Variable fallout effects can be obtained by using different salting isotopes. Gold has been proposed for short-term fallout (days), tantalum and zinc for fallout of intermediate duration (months), and cobalt for long term contamination (years). To be useful for salting, the parent isotopes must be abundant in the natural element, and the neutron-bred radioactive product must be a strong emitter of penetrating gamma rays.

Table 1.6-1 Candidate Salting Agents

Parent        Natural   Radioactive  Half-Life 
Isotope      Abundance    Product

Cobalt-59     100%        Co-60      5.26 years
Gold-197      100%        Au-198     2.697 days
Tantalum-181   99.99%     Ta-182     115 days
Zinc-64        48.89%     Zn-65      244 days

The idea of the cobalt bomb originated with Leo Szilard who publicized it in Feb. 1950, not as a serious proposal for weapon, but to point out that it would soon be possible in principle to build a weapon that could kill everybody on earth (see Doomsday Device in Questions and Answers). To design such a theoretical weapon a radioactive isotope is needed that can be dispersed world wide before it decays. Such dispersal takes many months to a few years so the half-life of Co-60 is ideal.

The Co-60 fallout hazard is greater than the fission products from a U-238 blanket because

1. many fission-produced isotopes have half-lives that are very short, and thus decay before the fallout settles or can be protected against by short-term sheltering;
2. many fission-produced isotopes have very long half-lives and thus do not produce very intense radiation;
3. the fission products are not radioactive at all.
   The half-life of Co-60 on the other hand is long enough to settle out before significant decay has occurred, and to make it impractical to wait out in shelters, yet is short enough that intense radiation is produced.

   Initially gamma radiation fission products from an equivalent size fission-fusion-fission bomb are much more intense than Co-60: 15,000 times more intense at 1 hour; 35 times more intense at 1 week; 5 times more intense at 1 month; and about equal at 6 months. Thereafter fission drops off rapidly so that Co-60 fallout is 8 times more intense than fission at 1 year and 150 times more intense at 5 years. The very long lived isotopes produced by fission would overtake the again Co-60 after about 75 years.

   Zinc has been proposed as an alternate candidate for the "doomsday role". The advantage of Zn-64 is that its faster decay leads to greater initial intensity. Disadvantages are that since it makes up only half of natural zinc, it must either be isotopically enriched or the yield will be cut in half; that it is a weaker gamma emitter than Co-60, putting out only one-fourth as many gammas for the same molar quantity; and that substantially amounts will decay during the world-wide dispersal process. Assuming pure Zn-64 is used, the radiation intensity of Zn-65 would initially be twice as much as Co-60. This would decline to being equal in 8 months, in 5 years Co-60 would be 110 times as intense.

   Militarily useful radiological weapons would use local (as opposed to world-wide) contamination, and high initial intensities for rapid effects. Prolonged contamination is also undesirable. In this light Zn-64 is possibly better suited to military applications than cobalt, but probably inferior to tantalum or gold. As noted above ordinary "dirty" fusion-fission bombs have very high initial radiation intensities and must also be considered radiological weapons.

   No cobalt or other salted bomb has ever been atmospherically tested, and as far as is publicly known none have ever been built. In light of the ready availability of fission-fusion-fission bombs, it is unlikely any special-purpose fallout contamination weapon will ever be developed.

   The British did test a bomb that incorporated cobalt as an experimental radiochemical tracer (Antler/Round 1, 14 September 1957). This 1 kt device was exploded at the Tadje site, Maralinga range, Australia. The experiment was regarded as a failure and not repeated.

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