Counters, dead time

COUNTER DEAD-TIME CORRECTION AND MEASUREMENT OF DEAD TIME... 

Because of counter dead time, the possibility exists that some particles will not be recorded since the counter will not produce pulses for them. Pulses will not be produced because the counter will be occupied with the formation of the signal generated by particles arriving earlier. The counting loss of particles is particularly important in the case of high counting rates. Obviously, the observed counting rate should be corrected for the loss of counts due to counter dead time. The rest of this section presents the method for correction as well as a method for the measurement of the dead time. 

Before results from a new calibration are accepted, it is essential that the accuracy over the full calibration range is validated independently. Although the intensity response of a WD spectrometer is expected to be linear over six orders of magnitude (subject to corrections for counter dead-time effects), the accuracy of the calibration function is as sensitive as that for any other instrumental technique to the effects of bias, which are likely to be most significant in the analysis of samples at the extremes (low or high) of the calibration range. Discrepancies of this nature are sometimes caused by the use of inaccurate values in reference samples. These discrepancies can only be overcome satisfactorily by a critical evaluation of all calibration data. If the analysis of independently characterized reference materials cannot be used in this evaluation (because, for example, these samples have had to be used as primary calibration samples) then it is possible, though not entirely satisfactory, to evaluate the self-consistency of calibration data in order to identify discrepant points. 

All data obtained from the proportional counters were corrected for background, absorption, counter dead time and nonlinearity, fluorescence, and atomic number. In this case the counter dead time, fluorescence, and atomic number corrections are negligible and the background can be accounted for by a simple subtraction. The absorption, however, requires more careful consideration. Detailed procedure and mass absorption coefficients were taken from Smith ( ). 

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. 

An alternative approach is to model the probability of coincidence as a function of either sample concentration or the fraction of time the sensing zone is occupied by passing cells (dead time). The model produces a coincidence-correction factor which is applied to the count. Still another approach is to reduce the size of the sensing zone, which reduces the probability of coincidence. In practice, commercial counters combine the latter two approaches. 

Time variations in the intensity of the flux during irradiation. This is an important consideration only when a single sample transfer system is used. Gas-filled BF3 neutron counter tubes are often used to monitor the neutron flux in order to normalize the data when the sample and the standard are not irradiated simultaneously. Gain shifts and dead-time effects associated with the use of neutron monitoring detectors also contribute to the errors associated with a single sample transfer system. 

By self-absorption, absorption in the air and the window, and dead time of the counter, rj is reduced, whereas it increases by the influence of backscattering. 

As proportional counters have dead times of only several ps, high counting rates can be measured without losses. Because the internal counting efficiency of proportional counters for y radiation is low (about 1%), they are not suited to measure y radiation. However, proportional counters of special design and operating at high gas pressure are applied to X-ray and low energy y-ray spectrometry. 

Finally, the influence of the dead time D in eq. (7.3)) has to be taken into account, particularly if the dead time of the detector is high (as in the case of Geiger Muller counters) and if the counting rates of the sample and the calibration source are markedly different. 

Dead time is particularly significant for Geiger counters they have the largest dead times (up to 200 ps) because they have the most charge to collect (maximum gas multiplication, maximum size ionization avalanche), which takes time. 

At very high dose rates, a Geiger counter with large dead time may become completely paralyzed its reading drops right down and stays there, even if the dose rate or activity is further increased. 

Geiger counters can be designed to electronically compensate for counts lost due to dead time. However, many are not. It is important to know whether a particular Geiger counter is dead time compensated, so that if not, its drawback can be kept in mind. 

The classical method of 4ji p counting with a 4jt gas flow proportional counter is still useful for the absolute measurements of P-emitting nuclides provided that good sources with small self-absorption can be prepared. From the observed counting rate, after fundamental corrections for background and counting loss due to dead time, the radioactivity, n, can be calculated as... 

The maximum intensity at which a counter can operate is determined by the dead time set by the electronics. In the case of an analogue device such as the television detector the detector can saturate in terms of the cumulation of charge in a picture element (pixel). The gain of the system can be reduced to allow these strong signals to be measured but to... 

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