A mechanism for creating an inversion of populations of energy levels

The possibility of the Gadolinium nuclei energy levels population inversion under neutron bombardment is theoretically shown. In the process two isotopes of gadolinium take part. Light isotope Gd¹⁵⁵ capturing neutron transforms to heavy isotope Gd¹⁵⁶ at that heavy nucleus is in an excited metastable state. The population inversion is investigating during several tens of second when the neutron flux intensity is 10¹³ cm^-2 · s^-1.