M. Nakhostin

Recursive Algorithms for Real-Time Digital

This paper reports on recursive algorithms for real-time implementation of CR-(RC)n filters in digital nuclear spectroscopy systems. The algorithms are derived by calculating the Z-transfer function of the filters for filter orders up to n=4 . The performances of the filters are compared with the performance of the conventional digital trapezoidal filter using a noise generator which separately generates pure series, 1/f and parallel noise. The results of our study enable one to select the optimum digital filter for different noise and rate conditions.

{\rm CR}\!-\!({\rm RC})^{\rm n}

A line-integrated neutron emission profile is routinely measured using the radial neutron collimator system in JT-60U tokamak. Stilbene neuron detectors (SNDs), which combine a stilbene organic crystal scintillation detector (SD) with an analog neutron-gamma pulse shape discrimination (PSD) circuit, have been used to measure collimated neutron flux. Although the SND has many advantages as a neutron detector, the maximum count rate is limited up to ∼1×105counts∕s due to the analog PSD circuit. To overcome this issue, a digital signal processing system (DSPS) using a flash analog-to-digital converter (Acqiris DC252, 8GHz, 10bits) has been developed at Cyclotron and Radioisotope Center in Tohoku University. In this system anode signals from photomultiplier of the SD are directory stored and digitized. Then, the PSD between neutrons and gamma rays is performed using software. The DSPS has been installed in the vertical neutron collimator system in JT-60U and applied to deuterium experiments. It is confirmed t...

Pulse Shaping

The problem of pulse pile-up is very often encountered in precise measurements of γ-rays using germanium detectors. The standard method of treating the pile-up events is to identify and reject them using an appropriate electronic system. Digital acquisition techniques now allow the recording of waveforms of pile-up events that can be analyzed and the contributing single pulses recovered, rather than simply tolerating the losses associated with pile-up. In this paper, a method for the off-line digital processing of pile-up events from germanium detectors is demonstrated. The method is based on an appropriate fitting of the detector signals, shaped with a suitable digital pulse shaper. It is shown that the method is able to recover the pile-up events with good accuracy even when the constituent signals are in close proximity. The method is very useful for γ-ray spectroscopy in nuclear physics experiments, where the low intensity signals can be lost due to the pile-up in a high-rate environment.

Fast collimated neutron flux measurement using stilbene scintillator and flashy analog-to-digital converter in JT-60U

We report on the application of digital pulse processing algorithms to improve the spectroscopic performance of a 1.2 mm thick planar HgI2 ?-ray detector. We have used offline processing of pulses which were recorded using a high resolution waveform digitizer. The recovery processes include long duration shaping to avoid ballistic deficit in the case of slow pulses, and the application of biparametric correction techniques to compensate for charge loss. Pulses of duration as long as 100 ?s were recorded to facilitate long duration shaping. Two different pulse processing algorithms, viz. semi-Gaussian and moving window deconvolution, were applied and their performance was compared. The application of long duration shaping and digital charge-loss correction improved the energy resolution at 662 keV by more than 20% and the peak to background ratio by a factor of two. The resolution and the peak to background ratio were further seen to improve drastically upon rejection of counts with very slow rise-time. A 2.6% energy resolution at 662 keV with 14:1 peak to background ratio was obtained.

A digital method for separation and reconstruction of pile-up events in germanium detectors

A new pulse-shape discrimination algorithm for neutron and gamma (n/γ) discrimination with liquid scintillation detectors has been developed, leading to a considerable improvement of n/γ separation quality. The method is based on triangular pulse shaping which offers a high sensitivity to the shape of input pulses, as well as, excellent noise filtering characteristics. A clear separation of neutrons and γ-rays down to a scintillation light yield of about 65 keVee (electron equivalent energy) with a dynamic range of 45:1 was achieved. The method can potentially operate at high counting rates and is well suited for real-time measurements.

Digital pulse height correction in HgI2 γ-ray detectors

Parallel-plate avalanche counters have long been recognized as timing detectors for heavily ionizing particles. However, these detectors suffer from a poor pulse-height resolution which limits their capability to discriminate between different ionizing particles. In this paper, a new approach for discriminating between charged particles of different specific energy-loss with avalanche counters is demonstrated. We show that the effect of the self-induced space-charge in parallel-plate avalanche counters leads to a strong correlation between the shape of output current pulses and the amount of primary ionization created by the incident charged particles. The correlation is then exploited for the discrimination of charged particles with different energy-losses in the detector. The experimental results obtained with α-particles from an 241Am α-source demonstrate a discrimination capability far beyond that achievable with the standard pulse-height discrimination method.

A new digital method for high precision neutron-gamma discrimination with liquid scintillation detectors

In computed tomography (CT) systems, it is desirable to know the X-ray energy spectra for various applications, including medical CT imaging, and diagnostic field and heavy ion therapy. However, because of the restricted space, the only practical solution is to use Compton spectroscopy, where the incident spectrum is inferred from the scattered spectrum. The geometry of the scatterer and its position within the CT can affect the spectrum of the secondary beam, making it difficult to determine the primary spectrum during operation of the CT system. A modified Compton spectrometer is described that allows measurement of the X-ray energy spectra during operation, and most importantly, in rotation mode. The geometry of the scatterer was optimized to reduce the energy broadening of the secondary beam. The performance of the system was evaluated by comparing the reconstructed exposure to that measured directly using an ion chamber.

Pulse-Shape Discrimination of Alpha Particles of Different Specific Energy-Loss With Parallel-Plate Avalanche Counters

A study of intrinsic state halflife measurements in the N=80 nucleus 138Ce has been made using the 130Te(12C,4n)138Ce fusion evaporation reaction at beam energy of 56 MeV. The fast-timing gamma-ray coincidence method was used with a mixed LaBr3(Ce)-HPGe array to establish the lifetimes of the yrast 6+ state at 2294 keV, the Iπ=5− state at 2218 keV, the Iπ=11+ state at 3943 keV and the 14+ state at that at 5312 keV, all of which are in the sub nanosecond regime. Reduced transition probabilities have been calculated for the electromagnetic decays from these states.

Compton spectroscopy for rotation-mode computed tomography

We developed a prototype of TlBr semiconductor detector array for high resolution PET scanner with less than 1mm FWHM. We obtained an energy resolution of ∼20% FWHM for 511 keV and a coincidence time resolution of ∼50nsec FWHM at a detector bias 100V. This result expects the capability of application of TlBr detector array to a high resolution PET scanner. The TlBr detector arrays with the present time resolution performance can be applied to a small PET scanner for the PET study using small animals such as rats and mice.

Electromagnetic Transition Rate Measurements in the N=80 Isotone, 138Ce

Applying downsized semiconductor detectors to PET cameras makes it possible to achieve high spatial resolutions.