Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

Response Normalized to Cs-137
10
Co-57
0.1
10 100
HEALTH PHYSICS INSTRUMENTATION 449
(Shield Open) (Shield Closed)
Ba-133
Am-241 Am-241
Cs-137
Co-60
Co-57
Ba-133
1000 10 100 1000
Gamma Energy (keV) Gamma Energy (keV)
Cs-137
Figure 9-18. Energy response curves for a Geiger-Müller counter whose wall is 30-mg/cm 2 -thick stainless steel.
The tube is in a cylindrical probe with a sliding beta shield. The figure on the left shows the instrument’s response
to photons of various energies with the shield open, and the curve on the right shows its response with the
shield closed. (Courtesy of Ludlum Measurements, Inc, Sweetwater, TX.)
Personal Monitoring
Co-60
The International Commission on Radiological Protection (ICRP) recommends
monitoring individual radiation workers for external exposure unless it is clear that
their doses will be consistently low. The International Atomic Energy Agency (IAEA),
in its Basic Safety Standards, interprets this to mean that workers whose occupational
dose may exceed 30% of the regulatory limit should be monitored. The regulations
in the United States require personal monitoring if the worker’s annual dose is likely
to exceed 10% of the regulatory limit. Individual monitoring for external radiation is
relatively simple. The worker wears a dosimeter, and the dose is tracked on the basis
of the individual readings of the dosimeter. The most widely used personal monitoring
devices include pocket dosimeters, film badges, thermoluminescent dosimeters,
opticoluminescent dosimeters, and electronic dosimeters.
The U.S. Nuclear Regulatory Commission (NRC) not only requires the monitoring
of licensees, but also that the personal dosimetry devices used to demonstrate
compliance with NRC regulations be processed by laboratories or commercial services
that have been accredited by the National Voluntary Laboratory Accreditation
Program (NVLAP) of the National Institute of Standards and Technology (NIST).
Earlier studies showed a great deal of disagreement between doses reported by various
commercial and in-house monitoring organizations and the actual doses delivered
to these test dosimeters. This concern about the reliability of the reported
doses led to the establishment, in 1984, of the NVLAP by the National Bureau of