Change In Count Rate Research Articles

Positron imaging of cerebral blood flow during continuous inhalation of C15O2.

This investigation tests the hypothesis that the normal cerebral image obtained non-invasively during continuous inhalation of C15O2 is related to cerebral blood flow. Trace amounts of CO2 labeled with the positron-emitting radionuclide 15O were administered to 4 normal subjects at normo- and hypocapnia and to 2 of these subjects at hypercapnia. Hypocapnia typically caused a marked decrease in cerebral 15O activity, and hypercapnia a small increase in activity. The relative difference in the change in count rate in response to hypo- and hypercapnia is what one would expect if the activity represented bloow flow, according to a mathematical model which assumes the 15O label enters the brain as water of perfusion. The findings in this study suggest that the normal cerebral image obtained during continuous inhalation of C15O2 is related to cerebral blood flow, but in a non-linear fashion, and that the technique would be more sensitive to ischemic events than to hyperemic phenomena.

Equilibrium (gated) radionuclide ventriculography.

Equilibrium (gated) radionuclide ventriculography is based on the fact that the heart is a periodically contracting organ. The amount of radioactivity in the heart is proportional to the amount of blood in its cavities, provided there is homogeneous distribution in the blood pool. Thus, the precordial count rate changes reflect the cyclic volume changes of the heart. Because the precordial count rate is too low for a reliable determination of a beat-by-beat time-volume curve, Hoffmann and Kleine applied a gating procedure, using the R-wave of the electrocardiograph (ECG) and a multichannel analyzer [24] to synchronize and sum hundreds of heart cycles. This resulted in a representative cardiac cycle and a well-delineated time activity curve, which is analogous to a time-volume curve. Since the left and right ventricles could not be differentiated by this technique, and an additional x-ray investigation was required for their delineation, Adam et al. [1, 4] and Bitter et al. [9] applied the gating procedure to a camera computer system. Representative time-activity (time-volume) curves of the left and right ventricle were obtained using an electronic cursor to isolate the ventricles.

Hourly variations in O VI P Cygni profiles of hot stars

view Abstract Citations (64) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hourly variations in O VI P Cygni profiles of hot stars. York, D. G. ; Vidal-Madjar, A. ; Laurent, C. ; Bonnet, R. Abstract Significant changes in the ultraviolet absorption profiles are reported in the spectra of Delta Ori A, Iota Ori, and Zeta Pup. The present data show changes in count rates in excess of 30 per cent at velocities more negative than -900 km/s, if these data are interpreted as being dominated by O VI absorption in stellar winds. The time scales range from 1 to 6 hours. Publication: The Astrophysical Journal Pub Date: April 1977 DOI: 10.1086/182410 Bibcode: 1977ApJ...213L..61Y Keywords: Hot Stars; Line Spectra; Periodic Variations; Stellar Spectra; Ultraviolet Absorption; Absorption Spectra; Early Stars; Oao 3; Photomultiplier Tubes; Satellite Observation; Stellar Mass Accretion; Stellar Winds; Astrophysics full text sources ADS | data products SIMBAD (13)

A new method of investigating recrystallization on the basis of positron annihilation

It is shown that the volume fraction of recrystallized material can be directly determined from the relative change of the peak counting rate of the angular correlation curve. Isothermal measurements of plastically deformed Ni of 99.99% purity are compared with the results available in the literature.

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.

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.

Reducing Gain Shifts in Photomultiplier Tubes

Gain shifts in photomultipliers from count-rate changes can be minimized by using only five active stages, making up the gain with a low-noise preamplifier. Further reductions sometimes result from a dim light shining on the dynode structure. Reductions to less than 1 part in 600 were seen.

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.

The effect of prolonged X-ray irradiation on lithium tetraborate glass discs as used in X.R.F. analyses

Prolonged irradiation by X-rays of lithium tetraborate glass discs causes significant changes in countrates for certain elements e.g. Na and Mg decrease while Al, Si and P increase. This effect is probably due to surface diffusion in the glass as grinding away about 10 μm of the surface of the disc restores the countrates to their original levels. Although no change in countrates will ever be noticed on individual glass discs undergoing routine analyses, where irradiation time is only a minute or so, a close watch must be kept on any specimens that have been irradiated for more than a few tens of hours.

A stereo magnetic tape system for radioactive tracer field experiments

A portable detection apparatus is described which makes it possible to undertake radioisotope experiments where transient changes in count rate must be observed even in difficult environmental conditions. In addition to conventional scaler, ratemeter and chart recorder facilities the apparatus includes a commercially available stereo tape recorder. The output pulses from either one or two scintillation detectors are recorded on one stereo track and time pulses from a crystal oscillator are recorded on the other. The data are read out at the laboratory into a multi-channel analyser operated in multiscaler mode using the time pulses to actuate the address advance. This system accurately reproduces the count rate as a function of time in the original data. The operating performance of the system is discussed and illustrated by means of results obtained from radioisotope tracer studies of sand movement on the sea floor near harbour installations.

The effects of abdominal decompression on vascular haemodynamics in pregnancy and labour.

SummaryThe use of radioisotopes in localization of the placental site facilitates study of alterations in vascular haemodynamics under varying conditions. Measurements demonstrated that during pregnancy the application of abdominal decompression results in a 30 per cent increase in count rate over the placental site. There was a simultaneous fall in count rate over the head and lower limbs during the application of decompression. Severe exercise did not produce any change in count rate over the placental site. Measurements carried out during labour showed no change in count rate over the placental site during a uterine contraction, but a 10 per cent fall in count rate over the uterine wall away from the placental site. The application of abdominal decompression during a uterine contraction in the first stage of labour resulted in a 15 per cent increase in count rate over the placental site and over the uterine wall away from the placental site.

Sudden changes in the intensity of high energy X-rays from Sco X-1

In this note we wish to report briefly the observation of sudden changes in the intensity of Sco X-1 by a factor of about 3 recorded in the energy interval 29.9–52.3 keV on December 22, 1968 between 04 h 27 m and 05 h 53 m UT. The observation was made with an X-ray telescope flown in a balloon from Hyderabad, India. The balloon was launched at 0200 hr UT and reached the ceiling of 7.5 g/cm2 of residual atmosphere at 0435 hr UT. The X-ray telescope consisted of a NaI(T1) crystal with an area of 97.3 cm2 and thickness 4 mm, surrounded by both active and passive collimators. The telescope was mounted on an oriented platform which was programmed to look in four specified directions successively, of azimuths, Φ=0°, 110°, 180° and 310° (Φ=0° being North and Φ=90°, West), spending about 4 min in each direction during a cycle of period of about 16 min. The axis of the telescope was inclined at an angle of 32° with respect to the zenith. A pair of crossed flux gate magnetometers provided information every 8.2 sec on the azimuth of the telescope. The pulse heights from the X-ray detector were sorted into several channels extending from 10 to 120 keV. An Am241 source came into the field of view of the telescope once in 15 min for about 30 sec to provide in-flight calibration of the detector. The meridian transit of Sco X-1 was at 0454 hr UT. Just before the balloon reached the ceiling Sco X-1 was in the field of view of the telescope for 3 min and 41 sec. After the balloon reached ceiling, Sco X-1 was in the field of view of the telescope on five occasions between 0443 and 0553 hr UT. During the last observation, however, the balloon had lost altitude by about 1 g/cm2. The excess counts due to Sco X-1 were obtained by subtracting the counting rates corresponding to the North direction which did not include any known X-ray sources. The observation on Sco X-1 in the 1st cycle was made while the balloon was still ascending and consequently the interposed grammage was changing from 10.5 to 9.7 g/cm2. However, for the energy range under consideration, the change in the background counting rate was not significant and there cannot be any doubt regarding the genuineness of the excess counts recorded.

Alpha half-life of 244Cm

The alpha half-life of 244Cm was measured by direct observation of the change in alpha counting rate of four samples over a period of seven years, and was found to be 18·099±0·015 yr.

Compensation automatique de l'extinction de fluorescence des scintillateurs liquides

A procedure for the liquid scintillation counting of weak β-emitters, which permits automatic compensation for the effects of quenching, is presented. Prior to each measurement the spectrum of an auxiliary radiation source, γ- or α-, is superposed on the sample. Should that sample be quenched there is a depression of the count rate of the auxiliary source measured in an analysis channel whose settings make the count rate therein strongly dependent on the degree of quenching. The count rate of the source may be restored to its unquenched value through readjustment of one of the efficiency controlling parameters, e.g. photomultiplier high voltage or amplifier input sensitivity, from the value P to a new value P+ ΔP, which is little different than the value P+ ΔP′ for which the depression of the sample count rate would be exactly compensated. Once the relationship between ΔP and ΔP′ is established automatic compensation is possible. The procedure described also corrects for count rate changes due to colour quenching, electronic gain changes, and changes in sample temperature.

Regional Heart K42 Clearance Determined by Computer Analysis of Serial Digital Scans

Medical scanning procedures which provide a visual image of the distribution of a radioisotope in internal organs are customarily done using nuclides which emit gamma rays of relatively low energy. In the study of deeply placed organs, such as the heart, most of the radiation is absorbed or scattered by tissue located between the site of interest and the detector. Through the use of K42 (1.5 mev gamma) and heavily shielded detectors, satisfactory visualization of the regional tracer uptake in the heart muscle has been obtained. In order to separate radioactivity originating in muscle from that in the circulating blood, a regression analysis of the changes in counting rate with time at each of 960 locations was made by computer processing of serial scans.

A Digital Ratemeter Controlled by the Input Data Rate

Many radiation counter applications require the rapid response speed of a digital ratemeterl & 2, but if large changes in count rate are encountered, a conventional digital ratemeter must be set for a short counting period, producing an excessive number of individual measurements. An instrument is described which avoids this difficulty by automatically adjusting its time base as the count rate changes, the counting period always being such that a preset statistical accuracy is achieved. Further, data readout is inhibited if a measurement does not differ by a statistically significant amount from the last measurement allowed to transfer to the readout line, or if a sudden increase in rate occurs while a measurement is being taken. For applications in which a single digital ratemeter scans the output lines of a number of detectors, a preset time base limit is provided to prevent time being lost in making measurements of very low count rates or where a counter has failed. Among the display modes is a prediction mode which indicates the rate with increasing accuracy as the count is accumulated. The display utilizes scientific notation, which is very economical of display components and output lines, resulting in an instrument which has fewer parts than a scaler-timer of conventional design.

Verstärkungsänderungen bei photovervielfachern vom typ Valvo 56 AVP

The gain changes of Valvo 56 AVP photomultiplier tubes (PM) after counting rate changes have been investigated with regard to their dependence to dynode current and temperature. The photomultipliers were combined with a NaI(Tl)-crystal. Changes of the counting rate cause a fast initial gain change, followed by a slow shift. Within 30 h, four tubes stabilize in gain, while one tube shows a further slow drift. All changes are reversible. Magnitude and time dependence of gain changes vary with dynode current and temperature of the PM. An increase of temperature or dynode current at a high counting rate speeds up the gain changes. The gain changes may be explained by caesium atoms covering the emitter surface of the dynodes.

Radiotracer Studies of Metal-Detergent Interaction by Means of a Dip Counter with an Evaporated Metal Film Window

Abstract The accumulation of solute on metal surfaces has been studied using a radiotracer technique. A dip GM-counter with an end window covered with an evaporated metal film was immersed in an aqueous solution of 35S-labeled sodium dodecyl sulfate, and the change in counting rate due to the accumulation of solute on the metal surface was traced with time for various metals and pH values of the solution. The accumulation curves obtained turned out to be composed of an initial plateau, representing the region of the physical adsorption of the detergent onto the the metal surface, and/or a rising portion of the accumulation curve, indicative of chemisorption. Assuming the closely packed adsorption of solute, the roughness factors of various metal surfaces have been calculated from these plateau characteristics. The rising portion of the curve corresponds to the formation of a thick metallic soap film. The rate of film growth is controlled by the migration of the detergent or metal ions through the film, and this rate decreases as the thickness of the surface film increases. This leads to the parabolic formula for the surface film growth, which is in good agreement with the experimental results. The effect of the pH of the solution on the rate of the uptake of dodecyl sulfate has also been measured for both copper and lead surfaces. For copper surfaces the rate of the uptake of a detergent shows a maximum at pH 7 and decreases in both acid and alkaline regions. For lead surfaces, the rate is large in the acid region and gradually decreases twoards the alkaline region.

Medical Radioisotope Scanning (Abstracts of Papers)

Medical Radioisotope Scanning (Abstracts of Papers)Published Online:29 May 2014https://doi.org/10.1259/0007-1285-38-448-315SectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InEmail AboutAbstractThe component parts of a scanner and the problems of interpreting the display and relating what is seen to the patient were enumerated. The necessity to analyse display systems in order to evaluate the count rate change which can be perceived, was pointed out and illustrated with reference to a colour display system where this has been performed. Previous article Next article FiguresReferencesRelatedDetails Volume 38, Issue 448April 1965Pages: 241-320 © The British Institute of Radiology History Published onlineMay 29,2014 Metrics Download PDF

Alloy analysis and coating weight determination using gamma-ray excited X-ray sources

The use of γ-ray excited X-ray sources in conjunction with X-ray fluorescence techniques for analysis of alloys and for determination of coating weight per unit area is described. The sensitivity of the technique is such that a 1 per cent change in counting rate is caused by introduction of about 100 ppm of cadmium in zinc or antimony in lead and by 0·002 mg/cm 2 of tin on steel (tinplate) or silver in photographic film emulsion.