Maryland health officials are investigating an unexplained incident 15 days ago in which a speck of radiaoactive cobalt was found beside a railroad track in the center of the small northern Montgomery County town of Dickerson.

Officials said the cobalt was found less than 100 yards from the headquarters of Neutron Products Inc., a company that makes cobalt power cells that are used to treat cancer patients and to sterilize medical instruments.

The speck was so radioactive that a person standing beside it for eight hours could have been subjected to as much as 500 millirems of radiation, Neutron Products officials acknowledged. The federal government recommends that workers in radioactivity-prone jobs limit their exposure to 500 millirems in an entire year.

State officials and top management of Neutron Products said they do not know how the cobalt got onto the railroad track, how long it had been there or whether other cobalt might be scattered around Dickerson.

Neutron Products President Jackson Ransohoff said there was "no rational way" the "hot spot" along the main B & O railroad tracks should have gotten there. "Right now we have no explanation," Ransohoff said.

He added, however, that the incident "should be kept in perspective . . . The town is not about to go down the drain. The biggest danger right now at that hot spot would be getting hit by a train."

Nevertheless, the cobalt discovery touched off an acrimonious meeting Thursday night of about 150 Dickersonians -- nearly 75 percent of the town's population.

Ransohoff was booed, heckled and charged with "incompetent management" by members of the audience, who packed a tiny Dickerson church. Residents urged state officials to withhold renewal of Neutron Products' license, and to schedule public hearings in the railroad-track incident.

The company, which grossed $3 million last year and is the only one of its kind in the eastern United States, has been in business in Dickerson, a farming community just south of the Frederick County line, since 1967.

Robert Corcoran, chief of radiation control for the Maryland Department of Health and Mental Hygiene, said he has "no reason at this time" to order Neutron Products closed or to revoke its license. He promised a "full investigation" lasting "a minimum of a month," which may include public hearings.

The "hot spot" was discovered by an informal committee of 10 Dickerson residents on the afternoon of Nov. 23. They said they had been receiving anonymous telephone calls for months, warning that "something was wrong at Neutron Products," according to Ted Miller, a 35-year-old electronics company executive who lives in Dickerson.

So Miller borrowed a Ludlum Micro R radiation detector from a friend who works at the National Bureau of Standards, "and we just started walking around. It started showing up right away. First the machine registered 8, then 12, then 200. Then it just went off the end of the scale. It was incredible," Miller said.

Carmine Smedira, a Neutron Products vice president, said he was alerted to the discovery by Miller the next day.

"I went out and found the hot spot right away. It was right beside the north rail of the eastbound tracks," Smedira said. He said he shoveled about three gallons of gravel and dirt -- including the speck -- into a pail, and locked the material in a secure vault at Neutron Products, where it remains pending analysis.

Smedira said the radioactive speck was not visible to the naked eye, and seemed to have a center of intensity betwen 5 and 15 inches below ground. Smedira said he has since remeasured the site, "and I know I got 99 percent of the activity in the bucket I took away."

Ransohoff insisted Thursday night that his company "has had a pretty good record for the 13 years we've been here." He characterized the radioactive speck incident as "spooky, but not that serious. The most serous risk I face at Neutron Products is driving to work every day [on two-lane Rte. 28]."

Nevertheless, the cobalt incident is the third involving radioactivity to have occurred at the Neutron Products plant in the last seven years.

In 1973, a concrete tank in which radioactive cobalt-60 is kept immersed, leaked several thousand gallons of radioactive water into a public storm drain. lThe radioactive water may have seeped into the underground wells of some Dickerson residents, although state officials never have been able to tell for certain. The leak was repaired about a month after it was discovered.

About three months ago, the same tank overflowed, again spilling several thousand gallons of water. Ransohoff said, however, that the liquid in that spill was "almost drinking-water quality." He said the overflow was caused by "human error."