Reinessa

Reinessa hunts ghosts with a lantern made of tanks. In 1956, near a nuclear reactor, one finally showed itself.

The Physics

Antineutrinos from a reactor almost never interact, but now and then one hits a proton and turns it into a neutron and a positron. The detector looked for a pair of flashes: first the positron annihilating, then the neutron being captured in cadmium. The signal rose with reactor power.

ν̄ + p → n + e⁺

• ν̄: electron antineutrino
• p: proton
• n: neutron
• e⁺: positron
• σ: cross section, ≈ 6.3 × 10⁻⁴⁸ m² (6.3 × 10⁻⁴⁴ cm²)

C. L. Cowan Jr., F. Reines and colleagues reported the detection from the Savannah River Plant in Science on 20 July 1956.

Honest caveat: The signal agreed (within 5 per cent) with a predicted cross section of 6.3 × 10⁻⁴⁴ cm²; that is an agreement, not a precise measurement.

On the shirt

Front: Reinessa, the pixel-art character, with the title. Back: the inverse beta decay reaction, the two-flash signature, the cross section, Savannah River Plant 1956 and the Science paper. Printed on a unisex tee.