Download Quality Control Procedures Applied to Nuclear

3.7. Pre calibration checks
Radiation survey meters should be calibrated with a radioactive source. Electronic calibration
alone is not acceptable [3]. The following items should be observed before exposing the
instrument to a source for adjustment and calibration: the instrument should be free of
significant radioactive contamination; the meter should be adjusted to zero or a point specified
by the manufacturer using adjustments provided; the batteries or power supply should comply
with manufacturers specification for the instrument; the instrument should be turned on and
allowed to warm up for the period specified by the manufacturer; electronic adjustments such
as high voltage should be set, as applicable, to the manufacturer's specifications; geotropism
should be known for orientation of the instrument in the three mutually perpendicular planes,
and this effect should be considered during calibration and performance testing; the
performance of any internal sampling time base in digital readout instruments should be
verified as being within the manufacturer's specification.
3.8. Test instruments
The test procedures for radiation survey meters should consider the use of the following
instruments: oscilloscope, pulse generator, frequency meter, power supplies (bench and HV
power supplies), digital multimeter and counter/timer.
3.9. Test conditions
Test conditions should consider the background radiation, temperature and radioactive source
employed.
4. TEST CIRCUITS
Manufacturer specifications and recommendations must always be followed. If not available
some test circuits can be adopted for use in testing different types of survey meters. Refer to
standard ANSI/IEEE test procedures [4] or NPL guide 14 (1999) for specific radiation
detectors.
4.1. Instruments based in ionization chamber detectors
Radiation detectors consist of a chamber filled with air or gas, in which an electric field inside
the detector is applied for the collection of charges associated with ions and electrons
produced in the measuring volume of the detector by the ionizing radiation. An electrometer
is used to measure very small electrical currents (in the range from 10-8 A to 10-15 A) or small
electrical charges (in the range from 10-12 C to 10-15 C).
Sensitivity in ionization chamber detectors is the ratio between the current produced by an
ionization chamber and the exposure rate, given for a radiation source. The isotope employed
must be specified.
Fig. 2 shows some of the setups for testing ionization chambers [5].
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