Frisch Grid Research Articles

Status of TACTIC: A detector for nuclear astrophysics

A new detector for nuclear astrophysics studies is being designed and built by TRIUMF and the University of York. The TRIUMF Annular Chamber for Tracking and Identification of Charged particles (TACTIC) is designed to detect low-energy charged particles from inverse kinematics reaction studies performed at the relevant astrophysical energies. TACTIC is a cylindrical ionisation/time-projection chamber with segmented anode strips, which allow the d E/d x of the particle to be determined along with the total energy. Information from drift times allows the particle trajectory to be reconstructed. This in turn identifies the interaction point along the beam axis and hence the centre of mass energy of the reaction. To amplify the expected weak signals, a gas electron multiplier (GEM) will be used in place of the usual Frisch grid. Full digital readout of the charge and timing of each anode strip will be achieved with flash ADC cards allowing pulse shape analysis of the signals.

Measurement of the cross sections of the reactions 14N(n, α)11B and 14N(n, t)12C at neutron energies 5.45–7.2 MeV

The cross sections of the reactions 14N(n, α)11B and 14N(n, t)12C have been measured for neutron energies 5.46–7.2 MeV. The neutrons are generated in the reaction D(d, n) on a solid titanium target. The work employs digital spectrometry. The charged-particle detector is a pulsed ionization chamber with a Frisch grid, filled with a kyrpton-nitrogen mixture. The cross sections are measured for four groups of α particles α0, α1, α2, and α3 from the reaction 14N(n, α)11B and for tritium from the reaction 14N(n, t)12C. The energy resolution of the spectrometer was 60 keV. The errors in determining the cross sections for the reactions (n, α) and (n, t) are 10–15%. The measurement results are compared with the ENDF/V VI evaluation. Good agreement is obtained in the neutron energy range 5.45–6.5 MeV. At higher energies, the discrepancy reaches 30%.

Enhanced High Speed SE Imaging in a VPSEM Using a Frisch Grid

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Micromegas neutron beam monitor neutronics

The Micromegas is a type of ionising radiation detector that consists of a gas chamber sandwiched between two parallel plate electrodes, with the gas chamber divided by a Frisch grid into drift and amplification gaps. Investigators have applied it to a number of different applications, such as charged particle, X-ray and neutron detection. A Micromegas device has been tested as a neutron beam monitor at CERN and is expected to be used for that purpose at the Spallation Neutron Source (SNS) under construction in Oak Ridge, TN. For the Micromegas to function effectively as neutron beam monitor, it should cause minimal disruption to the neutron beam in question. Specifically, it should scatter as few neutrons as possible and avoid neutron absorption when it does not contribute to generating useful information concerning the neutron beam. Here, we present the results of Monte Carlo calculations of the effect of different types of wall materials and detector gases on neutron beams and suggest methods for minimising disruption to the beam.

A new coplanar-grid high-pressure xenon gamma-ray spectrometer

High-pressure xenon (HPXe) gas is a desirable radiation detection medium for many reasons, including its large atomic number, high density, low mean energy to produce an electron-ion pair, and the ability to produce devices with large detection volumes. While past work in HPXe has produced relatively successful detectors with energy resolution at 662 keV as good as approximately 2% FWHM, an expected limitation of these chambers in field operation is resolution degradation due to the vibration of their Frisch grids. Progress on a detector without a Frisch grid is reported in this submission; it is expected that the proposed anode design will provide competitive energy resolution with minimal degradation from mechanical vibration. Simulations accounting for charge carrier statistics, changes in the charge induced on the anode as a function of interaction location, and electronic noise predict a best-case energy resolution of 2.3% FWHM at 662 keV. Experimental data is compared with these simulations.

Performance of a new CdZnTe portable spectrometric system for high energy applications

A new cadmium zinc telluride (CZT) portable spectrometric system called OMEGA has been developed at CEA/LETI for high-energy applications like spent fuel assay in response to needs in the nuclear industry. The unit has been designed to operate at room temperature under water. The CZT OMEGA system is based on a single 10/spl times/10/spl times/10 mm/sup 3/ CZT detector associated with a single electronics channel. To improve performance, the system incorporates several technologies including: an electrode design named the Mixed electrode which combines a noncontacting Frisch grid and the pixel field effect to obtain maximum sensitivity from the 10/spl times/10/spl times/10 mm/sup 3/ CZT detector, a single hybrid preamplifier designed to reduce noise, an electronics board configured to enhance count rate capabilities and finally, electron bi-parametric (BP) correction (pulse height corrected via measurement of rise time) to improve both energy resolution and detection efficiency. This paper reports the most recent results obtained with the CZT OMEGA spectrometric system at high energy (/sup 137/Cs and /sup 152/Eu sources) at both low and high count rate: spectrometric performance shows an energy resolution of 1% FWHM at 662 keV (0.8% at 1408 keV) from 1 kcps to 50 kcps, deteriorating to 2% FWHM at 200 kcps.

Factors limiting the performance of CdZnTe detectors

In the past few years, significant progress has been achieved in the development of room-temperature semiconductor detectors, particularly those based on CdZnTe (CZT) crystals. Several types of electron-transport-only detectors have been developed: pixel, coplanar-grid, cross-strip, drift-strip, orthogonal coplanar strip, and virtual Frisch grid, many of which are now commercially available. Despite all these varieties in the detector designs, they have many common features and problems. This review summarizes the common detector design constraints and related factors limiting performance of CZT detectors: bulk and surface leakage currents, surface effects, properties of Schottky contacts and surface interfacial layers, charge sharing and loss in multielectrode devices, charge transport nonuniformities, and fluctuations in the pulse height for long-drift-length devices. We also describe unique capabilities at Brookhaven National Laboratory, Upton, NY, for CZT device characterization and recent progress utilizing these tools.

A three-dimensional model of CdZnTe gamma-ray detector and its experimental validation

A fully three-dimensional (3-D) model of a semiconductor gamma-ray detector is presented. The model takes into account the gamma-ray and charge collection physical phenomena involved in the detection process and models the readout electronic response and noise. The model successively involves the Monte Carlo simulation of the photon transport, the finite element transient computation of the adjoint transport equation, and the electronic signal processing including an accurate noise model. The simulation is possible because the solution of the adjoint equation allows us to obtain a continuous mapping of the induced charge collection efficiency (at any time and any interaction point) with a single 3-D transient finite element computation. The simulation outputs are pulse height spectra and bi-parametric (rise time versus pulse height) spectra. The model has been validated on two applications. The first application is a CdZnTe monolithic pixelated detector at medical energies. The detector dimension is 10/spl times/10/spl times/5 mm, with a pixel size of 2 /spl times/ 2 mm, and a 2.5-mm pitch. The second application is a gamma-ray spectrometric single-channel probe for 200-keV to 2-MeV photons combining a small anode and the capacitive Frisch grid structure. The detector dimension is 10/spl times/10/spl times/9 mm. Simulated and experimental results are compared. In both cases, the comparison between simulated and experimental spectra is qualitatively good for pulse height spectra, as well as bi-parametric spectra.

Measurement of the probability of 232Th ternary fission induced by fast neutrons

Results obtained by measuring the energy dependence of the probability of 232Th ternary fission in the region of vibrational resonances are presented. The measurements were performed by using a double ionization chamber with Frisch grids and a CsI(T1) scintillation detector. The use of digital methods for pulse processing made it possible to obtain highly reliable results. The data analysis reveals that our value of the ternary-fission probability is compatible with existing systematics, which nevertheless need some correction. The results of our measurements unambiguously indicate that local variations in the total kinetic energy of fission fragments in the region of the individual vibrational resonances cannot be explained by corresponding variations in the ternary-fission probability.

HPXe ionization chambers for γ spectrometry at room temperature

High pressure xenon (HPXe) ionization chambers exhibit many characteristics which make them particularly suitable for industrial γ spectrometry at room or higher temperature. The use of a gas as detection medium allows one to reach very large effective volumes and makes these chambers relatively insensitive to radiation damage. Further, the high atomic number of xenon ( Z=54) enhances the total absorption of incident photons and provides, combined to high pressure, a good enough detection efficiency with respect to solid state detectors. Furthermore, such ionization chambers with Frisch grid appear to be very stable over wide periods (e.g. a research prototype has been used aboard MIR orbital station for several years) and temperature range (up to 180°), without maintenance. The characteristics of different prototypes are presented. Their detection efficiency and energy resolution are studied as a function of incident γ ray energy. New developments in electronics and signal processing are also investigated to improve their performances.

A high pressure xenon gamma-ray spectrometer using a coplanar anode configuration

A new design of a high pressure xenon ionization chamber has been fabricated in an attempt to eliminate the problems associated with acoustical vibrations of the Frisch grid. The function of the traditional Frisch grid has been accomplished by employing a coplanar anode system capable of single polarity charge sensing. Two different detector designs have been fabricated using both cylindrical and parallel plate geometries. Each is filled with highly purified xenon gas at a pressure of approximately 57atm. The designs of these new spectrometers and their measured characteristics will be presented.

A hemispherical high-pressure xenon gamma radiation spectrometer

A prototype hemispherical high-pressure xenon gamma radiation spectrometer was designed, constructed and tested. The detector consists of a pair of concentric hemispherical electrodes contained inside a thin-walled stainless steel pressure dome. Detector performance parameters such as energy resolution, linearity and vibration sensitivity were determined and compared to previous cylindrical and planar designs. Without a Frisch grid, the hemispherical detector provides a total room temperature energy resolution of 6% @ 662 keV and is relatively insensitive to acoustic interference.

A capacitive Frisch grid structure for CdZnTe detectors

Recent results with CdZnTe (CZT) gamma spectrometers have shown that Frisch grid structures yield an excellent performance. Hole trapping problems have been solved by this means. However, some difficulties persist concerning optimization of performance in terms of electronic noise and response uniformity. Also, the complexity of Frisch grid structures makes them hard to manufacture on an industrial scale. This paper presents some results obtained using a new Frisch grid structure based on capacitive coupling. Extremely easy to produce, it allows highly efficient detector screening without leakage current increase.

Development of a high-pressure xenon ionization chamber gamma-ray spectrometer for field deployment in cone penetrometers

It is sometimes necessary to measure gamma-ray spectra under difficult circumstances such as those encountered during in situ characterization of radioactive soils. For some classes of soil, various measurement instruments can be inserted to depths of 100 feet or more using the cone penetrometer technique. The problems for gamma-ray spectroscopy in this application include size limitations, elevated and/or variable temperature environment, vibration and shock, and remote operation. Measurement of gamma-ray spectra under these conditions has been done using scintillation detectors such as NaI(Tl) or BGO. However, these instruments suffer from poor energy resolution (ca. 8-10%), temperature sensitivity and, in the case of NaI(Tl), activation by neutrons. Sentor Technologies, Inc., working under Department of Energy sponsorship and in conjunction with Virginia Commonwealth University and the University of California, San Diego, has developed a high-pressure xenon ionization chamber spectrometer that is specifically designed for use in cone penetrometers. Key features of the detector design include a 29 mm O.D. cylindrical geometry with concentric cathode, Frisch grid, and anode, and ultra-purified (ca. ppb) xenon pressurized to a density of 0.6 g·cm-3. The results demonstrate the utility of high-pressure xenon ionization spectrometers for field use in cone penetrometers or similar applications including borehole logging.

Pulse rise-time characterization of a high pressure xenon gamma detector for use in resolution enhancement

Abstract High pressure xenon ionization chamber detectors are possible alternatives to traditional thallium doped sodium iodide [NaI(Tl)] and hyperpure germanium as gamma-spectrometers in certain applications. Xenon detectors incorporating a Frisch grid exhibit energy resolutions comparable to cadmium/zinc/telluride (CZT) (e.g., 2% @ 662 keV) but with far greater sensitive volumes. The Frisch grid reduces the position dependence of the anode pulse rise-times, but it also increases the detector vibration sensitivity, anode capacitance, voltage requirements and mechanical complexity. We have been investigating the possibility of eliminating the grid electrode in high-pressure xenon detectors and preserving the high energy resolution using electronic rise-time compensation methods. A two-electrode cylindrical high pressure xenon gamma-detector coupled to time-to-amplitude conversion electronics was used to characterize the pulse rise-time of deposited gamma-photons. Time discrimination was used to character...

An ionization chamber with Frisch grids for studies of high-energy neutron-induced fission

A gridded ionization chamber for fission fragment detection is described. The chamber has been specially designed for use at the quasi-monoenergetic 7Li(p, n) neutron source at the The Svedberg Laboratory, Uppsala, Sweden. The detector permits measurements of fission fragment energy and emission angle for two targets with diameter of up to 10 cm. The time response of the chamber (⩽5 ns FWHM) is adequate to apply time-of-flight discrimination against background events induced by non-peak neutrons. Results of angular anisotropy measurements for the 232Th (n, f) and 238U(n, f) reactions in the 20–160 MeV energy range are given.

Low-background spectrometer for the study of fast neutron-induced (n,α) reactions

An α-particle spectrometer based on an ionization chamber with Frisch grid and a waveform digitizer has been developed for studies of fast neutron-induced (n,α) reactions. The information on the energy of the α-particle and its emission angle can be obtained from the amplitude and rise time of the digitized anode signal. For a situation where both the solid target on the cathode of the ionization chamber and the working gas are the α-particle sources, a new method for the suppression of the neutron-induced background is proposed. The background due to gaseous α-particles was reduced by a factor of 30.

Performance of CdZnTe geometrically weighted semiconductor Frisch grid radiation detectors

Semiconductor Frisch grid radiation detectors have been manufactured and tested with encouraging results. Resolution enhancement occurs as a result of combining the geometric weighting effect, the "small pixel" effect and the Frisch grid effect. The devices are operated at ambient temperature without any pulse shape correction, rejection and compensation techniques. The new devices are manufactured from CdZnTe and do not require any cooling for operation. The geometrically weighted detectors have only one signal output to a standard commercially available Ortec 142A preamplifier. The detectors operate with simple commercially available NIM electronics, hence the device design can be coupled to any typical NIM system without the need for special electronic instruments or circuits. Geometrically weighted detectors that are 1 cubic centimeter in volume were fabricated from "counter grade" material, yet have shown room temperature energy resolution of 7.5% FWHM (at 29/spl deg/C) for /sup 57/Co 122 keV gamma rays and 2.68% FWHM (at 23/spl deg/C) for /sup 137/Cs 662 keV gamma rays.

A fast 1-D detector for imaging and time resolved SAXS experiments

A one-dimensional test detector on the principle of a highly segmented ionization chamber with shielding grid (Frisch grid) was developed to evaluate if this kind of detector is suitable for advanced small-angle X-ray scattering (SAXS) experiments. At present it consists of 128 pixels which can be read out within 0.2 ms with a noise floor of 2000 e-ENC. A quantum efficiency of 80% for a photon energy of 8 keV was achieved. This leads to DQE values of 80% for photon fluxes above 1000 photons/pixel and integration time. The shielding grid is based on the principles of the recently invented MCAT structure and the GEM structure which also allows electron amplification in the gas. In the case of the MCAT structure, an energy resolution of 20% at 5.9 keV was observed. The gas amplification mode enables imaging with this integrating detector on a subphoton noise level with respect to the integration time. Preliminary experiments of saturation behavior show that this kind of detector digests a photon flux density up to 10 12 photons/mm 2 s and operates linearly. A spatial resolution of at least three line pairs/mm was obtained. All these features show that this type of detector is well suited for time-resolved SAXS experiments as well as high flux imaging applications.

Geometrically weighted semiconductor Frisch grid radiation spectrometers

A new detector geometry is described with relatively high gamma ray energy resolution at room temperature. The device uses the geometric weighting effect, the small pixel effect and the Frisch grid effect to produce high gamma ray energy resolution. The design is simple and easy to construct. The device performs as a gamma ray spectrometer without the need for pulse shape rejection or correction, and it requires only one signal output to any commercially available charge sensitive preamplifier. The device operates very well with conventional NIM electronic systems. Presently, room temperature (23°C) energy resolutions of 2.68% FWHM at 662 keV and 2.45% FWHM at 1.332 MeV have been measured with a 1 cm 3 prism shaped CdZnTe device.