Ramsauer-Townsend effect in muonic atom scattering

Abstract

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.

Laser-induced nonresonant nuclear excitation in muonic atoms

“…Instead of using a fixed target of muonic atoms in a trap, it might therefore be more promising to employ a nonrelativistic beam of muonic atoms. Such beam experiments are in principle feasible and have recently led to the observation of the Ramsauer-Townsend effect in scattering of muonic hydrogen isotopes, for example [44]. Beams of 10 5 muons per second can be produced today [45] which could be converted into the same number of muonic atoms assuming 100% conversion efficiency.…”

Isotope effects in the harmonic response from hydrogenlike muonic atoms in strong laser fields

“…In the case of muonic helium isotopes, the harmonic signal from 4 He is expected to be larger than that from 3 He. For 4 He, though containing one more neutron, is a doubly magic nucleus of very compact size (see Table II).…”

“…The first x-ray spectroscopy of muonic atoms was performed in 1953 using a 4-meter cyclotron [3]. Today, large-scale facilities like TRIUMF (Vancouver, Canada) or PSI (Villingen, Switzerland) exist which are specialized in the efficient generation of muons and muonic atoms [4]. New developments aim at the production of radioactive muonic isotopes for conducting spectroscopic studies on unstable nuclear species [5].…”

“…HHG spectra calculated with the hardcore potential(9). (a) The black line shows the spectrum for muonic4 He at the laser parameters I ( 4 He) = 8.7×10 24 W/cm 2 and ℏω ( 4 He) = 255 eV. The grey (red) line represents the spectrum for muonic 3 He at the accordingly scaled values I( 3 He) = 8.3 × 10 24 W/cm 2 and ℏω ( 3 He) = 253 eV.…”

Differential cross sections for muonic atom scattering in solid hydrogenic targets

“…The solid hydrogenic targets have been used for the production of the muonic hydrogen atom beams, which have been applied to the time-of-flight measurements of various muonic-atom and muonic-molecular processes (see e.g., Refs. [1,2,3,4,5,6]). The bulk hydrogenic crystals are employed in the investigations of muon-catalyzed fusion of the hydrogen isotopes at high target densities and for various populations of the rotational molecular levels [7,8,9,10,11,12,13].…”