A high-efficiency two-dimensional angular correlation spectrometer for positron studies

West, R N; Mayers, J.; Walters, Philip

doi:10.1088/0022-3735/14/4/021

Cited by 84 publications

(30 citation statements)

References 23 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This is achieved by using position-sensitive gamma-ray detectors (known as Anger cameras, after their inventor, Hal Anger [386]) that are located far away (many meters) from the annihilation events to achieve a high angular resolution. This comes at the cost of the solid angle, however, and the count rates in such measurements are therefore invariably low [387]. In order to obtain cooling rates using this technique, the ACAR data must be time selected (TSACAR), which further reduces the data acquisition rate [239,294,388].…”

Section: Doppler and Time-of-flight Spectroscopymentioning

confidence: 99%

Experimental progress in positronium laser physics

2018

View full text Add to dashboard Cite

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 ≥10 6 ) 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 10 7 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.

show abstract

Section: Doppler and Time-of-flight Spectroscopymentioning

confidence: 99%

Experimental progress in positronium laser physics

2018

View full text Add to dashboard Cite

show abstract

“…9 The high-purity Cu sources were irradiated as 3-mmdiam spheres for 48 h at 8x 10 14 n° cm" 2 sec" 1 . The resulting 80-Ci sources were introduced into the UHV source chamber through an air lock by remote control.…”

Section: O Roellig City College Of the City University Of New Yormentioning

confidence: 99%

Positron or Positroniumlike Surface State on Al(100)?

et al. 1985

View full text Add to dashboard Cite

Using a high-intensity beam of 200-eV positrons we have measured the two-dimensional angular correlation of the 2y annihilation radiation from a clean Al(100) surface. The momentum distribution identified with the positron surface state has a nearly isotropic conical shape and a (7.1 ±0.5)-mrad full width at half maximum. The data are not consistent with a simple interpretation based on either the usual model of a positron bound in a surface state by its "image-correlation potential" or a positronium atom weakly bound to the surface. PACS numbers: 71.60.+ z, 73.20.-r, 71.25.HcPositron annihilation offers a unique way to observe the momentum distribution of electrons in solids. 1 The positron is not a weakly interacting probe and significantly increases the density of electrons in its vicinity. 2 Thus the positron annihilation rate in a metal is much greater than the independent-particle-model prediction. Nevertheless, theory and experiment have shown that the momentum distributions obtained from the angular correlation of the 2y annihilation radiation (ACAR) can yield valuable information about electron wave functions, and faithfully reflect the Fermi surface of metals and alloys. It is interesting to ask if an analogous picture will hold for positrons localized at the surface of a metal.The extension of the ACAR technique to the study of surfaces requires positrons with energy low enough to stop near a sample surface. It is well established that a large fraction of the positrons implanted into a single-crystal metal at a few hundred electronvolts of kinetic energy diffuse back to the surface and become trapped in a surface state. 3 The positrons in this surface state annihilate with near-surface electrons and thus can give new information about these electrons. Beams of slow positrons with sufficient intensity for practical surface ACAR measurements are now available. 4 The usefulness of such measurements will depend greatly on the nature of the positron surface state and how well we understand it. While there are many model calculations, we are presently lacking a firm theoretical foundation.Early ACAR results obtained on the internal surfaces of voids in metals 5 motivated the development of theories describing the positron interaction with surfaces. The most frequently used model has been that of a positron tightly bound to a surface in its imagecorrelation potential well. 6 ' 7 A second model is that of a positronium (Ps) atom weakly bound to the surface by Van der Waals forces. 8 It is not clear in either case that ACAR measurements will be uncontaminated by effects due to the momentum of the positron state. In order to find out more about the positron surface state we have made ACAR measurements on a simple s-p metal surface, Al(100). Our measurements show that neither picture is completely accurate.The apparatus for this experiment consists of a high-intensity reactor-produced 64 Cu positron source, an ultrahigh-vacuum (UHV) target chamber, and a pair of Anger cameras with associated electronics. 9 The ...

show abstract

“…They found a pronounced minimum in the low momentum region of ACAR. West et al [8] reported the first 2D-ACAR data for Ge which showed a marked anisotropy and a strong deviation from the polyhedral Jones-zone symmetry. For the observed minimum in ACARs in the low momentum region in semi 2D-ACAR [7] and full 2D-ACAR [8], any clear explanation had not been given until Chiba and Akahane [9] performed the theoretical calculation of 2D-ACARs in Ge, GaAs, and GaP by the LCAO scheme and Aourag et al [lo] by the pseudopotential method (where the electron wave functions are the relevant quantities).…”

Section: Introductionmentioning

confidence: 99%

Positron annihilation in narrow‐gap semiconductors

Bouarissa¹,

West²,

Aourag³

1995

Physica Status Solidi (b)

View full text Add to dashboard Cite

The valence electron and positron charge densities in InAs and InSb are obtained from wave functions derived in a model pseudopotential band-structure calculation. It is found that the positron density is maximum in the open interstices and is excluded not only, as usual, from the ion cores but also to a considerable degree from the valence bonds. Electron-positron momentum densities are calculated for the (001-110) plane. The results are used to analyze the positron effect in narrow-gap semiconductors.

show abstract

A high-efficiency two-dimensional angular correlation spectrometer for positron studies

Cited by 84 publications

References 23 publications

Experimental progress in positronium laser physics

Experimental progress in positronium laser physics

Positron or Positroniumlike Surface State on Al(100)?

Positron annihilation in narrow‐gap semiconductors

Contact Info

Product

Resources

About