ABSTRACT
Slow atoms in Rydberg states can exhibit specular reflection from a cylindrical surface upon which an azimuthally periodic potential is imposed. We have constructed a concave mirror of this type, in the shape of a truncated oblate ellipsoid of revolution, which has a focal length of (1.50±0.01) m measured optically. When placed near the center of a long vacuum pipe, this structure brings a beam of n=32 positronium (Ps) atoms to a focus on a position sensitive detector at a distance of (6.03±0.03) m from the Ps source. The intensity at the focus implies an overall reflection efficiency of â¼30%. The focal spot diameter (32±1) mm full width at half maximum is independent of the atoms' flight times from 20 to 60 µs, thus indicating that the mirror is achromatic to a good approximation. Mirrors based on this principle would be of use in a variety of experiments, allowing for improved collection efficiency and tailored transport or imaging of beams of slow Rydberg atoms and molecules.
ABSTRACT
The affinity A_{Ps} of positronium (Ps) atoms for a metal is the negative of the maximum kinetic energy with which Ps is emitted into vacuum when thermalized positrons in a metal encounter the surface. When this quantity is measured by ground state Ps time of flight (TOF), the precision is severely limited by the short triplet state lifetime of 142 ns. By quickly converting the emitted Ps atoms into long-lived Rydberg states, we are able to dramatically increase the TOF to allow precision measurements of A_{Ps}. From our measurements made on a Cu(110) sample at T=128 K, we find A_{Ps}(128 K)=(-2.476±0.010_{stat}±0.013_{syst}) eV, compared with the result A_{Ps}(128 K)=(-2.545±0.010_{num}±0.010_{syst}) eV found using highly accurate generalized gradient approximations for both electrons and positrons within density functional theory. Such precision opens up opportunities in the quest for an improved density functional.
ABSTRACT
We describe here the development and characterization of a micro-channel plate (MCP) based detector designed for the efficient collection and detection of Rydberg positronium (Ps) atoms for use in a time-of-flight apparatus. The designed detector collects Rydberg atoms over a large area (â¼4 times greater than the active area of the MCP), ionizing incident atoms and then collecting and focusing the freed positrons onto the MCP. Here we discuss the function, design, and optimization of the device. The detector has an efficiency for Rydberg Ps that is two times larger than that of the γ-ray scintillation detector based scheme it has been designed to replace, with half the background signal. In principle, detectors of the type described here could be readily employed for the detection of any Rydberg atom species, provided a sufficient field can be applied to achieve an ionization rate of ≥108/s. In such cases, the best time resolution would be achieved by collecting ionized electrons rather than the positive ions.
ABSTRACT
Recently it has been discovered that positronium (Ps), after forming in metal-organic framework (MOF) crystals, is emitted into vacuum with a high efficiency and low energy that can only be explained by its propagating as delocalized Bloch states. We show that the Ps atoms are emitted from MOFs in a series of narrow energy peaks consistent with Ps at Bloch-state energy minima being emitted adiabatically into the vacuum. This implies that the Ps emission energy spectra can be directly compared with calculations to obtain detailed information about the Ps band structure in the MOF crystal. The narrow energy width of the lowest energy Ps peak from one MOF sample (2-Methylimidazole zinc salt ZIF-8) suggests it originates from a polaronic Ps surface state. Other peaks can be assigned to Ps with an effective mass of about twice that of bare Ps. Given the immense catalog of available MOF crystals, it should be possible to tune the Ps properties to make vastly improved sources with high production efficiency and a narrow energy spread, for use in fundamental physics experiments.
ABSTRACT
Cholesteatoma of the upper urinary tract is a rare nonmalignant condition histologically characterized by keratinized desquamative squamous metaplasia. Most cases have been managed by extensive ablative surgery. We describe a new approach in the diagnosis and management of a patient with ureteral cholesteatoma using transurethral ureteroscopy and evacuation.
