There are two methods for monitoring surface contamination . One is the direct method, which measures both the fixed and loose contamination level, and the other is the smear method , which measures the loose contamination only.
For both methods a contamination monitor , such as
the RM-14 with an HP-210
detector or a Ludlum 177 with 44-9 detector,
may be used. Both of these detectors are pancake type Geiger
tubes with very thin mica windows
(2 mg cm 
). They are sensitive to beta
particles , and will detect beta
particles with energy greater than 40 keV. They can also ddetect energetic
alpha particles
and gamma-rays , however
the efficiency will be very different
for different radionuclides . Low-energy gamma
emitters are best detected with the LEG probe.
The counting efficiency also depends on how close to the contaminated surface
the detector is held. Fig. 9.1 
shows
the count rate indicated
by a pancake detector at different distances from a low energy beta source of
about the same dimension as the detector.
Figure 9.1: Effect on count rate of distance