Thorium-232 is a silvery, radioactive metal that is particularly good at absorbing X-rays. In the early days of X-ray imaging, doctors routinely injected patients with thorium dioxide because it produced high contrast images. Between the 1930s to the 1950s, some 10 million people received these doses.

The advantage of thorium dioxide, or Thorotrast as it was called, is that it had almost no immediate side effects on the patients, unlike other contrast agents, which were often dangerous. And the half-life of thorium is about 14 billion years, so it is relatively stable.

What doctors didn’t appreciate at the time were the long term effects on the body. Once injected, Thorotrast settles in various organs where it tends to stay. The biological half life of the stuff is 22 years.

When thorium eventually decays it sets in train a sequence of five further decays producing alpha particles. These all happen relatively quickly; four of them in a matter of hours or fractions of a second.

For that reason, Thorotrast turned out to be highly carcinogenic but often on a timescale measured in decades. It was eventually withdrawn as a contrast agent in the 1950s.

The problem for physicists is to calculate the effects of elements like thorium on the body. They’ve long known that the high energy particles released during a decay damage the body by smashing into and damaging molecules like DNA.

But today, Evandro Lodi Rizzini and pals at the University of Brescia in Italy say that physicists have missed another mechanism that may cause even more damage.

Polonium-212, for example, releases alpha particles with an energy of 8748 keV, which will then smash into any molecules nearby until its energy has been absorbed.

But Lodi Rizzini and co point out that there is another component to this reaction: a nucleus of lead-208 which recoils with an energy of 170 keV. In the case of Thorium-232, the result is an alpha particle with an energy of 4012 keV and a nucleus of radium-228 which recoils with an energy of 66 keV.

Nobody has considered the damage these recoiling nuclei can do to DNA. Until now.

Lodi Rizzini and pals say that being bigger and heavier, these nuclei will obviously travel less far within the body, perhaps a distance of a few hundred nanometers. That means that if the decay occurs near DNA, then it will do all its damage in that area.

By contrast, the alpha particle will release its energy in a much larger volume.

That has important consequences. “The nucleus recoiling after an α particle emission will give rise to an energy deposition (on a DNA structure nearby) even two orders of magnitude larger than the α particle itself,” says the Italian team.

So the damage from a recoiling nucleus can be one hundred times greater than the damage from an alpha emission.

That could change the way people think about the damage that radioactive decays can do inside the body. Lodi Rizzini promise a more detailed evaluation in the near future.

It may also lead to new strategies for controlling the damage that these substances can do. If it’s the nucleus rather than the alpha particle that does most of the damage, there may be ways of using this to an advantage.

Something for the readers of the arXiv Blog to speculate about, perhaps.

Ref: arxiv.org/abs/1107.3699: About The Importance of Nuclear Recoil In α Emission Near DNA