Vivian E. Psachos, 73, a senior research analyst with the Central Intelligence Agency who helped expose a former analyst as a Soviet spy, died of cancer Aug. 25 at her Falls Church home.

Mrs. Psachos interviewed William Kampiles, a former low-level CIA watch officer, after he revealed to a friend in the agency that he pilfered 13 copies of a top-secret manual for the state-of-the-art KH-11 spy satellite and sold them to the Soviet Union for $3,000.

According to her testimony in Kampiles's 1978 espionage trial, Mrs. Psachos said Kampiles first claimed that he got the money on the promise of information. But Kampiles, who had hoped the incident would make him an attractive candidate for a job as a CIA double-agent, eventually admitted turning over the purloined manual in exchange for cash.

The trial resulted in increased security at the CIA's Langley headquarters. Kampiles took the manuals by simply slipping them beneath his coat, according to the Jeffrey T. Richelson book "The Wizards of Langley: Inside the CIA's Directorate of Science and Technology."

After turning over the documents to the Soviet Union in Greece, Kampiles returned to the United States and told friend George Joannides how he had "conned" the Soviets. At Joannides's suggestion, Kampiles wrote a letter detailing the incident, but Joannides put it in his briefcase and forgot it for two months. When he rediscovered the letter, he sent it to Mrs. Psachos, who worked in the Soviet section. She set up an interview with the young spy in which he admitted the crime.

Kampiles, 23, was sentenced to 40 years in prison. Mrs. Psachos went on vacation. Her husband, Charles Psachos, said she was exhausted from the ordeal.

"I think [she was proud of nabbing a spy], even though it weighed heavily on her," he said. "This was just some stupid, dumb kid."

Mrs. Psachos was a native of Marlboro, Mass., and a graduate of Becker Junior College in Worcester, Mass. She joined the CIA in 1957 and was assigned to the directorate of operations. She had overseas assignments in Greece and Germany and short tours of duty in East Asia.

She received the Career Intelligence Medal and the Intelligence Commendation Medal for her work at the agency. She retired in 1985 and returned in 1987 as an independent contractor before retiring again in December.

Survivors include her husband of 35 years, of Falls Church; and a sister.

Vivian E. Psachos received two CIA medals.