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Why do current-mode ion chambers need sensitive electrometers?
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radiation measurement
Figure 1: (A) A simple equivalent circuit for the development of a voltage pulse at the output of a detector. R represents the resistance and C the capacitance of the circuit; V(t) is the time (t)-dependent voltage produced. (B) A representative current pulse due to the interaction of a single quantum in the detector. The total charge Q is obtained by integrating the area of the current, i(t), over the collection time, tc. (C) The resulting voltage pulse that is developed across the circuit of (A) for the case of a long circuit time constant. The amplitude (Vmax) of the pulse is equal to the charge Q divided by the capacitance C.
Current-mode ion chambers require sensitive electrometers because the magnitude of the electric current generated by incident radiation is extremely small.1
For example, a typical one-litre ion chamber exposed to a modest gamma-ray rate of roentgen per hour produces a current of only about 0.1 picoampere (pA). Because these signals are so minute, specialized instruments capable of measuring currents as low as amperes are necessary to ensure accurate detection and monitoring.12