The paper describes an experiment aimed at studying muon capture by 3

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.

Cross sections and forward/backward ratios in the laboratory reference system were measured for the 143 Nd(n,α) 140 Ce reaction at 4.0, 5.0, and 6.0 MeV and for the 147 Sm(n,α) 144 Nd reaction at 5.0 and 6.0 MeV. A twin-gridded ionization chamber and large-area back-to-back 143 Nd 2 O 3 samples and 147 Sm 2 O 3 samples were employed. Experiments were performed at the 4.5 MV Van de Graaff accelerator of Peking University, China. Fast neutrons were produced through the 2 H(d,n) 3 He reaction by using a deuterium gas target. A small 238 U fission chamber was employed for absolute neutron flux determination, and a BF 3 long counter was used as the neutron flux monitor. Present experimental data are compared with previous measurements, evaluations, and model calculations.

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We present the results of experimental and theoretical study of the scattering of low-energy p atoms in solid hydrogen cooled to 3 K. Strong effects resulting from the solid state interactions have been observed in the TRIUMF experiment E742 where muons were stopped in thin frozen layers of hydrogen. The resulting emission of low-energy p atoms from the hydrogen layer into the adjacent vacuum was much higher than that predicted by calculations which ignored the solid nature of the hydrogen. New differential scattering cross sections have been calculated for the collisions of p atoms on solid hydrogen to account for its quantum crystalline nature. Analysis of the experimental data performed using such cross sections shows the important role of the coherent scattering in p atom diffusion. For p energies lower than the Bragg cutoff limit (Ϸ2 meV) the elastic Bragg scattering vanishes which makes the total scattering cross section fall by several orders of magnitude, and thus the hydrogen target becomes transparent allowing the emission of cold p atoms to occur.