Instruments used to detect radioactive material
must be calibrated at least annually, and often more frequently.
This includes all portable rate meters (otherwise known as
'Geiger Counters') and dose-rate meters (or Ion Chambers). Liquid
scintillation counters and Gamma counters are not subject to the
same rules, but must be normalized on a regular basis. Without
these procedures the data produced by the instruments can not be
trusted. While the requirement for calibration is critical, where
the calibration takes place is not.
Register an Instrument with Radiation Safety
The UW Radiation Safety Office (RSO) operates an
instrument calibration facility (call 206.543.6328 or e-mail: firstname.lastname@example.org for more
information). Costs of meter calibration at the UW facility are
comparable to other calibration facilities. Some advantages to
using the UW calibration facility are:
- Shortened turn-around time.
- Avoidance of shipping charges for on-campus users.
It is not required that the RSO calibrate your instruments.
Calibrations may be performed by any qualified agency or by the
instrument owner, provided that it can be demonstrated that the
calibration is performed correctly.
To insure compliance with state regulations:
- All survey instruments must be calibrated at least
- A record of the results and the method of calibration must be
kept on file in the lab. Records shall be kept for a minimum of five years.
- The records must be made available to the RSO or the
Washington State Department of Health upon request.
If the Radiation Safety Office does not calibrate your
instruments, Washington state law and the UW's
Radioactive Materials License requires that they be provided with copies of
your calibration records. Call 206.543.6328 or e-mail: email@example.com for more
Instrument sensitivity is frequently raised as an
issue when instruments are calibrated. Your instrument should be
sensitive enough to detect the following radiation levels:
- 10 nCi at a distance of 1 cm from the surface for Group I radionuclides.
- 1 nCi at a distance of 1 cm from the surface for Group II radionuclides.
- 1 nCi at a distance of 1 cm from the surface for Group III and above radionuclides.
(See the Radionuclide Hazard Groups
Table for a list of radionuclides classified into Hazard
Radiation detection instruments detect
radioactive emissions by various processes. Without going into
too much detail, the fact is that they are capable of detecting
only a percentage of the emissions that they may encounter. For
example, if a source of radiation is emitting 100,000 particles
in one minute, the detection instrument might only see .05%, or
50 particles in that minute. The efficiency varies from
instrument to instrument and from radionuclide to radionuclide;
.05% is a fair generalization for portable rate meters. Liquid
scintillation counters produce efficiencies of between 50% and
90% for the radionuclides for which they are designed.
Since the instruments see only a portion of the
available detectable particles it is important to be able to
infer the correct number from the observed number. Using the
above example, you might infer that a reading of 50 observed
counts in one minute means that there were really about 100,000
What calibration does is measure
the instrument's response to a known number of counts. The first
part involves hooking the instrument to a pulsar, a device which
sends a discrete number of electronic pulses to the instrument.
The instrument's response is measured at several different levels
(i.e. 100, 1000, 10,000 pulses) and compared to make sure that
the observed counts are within 10% of the known counts. If so,
the instrument is considered to be in calibration. The instrument
is then exposed to different radioactive sources of known
activity to determine efficiencies for each.
Ion chambers are calibrated differently since it
is not the number of counts observed by them that is important
but their dose response to a known radiation dose. The
calibration is achieved by exposing the instrument to the
radiation produced by a large source of known activity at several
precisely measured distances. The ion chamber will produce a
reading at each distance which is compared to the dose that
actually is being given off by the source at each of those
distances. If the response is within 10% at every distance, then
the instrument is considered to be in calibration. If not,
adjustments are made until the instrument reads in the correct