Count Rate Variations Research Articles

Evaluation of the Reproducibility in Radioactivity Measurements with Liquid Scintillation Counters

Experimental studies were carried out to evaluate the reproducibility in 3H measurements with presently available liquid scintillation counters. A set of 3H samples which have activities ranging from 10(4) to 10(7) dpm were prepared and measured repeatedly in every possible different order with six counters of different models. The results demonstrate that the measured counting rates of the samples are subjected to much greater variations than expected from counting statistics. The poor reproducibility was found to be attributed to rate dependent photomultiplier gain variations. The variations in counting rate as large as 10 per cent was observed in a particular counter. It is also found that rate dependent photomultiplier gain variations can cause the external channel ratio to shift, giving rise to error as large as 9% in the efficiency determination.

Rudjer Bošković Institute Radiocarbon Measurements V

The following list contains dates of samples measured since our previous list (R, 1977, v 19, p 465-475). As before, age calculations are based on the Libby half-life, 5570 ± 30 years, and reported in years before 1950. The modern standard is 0.95 of the activity of NBS oxalic acid. Solid sample pretreatment, combustion and counting technique are essentially the same as described in R, 1971, v 13, p 135-140. Groundwater samples were prepared following the procedure adopted by the IAEA (IAEA, 1977). Carbonates and hydrocarbonates from water samples were precipitated by adding saturated barium chloride solution while the alkalinity was adjusted to pH = 8 by adding carbonate-free saturated solution of sodium hydroxide. The precipitation was enhanced by adding iron salts and Praestol as coagulating medium. Carbon dioxide was evolved by acidification of the precipitate and converted to methane. Statistical processing of data has been computerized (Obelić and Planinić, 1975). Sample descriptions were prepared with collectors and submitters. The errors quoted correspond to 1σ variation of sample net counting rate and do not include the uncertainty in 14C half-life.

The effect of absorber position on beta particle attenuation

A model is described which accounts for the variation in count rate as a thick absorber is moved between a beta particle source and a detector. The existence of an absorber position which yields a maximum count rate, a feature of considerable importance in thickness gauging, appears in results obtained using the model.

Resonant scattering of an intense beam of radiation by a scatterer of arbitrary size

Scattering of an intense beam of radiation by an assembly ofN atoms having a close transition frequency contained in a volume of arbitrary size is studied in the rotating-wave approximation exactly to all orders in the incident intensity. The atoms are taken in i) ground state, ii) excited state or iii) thermal equilibrium. It is seen that the scattered radiation is an equally spaced triplet, the middle frequency of which coincides with the incident frequency and the separation of which depends on the incident intensity. Differential counting rates for photons of the three frequencies are evaluated, and they are expressed as a sum ofN times the corresponding differential counting rates for a single atom plus the terms which depend on the shape of the scattering sample and are proportional toN(N−1). ForN≫1, thus only the latter term matters. Explicit results for a cylindrical scatterer are obtained and the angular variation of the differential counting rate is studied for some special cases.

The variations in the background counting rate of a sensitive whole-body radioactivity monitor based on plastic scintillators

The influence of the concentration of 214Bi in air on the background counting rate of a whole-body counter is studied. Small drifts in background counting rate may be identified on the basis of up-to-date control charts of background rate maintained routinely in the laboratory.

Influence of electrostatic precipitators on the background counting rate of a sensitive whole-body radioactivity monitor

Variability of the background counting rate of sensitive gamma-ray counters due to changes in the concentrations of the decay products of 222Rn in the shielded space may be reduced by filtering, from the air supply, all decay products by the help of electrostatic precipitators and by maintaining high flow rates of air to prevent build-up of decay products in the shielded space. This paper deals with the influence of a commercially available electrostatic precipitator (Honeywell type H-37) on the background counting rate of a multicrystal whole-body radioactivity monitor. Short-term variation in background counting rate is not likely to affect accuracy of measurements of body burden. But when the counter is used to determine extremely low radioactivity in materials the counting times are large and the changes in background counting rates are significant. Honeywell type H-37 precipitator helps to reduce the concentrations of the decay products of 222Rn. But a part of the natural radioactive aerosols penetrates the precipitator leading to change of background counting rate. Installation of a precipitator alone is not satisfactory.

Monovacancy formation energy in copper, silver, and gold by positron annihilation

Monovacancy formation energies in copper, silver, and gold have been deduced from the temperature variation of the peak counting rate in the angular correlation curve of positron annihilation radiation from these metals. The counting rate was temperature dependent over the entire temperature range, including temperatures so low that no trapping of positrons at vacancies is effective. At these temperatures the increase in counting rate results from thermal expansion of the lattice. By separating this thermal expansion effect from the vacancy trapping effect at higher temperatures, we obtained values for the monovacancy formation energyE 1v for copper, silver, and gold to 1.29±0.02 eV, 1.16±0.02 eV, and 0.97±0.01 eV, respectively.

Observation of a cosmic gamma-ray burst on Apollo 16. II - X-ray time profile and source location

A burst of X-rays was detected during the trans-earth coast phase of Apollo 16 on Apr. 27, 1972 at 10:68 UT, simultaneously with the observation of a transient event by a gamma-ray spectrometer aboard the same spacecraft. The two instruments provide a broad energy range of more than three orders of magnitude for describing the spectral distribution of this event. The conclusion that the incident flux was X-rays and not charged particles is based on the fact that the particle flux detectors in the Apollo gamma ray spectrometer and on the Vela 6A, which also observed the event, did not respond. The time variation of the total count rate in the X-ray range before and after corrections for detector geometry and the analysis for source direction is presented.

Automatic corrections for deadtime effects and lineshape changes due to source decay in positron-annihilation experiments

Correction systems are described for minimizing the changes in deadtime losses and in lineshape produced by the variation in count rate from intense, short-halflife sources. Application to the counting of 64Cu annihilation radiation in positron experiments employing angular-correlation and Doppler techniques is described. Both techniques have been improved by decreasing, by an order of magnitude, their sensitivities to change of counting rate.

A gamma transmission method for the determination of moisture content in soils

Soil samples from strata down to 164 m were studied with respect to the linear attenuation coefficient (μ) and the mass absorption coefficient (μ/ϱ) for gamma rays and their variation with moisture content. Details are given for three samples examined. The variation of gamma counting rate with moisture for different thicknesses of strata is also given. The reaction of clay samples was determined by the gamma transmission method and the explanation of such reaction is given. It is suggested that stratification can also be determined by this method with the aid of the gamma-ray linear attenuation coefficient, which will also provide a method for the determination of the density of the soils in the various strata. The method is suggested also for the study of seasonal variation of moisture content.

Observations on ratemeter controlled colour dot scan displays.

Abstract A colour scan is simply a recording of the ratemeter reading, each colour representing a small range of counting rate. The influence of the statistical significance of these colour ranges on the visual extraction of information from the scan is investigated. When each colour range extends over two to three standard deviations of a single reading of counting rate within the range, display is considered optimum in the sense that small variations in counting rate are most easily perceived. The standard deviation of an estimate of counting rate obtained by averaging readings over a small area of scan is approximated using a standard formula. This leads to a more accurate knowledge of the changes in counting rate which can be demonstrated and to a criterion for selection of the ratemeter time constant. Thus maximum assistance can be given to the observer enabling rapid and accurate interpretation of the scan.

Detection of hard β-emitting radionuclides in aqueous solutions using Cerenkov radiation: A review article

Cerenkov counting of solutions is advocated as a useful means of detecting hard β-emitters; the present work is dedicated to the practical aspects of Cerenkov detection with commercially available liquid scintillation spectrometers. The variations of count rates with the type of vials and the volume of solutions are pointed out. With a spectrometer equipped with S 11 phototubes, detection efficiencies from 16 calibrated sources are found to be 23–50 per cent for emitters of E β maximum in the range 1·3−2·3 MeV and 50–70 per cent up to 3·5 MeV. The results of experiments with various optical filters suggest a direct relationship between the β energy of the isotope and the fraction of Cerenkov light emitted in the ultra-violet region. Pulse height analysis either by calculation of the channel ratio obtained from the spectrometer or by direct oscilloscopic display of spectra enables the energy of any unknown β-emitter to be evaluated. Color quenching of biological solutions inhibits more widespread use of Cerenkov counting while solute quenching is shown not to be a problem. Changes induced by 31 wavelength shifters are assessed; we single out Esculin as one of the most effective. Stress is being put on the advantages of Cerenkov counting of 42K, 32P, 86Rb and 24Na and the applicability of the method to more than 40 β-emitters having maximum beta energies above 1 MeV.

University of Kiel Radiocarbon Measurements VII

This list contains data obtained during 1971. Unless otherwise stated, all organic samples or organic fractions are carefully washed with dil. HCl and dil. NaOH to remove all carbonates. Age calculations are based on 95% of NBS oxalic acid standard activity with modern value A.D. 1950. Results are calculated using the Libby half-life and are given in the B.P. scale. Also ages of shells are calculated with the NBS oxalic acid standard. Ages are not corrected for δC13 variations. Errors correspond to Iσ variation of sample net counting rate including statistics of modern standard and background. Sample activities, if given in per cent refer to 0.95 NBS oxalic acid standard activity.

Superconducting Thin-Film Detector of Nuclear Particles

Superconducting films of tin and indium on supporting substrates have functioned successfully as detectors of individual nuclear particles. The films are sufficiently thin and narrow that individual α-particle impacts initiate superconducting to normal transitions which spread, in the presence of a transport current, to span a full film cross section. The transitions are observed by means of the ir drop produced by a transport current. For low current densities self-terminating voltage pulses of a few nsec duration are observed. At higher current densities the boundaries of a normal region initiated by an α particle propagate by Joule heating to the ends of the film. The range of the 5.3-MeV α particles utilized for these experiments greatly exceeds the 0.1 μm thickness of the films and the resulting deposition of energy in the substrate affects the response of any film which is in direct contact with its substrate. The introduction of a thin thermally insulating layer of varnish between the film and its substrate, which in these experiments was either glass or crystalline quartz, increases the thermal decay constant to the extent that the film is thermally isolated from its substrate for a period comparable to that of pulse formation. The variation of count rate with film current has been studied and is shown to be consistent with the variation of critical current density along the length of the film. This model, coupled with classical heat diffusion within the film, accounts for the observed behavior of the thermally isolated films.

University of Kiel Radiocarbon Measurements VI

This list contains data obtained during 1970. Age calculations are based on 95% of NBS oxalic acid standard activity with modern value A.D. 1950. Results are calculated using the Libby half-life and are given in the B.P. scale. They are not corrected for δC13 variations. Errors correspond to 1σ variation of sample net counting rate including statistics of modern standard and background.

Investigation of Distortions in Gamma Camera Scintigraphic Pictures

SummaryThe aim of the work was to investigate the magnitude of distortions observed in a scintillation camera display. A Nuclear-Chicago Pho/Camera II combined with 1600-channel analyzer was used for these studies. The measurements were carried out with point, line and volume source of 99mTc. Significant variations of count rates registered, depending on the method of registration of radioactivity were observed across the face of the crystal.

A procedure for the Cherenkov counting of high energy ? and ? emitting radionuclides in coloured plant extracts

Elimination of count rate variations caused by the absorption of Cherenkov radiation by pigments in coloured solutions was achieved by placing these solutions in opaque walled cylinders immersed in colourless liquids contained in standard counting vials. Good counting efficiencies for a range of β and γ emitting radionuclides were obtained by selecting colourless liquids with high refractive index.

University of Kiel Radiocarbon Measurements IV

The following dates are samples measured since publication of Kiel III (Radiocarbon, v. 10, p. 328–332). Age calculations are based on 95% of the activity of NBS oxalic-acid standard as modern value of A.D. 1950. Results are calculated with Libby half-life and reported in yr before 1950. Error corresponds to 1σ variation of sample net counting rate as well as modern standard and background, but does not include the uncertainty in C14 half-life and in secular C14 variations. Dates are not corrected for isotopic fractionation.

Calibration of an excited nuclear state g-factor in terms of the proton g-factor

An apparatus is described which measures differentially in time the Larmor frequencies of both protons and excited nuclei in the same magnetic field with the same time to pulse height converter by transforming oscillations of the proton precession into a periodic variation of a pulse counting rate. Applied to the 198 keV 5/2 + state of 19F this method gave an accuracy of 0.2% for the ratio of the Larmor frequencies. The value for the 19F ∗ g-factor is g = 1.442 ±0.003.

Etude de la dispersion reelle des mesures d'un detecteur de neutrons type A.I.S.C. par comparaison entre les trois sections

The actual accuracy of the NM 64 neutron monitor, taking into account the statistical errors, the neutron multiplicity inside the detector and the instrumental errors, is studied from the Pic du Midi data. The detector is divided in three similar independent sections of three counters. The hourly counting rate is near to 2.5 × 10 6, the scaling factor beeing equal to 1000; this number is taken as variation unit. The level of significance of the variations of the total counting rate is estimated by the correlation between the algebraic increases observed in each section, for all cases in which the total increase is constant in absolute value. The result of the application of this method to the September-October 1965 Pic du Midi hourly data is given. It is shown that variations exceeding 4 or 5 units have surely a physical meaning. Then the theoretical value of the correlation is calculated; we found that the actual standard deviation of the variations is equal to the increase of the total counting rate which gives no correlation between the sections. It has been found 1.6 times greater than the statistical error. A method giving the standard deviation of the values of each section is given. The numerical results are quite similar to these of the preceding analysis. It is found that the amplitude distribution of the error is close to a Gauss distribution.