Conventional Proportional Counter Research Articles

Development of liquid scintillation based 4πβ(LS)-γ coincidence counting system and demonstration of its performance by standardization of 60Co

A single-vial, single-PMT 4πβ(LS)-γ coincidence counting system has been developed at the Radiation Safety Systems Division, BARC. It has advantages of simple sample preparation, higher counting efficiency and the absence of self absorption over the conventional proportional counter based 4πβ(PC)-γ coincidence counting system. The performance of the system is demonstrated by standardizing a 60Co solution using the 4πβ(LS)-γ coincidence counting system, 4πβ(PC)-γ coincidence counting system and CIEMAT/NIST method and comparing the results obtained by each method. The detection efficiency of liquid scintillation counter of the 4πβ(LS)-γ coincidence counting system was varied by color quenching, by chemical quenching and by varying the bias voltage applied to the LSC PMT. For the proportional counter based 4πβ(PC)-γ coincidence counting system the detection efficiency was varied by source self absorption. The activity concentrations obtained using the 4πβ(LS)-γ coincidence counting system, the 4πβ(PC)-γ coincidence counting system and the CIEMAT/NIST method are comparable within the uncertainty limits.

Monte Carlo simulation of β–γ coincidence system using plastic scintillators in 4 π geometry

A modified version of a Monte Carlo code called Esquema, developed at the Nuclear Metrology Laboratory in IPEN, São Paulo, Brazil, has been applied for simulating a 4 πβ(PS)–γ coincidence system designed for primary radionuclide standardisation. This system consists of a plastic scintillator in 4 π geometry, for alpha or electron detection, coupled to a NaI(Tl) counter for gamma-ray detection. The response curves for monoenergetic electrons and photons have been calculated previously by Penelope code and applied as input data to code Esquema. The latter code simulates all the disintegration processes, from the precursor nucleus to the ground state of the daughter radionuclide. As a result, the curve between the observed disintegration rate as a function of the beta efficiency parameter can be simulated. A least-squares fit between the experimental activity values and the Monte Carlo calculation provided the actual radioactive source activity, without need of conventional extrapolation procedures. Application of this methodology to 60Co and 133Ba radioactive sources is presented and showed results in good agreement with a conventional proportional counter 4 πβ(PC)–γ coincidence system.

Experimental gas amplification study in boron-lined proportional counters for neutron detection

Based on theoretical considerations related to the microscopic behaviour of electrons in gases in conjunction with a macroscopic hypothesis, a semi-microscopic gas gain formula for conventional proportional counters has been already developed and was afterwards successfully tested on gas gain data obtained in Ar–CH 4- and He–isobutane-filled proportional counters, using an X-ray radiation source. In the present work, it is verified that the semi-microscopic formula works as well in case of the detection of spread energy-spectrum of ionizing charged particles, such as involved within boron lined proportional counters, commonly used for thermal neutron detection. Indeed, we investigate experimentally the validity of the formula for the parameterization of the gas amplification factor data obtained in two boron lined proportional counters, filled with Ar–CH 4 (10%) and Ar–CO 2 (5%). The obtained results show clearly the ability of the gas gain formula to perfectly describe gas gain characteristics in such counters independently of the filling gas mixture used. Consequently, this provides further evidence of generality of the underlying gas amplification semi-microscopic model.

Application of a Microstrip Gas Counter in Energy-Dispersive X-Ray Fluorescence Analysis

Performance characteristics of a microstrip gas counter operated as an x-ray fluorescence spectrometer are reported. Gas amplification as a function of microstrip anode–cathode voltage was measured, and the breakdown threshold voltage was determined in pure xenon. The detector temporal stability and the effect of gas purity were assessed. Energy resolution and linearity, detection efficiency and uniformity of spatial response in the 2–60 keV x-ray energy range were determined from the pulse-height distributions of the fluorescence x-ray spectra induced in a variety of single- and multi-element sample materials. Energy resolution similar to that of conventional proportional counters was achieved at 6 keV. © 1997 John Wiley & Sons, Ltd.

Simple System for Measuring Tritium Ad/Absorption Using a 2πCounter and Thermal Desorption Spectrometer

In this paper, the authors develop a simple system using tritium tracer and thermal desorption techniques to measure the tritium adsorption and/or absorption on/in a material having typical surface conditions: namely, not cleaned surface. The tritium counting devices used were a 2{pi} counter and conventional proportional counter. With this system, the amounts of ad/absorption could be measured without exposing the samples to air after exposing them to tritium gas. The overall efficiency (F) of the 2{pi} counter was described at F = exp({minus}2.64h), where h is the distance from the sample to the detector. Ad/absorption measurements were carried out for several materials used for fabricating conventional vacuum systems. The results were, in the order of decreasing amounts of ad/absorption, as (fiber reinforced plastics(FRP)) {gt} (nickel(Ni), molybdenum disulfide(MoS{sub 2})) {gt} (stainless steel (SS304), iron(Fe), aluminum alloy(A2219)) {gt} (boron nitride(h-BN), silicon carbide (SiC), SS304 passivated by anodic oxidation layers(ASS) and that by boron nitride segregation layers (BSS)). The relative amounts were abut 100 for Ni and 0.1 for ASS and BSS, being normalized to Fe = 1.

2. X-Ray Astronomy

The period leading up to the last IAU General Assembly was dominated in X-ray Astronomy by the results from the Einstein Observatory. This first application of a large, satellite-borne, high resolution X-ray telescope to the study of cosmic sources had led, by the end of orbital operation in April 1981, to the detection of X-ray fluxes from a wide variety of astronomical objects and the full maturing of X-ray Astronomy. During the past three years a strong scientific output has continued to flow from the analysis of the more than 5600 separate Einstein observations, many of which are now widely available via the Einstein Data Bank. In sharp contrast, the peak of activity and scientific output from Einstein will apparently be followed, in the United States, by almost a decade with no new X-ray satellite. Fortunately, successful launches have been achieved in both the Japanese and European programmes, with Tenma launched in February 1983 (to join Hakucho in orbit) followed by EXOSAT three months later. Both these new missions have attracted world-wide interest and observing time on both is at a premium. Of a range of instruments on board the Tenma satellite the most productive is proving to be the large (~800 cm2) array of gas scintillation proportional counters. These detectors, flown for the first time on Tenma (and EXOSAT), provide a factor-of-two increase in spectral resolution over the more conventional proportional counters, an advantage which is particularly valuable in resolving the key emission line and absorption features associated with the K-shell of iron, near ~7 keV. One particularly exciting result reported from this Tenma instrument was an apparent absorption feature at ~4.1 keV in X-ray burst spectra from the source 2S 1636-536 (later supported by EXOSAT observations), the feature being attributed to red-shifted iron absorption in the strong gravitational field close to the neutron star burst source! At the time of writing (December 1984) a power storage problem is limiting the operational efficiency of Tenma, but the scientific payload continues to function well.

Studies with encapsulated sources of 125I III. Microdosimetry using a non-metallic wall-less proportional counter

A significant problem in measurement of microdosimetric spectra of low energy X rays has been the perturbing influence of the counters used in experiments. The conventional proportional counters used in microdosimetry have employed electrode structures of stainless steel. Since photoelectric cross-sections are highly sensitive to atomic number (approximately proportional to z 4, the photoelectrons, emitted from the steel structures, with correspondingly higher energies, may dominate over the softer Compton electrons emitted from the wall and gas. This is more serious for a wall-less type of counter, in which metallic supporting structures may contribute an additional source of photoelectrons entering the sensitive region. In order to perform accurate measurement with 125I photons and other soft X rays having energies less than 100 keV, a special counter was constructed having no metal parts. The construction is of the grid-defined “wall-less” type, contained inside a plastic spherical shell. The plastic was chosen to match the energy absorption coefficient of tissue at 25 keV as closely as possible; therefore, conventional tissue-equivalent A-150 was replaced by an “air-equivalent” type, C-552, which at 25 keV matches tissue to within a few percent. This material, based on the plastic polyvinylidene fluoride has additional advantages in terms of tensile strength and rigidity, for the purpose of molding delicate structures. The gas used was a propane-based tissue-equivalent type, modified to match the plastic. A human hair, coated with graphite, was used as the central collecting wire. The construction and details of this detector are described, and measurements are reported of 125I and 60 keV photons from 14 241Am, as well as high energy photons from 137Cs and 60Co. Values of Yd are compared to similar wall-less measurements with a conventional wall-less counter.

Individual electrons detected after the interaction of ionizing radiation with gases

The interaction of ionizing radiation (X rays or high-energy particles) with gases results in the creation of electron-ion pairs. For low-energy X rays a small cluster of primary ionization is produced close to the point of the initial photoionization. High-energy particles, however, lose a small fraction of their energy in each of a number of ionizing collisions resulting in a long track composed of clusters of ionization. We report here that the light signals emitted by a parallel-plate proportional counter are fast enough to allow the individual electrons in the ionization clusters to be resolved. By counting the number of electrons for low energy X ray events we demonstrate an improvement in energy resolution of more than a factor of two over conventional proportional counters.

The escape gated proportional counter — A low background and high energy resolution hard X-ray detector

Proportional counters, designed to detect X-rays, are generally filled with Xe or Kr or a mixture of Xe and Kr. In a large multichamber proportional counter the K-flourescent radiation of these gases, which escapes the cell of origin, but is absorbed in other cells of the counter, provides an additional signal. A coincidence technique, which makes use of this signal to reduce the charged particle background and to correct the energy spectrum for the escape peak, has been evaluated. The detector has an improved energy resolution over that of the conventional proportional counter and preliminary results indicate that a reduction in the background by a factor of about 1000 can be obtained. Its combined properties of very efficient background reduction and improved energy resolution make this type of instrument most suitable for spectroscopic studies of astronomical X-ray sources in the energy range 15–100 keV.

Fast gas mixtures for gas-filled particle detectors

Based on knowledge of the electron scattering cross sections at low electron energies, combinations of binary and tertiary gas mixtures have been discovered for which the electron energies and the overall scattering cross sections that correspond to these are reduced, resulting in an increase in the drift velocity, w, of the electrons in the gas mixture for a range of E/ P (pressure-reduced electric field) values. Several such mixtures of argon (Ar), carbon tetrafluoride (CF 4) and acetylene (C 2H 2) will be described that provide much faster response in gas-filled detectors compared to P-10 (90% Ar + 10% CH 4) over a wide and convinient range of E/ P values. The w( E/ P) for CF 4(100%), Ar(80%) + CF 4(20%), Ar(80%) + C 2H 2(20%), Ar(80%) + C 2H 2(10%) + CF 4(10%) are respectively, ∼15 × 10 6, ∼12 × 10 6, ∼5 × 10 6 and ∼9 × 10 6 cm s −1 at E/ P values of, respectively, ∼6, 0.8–1.5, 0.5–4 and 0.8–3.5 V cm −1 torr −1. Measurements with a conventional proportional counter showed that although the energy resolution of the Ar/CF 4 mixtures compares unfavorably with the energy resolution of P-10, that of the C 2H 2-containing binary and tertiary mixtures compares very favorably.

Further studies of cylindrical proportional counters for microdosimetric work

Walled and wall-less tissue-equivalent cylindrical proportional counters of unit elongation have been constructed and their performance extensively studied and compared with that expected from the theory of conventional proportional counter. The difference in behaviour between the walled and wall-less counters as regards energy resolution, gas gain, sensitive volume etc., is attributed to the wide grid structure of the latter. The transit time of single electrons released from the cathode walls by UV light has been measured in tissue-equivalent gas as a function of E/ p for working pressures of 81, 57, 30 and 20 torr.

The ability of a gas scintillation proportional counter to reject background events in a space environment

Results are presented on the ability of a xenon filled gas scintillation proportional counter (GSPC) to reject the charged particle background radiation found in a space environment. The optimisation of the detector to reject such radiation is discussed in detail. A rejection efficiency for 60Co induced events, used to simulate the space environment, of ∼97% was achieved, over the energy range 1–20 keV, for an X-ray acceptance of 82% at 6 keV. Such results compare favourably with conventional proportional counters which use xenon as the primary filling gas.

Some Experiments with a Radioisotope X-Ray Source for the Undergraduate Laboratory

Until recently x-ray experiments were done mainly in laboratories which had x-ray machines. With radioisotope x-ray sources now readily available, many experiments can now be performed in small universities with a minimum investment in equipment. In this paper such a source is used to excite fluorescent x rays in the 0–30-keV range in various metals. These x rays are studied with a conventional proportional counter and with one of the newly developed lithium-drifted silicon, Si(Li), surface barrier detectors. With this latter device resolutions as good as 450 eV are obtained for some of the x-ray measurements. The interaction of x rays with matter is studied by making half-value thickness measurements on aluminum, cobalt, and nickel, using Co57 and Zn65 sources.