Optimum counting time

In many laboratories, samples will not be submitted one by one for individual attention but in batches aU to be counted within as little time as possible. Efficient use of counting equipment in terms of the time devoted to counting each sample can pay dividends when time and equipment are limited. 

The first matter to be decided is the precision required of the final result. Let us suppose that, as an example, the reason for the count is to assess whether the Cs in a sample of lamb is above or below some action limit. It might be that a poor precision result from a count of only five or six hundred seconds might answer the question for the majority of samples where the amount of Cs was much lower, or indeed much greater, than the action limit. This would leave much more time available to achieve more precise results for those samples that are near to the action limit. 

For a single sample with unlimited counting time available, the optimum count period is that which will provide 

A special case arises when a background spectrum is necessary. An example might be the measurement of low levels of Co where a peaked-background correction must be made. Let us assume that we have a batch of 

Taking simple single channel counts as an example, if we measure C counts in time Ar and measure a background count B in Ar, then the net count rate (K) is  

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Therefore, if there is a fixed time T for the measurement, the optimum counting times are determined from the two equations... 

The only problem with this method is observed for weak reflections where (+)/(-) count-rates are similar (i.e. R 1). The (+)/(-) optimised counting-time proportions must be 50%/50%, but with low count-rates, we have observed that the lack of precision may lead to proportions which are not optimum (e.g. 47%/53%). The same behaviour has been observed for peak to background proportions. In fact, when measuring a flipping ratio in many steps, we observe oscillations of the time proportions which slow the decrease of the standard deviation. Of course, these time variations have no sense, and one should calculate the variances of the optimised counting-times (Equations (16)) to avoid such spurious fluctuations ... 

Influence of the ZnCFO contents (3,0 5,0 7,0 phr) on crosslink kinetics of the modelling unfilled rubber mixes from NBR-26 of sulfur, thiuram and peroxide vulcanization of recipe, phr NBR-26 - 100,0 sulfur - 1,5 2-mercaptobenzthiazole - 0,8 stearic acid - 1,5 tetramethylthiuramdisulfide - 3,0 peroximon F-40 - 3,0, is possible to estimate on the data of fig. 7. As it is shown, the increase of ZnCFO concentration results in increase of the maximum torque and, accordingly, crosslink degree of elastomeric compositions, decrease of optimum cure time, that, in turn, causes increase of cure rate, confirmed by counted constants of speed in the main period (k2). The analysis of vulcanizates physical-mechanical properties testifies, that with the increase of ZnCFO contents increase the tensile strength, hardness, resilience elongation at break and residual deformation at compression on 20 %. That is, ZnCFO is effective component of given vulcanization systems, as at equal-mass replacement of known zinc oxide (5,0 phr) the cure rate, the concentration of crosslink bonds are increased and general properties complex of rubber mixes and their vulcanizates is improved. 

These techniques are designed to minimize both the actual working time, required and the analytical uncertainties in sample analysis. Sample preparation and neutron activation procedures are based on proved analytical and microanalytical techniques. The unusually high sensitivity, reliability, and accuracy are achieved through a choice of optimum irradiation and counting times for the y-ray detection systems. 

Accordingly, in WDXRF, several counting strategies may be employed to keep this number as low as possible. In the optimum fixed time strategy, the minimum uncertainty is obtained within a time interval t = tp + tg when tj and tg are chosen in such a way that ... 

Recall that Eq. (11.2) applies for the optimum choice of counting time, where... 

Choosing optimum irradiation, decay, and counting times can improve sensitivity for many of the above significantly. However, some of the elements namely B, Be, Bi, C, Li, N, Nb, Ne, O, P, Pb, S, Si, and T1 are not ordinarily detected by this method. 

Now, if we have a fixed total count time, Ar. -f Ar, then the optimum sharing of the time will be found when the variance is at a minimum, i.e. when dV/d/J = 0. If the mathematics is followed through, we find that this condition is obtained when ... 

Optimum sharing of counting time between samples and background is achieved when the ratio of count times equals the ratio of sample to background activity. Decision limits are calculated according to the following equations ... 

Optimum resolution, i. e. low full width at half-maximum (FWHM), is a trade-off between high count rate, i. e. low dead-time, and good spectral resolution. 

The regular use of sodium pentachlorophenate has proved successful in many instances, but it is a persistent environmental hazard and cannot be recommended. However, o -benzy 1-p -chlorophenol can often produce goods results, with low bacteria counts and clean systems. But the timing of biocide applications needs to be matched carefully with the production cycles (and process leakage periods) for optimum product effectiveness. 

The intrinsically low intensity of Raman scattering strongly influences both the sensitivity and penetration depth of SORS and its variants. Dominant noise components (photon shot noise or thermal/dark count [1]) can be minimised relative to signal by increasing absolute signal levels. In many Raman systems, collection optics, laser power and other relevant parameters are usually maximised for optimum performance of the system current detectors (CCD devices), for example, have detection efficiencies approaching 100%. Typically, acquisition time provides the only straightforward means available... 

Calculate the optimum time from the selected strip and make this your base exposure time. An example is if you chose the fourth strip counting from the lightest strip and the exposure times were four seconds for each strip your base exposure time is 16 seconds. 

Measurement of soil activity there are a number of laboratory methods which are suitable for measuring the biological activity of the soil. In principle, a distinction is made between direct and indirect methods for the determination of soil activity. The biomass in the soil, for example, can be estimated by counting the individual organisms in the soil, or the measurement of respiration after the addition of a nutrient in excess can provide an indication of active biomass. Moreover, in determinations of activity, a distinction is made between actual and potential activity. Actual activity values are values measured at the time that the sample was taken. Determinations of potential activity, on the other hand, show the level of performance that microorganisms are capable of under optimum experimental conditions, after the addition of a nutrient substrate and prolonged incubation. 

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