The nature of the bright bolide and of the giant explosion happened on June 30, 1908, in Podkamennaya Tunguska river basin, Central Siberia, is still under discussion [1]. The area with trees fallen down is more than 2000 square km, while the explosive energy has been estimated [2] to be equal to no less than 30 million tons of TNT (or to 1500 Hiroshima bombs). Nevertheless,Kolesnikov et al. [3] has shown that the explosion could not be of nuclear nature. Its energy was, in fact, too big to be simple fissionable 235U nucleus explosion. Another two nuclear hypotheses, of annihilation and thermonuclear one, have been tested by measuring 39Ar radioactivity inducted from the K and Ca isotopes in rocks and soil at the explosion epicentre. This method has much more sensitivity when local neutron flow is detected than the method of 14C analysis in tree rings [4]. Kolesnikov et al. [3] did not detect 39Ar in the samples from the explosion epicentre although its estimated radioactivity was expected to be 100 times as many as the radiometric plant sensitivity (0.01 dpm).
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