Low-lying bound excited states of 17C and 19C

Complete text of publication follows. Theoretical calculations predict two low-lying excited states of both 17C and 19C, but only one excited state has been found in 17C. Recently, indications of two γ transitions of 17C have been reported, suggesting that both predicted excited states are bound in 17C. In addition, a γ peak in 19C was also observed. To obtain more information on the excited states in neutron rich odd carbon nuclei we have studied them by the (p,p'γ) process in inverse kinematics. The experiment was carried out at the RIKEN radioactive isotope separator RIPS. A 22Ne primary beam of 100 pnA intensity and 110 A x MeV energy hits a 9Be production target of 0.8 cm thickness. For optimizing the 19C beam, the secondary cocktail beam included 20% 19C and 25% 17B. By tuning the 17C beam, practically 100% purity could be achieved. On an event-by-event basis, the identification of the incoming beam was performed by energy-loss, time-of-fight (TOF) and magnetic rigidity (Bρ) measurements. The secondary beam bombarded a liquid hydrogen target of 3 cm diameter. A NaI(Tl) array surrounded the liquid hydrogen target to detect de-excited γ rays. A silicon telescope with layers of 0.5, 2 and 2 mm thicknesses was inserted in air atmosphere to identify scattered particles. To produce γ ray spectra, one fold events in the NaI(Tl) setup were selected (Fig. 1). In the (a) panel, two peaks are clearly visible at 72(4) and 197(6) keV which can be associated with the prompt decays of excited states of 19C. Fig. 1 shows two distinct peaks at 210(4) and 331(6) keV confirming two low-lying excited states of 17C. In the panel of the 1H(17C,17C) reaction, the higher energy peak is clearly visible, while the 210 keV peak is very faint, if it exists at all, and is situated on the Compton background of the 331 keV peak. The counting statistics in Fig. 1 allowed us to perform a γ-γ coincidence analysis, which showed that the two observed transitions of 17C are not in coincidence. The analyses of the spectra and the cross sections are now in progress. (author)