A charged particle beam is like a jet of water from a hose: it can be narrow or diffuse, steady or curved. To know exactly where particles are headed, you need a sensitive screen. In the WISArD experiment at ISOLDE, CERN's accelerator, that screen is a compact detector based on microchannel plates.

The palm-sized device is placed inside a magnetic field of 4 tesla — hundreds of thousands of times stronger than Earth's field. The magnet bends particle trajectories, and conventional monitors start to lie under such conditions. The new detector is not afraid of the field: it records the impact position of each ion with sub-millimeter accuracy. To achieve this, the team had to correct distortions caused by the square shape of the sensitive anode, like a photo stretched at the corners on a pillow.

Why such precision? WISArD studies the beta decay of argon-32 nuclei — one way to probe the weak interaction that governs particle transformations. From the beam shape, scientists reconstruct the angle at which the electron and neutrino fly apart after the decay. If this angle deviates even slightly from predictions, it signals that new forces beyond the Standard Model are at play. The new detector will measure the angle with an uncertainty of 0.1%—enough to catch even a faint hint of new physics.

The 4-tesla magnetic field doesn't hinder the device; it helps: it separates particles of different energies, so the impact position reveals their momentum. It's like a water jet breaking on a grid and splitting into individual drops.