L. Fabris

A FAST, COMPACT SOLUTION FOR LOW NOISE CHARGE PREAMPLIFIERS

Abstract A new structure for low noise charge sensitive preamplifiers, implemented using a transconductance gain stage plus a voltage gain stage, has been thoroughly investigated. Uniquely high values of open-loop gain have been demonstrated, together with good low noise performance and high load driving capability. Implementation of this scheme uses only a minimal number of commercial parts, resulting in a very compact and relatively low power circuit. A detailed theoretical analysis of the dynamic response and noise performance of the circuit is given. Several tests and measurements have been performed to validate the theory and the results are presented.

Real-time generation of images with pixel-by-pixel spectra for a coded aperture imager with high spectral resolution

The capabilities ofa coded aperture imager are significantly enhanced when a detector with excellent energy resolution is used. We are constructing such an imager with a 1.1 cm thick, crossed-strip, planar detector which has 38 strips of 2 mm pitch in each dimension followed by a large coaxial detector. Full value from this system is obtained only when the images are ‘‘fully deconvolved’’ meaning that the energy spectrum is available from each pixel in the image. The large number ofenergy bins associated with the spectral resolution ofthe detector, and the fixed pixel size, present significant computational challenges in generating an image in a timely manner at the conclusion ofa data acquisition. The long computation times currently preclude the generation ofintermediate images during the acquisition itself. We have solved this problem by building the images on-line as each event comes in using pre-imaged arrays ofthe system response. The generation ofthese arrays and the use off ractional mask-to-detector pixel sampling is discussed. r 2003 Published by Elsevier Science B.V.

Optimization of front-end design in imaging and spectrometry applications with room temperature semiconductor detectors

This paper addresses the optimization of front-end design in position sensing, imaging and high-resolution energy dispersive analysis with room temperature semiconductor detectors. The focus is on monolithic solutions able to meet the requirements of high functional densities set by multielectrode, finely segmented detectors. Front-end architectures featuring additional functions besides charge measurements, as demanded by the need of acquiring and processing multiparametric information associated with the detector signals will be discussed. Noise will be an issue of dominant importance in all the following analysis. The advent of CMOS processes featuring submicron gate length and gate oxide thicknesses in the few nanometers region is overturning some of the classical criteria in the choice of the front-end device. The achievement of the limits in resolution requires a strict control of the noise contribution from the current amplifier which ordinarily follows the front-end element in the charge-sensitive loop. This aspect becomes more crucial in designing front-end systems with submicron processes.

A Germanium Orthogonal Strip Detector System for Gamma‐Ray Imaging

A coded aperture, germanium‐detector based gamma‐ray imaging system has been designed, fabricated, and tested. The detector, cryostat, and signal processing electronics are discussed in this paper. The latest version orthogonal strip planar detector is 11‐millimeters thick, having 38×38 strips of 2‐millimeter pitch. The planar detector was fabricated using amorphous germanium contacts. The strips on each face of the detector lie in a chorded‐circular pattern to more efficiently utilize the area of the 10‐cm diameter germanium crystal. The detector is held in a mount that allows convenient installation and removal of the detector, lending itself to eventual tiling of such detectors into large arrays. The cryostat includes provisions to install a large volume coaxial germanium detector immediately behind the planar detector in the same cryostat. Many gamma rays Compton scatter from the planar detector into the coaxial detector. The energies of these coincident interactions are summed to increase the gamma‐r...

Simultaneous ballistic deficit immunity and resilience to parallel noise sources: A new pulse shaping technique

A new and different time variant pulse processing system has been developed based on a simple CR-RC filter and two analog switches. The new pulse processing technique combines both ballistic deficit immunity and resilience to parallel noise without a significant compromise to the low energy resolution, generally considered a mutually exclusive requirement. The filter is realized by combining two different pulse-shaping techniques. One of the techniques creates a low rate of curvature at the pulse peak, which reduces ballistic deficit, while the second technique increases the tolerance to low frequency noise by modifying the noise history. Several experimental measurements are presented, including tests on a co-planar grid CdZnTe detector. Improvements on both the resolution and line shape are shown for the 662 keV line of /sup 137/Cs.

Fast peak detector stretchers for use in XAFS applications

In this paper we present two different design approaches to the implementation of fast peak detector-stretchers for X-ray absorption fine structure fluorescence applications (XAFS). After describing the motivations for using peak detector-stretchers in high rate applications, we discuss in detail the design and benefits of their use in a modern nuclear spectroscopy system.

High-speed nuclear quality pulse height analyzer for synchrotron-based applications

High-speed Nuclear Quality Pulse Height Analyzer for Synchrotron-based Applications J-F. Beche, J. J. Bucher, L. Fabris, V. J. Riot Abstract--A high throughput Pulse Height Analyzer system for synchrotron-based applications requiring high resolution, high processing speed and low dead time has been developed. The system is comprised of a 120ns 12-bit nuclear quality Analog to Digital converter with a self-adaptive fast peak detector- stretcher and a custom-made fast histogramming memory module that records and processes the digitized data. The histogramming module is packaged in a VME or VXI compatible interface. Data is transferred through a fast optical link from the memory interface to a computer. A dedicated data acquisition program matches the hardware characteristics of the histogramming memory module. The data acquisition system allows for two data collection modes: “standard” data acquisition mode where the data is accumulated and read in synchronization with an external trigger and “live” data acquisition mode where the system operates as a standard Pulse Height Analyzer. The acquisition, standard or live, can be performed on several channels simultaneously. A two-channel prototype has been demonstrated at the Stanford Synchrotron Radiation Laboratory accelerator in conjunction with an X-ray Fluorescence Absorption Spectroscopy experiment. A detailed description of the entire system is given and experimental data is shown. I. S UMMARY A fast high resolution and low dead time Pulse Height Analyzer (PHA) system for Synchrotron-based applications has been developed. This module is an addition to and completes the previously designed custom electronics for XAFS applications [1, 2]. The PHA system receives analog information from a low dead time pulse processing front-end section. The low dead time PHA system is comprised of a fast high resolution Analog to Digital Converter (ADC), a fast histogramming memory and a VME or VXI compatible interface. The VME-based system consists of one complete data acquisition channel. The VXI-based PHA system addresses the problem of simultaneous data acquisition from multiple channels. Both systems were tested at the Stanford Synchrotron Radiation Laboratory in conjunction with an X- ray Fluorescence Absorption Spectroscopy experiment. The block diagram of a single acquisition channel is shown in Fig. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Science Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. J-F Beche, J. J. Bucher, L. Fabris and V. J. Riot are with E. O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA (telephone: 510- 495-2327, e-mail: JFBeche@lbl.gov). The analog input section is built around a fast self-adaptive peak detector-stretcher described in [1]. The peak detector- stretcher includes all the necessary logic to provide pileup rejection and gating functions. The stretched signal is fed to a commercial ADC chosen to maximize throughput without compromising the differential non-linearity (DNL). This ADC is an 8Msps unit, providing 14 bit of resolution with no missing code over the full temperature range, and with an intrinsic DNL of ½ LSB. In order to improve the DNL (not adequate for nuclear spectroscopy applications), only the first 12 most significant bits are used. A 6-bit sliding scale correction [3] lowers the DNL to less than 1%. Including delays, the conversion time of the ADC is 120ns. The electronic pulse-processing amplifier, in its fastest version, shapes the energy events coming from the detector with a 250ns peaking time fourth-order, pseudo-gaussian shaping function. The conversion time of the ADC does not add to the dead time associated with the pulse-shaping amplifier. Data collection is either gated by an external synchronization signal such as an accelerator beam clock or a precise clock signal to time-stamp the data collection and measure count rates. After an analog to digital conversion has taken place, the ADC board control logic sends a request to the histogramming memory module. When the histogramming memory controller acknowledges the request, it uses the ADC data to increment by one the content of the corresponding address memory location. The entire operation lasts 130ns, which is less than the dead time of the pulse- processing amplifier. Therefore, the histogramming operation does not increase the overall event-by-event processing time of the entire system. The histogramming memory module is packaged in a VME or VXI compatible interface and is built around a dual-port static random access memory. One port is dedicated to data accumulation while the second port is mapped to the VME or VXI address and data buses. The transfer to the computer is controlled by either the synchronization signal in the form of an interrupt signal sent to the VME or VXI controller, or by the computer accessing the memory at regular time intervals. The former corresponds to the “standard” acquisition mode and the latter to the “live” mode. In standard mode the content of the histogramming memory must be erased (zeroed) before collecting any new data set.

Germanium orthogonal strip detector system for gamma-ray imaging

A germanium-detector based, gamma-ray imaging system has been designed, fabricated, and tested. The detector, cryostat, electronics, readout, and imaging software are discussed. An 11 millimeter thick, 2 millimeter pitch 19x19 orthogonal strip planar germanium detector is used in front of a coaxial detector to provide broad energy coverage. The planar detector was fabricated using amorphous germanium contacts. Each channel is read out with a compact, low noise external FET preamplifier specially designed for this detector. A bank of shaping amplifiers, fast amplifiers, and fast leading edge discriminators were designed and fabricated to process the signals from preamplifiers. The readout system coordinates time coincident x-y strip addresses with an x-strip spectroscopy signal and a spectroscopy signal from the coaxial detector. This information is sent to a computer where an image is formed. Preliminary shadow and pinhole images demonstrate the viability of a germanium based imaging system. The excellent energy resolution of the germanium detector system provides isotopic imaging.

A fast zero dead-time single channel analyzer for nuclear spectroscopy applications

We discuss a new approach to the design of single channel analyzers (SCAs) for high rate nuclear spectroscopy with an emphasis on accelerator-based XAFS applications. A brief overview of the entire spectroscopy system and a detailed description of the SCA circuit design are given together with some insight into the application. The advantages of this instrument are then highlighted and the experimental results are presented.

Employing Thin HPGe Detectors for Gamma-Ray Imaging

We have evaluated a collimator‐less gamma‐ray imaging system, which is based on thin layers of double‐sided strip HPGe detectors. The positions of individual gamma‐ray interactions will be deduced by the strip addresses and the Ge layers which fired. Therefore, high bandwidth pulse processing is not required as in thick Ge detectors. While the drawback of such a device is the increased number of electronics channels to be read out and processed, there are several advantages, which are particularly important for remote applications: the operational voltage can be greatly reduced to fully deplete the detector and no high bandwidth signal processing electronics is required to determine positions. Only a charge sensitive preamplifier, a slow pulse shaping amplifier, and a fast discriminator are required on a per channel basis in order to determine photon energy and interaction position in three dimensions. Therefore, the power consumption and circuit board real estate can be minimized. More importantly, since the high bandwidth signal shapes are not used to determine the depth position, lower energy signals can be processed. The processing of these lower energy signals increases the efficiency for the recovery of small angle scattering. Currently, we are studying systems consisting of up to ten 2mm thick Ge layers with 2mm pitch size. The required electronics of the few hundred channels can be integrated to reduce space and power. We envision applications in nuclear non‐proliferation and gamma‐ray astronomy where ease of operation and low power consumption, and reliability, are crucial.We have evaluated a collimator‐less gamma‐ray imaging system, which is based on thin layers of double‐sided strip HPGe detectors. The positions of individual gamma‐ray interactions will be deduced by the strip addresses and the Ge layers which fired. Therefore, high bandwidth pulse processing is not required as in thick Ge detectors. While the drawback of such a device is the increased number of electronics channels to be read out and processed, there are several advantages, which are particularly important for remote applications: the operational voltage can be greatly reduced to fully deplete the detector and no high bandwidth signal processing electronics is required to determine positions. Only a charge sensitive preamplifier, a slow pulse shaping amplifier, and a fast discriminator are required on a per channel basis in order to determine photon energy and interaction position in three dimensions. Therefore, the power consumption and circuit board real estate can be minimized. More importantly, since...