[en] A new method for detecting the positronium minus ion is described, and the possibility of a long positronium mean free path in a solid is discussed

[en] Conservation of angular momentum and the isotropy of space prohibit orthopositronium (o-Ps) from annihilating into two photons. Nevertheless, an observed o-Ps decay rate in excess of theory tempts one to speculate on the existence of a ubiquitous field that would couple to Ps, allowing a small partial rate for the long-lived o-Ps state to annihilate into two photons. We apply a 3.4-kG magnetic field to a sample of Ps to quench the m=0 states in the laboratory frame of reference, and search for a 12-h periodicity in the 3γ yield. The observed effect is consistent with the assumption that space is isotropic and, at the 80% confidence level, is too small to explain the observed o-Ps decay-rate discrepancy with theory

[en] The microtron accelerator at Bell Laboratories presently produces 16-μs-long, 40-mA pulses of 18.5-MeV electrons at a repetition rate of 30 Hz. Energetic positron-electron pairs are produced at a 4-mm-thick W beam dump. The positrons are moderated to a few electron volts energy by a 9-mm-diam W(110) single crystal and bunched by a parabolic potential accelerator to 14-ns full width at half maximum bursts containing (7+-1) x 10⁴ positrons. The collection efficiency of the buncher is 63%. With the buncher turned off, the positron production efficiency is 4 x 10/sup -8/ positrons per electron

[en] Slow positron pulses are implanted at variable energies into a SiO₂ powder target to form positronium (Ps). By time-of-flight methods we estimate the energy spectrum of the Ps that diffuses out of the target into the vacuum. As expected, higher implantation energies and lower target temperatures result in a colder Ps energy spectrum. However, even with 19-keV positron implantation energies, the Ps escaping from a target at 4.2 K is only about 2% thermalized. No evidence was found for a low-energy cutoff due to Ps localization between the powder grains

[en] We have measured the yields and energy spectra of positronium (Ps) emitted from Al(111) surfaces treated by exposure to oxygen gas at low temperatures. We find that the oxygen induces the emission of Ps with kinetic energies of a few tenths of an eV, in agreement with previous work. We also find that Ps is thermally desorbed at low temperatures and has a velocity distribution characterized by a temperature that is the same as that of the sample. The intensity of the thermal Ps component is about 12% of the positrons that reach the surface. The velocity distributions may be interpreted as indicating that the Ps has a sticking coefficient of 1 in the limit of zero velocity, unlike any other system studied to date

[en] A 1-μsec pulsed proton beam is being used to study H⁺ thermalization and reemission from solid target surfaces in ultrahigh vacuum in order to help clarify analogous experiments using muon beams. Using a solid Ar target, vapor deposited on an ∼6-K Cu substrate, the reemission probability Y is 6x10^-4 at a proton implantation energy E_H⁺=1.4 keV and falls with increasing energy to 3x10^-4 at E_H⁺=5 keV and 2x10^-4 at E_H⁺=15 keV. Ne exhibits a 25% larger yield, while the yield for Kr is a factor of 4 lower. The reemitted protons are slow, with kinetic energies of order 1 eV. The reemitted proton yield Y decreases with an ∼100-m time constant, presumably due to deposition of neutral contaminants associated with the incoming beam, and thus ruling out the possibility that the slow protons originate from surface contaminants. For Ar, the observed variation of Y with E_H⁺ is interpreted with the help of a Monte Carlo calculation of the stopping and backscattering of the incident protons

[en] Positronium is the quasistable bound system consisting of an electron and its antiparticle, the positron. Its energy levels can be explained to a high degree of accuracy by the electromagnetic interaction, affording an ideal test of the quantum electrodynamic (QED) theory of bound systems. We have measured the 1 ³S₁--2 ³S₁ interval in positronium by Doppler-free two-photon spectroscopy to be 1 233 607 216.4±3.2 MHz. We employ continous-wave (cw) excitation to eliminate the problems inherent with pulsed laser measurements of nonlinear transitions. Positronium (Ps) atoms generated in vacuum are excited to the 2S state using cw laser light built up to 2.5 kW circulating power in a resonant Fabry-Perot cavity. The excited-state atoms are photoionized using a pulsed laser at 532 nm, and the liberated positrons counted as the cw laser is tuned relative to a reference line in tellurium (Te₂) molecular vapor. The fit of a detailed theoretical model to the measured line shape determines the Ps resonance frequency relative to the Te₂ reference line. The Monte Carlo model includes details of the excitation and detection geometry, the Ps velocity distribution, and the dynamic Stark shift, and gives excellent agreement with the measured line shapes. The quoted 2.6 parts per billion (ppb) uncertainty is dominated by the measurement of the Ps line center relative to the Te₂ reference line, with a 1.0-ppb contribution from a recent calibration of our Te₂ cell relative to the hydrogen 1S-2S transition frequency. The measurement is in excellent agreement with theory and sufficiently accurate to provide a test of the as-yet-uncalculated α⁴R_∞ QED correction

[en] Using continuous-wave excitation to eliminate the problems inherent with pulsed laser measurements of nonlinear transitions, we have measured the 1³S_1-2³S₁ interval in positronium (Ps) to be 1 233 607 216.4±3.2 MHz. The quoted 2.6 ppb (parts per 10⁹) uncertainty is primarily due to the determination of the Ps resonance relative to the Te₂ reference line, with a 1.5 ppb contribution from a recent calibration of the Te₂ line relative to the hydrogen 1S-2S transition. The uncertainty corresponds to 3.5x10^-5 of the α²R_∞ QED contribution to the 1³S_1-2³S₁ interval. Our measurement is sufficiently accurate to provide a test of the as yet uncalculated α⁴R_∞ QED corrections

[en] Using a spatially filtered pulsed laser at 486 nm we have photoexcited positronium to the 2S state and detected it via a delayed and spatially separated 532-nm photoionization pulse. We have extended our observations down to low 486-nm power to explore the possibility of a cw measurement of the positronium 1S-2S interval. A model calculation taking into account the velocity distribution of the 235-K positronium from an oxygenated cold Al(111) target, the parameters of the laser beams, and the nonlinear excitation probability explains our data. The same model indicates that a cw measurement is presently feasible