Researchers are achieving unprecedented levels of precision in measuring the energies released during nuclear decay, employing a suite of advanced techniques that refine our understanding of fundamental physics. These breakthroughs, spanning multiple experiments and isotopes, are resolving long-standing uncertainties in nuclear data and paving the way for new inquiries into the nature of neutrinos and the fundamental forces that govern the universe. The new methods replace older, less direct detection techniques with technologies that can capture the subtle signatures of decaying atoms with remarkable clarity.
The drive for higher precision is critical for testing the limits of the Standard Model of particle physics and has significant practical implications. By accurately characterizing the decay of various isotopes—from hydrogen and helium to technetium and copper—scientists are not only creating a more reliable set of standards for nuclear applications but also developing sensitive probes for some of the biggest mysteries in physics. Whether by trapping individual ions in magnetic fields or by detecting the faint radio waves emitted by single electrons, these experiments are pushing the boundaries of what can be measured and, in doing so, are opening new windows on the subatomic world.
A New Spectroscopy for Fundamental Particles
One of the most innovative approaches is Cyclotron Radiation Emission Spectroscopy (CRES), a technique that detects the radio waves emitted by electrons as they spiral in a magnetic field. This non-destructive method allows for a very precise measurement of the electron’s kinetic energy and provides a clear signal with virtually no background noise. Two separate collaborations, Project 8 and He6-CRES, have pioneered this technique to explore different aspects of fundamental physics. By capturing the radio signals, scientists can reconstruct the energy of the decay products far more accurately than with traditional detectors that require the particles to collide with a solid material.
Probing Neutrino Mass
The Project 8 collaboration is using CRES to study the beta decay of tritium, a radioactive isotope of hydrogen. The primary goal is to determine the mass of the neutrino, one of the most elusive and lightweight particles in the universe. The precise energy spectrum of the electrons emitted during tritium decay is sensitive to the neutrino’s mass; a more accurate measurement of this spectrum can place a tighter upper limit on it or potentially measure it directly. The high-resolution capabilities of CRES are essential for detecting the tiny distortion in the spectrum that would reveal the neutrino’s mass.
Testing the Weak Force
Separately, the He6-CRES collaboration is applying the same technique to observe the decay of helium-6 and other radioactive isotopes. Their goal is to investigate the weak interaction, one of the four fundamental forces of nature. By precisely measuring the energies of electrons and positrons emitted in these decays at up to 2.1 MeV, the team is laying the groundwork for a high-precision search for a theoretical component of the weak force that has not yet been observed. This work demonstrates the versatility of the CRES method across a range of typical nuclear decay energies.
Trapping Ions for Precise Mass Differences
Another powerful method for measuring decay energies involves using a Penning trap, a device that confines charged particles using a combination of electric and magnetic fields. This technique allows for extremely precise measurements of an ion’s mass by detecting its resonant frequency within the trap. The JYFLTRAP double Penning trap mass spectrometer was recently used to conduct a direct measurement of the decay energy, or Q-value, of the electron-capture decay of technetium-97 ($^{97}$Tc).
By measuring the masses of the parent atom ($^{97}$Tc) and the daughter atom (molybdenum-97) with high precision, researchers can calculate the energy released in the decay. This direct measurement resulted in a value of 324.82(21) keV for the ground-state decay. This result is approximately 19 times more precise than the value listed in the 2020 Atomic Mass Evaluation, highlighting a significant improvement in nuclear data. The refined Q-value also allowed scientists to evaluate a potential ultra-low energy decay path to an excited state in the daughter nucleus, a type of transition that could be useful in future neutrino mass experiments.
Improving Gamma-Ray and Decay Standards
Significant progress is also being made in the high-precision measurement of gamma-rays, the high-energy photons often released in nuclear decay. These efforts are crucial for updating and correcting the standardized nuclear data that many fields rely upon. In some cases, modern techniques are revealing significant discrepancies in long-accepted values, while in others they are providing a much-needed increase in accuracy for reference isotopes.
Correcting Critical Data Discrepancies
Recent work on copper-61 ($^{61}$Cu), an isotope used in medical imaging and as a monitor for deuteron beams, uncovered an 11% error in the listed intensity ratio of its two strongest gamma-rays in the official Nuclear Data Sheets. To resolve this, researchers collected over a hundred separate measurements using a variety of detectors and experimental setups. The weighted average of these meticulous measurements provides a corrected value that will improve the accuracy of medical dose calculations and cross-section measurements in nuclear physics that depend on this isotope.
Establishing New Reference Standards
Similar high-precision gamma-ray spectroscopy is being applied to other isotopes to build a more robust library of nuclear standards. For example, new measurements of gamma-rays from the decay of hafnium-181 ($^{181}$Hf) have determined its transition energies to within an accuracy of several electron-volts, a substantial improvement. A new evaluation of the decay data for holmium-166 ($^{166}$Ho) has also been performed by comparing and combining results from five primary sources to produce updated and more reliable recommended values for its decay properties. These comprehensive evaluations ensure that the foundational data used across nuclear science is as accurate as possible.
The Broader Impact of Precision Science
The ongoing push for greater precision in measuring nuclear decay energies reflects a fundamental drive to test physical laws at their limits. Each improvement, whether through innovative techniques like CRES or the systematic re-evaluation of existing data, provides a more stringent test of the Standard Model. These measurements are essential for the global search for physics beyond the Standard Model, as tiny deviations from theoretical predictions could signal the presence of new particles or interactions.
Furthermore, the impact extends beyond fundamental research. Accurate decay data is the bedrock of numerous applications, including nuclear medicine, where precise knowledge of radiation intensity is vital for patient safety and treatment efficacy. It is also critical for reactor physics, astrophysics, and the monitoring of nuclear materials. The work of these research teams around the world is collectively building a more accurate and reliable picture of the atomic nucleus, ensuring that both our scientific understanding and our technological capabilities can continue to advance.