Timing Characteristics of the Pulse

For certain measurements, like coincidence-anticoincidence counting or experiments involving accelerators, the time resolution of the signal is also important, in addition to energy resolution. For timing purposes, it is essential to have pulses with constant risetime. 

No detector produces pulses with exactly the same risetime. This variation is due to the fact that electrons are produced at different points inside the detector volume, and thus traverse different distances before they reach the point of their collection. As a result, the time elapsing between production of the charge and its collection is not the same for all the carriers 

Consider a true coaxial detector, shown in Fig. 12.39a (see also Fig. 7.26). Since the electric field is radial, electrons and holes will follow a trajectory perpendicular to the axis of the detector. The maximum time required for collection of the charge corresponds to electron-holes being produced either at A or C. That time t is equal to I AC)/v, where 4C is the detector thickness and V is the speed of electrons or holes. For a detector bias of about 2000 V and the size shown in Fig. 12.39a, v 0.1 mm/ns = 10 m/s, which gives a maximum collection time of 120 ns. The best risetime corresponds to electron-holes generated at point B (Fig. 12.39a) and is equal to about 60 ns. 

The pulse risetime is essentially equal to the collection time. For the detector shown in Fig. 12.39a, the risetime will vary between 60 and 120 ns. For other detector geometries the variation in risetime is greater because the electrons and holes, following the electric field lines, may travel distances larger than the thickness of the detector core (Fig. 12.396). The variation in risetime for the detector of Fig. 12.396 will be between 60 and 200 ns. The distribution of pulse risetimes for commercial detectors is a bell-type curve, not exactly Gaussian, with a FWHM of less than 5 ns. 

A great advantage for CdTe and Hgl2 detectors, compared to Ge and Si(Li) detectors, is that they can operate at room temperature (see also Sec. 7.5.6). At this time, they can be obtained in relatively small volumes, but they still have an intrinsic efficiency of about 75 percent at 100 keV because of the high atomic number of the elements involved. The energy resolution of CdTe detectors is 18 percent at 6 keV and 1.3 percent at 662 keV. The corresponding numbers for Hgl2 are 8 percent and 0.7 percent.  

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Notice that the rate at which the pulse rises (risetime) is determined by the decay time T. In certain measurements, e.g., coincidence-anticoincidence measurements (Chap. 10), the timing characteristics of the pulse are extremely important. 

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