Absolute Differential Positronium-Formation Cross Sections

Abstract

The first absolute experimental determinations of the differential cross-sections for the formation of ground-state positronium are presented for He, Ar, H2 and CO2 near 0 ○ . Results are compared with available theories. The ratio of the differential and integrated cross-sections for the targets exposes the higher propensity for forward-emission of positronium formed from He and H2.PACS numbers: 36.10. Dr, 34.80.Bm, 34.80.Uv The formation of positronium (Ps, the bound state of an electron and a positron) is an important channel in the scattering of positrons from atoms and molecules e.g. [1][2][3], accounting for up to 50% of the total cross-section, with experimental and theoretical investigations of its integrated formation cross-sections available for a wide range of atoms and simple molecules, e.g. [3][4][5]. Recent experimental studies also include its formation in an excited state [6] or accompanied by ionic excitation [7]. However, whilst theoretical predictions for the differential Ps formation cross-section ( dQPs dΩ ) are available for atomic [8][9][10][11][12][13][14][15][16] and molecular [17,18] hydrogen, the noble gases [9,16,[19][20][21][22][23][24][25][26][27][28][29] Details of the experimental arrangement employed at UCL for producing a beam of Ps atoms, together with a review of recent advances, may be found in [41]. In brief, the Ps beam is produced by charge-exchange of positrons (e + ) with a target gas (A), i.e. e + + A → Ps + A + , and detected downstream by a channel-electronmultiplier (CEM or CEMA) in coincidence with one or more γ-ray detectors (e.g. CsI or NaI) where it has been found to be composed predominantly of groundstate atoms [42,43].Depending on the relative spin orientation of its constituents, ground-state Ps may be formed in an ortho-( 3 S 1 ) or para-( 1 S 0 ) state. The two are characterized by lifetimes differing by three orders of magnitude (142 ns and 125 ps, respectively) and different annihilation modes (dominantly 3-γ and 2-γ, respectively). Only ortho-Ps reaches the detection region.In order to determine dQPs dΩ (a measure of the probability that Ps is emitted within a solid angle dΩ = 2πsinθdθ), we have measured the number of Ps atoms (ε where τ Ps is the lifetime of ortho-Ps, t its flight-time to the detector and ǫ Ps the 'true' Ps beam production efficiency. In Eq.(1), ǫ m Ps may be seen to depend on the (energy-dependent) ratio of the positron to positronium detection efficiencies R d also determined by our group [38][39][40].By studying the variation of ǫ Ps with gas pressure, optimum beam operating conditions may be determined for a given target and Ps energy [39,40,44,45]. An example is shown in Figure 1 for production of 20 eV Ps from CO 2 . Here ǫ Ps may be seen to increase and then decrease with increasing pressure. This variation may be expressed as:

Differential positronium-formation cross sections for Ne, Ar, Kr, and Xe

“…Specifically, data for Q + T in [14][15][16][17] and Q Ps T in [3,9,18,19] for Ne, Ar, Kr, and Xe were used. As expected and verified in our previous work [2], the differential Ps formation cross section was found to be pressure independent within errors for the majority of the data. An apparent pressure dependence may arise if inaccurate values of Q + T and Q Ps T are used as input to Eq.…”

“…From Eq. (1), Shipman et al [2] obtained the following expression for the differential cross section near zero degrees:…”

“…By studying its properties, formation and scattering (e.g., [1][2][3], respectively), insights into basic matter-antimatter interactions can be gained (e.g., [4,5] and references therein). The first absolute experimental determinations of the differential Ps formation cross section ( dQ Ps d ) near 0 • were obtained for He, Ar, H 2 , and CO 2 [2] from measurements of the Ps production efficiency made on the UCL Ps beam line [6]. In the current paper, we present the extension of these studies to Ne, Kr, Xe, and additional data for Ar.…”

Positronium decay fromn=2states in electric and magnetic fields

“…If high Rydberg states [64] can be produced more efficiently using these approaches, then many other experimental areas may benefit, for example, electric-field control of the translational motion of Ps [72] or Ps scattering measurements, which are currently conducted using only ground-state atoms (e.g., [74,75]). Controlled and slowed Ps beams may also find application in the production of antihydrogen atoms [76][77][78][79], in Ps Bose-Einstein condensates [80][81][82][83], Ps gravity measurements [84,85], and in Ps spectroscopy and QED tests [86][87][88][89].…”

Resonant scattering of positronium as a quasifree electron

“…Slow positrons are obtained from a 22 Na source covered with a solid rare-gas moderator (krypton or neon), accelerated to the required energy by a positive potential applied directly to the source, and radially confined by an axial magnetic field from the moderator to the scattering region. Ps is produced via the charge exchange reaction from a gas X, e + + X → Ps + X + [27]. In this work, H 2 was used as the production gas [28] except at the lowest energies where Ar was used [25].…”