It is suggested that the anomalous leakage of ultra-cold neutrons from a neutron trap may be associated with the formation of dineutrons in it. It is shown that the gas of ultracold neutrons at temperatures T less than 10-3 K should form a dineutron Bose condensate even in the absence of dineutrons as free stable particles. The consequences arising from the assumption of the stability of dineutrons in neutron stars are considered. The conditions for the formation of a Bose condensate from dineutrons in them and in heavy nuclei are discussed
We study theoretically some effects caused by a propagation of the charged particle in a dilute gas of alkali-metal atoms in the state with Bose-Einstein condensates. The energy change of the high-speed (relativistic) particle based on the Cherenkov effect in this system is investigated. We note that the particle in some cases not only emits the energy but also can be accelerated by the ultracold atomic gas. The necessary conditions for the acceleration of the particle by the condensate are found. We study also a possibility of defining the spectral characteristics of atoms by the detection of the Cherenkov radiation in the condensate.