= Mössbauer discovery experiment
{c}
<Rudolf Ludwig Mössbauer> found the effect during his Heidelberg work in 1957.<Rudolf Ludwig Mössbauer, CV by Lindau Nobel Laureate Meetings>[CV, opening paragraphs: Heidelberg discovery in 1957]{ref} His 1961 lecture \i[Recoilless nuclear resonance absorption of gamma radiation] describes crystalline preparations using the $129\,\mathrm{keV}$ transition of the <iridium> <isotope> $^{191}\mathrm{Ir}$. Separate cryostats, lead collimation and a <particle scintillation> detector measured transmission. Liquid-air cooling of both preparations enhanced absorption; cooling only the source against a room-temperature absorber slightly reduced it. A rotating source cryostat supplied nearly constant projected velocities over selected arcs. Centimetres per second suppressed absorption relative to zero <velocity>; a nonresonant absorber supplied the comparison. This tested a narrow common emission/absorption <energy> beyond the detector's resolution. Temperature data alone had not established its width. The lecture cites the initial and direct-verification publications from 1958.<Recoilless nuclear resonance absorption of gamma radiation by Rudolf Ludwig Mössbauer>[Authorized 2012 reprint, pp. 6–8, 13–15, 17; Figs. 1.3–1.4, 1.7–1.8]{ref}
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