We present measurements of absolute cross sections for low-and intermediate-energy elastic electron scattering from CO 2 , which have been measured independently on different experimental apparatus in two laboratories. The results are compared with a number of previous measurements and calculations. Whilst there are some interesting differences between recent experimental determinations, the largest discrepancies are observed between experiment and contemporary scattering theory at very low energies.
Absolute differential cross sections for elastic scattering of electrons from sulfur hexafluoride (SF 6 ) have been measured at 11 incident energies between 2.7 and 75 eV and for scattering angles between 10 • and 130 • . The magnetic angle-changing device of Read and coworkers has been used to extend these measurements to backward angles (130 • -180 • ) for incident energies below 15 eV. The measurements reveal some substantial differences with several previous determinations and a reasonably good level of agreement with a recent close-coupling calculation.
The formation and decay of the two lowest-lying, triply excited resonances in the autoionizing region of the helium spectrum (57-60 eV) have been studied by measuring electron-impact excitation functions for the n = 2 singly excited states of helium as a function of electron scattering angle. These results offer unambiguous confirmation of the classification of these states as and , respectively. Furthermore, the observation of the relative strengths of the decay of these features into the various final states enables some speculation as to the structure of the three excited electrons.
Nuclear physics experiments at Thomas Jefferson National Accelerator Facility's CEBAF rely on high polarization electron beams. We describe a recently commissioned system for prequalifying and studying photocathodes for CEBAF with a load-locked, low-voltage polarized electron source coupled to a compact retarding-field Mott polarimeter. The polarimeter uses simplified electrode structures and operates from 5 to 30 kV. The effective Sherman function for this device has been calibrated by comparison with the CEBAF 5 MeV Mott polarimeter. For elastic scattering from a thick gold target at 20 keV, the effective Sherman function is 0.201(5). Its maximum efficiency at 20 keV, defined as the detected count rate divided by the incident particle current, is 5.4(2) x 10 -4 , yielding a figure-of-merit, or analyzing power squared times efficiency, of 1.0(1) x 10 -5 . The operating parameters of this new polarimeter design are compared to previously published data for other compact Mott polarimeters of the retarding-field type.
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