| 1. |
- Backis, A., et al.
(författare)
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Time- and energy-resolved effects in the boron-10 based multi-grid and helium-3 based thermal neutron detectors
- 2021
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Ingår i: Measurement science and technology. - : IOP PUBLISHING LTD. - 0957-0233 .- 1361-6501. ; 32:3
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Tidskriftsartikel (refereegranskat)abstract
- The boron-10 based multi-grid detector is being developed as an alternative to helium-3 based neutron detectors. At the European Spallation Source, the detector will be used for time-of-flight neutron spectroscopy at cold to thermal neutron energies. The objective of this work is to investigate fine time- and energy-resolved effects of the Multi-Grid detector, down to a few mu eV, while comparing it to the performance of a typical helium-3 tube. Furthermore, it is to characterize differences between the detector technologies in terms of internal scattering, as well as the time reconstruction of similar to mu s short neutron pulses. The data were taken at the Helmholtz Zentrum Berlin, where the Multi-Grid detector and a helium-3 tube were installed at the ESS test beamline, V20. Using a Fermi-chopper, the neutron beam of the reactor was chopped into a few tens of mu s wide pulses before reaching the detector, located a few tens of cm downstream. The data of the measurements show an agreement between the derived and calculated neutron detection efficiency curve. The data also provide fine details on the effect of internal scattering, and how it can be reduced. For the first time, the chopper resolution was comparable to the timing resolution of the Multi-Grid detector. This allowed a detailed study of time- and energy resolved effects, as well as a comparison with a typical helium-3 tube.
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| 2. |
- Bull, James N., et al.
(författare)
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Ultraslow radiative cooling of Cn- (n=3-5)
- 2019
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 151:11
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Tidskriftsartikel (refereegranskat)abstract
- Ultraslow radiative cooling lifetimes and adiabatic detachment energies for three astrochemically relevant anions, Cn- (n = 3-5), are measured using the Double ElectroStatic Ion Ring ExpEriment (DESIREE) infrastructure at Stockholm University. DESIREE maintains a background pressure of approximate to 10(-14) mbar and temperature of approximate to 13 K, allowing storage of mass-selected ions for hours and providing conditions coined a molecular cloud in a box. Here, we construct two-dimensional (2D) photodetachment spectra for the target anions by recording photodetachment signal as a function of irradiation wavelength and ion storage time (seconds to minute time scale). Ion cooling lifetimes, which are associated with infrared radiative emission, are extracted from the 2D photodetachment spectrum for each ion by tracking the disappearance of vibrational hot-band signal with ion storage time, giving 1e cooling lifetimes of 3.1 +/- 0.1 s (C3-), 6.8 +/- 0.5 s (C4-), and 24 +/- 5 s (C5-). Fits of the photodetachment spectra for cold ions, i.e., those stored for at least 30 s, provide adiabatic detachment energies in good agreement with values from laser photoelectron spectroscopy on jet-cooled anions, confirming that radiative cooling has occurred in DESIREE. Ion cooling lifetimes are simulated using a simple harmonic cascade model, finding good agreement with experiment and providing a mode-by-mode understanding of the radiative cooling properties. The 2D photodetachment strategy and radiative cooling modeling developed in this study could be applied to investigate the ultraslow cooling dynamics of a wide range of molecular anions.
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| 3. |
- de Ruette, Nathalie, et al.
(författare)
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Mutual Neutralization of O- with O+ and N+ at Subthermal Collision Energies
- 2018
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 121:8
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Tidskriftsartikel (refereegranskat)abstract
- We have measured total absolute cross sections for the mutual neutralization (MN) of O- with O+ and N+. A fine resolution (of about 50 meV) in the kinetic energy spectra of the product neutral atoms allows unique identification of the atomic states participating in the mutual neutralization process. Cross sections and branching ratios have also been calculated down to 1 meV center-of-mass collision energy for these two systems, with a multichannel Landau-Zener model and an asymptotic method for the ionic-covalent coupling matrix elements. The importance of two-electron processes in one-electron transfer is demonstrated by the dominant contribution of a core-excited configuration of the nitrogen atom in N+ + O- collisions. This effect is partially accounted for by introducing configuration mixing in the evaluation of coupling matrix elements.
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| 4. |
- Kristiansson, Moa K., et al.
(författare)
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Experimental and theoretical studies of excited states in Ir
- 2021
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Ingår i: Physical Review A. - : American Physical Society. - 2469-9926 .- 2469-9934. ; 103:6
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Tidskriftsartikel (refereegranskat)abstract
- The properties of atomic negative ions are to a large extent determined by electron-electron correlation which makes them an ideal testing ground for atomic many-body physics. In this paper, we present a detailed experimental and theoretical study of excited states in the negative ion of iridium. The ions were stored at cryogenic temperatures using the double electrostatic ion ring experiment facility at Stockholm University. Laser photodetachment was used to monitor the relaxation of three bound excited states belonging to the [Xe] 4f(14)5d(8)6s(2) ionic ground configuration. Our measurements show that the first excited state has a lifetime much longer than the ion-beam storage time of 1230 +/- 100 s. The binding energy of this state was measured to be 1.045 +/- 0.002 eV. The lifetimes of the second and third excited states were experimentally determined to be 133 +/- 10 and 172 +/- 35 ms, respectively. Multiconfiguration Dirac-Hartree-Fock calculations were performed in order to extract binding energies and lifetimes. These calculations predict the existence of the third excited bound state that was detected experimentally. The computed lifetimes for the three excited bound states agree well with the experimental results and allow for a clear identification of the detected levels.
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