The first measurement of the cross section of the quasifree pn→pn reaction has been carried out at the CELSIUS storage ring using a deuterium internal cluster jet target. The energy dependence of the cross section is extracted using a fixed incident proton energy of T p ϭ1350 MeV and exploiting the Fermi momentum of the struck neutron. The data cover a range of center-of-mass excess energies from 16 to 109 MeV. The shape of the excitation function is broadly similar to that of the pp→pp reaction, though with a cross section about a factor of 6.5 larger. ͓S0556-2813͑98͒02811-8͔

We present a spectrometer for inverse photoemission in the VUV range with variable energy resolution between 400 and 165 meV FWHM (full width at half maximum). The energy distribution of the electron beam used for excitation can be adjusted between 300 and 125 meV by the use of a toroidal 90 o electrostatic deflector combined with a slit aperture. The emitted photons are detected by Geiger-Müller counters filled with either acetone or iodine as counting gas. The optical bandpasses of the detectors can be tuned between 100 and 330 meV by varying the temperature of their entrance windows. The overall resolution of the spectrometer is determined by measuring the Fermi-level onset in inverse-photoemission data of polycrystalline gold. Furthermore, the resolution enhancement is demonstrated by spectra of image-potential-induced surface states at Cu(001).

The differential cross section for the pd→ 3 He reaction has been measured at four beam energies 930, 965, 1037, and 1100 MeV. The total cross section is nearly constant throughout the whole energy region, despite the c.m. angular distribution becoming more anisotropic with increasing energy. The data join smoothly onto the results of near-threshold measurements, which are dominated by the S-wave 3 He final state interaction. At all the energies the differential cross section is maximal for cos Ϸϩ0.5 and this may provide a clue to the reaction mechanism and, in particular, to two-step contributions.

The quasifree p 1 n ! d 1 h reaction cross section has been measured in the near-threshold region using deuterium from an internal cluster-jet target and 1350 MeV protons in the CELSIUS storage ring of the The Svedberg Laboratory, Uppsala. The energy dependence of the cross section is extracted by exploiting the Fermi momentum of the target neutron and reconstructing the kinematics on an event-by-event basis. The data cover center of mass excess energies from 16 to 113 MeV. [S0031-9007(97)04165-3] PACS numbers: 13.75.Cs, 14.20.Gk, 25.10. + s, 25.40.VeAs more data are becoming available on h-meson production near the kinematical threshold, the interest in hmeson physics has increased considerably. In particular, attention has been given to the relatively strong and attractive S-wave h-nucleon interaction. The near-threshold region is appropriate for S-wave interaction studies since the final state involves primarily the lowest partial wave. Both the production mechanism and the h-N final state interaction are expected to be dominated by the presence of the N ‫ء‬ (1535) S 11 resonance.Precise h cross section data for nucleon-nucleon collisions are available only for the proton-proton reaction channel. It is therefore of interest to get complementary data from the neutron-proton channel. Such data will contribute to the understanding of the h production process, its isospin and spin dependence, and the importance of different meson exchanges. Available data for the neutron-proton channel are as yet very limited. Cross sections have been unfolded either from data obtained using the upper energy tail of a neutron beam [1] or from

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