Observed Count Rate Research Articles

Local realist model for the coincidence rates in atomic-cascade experiments

We assume that the two photons emerging from an atomic cascade process are classical wave-packets with well defined intensities and states of elliptical polarization. Such weak light signals, when interacting with optical devices such as linear polarizers and quarter-wave plates, are assumed to undergo stochastic changes of intensity in addition to the change in polarization experienced by macroscopic signals. We show how such assumptions can lead, in a natural way, to an enhancement in the detection probability of certain signals, and hence to a family of models which fit the observed counting rates in all atomic-cascade experiments so far performed, including those which investigate the circular polarizations of the photons.

The effect of sample composition and vial type on Ĉerenkov counting in a liquid scintillation counter

The effect of sample vial type and sample composition on the Ĉerenkov count rate detected from 32P and 36Cl was studied using a liquid scintillation counter. When counting was done in the noncoincident mode, glass vials allowed higher counting efficiency than plastic vials. In the coincident mode light scattering caused by polyethylene and polyproplyene vials allowed higher counting efficiency than glass vials. Highest coincident counting efficiency was from plastic minivials in a glass carrier vial. Increased solute concentration in samples caused increased counting efficiency due to changes in the refractive index of the solution. This can cause significant counting efficiency changes with no sample channel ratio change in density gradient fractions. The use of wavelength shifters is shown to be inappropriate when the sample pH varies, as this can change the fluorescent properties of the shifters and thereby the observed count rate.

A Kalman filtering approach to natural gamma ray spectroscopy in well logging

This paper describes an application of (adaptive) Kalman filtering to a geophysical subsurface estimation problem. The NGT® is a sonde designed to detect the natural gamma rays of various energies emitted from a formation by the radioactive nuclei of potassium (K) and the thorium (Th) and uranium-radium (U) series. Using a minicomputer at the surface, the (Th,U,K) concentrations along the borehole have to be estimated on-line from the detection of the gamma rays in five energy windows. The standard technique in the logging industry has been to compute the elemental concentrations at a given depth using only the observed counting rates at the same depth. The resulting estimates have fairly large statistical errors which have limited the application of the NGT in computer reservoir evaluation. In this paper, it is shown that a Kalman filter based on a dynamical model of the (Th,U,K) vertical variations can produce real-time estimates that are readily usable on a quantitative basis. The paper focuses on the usual critical issues in applying Kalman filtering to real data, namely modeling, adaptivity, and computational aspects.

Non‐E × B ordered ion beams upstream of the Earth's bow shock

The unexpected appearance of spin modulations in our fixed voltage electrostatic analyzer detectors on ISEE 1 and 2 has given us the opportunity to investigate the plasma properties of the upstream ions in high time resolution. The detectors are narrowly collimated and point southward, antiparallel to the satellite spin axis covering a region in phase space not viewed by the other plasma experiments on the satellites. The count rate modulations (up to three orders of magnitude) are due to the convolution of the anisotropic detector response function with the local plasma distribution function and reflect the existence of strong velocity gradients in the local distribution function. By assuming a form for this distribution function, we can simulate the output of any of our detectors. Comparing the simulated and observed count rates, we can then estimate several of the local plasma parameters. This of course is dependent upon the distribution function chosen. Using an isotropic ‘flowing’ Maxwellian distribution (which should be thought of as a ‘local’ approximation to the actual distribution near the region in phase space occupied by our detector response functions), we have obtained estimates of the local plasma temperature and three‐dimensional flow velocity of the upstream ion population we observe. We find that in almost all of our observations of upstream particles there exist beam‐like ions with temperatures in the range 5–80 eV. They are extremely dynamic in velocity with changes of 200–300 km/s occurring in 3–6 s being common. Most significantly, their ‘flow’ velocities cannot be ordered by E × B. This last point separates these observations from the previously reported reflected and diffuse populations of upstream ions. Mechanisms that can gyrophase bunch the ions reflected at the bow shock are discussed as a method of explaining the data.

Microprocessor crosstalk correction in multiple-detector gamma counters

A method for compensating for crosstalk between nuclear counters and its application to a multidetector gamma counter is described. The standardisation procedure employed measures the relative efficiency of each detector for a sample within that detector and for the same detector when the sample is located in each of the other detectors. Results with a 12-element detector system show that the stored calibration data obtained in this way can be applied successfully to correct the observed count rates for crosstalk as well as detector sensitivity differences when using gamma emitters having energies up to 1.3 MeV.

Correction for deadtime losses in a gamma camera/data analysis system.

In quantitative analyses of dynamic radionuclide studies, using large activities of 99mTc flowing through organs it is necessary to determine accurately count-rate losses and to provide appropriate corrections. A method is described for estimating deadtimes and applying corrections to the observed counting rates using a two component model where the gamma camera is considered to have a purely paralyzable deadtime and the data analysis system a purely non-paralyzable deadtime.

New Anger scintillation cameras with improved count rate capability.

This paper proposes scintillation cameras which can handle input pulse rates beyond 1 Mcps with negligible as spatial resolution and energy resolution at low pulse rates. The cameras are based on the "variable sampling-time technique", in which phototube current pulses are first shortened in width and then selectively integrated by gated integrators followed by sample-and-hold circuits. The period of integration before sampling is not fixed but shortened by the arrival of the following pulse to keep a high pulse-pair time resolution. An approximate expression for the count rate characteristics has been derived, and its validity was confirmed by Monte Carlo simulation. The results indicates that the maximum observed count rate will reach 420-520 kcps with an energy window fraction of 0.5 for typical design parameters.

A limit to the X-ray luminosity of nearby normal galaxies

The hypothesis that normal galaxies are on the average more luminous in the X-ray region than the Milky Way galaxy or M31 and therefore are possible candidates for the low-luminosity sources of the 2 to 60 keV extragalactic diffuse background is tested. Data from the A-2 detectors on the HEAO-1 spacecraft were examined for emission from positions coincident with 76 selected normal galaxies, and upper limits to the average galactic luminosity for various luminosity distributions resulting in the observed count rate distribution were determined. For uniform and exponential galactic luminosity distributions, limits of 2.7 x 10 to the 38th erg/sec and 3.4 x 10 to the 38th erg/sec, respectively, at the 90% confidence level were obtained. It is shown that the Hubble-constant-independent upper limit to galactic emissivity is less than 1% of the diffuse background emissivity, indicating that normal galaxies are not responsible for the diffuse X-ray background and have luminosities comparable to that of the Galaxy.

The MIT/OSO 7 catalog of X-ray sources - Intensities, spectra, and long-term variability

This paper is a summary of the observations of the cosmic X-ray sky performed by the MIT 1-40-keV X-ray detectors on OSO 7 between October 1971 and May 1973. Specifically, mean intensities or upper limits of all third Uhuru or OSO 7 cataloged sources (185 sources) in the 3-10-keV range are computed. For those sources for which a statistically significant (greater than 20) intensity was found in the 3-10-keV band (138 sources), further intensity determinations were made in the 1-15-keV, 1-6-keV, and 15-40-keV energy bands. Graphs and other simple techniques are provided to aid the user in converting the observed counting rates to convenient units and in determining spectral parameters. Long-term light curves (counting rates in one or more energy bands as a function of time) are plotted for 86 of the brighter sources.

Fuel content characterization and pressure retention measurements of DT-filled laser fusion microballoon targets

We have developed a nondestructive assay of the fule content of deuterium-tritium (DT) -filled microballoon laser fusion targets, which is based on β-particle counting rates. Using a model employing transmission measurements of kilovolt electrons through thin films, observed count rates are correlated with the amount of tritium in the glass walls and hollow interior of the microballoons. It has been shown that gas pressure in balloons can be calculated from tritium content and a knowledge of the initial gas composition since D and T were found to leak at the same rate. This assay technique is primarily applicable for balloons with glass wall thicknesses <1.5 μm where the number of escaping β particles is large compared with the number of x-ray photons generated in the glass. The technique has been applied to measure the pressure retention characteristics of individual targets. At room temperature, the balloons exhibited widely diverse and rapid leakage rates which could not be correlated with a model based on molecular diffusion and the assumption that all balloons had a homogeneous composition. Cryogenic storage greatly reduced the leakage rates with pressure retention half-lives ranging from 5 to approximately 12 years. Finally, it was determined that tritium in the glass wall is trapped and evidence is presented to support the hypothesis that it is uniformly distributed across the shell.

Low-Speed Calibration of the Bendix Q15R Current Meter

Recent calibrations of the Bendix Q15R ducted impeller current meter show a strong nonlinear relationship between the observed count rate conversion factor and the true water speed in the range 0.0-0.1 m/s. An empirical response curve is fitted (using least squares) to the data and the fitted coefficients are presented for all current meters calibrated by Argonne National Laboratory. Separate calibrations of every current meter are recommended wherever possible. If this is not possible, use of the composite fitted curve is recommended in preference to the manufacturer's conversion factor.

An efficient spectroscopic detection system for cathodoluminescence mode scanning electron microscopy (SEM)

SUMMARYA detection system for analytical cathodoluminescence (CL) mode scanning electron microscopy (SEM) is described. This incorporates a cold stage, an efficient light collector, a monochromator and a photomultiplier and used the photon counting technique. The efficiency of the component subsystems was optimized and calibrated, and the performance of alternative light collecting and monochromating equipment is compared. The operation of the photon counter is discussed. The digital output of the photon counter was fed into a multichannel scaler and thence to a computer. This was used to correct the observed count rate with the calibrated spectral variations in the performance of the detection system.Spectra obtained at both room temperature and liquid nitrogen temperacure and monochromatic and panchromatic SEM micrographs are given as examples illustrating the value of this technique.The factors governing the performance of the system are discussed. The forms of noise in the signal and in the detection system are described and the means for minimizing, avoiding or correcting for them are dealt with. Sources of spurious signals in the SEM are treated.

Energetic electrons in the Jovian magnetosphere

Results are reported for a detailed analysis of Pioneer 10 data on energetic particle species in the magnetodisk region of Jupiter's magnetosphere. It is shown that the observed counting rates in the magnetodisk (beyond 20 Jupiter radii) were caused primarily by electrons with energies exceeding 0.06 MeV. Absolute omnidirectional electron intensities in the magnetodisk are presented for five integral energy ranges, and a model electron differential energy spectrum is found to fit the intensities throughout most of the encounter trajectory. It is suggested that the observed spectral shape results from losses of high-energy electrons by pitch-angle scattering. Observed equatorial energy spectra are used to compute distribution functions for several values of the first adiabatic invariant, mu. The radial profiles of the functions are found to have maxima at about 50 Jupiter radii inbound as well as at about 90 radii outbound and to diminish strongly for lesser radii. The large decreases in density are shown to require strong losses, and resonant electron whistler-mode pitch-angle scattering is suggested as a loss mechanism.

Echo I: An experimental analysis of local effects and conjugate return echoes from an electron beam injected into the magnetosphere by a sounding rocket

On 13 August 1970, a sounding rocket carrying a high voltage electron accelerator and several electron detectors was launched from Wallops Island, Virginia. 16 msec long, 70 mA pulses were injected into the magnetosphere at pitch angles near 90°. In each pulse the electron energy was modulated between 35 and 43 keV. The electrons were trapped in the Earth's magnetic field and bounced between the northern and southern conjugate points with a period of ∼0.65 sec and drifted eastward with a gradient-curvature drift velocity of ∼765 m/sec. For about 90 seconds the rocket intercepted the returning echoes. Careful study of the rocket trajectory has allowed a partial space-time picture of an echo to be constructed. The bounce time and drift velocity observations are consistent with predictions based on internal magnetic field models with no electric fields. The flux has the spatial variations predicted by atmospheric scattering models at the southern conjugate point but is about a factor of 10 too low. After some injections delayed echoes are observed, apparently 40 keV electrons whose bounce time has been increased by ∼75 msec, but with no change in their bounce averaged drift velocity. Study of detector response during gun pulses revealed three unexplained features: (1) a field aligned upward moving flux after downward injections; (2) a downward moving, time dependent, flux after injections at some upward pitch angles; (3) a lack of altitude (or atmospheric density) dependence on observed count rates.

The accurate dead time measurement of single channel gamma ray counters by means of the proportional source method

A method is presented to fit a dead time model to observed count rates. The elimination of errors in the proportional source method, the choice of a correct dead time model based on two dead times θ, τ, the mathematical methods involved in the estimation of the dead times and the stability of dead time as a function of time are examined. It is demonstrated that in this way the aim of the present investigation of measuring true count rates up to 1·5 × 10 5 s −1 with an overall uncertainty of the order of 0·1 per cent is met.

Cosmic Ray Stömer Cut-offs on the Surface of Jupiter

It has recently been reported (Cini-Castagnoli et al. 1973; Jacklyn and Cini-Castagnoli, 1974) that Jupiter may be responsible for a modulation component in the counting rate of underground cosmic ray detectors on the earth. The evidence suggests that a screening mechanism is operating, in as much as the observed counting rates are diminished when Jupiter is within the field of view of these detectors. The magnitude of the effect is larger than can be accounted for by simple line-of-sight obscuration by the planet itself, and it has been suggested that the intense and extensive Jovian magnetic field is producing the effect.

Deadtime of scintillation camera systems--definitions, measurement and applications.

Fast quantitative dynamic studies with scintillation camera systems require deadtime correction. Although such systems are semiparalyzable, they are most conveniently treated either as paralyzable or nonparalyzable. Paralyzing deadtime (τ) is that deadtime during which a system is unable to provide a second output pulse unless there is a time interval of at least τ between two successive events. Paralyzable systems are characterized by Poisson statistics, so that the “true” counting rate , where R is the observed counting rate. Nonparalyzing deadtime (T) is that deadtime during which a system is insensitive after each observed event. The period of insensitivity is not affected by any additional “true” events before full recovery occurs. For nonparalyzable systems the corrected counting rate . A two‐source method protocol is presented for deadtime measurement. The paralyzing deadtime is calculated by a 5‐dimension Newton–Raphson iteration. The nonparalyzing deadtime is calculated by a quadratic equation. Approximation equations are also presented not requiring a computer. Deadtimes are fitted to polynomial equations as dependent variables of measured counting rate. Algorithms incorporating the polynomials are presented for the deadtime correction of histogram curves. Using either the paralyzing or the nonparalyzing approach, precise deadtime corrections are demonstrated.

The reduction of coincidence loss in radionuclide imaging cameras through the use of composite filters

The use of a composite filter makes it possible, by selectively attenuating the input gamma -ray intensity distribution, to reduce coincidence loss and thereby increase the observed count rate of an Anger type camera. A theoretical analysis is presented to assist in estimating the crossover count rate which is that count rate above which the composite filter will be useful. A vial of 99Tcm within a scattering medium was used to obtain experimental data. Data are presented for a two layer filter consisting of a 0.23 mm thickness of tin and 0.13 mm of copper. This filter attenuates the primary 140 keV gamma -ray by about 12%.

Statistical Errors in Subcritical Reactivity Inferred from Inverse Kinetics Rod Drop Measurements Using the Three-Point Method

A study was made to determine the uncertainty in subcritical reactivity as inferred from inverse kinetics rod drop experiments (using the three-point method) due to the statistical uncertainty inherent in the observed count rate of the neutron sensor. The two methods employed were a classical propagation of error analysis, and an analysis of simulated repeated rod drops, with an assumption that the uncertainty in reactivity was due to the detection process itself for both techniques, To test the analysis methods, the reactivity uncertainties for various experimental rod drop data sets were computed by both methods. There was excellent agreement of the results. The propagation of error analysis may be used on three-point subcriticality measurements to provide an experimenter with an index to the statistical reliability of the inferred reactivity estimate.

Low energy atmospheric gamma rays near geomagnetic equator

Based on the results from three balloon flights, made at Hyderabad(7.6°N geomagnetic latitude) using omnidirectional gamma ray spectrometers, the different aspects of the low energy atmospheric gamma rays at equatorial latitudes in the energy interval 100 keV to 1 MeV are investigated and detailed discussion is presented. The energy loss spectrum in this energy range is found to consist of a continuum superimposed on which is a photopeak due to 0.51 MeV line arising from electron positron annihilation. The continuous background spectrum is similar to that observed at mid and high latitudes. The intensity of 0.51 MeV line is estimated to be 0.079 ± 0.01 photons cm −2 sec −1 at 6 g cm −2 over Hyderabad and the altitude dependence of its intensity is established for this low latitude station. The latitude effect of the intensity of this line at 6 g cm −2 is derived for the first time by comparing the results of the present measurements with those available for mid and high latitudes. The contribution of the cosmic gamma rays to the observed count rates at 6 g cm −2 is shown to be negligible in the case of the omnidirectional spectrometers of the type used in the present observations even for low latitude stations.