Nuclear waste: Cold does not accelerate radioactive decay

Half-life can not be influenced by cooling

Accelerator module at the electron accelerator ELBE in the FZD. At ELBE, activation of the gold strips and their measurements were performed at room temperature. © FZD
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Exactly two years ago, the announcement caused a sensation that the half-life of nuclear waste could be drastically reduced if it is cooled - embedded in metal. Responsible should be a controversial effect in the professional world, which increases the responsiveness of the radioactive nucleus and thus accelerates the decay. However, new experimental results from researchers now show that radioactive decay can not be influenced by cooling.

Radioactive nuclei are unstable, they decay at a rate determined for each core. The gold atomic nucleus with the mass number 196, for example, has a half-life of three days, Gold-198 one of six days. During this time, half of the atomic nuclei decays into other, stable nuclei, after about ten half-lives, the radioactivity has subsided to one-thousandth of its original value.

Important role for conduction electrons of the metal?

Nuclear physicists at Forschungszentrum Dresden-Rossendorf (FZD) and from Vancouver, Canada, have now studied this decay process at room temperature and at low temperatures on the radioactive metals Gold-196, Gold-198 and Sodium-22 radioactive nuclei embedded in metal should decay faster if cooled sufficiently.

According to this work, the conduction electrons of the metal should be responsible for the change in radioactive decay at low temperatures. Each atomic nucleus has a barrier around it due to its positive charge, which must overcome positively charged particles to penetrate into the nucleus. It is acknowledged in the physicists' guild that the electron cloud surrounding the core slightly lowers this barrier. Other particles can then get into the nucleus more easily. This effect is called electron screening effect. It seems to increase when the atomic nuclei are cooled.

Dense electron clouds increase the reactivity of atomic nuclei

From the fact that dense electron clouds increase the reactivity of atomic nuclei, physicists concluded two years ago that the effect could also be transferred to radioactive decay. Initial experiments appeared to prove a change in half life through cooling. The controversial conclusion, however, was refuted by two papers, which were carried out in 2007 and 2008 in Texas (USA) and Israel, for initially an atomic nucleus (Gold-198). display

The new experiments in Dresden and Vancouver, presented in the journal Physical Review C, show that the decay rate of the three investigated radioactive metals at low temperatures corresponds exactly to the rate of decomposition at room temperatures. Conclusion of the researchers: The two years ago established thesis is baseless.

Bible of the nuclear physicist needs to be corrected

The theory predicted that metal-embedded radioactive alpha and beta plus emitters, if cooled hard enough, would decay faster, whereas beta-minus emitters and nuclei decay by electron capture, an extension of the half-life was predicted. In the just published experiments, sodium-22 (beta-plus decay) and gold-196 (electron capture decay) embedded in metal have been studied for the first time.

For the third metal investigated, gold-198 (beta-minus decay), the researchers from Dresden and Vancouver were able to determine the half-life more precisely than ever before, so that the value in the Table of Isotopes, the bible of nuclear physicists, must be corrected from 2.6952 to the value 2.6937 days.

Gold samples were examined at room temperature

The gold samples were prepared at Forschungszentrum Dresden-Rossendorf and examined at room temperature. The sodium measurements and the experiments at -263 C took place in Vancouver. All measurements disproved the controversial thesis. Radioactive metal decomposes at low temperatures as quickly as at room temperature.

Thus, it has been proven that the half-life is a physical quantity that can hardly be influenced by such simple means as cooling. The atomic bubble can only be tackled with more complex processes such as transmutation. For this purpose, we have our own laboratory at the electron accelerator ELBE, where we want to use neutron bombardment to convert radioactive atomic nuclei into harmless ones. At ELBE, we also performed the activation of the gold samples and the measurements at room temperature, says Daniel Bemmerer from the Institute of Radiation Physics at the FZD.

(idw - Research Center Dresden - Rossendorf, 04.07.2008 - DLO)