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THE
ORCBS > Radiation
Safety
> Radiation
Safety Manual
>
 Monitoring
Instruments
Every laboratory
using radioactive materials must possess or have available for immediate
use appropriate radiation monitoring equipment. This equipment must
be in good working order, and must be calibrated yearly by the ORCBS
Health Physics staff. Results of this calibration will be forwarded
to the project leader. Equipment that has not passed this annual examination
must be removed from service until it is repaired or replaced. If
you believe that there is a problem with your equipment, contact the
ORCBS and arrange a time when the equipment can be inspected and calibrated.
Radioactive
monitoring instruments must be capable of detecting the radioisotope
being monitored at or below the contamination limits listed in the
section on Radiation Surveys. To calculate the sensitivity of the
survey instrument, the following formula may be used.
(Background
of instrument x 2) ÷ (Efficiency* for the isotope being measured)
= Minimum Detectable
Activity in DPM
*Note that efficiencies are given in the ORCBS calibration results.
There are several
types of monitoring instruments commonly used in teaching and research
laboratories. The most widely used instrument is the Geiger counter,
a portable instrument capable of detecting beta or gamma radiation,
providing the appropriate detector is used. The Geiger counter is
the least expensive, fastest and generally the most reliable means
of detecting and measuring radioactive contamination.
The beta pancake
detector is used with the Geiger counter for finding and measuring
beta radiation, and will detect all beta radioisotopes used at Michigan
State University except 3H and 63Ni. It does
not detect those nuclides because their betas are too low in energy
to penetrate the window of the detector. Radioisotopes which may
be detected reliably with the beta pancake are 14C, 35S,
33P, 32P, 45Ca, 36Cl,
and other beta emitting nuclides.
The low energy
gamma (LEG) probe is used with the Geiger counter to detect and
measure gamma radioisotopes of various energies. It is most efficient
for 125I, but will perform adequately for 51Cr,
111In, 60Co and other gamma emitting nuclides.
These detectors will also detect low energy x-rays, such as those
emitted by beta emitters producing Bremsstrahlung radiation.
Another instrument
in common use is the liquid scintillation counter. It is necessary
to use it in radiation safety surveys for 3H and 63Ni,
since no other instrument will detect these nuclides. Liquid scintillation
counters work for both beta and gamma nuclides for quantifying what
is in a sample. It is not an adequate primary method of evaluating
contamination surveys, however, since samples measured consist of
wipes of the areas of suspected contamination. If the contamination
is not removable, the wipe will not pick it up, and contamination
will not be detected. It is also possible for only part of
the contamination present to come up on a wipe, not giving an accurate
measurement of the contamination present.
A third instrument
which may be used to evaluate contamination is the gamma well counter.
Again, this is used to gather data in samples, but for the same
reasons as the liquid scintillation counter, it is not a good radiation
survey instrument.
Ion chambers
are used commonly by the Radiation Safety staff and in locations
where frequent and higher flux external radiation hazards are present;
they are typically not used for contamination surveys by laboratory
staff. These instruments measure the ions produced in air (of one
sign) by gamma radiation, and are a good indicator of radiation
exposure fields. They are useful for exposure potential screening
on shipments, hot parts at the Cyclotron, drums of waste at the
Radioactive Waste Building, packages prior to shipment and sources
and stocks of radioactive materials.
Other more sophisticated
instruments used to detect and quantify radiation are the gamma
spectrometer or multi-channel analyzer, neutron detectors, alpha
detectors, and a wide array of electronic dosimeters, area monitors,
and even portal monitors (which a person walks through to detect
any contamination on the body or clothing; these are used at reactors).
For effective
and accurate data gathering in radiation, follow a few simple guidelines:
- Survey at
the proper geometry. Hold the detector about 1 cm. or 1/2 inch
above the surfaces monitored. If the detector is too far away,
serious underestimation of activity or no detection of activity
present may occur. If the detector is too close, contamination
of the detector may occur.
- Use the correct
detector. Do not survey for beta radiation with a gamma probe,
or for gamma radiation with a beta probe. NO GEIGER COUNTER WILL
DETECT TRITIUM; LIQUID SCINTILLATION TECHNIQUES MUST BE UTILIZED.
- Survey slowly;
do not race the detector over the surface or wave it like a magic
wand; the sensitivity of the detection is inversely proportional
to increasing survey speed.
- Do not cover
the detector while surveying; covers decrease or eliminate detection,
since they act as a shield.
Calibrations
Radiation
Safety Manual Table of Contents
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