A high-resolution, variable-energy electron beam from a Penning–Malmberg (Surko) buffer-gas trap

THE EUROPEAN PHYSICAL JOURNAL D Eur. Phys. J. D (2022)76:33 https://doi.org/10.1140/epjd/s10053-022-00349-y Regular Article – Atomic and Molecular Collisions A high-resolution, variable-energy electron beam from a Penning–Malmberg (Surko) buffer-gas trap J. R. Machacek1 , T. J. Gay2 , Stephen J. Buckman1,3,a , and Sean S. Hodgman1 1 Research School of Physics, Australian National University, Canberra, ACT, Australia Department of Physics and Astronomy, University of Nebraska, Lincoln, NE 68588-0299, USA 3 Department of Actuarial Science and Applied Statistics, Faculty of Business and Information Science, UCSI University, 56000 Kuala Lumpur, Malaysia 2 Received 25 October 2021 / Accepted 13 January 2022 © The Author(s) 2022 Abstract. We describe the production of a high-resolution electron beam using a Penning–Malmberg buffergas trap, or Surko trap as they have become known. A high-flux beam with an energy width of ∼ 30 meV (FWHM) is readily achieved and the efficiency of production is considerably higher than that for positrons in a similar trap configuration. The reasons for this become apparent when one considers the molecular collisions and the respective selection rules involved, for electrons and positrons. We demonstrate the production of the beam and the capacity that it realises for absolute scattering measurements and for high-resolution electron spectroscopy. 1 Introduction and background The field of positron atomic physics has been revolutionised over the past few decades through the use of buffer-gas traps to produce high-resolution, variableenergy positron beams that can be used for a broad range of experiments [1]-from the production of antihydrogen [2] to low-energy atomic physics [3] and annihilation studies [4]. Such traps, in conjunction with a high-activity 22 Na radioactive positron source, can routinely provide a high-intensity positron beam with an energy width parallel to the guiding axial magnetic field which reflects the temperature of the trapping gases, (3/2 kB T) ∼ 32 meV (FWHM) at room temperature. These traps, which use high solenoidal magnetic fields (0.5–1 kG) for radial confinement of particles and electrostatic potential for axial confinement, typically use molecular nitrogen (N2 ) and carbon tetrafluoride (CF4 ) as the trapping and cooling gases, with the main energy-loss mechanisms being electronic, vibrational and rotational excitation. Nitrogen is favoured for a number of reasons [5]—it has relatively low-lying electronic excited states which, when excited by positron impact, provide the principal energy loss mechanism to enable trapping of the positrons. The lowest lying of these states, the a1 Π state, has an excitation threshold of 8.59 eV, which is below the threshold of 8.78 eV for the formation of positronium (Ps) in N2 [5]. This makes it possible, to some extent, to balance the trapping of a e-mail: author) (corresponding 0123456789().: V,-vol the inelastically scattered positrons against the potential loss of positrons through Ps formation and subsequent annihilation. It is important to note here (for the discussion that is to follow) that it is only the manifold of singlet excited states that can be excited by positron impact on the singlet (X 1 Σ+ g ) ground state of N2 . This is because the spin–orbit interaction, which can lead to spin-flip excitations, is negligible for positron collisions, and the exchange interaction, which can also lead to singlet–triplet transitions, is absent altogether for positron collisions. The principal role of the CF4 gas is to then provide further cooling of the positrons through vibrational excitations [5]. It is interesting to note that the choice of these two trapping gases, from a host of possibilities, was largely based on empirical experimental observations of trapping and cooling efficiency. The measurement of the collision cross sections for these excitation processes [6, 7], and thus a full understanding of the underlying molecular scattering mechanisms that enable the efficient operation of the trap [8–10], was not possible until a high-flux, high-resolution positron beam was actually realised. Much has been written about the success of such traps for positrons and, while the trapping of electrons has also been demonstrated and explored in a number of configurations [11], to our knowledge such a trap has only been used once to produce a high-resolution electron beam for low-energy collision studies of CF4 [12]. • When one considers the various collision cross sections for electron and positron scattering from N2 and CF4 , it becomes immediately obvious that the conditions for trapping and cooling electrons are far 123 33 Page 2 of ?? more favourable. This is due to a range of collisional and spectroscopic factors, namely: The exchange interaction For electron scattering, the indistinguishability of the incident and target electrons means that electron collisions with N2 can result in the excitation of both singlet (direct excitation) and triplet (via the exchange interaction) electronic states from the singlet ground state. For positron impact, only the singlet manifold of states can be excited, due also to the absence of any significant spin–orbit interaction. In Fig. 1a (from [13]) we show the calculated potential energy curves for the singlet and triplet manifolds of N2 which identify their excited states and thresholds. In Fig. 1b, we show the corresponding electron excitation cross sections for these states [14] as well as the single ionisation cross section [15]. All of these processes lead to significant energy loss, which can then result in the trapping of the scattered or ionised electrons. A summary of the cross sections that relate to predicting the trapping of both positrons and electrons via their energy loss when exciting the various N2 scattering channels is shown in Fig. 1c. In this case, the only measured cross sections for positron scattering are those for the a1 Π state [7] and for direct ionisation [5]. These are similar to the cross sections for electrons, so we have assumed the other singlet excitations to be the same as those for electrons. The result of this comparison shows clearly that the probability of an energy-loss collision that can lead to trapping is significantly higher for electrons than for positrons. Finally, it should be noted that the positron ‘trapping’ curve in Fig. 1c does not consider the losses due to positronium formation-a cross section which grows strongly above the threshold of 8.78 eV (N2 ) to dominate the positron ‘trapping’ cross sections. This imposes the additional constraint of a rather narrow incident energy range of operation for the positron trap in order to maximise energy loss and trapping through electronic excitation, and minimise positron loss due to positronium formation. • Resonant electron scattering via transient negative ions Scattering resonances, where a projectile electron is trapped temporarily in the field of the m (...truncated)