11.7. Radiation Protection 667
Problems
1.In laboratory environments, radioactive samples are tagged with their initial
activities. If considerable time has elapsed as compared to its half life, the
current activity of a sample should be computed using the decay rate equation.
This and the next example would refresh the reader’s mind as to how simply it
can be done.A radioactive sample having a half life of 20.6 days has an initial
activity of 80μCi. What would be its activity after 200 days?2.The half life of a radioactive sample is found to be 21.5 days. How long would
it take for its activity to decrease by 70%?3.A radiation worker receives an exposure of 25Rafter working for 3 hours at a
distance of 5.5cmfrom a cobalt-60 source. Compute the activity of the source.4.If the worker of the previous exercise had to work at the same distance for the
same amount of time in front of a cesium-137 source, estimate the activity of
the source if in the end he received the same amount of exposure as in the case
of cobalt-60 source.5.In a radiation therapy setup it was assumed that the incident radiation beam
was circular with a radius of 0.3mm. However the actual beam was rectangu-
lar. If one side of the rectangle was 0.05mm, compute the other side. What
would be the area of an equivalent square beam?6.Compute the energy released after the capture of thermal neutrons by oxygen-
12 and carbon-14 nuclei.7.Estimate the dose absorbed by a soft tissue if an internal source of thermal
neutrons remains in a soft tissue for 24 hours. Assume that the source emits,
on the average, 10^3 neutrons/cm^2 s−^1.8.A patient is exposed to 0.02nmwavelength photons for a period of 2.5 seconds.
If 6. 7 × 106 photons strike the patient per second, compute the particle and
energy fluence rates.9.Calculate the exposure rates from a 30mCicobalt-60 source at distances of
10 cmand 100cm.10.For the exposure computed in the previous exercise, determine the air Kerma.
Assume the quality factor to be 0.01.11.In a laboratory experiment, a 20mCiradioactive source emittingγ-rays is to
be shielded. Estimate the width of lead blocks needed to shield the source such
that the exposure rate falls to 5% of its value at 10cmin air.12.If the mass attenuation coefficient of lead forγ-rays emitted by^241 Am(60
keV)is50cm−^1 , calculate the half- and tenth-value layers.13.Suppose a GM counter having an efficiency of 15% measures a count rate of
3500 counts per minute in a radiation environment. Determine the activity it
would correspond to if the measurement is made at a distance of 1.6mfrom
the source.