Valentin T. Jordanov

Digital synthesis of pulse shapes in real time for high resolution radiation spectroscopy

Abstract Techniques have been developed for the synthesis of pulse shapes using fast digital schemes in place of the traditional analog methods of pulse shaping. Efficient recursive algorithms have been developed that allow real time implementation of a shaper that can produce either trapezoidal or triangular pulse shapes. Other recursive techniques are presented which allow a synthesis of finite cusp-like shapes. Preliminary experimental tests show potential advantages of using these techniques in high resolution, high count rate pulse spectroscopy.

Digital techniques for real-time pulse shaping in radiation measurements

Abstract Recursive algorithms for real-time digital pulse shaping in pulse height measurements have been developed. The differentiated signal from the preamplifier (exponential pulse) is amplified and then digitized. Digital data are deconvolved so that the response of the high-pass network is eliminated. The deconvolved pulse is processed by a time-invariant digital filter which allows trapezoidal/triangular or cusp-like shapes to be synthesized. A prototype of a digital trapezoidal processor was built which is capable of sampling and processing digital data in real time at clock rates up to 50 MHz.

A ring Compton scatter camera for imaging medium energy gamma rays

Characteristics of a novel ring Compton scatter camera are described. This camera is designed to image medium-energy (0.5 to 3 MeV) gamma-ray fields. It consists of two position-sensitive detector arrays: a 4*4 planar array of high purity germanium (HPGe) crystals and a ring array of up to 64 NaI(Tl) crystals. Past evaluations of Compton cameras have used a planar second detector, which is subjected to a large flux of gamma rays that either pass directly through or undergo small angle scatter in the first detector. A ring array significantly reduces direct and small angle scattered events in the second detector. An analytical model for ring camera systems has been developed to predict angular resolution and efficiency, and has been benchmarked against measurements made with a prototype system consisting of the 4*4 HPGe array and an eight-element ring. Predictions are made for a system with 64 crystals in the second detector ring. >

Digital pulse processor using moving average technique

A digital pulse processor with improved differential linearity and reduced dead time has been designed. The circuit uses an 8-b flash ADC (analog-to-digital converter) running at 36 MHz and continually sampling the signal from the preamplifier or shaping amplifier. The digitized signal is then processed by a digital moving averager. A digital peak detector is used for measuring the amplitude of the shaped pulses. A novel, threshold-free circuit has been designed that combines both the moving average and peak detection functions. The circuit also provides a timing signal with an uncertainty of one sampling period. The number of the averaged samples (equivalent to the shaping time constant) is digitally controlled. The resolution of the processor is limited by the finite ADC resolution and the finite sampling frequency. Increasing the sampling frequency should improve the resolution in pulse height analysis and the timing precision. >

Digital pulse-shape analyzer based on fast sampling of an integrated charge pulse

A novel configuration for pulse-shape analysis and discrimination has been developed. The current pulse from the detector is sent to a gated integrator and then sampled by a flash analog-to-digital converter(ADC). The sampled data are processed digitally, thus allowing implementation of a near-optimum weighting function and elimination some of the instabilities associated with the gated integrator. The analyzer incorporates pileup rejection circuit that reduces the pileup effects at high counting rates. The system was tested using a liquid scintillator. Figures of merit for neutron-gamma pulse-shape discrimination were found to be: 0.78 for 25 keV (electron equivalent energy) and 3.5 for 500 keV. >

Compact circuit for pulse rise-time discrimination

Abstract A compact low-power circuit has been developed for rise-time selection in protable systems using thermoelectrically cooled Si-PIN photodiodes and CZT detectors. The principle of operation of the circuit is based on the comparison of the delayed detector current waveform (current signal) with the shaped preamplifier signal representing the total delivered charge (charge signal). The delay time of the detector current waveform is chosen to be shorter than the peaking time of the main shaping amplifier. If the amplitude of the current signal exceeds the amplitude of the charge signal a gate signal is generated allowing pulse acceptance from the multichannel analyzer (MCA); otherwise the pulse is rejected. The circuit comprises a small number of components mounted on a 50 × 80 mm printed circuit board. The maximum power consumption is 200 mW. Tests performed using Si-PIN photodiodes and CZT detectors indicate a significant reduction of the spectrum background. In the case of the CZT detector an improvement of the energy resolution was also observed.

Real time digital pulse shaper with variable weighting function

Abstract Real time digital pulse shaper has been developed that provides digital control of the synthesized weighting function (WF). The shaper can synthesize in real time shapes that are optimum or near optimum in the presence of 1/ f noise. Other pulse shapes including trapezoidal and triangular can be realized. The WF is synthesized by algebraically adding a concave and a convex pulse shapes. The shaper is implemented in a single chip and is a part of a spectroscopy system on a programmable chip.

Deconvolution of pulses from a detector-amplifier configuration

Abstract A technique for deconvolution of pulses from a detector-amplifier configuration is presented. The method also allows deconvolution of the pulses obtained using CRRC shaping networks. Analog and digital realizations of the technique are described.

Charge calibration of CsI(Tl)/photodiode spectroscopy systems

Abstract A charge calibration method for spectroscopy systems that use inorganic scintillators, photodiodes, and charge-sensitive preamplifiers is presented. The shaped square wave (SSW) method accounts for ballistic deficit when long decay time constants are present. The SSW method is demonstrated for CsI(T1) and compared to other calibration methods.

A data acquisition system for a Ring Compton-scatter Camera

A data acquisition system for a Ring Compton scatter Camera (RCC) has been developed. The RCC is composed of two detector arrays. A personal computer is used for storing the detector energy data and controlling the Compton camera. The camera output interface uses a pipelined hardware architecture to reduce the counting losses due to the finite data transfer time, and to provide variable length data records to the computer. Data transfers are controlled by either hardware or software generated interrupts. The acquisition modes include: the collection of coincident spectra, delayed coincident counting curves, and pulse height spectra from the individual detectors. Diagnostic procedures have also been developed. The software supports all of the hardware functions, and provides routines for preprocessing and displaying the acquired data. >