Limiting count rate

For some experiments, the solar neutrino flux and the rate of decay of the proton being extreme examples, tire count rate is so small that observation times of months or even years are required to yield rates of sufficiently small relative uncertainty to be significant. For high count rate experiments, the limitation is the speed with which the electronics can process and record the incoming infomiation. 

In the presented work an algorithm for the primary radiation filter optimization has been developed and realized in the Mathcad envelope which provides a minimal detection limit of a critical element both at the given X-ray tube power and at the given maximal acceptable count rate. 

Because X-ray counting rates are relatively low, it typically requires 100 seconds or more to accumulate adequate counting statistics for a quantitative analysis. As a result, the usual strategy in applying electron probe microanalysis is to make quantitative measurements at a limited collection of points. Specific analysis locations are selected with the aid of a rapid imaging technique, such as an SEM image prepared with backscattered electrons, which are sensitive to compositional variations, or with the associated optical microscope. 

Resistive Anode Encoder (RAE). This detector has the advantage that the single-ion events are detected digitally. It therefore it delivers quantitative results, irrespective of local differences in the amplification of the channel plate. One disadvantage is that the count rate is limited to 200000. 

The cause of this difficulty therefore resides within the counter itself. The difficulty is described by saying that the Geiger counter has a dead time, by which is meant the time interval after a pulse during which the counter cannot respond to a later pulse. This interval, which is usually well below 0.5 millisecond, limits the useful maximum counting rate of the detector. The cause of the dead time is the slowness with which the positive-ion space charge (2.5) leaves the central wire under the influence of the electric field. This reduction in observed counting rate is known as the coincidence loss. 

The promise of 3-D detectors The ideal detector would be able to measure the wavelength dimension of each photon in addition to the two spatial dimensions - a 3-D detector . There is progress being made on 3-D detectors - for example, the superconducting tunneling junction (STJ). However, at present, 3-D detectors are experimental and have only achieved limited wavelength resolution ( /5 < 25) with relatively slow count rates (few kHz per pixel) on a small number of pixels 100 j. Since these devices are experimental and are not being used... 

C22-0114. The amount of radioactive carbon in any once-living sample eventually drops too low for accurate dating. This detection limit is about 0.03/g min, whereas fresh samples exhibit a count rate of 15.3/g min. What is the upper limit for age determinations using carbon dating ... 

Quantitative Analysis. The efficiency of the detector is such that almost 100% of the X-rays entering it will produce a pulse, but the pulse processing speed limits the rate at which X-rays can be counted. If the count rate is less than a few thousand counts per second, then most of the incoming pulses are processed, but as the count rate rises an increasing fraction of the pulses are rejected. The live time during an analysis when the detector was counting is thus less than the elapsed time, and the EDS system records both times in order that the true count rate may be measured. 

D Position Sensitive Detectors are multi-wire electrical-field detectors. The principal limitation of the total counting rate reduces the applicability at a synchrotron beamline in particular for 2D detectors. But even strong, narrow peaks pose a problem, because the whole image is distorted as soon as local saturation occurs. The detector response is changing, because the wires are worn out by use. 

SPC techniques offer the advantage of low noise detection, providing / -values of 1-2 times lower than in analog detection. Although this can in principle result in four times faster acquisition speed this gain in speed is not realized in practice. In SPC, the comparatively high dead-times of detectors and electronics limits the acquisition speed. SPC system should be operated at count rates below the inverse of the dead-time of the system (electronics... 

Source range instrumentation also measures the rate of change of the count rate. The rate of change is displayed on meters in terms of the startup rate from -1 to +10 decades per minute. Protective functions are not normally associated with source range instrumentation because of inherent limitations in this range. However, interlocks may be incorporated. 

The natural cosmic ray background and the environmental radioactivity set a lower limit on measurable counting rates and thus the minimum number of radioactive atoms in the sample. This minimum number increases linearly with the half-life. For 14C the number of atoms present in a sample is given by ... 

Thus the counting rate should be sufficiently high to ensure a precision sufficient for isotope effect measurements a target precision of 0.1% is obtained for total counts of 106. Radiolabeled material used for isotope effect measurements usually has counting rates of 15,000 counts per minute (cpm). To achieve 0.1% precision such samples must be counted for 8 h or more. More often counting time is limited to several cycles of about 10 min each with a commensurate increase in the statistical error. 

Dead time considerations in the alpha particle detection limit the count rate, and hence limit the neutron flux that can be used with this approach. This means that large scan times will probably be required with most implementations of this approach. 

Although in principle the time structure of the SR beam may be exploited in time resolved studies the major limiting factor is the rate at which three dimensional data may be accumulated. In this respect time resolved methods are bound to develop in tandem with the development of high count rate/fast refresh rate electronic area detectors. This applies to both monochromatic and white beam methods. For the latter the use of an integrating detector such as a CCD or image plate are the main expected improvements over film. 

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