According to the United States Nuclear Regulatory Commission, the sale was legal even though the clinic in Mexico had no license to receive radioactive materials.

Once in Juarez, however, the machine sat in a warehouse. Doctors at the clinic said they were never able to hire the specialist required to operate the device.

Device Sold to Junkyard for $9

Then, late last November, an electrician, Vicente Sotelo, said he was told to go to the warehouse to pick up some material and to take it to the Jonke Fenix junkyard in Juarez. He and a co-worker heaved the radioactive tungsten wheel, which had been removed from its enormous lead casing, into the back of a pickup truck. Mr. Sotelo later told the authorities he forced the unmarked capsule open on the back of his truck.

Doctors at the clinic say Mr. Sotelo was not authorized to take the wheel containing the capsule, which he sold to the junkyard for $9. No charges have been filed, and the Mexican authorities said they were still investigating the incident.

When the capsule was breached it held about 400 curies of radioactive cobalt in the 6,010 pellets. Each pellet, according to Joel Lubenau, a health physicist at the Nuclear Regulatory Agency, produced a radiation dose of 25 rads per hour two inches from the pellet. One to 50 rads per hour is considered a significant radiation dose. In comparison, the highest exposure a bystander could have received from the nuclear accident at Three Mile Island was 100 millirads, or about one-tenth of a rad.

A rad is a unit of absorbed radiation. An average chest X-ray produces 20 to 30 millirads instantaneously. A lethal dose for half the population is 450 rads received instantaneously over the whole body.

''If you sat next to a pellet over many hours, you might several days later develop a reddening of the skin,'' Dr. Lubenau said. ''If a pellet became embedded in your shoe you could get localized high exposure to the body.''