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

Solution
HEALTH PHYSICS INSTRUMENTATION 479
Substituting the values for I1 = 0.88, I2 = 0.11, and d1 = 1 into Eq. (9.22), we
have
0.88
0.11 = d2 2
1 2
d2 = 2.83 m.
When applied to a point source (for practical purposes, a real source approximates
a “point” at distances from the source greater than 10 times the longest linear
dimension of the source) the inverse square law is strictly applicable only in a free
field, that is, in a field where there is no scattered radiation that can reach the detector.
If there is significant scattered radiation, then the fall-off of radiation intensity
with distance will be less than that predicted by the inverse square law. Allowance
for the scatter can be made in several ways. One method is to measure the radiation
at various distances from the calibration source with another calibrated instrument,
then plot the radiation level versus distance on log–log paper and calculate the slope
of the line. For example, if the slope is found to be –1.8, then instead of an inverse
square relationship, we have
I1
I2
= d1.8 2
d1.8 1
.
Under this condition, d2, the distance from the source in Example 9.9, would be
3.19 m.
Another method for accounting for scattered radiation is to make two measurements
at each calibration distance. One measurement is made with the detector in
the radiation beam, and the second reading is made with the direct beam blocked
out by a thick piece of lead, whose cross section is that of the detector, placed midway
between the detector and the source. The difference between the two readings is
due to the scattered radiation, and the appropriate correction can be made. When
properly calibrated, a gamma- or X-ray survey meter should measure the radiation
within ±10% about 95 times out of 100 measurements.
Beta Particles
Dose–response characteristics of most portable survey instruments for beta particles
are strongly energy dependent. As a consequence, survey instruments are used
mainly to detect beta radiation rather than to quantify the beta-radiation dose rate.
Such instruments are therefore not usually calibrated for beta dose. For those cases
where beta calibration is desired, sources of various surface areas are commercially
available. These sources, whose calibration is traceable to the NIST, may be calibrated
in transformations per minute per square centimeter for use in contamination monitoring
or in microsieverts per hour or millirems per hour at the surface and at several
distances above the surface. In external dosimetry, we are not concerned with measuring
dose from beta particles whose energy is less than 70 keV, because the range