3.6 Reaction Cross-Section
41
specific activity of an isotope, thus, produced will be constant, and independent of
time of irradiation. This suggests that maximum specific activity of the product can
be obtained by irradiation (for a given amount of target material) for a period of 5–6
times the half-life of the expected radioactive isotope (which is to be produced by the
nuclear reaction). It is also obvious from Eq. (3.4) that the activity of atom B can be
increased by increasing weight “w” of the target. Alternatively, a high specific activity
of atom B can be obtained by increasing the neutron flux. It must be remembered
that with a long-lived isotope, it is not possible to irradiate material for a long time
in the neutron source, because it will become very expensive to produce long-lived
isotopes, e.g., for the isotope of half-life greater than few days or impossible for
isotope with a half-life of a few years. With these types of materials, either neutron
flux or amount of material or both are increased. Alternatively, for such cases, the
Szilard–Chalmers process may be adopted to get high specific activity in a relatively
shorter duration of irradiation. This aspect has been dealt with in a separate chapter.
If there are possibilities of the formation of more than one product during the
nuclear reaction and the half-life of each radioactive isotopes is different, then the
production of unwanted long-lived isotopes can be controlled by controlling the time
of irradiation.
3.7 Some Features of Nuclear Reactions
In the study of nuclear reactions with any target material, it is advantageous to
consider the following factors.
3.7.1 The Purity of the Target Material
If the target material is not isotopically pure, the products formed may be a mixture
of the various nuclei (which in fact depends upon the cross-section and the isotopic
abundance of each species), and the study of the reaction and the products formed
becomes a mixture of products.
3.7.2 Conservation of Mass Relationship
The study of masses of reactants and the product involved in the nuclear reaction
can sometimes help to rule out the possibility of the occurrence of certain reactions.
For example, in the case of Bismuth-209 (mentioned earlier), the α-particle must
have at least 20 MeV energy in order to give Astatine-211. In other words, Astatine211 cannot be formed from Bismuth-209 by the nuclear reaction with helium ions
of energy less than 20 MeV. A calculation based on the conservation of masses of
41
specific activity of an isotope, thus, produced will be constant, and independent of
time of irradiation. This suggests that maximum specific activity of the product can
be obtained by irradiation (for a given amount of target material) for a period of 5–6
times the half-life of the expected radioactive isotope (which is to be produced by the
nuclear reaction). It is also obvious from Eq. (3.4) that the activity of atom B can be
increased by increasing weight “w” of the target. Alternatively, a high specific activity
of atom B can be obtained by increasing the neutron flux. It must be remembered
that with a long-lived isotope, it is not possible to irradiate material for a long time
in the neutron source, because it will become very expensive to produce long-lived
isotopes, e.g., for the isotope of half-life greater than few days or impossible for
isotope with a half-life of a few years. With these types of materials, either neutron
flux or amount of material or both are increased. Alternatively, for such cases, the
Szilard–Chalmers process may be adopted to get high specific activity in a relatively
shorter duration of irradiation. This aspect has been dealt with in a separate chapter.
If there are possibilities of the formation of more than one product during the
nuclear reaction and the half-life of each radioactive isotopes is different, then the
production of unwanted long-lived isotopes can be controlled by controlling the time
of irradiation.
3.7 Some Features of Nuclear Reactions
In the study of nuclear reactions with any target material, it is advantageous to
consider the following factors.
3.7.1 The Purity of the Target Material
If the target material is not isotopically pure, the products formed may be a mixture
of the various nuclei (which in fact depends upon the cross-section and the isotopic
abundance of each species), and the study of the reaction and the products formed
becomes a mixture of products.
3.7.2 Conservation of Mass Relationship
The study of masses of reactants and the product involved in the nuclear reaction
can sometimes help to rule out the possibility of the occurrence of certain reactions.
For example, in the case of Bismuth-209 (mentioned earlier), the α-particle must
have at least 20 MeV energy in order to give Astatine-211. In other words, Astatine211 cannot be formed from Bismuth-209 by the nuclear reaction with helium ions
of energy less than 20 MeV. A calculation based on the conservation of masses of