Live time clock

For an ideal live time clock, if a livetime tt is measured as the time required to count n events, then the most probable estimate of the mean rate p is... 

In Section 4.6.3, I explained that an allowance must be made for detector pulses that reach the MCA system but are not analysed because the ADC is already busy measuring a previous pulse. Because of the lost pulses, simply dividing the number of pulses within a channel, aregion-of-interesL a peak, or indeed the whole spectrum, by the real count period will underestimate count rates. The lost pulses can be accounted for by dividing numbers of counts by the live time, as measured by the live time clock (LTC - see Section 4.6.3 and Figure 4.30). 

This works well at low to moderate count rates but is limited at high count rate. Dead time is discussed further in Chapter 14 in this connection. For now, it is sufficient to say that, in general, high dead times are to be avoided. Each laboratory has its own arbitrary limit. My own was about 30 %, but many laboratories have much lower limits. At very high dead times, measurement of hve time may be inaccurate because of differences in shape between the detector pulses and the live time clock pulses, which are not blocked in the same way by the input gate. Inaccurate live time measurement does not prevent nuclide identification but does affect the quality of quantitative nuchde measurements. 

The ORTEC ultra-high count rate Mercury system uses this method and the accuracy of the live time clock at maximum throughput is said to be better than 3 %. There is further discussion of high pulse rates and dead time in Chapter 14. 

LIVE TIME CLOCK The clock within the MCA that allow correction to be made for dead-time. 

LIVE TIME CORRECTION (LTC) The process of stopping the live time clock in the MCA whenever the pulse processing circuits are busy. A common method of allowing for dead time. 

LTC See Live Time Correction and Live Time Clock. 

Here X denotes the radionuclide decay constant,fo denotes the time from sample collection to the start of sample counting, and denotes the real time, or clock time, of the sample counting measurement, which may be longer than the live time Ic- When the time r is short relative to the half-life of the radionuclide, the product XfR may be very small, and as XtR approaches zero, the correction factor for decay during counting, (1 - e )/XtR, approaches 1. If the value of this factor is sufficiently close to 1, it may be omitted from the expression for D. When it is included, it must be calculated carefully to avoid large round off errors when XtR is small. 

Uncertainty due to dead time can be avoided by keeping the dead time rate low (e.g., less than 10%). If the dead time rate is not negligible, the gross count rate must be calculated for the live time rather than the clock time (real time) if the dead time rate is not excessive, no additional uncertainty component for dead time is usually needed. 

It is obvious that the time to effect this measurement -the eonversion time - is proportional to the pulse height. This variable dead time must be taken into account within the live time measurement system. The resolution of the ADC depends upon the relationship between the rate of discharge of the capacitor and the clock rate. If the discharge rate is decreased to shorten the conversion time, the resolution will be decreased because fewer clock pulses will be recorded. A faster clock would be needed to maintain the resolution. Clock rates of 100 MHz and 450 MHz are typical. (Note that this clock is separate from and independent of the live time correction clock.) Wilkinson ADCs are considered to have excellent linearity, and at one time were regarded as significantly better than other types of ADC. 

C2.14.4 Kineticsit has already been emphasized (section C2.14.1, section C2.14.2.2 and section C2.14.3.1) tliat kinetics are of paramount importance in describing living systems [76]. The root of tliis may ultimately he in tire fact tliat whereas inanimate matter has endless time in which to undergo its transfonnations, mortal, animate matter is constantly racing against tire clock. 

Chemical manufacturing and petroleum refining have enriched our lives. Few individuals in the developed world stop to realize how the chemical industry has improved every minute of their day. The benefits of the industries are apparent from the time our plastic alarm clock tells us to wake up from a pleasant sleep on our polyester sheets and our polyurethane foam mattresses. As our feet touch the nylon carpet, we walk a few steps to turn on a phenolic light switch that allows electrical current to safely pass through polyvinyl chloride insulated wires. At the bathroom sink, we wash our face in chemically sanitized water using a chemically produced soap. 

Doolittle R. F., Feng D. F., Tsang S., Cho G., and Little E. (1996) Determining divergence times of the major kingdoms of living organisms with a protein clock. Science 271(5248), 470-477. 

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