Positronium formation from Ar and Xe

Positron scattering on molecular hydrogen: Analysis of experimental and theoretical uncertainties

“…This in turn validates an almost constant TCS value in-between 3 and 10 eV, as already noticed for several other atomic and molecular targets [65,66]. A hypothesis of the virtual-positronium effects in this energy range is to be exploited theoretically (and experimentally as indicated by the work of Szluinska and Laricchia [67] on noble gases).…”

“…In the range of 3-10 eV, where experimental TCSs have almost constant values, a good compatibility between DCSs and TCSs reported by the ANU group [6], as proved by MERT, indicates that the inclusion of virtual-positronium formation inside the elastic scattering channel is probably needed in order to describe total and differential cross sections using ab initio procedures. Experiments on positron-H 2 scattering combined with the tracing of positronium-annihilation processes [67] are needed as well to prove this hypothesis.…”

Energy scale determination and energy resolution in positron total cross section measurements

“…In particular, in Ar no (rotational, vibrational or electronic) excitations are possible below the Ps threshold (8.96 eV) so the onset is well visible, see figure 2. Furthermore, as seen in our data for Ar, as noted by Szłuińska and Laricchia for Xe [23] and as recently confirmed by Buckman and collaborators in Ar [24], threshold structures are present near the positronium threshold. From Ar measurements we deduced the potential bias of +2.4 eV between the W moderator and the scattering cell.…”

Simultaneous Ionization and Excitation of Molecules by Positron Impact

“…A beam of approximately 10 4 slow positrons s À1 , generated by moderating the fast þ output of a 22 Na source with an annealed W-mesh moderator, is constrained radially by a magnetic field of $10 À2 T. By applying a positive potential V m to the moderator, the peak energy of the beam can be varied according to E þ % eV m þ 2 eV, as established by a retarding-potential analysis. This value is consistent with other determinations for W under similar non-UHV conditions, including the absolute determinations of 2:4 AE 0:1 eV [13] and 2:4 AE 0:3 eV [14] using time-of-flight methods. The transmission of unwanted components, for example, unmoderated positrons or secondary electrons, is substantially reduced by a Wien filter and a cylindrical electrode held at À500 V, the latter being grounded during measurements with electron projectiles which may be obtained by reversing the polarity of the moderator.…”