Elastic electron scattering from neon at backward angles H. Cho * Physics Department, Chungnam National University, Daejeon 305-764, Korea R. P. McEachran and S. J. Buckman Atomic and Molecular Physics Laboratories, Research School of Physical Sciences and Engineering, Australian National University, Canberra, Australian Capital Territory 0200, Australia H. Tanaka Department of Physics, Sophia University, Chiyoda-ku, Tokyo, Japan Received 4 June 2008; published 11 September 2008 We present experimental and theoretical differential cross sections for the elastic scattering of electrons from neon at four incident electron energies from 5 to 50 eV. The magnetic angle-changing device has been used to extend the present measurements from mid-angles to backward angles up to 180°. The results reveal some small differences between experiment and theory at backward angles in some cases; however, the agreement at 5 and 50 eV is excellent. DOI: 10.1103/PhysRevA.78.034702 PACS numbers: 34.80.Bm I. INTRODUCTION There have been many experimental investigations of electron-neon scattering. Focusing on only those which are relevant to the present study, the results of Register and Tra- jmar 1 cover elastic differential cross sections DCSs for 11 incident electron energies from 5 to 100 eV and for scat- tering angles in the range of 10°–145°. Shi and Burrow 2 measured the DCS in the very low-energy region of 0.25– 7.0 eV for scattering angles from 30° to 120°. Gulley et al. 3 also did measurements in the low-energy range from 0.1 to 7.0 eV and for angles up to 130°. Zubek and co-workers 4 reported measurements that are more relevant to this experiment. They measured the elastic DCS for neon at the two electron energies of 5 and 7 eV from 130° to 180° using an angle-changing device, which is basically the same as the device we used in this experiment. However, their DCSs increased rapidly toward backward angles at both incident energies, a trend that was unexpected. Later, in 2006, the same group reported a new set of DCSs at 7, 10, and 15 eV for the backward angles 5. In this new result at 7 eV, the previous increase at backward angles was no longer observed, and, consequently, the puzzle was par- tially resolved. However, the puzzle at 5 eV still persists and this was, in part, a direct motivation for the present study. On the theoretical side, there have also been many stud- ies, the most successful of which are those of Fon and Ber- rington 6, McEachran and Stauffer 7, and Saha 8. An extensive review of all previous theoretical work on neon is provided in Ref. 5. In this paper, we present absolute measurements as well as theoretical calculations of the differential cross section for elastic scattering from neon at four incident electron energies from 5 to 50 V and for scattering angles from mid-angles 90°, 100°, or 120° to 180°. This will be the final in a series of reports on the elastic DCSs of the rare gases, which have been published over the last several years 9–11. II. EXPERIMENT Since the experimental study is focused on measuring the DCS at backward angles up to 180°, where cross-section measurements are typically inaccessible due to the mechani- cal constraints of the electron spectrometer, a version of the magnetic angle-changing device developed by Read and Channing 12 has been used with a conventional electron spectrometer. The electron spectrometer used in the present experiment is described in detail elsewhere 13,14. Relative measurements of the angular distribution are placed on an absolute scale by the use of the relative flow technique, which relies on measurements of the ratio of scattered elec- tron intensities for the gas of interest relative to that for a standard gas, in our case helium. In using this technique, the * Corresponding author. FAX: +82 42 823 0919. hcho@cnu.ac.kr TABLE I. Differential cross sections for elastic electron scatter- ing in units of 10 -16 cm 2 sr -1  from neon. Figures in parentheses indicate estimated percentage uncertainties. Angle deg Energy eV 5 10 20 50 90 0.050 15 100 0.136 12 0.014 17 110 0.085 13 0.014 15 120 0.120 15 0.082 14 0.073 16 0.080 13 130 0.087 16 0.073 14 0.107 15 0.196 15 140 0.066 16 0.076 13 0.191 14 0.311 14 150 0.055 15 0.093 14 0.242 14 0.449 14 160 0.051 15 0.109 15 0.325 12 0.559 13 170 0.049 16 0.121 13 0.397 13 0.653 12 180 0.047 17 0.126 13 0.432 12 0.770 10 PHYSICAL REVIEW A 78, 034702 2008 1050-2947/2008/783/0347024 ©2008 The American Physical Society 034702-1