Gain without inversion for gamma radiation

Abstract

Gain without inversion for a level-mixing scheme is studied for gamma-optics. In this scheme nuclear level mixing is created by misalignment of a dc magnetic field with respect to the c-axis of a noncubic crystal. Axially symmetric electron nuclear coupling and nuclear quadrupole interaction with an electric field gradient produce two electro-nuclear levels that are equally mixed and split, with the energy gap dependent on the tilting angle of the magnetic field. By laser excitation of an electron transition, nuclear spin coherence can be created between these two levels. A condition is found for the predominant population of the dark state by spontaneous emission. This state is a particular superposition of the mixed states, such that, by selection rules, the transition from it cannot be excited by γ-radiation. If all absorbing nuclei are in the dark state, resonant γ-absorption is suppressed in the sample. At the same condition, γ-emission of the excited nuclei is allowed because the corresponding transition terminates in another component of the mixed states superposition. The constraints on the tilting angle and reciprocal gap between two mixed electro-nuclear levels are found, setting the limits to these values beyond which the gain without inversion becomes impossible.