Gamma rays are the most energetic flashes of light born deep within atoms. Their energies are like fingerprints: each radioactive element has its own pattern. But measuring that pattern with high precision is like weighing a grain of sand on a scale sitting on a vibrating truck.

The new setup works like an ultra-sensitive thermometer. Magnetic microcalorimeters catch a gamma quantum and measure the tiny temperature jump — less than one millionth of a degree. Eight such detectors look at four sources containing a mix of different radioactive elements. From their emissions, the instrument calibrates itself and corrects for inaccuracies.

In the end, the energies of 27 gamma rays became known with new precision. For four of them, the uncertainty decreased tenfold. Previously, such measurements were done with semiconductor detectors: they were like a ruler with coarse markings. Now we have a microscope. This level of precision is needed to test nuclear theories, calibrate medical scanners, and even search for signs of new physics.