Precise Radioactivity Measurements: A Controversy Settled

The Science

For more than a decade, scientists debated if the structure of a decaying atom’s nucleus influenced whether it released electrons or gamma rays. Sometimes, rather than a gamma ray being emitted, the nucleus conveys its energy to an electron, causing it to be ejected at high speed instead. Scientists have now measured these decays with enough precision to show that whether an electron or gamma ray is produced is independent of the nuclear structure. Settling a controversy, they also show that only theories that include the vacancy in the atomic shell give correct predictions.

The Impact

In nuclear medicine and elsewhere, decay schemes are vital as they offer a complete picture of how radioactive materials release energy. The schemes include the contribution of electrons as well as gamma-ray emission for every radioactive decay. The new measurements have already led the U.S. National Nuclear Data Center to change the way it calculates the probability of electrons vs gamma rays. The center provides this probability for the nuclear decay schemes it makes available to users.

Summary

The probability of electron emission relative to that of gamma-ray emission is called the internal conversion coefficient (ICC). The new ICC measurements exploit the fact that whenever an electron is ejected from its atomic orbit, the vacancy created is soon filled by another atomic electron dropping into that orbit, a process that leads to the emission of an x-ray. This x-ray acts as a signal that an electron has been ejected. When scientists study the radioactive decay of a prepared sample that contains many identical atoms, they can compare the number of x-rays observed with the number of gamma rays to deduce the ICC, the relative number of electrons to gamma rays. Unlike the electrons themselves, the x-rays can be recorded in the same detector as the gamma rays. This makes it possible to measure the x-rays and gamma rays relative intensity much more precisely than if two different detectors had to be employed. The detector used was a uniquely well-calibrated high-purity germanium detector.

So far, ICC measurements have been made on decays of eight nuclei, with different structures, covering a wide range of the Periodic Table. In all cases, the results agree with one set of theoretical predictions to within a fraction of a percent, thus confirming the ICC’s independence from nuclear structure. Before these results became available, there was controversy between two sets of predictions, one that included the effects of the atomic vacancy on the departing electron and one that ignored it. The new results have convincingly settled that controversy in favor of including the vacancy effects.

Funding

This work was supported by the Office of Nuclear Physics, Office of Science, U.S. Department of Energy, and by the Welch Foundation.

Publication

N. Nica, J.C. Hardy, V.E. Iacob, H.I. Park, K. Brandenburg, and M.B. Trzhaskovskaya, “Precise measurement of α_k for the 88.2-keV M4 transition in ¹²⁷Te: Test of internal-conversion theory.” Physical Review C 95, 034325 (2017). [DOI: 10.1103/PhysRevC.95.034325]