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How does the recoil of an atomic nucleus compare with the recoil of a rifle?

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Figure 1: Spectrometer utilizing Mössbauer effect concept Effect is usually observed by measuring transmission of gamma rays from radioactive source through absorber containing resonant isotope. — Mössbauer effect Figure 1: *Spectrometer utilizing Mössbauer effect concept* Effect is usually observed by measuring transmission of gamma rays from radioactive source through absorber containing resonant isotope.

The recoil of an atomic nucleus is fundamentally governed by the same principles of momentum conservation that apply to a rifle.1

When a rifle is fired, it experiences a backward "kick" because the momentum of the projectile must be balanced by an equal and opposite momentum in the firing system. If the rifle is held loosely, the recoil is violent; however, if it is held firmly against a marksman's shoulder, the marksman’s body adds significant mass to the system, which correspondingly reduces the backward velocity of the rifle.1

An atomic nucleus behaves similarly when it emits or absorbs a gamma ray. The nucleus experiences a recoil due to the momentum of the photon. In a free atom, this recoil carries away kinetic energy, which prevents the exact energy match required for resonance absorption. The Mössbauer effect overcomes this by embedding the nuclei in a solid lattice. In this configuration, the recoil momentum is taken up by the entire crystal lattice rather than a single nucleus, effectively reducing the recoil energy to nearly zero and allowing resonance to occur.123