There is a nagging suspicion among scientists studying the mysterious nuclear explosion last Sept. 22 near South Africa that it was a neutron bomb.

"We have no evidence that it was a neutron bomb but the possibility has been discussed more than once," said one source close to the White House Office of Science and Technology Policy. "It's one of the few things that could explain why we've never found any radioactive fallout from the explosion."

Designed for use on the battlefield, a neutron bomb generates a small fireball that produces little physical destruction but a huge excess of neutrons. The neutrons are lethal to people in the vicinity but do not travel far and do not persist in the atmosphere the way radioactive fallout does.

A neutron bomb would also explain the low strength of the explosion seen last Sept. 22 by an orbiting Vela satellite, whose optical sensors (called "bangmeters") witnessed what scientists say was an explosion with a force between two and four kilotons.

"An explosion of a neutron weapon would certainly explain the low yield of that event," one nuclear weapons scientist from the Los Alamos Scientific Laboratory said. "It takes a complicated design process to build a neutron bomb but I wouldn't be shocked at the idea that somebody was smart enough to test one."

The Central Intelligence Agency has told Congress that if the event seen in the South African skies Sept. 22 was a nuclear explosion, then it suspects Israel or South Africa or both as the source of the event.

Weapons analysts say they can understand why both Israel and South Africa would be interested in developing a neutron bomb, which could be an effective last-ditch battlefield defense for both countries in all-out war with their neighbors.

Insofar as anybody knows, no country has ever tested a neutron bomb. There was some speculation that France may have tested one two years ago at its South Pacific underground test site but the French government has never confirmed it.

Still another suspicion that nags scientists about the mystery explosion is the possibility that it was not the first in the skies near South Africa. There is a distinct possibility that other nuclear explosions took place below the Cape of Good Hope before Sept. 22.

Sources point out that the Vela that picked up the fireball was literally out of position the night of the event. Put into orbit 10 years ago, the Vela had lost altitude with time and was circling Earth in a way that brought it over the same spots on Earth at unpredictable times.

"We were lucky the Vela picked up the fireball because if it had not been out of position it would have missed it," one source said, "There is a chance the same Vela missed other explosions before Sept. 22 when it was in the position it was supposed to be in."

One source said that whoever set off the explosion in the middle of the night on Sept. 22 could have timed it so that Vela would miss it. The source said the panel of scientists convened by the White House to unravel the mystery even looked into the possibility that somebody tried to fool the Vela satellite into mistaking the explosion for some other phenomena.

"There were many light flashes Vela saw that had some of the characteristics of nuclear explosions," the source said. "The panel considered the possibility that some were deliberate flashes of laser beams to confuse the satellite but the possibility was set aside. There wasn't enough similarity between the Sept. 22 flash and the hundreds of other flashes it saw."

So much of the puzzling explosion remains a mystery that next month the White House will reconvene the panel of outside scientists it brought together last November to investigate it. The White House thought the panel had finished its work in January and even drafted a report that was said to summarize the panel's findings. The report has never been made public.