Dead time

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In gaseous ionization detectors the dead time is the time after each event, detected by the detector, in which the detector is not able to reveal another event if it happens.

In this time the detector is as if it were dead or frozen or paralysed. Even if it is really live, it is not able to count. This may be due both to real events and spurious events. In some cases the detector is not even sensitive to another event (for example in a spark chamber until the potential between the plates recovers above a high enough value); in other cases, the detector is sensitive but is not able to give out information or the information is not trustworthy.

An easy example in every day life is given by the screen of Cathode ray tube television set. When a beam arrives on the screen and lights the pixel, then it remains on for a fraction of second even after the beam is stopped. This effect and the persistence effect in the retina of human eyes make it possible to view a complete image though it is composed of points of light that are illuminated at different moments.

From the total time a detector is running, the dead time must be subtracted to obtain the live time of the experiment.

The total dead time of a detection system is usually due to the contributions of the intrinsic dead time of the detector (for example the drift time in a gas detector), of the analog front end (for example the shaping time of a spectroscopy amplifier) and of the DAQ (the conversion time of the ADCs and the readout and storage times).

[edit] Paralizable behaviour

A detector, or detection system, can be characterized by a paralizable or non-paralizable behaviour.

In a non-paralizable detector, an event happening during the dead time since the previous event is simply lost, so that with an increasing event rate the detector will reach a saturation rate equal to the inverse of the dead time.

In a paralizable detector, an event happening during the dead time since the previous one will not just be missed, but will restart the dead time, so that with increasing rate the detector will reach a saturation point where it will be incapable of recording any event at all.

A semi-paralizable detector exhibits an intermediate behaviour, in which the event arriving during dead time does extend it, but not by the full amount, resulting in a detection rate that decreases when the event rate approaches saturation.