Title Study of light nuclei by 13C beam induced reactions Title (croatian) Lake atomske jezgre proučavane reakcijama izazvanim snopom 13C Author Lovro Prepolec Mentor Neven Soić (mentor)Committee member Tamara Nikšić (predsjednik povjerenstva)Committee member Matko Milin (član povjerenstva)Committee member Neven Soić (član povjerenstva)Granter University of Zagreb Defense date and country 2015-09-28, Croatia Scientific / art field, NATURAL SCIENCES Universal decimal classificationUDC ) 53 - Physics Abstract The study of light nuclei is one of the most active areas in the nuclear physics today. Nuclei in the mass region A < 20 have been used as a testing sample for different theoretical models since the early days of nuclear physics. Nowadays the advent of ab initio calculations in various frameworks and the astrophysical interest makes the precise experimental data even more important. One of the topics of interest in the structure of light nuclei are the neutron-rich oxygen isotopes, starting already with astrophysically important 17O. One of the challenging questions in this field of research is how does the well developed cluster structure in 16O change with addition of neutrons in heavier oxygen isotopes. Therefore, the topic of this work is the study of the cluster-structure of the 17O and 18O. In this work results of two experiments are presented: the 13C+4He resonant elastic scattering on a thick gas target measurement and the 13C+9Be resonant particle spectroscopy measurement at E13C = 72 MeV. Both measurements were used due to their selectivity in populating the cluster-state candidates in 17O and 18O. The known 13C(4He,4He) elastic scattering cross-section has been extended both in angle (to the 175∘ in direct kinematics) and energy (up to Ex(17O)=13.8 MeV). The measured cross-section can be reproduced by using published set of parameters from the comprehensive Rmatrix fit. The new part of the spectrum contains several resonances of interest, for which the tentative values of spin, parity and Wigner ratio were determined using a simplified R-matrix fitting method. The resonant particle spectroscopy study of the 13C+4He→ 13C+ 4He+ 5He reaction has provided an improvement in statistics and the energy range covered by the previous study. Resonances decaying to the 13C*+4He decay channel were observed as well. The results of both measurements combined point to the pronounced a-cluster nature of the 8.9 MeV, 9.15 MeV and the 13.57 MeV 17O excited states. This is in accordance with predictions of the 17O rotational band based on the weakly coupled neutron to the well-known a-cluster structure in the 16O. However, precise determination of exact spins and parities, as well as partial and total widths are crucial to confirm the existence of the proposed rotational band. The results for the 18O nucleus were obtained from the 13C+9Be reaction measurements using the resonant particle spectroscopy method. The decays of the 18O* nucleus to the 14C+4He and 14C*+4He channels were studied, confirming the results and extending the excitation energy range of previous studies, as well as observing the a-decay of highly excited states for the first time. The results suggest that the 18O states decaying to the 14C+4He channel do not decay to the 14C*+4He channel and vice-versa, possibly due to a different structure. The 6He decay of the 18O has been observed for the first time, at an excitation energy Ex(18O) ≈ 26:5 MeV. Additional studies are necessary to assess the possibility of the 12C+2n+4He molecular structure of the 18O nucleus. In the study of the 13C+9Be→ 10Be+ 8Begs+ 4He reaction a few possible peaks that could indicate the 10Be+8Begs decay of the 18O nucleus were observed, but statistics was too low to obtain conclusive results. Therefore, additional measurements have to be performed. The resonant particle spectroscopy method may be used to collect the high quality data that would allow to compare the partial widths of the states, as well as to extract additional information using a DWBA calculation. The resonant elastic scattering may be used in order to clarify the spin and parity assignments of the cluster or molecular state candidates at higher 18O excitation energies, but the data on all the relevant inelastic channels have to be measured as well.
Abstract (croatian) Proučavanje lakih jezgra danas je jedno od najaktivnijih područja nuklearne fizike. Jezgre masenog područja A<20 korištene su za ispitivanje raznih modela nuklearne strukture još od ranih dana nuklearne fizike. Danas su pojavom ab initio računa u raznim modelima te zbog astrofizičke važnosti lakih jezgara precizni eksperimentalni podaci još potrebniji. Jedno od izazovnih pitanja u ovom području istraživanja je kako se naglašena klasterska struktura jezgre 16O mijenja dodavanjem neutrona u težim izotopima kisika. Stoga je tema ovog rada proučavanje klasterske građe izotopa 17O i 18O. U ovom su radu predstavljeni rezultati dvaju eksperimenata: rezonantnog elastičnog raspršenja 13C+4He na debeloj plinskoj meti te mjerenja reakcija 13C+9Be, koje su analizirane metodom rezonantne čestične spektroskopije. Oba su mjerenja odabrana zbog visoke selektivnosti za pobuđivanje stanja moguće klasterske građe u jezgrama 17O i 18O. Područje poznatog udarnog presjeka elastičnog raspršenja 13C(4He,4He) prošireno je u kutnom rasponu (ovo mjerenje odgovara kutu od 175∘ u direktnoj kinematici) te energiji (do Ex(17O) = 13:8 MeV). Izmjeren udarni presjek može se reproducirati korištenjem objavljenih parametara koji su rezultat opsežne prilagodbe višekanalnom R-matricom. Novoizmjereni dio spektra sadrži nekoliko zanimljivih rezonanci za koje su pojednostavljenom prilagodbom R-matrice određeni provizorni spin i paritet te Wignerov omjer. Metodom rezonantne čestične spektroskopije proučena je reakcija 13C+4He → 13C+4He+5He a dobiveni rezultati imaju bolju statistiku i pokrivaju šire područje energija pobuđenja od ranije objavljenih. Također su opažene rezonance koje se raspadaju kanalom 13C*+4He. Rezultati oba mjerenja ukazuju na izraženu a- klastersku građu pobuđenih stanja jezgre 17O na energijama 8.9 MeV, 9.15 MeV i 13.57 MeV. Taj je rezultat u skladu sa predviđanjem rotacijske vrpce 17O zasnovane na poznatoj a-klasterskoj strukturi jezgre 16O i slabo vezanom neutronu. Ipak, precizno određivanje spinova i pariteta stanja, kao i njihovih parcijalnih i ukupnih širina, ključno je za potvrđivanje postojanja predložene rotacijske vrpce. Rezultati za jezgru 18O dobiveni su mjerenjem reakcija 13C+9Be, pomoću metode rezonantne čestične spektroskopije. Raspadi jezgre 18O* kanalima 14C+4He i 14C*+4He potvrdili su informacije o poznatim stanjima te proširili područje energija pobuđenja iz ranijih istraživanja. Pritom je za visokopobuđena stanja jezgre 18O po prvi puta opažen a-raspad. Dobiveni rezultati pokazuju da se stanja koja se raspadaju kanalom 14C+4He ne raspadaju kanalom 14C*+4He i obrnuto, vjerojatno zbog različite strukture. Po prvi je puta opažen 6He raspad jezgre 18O, na energiji pobuđenja Ex(18O) ≈ 26:5 MeV. Potrebna su dodatna istraživanja kako bi se donio zaključak o postojanju moguće molekularne strukture oblika 12C+2n+4He. Analizom reakcije 13C+9Be → 10Be+ 8Begs + 4He pronađeni su mogući vrhovi koji bi mogli ukazivati na 10Be+8Begs raspad pobuđenih stanja jezgre 18O, no broj događaja je premalen da bi se mogao donijeti zaključak. Dakle, potrebna su dodatna mjerenja. Tehnikom rezonantne čestične spektroskopije mogli bi se prikupiti visokokvalitetni podaci, iz kojih bi bilo moguće usporediti parcijalne širine stanja, kao i izdvojiti podatke o strukturi korištenjem DWBA računa. Rezonantno elastično raspršenje može se iskoristiti za razjašnjavanje vrijednosti spina i pariteta klasterskih ili molekulskih stanja na višim energijama pobuđenja 18O, no pritom treba prikupiti i podatke o neelastičnim kanalima.
Keywords
nuclear structure
resonant elastic scattering
resonant particle spectroscopy
cluster states
molecular states
rotational bands
Keywords (croatian)
nuklearna struktura
rezonantno elastično raspršenje
rezonantna čestična spektroskopija
klasterska stanja
molekulska stanja
rotacijske vrpce
Language english URN:NBN urn:nbn:hr:217:443849 Study programme Title: Physics Type of resource Text Extent 209 str. File origin Born digital Access conditions Open access Terms of use Created on 2017-08-22 08:14:20
