Radioactive decay is ubiquitous. It occurs everywhere on Earth and throughout the universe. The most common forms occur when an unstable nucleus spits out an alpha particle, consisting of two neutrons and two protons, a beta particle consisting of an energetic positron or electron, or a gamma ray, consisting of a high energy photon.

When these powerful forms of radiation pass through matter, they strip electrons from atoms and molecules, leaving a trail of charged particles in their wake. Detecting these charged particles is simply a matter of mopping them up with an electric field, in a device known as a Geiger counter.

This gives physicists a good indication of local levels of radiation. But they also have other techniques for characterizing the decays in more detail. For example, they can measure the amount of energy a decay particle deposits in a material by studying the light it produces as it passes through, or by the heat it generates.

Radiation Detector

But some forms of radiation are still hard to characterize. For example, neutrinos are commonly produced in nuclear decays but do not interact significantly with matter. So the energy they carry cannot be easily measured.

Because physicists are blind to the behavior of this kind of radiation, they would dearly love a practical way to characterize it.

Enter Jiaxiang Wang and colleagues at Yale University in New Haven who have developed an entirely new way to measure the energy of nuclear decays, based on the behavior of the daughter nucleus left behind.

The team’s idea is that the release of decay particles produces an equal and opposite reaction in the daughter nucleus. In other words, the daughter nucleus recoils. However, the force is tiny and the recoil hard to see, particularly when thermal noise can swamp such movement.

In recent years, physicists have developed laser techniques that can suspend single particles in a vacuum and cool them to the point where thermal noise is negligible. Now Wang and co have used the technique to watch these particles recoil as nuclei within them decay.

The team start with tiny silica spheres with a diameter of about 3 micrometers, less than than the width of a human hair. They then plant radioactive nuclei into the surface of these spheres by allowing radon-220 gas to decay nearby. This produces polonium-216 ions that collect on the surface of the charged silica spheres.

Polonium-216 has a half-life of about a tenth of a second and so quickly decays by alpha emissions into lead-212. This process has the effect of embedding the lead nuclei some 60 nanometers into the surface of the sphere.

The team then suspend the spheres in an optical trap and then watch and wait. Lead-212 is itself unstable with a half-life of about 10 hours and decays via electron emission into bismuth-212, which also decays by electron emission into polonium-212.

The recoil from beta emissions is not yet observable because the mass of an electron and a neutrino are so small. So it is the decay of polonium-212 via alpha emission into the stable lead-208 isotope, that the team are interested in.

Sure enough, each decay of a polonium-212 nucleus produces a recoil of just a few nanometers, which they can observe. By measuring the momentum change of the sphere, they can characterize the decay that caused it. “This demonstrates the detection of single nuclear decays in optically trapped, micron-sized spheres through both the change in the sphere’s charge and its coincident recoil,” they say.

(The decays also change the charge on the spheres, which the team can also observe. When the charge becomes too large, the team neutralize it by adding or removing electrons using a thermal filament or an ultraviolet lamp.)

Half Life

Of course, that’s just an alpha decay, which physicists can already detect. Wang and co say the real value of their method is in the possibility that it can also work for decays that produce high energy neutrinos. “Extending the same techniques to femtogram mass spheres will allow reconstruction of the momentum of a single neutrino leaving the sphere,” they say.

That will take some development but the team is confident that improvements will be straightforward to make. “The ongoing rapid progress in the field of levitated optomechanics promises to extend the future sensitivity of these techniques by orders-of-magnitude,” they say.

If that happens, the technique could have a variety of applications. It may find applications in nuclear forensics, which aims to determine the isotopic composition of a nuclear material, say Wang and co.

But the real prize may be access to new science. The team suggest the technique may be able to detect more unusual particles emitted in nuclear decays, including sterile neutrinos or even particles that may be related to dark matter.

That’s interesting because current experiments for detecting neutrinos and dark matter are huge, some the size of city blocks.

Given that nobody has detected dark matter or the effect it might have on visible matter, that’s a mouth-watering suggestion.

Ref: Mechanical detection of nuclear decays : arxiv.org/abs/2402.13257