218
14 Hot Atom-Nuclear Reaction
radioactive nuclide thus formed is an isotope of the target element and is in a free
state, which can be separated easily. Ethyl iodide was the first compound studied by
them. From the irradiated ethyl iodide, Iodine-128 was separated in the aqueous phase
with a high specific activity by using chloroform and sodium thiosulfate solution as
immiscible solvent (i.e., by solvent extraction technique). This reaction is named
after the discoverers, the Szilard-Chalmers reaction.
The mechanism of this reaction can be explained by considering the laws of
conservation of momentum. When a nucleus undergoes an (n, γ ) reaction, the γ -
rays (usually referred to as the γ -rays of capture) are emitted within the time of
the order of 10 picosecond of neutron capture. The energy of the γ -rays equals the
binding energy of a neutron within the nucleus and is usually of the order of 7–8
MeV. To conserve momentum, the nucleus must recoil with an energy which can be
shown by a simple calculation to be, in general, greater than the binding energy of
atoms in molecules.
The energy of γ -photon (E γ in MeV) is given by hv, where h is a Planck’s
constant and v is the frequency of the photon. Momentum of the recoiling nucleus
( p) is equal to the momentum of the photon and may be given as
p = mv =
hv
c
=
E γ
c
(14.1)
The energy of recoil E R , is therefore
E R =
1
2
(mv
2
) =
p
2
2m
=
E
2
γ × 536
M
eV
(14.2)
where m, c, M and 536 are mass of recoiling nucleus, velocity of light, atomic mass
unit of recoiling atom and a conversion factor of units which also takes into account
the velocity of light, respectively.
From this equation, the recoil energy E R can be calculated, provided the energy of
γ -photon is known. For example, the energy of the emitted γ -photon from bromine
atom being 5.1 MeV, the recoil energy E R from the Eq. (14.2) comes to 178 eV. A
comparison of this recoil energy with the chemical bond energy (which is about 1–5
eV) shows that the separation of the radioactive nuclide thus formed is likely to be
complete. However, in practice, usually only about 50–60% of the total activity is
separable and the rest are found to be present in their original chemical form. The
amount of activity, which remained with the parent chemical form, is also called the
retention activity.
14 Hot Atom-Nuclear Reaction
radioactive nuclide thus formed is an isotope of the target element and is in a free
state, which can be separated easily. Ethyl iodide was the first compound studied by
them. From the irradiated ethyl iodide, Iodine-128 was separated in the aqueous phase
with a high specific activity by using chloroform and sodium thiosulfate solution as
immiscible solvent (i.e., by solvent extraction technique). This reaction is named
after the discoverers, the Szilard-Chalmers reaction.
The mechanism of this reaction can be explained by considering the laws of
conservation of momentum. When a nucleus undergoes an (n, γ ) reaction, the γ -
rays (usually referred to as the γ -rays of capture) are emitted within the time of
the order of 10 picosecond of neutron capture. The energy of the γ -rays equals the
binding energy of a neutron within the nucleus and is usually of the order of 7–8
MeV. To conserve momentum, the nucleus must recoil with an energy which can be
shown by a simple calculation to be, in general, greater than the binding energy of
atoms in molecules.
The energy of γ -photon (E γ in MeV) is given by hv, where h is a Planck’s
constant and v is the frequency of the photon. Momentum of the recoiling nucleus
( p) is equal to the momentum of the photon and may be given as
p = mv =
hv
c
=
E γ
c
(14.1)
The energy of recoil E R , is therefore
E R =
1
2
(mv
2
) =
p
2
2m
=
E
2
γ × 536
M
eV
(14.2)
where m, c, M and 536 are mass of recoiling nucleus, velocity of light, atomic mass
unit of recoiling atom and a conversion factor of units which also takes into account
the velocity of light, respectively.
From this equation, the recoil energy E R can be calculated, provided the energy of
γ -photon is known. For example, the energy of the emitted γ -photon from bromine
atom being 5.1 MeV, the recoil energy E R from the Eq. (14.2) comes to 178 eV. A
comparison of this recoil energy with the chemical bond energy (which is about 1–5
eV) shows that the separation of the radioactive nuclide thus formed is likely to be
complete. However, in practice, usually only about 50–60% of the total activity is
separable and the rest are found to be present in their original chemical form. The
amount of activity, which remained with the parent chemical form, is also called the
retention activity.