Single ionization of He, Ne and Ar by positron impact

Abstract

The single ionization cross sections of He, Ne and Ar by positron impact are re-examined. The new results differ from earlier published data for impact energies from the threshold to about 100 eV with the new cross sections being smaller.

Strong Suppression of the Positronium Channel in Double Ionization of Noble Gases by Positron Impact

“…1(c) and 1(d) we also show values for s 21 I as derived from measurements of s 21 I ͞s 1 I by Charlton et al [15,16] and s 1 I by Jacobsen et al [6]. Figure 1(d) also includes values of s 21 I obtained by Kara et al [9].…”

“…This intensity fell by a factor of 5 at the lowest energies used in this investigation. A retarding field analyzer was used to measure the positron impact energy and the energy spread, which was typically 0031-9007͞98͞81(1)͞73(4)$15.00 ; ͑᭺͒ s 1 I , Jacobsen et al [6]. This cross section was also measured by Moxom et al [7], but as their results agree with those of Jacobsen et al, they are not shown in this figure. (b) Single ionization of Ne by e 1 impact: ͑᭹͒ s 1 tot , Laricchia [8]; ͑᭺͒ s 1 I , Kara et al [9].…”

“…The single ionization cross section, both including the positronium channel ͑s 1 tot ͒ and excluding it ͑s 1 I ͒, has previously been measured for helium and neon [5][6][7][8][9].…”

The Physics of Positron Annihilation in the Solar Atmosphere

“…These results are significantly lower than those of Spicher et al (1990), which were used by Wallyn et al (1994) in their study of annihilation in the ISM. The cross section for impact ionization of He was measured by Jacobsen et al (1995) from threshold to 1500 eV. From threshold to $100 eV, these results are somewhat less than earlier published data (e.g., those of Fromme et al 1986used by Chapuis et al 1994, who first included He reactions for the ISM).…”

Strong suppression of positron-induced double ionization of helium at low-to-intermediate velocities

“…Fromme [1], Jacobsen [2], Moxom [3,4], Knudsen [5], and Ashley [6] had measured the single-ionization cross section of positronhelium collision from the threshold to intermediate and high energies. Charlton [7] measured the double-to-single ratio /?2i of positron-helium impact and electron-helium impact and proposed the assumption that the difference of double ionization between the electron and proton was attributed to their opposite charge.…”