Radioactivity gets fast-forward
Date: Thursday, September 23, 2004 @ 21:43:55 MST
by Philip Ball
A radioactive element's rate of decay has been speeded up.
Scientists in Japan have persuaded a radioactive material to decay significantly faster than normal.
The rate of decay of radioactive atoms is often regarded as something preordained and beyond our power to manipulate. It has been known for some time that that is not really the case, but this change, almost 1%, is by far the most dramatic effect achieved so far. It decreased the half-life of beryllium-7 by about half a day.
In principle, the result suggests that we might be able to neutralize nuclear waste faster. The researchers admit, however, that the possibility of magnifying the effect enough to significantly speed up this process remains "somewhat remote".
Atoms of beryllium-7 decay by grabbing electrons from their surroundings. The electron is absorbed into the nucleus, where it combines with a proton to make a neutron, transforming the atom into a different element, lithium-7.
The rate of this kind of decay depends on the chance of an electron straying into the nucleus and getting absorbed. So increasing the density of electrons surrounding the atomic nucleus can speed up the decay. The reverse is true for the types of decay that involve expelling a neutron: increasing the electron density around that type of atom slows the process down.
At least, that is the idea. But the changes seen previously have been tiny. Now Tsutomu Ohtsuki of Tohoku University in Sendai, Japan, and colleagues have boosted the effect by trapping beryllium-7 atoms in molecular cages. They report their results in Physical Review Letters.
The researchers induced a nuclear reaction to produce beryllium-7 atoms with a lot of energy, which were able to bash their way through the walls of cage-like carbon molecules called buckminsterfullerenes.
Once the beryllium atoms are trapped, the carbon cage surrounds them with a dense cloud of electrons. This makes it more likely for an electron to get into the trapped atom's nucleus and induce decay. The researchers found that beryllium-7 encased in buckminsterfullerene has a half-life of about 52.5 days, compared with 53 days for pure beryllium-7. The half-life is the time it takes for half of the initial amount of material to decay.
Speeding up decay by less than 1% will not help much in disposing of radioactive waste with half-lives of thousands or millions of years. So could the effect be made much bigger?
Peter Möller, a physicist at the Los Alamos National Laboratory in New Mexico, says that if you could recreate conditions such as those found in the interior of a hot star, changes in nuclear decay rates could be much more dramatic.
"People don't know how to engineer such an environment," he admits. But squeezing a radioactive substance to very high pressures might enhance the effect that Ohtsuki and colleagues have seen.
1. Ohtsuki T., Yuki H., Muto M., Kasagi J. & Ohno K. Physical Review Letters, 93 (2004).
Story from: http://www.nature.com/Physics/Physics.taf?g=&file=/physics/highlights/7007-1.html&filetype=&_UserReference=C0A8041B46B42219E127551C7DE941530C13