With the 2-meter curved crystal focusing spectrometer three nuclear gamma-ray lines from

I^{131}

have been found to have wave-lengths (34.033±0.01)·

10^{- 11}

cm, (154.671±0.01)·

10^{- 11}

cm, (43.622±0.02)·

10^{- 11}

cm which when converted to energies give 364.18±0.1, 80.133±0.005, 284.13±0.1 expressed in kev. The sum of the last two energies equals the first to an accuracy of a part in 4500, thus verifying a decay scheme proposed by Metzger and Deutsch in which the 80- and 284-kev lines form a cascade between the same pair of excited levels of

{Xe}^{131}

as the initial and final levels for the 364-kev line. An alternative proposal seems also thus to be invalidated. The intensities of the lines are discussed, and a method of calibrating the efficiency of the multicellular Geiger counter as a function of wave-length depending on the use of such cascades is proposed. This is probably the first time that a precision test of the Ritz combination principle as applied to nuclear gamma-rays has been possible. It also furnishes an excellent internal check of the reliability of the wave-length scale of the 2-meter curved crystal spectrometer.

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