For radiation detection systems that record pulses (discrete events), the dead time is the time after each event during which the system is not able to record another event. This phenomenon is very important, for example, for Geiger counters. Because of the large avalanche induced by any ionization, a Geiger counter takes a long time (about 1 ms) to recover between successive pulses. Therefore, Geiger counters are not able to measure high radiation rates due to the “dead time” of the tube.
In other words, dead-time is the period during which the detector is busy and cannot accept and process pulses. In case of detectors of ionizing radiation, this phenomenon can have serious consequences, since dead-time distorts outputs at high activities or high dose rates. The total dead time of a detection system is usually due to the contributions of the intrinsic dead time of the detector, of the analog front end and of the data acquisition.
Paralyzable and Non-paralyzable Detector
Because of the random nature of radioactive decay, there is always some probability, that a true event will be lost, because another event is just recorded and the detector cannot accept and process more than one pulse.
There are two main dead-time characteristics of each detecting system:
- Paralyzable. In a paralyzable detector, an event happening during the dead time 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.
- Non-paralyzable. In a non-paralyzable detector, an event happening during the dead time 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.
Quenching – Dead Time – Geiger Counters
Dead time phenomenon is very important also for Geiger counters. Typically it has a value of about 100 μs (for proportional counters it is much less), and so after each ionising event a Geiger tube is basically switched off for 100 μs.
In a Geiger counter the fill gas of the chamber is an inert gas which is ionized by incident radiation, and a quench gas of 5–10% of an organic vapor or a halogen gas to prevent spurious pulsing by quenching the electron avalanches. The Geiger counter should not give spurious pulses, and should recover quickly to the passive state, ready for the next radiation event. Argon and helium are the most frequently used fill gases and allow for the detection of alpha, beta, and gamma radiation. For neutron detection He-3 and BF3 (Boron Trifluoride) are the most commonly employed gases.
However, for each electron collected in the chamber, there is a positively charged gas ion left over. These gas ions are heavy compared to an electron and move much more slowly. Free electrons are much lighter than the positive ions, thus, they are drawn toward the positive central electrode much faster than the positive ions are drawn to the chamber wall. The resulting cloud of positive ions near the electrode leads to distortions in gas multiplication. Eventually the positive ions move away from the positively charged central wire to the negatively charged wall and are neutralized by gaining an electron. These atoms then return to their ground state by emitting photons which in turn produce further ionisation and thereby spurious secondary discharges. The electrons produced by this ionization move toward the central wire and are multiplied en route. This pulse of charge is unrelated to the radiation to be detected and can set off a series of pulses. In practice the termination of the avalanche is improved by the use of “quenching” techniques.
The quenching gas molecules have a weaker affinity for electrons than the chamber gas does; therefore, the ionized atoms of the chamber gas readily take electrons from the quenching gas molecules. Thus, the ionized molecules of quenching gas reach the chamber wall instead of the chamber gas. The ionized molecules of the quenching gas are neutralized by gaining an electron, and the energy liberated does not cause further ionization, but causes dissociation of the molecule. This type of quenching is known as self-quenching or internal-quenching, since tubes stop the discharge without external assistance.
For Geiger counters, external quenching, sometimes called “active quenching” or “electronic quenching“, is also a possibility. Electronic quenching uses simplistic high speed control electronics to rapidly remove and re-apply the high voltage between the electrodes for a fixed time after each discharge peak in order to increase the maximum count rate and lifetime of the tube.