Mossbra

Mossbra, armoured in crystal, looses a gamma arrow and does not rock back at all. The whole lattice takes the kick for her.

The Physics

A free nucleus that emits a gamma ray recoils and steals a little energy, so the photon misses the absorber's resonance. For iron-57's 14.4 keV line the recoil is about 2.0 × 10⁻³ eV, while the natural linewidth is only about 4.6 × 10⁻⁹ eV: the miss is roughly 400 000 linewidths.

Rudolf Mössbauer found in 1958, working with iridium-191, that a nucleus bound in a crystal can emit and absorb without recoil. The whole lattice takes the momentum, and with some probability no phonon is created. That gives a line of fractional width about 3 × 10⁻¹³, which made Mössbauer spectroscopy possible.

E_R = E_γ² / (2Mc²)

• E_R: recoil energy of a free nucleus (eV)
• E_γ: gamma-ray energy (eV)
• M: mass of the nucleus (kg)
• c: speed of light (m/s)

Mössbauer, Z. Phys. 151, 124 (1958). He shared the 1961 Nobel Prize in Physics for this discovery.

Honest caveat: Only a fraction of the gammas are recoil-free, not all of them. Even then, the crystal does recoil, just by a negligible amount, because it is so heavy.

On the shirt

Front: Mossbra, the pixel-art character, with “NO RECOIL”. Back: the recoil formula, the iron-57 energy scales, the recoil-free crystal note and Mössbauer's 1958 paper. Printed on a unisex tee.