The FBI arrested a debt-ridden Northrop Corp. engineer today after he allegedly tried to sell billion-dollar "Stealth" technology to shield bombers from radar to undercover FBI men posing as Soviet officials.

According to an FBI affidavit, engineer Thomas Patrick Cavanagh, 40, of Downey, Calif., received $25,000 from the agents to pay off debts that would prevent him from obtaining top-level government security clearance.

The affidavit, filed in federal court in Los Angeles along with a complaint charging Cavanagh with trying to sell national defense secrets to the Soviet Union, said Cavanagh was arrested at a hotel moments after delivering classified material and receiving the cash. It said he came to the hotel carrying an automatic pistol and the classified documents.

Cavanagh was charged with attempting to transmit classified national defense information regarding an advanced military project that uses low observable technology.

FBI spokesman Lane Bonner said low observable technology, or "Stealth" technology, is designed to prevent radar installations from detecting aircraft. The United States has had a secret project to build such a plane for several years.

Cavanagh is accused of taking technical manuals, blueprints, drawings and lists of subcontractors without permission from his employer, Northrop's Advanced Systems Division in Pico Rivera, Calif.

FBI Director William H. Webster said disclosure of the documents would have caused irreparable damage to national security, but assistant FBI director Bill Baker said Cavanagh "never actually contacted Soviet officials" and no information fell into Soviet hands.

The affidavit said two FBI agents, acting on "confidential information," posed as Soviet agents and approached Cavanagh. During a Dec. 10 meeting, Cavanagh said he would not receive top-secret clearance unless he got "the creditors off my back,' " the affidavit said, adding: "Cavanagh stated he needed '$25,000 . . . cash for information and it's that simple . . . . You get the cash, I give you the information.' "

Cavanagh has a government "secret" clearance and a pending request for "top-secret" clearance, the affidavit said.

A spokesman for Northrop declined to answer questions about Cavanagh, who was denied bail at an arraignment hearing today.