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Decay of single-E-shell-vacancy states of xenon atoms by the emission of photon pairs that continuously share the transition energy was studied. The vacancy states were generated by electron-capture decay in ' 'Cs. A pair of high-purity germanium detectors and a fast-slow coincidence system with a (128 X512X512)-channel pulse-height analyzer were used in the measurements. Accurate energy calibration was made by a careful determination of positions of peaks due to crosstalk among the detectors via GeEx rays. Identification of events due to the two-photon decay is based on the shifts of energy sums. From the numbers of events in the ridges, the relative differential probabilities of 2s -+1s, 3s -+1s, 3d -+1s, and 4sd -+1s two-photon decay were derived. They are compared to the values derived from the results of non-relativistic calculations for hydrogen and hydrogenic ions of Shapiro and Breit [Phys. Rev. 113, 179 (1959)], of Tung, Ye, Salamo, and Chan [Phys. Rev. A 30, 1175 (1984)], and of Florescu [Phys. Rev. A 30, 2441(1984],and to the results of the following studies of two-photon decay of single-1t-shell-vacancy states of xenon atoms: a calculation based on the theory of Bannett and Freund [Phys. Rev. Lett. 49, 539 (1982); Phys. Rev. A 30, 299 (1984)], a nonrelativistic calculation of Wu and Li [J. Phys. B 21, 1509Phys. B 21, (1988], and relativistic calculations of Mu and Crasemann [Phys. Rev. Lett. 57, 3039 (1986); Phys. Rev. A 38, 4585 (1988)], and of Tong, Li, Kissel, and Pratt [Phys. Rev. A 42, 1442 (1990)].As expected, no events due to the np~ls two-photon transitions were observed and upper limits are given.PACS number(s): 31.30.Jv, 32.30.Rj, 32.80.Wr INTRA DUCTIQNThe process of emission of photon pairs that continuously share the transition energy is closely related in two atomic systems: hydrogenic systems and many-electron systems with a single inner-shell vacancy. In many systems single-particle and single-hole states have many

The high-energy part of the proton spectrum at 4° from the reaction D (n,p)2n at 14.4 Mev has been analyzed using the final-state interaction formalism, taking into account only the n -n interaction in the final state. The primary interaction has been calculated in a kind of Born approximation. It has been shown that the shape of the high-energy part of the proton spectrum is dependent mostly on the n-n scattering length, a nn , and that by varying a nn , a good fit to the shape of this part of the spectrum is obtained. From this analysis, a value of a nn -( -22±2)f has been obtained. However, this value of a nn is to some extent dependent on the accuracy of the approximations used. A more refined theoretical treatment would therefore be desirable.

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