A Fairfax County judge sentenced a 20-year-old Fairfax County man yesterday to 50 years in prison for shooting his former girlfriend, leaving the teen-ager a quadriplegic.

Circuit Court Judge Lewis H. Griffith imposed the jail term on Charles R. Brewer Jr.

"I have spent days thinking about the consequences of this act on the victim," Griffith told Brewer, after turning down his attorney's requests to hospitalize him for mental illness rather than sentence him to prison. Under Virginia law, Brewer could be eligible for parole in eight or nine years.

Brewer pleaded guilty to the four charges resulting from the shooting last Aug. 28 of Joy Keo, 17, a Fairfax County girl he had dated for about six months.

After Keo tried to break off the romance, Brewer broke into a house where she was visiting, kicked in the door to the bedroom where she was hiding and fired four bullets into her body. The first severed her spinal cord. Doctors say she will be paralyzed from the neck down for the rest of her life.

"It would be hard to envision a more vicious crime . . . " Commonwealth's Attorney Robert F. Horan Jr. told the court. "A young, active, vibrant girl cut down in the bloom of her life."

Defense attorneys argued that Brewer had suffered from an organic brain disorder since he was an infant; a disease that they said causes him to act irrationally sometimes. "There's got to be something wrong with this boy," defense attorney Brooke Howard said. "Call it what you like--there's no question about that."

The sentencing hearing attracted a courtroom full of assistant prosecutors, defense attorneys and judges, as well as friends and relatives of the Brewer and Keo families. Brewer's father and brother sat in the front row and Keo's mother and sister sat in the rear rows as Horan recounted details of the shooting.

"Brewer ran up the stairs and kicked in the door," Horan said. "He pointed the gun at her and said, 'You're dead, bitch!' "

At that point, Brewer and members of his family burst into sobs. His father and brother clutched each other and Brewer bent over the defense table, shoulders heaving. The judge recessed the hearing temporarily.

Joy Keo, who is undergoing therapy at a rehabilitation center in Fishersville, wrote Griffith in a letter: "I don't seek revenge, but I'm hoping to avoid any future contact with Brewer. He must have his way or he becomes violent."

She also told the judge that after the shooting, "I suffered in the intensive-care unit for two months where the doctors and nurses struggled to keep me alive even though I often felt it would be better to die than to be left a quad at age 17."

Griffith sentenced Brewer to a total of 71 years in jail: 48 years for burglary using a deadly weapon, 20 years for maiming, two years for use of a firearm and one year for assault. He ordered the terms to be served concurrently with the exception of the firearms charge.