The 28-year-old son of Interior Secretary Manuel Lujan Jr. was convicted yesterday of breaking into the apartment of an Alexandria woman and raping her in November. The jury in the case recommended a 20-year prison term.

Robert Jeffrey Lujan, who testified in his Alexandria Circuit Court trial that his sexual encounter with the woman was consensual, was acquitted of a charge that he threatened the woman with a shotgun during the incident. Judge Donald Kent scheduled sentencing for Feb. 21.

By law, the judge may impose a sentence shorter than that recommended by the jury, but not longer.

Manuel Lujan, who showed no emotion as the verdicts in his son's case were read shortly after 4 p.m. yesterday, said later that he and family members were "very saddened."

"It's been a terrible time for everyone," he said, speaking barely above a whisper in the hallway outside Kent's courtroom.

The incident occurred in the early hours of Nov. 11 in the 48-year-old victim's apartment. She lived in the same Alexandria high-rise building where Jeff Lujan lived with his parents.

The woman, who testified Wednesday on the first day of Jeff Lujan's trial, said she woke up about 4:30 a.m. and saw Lujan naked on the edge of her bed with a shotgun. Assistant Commonwealth's Attorney Cary F. Greenburg told the jurors that the woman had never met or seen Lujan before he broke into her apartment and held her captive for four hours before raping her.

Lujan, however, said he saw the woman about 3 a.m. when he went to the basement vending machines to buy a soda and cigarettes. A short time later, he testified, he saw her again on an elevator. Lujan said she invited him into her apartment and they engaged in consensual sex. Manuel Lujan testified that days after the encounter between his son and the woman, he found the family's shotgun "exactly" where he had hidden it weeks before the incident. The Interior secretary said he kept the gun in his wife's closet so that no intruders, children or others, would find it.

He told the jury that he doubted his son would have gone into the closet to take the weapon out.

One juror said after yesterday's verdict that he believed "beyond a reasonable doubt" that Jeff Lujan had used a shotgun during his encounter with the woman. But he declined to say whether any other jurors shared that belief or why he finally gave in and voted to acquit Lujan of possessing a firearm during a crime of violence.

"There were a lot of sticking points" among members of the panel, the juror said, meaning the jurors disagreed on other points beside the shotgun during 16 hours of deliberations that began Friday. "There's a question of what is reasonable doubt. And different people had different levels of reasonable doubt."

Another juror, who also did not want to be identified, called the verdicts "the toughest decision we had to make in our lives." The jurors, who deliberated for eight hours Friday, returned to the Alexandria courthouse at 8 a.m. yesterday and issued their verdicts shortly after 4 p.m. They convicted Lujan of burglary, sodomy and rape.

Jeff Lujan, a part-time house painter, was convicted in 1983 in New Mexico of possessing cocaine with intent to sell and using a telephone to conduct a drug transaction. He served 16 months in jail on those charges and an additional 18 months in a federal prison after a 1987 drunken driving conviction.

In all, Jeff Lujan has been convicted of seven charges of driving while intoxicated and two charges of driving with a suspended license.

In the trial, defense lawyer Robert Stanley Powell, in aggressive cross-examination of the victim, accused her of fabricating her story.

Powell accused the woman of filing a complaint only after learning that Lujan's father was an affluent member of President Bush's Cabinet. He suggested that the woman, in bringing the charges, hoped eventually to obtain money from the Lujan family or from the apartment complex by filing a lawsuit.

The woman was not present at yesterday's verdict.