Page
4
3.0. CIRCUIT
DESCRIPTION
3.1. IONIZATION
CHAMBERS
In its simplest form, an ionization chamber is an enclosed volume with two electrodes. Voltage is
applied between the electrodes, generating an electric field, which will segregate and collect
electric charges, which are created by nuclear events occurring inside the chambers. Nuclear
events may consist of ionization of air molecules by external or internal alpha, beta or gamma
radiation.
The RS400 monitors are designed to measure tritium. Activity of tritium decay is such that a
concentration of 1
μ
Ci/m
3
in a volume of one liter will generate an ionization current of about 0.95
x 10
-15
amperes. This is a very weak current.
Alpha pulses from naturally occurring radon, are much more energetic, they can produce short
current bursts of up to 10
-13
coulombs during decay, and therefore appear as large noise "spikes"
which can seriously impair tritium measurement.
Gamma radiation also has a strong effect. In practice, a gamma radiation field of 1 mR/hr will
create the same amount of ionization as 90
μ
Ci/m
3
of tritium.
A tritium monitor, in order to measure to low concentrations, must be able to respond only to tritium
and be immune to alpha or gamma radiation. For this purpose, a second ionization chamber
system has been included to balance out any ionization current contribution from external gamma
radiation.
In the RS400 instruments, the four ionization chambers are arrayed in a cruciform pattern, ensuring
almost perfect gamma compensation in all directions and even for high gradient non-uniform fields.
The ionization chamber polarizing voltage is supplied by an electronic high voltage power supply.
The surfaces of the ionization chambers have a thin coat of paint for insulation, but it is best to
avoid touching them.
3.2. ELECTROMETER
Also known as a transimpedance amplifier, it serves the purpose of converting the extremely feeble
ionization current into a voltage suitable for further signal processing and measurement display.
The heart of the electrometer consists of a specially selected dual ultra high impedance
semiconductor device which has been chosen both for ultra low internal current leakage as well
as long term d.c. stability. The semiconductors used in the RS400 instruments are suitable for
measurement of currents as low as10
-16
amperes.
In the RS400 instruments, the electrometer is directly attached to the ionization chamber cluster
and is protected by a rectangular metal cover.
CAUTION:
This instrument has not been designed for indiscriminate opening or
disassembly of the internal parts. Electrostatic charge from the bias voltage on the
ionization chamber cans will be present even when the instrument is switched OFF.
The detector pre-amp contains highly sensitive semiconductors which are destroyed
by even the slightest electrostatic discharge.