1.6. General Properties and Sources of Particles and Waves 59
17.How many mean lives should pass for a radioactive substance to decay 99.9%
of its atoms.18.The activity of a radioactive sample is found to be 47Bq. Suppose that the
half life of the material is 4 days. Compute its activity 2 days before and 2
days after the measurement was made.19.The activity of a radioactive material composed of only one type of material is
monitored through a detector. The detector is configured in such a way that
each output count represents one decay. If the detector count rate decreases
from an initial value of 20,000 counts per minute to 3,000 counts per minute
after 3 days, estimate the mean life of the element.
20.Estimate the number of thorium-232 atoms present in a sample that also con-
tains 2. 5 × 1015 atoms of its daughter radium-228. Assume the two isotopes to
be in secular equilibrium.21.A1mCi sample of pure thorium-227 goes through the following series of
decays:
227
90 Th(T 1 / 2 =18.5days) →
223
88 Ra(T 1 / 2 =11.4days)→
211
82 Pb(T^1 /^2 =36.1min)→211
83 Bi(T^1 /^2 =2.1min)→207
81 Tl(T^1 /^2 =4.8min)Compute the activities of its first four decay products after 10 days.22.How much uranium-238 is needed to to have an activity of 3.5μCi.(Takethe
half life of uranium-238 to be 4. 47 × 109 years.)