Gamma decay is one of the three radioactive decay modes available to atomic nuclei, along with alpha and beta decay. In alpha and beta decay, the atomic number of the nucleus changes, but in gamma decay the atomic number does not change. In gamma decay, a nucleus in an excited energy state decays to a lower-energy state by emitting a high-energy photon. The photons produced in this decay are consequently known as gamma rays and have a wavelength with an order of magnitude of about 1,000 femtometers, or 10-12 meters. The decay process is very similar to the absorption and emission of light by atoms in the ultraviolet, visible, and infrared spectrums.
Gamma decay often follows either alpha or beta decay in a decay process. Following an alpha or beta decay, the number of protons and neutrons in the nucleus has changed, and the resulting nucleus may not be in its lowest energy state (called the ground state). As a result, the nucleus will decay to the ground state by emitting one or more gamma-ray photons.
The lifetime of a nucleus that can undergo gamma decay is short, and direct measurements are difficult. A method called resonance fluorescence can be used to measure the lifetime of the excited state. In this method, an experimenter shines gamma rays on a nuclear target to excite it to a higher energy level. The probability of absorbing the gamma rays will be highest when they have the same energy as an energy level difference in the nucleus, called a resonance energy. The nuclei then re-emit the gamma rays, which are collected using a detector. Gamma rays with energy close to a resonance energy, however, may also be absorbed and emitted, leading to a peak with nonzero width in the measured spectrum. The Heisenberg uncertainty principle can be applied to this problem and by measuring the width of the spectrum one can find the lifetime of the nuclear energy state.
Although this process is straightforward, extra steps must be taken when actually performing the experiment. Absorbing and emitting a photon results in recoil energy due to conservation of momentum, and the nuclear recoil energy is much larger than the width of a typical gamma ray spectrum peak. Resonance fluorescence, therefore, cannot be easily observed in many nuclear systems. By a special technique called Mössbauer spectroscopy, the effects of nuclear recoil are removed by cooling the entire experiment, allowing a precise measurement of the nuclear lifetime.
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