with up to 16 time slots. These 16 time slots will have adjustable width and time delay that can be arbitrarily allocated within the ON and OFF periods. The conceptual system design and considerations for performing gate signal testing and tracking together with pulse height analysis and bin allocation into spectra in real time will be presented.

Time Resolution Studies using Digital Constant Fraction Discrimination
A. Fallu-Labruyere, H. Tan, W. Hennig, and W.K. Warburton
Nuclear Instruments and Methods in Physics Research A, Volume 579 (2007) pp. 247-251
ABSTRACT: Digital Pulse Processing (DPP) modules are being increasingly considered to replace modular analog electronics in medium scale nuclear physics experiments (100’s to 1000’s of channels). One major area remains, however, where it has not been convincingly demonstrated that DPP modules are competitive with their analog predecessors – time-of-arrival measurement. While analog discriminators and time to amplitude converters can readily achieve coincidence time resolutions in the 300 – 500 ps range with suitably fast scintillators and photomultiplier tubes (PMTs), this capability has not been widely demonstrated with DPPs. Some concern has been expressed, in fact, that such time resolutions are attainable with the 10 ns sampling times that are presently commonly available. In this work we present time coincidence measurements taken using a commercially available DPP (the Pixie-4 from XIA LLC) directly coupled to pairs of fast PMTs mated with either LSO or LaBr3 scintillator crystals and excited by 22Na gamma-ray emissions. Our results, 886 ps for LSO and 576 ps for LaBr3, while not matching the best literature results using analog electronics, are already well below 1 ns and fully adequate for a wide variety of experiments. These results are shown not to be limited by the DPPs themselves, which achieved 57 ps time resolution using a pulser, but are degraded in part both by the somewhat limited number of photoelectrons we collected and by a sub-optimum choice of PMT. Analysis further suggests that increasing the sampling speed would further improve performance. We therefore conclude that DPP time-of-arrival resolution is already adequate to supplant analog processing in many applications and that further improvements should be achieved with only modest efforts.

A high speed fully digital data acquisition system for Positron Emission Tomography
P.D. Olcott, A. Fallu-Labruyere, F. Habte, C.S. Levin, W.K. Warburton
2006 IEEE Nuclear Science Symposium Conference Record
ABSTRACT: The availability of compact arrays of high speed analog to digital converters and fast field programmable gate arrays allow much of the complex analog signal processing chain in nuclear pulse processing data acquisition to be replaced with digital algorithms. The fully digital approach allows us to evaluate APD based PET detectors and time of flight (TOF) capable fast PMTs using the same digital data acquisition (DAQ) system. We have developed digital processing algorithms for recording the time, position, and energy from a 511 keV photon interaction with a lutetium-oxyortho-silicate (LSO) crystal array coupled to a Position Sensitive Avalanche Photodiode (PSAPD) or to a XP2020 Photonics PMT as the scintillation light photodector. Implementing these algorithms in a Pixie-4 setup, we compared its performance to an analog setup based on NIM electronics modules, and we evaluated its coincidence time performance using utra-fast PMTs. From our initial experiments, the Pixie-4 system performs well in high spatial resolution, high count rate PET applications with our PSAPD detectors and with fast PMTs that are capable of TOF PET applications.

Digital Pulse Processing: New Possibilities in Nuclear Spectroscopy
W.K. Warburton, M. Momayezi, B. Hubbard-Nelson and W. Skulski
Applied Radiation and Isotopes, Volume 53, Issues 4-5 (2000) pp. 913-920
ABSTRACT: We describe the application of the DGF-4C to a series of experiments. The first, for which the DGF was originally developed, involves locating gamma-ray interaction sites within large segmented Ge detectors. The goal of this work is to attain spatial resolutions of order 2 mm within 70 mm x 90 mm detectors. We show how pulse shape analysis allows ballistic deficit to be significantly reduced in these detectors. A second experiment involves studying exotic nuclei by observing their 1 MeV direct proton decays following implantation in a Si crossed stripe detector at 35 MeV. Whereas the implantation paralyzes analog electronics for almost 10 microseconds, the DGF allows the study of decay times as short as 1 microsecond. Initial energy and time resolution results are presented. Finally, we show how the DGF’s precise timing and coincidence capabilities lead to significant experimental simplifications in dealing with phoswich detectors, low background counting work, and trace Pb detection by coincident photon detection.

General purpose pulse shape analysis for fast scintillators implemented in digital readout electronics
Stephen Asztalos, Wolfgang Hennig, William K. Warburton
Nulear Instruments and Methods in Physics Research A, Vol 806, 2016, pp 132-138

ABSTRACT: Pulse shape discrimination applied to certain fast scintillators is usually performed offline. In sufficiently high-event rate environments data transfer and storage become problematic, which suggests a different analysis approach. In response, we have implemented a general purpose pulse shape analysis algorithm in the XIA Pixie-500 and Pixie-500 Express digital spectrometers. In this implementation waveforms are processed in real time, reducing the pulse characteristics to a few pulse shape analysis parameters and eliminating time-consuming waveform transfer and storage. We discuss implementation of these features, their advantages, necessary trade-offs and performance. Measurements from bench top and experimental setups using fast scintillators and XIA processors are presented.

CLYC versus Stilbene: Optimization and comparison of two neutron-gamma discriminating scintillators
Stephen Asztalos, Wolfgang Hennig
In press

ABSTRACT: CLYC is a novel scintillator with excellent pulse shape discrimination attributes. When introducing a new scintillator it is helpful to derive a quantitative measure of its performance. A detailed analysis is performed with CLYC and another common material Stilbene – two materials capable of neutron/photon discrimination. Using one particular choice of a figure of merit we find that CLYC outspecifies Stilbene by a factor of 50.

A Fast Pulsed Neutron Source for Time-of-Flight Detection of Nuclear Materials and Explosives 
M. Krishnan, B. L. Bures, C. James, R. E. Madden W. Hennig, D. Breus, S. Asztalos, K. Sabourov, and S. Lane
AIP Conference Proceedings 1412, 47 (2011)
ABSTRACT: AASC has built a fast pulsed neutron source based on the Dense Plasma Focus (DPF). One version stores 500J, fires at 0.5Hz and emits ~3×106n/pulse at a pulsed current peak of 130kA. A more advanced version stores only 100J, but fires at ~10-50Hz, and emits ~106n/pulse at a peak current of 100kA. Both sources emit 2.45±0.1MeV (DD) neutron pulses of ~25-40ns width. Such fast, quasi-monoenergetic pulses allow time-of-flight detection of characteristic emissions from nuclear materials or high explosives. A test is described in which iron targets were placed at different distances from the point neutron source. Detectors such as Stilbene and LaBr3 were used to capture inelastically induced, 847 keV gammas from the iron target. Shielding of the source and detectors eliminated most (but not all) source neutrons from the detectors. Gated detection, pulse shape analysis and time-of-flight discrimination enable separation of gamma and neutron signatures and localization of the target. A Monte Carlo simulation allows evaluation of the potential of such a fast pulsed source for a field-portable detection system.

Radioxenon Measurements with the PhosWatch Detector System
W. Hennig, W.K. Warburton1, A. Fallu-Labruyere, K. Sabourov, M.W. Cooper, J.I. McIntyre, A. Gleyzer, M. Bean, E.P. Korpach, K. Ungar, W. Zhang, P. Mekarski, R.M. Ward, S.R.F. Biegalski, D.A. Haas
2009 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies
ABSTRACT: Many of the radioxenon detector systems used in the International Monitoring System (IMS) and in other applications employ beta/gamma coincidence detection to achieve high sensitivity. In these systems, the coincidence detection is implemented by requiring simultaneous signals from separate beta and gamma detectors. While very sensitive to small amounts of radioxenon, this approach requires careful calibration and gain matching of several detectors and photomultiplier tubes. An alternative approach is the use of a phoswich detector in which beta-gamma coincidences are detected by pulse shape analysis. The phoswich requires only a single photomultiplier tube and thus is easier to set up and calibrate, and can be assembled into a more compact and robust system. In the past, we developed a COTS detector system, named PhosWatch, which consists of a CsI(Tl)/BC-404 phoswich detector, digital readout electronics, and on-board software to perform the pulse shape analysis. Several units of this system have now been manufactured and are currently being evaluated at several radioxenon research laboratories. In this paper, we will report results from production tests and some of the evaluations, including a side- by-side comparison of a SAUNA detector and a PhosWatch system using atmospheric radioxenon samples and a comparison of an ARSA-type detector and a PhosWatch system with samples made by irradiating stable xenon isotopes. In addition, we will show initial results obtained with a higher speed version of the readout electronics, digitizing at 500 MHz and thus able to better resolve the BC-404’s fast pulses.

CHELSI : recent developments in the design and performance of a high-energy neutron dose meter
Thomas D. McLean, Richard H. Olsher, Robert T. Devine, Leonard L. Romero, Anthony Fallu-LaBruyere, Peter Grudberg, Hui Tan and Yunxian Chu
Nuclear Instruments and Methods in Physics Research Section A, Volume 562, Issue 2, p. 793-796
ABSTRACT: The intrinsic pulse shape discrimination properties of CsI(Tl) form the basis of a high-energy neutron (>20MeV) spectrometer (CHELSI) currently being developed at LANL that shows promise in satisfying the requirements of an ideal survey meter; lightweight, portable and real time display of dose. Charged particle spallation products generated in the scintillator via neutron interactions are identified on the basis of pulse shape using digital pulse processing. Conservative estimates of dose rate can be given in real time based on count rates and pulse height distributions. More accurate dose measurements can be done offline using unfolding methods to analyze stored pulse shape versus energy data. As a precursor to the development of a portable instrument, data has been obtained using a 1”x1” CsI based probe and a digital spectrometer. This system has been used to collect data on the 90-meter flight path at the LANSCE/WNR facility at average neutron energy of 335 MeV. The spectrometer has the capability, in addition to storing individual waveforms for off-line analysis, to record time-of-flight data and calculate a pulse shape parameter and pulse height for every scintillation event. Combining this data with traditional multichannel analyzer data has yielded a set of empirical response functions. Analysis of the charged particle spectra has shown that at in this neutron field an average count rate of 1.1 cps per mrem/hr is obtained for a 1”x1” CsI(Tl) scintillator.Data recorded using monoenergetic protons from 30 to 75 MeV has been used to further characterize scintillator performance.

Single Channel Beta-Gamma Coincidence Detection of Radioactive Xenon Using Digital Pulse Shape Analysis of Phoswich Detector Signals
IEEE Transactions on Nuclear Science, Volume 53, 2, (2006) pp. 620-624
Wolfgang Hennig, Hui Tan, William K Warburton, and Justin I McIntyre
ABSTRACT: Monitoring radioactive xenon in the atmosphere is one of several methods used to detect nuclear weapons testing. To increase sensitivity, monitoring stations use a complex system of separate beta and gamma detectors to detect beta-gamma coincidences from the Xe isotopes of interest, which is effective, but requires such careful gain matching and calibration that it is difficult to operate in the field. To simplify the system, a phoswich detector has been designed, consisting of optically coupled plastic and CsI scintillators to absorb beta particles and gamma rays, respectively. Digital pulse shape analysis of the detector signal is used to determine if radiation interacted in either or both parts of the detector and to measure the energy deposited in each part, thus using only a single channel of readout electronics to detect beta-gamma coincidences and to measure both energies. Experiments with a prototype detector show that the technique can clearly separate event types, does not degrade the energy resolution, and has an error rate for detecting coincidences of less than 0.1%. Monte Carlo simulations of radiation transport and light collection in the proposed detector were performed to obtain optimum values for its design parameters and an estimate of the coincidence detection efficiency (82-92%) and the background rejection rate (better than 99%).

Digital Pulse Shape Analysis with PHOSWICH Detectors to Simplify Coincidence Measurements of Radioactive Xenon
W. Hennig, H. Tan, W.K. Warburton and J.I. McIntyre
27th Seismic Research Review: Ground-Based Nuclear Explosion Monitoring Technologies (2005)
ABSTRACT: The Comprehensive Nuclear-Test-Ban Treaty establishes a network of monitoring stations to detect radioactive xenon in the atmosphere from nuclear weapons testing. One such monitoring system is the Automated Radioxenon Sampler/Analyzer (ARSA) developed at Pacific Northwest National Laboratory, which uses a complex arrangement of separate beta and gamma detectors to detect beta-gamma coincidences from the xenon isotopes of interest. The coincidence measurement is very sensitive, but the large number of detectors and photomultiplier tubes requires careful calibration. Simplifying this coincidence measurement system while maintaining its performance is the objective of the research described here. It has been suggested that beta-gamma coincidences could be detected with only a single photomultiplier tube and electronics channel by using a phoswich detector consisting of optically coupled beta and gamma detectors. In that work, rise time analysis of signals from a phoswich detector was explored as a method to determine if interactions occurred in either the beta or the gamma detector or in both simultaneously. However, this approach was not able to detect coincidences with the required sensitivity or to measure the beta and gamma energies with sufficient precision for radioxenon monitoring. In this paper, we present a new algorithm to detect coincidences by pulse shape analysis of the signals from a BC-404/CsI(Tl) phoswich detector. Implemented on fast digital readout electronics, the algorithm achieves clear separation of beta only, gamma only and coincidence events, accurate measurement of both beta and gamma energies, and has an error rate for detecting coincidences of less than 0.1%. Monte Carlo simulations of radiation transport and light collection were performed to optimize design parameters for a replacement detector module for the ARSA system, obtaining an estimated coincidence detection efficiency of 82-92% and a background rejection rate better than 99%. The new phoswich/pulse shape analysis method is thus suitable to simplify the existing ARSA detector system to the level of a single detector per sample chamber while maintaining the required sensitivity and precision to detect radioxenon in the atmosphere.

An approach to sub-pixel spatial resolution in room temperature X-ray detector arrays with good energy resolution
W.K. Warburton
2005 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies
ABSTRACT: In this paper we examine a recently proposed concept for obtaining sub-pixel spatial resolution in compound semiconductors where hole transport properties are relatively poor. This approach uses weighted sums and differences of local pixel signals to extract both accurate x-ray energy estimates and interpolate location at the sub-pixel level. A simple analysis, including noise estimates, suggests the possibility of obtaining locations at the 50-100 micron level using 1-2 mm wide stripe electrodes while obtaining 1-2% energy resolution for x-rays up to 100 keV. Following this examination, we will present the most recent experimental results from our program to develop electronics to implement this scheme.

Towards Digital Gamma-Ray and Particle Spectroscopy
W. Skulski, M.Momayezi, B.Hubbard-Nelson, P.Grudberg, J.Harris, W.Warburton
Acta Physica Polonica B, vol. 31 Issue 1, (2001) p.47
ABSTRACT: Digital spectroscopy is an experimental technique for directly processing detector signals without analog signal shaping. Digital spectrometers capture the detailed shape of preamplifier signals with high speed ADCs, and then process captured waveforms in real time with field–programmable gate arrays and digital signal processors, that perform digitally all essential data processing functions, including precise energy measurement and event timing, ballistic deficit correction, pulse shape analysis, and time stamping the output data for offline analysis. Applications of this novel technology include position sensitive gamma-ray spectroscopy with arrays of Ge detectors and high-speed particle emission spectroscopy. In both applications digital spectrometers process signals from semiconductor detectors in order to measure the interaction energy, time, and location within the detector volume. Excellent energy resolution and essentially zero dead time can be easily obtained with XIA digital spectrometer devices, even when time separation between consecutive events in a decay chain is shorter than 1 microsecond. These and other applications of digital spectroscopy are at the frontier of experimental nuclear chemistry and nuclear physics.

Particle Identification in CsI(Tl) Crystal Using Digital Pulse Shape Analysis
Nuclear Instruments and Methods in Physics Research A, Volume 458 (2001), pp. 759-771 W. Skulski, M. Momayezi
ABSTRACT: Particle identification in a CsI(Tl) crystal has been achieved using digital pulse shape analysis. We used an 1x1x1 cmunpolished CsI(Tl) crystal coupled to a Hamamatsu photo diode type S3590-08 of 1×1 cm2 active area. The diode was connected to a charge integrating preamplifier with a 250 microsecond RC constant. The output pulses from the preamp were digitized at 40 MHz rate and 12 bit precision by the XIA DGF-4C digital spectrometer and wave form digitizer, described in detail in the DGF Instruments section. Standard DGF control software DGF Viewerwas used both to collect the data in the list mode and to perform offline analysis. The CsI(Tl) crystal was irradiated with alpha particles and gamma rays emitted by 241Am, 137Cs, 60Co, and natTh. In order to develop proton/alpha discrimination algorithms we irradiated the crystal with protons produced by elastic knock out of 1H atoms from mylar by alpha particles.

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