Silicon-25 lives for just 219 milliseconds. In that time, it manages to emit a beta particle and sometimes a proton. Those protons, flying out of the nucleus after beta decay, have become the calibration standard for a new microscope that physicists have assembled at the FRIB accelerator.

Previously, precise measurements of beta-delayed protons ran into two problems: beam contaminants and thick targets that smeared the energy resolution. Now, a beam of argon-36 is accelerated to 210 MeV per nucleon, hits a carbon target, and the resulting silicon-25 is separated in the ARIS magnetic separator. The ions are then slowed down in the cryogenic gas stopper ACGS and implanted into a carbon foil just 150 nanometers thick, thinner than the wall of a soap bubble.

Around the foil sit six silicon telescopes and two germanium detectors. Over four hours, they collected 780 thousand protons. The energy of each proton was reconstructed, accounting for losses in the foil and detector dead layers, using SRIM tables. The result was a spectrum with a resolution of about 3 keV, which is like distinguishing individual hairs on a person's head from a kilometer away.

The new data showed that transitions to the first excited state of magnesium-24 occur more often than to the ground state. Interference patterns in the proton lines gave reliable spin-parity assignments for high-lying states of aluminum-25. The extracted beta strength up to 10 MeV required a quenching factor of 0.75 instead of the standard 0.60. This means that Gamow-Teller quenching depends on how far the nucleus is from the valley of stability.

The technique is ready for more exotic nuclei, such as aluminum-22 and phosphorus-26, which are suspected of having a proton halo. The same calibration standard that validated the new microscope will now reveal details that no one has seen before.