I stumbled upon a fascinating paper on arXiv the other day. Anyone who works with lasers should be familiar with the concept of stimulated emission – the means by which photons excite electrons which, in turn, emit more photons (identical, in phase, energy and velocity, to the original photon). But could you elicit a similar effect with nuclear decay? Specifically, can you make α decay happen?
To take a step back momentarily, as we all probably know, helium is the second most common element in the Universe. It’s created in stars during hydrogen fusion and the CNO cycle and it’s nucleus is so stable that radioactive heavy atoms decay by spitting out helium nuclei – α particles. To give you some idea of how common these are in the Universe, about 9% of cosmic rays are α particles (90% of the rest are protons, with 1% being electrons).
So to stimulate the emission of an α particle, you’d need a collection of α particles in the same quantum state. Superfluid helium fits the bill quite well! I’ve mentioned before what superfluid helium is, but I haven’t talked about why it is. Any given particle has a de Broglie wavelength, relating to it’s energy. This wavelength determines whether it behaves like a particle or a wave when it interacts with other particles (or waves). A Bose-Einstein condensate (a.k.a. a superfluid) will occur if the de Broglie wavelength exceeds the distance between two particles. If you cool liquid helium down below its lambda point (2.17K), the de Broglie wavelength of the actual atoms increases above the distance between those atoms and the helium becomes superfluid.
The result is a collection of helium atoms in an identical quantum state. So then, can atoms cause stimulated emission the way photons do? Judging by this paper, Barabanov certainly thinks so! The effect is described for polonium and astatine atoms (which are α emitters). If he’s right, it means that an atom travelling through superfluid helium will back-emit an α particle at the same velocity the atom’s travelling through the superfluid. In other words, the emitted α particle is at rest with respect to the superfluid. You can think or it a bit like kicking a crate off the back of a moving truck. Kick it with just enough force and it will quickly come to rest, with respect to the ground.
Barabanov’s work appears quite speculative, but it certainly does seem to make sense – even if it seems an unusual concept. The one problem would be detecting these α particles. Sadly, Barabanov doesn’t elaborate much on how he’d like to do this, stating that “To detect the effect significant efforts and additional ideas may be needed”. I must say, I’d very much like to see if anyone can come up with said ideas…
A. L. Barabanov (2009). Is it possible to stimulate nuclear alpha decay by superfluid helium? preprint arXiv: 0902.4628