GAMMA RAY LASERS


Tantalum-180 is the only isotope to have a measurable abundance as a nuclear isomer; natural tantalum comprises 0.012% of the isomeric form of tantalum-180. Tantalum-180 has a halflife of only 8.152 hours and has long since decayed, whereas the metastable isomer Ta-180m has an exceedingly long halflife of 1.2×1015 years, accounting for its still detectable presence since its formation less than 1010 years ago in a nearby supernova explosion. Ta-180m has such a phenomenally long halflife because the isomer has much greater spin than its ground state, 8 units higher in this case, and if it decays into its ground state, the emitted gamma ray must inherit this high spin difference, but gamma rays rarely carry more than unit spin, so the isotope is trapped in this high spin state. A further trapping mechanism exists for prolate or oblate nuclei where the spin axis alignment between the nuclear rotation and the individual constituents differs for metastable and ground states, where any escaping gamma ray must also change the spin of the nucleus in just the right way, and this is so for Ta-180m/Ta-180.

Metastable isomeric states have a higher energy than their ground states, by only 0.074MeV in the case of Ta-180m, and if the isomer could be stimulated (by external gamma-rays) into decaying sooner, the result would be a nuclear laser, a million times more powerful than its atomic equivalent. The gamma-rays boost the energy of the isomeric state such that the nucleus briefly becomes spherical in the switch from prolate to oblate, and during this brief moment, there is no preferred spin axis, and the decay to the ground state is un-inhibited. The isomer decays. The beauty of this technique is that the substance is non-radioactive until stimulated. Unfortunately, in the case of Ta-180m, the energy of the gamma-ray required for this boost, 2.4MeV, the activation energy, is 30 times the extra energy released by the subsequent stimulated decay, 0.074MeV.

The activation energy of 2.4MeV boosts the Ta-180m into it's so-called 'k-mixing state', where it is in two states at the same time, a quantum superposition of states, and in this state it is neither prolate nor oblate, but both at the same time, and it spends a proportion of the time in the spherical state where there is no preferred spin axis. It is during the time it is in the spherical state that it can decay easily because there is no constraint on the spin carried away by the emitted gamma ray. The gamma ray is emitted with an energy of 0.074MeV, the difference in energy between the metastable and ground states.



The isotope of another element, hafnium-178m, has a much higher metastable state energy, 2.447MeV, and a much lower activation energy, just 0.2MeV, in the X-ray region, releasing 100 times the activation energy. In fact hafnium-178 has two metastable states, Hf-178m and Hf-178m2. The spin of the first excited state, Hf-178m, is spin-8 but the halflife is a very short 4 second. This state is of no use. It is the second metastable state, Hf-178m2 that is important. This state has spin-16 and a halflife of 31 years, and is a high 2.447 MeV higher than the ground state, Hf-178. Unfortunately, because of the short halflife of only 31 years, it is not present on Earth and has to be manufactured artificially. The spin difference between ground state and metastable state is 16 units. Stimulating this metastable isomer into decaying by irradiating it with 0.2 MeV Xrays (the activation energy required to bump Hf-178m2 to it's k-mixing state) would yield a much higher energy gain. So far, a shortening of the halflife by only 4% has been observed by stimulating hafnium-178m with X-rays. Research continues for more promising isomers.

It should be noted that this mechanism of stimulated emission is not the same as the stimulated emission obtainable in optical lasers; in the case of lasers the stimulating radiation is the same wavelength as that emitted, thus each emitted photon stimulates the emission of more identical photons, in unison, amplifying the beam which is coherent. In the case of the stimulated gamma emission here, a different wavelength photon is emitted from the stimulating one, a crucial difference. Also, in an optical laser, the beam is bounced back and forth between two parallel mirrors, effectively increasing the path length of the amplifying material such that the gain is above unity. In the case of any possible gamma ray laser, there are no known mirrors for gamma rays (just grazing-incidence reflectors) so the length of material the gamma ray beam passed through would have to be sufficientlu long that the gain is above unity. Research continues to find nuclear isomers that may be stimulated by the same wavelength that they emit, to produce what would be a gamma ray laser.

GAMMA RAY WEAPONS

The emission of only gamma rays by the two isotopes above has stimulated interest by the military, for use as a weapon. The release of spin energy can be triggered almost instantaneously by giving it a quick blast of low-energy X-rays, releasing the energy in an explosive blast of gamma rays. The weapon can be made in all yields, unlike fission bombs where there is a minumum yield due to the (minumum) critical mass of the isotopes. This makes devices like this very worrying, for the lower the yield, the more willing will be trigger-happy battle commanders to use such devices. Luckily, hafnium-178m2 is only manufacturable in milligram quantities at present, but one gram of this isotope would yield the equivalent of 50kg of TNT. Hafnium-178m2 does not exist naturally, and has to be made by pumping energy into Hafnium-178. At present hafnium-178m2 is obtained by the radioactive decay of other isotopes, which must themselves be manufactured by irradiating them with protons, but there may be cheaper ways of manufacturing it directly from Hf-178 by irradiation with photons.

See Quantum Nucleonic Reactor.