Measurement of the Width of the First Excited State in ¹³³Cs

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The line width of the first excited state of ¹³³Cs has been measured using the Mössbauer method. Our gamma ray source was ¹³³BaTiO₃ in free space. When the absorber, CsCl, was placed between two parallel flat plates, narrowing of the net width by as much as 8.7% was observed for the plate separation distance b=2.13 mm. This result implies that the lifetime of the 81 keV energy level of ¹³³Cs is increased to 6.87 ± 0.11 ns. This value is 9.6% larger than the standard value of 6.27 ±0.02 ns.