A resistive-anode based position-sensitive Rydberg atom detector.

We describe here the development and characterization of a position-sensitive detector for Rydberg atom experiments. The detector builds on an earlier design that field-ionized incident Rydberg positronium (Ps) atoms and then electrostatically focused the freed positrons onto a micro-channel plate (MCP) detector without the use of a position sensitive anode. In this design, pulses from the MCP are deposited onto a resistive anode, providing a means of measuring the incident particles' x, y positions. The first detector constructed utilized a pair of MCPs in a chevron configuration and was used to observe the focusing of Rydberg Ps atoms from an electrostatic mirror. A second detector, developed for use in a measurement of the 1S-2S interval of Ps, incorporates three MCPs in a Z-stack configuration to produce larger pulses. Using a UV-induced signal, we have characterized the performance of the assembled detectors, finding a spatial resolution of ∼1.4 mm for the largest induced pulses and for pulse widths of ∼7-10 ns FWHM; pulse times can be resolved to better than 1 ns. The Ps induced signal is anticipated to yield pulses ∼5 times larger, which are expected to achieve a spatial resolution of <1 mm. Appropriate lenses could make possible applications involving either imaging a large area or magnifying a small area of the incident Ps spatial distribution.

Detector for positronium temperature measurements by two-photon angular correlation.

We report on the design and characterization of a modular γ-ray detector assembly developed for accurate and efficient detection of coincident 511 keV back-to-back γ-rays following electron-positron annihilation. Each modular detector consists of 16 narrow lutetium yttrium oxyorthosilicate scintillators coupled to a multi-anode Hamamatsu H12700B photomultiplier tube. We discuss the operation and optimization of 511 keV γ-ray detection resulting from testing various scintillators and detector arrangements concluding with an estimate of the coincident 511 keV detection efficiency for the intended experiment and a preliminary test representing one-quarter of the completed array.

Focusing of a Rydberg Positronium Beam with an Ellipsoidal Electrostatic Mirror.

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 ∼30%. 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.