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The muonic atom 2p,~, -ls,~, and 2p3/p 1s,~2 transition energies were measured with an experimental accuracy of better than 20 ppm for the isotope chain "'" " """""' '" Sn. Precise values for the Barrett equivalent nuclear radii Rt, and their differences as well as root-mean-square radii were deduced. The hN =2 isotope shifts between the even Sn isotopes show a subshell effect at the neutron number N =64. Otherwise, there is a nearly linear decrease with increasing N, in accordance with the general systematics of nuclear charge radii. Our muonic atom results are in a good agreement with recent optical data, including odd-even staggering. Hartree-Fock calculations reproduce the general trend but not the subshell effect. Regarding the nuclear polarization corrections, the problem in the 2p splitting found earlier in p -Zr and p -Pb seems also to persist in p Sn.

The muon exchange reactions from the ground state of muonic protium and deuterium atoms to neon are studied. Measurements have been performed in binary gas mixtures at room temperature. The transfer rate from thermalized muonic deuterium is found to exceed by about an order of magnitude the one from muonic protium. On the other hand, an energy dependence of the rate from pd is revealed, while none is observed from pp. The intensity patterns of the muonic Lyman series of neon resulting from the muon exchange difFer from one hydrogen isotope to the other, the most obvious discrepancy being the presence of the muonic Ne(7-1) line after transfer from pd, whereas this line is absent by transfer from pp. This indicates that the muon is transferred to the level n"= 6 in neon from protium, respectively, n& --7 from deuterium~. This observation disagrees with the theoretical predictions. This study sheds some light on the surprising behavior of muon transfer to neon.PACS number(s): 36.10. Dr, 34.70.+e, 82.30.Fi

We present the final results of an experimental study of d and t atom scattering in solid hydrogen cooled to 3 K. Strong effects resulting from the Ramsauer-Townsend effect have been observed in the TRIUMF experiment E742 where muons were stopped in thin frozen layers of hydrogen. The measured RamsauerTownsend minimum energy for both d and t atoms and the minimum cross section are in agreement with theory. Negative muons stopping in hydrogen can form muonic hydrogen ͑h͒ atoms, where h = p , d, or t depending on the hydrogen isotope. Although created in excited states, such atoms cascade to the ground state quickly ͑10 −9 s͒, where their kinetic energy is of the order of several eV, much higher than thermal equilibrium energies. The muonic hydrogen atom is about 200 times smaller ͑m / m e scaling͒ than the size of ordinary electronic hydrogen. The small neutral atom can easily diffuse through the surrounding medium, undergoing different types of interactions with atoms and molecules including elastic and inelastic scattering. The scattering processesare predicted ͓1-4͔ to show a dramatic minimum in scattering, the so-called Ramsauer-Townsend ͑RT͒ effect. This effect is due to a minimum scattering cross section ͑ϳ10 −21 cm 2 ͒ at collision energies between 2 and 30 eV.These cross sections are more than two orders of magnitude smaller than the p +H 2 one ͓5͔. The result is a quasitransparency of the H 2 molecules for incident d or t atoms. Theoretical calculations for muonic atoms scattering in solid or gas are performed in Refs. ͓4-7͔. However, the improvements in those calculations are important for muonic atoms of low energy and negligible for energies above eV ͓5͔. The development at TRIUMF of multilayer thin frozen hydrogen film targets ͓8-14͔, which produce muonic atom beams emitted into vacuum, permitted the cross sections to be probed in an interesting way. We have studied several isolated muon induced processes using a time-of-flight ͑TOF͒ method developed by the frozen target geometry ͓14͔.The aim of the present experiment was to study and verify the cross sections in the RT minimum of d and t elastic scattering in solid hydrogen. The experiments were performed at the M20B muon channel at TRIUMF. The layout of the apparatus was shown and described in Refs. ͓5,15-17͔. Gaseous hydrogen ͑or neon͒ was sprayed, using a special diffusion system, onto a 51 m thick gold foil, kept at 3 K, where it froze creating thin solid films which could be maintained in high vacuum. Details of the target construction and working procedure are given in Refs. ͓9,18͔.The muons eventually stop either in the gold target support foil or in the 800 m thick solid hydrogen target where they finally form muonic atoms. The hydrogen target frozen on that foil, which is placed perpendicularly to the muon beam axis, is called the upstream target ͑US͒, and is made of pure protium or of protium with a small admixture of deuterium or tritium, depending on the experiment.