Chadwick's Neutron

Plate 47 speaks for itself: “Seen by what it knocks.”

The Physics

Alpha particles on beryllium gave a penetrating neutral radiation that knocked fast protons out of paraffin. Chadwick showed the protons were too fast for gamma rays and that a neutral particle of about the proton's mass fit.

⁹Be + ⁴He → ¹²C + n

• ⁹Be: beryllium-9 nucleus
• ⁴He: alpha particle
• ¹²C: carbon-12 nucleus
• n: neutron, mass ≈ 1.675×10⁻²⁷ kg

Neutral; about the mass of a proton. Paraffin protons: too fast for gammas.

Sources

• J. Chadwick, Nature 129, 312 (1932); Proc. R. Soc. A 136, 692 (1932).

On the poster

Plate 047, “Chadwick's Neutron”: the pixel-art plate with its slogan “Seen by what it knocks.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.