B.L. Brown

Bio: B.L. Brown is an academic researcher from Bell Labs. The author has contributed to research in topics: Positron & Positronium. The author has an hindex of 5, co-authored 10 publications receiving 188 citations.

Observation of positronium specular reflection from LiF.

Abstract: A monoenergetic positronium (Ps) beam of 0-60-eV energy and an angular width of \ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{} is created by charge-exchange collisions of a slow positron beam passing through an Ar gas cell. The Ps beam is directed at a Lif(100) crystal, and reflected Ps atoms are detected at an annihilation target. With angles of incidence of 50\ifmmode^\circ\else\textdegree\fi{} to 60\ifmmode^\circ\else\textdegree\fi{} we observe a specularly reflected beam with a maximum reflected fraction $R=(30\ifmmode\pm\else\textpm\fi{}5)%$ at a Ps energy of 7 eV. At higher energies (20-60 eV) the reflectivity ($R=0.5%$ at 60 eV) can be ascribed principally to a short Ps mean free path $\ensuremath{\lambda}=(0.75\ifmmode\pm\else\textpm\fi{}0.15)$ \AA{}, and to a lesser extent a Ps inner potential of about 4 eV.

Laboratory simulation of direct positron annihilation in a neutral-hydrogen galactic environment.

Abstract: The linewidth for thermalized positrons directly annihilating with bound electrons in low-density molecular hydrogen gas has been found to be 1.56 \ifmmode\pm\else\textpm\fi{} 0.09 keV, by use of a Ge detector with a low-energy pulsed positron beam. This linewidth measurement together with previous experimental results makes it possible to determine precisely the composite spectrum due to positrons annihilating in a neutral-hydrogen galactic medium.

Positron annihilation in a simulated low-density galactic environment

Abstract: On utilise un faisceau de positons a basse energie pour etudier le comportement des positons dans H 2 et He a basse energie (10 −3 Torr)

The 511 keV emission from positron annihilation in the Galaxy

Abstract: The first $\ensuremath{\gamma}$-ray line originating from outside the Solar System that was ever detected is the 511 keV emission from positron annihilation in the Galaxy. Despite 30 years of intense theoretical and observational investigation, the main sources of positrons have not been identified up to now. Observations in the 1990s with OSSE/CGRO (Oriented Scintillation Spectrometer Experiment on GRO satellite/Compton Gamma Ray Observatory) showed that the emission is strongly concentrated toward the Galactic bulge. In the 2000s, the spectrometer SPI aboard the European Space Agency's (ESA) International Gamma Ray Astrophysics Laboratory (INTEGRAL) allowed scientists to measure that emission across the entire Galaxy, revealing that the bulge-to-disk luminosity ratio is larger than observed at any other wavelength. This mapping prompted a number of novel explanations, including rather ``exotic'' ones (e.g., dark matter annihilation). However, conventional astrophysical sources, such as type Ia supernovae, microquasars, or x-ray binaries, are still plausible candidates for a large fraction of the observed total 511 keV emission of the bulge. A closer study of the subject reveals new layers of complexity, since positrons may propagate far away from their production sites, making it difficult to infer the underlying source distribution from the observed map of 511 keV emission. However, in contrast to the rather well-understood propagation of high-energy ($g\mathrm{GeV}$) particles of Galactic cosmic rays, understanding the propagation of low-energy ($\ensuremath{\sim}\mathrm{MeV}$) positrons in the turbulent, magnetized interstellar medium still remains a formidable challenge. The spectral and imaging properties of the observed 511 keV emission are reviewed and candidate positron sources and models of positron propagation in the Galaxy are critically discussed.

Plasma and trap-based techniques for science with positrons

Abstract: In recent years, there has been a wealth of new science involving low-energy antimatter (i.e., positrons and antiprotons) at energies ranging from 10 to less than 10 eV. Much of this progress has been driven by the development of new plasma-based techniques to accumulate, manipulate and deliver antiparticles for specific applications. This article focuses on the advances made in this area using positrons. However many of the resulting techniques are relevant to antiprotons as well. An overview is presented of relevant theory of single-component plasmas in electromagnetic traps. Methods are described to produce intense sources of positrons and to efficiently slow the typically energetic particles thus produced. Techniques are described to trap positrons efficiently and to cool and compress the resulting positron gases and plasmas. Finally, the procedures developed to deliver tailored pulses and beams (e.g., in intense, short bursts, or as quasi-monoenergetic continuous beams) for specific applications are reviewed. The status of development in specific application areas is also reviewed. One example is the formation of antihydrogen atoms for fundamental physics [e.g., tests of invariance under charge conjugation, parity inversion and time reversal (the CPT theorem), and studies of the interaction of gravity with antimatter]. Other applications discussed include atomic and materials physics studies and study of the electron-positron many-body system, including both classical electron-positron plasmas and the complementary quantum system in the form of Bose-condensed gases of positronium atoms. Areas of future promise are also discussed. The review concludes with a brief summary and a list of outstanding challenges. ∗ Contact information: jrdanielson@ucsd.edu † Contact information: ddubin@ucsd.edu ‡ Contact information: rgreaves@fpsi.edu § Contact information: csurko@ucsd.edu

Use of the positron as a plasma particle

Abstract: The use of positrons in laboratory plasma physics experiments is considered. Recent progress in this area is discussed, including the creation of a single‐component positron plasma in the laboratory. Specific applications of such antimatter plasmas are also discussed, with emphasis on areas where existing plasma physics technology and that currently under development are likely to produce results in the next few years.

Experimental progress in positronium laser physics

Abstract: The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.

Brightness enhancement of slow positron beams

Abstract: The brightness of slow positron beams can be enhanced significantly by repeated stages of moderation, acceleration and focusing. Presently available data suggest that the source spot area should decrease by 10−4 after each stage with only a modest loss of intensity. Beams with very small angular divergence, which could be made with this technique, would be useful for characterizing surfaces by positron diffraction and microscopy. Using such beams it is possible to envision the study of new exotic systems such as thee +-e − plasma and the positronium molecule.