The “gamma-ray laser”—a much discussed but never before realized construct of nuclear physics—looks more plausible thanks to a new theoretical study. Key to the gamma-ray laser idea is that the population inversion necessary for lasing happens with atomic nuclei, rather than the electrons whose transitions are familiar to laser physicists.
These transitions, instead of involving photons in the visible range, can be at extraordinary energies—up to the mega-electron-volt range. A coherent source of photons at such energies would be useful for metrology and quantum information purposes, and would be of particular interest for defense applications.
However, atomic nuclei present a special set of experimental difficulties. First a population of nucleons (neutrons and protons) has to be assembled, and excitation must match their so-called isomeric energy levels. Furthermore, the linewidth of any emission must be maintained at a level narrow enough for the cascade of stimulated emission characteristic of a laser, and—as laser physicists know well—creating a population inversion with just two states is not feasible.
Eugene Tkalya of Moscow State University (Moscow, Russia) has been researching what appears to be a uniquely suitable atom for the process: thorium-229 (229Th). “It was a long story,” he says, describing early studies on a dielectric oxide of 229Th. “In 2000 I understood that inside the dielectric, the main decay channel of the low-energy isomeric level of the 229Th nucleus was nuclear gamma-ray emission.”
The exact energy gap between the ground and excited states remained elusive, however. The important criterion was that the energy gap of the nucleons’ isomeric levels was lower than the energy gap of the material in which it was incorporated. The thorium oxide dielectric was later found not to satisfy that criterion, but the important point that Tkalya noted was that the gap contained no electronic states. “For this reason, electronic conversion—the main decay channel of the low-energy nuclear isomeric levels—is forbidden,” he says. “It was a key idea, because I understood that we could excite the low-energy nuclear level in 229Th by usual laser radiation with a suitable wavelength.”
Crystal with thorium added
What remained was finding a suitable host for the nuclei. Then, last year, a group from US universities found one in the form of a lithium-calcium-aluminum-fluorine (LiCaAlF6) crystal, in which they replaced some of the calcium atoms with thorium-232.1 The thorium in the LiCaAlF6 crystal showed an energy gap of about 10 eV, meaning that input photons of the right wavelength could, without disturbing any electronic levels, excite the thorium nuclei.
That sparked Tkalya’s renewed interest in thorium as the basis for the first nuclear laser. In the current study, he proposes creating a ground-state energy-level splitting in the crystal by employing a high magnetic field of about 100 Tesla, or an electric field gradient of 1018 V/cm2.2 This would create the third level necessary for creating a population inversion, and would lase with the 229Th’s energy gap of 7.6 eV, or 163 nm (see figure). This value, which is in the vacuum-ultraviolet range, means the approach is still not the much-vaunted gamma-ray laser. But the thorium system represents the first viable route toward a nuclear laser.
“The real gamma-ray laser, which emits photons in the mega-electron-volt range, is a dream of military science,” says Tkalya. “Luckily for us, the toy gamma-ray laser based on the 7.6 eV transition in the 229Th nucleus cannot be used for military purposes.”
What still remains to be measured is the pumping energy that will be required—Tkalya notes that the necessary wavelength may already be achievable with existing UV lasers, but says that the wide tunability of a free-electron laser may be required. So, while this first proposal for a nuclear laser seems valid, a full-power gamma-ray laser for now remains a theoretical construct.
REFERENCES
1. W.G. Rellegert et al., Phys. Rev. Lett.; doi: 10.1103/PhysRevLett.104.200802.
2. E. Tkalya, Phys. Rev. Lett.; doi: 10.1103/PhysRevLett.106.162501.