Instrument Dead Time

From a piece of cardboard or in. plastic cut a circle 3.0 cm in diameter (this will fit most planchets). If necessary decrease the size of plastic disk so that it can be easily removed from the planchet. 

Suspend a small amount of uranyl nitrate in airplane glue, spread a film of this mixture on the disk, and allow it to dry. 

Count each half of the disk separately 10 times for 5 minutes each. 3-98. Count both halves together 10 times for 5 minutes each. 

The dead time, t, may be calculated from these data using the formula 

In this experiment the following data were obtained  

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It is common to prepare standards that contain approximately the same or slightly larger amounts of the element of interest than the amount estimated to be found in the sample. In this way errors associated with varying sample and standard instrumental dead-time corrections in the counting system are minimized. This is an important consideration when counting indicator radionuclides having half-lives short with respect to the time of counting. 

If Tg is very short, it is difficult to see the signal because the FID will decay before the instrumental dead time is over. This situation will usually occur only for solids, and one way to deal with it is to try to form an echo which occurs later than the end of the dead time. Such echoes are fairly easy to form in the presence of a large inhomogeneous broadening such as in metals and with many quadrupolar nuclei in NQR. You will need a high speed digitizer because the echoes will be very sharp. For some other solid state echoes, see IV.B.3. and IV.B.4. Another possible solution is to use the zero time resolution method described in VI.D.4. 

The dead time for the instrument used is estimated to be 1 sec based on the appearance of a signal following injection of nitric oxide into solution. Determination of the rate of disappearance of nitric oxide following injection of ONOO" is limited by instrument dead time. 

The main limitation of the above described two-pulse experiment originates from phase relaxation processes. T may become too short with respect to the instrumental dead time (about 150 ns), resulting in an overlap with the ESE spectrum. This may be overcome by applying a three-pulse excitation scheme (Fig. 7), in which the second pulse of the two-pulse experiment is divided into two n/2... 

FIGURE 5.5 Golay plots for a 5.0-m-long, 0.25-mm-i.d. thin-film column using hydrogen carrier gas showing the effects of extracolumn band broadening defined by the total instrumental dead time Af. A binary diffusion coefficient of 0.4 cm /s and a retention factor of 2.0 are assumed. 

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