Counting loss correction

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... 

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. 

It should be noted that the a count rate of 4.510 s is close to the limit of a single channel in currently available TCSPC devices. Intensity measurements at rates this high require a correction for counting loss (see Sect. 7.9.2, page 338). The moments of the time-of-flight distributions are not influenced by eounting loss. Certainly, there is a small pulse-shape error due to classic pile-up. However, because only small changes in the moments are of interest, the pile-up-error is not substantial. 

Most energy-dispersive spectrometers include a means for automatically correcting for deadtime losses. The residual error in the deadtime loss correction is generally an increasing function of counting rate. Thus, systematic errors in the deadtime correction scheme can limit the counting rates that can be employed. 

A second problem to avoid is that of deadtime. Where equipment without automatic deadtime correction is employed, a large error may accrue since measurement pairs are all being done at low and high count rates, respectively. Thus, the high count rate (i.e., the lower dilution) will have a greater count loss than the low count rate (i.e., the higher dilution ratio). It is thus essential to correct all of the count data for deadtime. 

FIG. 9—Effect of precorrosion on inhibitor performance. Constant pH kettle test, 55°C, 1 bar CO2,100 ppm inhibitor. Corrosion rates monitored by Fe-counts or weight loss corrected LPR measurements. 

If counting losses due to random summing and/or self-absorption are corrected for, then these corrections will themselves have an uncertainty that must be accounted for. If these corrections are made by the spectrum analysis program, you should make sure, by reading the manual and by validation measurements, that the uncertainties assigned by the program are reasonable. 

Correct for counting losses due to dead time and random summing. 

The activity estimate at this point may need correction for a number of counting losses which were covered in Chapter 6. Only the following are routinely catered for in spectrum analysis programs (numbers in parenthesis refer to the equation used to make the correction) ... 

Apart from the normal routine checks, one would have to assure oneself that the electronic system was working satisfactorily - at high count rate, there is the additional burden of confirming that counting losses are being adequately accounted for. There are procedures that have been widely used for many years. In 1990, Gehrke proposed the particular procedure below, which is now enshrined in the US standards ANSI N41.14 (revised). It is a test of the precision of automatic or semi-automatic dead time correction of whatever type - by a measured correction factor, by PUR and ETC circuits, by the pulser method, by the virtual pulser, or by any combination of these. The procedure is as follows ... 

After the end of the 4-day exposure, the detectors were returned to EML for analysis. The amount of radon adsorbed on the carbon device was determined by counting the gamma rays of radon progeny in equilibrium with radon. The concentrations of radon in the buildings were determined from the radioactivity in the device and the calibration factor, obtained in a radon chamber, that takes into consideration the length of exposure and a correction for the amount of water vapor adsorbed during the exposure. The lower limit of detection with this technique is 0.2 pCi/1 for a measurement period of 4 days when the test sample is counted for 10 min, 4 days after the end of exposure. More than 90% of the radon monitoring devices were analyzed successfully. Most of the unsuccessful measurements were due to delays or losses caused by the participants. 

Whether this condition can be fulfiUed depends on the electron count of the metal, and the stereochemistry of the elimination. For instance, in m-elimination from octahedral d , or square planar d , systems, metal ndipP -y ) acts as acceptor, and this should be a facile process ( e Fip. 1, 2). For /rans-elimination, on tiie other hand, the lowest empty orbital of correct symmetry is (n + l)p. Such elimination Kerns energetically less Ukely, unless a non-concerted pathway (such as successive anionic and cationic loss) is available. The same arguments apply, of course, to oxidative additions. It foUows that the many known cases of traits oxidative addition to square planar t/ systems are unlikely to take place by a concerted mechanism, and this conclusion is now generally accepted There are special complexities in reductive elimination from trigonal systems, and these are discussed furdier in Part III. 

---