Theory for the Szilard-Chalmers Reaction

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As is shown in the discussion of recoil energies in thermal n-capture reactions, chemical bounds can be broken due the recoil energy caused by the prompt γ-emission. This will lead to the creation of free radicals, which is very reactive. Therefore, a long list of new componds can be formed during n capture.

In 1934 L. Szilard and T. A. Chalmers showed that when Ethyliodide is irradiated with thermal neutrons, a large fraction of the radioactive nuclei created in the process will be present as free iodine atoms or iodine ions.

The bond breakage can typically lead to the following reacions:

[math]1) \quad C_2H_5 \, ^{127}\!I + ^{128}\!I \rightarrow C_2H_5 \cdot + ^{127}\!I \cdot + ^{128}\!I \cdot[/math]

[math]2) \quad ^{127}\!I \cdot + ^{128}\!I \cdot \rightarrow ^{127}\!I ^{128}\!I[/math]

[math]3) \quad C_2H_5 \cdot + ^{128}\!I \cdot \rightarrow C_2H_5 ^{128}\!I[/math]

[math]4) \quad C_2H_5 ^{127}\!I + ^{128}\!I \cdot \rightarrow C_2H_5 ^{128}\!I + ^{127}\!I \cdot[/math]

[math]5) \quad C_2H_5 ^{127}\!I + ^{128}\!I \cdot \rightarrow CH_2 ^{127}\!I CH_2 ^{128}\!I + H \cdot[/math]

[math]6) \quad C_2H_5 ^{127}\!I + ^{128}\!I \cdot \rightarrow CH_2 ^{127}\!I CH_3CH ^{127}\!I ^{128}\!I + H \cdot[/math]

[math]7) \quad ^{128}\!I \cdot + H \cdot \rightarrow H^{128}I[/math]

These reactions are listed according to diminising kinetic energy on the reaction products. Reaction 3) to 6) represent organically bound iodine. This part is named the retention. It denotes radionuclei which cannot easily be separated from the inactive iodine.

The substitutions reactions 5) and 6) represent a method for radio labeling a compound.

The reactions 2) and 7) represent ways to produce inorganic iodine.

The Szilard-Chalmers Method

The idea behind the Szilard-Chalmers method is to produce a carrier-free source. All the various inorganic iodine and iodine compounds in the reaction mixture will have been formed from free-radical reactions after the n irradiation. Therefore, it only contains tracer amounts of the iodine isotopes (127I and 128I). The bulk of the non-radioactive iodine will be organic.

If we have a (chemical) method to separate organic and inorganic iodine, the inorganic part will be carrier free. I.e. under these special conditions we are able to perform isotope separation by chemical means.

In general, this method for producing carrier free amounts of radioactivity can be used provided the following conditions are met:

  1. In the nuclear reaction the radioactive atom created must receive a big enough recoil energy to break the chemical bound.
  2. The likelihood of the free radical to return to it's original compound must be small.
  3. We must have a chemical separation-method which can separate the created radioactive compounds or atoms from the initial compound.

Compounds which are well suited as targets for Szilard-Chalmers reactions is e.g. organic halides and inorganic ions of the type MnO4-, PO43-, ClO4-, IO3-, IO4-, BrO3-, etc.