A pot of boiling soup bubbles away, and you'd love to know how hot individual droplets are inside. But you can only dip your thermometer at the edge, where things are calmer. Laser fusion is a bit like that: a powerful laser pulse hits a target, creating a cloud of fast protons. You can only catch them for measurements once they fly out, having already lost some energy.

The authors of a new study found a way to peek inside the 'pot.' They turned the target itself into a measuring device. The target contains boron — the same element found in borax and eye drops. When a fast proton slams into a boron nucleus, it sometimes produces radioactive carbon-11, and sometimes beryllium-7. These newborn atoms don't live long and emit characteristic radiation that can be counted after the shot. The more energetic the protons, the more carbon-11 is produced relative to beryllium-7. It's a kind of radioactive 'scale' for energy.

By collecting data on isotope yields, the physicists built the proton energy spectrum. It fits an exponential decay: the higher the energy, the fewer particles. They also pinned down the number of key fusion reactions where a proton splits boron into three alpha particles. Previously, this number was estimated indirectly; now it's measured via a side channel with known accuracy.

So the target has gone from a passive lump to a nuclear diary. It records the history of every proton strike. And if experimenters used to be like cooks tasting soup only from the surface, now they can scoop right from the thick of it.