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
(PDF format, 297 KB in size)

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.

CHELSI: A PORTABLE NEUTRON SPECTROMETER FOR THE 20- 800 MEV REGION
T.D. McLean, R.H. Olsher, L.L. Romero, L.H. Miles, R.T.Devine, A. Fallu-Labruyere, P. Grudberg
(PDF format, 491 KB in size)

ABSTRACT: CHELSI is a CsI-based portable spectrometer being developed at Los Alamos National Laboratory for use in highenergy 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.

Time Resolution Studies using Digital Constant Fraction Discrimination
A. Fallu-Labruyere, H. Tan, W. Hennig, and W.K. Warburton
(PDF format, 205 KB in size)

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.

Single Channel Beta-Gamma Coincidence Detection of Radioactive Xenon Using Digital Pulse Shape Analysis of Phoswich Detector Signals
Wolfgang Hennig, Hui Tan, William K Warburton, and Justin I McIntyre
(PDF format, 324 KB in size)

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%).

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
(PDF format, 375 KB in size)

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.

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
(PDF format, 1,142 KB in size)

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” CsIbased
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 75MeV has been used to further characterize
scintillator performance.

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
(PDF format, 147 KB in size)

ABSTRACT: 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
(PDF format, 178 KB in size)

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 N 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.

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
(PDF format, 517 KB in size)

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.

Particle Identification in CsI(Tl) Crystal Using Digital Pulse Shape Analysis
M. Momayezi, W. Skulski
(PDF format, 463 KB in size)

ABSTRACT: Particle identification in a CsI(Tl) crystal has been achieved using digital pulse shape analysis. We used an 1x1x1 cm³ unpolished CsI(Tl) crystal coupled to a Hamamatsu photo diode type S3590-08 of 1x1 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 Viewer was 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.

Digital Pulse Processing: New Possibilities in Nuclear Spectroscopy
W.K. Warburton, M. Momayezi, B. Hubbard-Nelson and W. Skulski
(PDF format, 1.13 MB in size)
Zipped version of the PDF file
(PDF format, 182 KB in size)

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.

Towards Digital Gamma-Ray and Particle Spectroscopy
W. Skulski, M.Momayezi, B.Hubbard-Nelson, P.Grudberg, J.Harris, W.Warburton
(PDF format, 242 KB in size)

ABSTRACT: Digital spectroscopy is an experimental technique for directly processing of 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. 

Position resolution in a Ge-strip detector
M.Momayezi, W.K. Warburton, R.Kroeger
(PDF format, 246 KB in size)

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. 

An approach to sub-pixel spatial resolution in room temperature X-ray detector arrays with good energy resolution
W.K. Warburton
(PDF format, 488 KB in size)

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.