A school janitor who pleaded guilty last May to second-degree rape of an emotionally disturbed 12-year-old Cambodian refugee was sentenced yesterday to 18 months in the Montgomery County Detention Center.

Floyd Reed Dorsey, 23, who confessed to having a long-term relationship with the girl, could have received a maximum penalty of 20 years. Under Maryland law, having sex with a child under 14 is considered second-degree rape if the person is at least four years older than the child.

The girl, now 14, is undergoing long-term treatment for multiple personalities syndrome at a Maryland state institution. "She's not doing well," said prosecutor Matthew Campbell, who has followed the girl's progress through her American adoptive parents. "She's in an institution and probably will be for many years. It's a very sad case."

Campbell said in an interview that psychiatrists had some early success treating the girl, who doctors said developed multiple personalities syndrome as a defense mechanism after watching her parents die when they stepped on land mines, and seeing her grandfather shot, during the height of the Cambodian war in 1974.

Recently, however, the girl has regressed, he said, and is not responding to psychiatrists' efforts to treat her by "fusing" the various personalities into one, healthier personality.

"There's very little known about child multiples because there have been so few," Campbell said. The multiple personalities disorder first received national attention in 1957, when actress Joanne Woodward won an Academy Award for "The Three Faces of Eve."

Among the five or six personalities the Cambodian child developed was a sexually promiscuous girl who called herself "Bunny," according to Charles M. Cockerill, Dorsey's defense attorney.

In September 1982 that personality began a relationship with Dorsey, Campbell said. Eventually, the child began sneaking out of her house to meet Dorsey, who would drive her to his Wheaton home, Campbell said.

The girl's adoptive parents learned of the relationship in August 1983, after a neighborhood boy spied on the pair's meetings and demanded that the girl have sex with him and his friends, Campbell said.

When the girl refused, the boy told her father of the relationship with Dorsey. The parents then read about the relationship in a diary the girl kept, Campbell said. They confronted the girl and informed county police.

At the sentencing yesterday, Dorsey told Montgomery Circuit Court Judge John J. Mitchell that he was sorry and "didn't mean to hurt" anyone.

"There are many victims in this case," Mitchell replied. The judge then sentenced Dorsey to 10 years in prison, with all but 18 months suspended. He also sentenced Dorsey, who no longer works at the school, to five years of supervised probation after he gets out of prison. During that time Dorsey is to receive psychological counseling and make restitution to the girl's adoptive parents for medical bills.

While not all the girl's psychiatric and medical problems are related to Floyd Dorsey, "a lot are," Campbell said.

Cockerill disagreed. "I think her psychiatric condition probably predated any relationship with Dorsey, he said, adding, "It's a tragic case for him and for the young woman. It would have been very difficult for everyone if it had gone to trial."