Dead time

In nearly all detector systems there is a fraction of time when, for some reasons, the system is not active, namely not able to detect events. For instance, a minimum dead time must separate two events in order that they be recorded as two separate pulses. The dead time can be set by processes in the detector itself or may arise in the associated electronics, as is for the GRBM. The dead time gives a probability that if an event occurs too close with respect to a preceding event it will be no recorded by the instrument. The dead time losses can become severe in case of very bright sources.
The relation between the true interaction rate n, the recorded count rate m and the system dead time $\tau$ is (e.g. [Knoll 1989]):

$$n=\frac{m}{1-m\tau}$$ (15)

As anticipated in section 1.2.4, the GRBM 1s ratemeters and 240 channels spectra dead time is 4$\mu$s and 14$\mu$s respectively. Applying the above formula, we find that for a count rate of 10000 counts/s the dead time loss in the ratemeters in 4%, while in the 240 channel spectra it is 14%. Thus, the energy spectra data are significatively affected by dead time losses in the case of bright sources, like GRB970111 or GRB980329. The ratemeters data, in turn, become severely affected by dead time when the count rate reach very high value (40% for a count rate of 100000 counts/s), as in the case of SGR1900+14 (see section 5.4), which had a peak count rate (corquickly)rected for counter recycle as discussed below) in the GRBM band of 147000 counts/s.