Gas Proportional Counter Research Articles

High-precision measurement of oceanic 226Ra

A system capable of oceanic 226Ra measurements with a precision of ±1% is described, which represents an improvement of approximately a factor of three over existing techniques. 222Rn grown-in from 226Ra decay in 14-l seawater samples is quantitatively transferred to, and measured in, proportional gas counters. Errors other than counting statistics are estimated not to exceed ±0.5%, which is consistent with repeated 226Ra measurements on the same samples. A NE Atlantic 226Ra depth profile (2000–5000 m) is reported as an example. It is found that with the precision reported here, certain hitherto unresolved features of the 226Ra distribution in deep water become apparent.

A gas proportional counter system for the detection of 13N and other beta emitting gases

A simple gas proportional counter system is described that has been developed for measurements of 13N gases in the labeled effluent from gas chromatographs and in recirculating gas atmospheres from soil cores. The gas proportional counter is designed to detect the positrons from the decay while remaining fairly insensitive to the annihilation gamma-rays. A commercial high density amplifier-comparator was used in place of traditional NIM electronics. The general properties of the system are described and compared to those employing traditional low density NIM electronics. With this system a multiple detector array can be built at low cost and without concern for cross-talk.

Microdosimetry of 10-15 MeV bremsstrahlung x rays.

Experimental techniques have been developed for obtaining microdosimetric spectra on a hospital-based linear accelerator. Teletherapy beams of 10 and 15 MeV bremsstrahlung x rays from a Varian Clinac-18 and Clinac-20, respectively, have been produced at ultralow dose rates (50-200 microGy/h) which enables direct measurements of lineal energy distributions with a conventional Rossi-type gas proportional counter. Extensive measurements have been made to insure that the dosimetric properties of these low dose rate beams are nearly identical to those produced under high dose rate clinical conditions. Analytical procedures have been developed to correct measured lineal energy spectra for pileup caused by the low duty factor of the linear accelerator. The lineal energy spectra of these megavoltage beams differ significantly from Co-60, with dose averaged lineal energies (yD) being 20%-30% lower than for Co-60. Although such differences may not be important at clinical doses, the theory of dual radiation action does predict a lower biological effectiveness for these beams at very low dose levels.

A Silicon Photodiode Based Gas Proportional Scintillation Counter

A commercial available Schottky barrier silicon photodiode is coupled to a Gas Proportional Scintillation Counter through a quartz window and is used as an alpha particle room temperature detector. An Ar+N2(2%) gas mixture at 1470 Thrr is used. Resolutions of 3.2% are achieved. A discussion of the results obtained is presented and future improvements considered.

University of Saskatchewan Radiocarbon Dates X

This series reports some of the measurements made since publication of the last list (R, 1981, v 23, p 94–135). Also included are a number of earlier dates previously withheld pending submitter evaluation. For some the authors have been unable to obtain appropriate comment. References to other publication of these dates are given where known.Acetylene proportional gas counting methods essentially remain as described in Saskatchewan II (R, 1960, v 2, p 73). Bone dating is carried out on soluble collagen extract (Longin, 1971) since 1978.

Secondary light emission by noble gases

Drift velocities and excitation coefficients α, for electrons drifting in Ar, Kr and Xe, under the influence of uniform electric fields such that essentially no charge multiplication occurs, were obtained by calculating their distribution function through an analytical solution of the Boltzmann transport equation. A compilation of the previous experimental data, arising mainly from work related with the gas proportional scintillation counter, is made, and these data are compared with the calculated results.

Two-dimensional focal plane counter with induction readout

A two-dimensional position sensitive gas proportional counter was constructed as a focal plane counter for a spectrograph with large dispersion. Position information along the focal plane is obtained by the charge division method using induced signals on resistive electrodes placed near the anodes. The vertical incidence position of a particle is determined from the drift time measurement. The usual deterioration of position resolution for non-normal incidence of the lightly ionizing particles is minimized by using a multi-anode design. The “induction readout” makes it possible to realize fast rise time of position pulses as well as of timing signals for the drift time measurement even for a 1.5 m long counter of multi-anode design.

The decay scheme of 64Cu

The branching ratios in the decay scheme of 64Cu, which decays by the emission of positrons and electrons, and also by electron capture, have been determined by six distinct but partially correlated measurements. From this data, best estimates, including covariances, are obtained by a generalised least-squares criterion. The electron-to-positron ratio was determined by β spectrometry and separately by a 4πβ-γ coincidence-counting method using a gas proportional counter. The total β-ray-branching ratio was determined by a 4πβ-γ method using a liquid-scintillation technique. The positron branching ratio was estimated firstly by comparison with a Ge(Li) detector of the emission rates of annihilation γ-rays from sources of 64Cu and 22Na of known activity, and secondly by counting with calibrated NaI well crystal the total γ-rays from a copper foil of known activity which had been irradiated in a standard thermal-neutron field. In addition, the yield of the low-intensity 1.34 MeV γ-ray was determined with a calibrated Ge detector. Furthermore, an absolute counting technique using the liquid-scintillator system was devised to determine the disintegration rate of 64Cu, and from the β-ray-spectrometry, the electron and positron end-point energies were accurately measured.

Silicon avalanche radiation detectors: the basis for a new in vivo radiation detection probe.

Recent advances in semiconductor technology have made it possible to develop practical silicon avalanche radiation detectors. These detectors are analogous in operation to a gas proportional counter, but are capable of extreme miniaturization. Most importantly these devices have overcome the in vivo limitations of past semiconductor detectors with respect to noise, microphonics, and adaptability to relatively harsh environments. The operation and some useful characteristics of an avalanche detector are outlined. The performance of a probe mounted detector in an in vivo setting is described which illustrates one application of the silicon avalanche detector in this milieu.

The determination of technetium-99 in urine

A method has been developed for the routine radiochemical analysis of technetium-99 in human urine. The excreted technetium is coprecipitated from a 200 mL urine sample with CuS. Valence state adjustments are performed with K2S2O8. The precipitate is dissolved in ammoniacal peroxide and, after the Cu(II) is removed by ion exchange, the Tc-99 is extracted with 2-butanone (MEK). Subsequent evaporation of the organic solvent gives an essentially weightless source that if counted with a gas proportional counter will give a better limit of detection than liquid scintillation methods. The recovery of Tc-99 using this procedure is 80% (σ = 4%) and the limit of detection is 20 mBq (0.6 pCi) at the 95% confidence level for a routine sample – counting time 20 minutes, counting efficiency 24%, ambient background 1.3 counts per minute (cpm).

The half-lives of 26mAl, 34Cl, 38mK: Precision measurements with isotope-separated samples

The half-lives of 26mAl, 34Cl and 38mK have been measured with isotopically pure samples prepared with an on-line isotope separator. Emitted β + particles were observed with a large-solid-angle, high-efficiency, low-background gas proportional counter, from which signals were multiscaled. The counting electronics had a well-defined non-extendable dead time. Each sample decay curve was individually corrected for dead time before being summed with other accumulated data, no pile-up corrections being necessary. The analysis procedure was tested on hypothetical data. The effect of contaminant isobars on the resultant half-lives has been investigated. Our half-life results are 26m Al: 6346.2 ± 2.6 ms, 34Cl: 1527.7 ± 2.2 ms and 38mK: 924.2 ± 0.3 ms. The quote uncertainties include — for the first time — explicit provision for all possible contaminants.

Electron-energy absorption used for the measurement of superficial densities of air-pollution samples on thin membrane filters

A sampling and analysis method to measure dust concentrations in air is described. The dust was deposited on membrane filters, the superficial density of which was measured by an electron-energy-absorption method. Internal-conversion electrons from the 109Cd decay were used as the electron source. The electron detector was a gas-proportional counter operated at atmospheric pressure with a 90% Ar + 10% CH 4 gas flow. Detected superficial-density differences ranged from 0 to 400 μg/cm 2 with an estimated accuracy of 20 μg/cm 2. The measurement process was automated using a computer-controlled sample changer. About 8,000 samples were taken and analysed during 18 months. In addition to the total amount of dust the blackening index of the dust stains was also measured by an optical reflection method.

A dating method with 39Ar

The principles of a dating method based on the cosmic-ray-produced radioisotope 39Ar are given. Technical requirements such as background and standard gas samples and gas proportional counting systems are described. With samples extracted from Greenland ice it can be demonstrated that 39Ar ages agree with ages obtained by other methods. First results on ocean water samples show that with this isotope valuable information on ocean mixing and circulation can be expected. 39Ar results on groundwater samples disagree for some aquifers with conventional 14C ages; possible explanations are discussed, especially underground production of 39Ar.

Replacement of monochromator and proportional gas counter by mercuric iodide detector in X-ray powder diffraction

Low-resolution and therefore low-cost mercuric iodide detectors have successfully been applied to replace the combination of a graphite monochromator and a proportional gas radiation counter used in X-ray diffractometers. The mercuric iodide detector requires a lower DC bias of only 200 V rather than the 1500 V bias needed for the proportional gas counter. The much better stopping power of HgI2 allows higher counting efficiency and therefore a better signal-to-noise ratio. Results are shown for X-ray powder diffractions of polycrystalline cubic silicon and tetragonal HgI2.

Unidimensional Monte Carlo Simulation of the Secondary Scintillation of Xenon

An unidimensional Monte Carlo method is used to calculate the secondary scintillation (electroluminescence) intensity in xenon gas proportional scintillation counters. Other transport parameters like electron drift time, average number of collisions, efficiency for light production and induced charge pulse amplitude are also calculated. The values obtained agree well with the experimental data available.

Radiocarbon Dating Archaeologic and Environmental Samples Containing 10 to 120 Milligrams of Carbon

A conventional 14C system with either a 15cm3 or a 100cm3 methane gas proportional counting tube, each constructed from high purity copper, has been used at the Washington State University 14C laboratory for three years (Sheppard, Hopper, Westberg, 1981). The electronic components of this system included highly stable (John Fluke) power supplies, modified pre-amplifiers (Canberra), and NIM amplifiers, single channel analyzers, scalers, timers, etc, (ORTEC). Modules were selected for a highly stable low-noise system. The system was designed to minimize problems generated by ground-loops, electromagnetic noise pick-up, and line noise. It operates in a copper-lined basement room of a four-story brick and concrete building along with a second and older gas proportional system which has three 500cm3 copper counting tubes operated at 2 or 3 atmospheres of methane. Analysis of the older system's background data indicates that the background counting rate of 0.8 counts per minute is virtually independent of atmospheric pressure. It has not been possible to determine the pressure dependence of the small counting tubes.

Suppression of X-Ray Fluorescence Background in X-Ray Powder Diffraction by a Mercuric Iodide Spectrometer

AbstractWe have explored the merits of using a Hgl2 spectrometer as a detector in x-ray diffraction systems instead of a proportional gas counter, or a scintillation counter. The full width at half maximum energy resolution of the HgI2 spectrometer used was about 1.1 keV for the CuKα line (8.1 keV), and about 1.5 keV for the MoKα line (17.4 keV), The energy resolution was utilised to eliminate x-ray fluorescence background from powder diffraction spectra. We demonstrate the suppression of Fe x-ray fluorescence in diffraction patterns of ErFe03 obtained with a Cu x-ray tube, and of Y x-ray fluorescence in diffraction patterns of Y2O3 obtained with a Mo x-ray tube. The peak height to background ratios were improved by about an order of magnitude in both cases.

International Comparison of Proportional Gas Counters for 14C Activity Measurements

In early 1982 we attempted to compile information on 14C counting equipment from all laboratories listed in RADIOCARBON. A gratifying number of laboratories cooperated in communicating their data: more than 65% of those in which proportional counters are in operation. Our compilation covers more than 80% of all gas counters used for 14C dating. Unfortunately, the number of parameters determining counter quality as well as the spread in these factors is so large that their separate influence cannot be established beyond statistical doubt. Nevertheless qualitative and semi-quantitative trends are clearly observed. Laboratories are not identified in the data but the calculations and standardization procedures allow the laboratories to identify their own counter data in the graphs. Each plot does not contain a complete compilation of all counters. This is either because of lack of or internally inconsistent information.

Miniature Signals and Miniature Counters: Accuracy Assurance Via Microprocessors and Multiparameter Control Techniques

When14C signals approach background levels, the validity of assumptions concerning Poisson counting statistics and measurement system stability becomes crucial in interpreting the resultant low-level counting observations. This has been demonstrated in our previous work on detection limits for non-Poisson error and it is critical in our current studies of carbonaceous pollutants, where the14C signal from just 5 mg C is comparable to that of the background for our miniature gas proportional counters. To assure data quality, our multi-detector system is designed for the on-line monitoring of critical parameters that reflect both the (statistical) nature of the non-Poisson errors and the underlying (physical) causes. It sends >60 bits of information/pulse to a microprocessor which automatically generates, for each counting period, two-dimensional spectra and multiparameter correlation and control charts. To evaluate the validity of long-term counting of 1–10 mg C we use robust (statistical) estimators, optimal counting interval subdivision, and time series analysis of the individual pulses. New opportunities for selective sampling and chemical fractionation which come with the small sample measurement capability have led us to give special attention also to higher control levels, involving e g, isotonic heterogeneity and representative standard materials.

Radio gas chromatography on capillary columns using a multi-column system (column-switching)

A gas chromatographic system with capillary columns (fused silica) for the analysis of radiolabelled compounds is described. The system presented is based on a dual column gas chromatograph equipped with column switching facllity and a variable splitter at the column outlet combined with a dead-volume free adapter for the radioactivity monitor for continous measurement of radioactivity in the column effluent. The first column works as a separation column and the second is roughly shortened and used as a feed to the mass detector. The adjustment of the split ratio is regulated by the inlet pressures for the carrier gas supplying both columns. For mass detection all conventional systems can be used. Detection of radioactivity by a gas proportional counter (system based on a combustion technique). Three flow modes can be adjusted: a) total column effluent to the mass detector or b) to the radioactivity monitor, and c) simultaneous flow (dependent on the chosen split ratio) to mass-and radioactivity detectors. The system was developed for use in clinical chemistry and tested with labelled and unlabelled steroids. The method for peak identification by means of relative retention times and methylene units was possible also for radioactive peaks when a heart cutting technique was used. The radio gas chromatographic system presented allows the development of radiochromatograms with the same peak characteristics as in conventional capillary gas chromatography.