Glenn F. Knoll

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.

Light collection in scintillation detector composites for neutron detection

The authors have been investigating the heterogeneous combination of neutron-conversion materials in a plastic or liquid scintillation matrix with the goal of developing neutron detectors of high detection efficiency and fast response. They report on one such combination, consisting of thin-wall glass microspheres filled with high-pressure /sup 3/He gas dispersed in plastic scintillation. They have developed glass formulations that are capable of containing the helium in shells with wall thickness as low as 1 mu m with leak rates at room temperature that correspond to several years half life. The energy loss of the reaction products in the shell wall and the low light yield of plastic scintillations for low-energy protons result in low light yield per event. The authors discuss the use of an optical modeling code to investigate the incorporation of wavelength-shifting elements to aid in the collection of this light from highly scattering composite scintillation media. >

1-D POSITION SENSITIVE SINGLE CARRIER SEMICONDUCTOR DETECTORS

Abstract A single polarity charge sensing method has been studied using coplanar electrodes on 5 mm cubes of CdZnTe γ-ray detectors. This method can ameliorate the hole trapping problem of room-temperature semiconductor detectors. Our experimental results confirm that the energy resolution is dramatically improved compared with that obtained using the conventional readout method, but is still about an order of magnitude worse than the theoretical limit. A method to obtain the γ-ray interaction depth between the cathode and the anode is presented here. This technique could be used to correct for the electron trapping as a function of distance from the coplanar electrodes. Experimental results showed that a position resolution of about 0.9 mm FWHM at 122 keV can be obtained. These results will be of interest in the design of higher performance room-temperature semiconductor γ-ray detectors.

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.

POSITION-SENSITIVE SINGLE CARRIER CDZNTE DETECTORS

Abstract Single polarity charge sensing on room temperature semiconductor gamma-ray detectors can be achieved by using the coplanar electrode read-out technique. This mehod can eliminate the hole-trapping problem of the wide band gap semiconductors which are currently available. Our previous results on 5 mm cube CZT detectors confirmed [6] that the energy resolution can be dramatically improved compared with that obtained using the conventional read-out method. This paper explores the application of this technique to CdZnTe detectors of larger volume, namely 1 cm 3 . In our previous work, we suggested a method to obtain γ-ray interaction depth and further progress is reported here. This technique can be used to correct for the electron trapping as a function of distance from the anode. The intrinsic position resolution has been analyzed and energy resolutions of less than 2% FWHM at 662 keV were obtained on both detectors tested. Finally, the factors which inhibit attaining the statistical energy resolution limit of CdZnTe detectors have been explored. These results will be of interest in the design of higher performance, portable and imaging-related, room-temperature semiconductor γ-ray detectors.

Evaluation of a Compton scattering camera using 3-D position sensitive CdZnTe detectors

Abstract A CZT Compton Camera (CCC) is being built using two three-dimensional (3-D) position-sensitive CZT detectors. Expected system performance was analyzed by analytical and Monte Carlo approaches. Based on the measurement of detector energy and position resolution, the expected angular resolution is ∼3° and ∼2° for a ±30° FOV for 511 keV and 1 MeV γ -rays, respectively. The intrinsic efficiency for a point source 10 cm from the first detector surface ranges from 1.5×10 −4 to 8.8×10 −6 for 500 keV–3 MeV.

Calculated gamma ray response characteristics of semiconductor detectors

Abstract A Monte Carlo computer program has been used to calculate characteristics of the response of fully depleted silicon and germanium radiation detectors to monoenergetic gamma rays. Data for total absorption probability, intrinsic efficiency, escape peak efficiency and pulse height spectra are presented as functions of detector thickness and photon energy. Other parameters of interest in analysing detector response are also given. The results of a second Monte Carlo calculation of electron migration in silicon and germanium are employed to account for the leakage of secondary electrons from the detector volume. Bremsstrahlung energy loss by electrons is also simulated. The calculations are expected to be applicable in those cases in which secondary electron energies do not exceed 2 MeV. Comparison with experiment shows good agreement within this limitation.

Temperature dependence of CsI(Tl) gamma-ray excited scintillation characteristics

The gamma-ray excited, temperature dependent scintillation characteristics of CsI(Tl) are reported over the temperature range of −100 to + 50°C. The modified Bollinger-Thomas and shaped square wave methods were used to measure the rise and decay times. Emission spectra were measured using a monochromator and corrected for monochromator and photocathode spectral efficiencies. The shaped square wave method was also used to determine the scintillation yield as was a current mode method. The thermoluminescence emissions of CsI(Tl) were measured using the same current mode method. At room temperature, CsI(Tl) was found to have two primary decay components with decay time constants of τ1 = 679±10 ns (63.7%) and τ2 = 3.34±0.14 μs (36.1%), and to have emission bands at about 400 and 560 nm. The τ1 luminescent state was observed to be populated by an exponential process with a resulting rise time constant of 19.6±1.9 ns at room temperature. An ultra-fast decay component with a < 0.5 ns decay time was found to emit about 0.2% (about 100 photons/MeV) of the total scintillation light. Except for the ultra-fast decay time, the rise and decay time constants were observed to increase exponentially with inverse temperature. At −80°C τ1 and τ2 were determined to be 2.22±0.33 μs and 18.0±2.59 μs, respectively, while the 400 nm emission band was not observed below −50°C. At +50°C the decay constants were found to be 628 ns (70.5%) and 2.63 μs (29.3%) and both emission bands were present. The scintillation yield of CsI(Tl) was observed to be only slightly temperature dependent between −30 and +50°C, peaking at about −30°C (about 6% above the room temperature yield) and monotonically decreasing above and below this temperature. Four different commercially available CsI(Tl) crystals were used. Minimal variations in the measured scintillation characteristics were observed among these four crystals. Thermoluminescence emissions were observed to have peak yields at −90, −65, −40, +20, and possibly −55°C. The relative magnitudes and number of thermoluminescence peaks were found to vary from crystal to crystal.

Evidence for field enhanced electron capture by EL2 centers in semi-insulating GaAs and the effect on GaAs radiation detectors

The performance of Schottky contact semiconductor radiation detectors fabricated from semi‐insulating GaAs is highly sensitive to charged impurities and defects in the material. The observed behavior of semi‐insulating GaAs Schottky barrier alpha particle detectors does not match well with models that treat the semi‐insulating material as either perfectly intrinsic or as material with deep donors (EL2) of constant capture cross section compensated with shallow acceptors. We propose an explanation for the discrepancy based on enhanced capture of electrons by EL2 centers at high electric fields and the resulting formation of a quasineutral region in the GaAs. Presented is a simple model including field enhanced electron capture which shows good agreement with experimental alpha particle pulse height measurements.

COPLANAR GRID PATTERNS AND THEIR EFFECT ON ENERGY RESOLUTION OF CDZNTE DETECTORS

Abstract This paper describes diagnostic techniques using depth and radial position-sensing methods that have been applied to identify and remove the non-symmetric effect of coplanar grid electrodes. Our experimental results show that the non-symmetric effect can degrade significantly the energy resolution of single polarity charge sensing CdZnTe detectors and can be minimized by balancing the weighting potentials of coplanar anodes. The coplanar electrode design has been modified based on the knowledge gained from this study and improvements in detector performance have been achieved.