Detecting the “Kick” from a Single Nuclear Decay

The Science

Radioactivity is all around us. Even bananas contain trace amounts of radioactive potassium, with approximately 10 nuclei decaying every second in a typical banana. While these tiny amounts of radioactivity are not dangerous, scientists are interested in developing more precise tools for detecting such nuclear decays. In this work, scientists have for the first time mechanically detected individual nuclear decays occurring in a microparticle (the size of a single grain of dust). The research used a new technique. Rather than detecting the radiation emitted by the nuclei, the researchers measured the tiny “kick” to the entire microparticle that contained the decaying nucleus as the radiation escaped.

The Impact

These techniques can help us learn about particles emitted in nuclear decays that would otherwise be hard to detect. For example, the potassium decays in a banana emit particles called neutrinos that interact so weakly with matter that they escape undetected. One way to learn about these neutrinos is to see how much they kick the microparticle when they leave. In addition, these techniques can identify radioactive material in a single dust particle. This could enable new tools for nuclear monitoring and nonproliferation. Finally, the ability to see these tiny kicks is ultimately limited by quantum mechanics and the Heisenberg uncertainty principle. In the future, quantum sensing techniques can further improve the method.

Summary

In this work, researchers implanted radioactive lead-212 nuclei into silica microparticles with a diameter of approximately 3 microns. These microparticles were trapped in high vacuum at pressures of less than 10^-10 atmospheres, to minimize noise from thermal fluctuations in the position of the microparticle. The researchers performed the trapping using a laser focused on the center of the vacuum chamber, which confines the microparticle to a small region near the laser focus (forming an “optical tweezer”). The researchers used the light scattered by the microparticle to image its position and look for any small jumps in the microparticle’s motion that could arise from nuclear decays.

The lead-212 decays produced further unstable daughter nuclei, which eventually decayed by emitting an alpha particle. When the alpha particles escaped the microparticles, two signatures were detected. First, the electric charge on the microparticle changed, which was detected with precision better than a single elementary charge. Second, the tiny recoil of the entire microparticle (more than a trillion times heavier than the alpha particle itself) could be detected.

By scaling these same techniques to smaller nanoparticles, it will also be possible to detect the kick from a single beta, gamma, or neutrino exiting the sphere. Fundamental constraints on this measurement are imposed by quantum mechanics. Measurement of the nanoparticle position using light introduces noise, due to the fluctuations in the number of light quanta (“photons”) interacting with the nanoparticle. Quantum sensing techniques can eventually be used to surpass the corresponding “standard quantum limit” that applies to simultaneous measurements of the nanoparticle position and momentum. By employing squeezed light or similar methods to focus solely on measuring the particle's momentum—despite the trade-off of increased noise in the position which is less critical — it's possible to detect even smaller recoils.

Funding

This research was supported through the Department of Energy Office of Science, Nuclear Physics program through the Quantum Horizons: QIS Research and Innovation for Nuclear Science program. Development of the supporting levitated opt mechanics technologies was funded in part through the Office of Naval Research and the National Science Foundation.