Hidden amid the mountains and mesas of northern New Mexico lies Los Alamos National Laboratory. It's shrouded in secrecy. Once known simply as Project Y, it was a classified lab where scientists built the atomic bomb.

Now, 70 years later, scientists there still work on nuclear weapons, but they're also using some of that same knowledge to battle cancer.

NBC News got exclusive access to the secure facility, where physicist Eva Birnbaum is working to use radioactive elements to battle cancer.

Los Alamos National Laboratory in New Mexico. NBC News

She works with actinium 225 or ac-225, one of a new class of radioactive therapies called alpha-emitting isotopes. It's unique because of its ability to destroy cancer without hurting healthy tissue.

The alpha particles emitted by ac-225 are relatively feeble, so they don't do much damage past the targeted area. Even a little healthy skin stops them dead. And its half-life is 10 days, meaning half of it's gone after 10 days, so it doesn't hang out in the body for long.

"I think we're very hopeful that this will have tremendous impact for some cancers right now that really don't have highly effective treatments," Birnbaum said.

Early clinical trials have found that ac- 225 has the potential to treat leukemia, melanoma and other cancers, like those of the breast and the prostate.

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But the issue that concerns cancer experts isn't whether actinium is promising. It's whether there's enough of the precious substance to use in clinical trials. The supply has been quite limited because it's so hard to produce.

The scientists at Los Alamos can use particle accelerators built to make nuclear products for weapons and energy generation to make ac-225 from another radioactive element called thorium.

Los Alamos National Laboratory in New Mexico. NBC News

"This project is aimed at making 50 times more material available than is available right now," said Kevin John, the project manager at Los Alamos.

It takes Los Alamos' half-mile-long proton accelerator. The team starts with a disc of thorium, blasting it with the stream of protons. This knocks off the charged particles called isotopes — some of which are Ac-225 particles.

To use it for cancer therapy, scientists outside Los Alamos bind the isotope with an antibody — a human immune system protein engineered in the lab to find tumor cells. When injected into the body, it homes in on the targeted tumor and destroys the cancer.

"This is more focused radiation. It's more precise, and it allows for efficient killing of the targeted cell alone," said Joseph Jurcic, a hematologist and oncologist at New York-Presbyterian/Columbia University Medical Center.

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Jurcic conducted some of the early clinical trials, and he's on the board of Actinium Pharmaceuticals, a company that's working to develop cancer treatments using ac-225.

He said it can help people with smaller tumors or those that have metastasized. More research is needed before it can be approved.

And that research can now be done in large part thanks to the scientists at Los Alamos who are helping create a new type of weapon. This time, though, it's to fight cancer.