A role of the unstable nuclei ${ }^{ 6}$Be, ${ }^{ 8}$Be and ${ }^{ 9}$B in the dissociation of relativistic nuclei ${ }^{ 7,9}$Be, ${ }^{ 10}$B and ${ }^{ 10,11}$C is under study on the basis of nuclear track emulsion exposed to secondary beams of the JINR Nuclotron. Contribution of the configuration ${ }^{ 6}$Be + $\mit{ n}$ to the ${ }^{ 7}$Be nucleus structure is 8 $\pm$ 1% which is near the value for the configuration ${ }^{ 6}$Li + $\mit{ p}$. Distributions over the opening angle of $\alpha$-particle pairs indicate to a simultaneous presence of virtual ${ }^{ 8}$Be$_{ g.s.}$ and ${ }^{ 8}$Be$_{ 2^+}$ states in the ground states of the ${ }^{ 9}$Be and ${ }^{ 10}$C nuclei. The core ${ }^{ 9}$B is manifested in the { ${ }^{ 10}$C} nucleus with a probability of 30 $\pm$ 4%. Selection of the ${ }^{ 10}$C "white" stars accompanied by ${ }^{ 8}$Be$_{ g.s.}$ (${ }^{ 9}$B) leads to appearance in the excitation energy distribution of 2$\alpha$2$\mit{ p}$ "quartets" of the distinct peak with a maximum at 4.1 $\pm$ 0.3 MeV. ${ }^{ 8}$Be$_{ g.s.}$ decays are presented in 24 $\pm$ 7% of 2He + 2H events of the ${ }^{ 11}$C coherent dissociation and 27 $\pm$ 11% of the 3He ones. The channel ${ }^{ 9}$B + H amounts 14 $\pm$ 3%. The ${ }^{ 8}$Be$_{ g.s.}$ nucleus is manifested in the coherent dissociation ${ }^{ 10}$B $\to$ 2He + H with a probability of 25 $\pm$ 5% including 14 $\pm$ 3% of ${ }^{ 9}$B decays. A probability ratio of the mirror channels ${ }^{ 9}$B + $\mit{ n}$ and ${ }^{ 9}$Be + $\mit{ p}$ is estimated to be 6 $\pm$ 1.
Categories: nucl-ex