Fluke Biomedical 440RF - Manual (Page 27)

Circuit Description
Ion Chamber
4
4-1
Section 4
Circuit Description
4.1 Ion Chamber
The ion chamber serves as the input transducer of the instrument. It responds to an ionizing radiation
field with the generation of a small direct current.
X or gamma radiation is comprised of photons, or electromagnetic packets of radiation. When the photon
strikes atoms of chamber wall material (or air), some or all of the photon energy is transferred into
ejecting a high-speed electron from one of the inner orbits of the target atom. This electron speeds off
through space striking the shells of other atoms, until all of the original high-speed electron energy is
converted into a long jagged track of ionized atoms. Collection potential, applied between the chamber
electrodes, collects the ions and electrons generated in the air volume before they can locally recombine
and delivers them to the external circuit. Due to electrode geometry and spacing, and the chamber
collection voltage, less than 0.1% of the ions are not collected at the maximum measurable exposure
rate.
The ion chamber behaves in a bandpass manner. This means it will respond evenly only for a specific
range of photon energies.
At very low energies, photons are unable to penetrate the chamber wall material, so there are no internal
high-speed electrons or ion chamber current. In practice, the wall is made of the thin, low-density
material necessary to achieve the desired low energy response.
At very high energies, the situation is reversed and a minimum wall thickness is necessary to prevent
high-speed electrons generated outside the chamber entering and adding spurious contributions to the
ion current. As a condition of measurement, all high-speed electrons are generated in the chamber wall
or the internal air volume. Therefore, above a certain energy (about 250 keV), additional wall material
must be added to insure non-response to outside electrons.
The electrometer preamplifier circuit receives the current from the ion chamber. At typical temperatures
and pressures, the ion chamber delivers about 5 x 10
-15 A when a uniform exposure rate of 1 mR/h
permeates the chamber volume.
An -18 V collection potential is supplied by two 9 V alkaline transistor batteries, connected in series. This
potential is always present in the circuit.
Refer to Figure 4-1 for a block diagram of the 440RF/D Exposure Rate Measuring System.
4.2 Probe
The electrometer and buffer amplifier are located in the probe assembly. The signal from the ion
chamber is applied to the electrometer and buffer amplifier. The amplified signal is applied to circuitry on
the analog board.
4.3 Analog Board
The analog board contains circuitry for two programmable gain amplifiers, an analog multiplexer, and + 5
V voltage regulators. (See Figure 4-2.)