3.0. CIRCUIT DESCRIPTION
CAUTION: This instrument has not been designed for indiscriminate opening
or disassembly of the internal parts. It contains highly sensitive
semiconductors which are destroyed by even the slightest electrostatic
discharge.
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.
Model 200SB monitors are designed to measure tritium. Activity of tritium decay is such
that a concentration of 1 μCi/m3in 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 amperes 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 μCi/m3(3.3 MBq/m3) 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 200SB instruments, two ionization chambers are arrayed in a side by side
ensuring good gamma compensation in all directions.
The ionization chamber polarizing voltage is supplied by a set of dry batteries with a long
life. The surfaces of the ionization chambers themselves are bare, and, to avoid damage to
the electrometer, must NOT be touched by hand.
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 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 Model 200SB instruments are
suitable for measurement of currents as low as10-16 amperes.
In Model 200SB, the electrometer is directly attached to the ionization chamber assembly,
which is protected by a rectangular metal cover.
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