Why was Anna Akhmatova's son put into prison?

Anna Akhmatova's son, Lev Gumilev (also transliterated as Gumilyov), was first imprisoned effectively after someone informed on Gumilev for Gumilev's failure to inform on his family members (who were considered dissidents). The second time, Gumilev was arrested and imprisoned, again, based on the testimony of an informer. These were the formal reasons for which Gumilev was imprisoned; however, Gumilev's imprisonment was also a means by which the Soviet state could control his mother's dissidence.