Μελέτη και εγκατάσταση συστήματος απογύμνωσης ιόντων στον 5.5 MV επιταχυντή Tandem Van de Graaff του Ε.Κ.Ε.Φ.Ε. "Δημόκριτος".

Εθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Φυσική και Τεχνολογικές Εφαρμογές

Magnetospheric effects on cosmic rays during the magnetic storm of March 2015

Cosmic ray variations of magnetospheric origin during the magnetic storm on 17th of March 2015 were studied. Cosmic ray intensity data were obtained from the neutron monitor database (NMDB) and the data of the Dst index were taken from World Data Center for Geomagnetism, Kyoto. The global survey method was employed for the calculation of changes in the cutoff rigidities throughout the storm. A correlation analysis between the Dst index and the calculated cutoff rigidity variations was performed for each cosmic ray station. The most essential decrease in cutoff rigidities occured when the Dst index was around the value of -234nT. A latitudinal distribution of the cutoff rigidities was acquired, showing that the maximum effect took place at mid-latitude stations with rigidities around 8-10GV. During the examined event the maximum change in cutoff rigidity was observed at Athens station where the decrease of the cutoff rigidity reached the value of 1.07GV. Furthermore, corrections of cosmic ray intensity due to the magnetospheric effect were calculated using the derived cutoff rigidities showing a discperancy with the observed values at mid- and low- latitude stations

Μελέτη ατομικών διεγέρσεων δέσμης ηλιοειδών ιόντων C4+ και O6+ σε κρούσεις με στόχους He

Using the APAPES (Atomic Physics with Accelerators: Projectile Electron Spectroscopy) project zero-degree Auger projectile spectrograph setup and the C4+ and O6+ He-like ion beams supplied by the 5.5 MV “Demokritos” tandem Van de Graaff accelerator, K-Auger projectile electron spectra measurements were performed, for 0.5-1.5 MeV/u collisions with He gas targets. He-like beams in general contain a mixture of 1s2 1S, 1s2s 1S and 1s2s 3S components, so all three initial channels are considered. In particular, this work focuses on the following two collision processes: Xq+(1s2) + He →X(q+1)+(1s2p2 2D) + He+(All) (Transfer-Excitation) (1) ↳Xq+(1s2) + e-A (θ=0ο) (2) and Xq+(1s2, 1s2s 3,1S) + He →Xq+(2s2p 3,1P) + He(All) (Projectile Excitation) (3) ↳ X(q-1)+(1s) + e-A (θ=0ο) (4) where Xq+ refers to carbon C4+ or oxygen O6+ projectiles. The second line of each process indicates the emission of the Auger electron and its detection at theta=0 degrees. In this thesis, the cross section for the production of the 1s2p2 2D, 2s2p 3P and 2s2p 1P states were determined, in order to study the transfer-excitation (TE), single and double projectile excitation atomic processes. The measurements are also accompanied by close-coupling (CC) calculations in which three-active electrons have been included for the first time (3eAOCC) , in an attempt to cast further light on these complex multi-electron processes. The calculated cross sections for the production of 1s2p2 2D from 1s2 1S through TE, both for carbon and oxygen are found to be in agreement with zero-degree Auger projectile spectroscopy measurements covering the maximum of the resonance peak and its high energy wing. At lower energies, the theoretical results show a second maximum which is interpreted recently [1] through a nonresonant one-step transfer excitation mechanism. In the case of 2s2p 3P and 2s2p 1P the 3eAOCC calculated cross sections were converted to Auger electron normalized yields for a direct comparison between theory and experiment, as 2s2p 3P and 2s2p 1P states can be populated by all three 1s2 1S, 1s2s 1S and 1s2s 3S components through different processes. Although both theory and experiment trends appear to have a similar energy dependence, the theoretical results in this case are in general smaller than the experimental, leaving room for discussion on the possible reasons that this happens.[1] A. Laoutaris, S. Nanos, I. Madesis, S. Passalidis, E. P. Benis, A. Dubois, and T. J. M. Zouros, Phys. Rev. A 106, 022810 (2022).Αξιοποιώντας τον ήδη υπάρχοντα φασματογράφο ηλεκτρονίων δέσμης μηδέν μοιρών της πειραματικής διάταξης APAPES (Atomic Physics with Accelerators: Projectile Electron Spectroscopy) και τις ηλιοειδείς δέσμες ιόντων C4+ και O6+ προερχόμενες από τον 5.5 MV επιταχυντή tandem Van de Graaff του «Δημόκριτου», πραγματοποιήθηκαν μετρήσεις K-Auger ηλεκτρονίων δέσμης ιόντων, μέσω κρούσεων με στόχους αερίου He στην ενεργειακή περιοχή 0.5-1.5 MeV/u. Οι ηλιοειδέις δέσμες εν γένει συνυπάρχουν σε ένα μείγμα των 1s2 1S, 1s2s 1S και 1s2s 3S καταστάσεων, οπότε συμπεριλαμβάνονται διαδικασίες προερχόμενες και από τις τρεις αυτές αρχικές καταστάσεις. Ποίο συγκεκριμένα η παρούσα δουλειά εστιάζει στις επόμενες δύο διαδικασίες κατά την διάρκεια κρούσεων:Xq+(1s2) + He →X(q+1)+(1s2p2 2D) + He+(All) (Transfer-Excitation) (1) ↳Xq+(1s2) + e-A (θ=0ο) (2)καιXq+(1s2, 1s2s 3,1S) + He →Xq+(2s2p 3,1P) + He(All) (Projectile Excitation) (3) ↳ X(q-1)+(1s) + e-A (θ=0ο) (4) όπου το Xq+ αναφέρεται στη δέσμη ιόντων άνθρακα C4+ ή οξυγόνου O6+. Η δεύτερη γραμμή κάθε διεργασίας δείχνει την εκπομπή του ηλεκτρονίου Auger και την ανίχνευσή του σε θ=0 μοίρες.Στην παρούσα διατριβή προσδιορίστηκαν οι ενεργές διατομές για την παραγωγή των καταστάσεων 1s2p2 2D, 2s2p 3P και 2s2p 1P, προκειμένου να μελετηθούν οι ατομικές διεργασίες transfer excitation (TE) και single ή double excitation της δέσμης ιόντων. Οι μετρήσεις συνοδεύονται επίσης από close coupling (CC) υπολογισμούς στους οποίους έχουν συμπεριληφθεί για πρώτη φορά τρία ηλεκτρόνια (3eAOCC) με ενεργό ρόλο κατά την κρούση, σε μια προσπάθεια να ριχθεί περαιτέρω φως σε αυτές τις πολύπλοκες διεργασίες πολλών ηλεκτρονίων. Οι υπολογισμένες ενεργές διατομές για την παραγωγή 1s2p2 2D από 1s2 1S μέσω TE, τόσο για άνθρακα όσο και για οξυγόνο, βρέθηκαν να συμφωνούν με τις πειραματικές μετρήσεις φασματοσκοπίας Auger μηδέν μοιρών που καλύπτουν το μέγιστο της resonance κορυφής και την πλευρά των υψηλών ενεργειών. Σε χαμηλότερες ενέργειες, τα θεωρητικά αποτελέσματα δείχνουν ένα δεύτερο μέγιστο που ερμηνεύεται πρόσφατα [1] μέσω ενός nonresonant one-step transfer excitation μηχανισμού. Στην περίπτωση των καταστάσεων 2s2p 3P και 2s2p 1P οι 3eAOCC υπολογισμοί ενεργών διατομών μετατράπηκαν σε Auger electron normalized yields για μια άμεση σύγκριση μεταξύ θεωρίας και πειράματος, καθώς οι καταστάσεις 2s2p 3P και 2s2p 1P μπορούν να εποικηθούν και από τις τρεις 1s2 1S, 1s2s 1S και 1s2s 3S καταστάσεις μέσω διαφορετικών διεργασιών. Παρόλο που θεωρία και πειραματικές μετρήσεις φαίνεται να έχουν παρόμοια ενεργειακή εξάρτηση, τα θεωρητικά αποτελέσματα σε αυτήν την περίπτωση είναι γενικά μικρότερα από τα πειραματικά, αφήνοντας χώρο για συζήτηση σχετικά με τους πιθανούς λόγους που συμβαίνει αυτό[1] A. Laoutaris, S. Nanos, I. Madesis, S. Passalidis, E. P. Benis, A. Dubois, and T. J. M. Zouros, Phys. Rev. A 106, 022810 (2022)

Mixed-State Ionic Beams: An Effective Tool for Collision Dynamics Investigations

The use of mixed-state ionic beams in collision dynamics investigations is examined. Using high resolution Auger projectile spectroscopy involving He-like ( 1 s 2 1 S , 1 s 2 s 3 , 1 S ) mixed-state beams, the spectrum contributions of the 1 s 2 s 3 S metastable beam component is effectively separated and clearly identified. This is performed with a technique that exploits two independent spectrum measurements under the same collision conditions, but with ions having quite different metastable fractions, judiciously selected by varying the ion beam charge-stripping conditions. Details of the technique are presented together with characteristic examples. In collisions of 4 MeV B 3 + with H 2 targets, the Auger electron spectrum of the separated 1 s 2 s 3 S boron beam component allows for a detailed analysis of the formation of the 1 s 2 s ( 3 S ) n l 2 L states by direct n l transfer. In addition, the production of hollow 2 s 2 p 1 , 3 P doubly- and 2 s 2 p 2 2 D triply-excited states, by direct excitation and transfer-excitation processes, respectively, can also be independently studied. In similar mixed-state beam collisions of 15 MeV C 4 + with H 2 , He, Ne and Ar targets, the contributions of the 1 s 2 , 1 s 2 s 3 , 1 S beam components to the formation of the 2 s 2 p 3 , 1 P states by double-excitation, 1 s → 2 p excitation and transfer-loss processes can be clearly identified, facilitating comparisons with theoretical calculations

Radiative Cascade Repopulation of 1s2s2p 4P States Formed by Single Electron Capture in 2–18 MeV Collisions of C4+ (1s2s 3S) with He

This study focuses on the details of cascade repopulation of doubly excited triply open-shell C3+(1s2s2p)4P and 2P± states produced in 2–18 MeV collisions of C4+(1s2s3S) with He. Such cascade calculations are necessary for the correct determination of the ratio R of their cross sections, used as a measure of spin statistics [Madesis et al. PRL 124 (2020) 113401]. Here, we present the details of our cascade calculations within a new matrix formulation based on the well-known diagrammatic cascade approach [Curtis, Am. J. Phys. 36 (1968) 1123], extended to also include Auger depopulation. The initial populations of the 1s2snℓ4L and 1s2snℓ2L levels included in our analysis are obtained from the direct nℓ single electron capture (SEC) cross sections, calculated using the novel three-electron close-coupling (3eAOCC) approach. All relevant radiative branching ratios (RBR) for n≤4 were computed using the COWAN code. While doublet RBRs are found to be very small, quartet RBRs are found to be large, indicating cascade feeding to be important only for quartets, consistent with previous findings. Calculations including up to third order cascades, extended to n→∞ using an n−3 SEC model, showed a ∼60% increase of the 1s2s2p4P populations due to cascades, resulting, for the first time, in R values in good overall agreement with experiment