Figure 1
Experimental data and theoretical calculations of the cross section for single ionization by antiproton impact on atomic hydrogen, molecular hydrogen and helium as a function of the projectile laboratory velocity. For atomic hydrogen, we show as dashed-dotted black curves calculations by Lühr and Saenz [
12] and by Igarashi et al. (green dash-dot-dot) [
11]. Also shown as a brown solid curve is the low-velocity calculation of the total cross section for ionization by Cohen [
4]. In the case of the molecular hydrogen target, we show the experimental data of the present work (▴ run in 2010, ▵ run in 2009) as well as previous data obtained by our group, (○), published in Hvelplund et al. [
9]. The straight, solid line is a linear fit to these data below a velocity of 1 a.u. The data points marked ▪ indicate the sum of the cross sections for nondissociative and dissociative ionization of molecular hydrogen by Hvelplund et al. [
9]. The long-dashed dark blue curve shows a calculation of this cross section by Lühr and Saenz [
3]. The vertical line indicates the projectile energy above which more than 90% of the
ions created and more than 90% of the projectiles deflected in the collisions are collected by our apparatus. For the helium target, we show experimental data by Hvelplund et al. (□) [
9] and by Knudsen et al. (▾) [
1]. The short-dashed, nearly horizontal line is a straight-line fit to these data below 1 a.u. to guide the eye.
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