Understanding Neutron Radiography Post Exam Reading VIII-Part 2a of 2A

Output Current Measurements
The ionization current collected in the ion chamber flows through an external
circuit for measurement. Although in principle an ammeter could be placed in
the external circuit to read the ion current, in practice the ammeter is not
placed there, for the current is very small.
A 440 cm 3 (27 in. 3 ) ion chamber typically produces about 4 × 10 –15 A·μSv –1
(4 × 10 –14 A·mR –1 ) at standard temperature and pressure.
A high valued load resistor (on the order of 10 10 Ω) is placed in the circuit and
the voltage drop across the resistor is measured with a sensitive electrometer.
A metal oxide silicon field effect transistor (MOSFET) is used in some
electrometers. The metal oxide silicon field effect transistor produces an input
impedance on the order of 10 15 Ω to amplify the collected current (Fig. 5).
Charlie Chong/ Fion Zhang