A ruler for measuring the invisible must be checked against something familiar. In physics, that check has become the decay of technetium-99, an artificial isotope that lives for 211 thousand years. Its decay is called "forbidden" because it is hard for the particle to escape the nucleus: it must overcome a complex angular barrier. That very difficulty makes the process sensitive to details of the theory, like a fine string responds to the slightest touch.
The detector caught these electrons, while a neighboring crystal with lutetium gave flashes of light at strictly defined energies. By comparing signals, the scientists calibrated the instrument on the fly, like a tuner checks a piano against a tuning fork. Over six days, 180 thousand events were collected. Comparison with theory showed that particles inside the nucleus behave more weakly than models predict. The suppression factors came out as 0.40 and 0.47, noticeably below the theoretical values.
This result is important for the search for neutrinoless double beta decay, an event so rare that it has never been seen. If models overestimate the strength of interaction in the nucleus, then plans for future experiments need adjustment. A ruler calibrated with technetium helps avoid mistakes in measuring the invisible.
Technetium-99 is an artificial isotope with a half-life of 211 thousand years. It is produced in significant quantities as a fission product of uranium in nuclear reactors and is one of the main long-lived radionuclides in spent nuclear fuel. Its beta decay leads to ruthenium-99, which is stable.