Given the task definition and input, reply with output. In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.

The approximate magnitude of this effect is given by a factor equal to the expectation value of the exponential of minus the square of the direct product of the scattering vector and initial scattering center displacement. That factor gives the fraction of scattered particles experiencing this effect and is named for Waller and Debye. Zeeman splitting due to nuclear magnetic fields can be measured by this effect combined with the Doppler effect, a measurement known as its namesake spectroscopy. This effect is a momentum-space analogue of zero-phonon optical emission. It was first observed in solid iridium, which was found to exhibit sharp resonances like gases in x-rays. For 10 points, name this phenomenon in which a certain fraction of gamma rays emitted by atoms in a solid are emitted recoil-free, an effect named for its German discoverer who won the 1961 Nobel Prize in Physics.
Science