Calculations of positron scattering from small molecules

Eur. Phys. J. D (2024)78:58 https://doi.org/10.1140/epjd/s10053-024-00853-3 THE EUROPEAN PHYSICAL JOURNAL D Regular Article Calculations of positron scattering from small molecules N. A. Moria , I. Bray , and D. V. Fursa Department of Physics and Astronomy, Curtin University, Perth, WA 6102, Australia Received 5 February 2024 / Accepted 19 April 2024 © The Author(s) 2024 Abstract. Recently, convergent close-coupling calculations have been completed for positron scattering from the carbon and oxygen atomic targets. These, together with previously completed calculations for atomic hydrogen, are utilized to perform positron scattering calculations for molecular hydrogen (H2 ), molecular oxygen (O2 ), diatomic carbon (C2 ), carbon monoxide (CO), carbon dioxide (CO2 ), ozone (O3 ), water (H2 O), and methane (CH4 ) through a modified independent atom approach. For these molecules, positronium-formation, direct ionization, electron-loss, elastic, total electronic excitation, total inelastic, and total cross sections are obtained for energies between 0.1 and 5000 eV. There is, in general, good agreement between the current results and past experiments for most transitions, particularly at high energies where this approach is expected to be most accurate. 1 Introduction Positron scattering from molecular targets is of significant interest to astronomical, atmospheric, and medical research. In medical research, positrons have become essential in medical imaging due to their use in positron emission tomography (PET) scans. Beyond PET scans, the other main use of positrons in medicine is positherapy, which utilizes positrons to destroy cancer cells [1]. The H2 O molecule is by far the most important biologically active molecule, with it representing approximately 60% of the human body. Cross sections for this molecule are required to accurately model the energy deposition and transport of a positron as it loses energy and annihilates within the body. With a better understanding and accurate modeling of the processes these positrons undergo upon emission into the body, image blur can be reduced in PET scans [2], and damage to healthy tissue minimized in positherapy treatments. As 80% of the gamma rays detected by PET scans are emitted from the decay of positronium [3], the positronium-formation cross section (σPs ) is of particular interest. Some of the molecules considered in this work (H2 , CO, O2 , O3 , CO2 , H2 O, and CH4 ) are important components of the atmospheres of Earth, planets within our solar systems [4], several of the moons of Jupiter and Saturn [4,5], and other exoplanets [6]. Due to the interactions of cosmic rays, thunderstorms, and other high-energy processes occurring in the upper atmospheres of these bodies, positrons are continuously proa e-mail: author) (corresponding 0123456789().: V,-vol duced [7–10]. Accurate scattering cross sections for these molecules, therefore, are required for atmospheric research of positron transport. Diatomic carbon (C2 ) is typically only found in extreme conditions, usually as a component of carbon vapor, as it forms at temperatures > 3500◦ C [11]. Consequently, its presence in nature is constrained mainly to comets, stellar atmospheres, and the interstellar medium (ISM) [12]. In industry, this molecule typically forms when carbon is introduced to plasmas and is involved in the production of fullerenes [13]. Alongside C2 , all of the considered molecules, except for O3 , have been detected in the ISM [14,15]. Substantial quantities of positrons are known to propagate through the ISM, and there is much interest in the transport, interactions, and source of these particles [16–18]. There exist numerous positron experiments for several processes for each of the considered molecules, except for C2 and O3 . At high energies, the inelastic results for positrons and electrons are expected to become equivalent. For elastic cross sections, the electron results provide an upper bound on the positron results for the considered energies. As the elastic cross section at large energies is small, the total cross section for these projectiles will also be close at such energies. Therefore, electron experiments are presented either where appropriate or when a transition has no existing positron measurements. Due to the extreme conditions required to create and maintain C2 , there exists no experimental results for electron or positron scattering for any of the considered processes. Previous calculations for positron scattering on the molecules considered in this work have been conducted 123 58 Page 2 of 28 in either the independent atom model (IAM) [19,20], IAM with screening-corrected additivity rule (IAMSCAR) [21,22], IAM-SCAR with the inclusion of interference terms (IAM-SCAR+I) [23–25], model potentialbased approaches [26–33], many-body theory (MBT) [34], binary-encounter Bethe (BEB) [35,36], distortedwave [37–41], Schwinger multichannel (SMC) [42–46], body-frame vibrational close-coupling (BF-VCC) [47, 48], R-matrix [49,50], and close-coupling [51] methods. Of these, BEB and distorted-wave methods have been utilized solely for the calculation of direct ionization cross sections (σion ), whereas the close-coupling, BF-VCC, R-matrix, and SMC calculations have been conducted only for low energies. From the remaining approaches, almost all possible cross sections have been calculated previously. The exceptions to this are O3 , in which positron calculations have been completed for only the total and elastic cross section, and the total electronic excitation cross sections (σexc ) of CH4 . As with experiment, current calculations are compared to electron calculations either where appropriate or in the absence of positron results. For molecular systems, the molecular convergent close-coupling (MCCC) method is currently limited to molecular targets with a few electrons. This includes H2 and H+ 2 for positron [52–55] and electron scattering [56– 59], and HeH+ for electron scattering [60]. The extension of this approach to the other molecules considered in this study would be a significant undertaking and calculations would require considerable computational resources. As CCC calculations have been completed for atomic hydrogen [61,62], carbon [63], and oxygen [64], a simpler approach is instead to use the IAM-SCAR method [65]. The IAM-SCAR method allows us to approximate molecular cross sections with only the relevant atomic cross sections and interatomic distances of the molecules. It has previously been applied to a wide range of molecular systems where its accuracy was tested [22,65– 69]. Typically the atomic values utilized for these calculations are from model potential approaches. Here, we instead use the results obtained from the relevant atomic convergent close-coupling (CCC) calculations [61–64]. Unlike the CCC approach, some model potential approaches are not ab initio and rely upon the choice of parameters to model the sc (...truncated)