Apr 3, 2008 - regions reveal no evidence of the structures in the cross section reported by ..... and it is a pleasure to acknowledge Cliff Surko and Mike.
IOP PUBLISHING
JOURNAL OF PHYSICS B: ATOMIC, MOLECULAR AND OPTICAL PHYSICS
doi:10.1088/0953-4075/41/8/081001
J. Phys. B: At. Mol. Opt. Phys. 41 (2008) 081001 (4pp)
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High-resolution, low-energy positron scattering from helium: measurements of the total scattering cross section J P Sullivan1, C Makochekanwa1,2, A Jones1, P Caradonna1 and S J Buckman1 1
Center for Antimatter-Matter Studies, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia 2 Center for Antimatter-Matter Studies, SoCPES, Flinders University, GPO Box 2100, Adelaide, South Australia, Australia
Received 20 February 2008, in final form 3 March 2008 Published 3 April 2008 Online at stacks.iop.org/JPhysB/41/081001 Abstract We present high-resolution (80 meV FWHM) measurements of the positron–helium total cross section in the energy range between 1 and 15 eV. The absolute magnitude of the cross section is in excellent agreement with recent state-of-the-art theoretical calculations and with previous absolute measurements (e.g. Mizogawa et al). Specific scans in the 1–3 eV and 5–8 eV energy regions reveal no evidence of the structures in the cross section reported by Karwasz et al. (Some figures in this article are in colour only in the electronic version)
A few cases exist in the low-energy scattering of positrons from atoms and molecules where one could consider a ‘benchmark’ cross section, i.e. one where experiment and theory agree, to within 10%, to exist. The helium atom may well be one such case, as has been demonstrated in recent reviews by Surko et al (2005) and Buckman and Sullivan (2006). In the case of the total positron scattering cross section for helium, there is excellent agreement between the most accurate experimental measurement to date (Mizogawa et al 1985) and three state-ofthe-art theoretical calculations, as demonstrated in figure 17 of Surko et al. In this figure it is apparent that the experimental cross section is completely consistent with a Kohn variational calculation (van Reeth and Humberston 1999), a convergent close coupling (CCC) calculation (Wu et al 2004) and a many body theory (MBT) calculation (Ludlow and Gribakin 2004), at energies below the positronium formation threshold. There have been many other measurements of this cross section (well summarized by Charlton 1985) but it is interesting to note the very first absolute measurement of positron–helium scattering was published in this journal more than 35 years ago (Canter et al 1972). Against this background, the measurements of Karwasz (2005) and Karwasz et al (2005) of the total cross section 0953-4075/08/081001+04$30.00
(TCS) for helium are somewhat perplexing. They show a significantly larger magnitude in the region of the cross section minimum around 2 eV, and also indicate the presence of several strong, resonance-like features between 1 and 3 eV, and further features at around 7 eV. In these measurements, not only is the cross section magnitude larger than both the experiments and theory described above, by a factor of more than two, but the presence of a ‘resonance’ is something that is clearly not observed in either the previous experiment or any of the calculations. These measurements have led to some discussion, including a claim by Zecca (2006) that both the increased magnitude of the cross section observed by Karwasz et al, and the presence of ‘resonances’, were a result of flawed experimental procedures. Karwasz (2006) responded to the claims of Zecca, but the situation remains largely unresolved. The aim of the present work was to apply a recently constructed, high-resolution, trap-based positron beam to measurements of the total cross section for helium in the energy region below 10 eV. The experimental apparatus will be described fully in a forthcoming communication and only a brief description will be given here. Positrons from a ∼35 mCi 22 Na radioactive source are slowed with a solid neon moderator and then collisionally cooled in a three-stage Surko-type trap 1
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J. Phys. B: At. Mol. Opt. Phys. 41 (2008) 081001
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(see Murphy and Surko 1992). The trapped positrons are released in a pulsed beam, at a frequency of approximately 100 Hz, with typically 500 positrons per pulse. The energy resolution of the beam is 80 meV. The absolute cross sections have been measured using the techniques developed by the San Diego group, and explained in Sullivan et al (2002), for scattering in a high magnetic field. The pulsed positron beam is directed through a 200 mm long scattering cell, which has an internal diameter of 50 mm and entrance and exit apertures of 5 mm diameter, resulting in a well-defined target pressure throughout the region where the positrons have a well-defined interaction energy. End effects are minimized by the presence of cylindrical meshes at each end of the cell, maintaining the energy of the positron beam at the desired collision energy in the region of changing gas density at the ends of the cell. The target gas, in this case helium, is admitted into the cell via a manually controlled leak valve and the pressure is monitored using a high accuracy (±0.05%), MKS model 690A Baratron capacitance manometer (resolution of ∼1×10−5 Torr). Typical pressures for the experiments presented in this paper range from 10 to 25 mTorr. The pressure was chosen such that no more than 10% of the incident positrons underwent a scattering event inside the cell. A retarding potential analyser is located downstream of the gas cell, and measures the parallel energy distribution of the beam (E) after it passes through the gas cell. Any positrons that have been elastically scattered through some angle and, as a result, lost E, are rejected, allowing the scattering fraction to be determined. The Beer–Lambert law is used to calculate the cross section (see Sullivan et al 2002), with the main contribution to the errors in the absolute determination of the cross section coming from determining the scattering fraction. The magnetic fields in the trap, scattering region and at the retarding potential analyser are all 530 Gauss and, consequently, corrections due to the helical path length through the cell are small (∼1% at 1 eV scattering energy and reducing as the energy increases), and have not been applied to the data. The other main source of error in the determination of the cross section magnitude lies in the determination of the pressure, which has an error of
