A 38-year-old Rockville man was sentenced yesterday in D.C. Superior Court to serve 10 to 30 years in prison after being convicted of charges that he raped the 8-year-old daughter of the woman with whom he was living.

Rux D. Mitchner, who worked as a corrections officer in the maximum-security prison at Lorton Reformatory, was convicted by a D.C. jury on Oct. 7 of rape, carnal knowledge and taking indecent liberties with a minor.

Mitchner's lawyer, Leroy Nesbitt, asked Judge Fred B. Ugast to place Mitchner on probation. Nesbitt argued that Mitchner's troubles stemmed from a long-term alcoholism problem and that his condition had improved since his arrest a year ago.

However, Assistant U.S. Attorney William J. Bowman urged Ugast to impose a long sentence, arguing that Mitchner's acts left permanent emotional scars on the girl and her mother. Mitchner faced a maximum sentence of life in prison on both the rape and carnal knowledge charges.

Ugast called the situation a tragedy for the girl and her family and for Mitchner, but said his responsiblity to the community required him to impose a substantial sentence.

Ugast said he would order Mitchner to serve the sentence at a federal prison rather than at Lorton, apparently because Mitchner might face retaliation from prisoners or guards if he were imprisoned at Lorton.