Focusing a beam of ultracold spin-polarized hydrogen atoms with a helium-film-coated quasiparabolic mirror

Abstract

We formed the first "atomic-optics" beam of electron-spin-polarized hydrogen atoms using a quasiparabolic polished copper mirror coated with a hydrogen-atom-reflecting film of superfluid 4 He. The mirror was located in the gradient of an 8-T solenoidal magnetic field and mounted on an ultracold cell at 350 mK. After the focusing by the mirror surface, the beam was again focused with a sextupole magnet. The mirror, which was especially designed for operation in the magnetic field gradient of our solenoid, increased the focused beam intensity by a factor of about 7.5. PACS numbers: 29.25.Pj, 67.70.+n Many high energy spin physics experiments require a high intensity spin-polarized atomic hydrogen source, which is either accelerated as a high energy polarized proton beam, or used as a polarized internal target placed in a stored high energy beam [1]. We are developing an ultracold high density jet target [2] of proton-spin-polarized hydrogen atoms for the experiments NEPTUN-A [3] and NEPTUN [4] at the 400 GeV to 3 TeV UNK proton accelerator in Protvino, Russia. This relatively new ultracold method uses a cell coated with superfluid 4 He and a high magnetic field to produce electron-spin-polarized atomic hydrogen [5]. Depolarization and recombination into molecular hydrogen are strongly suppressed because the average thermal energy is much too small to flip the electron spin. Using the Michigan ultracold prototype jet [2], we recently investigated "no microwave" extraction, which uses a steep magnetic field gradient to separate the cold hydrogen atoms of different electron-spin states [6,7]. This method yielded about the same dc flow of almost 10 15 electron-spin-polarized hydrogen atoms per sec (Hs" 1 ) into a compression tube (CT) detector [6] as our earlier "microwave" extraction method [2].The quantum reflection of cold hydrogen atoms from a helium-film-covered surface was first demonstrated by Berkhout et al. [8]. They measured about 80% specular reflectivity for normal incidence on a hemispherical optical quality concave quartz mirror coated with a 100-mK saturated 4 He film. The quantum reflection occurs because each hydrogen atom is light and interacts very weakly with the helium film.We now report the first formation of an external beam of ultracold electron-spin-polarized hydrogen atoms using a highly polished quasiparabolic copper mirror coated with a 4 He film. This mirror focusing significantly improved our jet's beam transport efficiency and thus increased the detected beam intensity by a factor of about (7.5 to 3.7)xlO 15 Hs" 1 . This is an important step to-wards our goal of 10 17 Hs"" 1 .The Michigan prototype jet [2] using the nomicrowave-extraction method [6] is shown in Fig. 1. The atomic hydrogen was produced in a room temperature rf dissociator and guided to an ultracold stabilization cell through a Teflon tube with a Teflon-coated copper nozzle held at about 20 K. The double walls of the cell formed the horizontal mixing chamber of the dilution refrigerator; its cooling power was about 20 mW...

Ultra-cold methods for polarized atomic hydrogen

“…Electron-spin-polarized hydrogen atoms in the lower hyperfine states 13) and 14) can be stabilized for a long time in a magnetic field above 5 T at a temperature below 500 mK (see Fig.l). Under these conditions, the electron-spin magnetic energy difference between states with the electron spin "up" and "down" (2geB) is much larger than the thermal energy (kT); therefore, only the two lowest states are significantly populated.…”

Electric and Mass Transport of a Suspended Multiwall Carbon Nanotube Studied by In situ Transmission Electron Microscopy

“…Various concave surfaces have been suggested as possible mirrors for atom focusing [8][9][10]. The small de Broglie wavelength of thermal-energy atoms places particular constraints on the materials and techniques that can be used.…”

“…Successful focusing requires a surface with the correct macroscopic shape as well as one that is flat on a local atomic scale. In the earliest attempts to demonstrate focusing [8], a diffuse beam of hydrogen atoms was concentrated by reflection from a macroscopic concave surface covered by a film of liquid helium. The liquid film provided a surface that was atomically flat and prevented chemical interaction with the incident atoms.…”

Michigan ultra-cold polarized atomic hydrogen jet target

“…The atoms in the two higher hyperfine states 1 and 2 are repelled toward the low field region and effuse from the exit aperture, forming an electron-spin polarized beam. To increase the jet density we use a polished gold-coated copper focusing mirror covered with a helium-4 superfluid film similar to the prototype mirror [2]. After an rf transition unit, which interchanges atoms in states 2 and 4, the beam passes through a superconducting sextupole.…”