Problem
227
(b) 1.25 g of calcium iodide was irradiated in a nuclear reactor for two hours.
Calculate the activity of the Iodine-128 isotope produced. What would be
the specific activity of this isotope?
Given: cross-section for Iodine-128 is 6.4 barns, thermal neutron flux is
3.0 × 10
11 neutron cm
−2 s
−1 , and half-life of Iodine-128 is 26.5 min.
42. Explain the principle of a scintillation counter. Describe how the pulse height
analyzer is used together with the scintillation counter to measure the energy of
γ -rays.
43. To a crude mixture of organic compounds containing some benzoic acid and
bezoate 40.0 mg of benzoic acid labeled with
14 C (activity 2000.0 cpm) was
added. After equilibrium, the mixture was extracted with immiscible solvents.
The extracted solid, following the removal of the solvent, was purified by the
recrystallization of the benzoic acid to a constant melting point. The purified
material weighed 60.0 mg and gave a count rate of 500 cpm. Compute the
weight of benzoic acid in the mixture.
44. How does an ionization counter differ from a scintillation counter? Discuss in
five lines.
45. What are the basic differences among a G.M. counter, a proportional counter,
and an ionization counter?
46. Which counter will you select to count the activity of a liquid sample giving
100% of energy 0.55 MeV as well as 100% particles of E max 0.55 MeV and
why?
47. How can a mixture of radioactive sample emitting an α-particle and a β-particle
of the same energy (0.98 MeV) be counted by a proportional counter without
performing any chemical separation while it is not possible with an end-window
mica G.M. counter? Explain and discuss.
48. Why does a G.M. counter need no amplification of signals as needed in a proportional counter? Why is a pulse height analyzer not used in a G.M. counter to
differentiate high energetic β-particles from γ -rays? Explain.
49. Why does one find out a plateau in an ionization-type counter and why does
a proportional counter give two plateaus, one for α-particles and another for
β-particles?
50. What information do you get from the plateau of a G.M. counter? A G.M. counter
has a dead time of 450 ms. A Cesium-137 isotope was counted for 10 min and
it gave an activity of 10,000 counts. Calculate the % loss of activity with this
G.M. counter.
51. What information regarding nuclear energy levels is obtained by the measurement of the energies of β-particles and γ -radiation in a β-decay? How does it
help in understanding the decay scheme of a radioactive isotope?
52. A
27 Mg isotope undergoes β-decay and in about 70% of the disintegration, the
β-particles have E max of 1.78 MeV while in about 30% of the disintegration the
E max is 1.59 MeV. γ -rays of energies 0.834 MeV, 1.015 MeV, and 0.181 MeV are
also observed to be emitted during the disintegration. Construct a decay scheme
consistent with these observations.
227
(b) 1.25 g of calcium iodide was irradiated in a nuclear reactor for two hours.
Calculate the activity of the Iodine-128 isotope produced. What would be
the specific activity of this isotope?
Given: cross-section for Iodine-128 is 6.4 barns, thermal neutron flux is
3.0 × 10
11 neutron cm
−2 s
−1 , and half-life of Iodine-128 is 26.5 min.
42. Explain the principle of a scintillation counter. Describe how the pulse height
analyzer is used together with the scintillation counter to measure the energy of
γ -rays.
43. To a crude mixture of organic compounds containing some benzoic acid and
bezoate 40.0 mg of benzoic acid labeled with
14 C (activity 2000.0 cpm) was
added. After equilibrium, the mixture was extracted with immiscible solvents.
The extracted solid, following the removal of the solvent, was purified by the
recrystallization of the benzoic acid to a constant melting point. The purified
material weighed 60.0 mg and gave a count rate of 500 cpm. Compute the
weight of benzoic acid in the mixture.
44. How does an ionization counter differ from a scintillation counter? Discuss in
five lines.
45. What are the basic differences among a G.M. counter, a proportional counter,
and an ionization counter?
46. Which counter will you select to count the activity of a liquid sample giving
100% of energy 0.55 MeV as well as 100% particles of E max 0.55 MeV and
why?
47. How can a mixture of radioactive sample emitting an α-particle and a β-particle
of the same energy (0.98 MeV) be counted by a proportional counter without
performing any chemical separation while it is not possible with an end-window
mica G.M. counter? Explain and discuss.
48. Why does a G.M. counter need no amplification of signals as needed in a proportional counter? Why is a pulse height analyzer not used in a G.M. counter to
differentiate high energetic β-particles from γ -rays? Explain.
49. Why does one find out a plateau in an ionization-type counter and why does
a proportional counter give two plateaus, one for α-particles and another for
β-particles?
50. What information do you get from the plateau of a G.M. counter? A G.M. counter
has a dead time of 450 ms. A Cesium-137 isotope was counted for 10 min and
it gave an activity of 10,000 counts. Calculate the % loss of activity with this
G.M. counter.
51. What information regarding nuclear energy levels is obtained by the measurement of the energies of β-particles and γ -radiation in a β-decay? How does it
help in understanding the decay scheme of a radioactive isotope?
52. A
27 Mg isotope undergoes β-decay and in about 70% of the disintegration, the
β-particles have E max of 1.78 MeV while in about 30% of the disintegration the
E max is 1.59 MeV. γ -rays of energies 0.834 MeV, 1.015 MeV, and 0.181 MeV are
also observed to be emitted during the disintegration. Construct a decay scheme
consistent with these observations.