Going beyond radiation

Coupling between different nuclei could potentially transform nuclear reactions into controllable, radiation-free processes

In a previous post, we explored why the term “decay constant” - that measures the speed of nuclear transitions (and also nuclear reactions) - is a misnomer when considering nuclei within solids. Nuclear transitions have been accelerated in the laboratory by taking advantage of couplings between identical nuclei in a solid environment. In this post, we'll consider the consequence of couplings between different nuclei - namely the possibility of controlling nuclear radiation.

When a nuclear transition happens, the nucleus rearranges itself into a more preferable, lower energy configuration. The leftover energy typically gets liberated from the nucleus as "nuclear radiation" in the form of gamma rays, high energy neutrons or charged particles in a process that's best known as "radioactivity". The most challenging forms of radiation are the highly penetrating gamma rays and neutrons. Neutrons in particular degrade the materials they encounter and can cause secondary nuclear reactions that induce further radioactivity. This is a problem that traditional nuclear fission reactors and proposed nuclear fusion reactors have to contend with.

Much like transition rates, the type and energy of nuclear radiation have historically been regarded as fixed for a given nuclear process - in other words, nuclear radiation is immutable and inevitable. But is this really true? Our answer can be guided by known examples of large- and small-scale systems that control radiative processes.

Consider the difference between radio transmission and wireless charging. Both electronic systems move energy from one place to another, but a radio transmitter sends energy indiscriminately into the environment - we say it "radiates" - whereas wireless charging transfers energy directly from the transmitter to the receiver. The "non-radiative" energy transfer that makes wireless charging possible is much more efficient but it requires precisely matched and strongly coupled transmitter and receiver systems - something that requires careful engineering and has only been developed in recent years.

Such careful "engineering" was, perhaps unsurprisingly, achieved in nature long ago as a way of maximising the efficiency of photosynthesis in plants. When plants absorb light energy, they move it from where it has been absorbed (a donor molecule) to where it can be used (an acceptor molecule) without radiating the energy back into the environment along the way. This process is so fast that the energetically excited donors don't have time to de-excite and radiate the energy back out where it would be lost. It's like a quantum version of the game "hot potato" but with the less catchy name "Resonance Energy Transfer" (RET).

Not to be outdone by Nature, we humans have recently begun applying non-radiative RET principles to engineer more efficient organic solar cells, which face similar challenges to photosynthetic systems. RET allows energy to move more quickly from the light absorbing “donors” to the “acceptors” where the energy can be converted into electricity.

So, is nuclear radiation immutable and inevitable? Could we instead engineer a form of nuclear resonance energy transfer? If so, what could it look like? One could imagine taking a radioactive material that emits harmful gamma radiation and doping it with receiver nuclei that are coupled to and energetically matched to the donors, but that emit more easily controlled charged particles instead. Or, one could imagine the receiver systems not only well matched to the donors but also strongly coupled to the solid vibrations such that nuclear energy can be exchanged with lattice modes and dissipate as heat.

These are the kinds of ideas and mechanisms that we have explored and described in much detail in our recent New Journal of Physics article — especially in its extensive supplementary notes. Our goal is that this approach will ultimately lead to the design of many variants of useful technology, where nuclear energy is released in a much more controlled and — in some manifestations — in a radiation-free way.