Christopher Boyce, a convicted spy serving a 40-year sentence for passing satelite secrets to the Soviets, escaped Monday night from what appears to be a not-so-maximum security federal prison in California.

The 26-year-old Boyce was at large yesterday after using tin snips and a makeshift ladder to scale a fence at Lompoc prison. Prison officials in Washington and California are trying to determine why armed guards in nearby towers did not see the escape.

Norman Carlson, director of the federal Bureau of Prisons, said yesterday that the Lompoc facility was in the midst of being changed from a youth corrections center to a maximum security prison. "We try to run a humane institution, we don't lock people up 24-hours a day. These things are going to happen. This guy just beat the system," he said.

Carslon added that there was no indication that anyone inside or outside the prison assisted in the escape.

Carlson said the decision to increase security at Lompoc was made about a year ago, to replace the closed McNeil Island, Wash., prison.

Towers were built and manned and concertina wire woven between two exterior fences. But an electronic detection system and better lighting have not been added yet, he said.

Buck Samples, acting warden at Lompoc, said in a telephone interview that "if we'd had the detection system in, an alarm bell would have rung in the control room as soon as he touched that fence."

Last year nine inmates broke out of Lompoc in three incidents. One inmate, American Indian Movement activist Leonard Peltier, escaped while friends pinned down prison guards with rifle fire, Samples said. Peltier was recaptured.

Boyce was convicted in 1977 of passing classified material to the Soviets while a civilian employe in a code room operated for the Central Intelligence Agency by TRW Inc. in California.

His childhood friend, Andrew Dalton Lee, is serving a life term at Lompoc. He acted as the messenger in the scheme, delivering the information to the Soviet Embassy in Mexico City starting in 1975, according to testimony at the trial.

Lee was arrested in January, 1977 by a Mexican policeman who thought he was littering the Soviet Embassy grounds when he threw a message inside, Microfilm marked "top secret" was found in his pocket. He soon implicated Boyce.

Robert Lindsey, the New York Timies reporter who covered the trials, wrote a widely acclaimed book on the two men titled "The Falcon and the Snowman." This refereed to Boyce's love of the outdoors and falconry, and Lee's career as a dealer in cocaine.

Lindsey described Boyce as a youth from an upper-middle-class family -- his father was a former FBI agent -- who was disillusioned with his country and Western society. Lee, on the other hand, was motivated by the need for money to finance his drug deals, according to the book.

William Dougherty, Boyce's defense attorney, said yesterday that his client may have decided to escape after a federal judge last month turned down a motion to reduce his 40-year sentence.

The lawyer said he talked with Boyce by telephone 10 days ago, but got no feeling that he was planning an escape from the facility about 100 miles northwest of Lost Angeles.

Boyce's parents in Palos Verdes, Calif., called on their son to turn himself in. They refused to talk to reporters, but said through a spokesman that they were concerned for their son's safety. They added that he had not contacted them.

Government attorneys said at the trial that some of the material Boyce passed through Lee to the Soviets was "so extremely sensitive" that it could not be included in the indictment or mentioned at trial.

The two men were convicted of selling the Soviets an outdated study for a satellite project.

At separate trials, Boyce and Lee turned on each other. Boyce claimed he was blackmailed by Lee into giving up the documents. Lee said he had been set up by Boyce, who, he alleged, was working for the CIA in an effort to penetrate Soviet intelligence.