14.2 Szilard-Chalmers Reactions
219
Scientists have been enquiring as to why only a very few percentage of total
activity was available in the form, different from the parent compounds. Because
one would expect to get 100% bond rupture in the nuclear recoil process. There have
been several proposed mechanisms to explain the process by which the recoil atoms
recombine with the target molecules. According to Libby, when the halogen atom of
an alkyl-halide captures a neutron, it recoils with an energy considerably in excess
of the chemical bond energy. It loses energy, largely by elastic collision (no transfer
of energy) most effectively when it hits an atom of similar mass. If sufficient energy
is lost from the radioactive atom during collision, the struck molecule gets disrupted
and if the radioactive atom has insufficient energy after the collision to escape from
the surrounding molecules (the so-called molecular cage), it is held in the vicinity
of the molecular residue with a high probability of combination (i.e., retention of
radioactive nuclide results). However, if the energy of the radioactive nuclide after
collision is sufficiently large to escape from the cage, but not large enough to produce
further dissociation, it will not be able to combine after another collision and it will
emerge as a free halogen atom unless the side reaction occurs. Free halogen atom
will then combine to form halogen molecules.
If an alkyl-halide is irradiated in the gas phase, a very low retention value is
obtained supports the above statement, because the energy required to escape from the
surrounding molecules in the gas at a site of collision is much less. Even by diluting
the material with the solvent, the retention can be decreased, since the molecules are
dispersed further away from each other.
In general, the Szilard-Chalmers reactions can satisfactorily be observed provided:
1. The recoil energy is greater than the chemical bond energy,
2. The radioactive atoms combine with the parent molecules inefficiently i.e., the
retention value is low.
3. Chemical exchange does not take place between the stable and the radioactive
species. If there is an exchange, the retention of activity will be appreciable. For
example, exchange of stable bromine with the radioactive bromine in aromatic
nitro halogen derivatives may result in a loss in the recovery of free radioactive
halogen molecules produced in this reaction.
4. There is a suitable method for the separation of the radioactive nuclide existing
in a different chemical state to the parent molecules.
14.2.1 Szilard-Chalmers Reaction with Organic Substances
A considerable amount of work has been done on the Szilard-Chalmers reaction with
organic molecules, perhaps because the radioactive atom can be separated easily
from the parent one. An interesting effect of this reaction is the production of labeled
compounds with a high specific activity, such as bromobenzene, dichlorobenzene etc.
This is very important because the production of high specific activity of a long-lived
nuclide is not possible by ordinary irradiation methods. For example, 10% of the total
219
Scientists have been enquiring as to why only a very few percentage of total
activity was available in the form, different from the parent compounds. Because
one would expect to get 100% bond rupture in the nuclear recoil process. There have
been several proposed mechanisms to explain the process by which the recoil atoms
recombine with the target molecules. According to Libby, when the halogen atom of
an alkyl-halide captures a neutron, it recoils with an energy considerably in excess
of the chemical bond energy. It loses energy, largely by elastic collision (no transfer
of energy) most effectively when it hits an atom of similar mass. If sufficient energy
is lost from the radioactive atom during collision, the struck molecule gets disrupted
and if the radioactive atom has insufficient energy after the collision to escape from
the surrounding molecules (the so-called molecular cage), it is held in the vicinity
of the molecular residue with a high probability of combination (i.e., retention of
radioactive nuclide results). However, if the energy of the radioactive nuclide after
collision is sufficiently large to escape from the cage, but not large enough to produce
further dissociation, it will not be able to combine after another collision and it will
emerge as a free halogen atom unless the side reaction occurs. Free halogen atom
will then combine to form halogen molecules.
If an alkyl-halide is irradiated in the gas phase, a very low retention value is
obtained supports the above statement, because the energy required to escape from the
surrounding molecules in the gas at a site of collision is much less. Even by diluting
the material with the solvent, the retention can be decreased, since the molecules are
dispersed further away from each other.
In general, the Szilard-Chalmers reactions can satisfactorily be observed provided:
1. The recoil energy is greater than the chemical bond energy,
2. The radioactive atoms combine with the parent molecules inefficiently i.e., the
retention value is low.
3. Chemical exchange does not take place between the stable and the radioactive
species. If there is an exchange, the retention of activity will be appreciable. For
example, exchange of stable bromine with the radioactive bromine in aromatic
nitro halogen derivatives may result in a loss in the recovery of free radioactive
halogen molecules produced in this reaction.
4. There is a suitable method for the separation of the radioactive nuclide existing
in a different chemical state to the parent molecules.
14.2.1 Szilard-Chalmers Reaction with Organic Substances
A considerable amount of work has been done on the Szilard-Chalmers reaction with
organic molecules, perhaps because the radioactive atom can be separated easily
from the parent one. An interesting effect of this reaction is the production of labeled
compounds with a high specific activity, such as bromobenzene, dichlorobenzene etc.
This is very important because the production of high specific activity of a long-lived
nuclide is not possible by ordinary irradiation methods. For example, 10% of the total