RAPID COMMUNICATIONS PHYSICAL REVIEW C 75, 051304(R) (2007) Decay of 10 C excited states above the 2 p + 2α threshold and the contribution from “democratic” two-proton emission R. J. Charity, 1 K. Mercurio, 2 L. G. Sobotka, 1,2 J. M. Elson, 1 M. Famiano, 3 A. Banu, 4 C. Fu, 4 L. Trache, 4 and R. E. Tribble 4 1 Department of Chemistry, Washington University, St. Louis, Missouri 63130, USA 2 Department of Physics, Washington University, St. Louis, Missouri 63130, USA 3 Department of Physics, Western Michigan University, Kalamazoo, Michigan 49008, USA 4 Cyclotron Institute, Texas A&M University, College Station, Texas 77843, USA (Received 28 February 2007; published 31 May 2007) The decay of 10 C excited states to the 2p + 2α exit channel has been studied using an E/A = 10.7 MeV 10 C beam inelastically scattered from a 9 Be target. Levels associated with two-proton decay to the ground state of 8 Be have been observed. These include states at 5.18 and 6.54 MeV which decay by sequential two-proton emission through the long-lived ground state of 9 B. In addition, states at 5.3 and 6.57 MeV were found in which there is no long-lived intermediate state between the two proton emissions. For the 6.57 MeV state, the two protons are preferably emitted on the same side of the decaying 10 C fragment. DOI: 10.1103/PhysRevC.75.051304 PACS number(s): 27.20.+n, 23.50.+z, 23.60.+e, 25.60.Gc The level structure of 10 C is not well known. Figure 1 shows the low-lying levels that have been identified so far [1]. Only the first excited state (2 + ,E ∗ = 3.351 MeV), which decays by γ -ray emission, is fully characterized and all other excited states are particle unstable. Common particle decay modes, such as proton or α-particle emission, produce daughter nuclei, 9 B and 6 Be, which are also particle unstable. Both of these nuclei decay, either directly, or via an intermediate step, and lead to the 2p + 2α exit channel. The threshold for 10 C → 3 He + 7 Be decay is 15.0 MeV. This is the first binary decay mode that leads to two particle-stable fragments. All excited states with excitation energies between this threshold and the 2p + 2α threshold must decay in some manner to the 2p + 2α exit channel. Figure 1 also shows possible intermediate states in neighboring nuclei and their decays which could contribute to the 2p + 2α channel. The structure of 10 C is expected to be similar to its mirror nucleus 10 Be. Both nuclei are predicted to have strong α-particle cluster structure [2]. Rotational bands based on the two molecular configurations each consisting of a two-alpha ( 8 Be) core and two valence neutrons [2] have been identified in 10 Be [3]. The 0 + ground and 2 + first excited state in 10 C are presumably members of one of these rotational bands. One would like to identify members of the higher-lying rotational band built on a second more deformed 0 + level. The excited states of 10 C may undergo two-proton decay. This could be a sequential two-proton emission passing through a long-lived intermediate as in the decay of the ground state of 12 O[4]. Alternatively, a more direct three-body breakup or democratic decay could occur as in the case of 6 Be [5,6]. Due to the potentially interesting features of 10 C levels, we have studied the decay of inelastically excited 10 C nuclei to the 2p + 2α exit channel. Correlations between the four detected particles were analyzed and used to assign the decay paths of all observed levels. At the Texas A&M University cyclotron facility, a primary beam of E/A = 15.0 MeV 10 B of intensity 40 pnA was extracted from the K500 cyclotron. This beam impinged on a hydrogen gas cell held at a pressure of two atmospheres and kept at liquid-nitrogen temperature. A secondary beam of E/A = 10.7 MeV 10 C was produced through the 10 B(p,n) 10 C reaction and separated from other reaction products using the MARS spectrometer [7]. This secondary beam, with intensity of 5 × 10 4 s −1 , purity of 99.5%, and an energy spread of 3%, impinged on a 14 mg/cm 2 target of 9 Be. The beam spot on the target was 3.5 mm × 5.3 mm in area. Charged particles produced in the interactions were de- tected in four Si E-E telescopes located in a plane 14 cm downstream of the target. The telescopes, part of the HIRA array [8], consisted of a 65 µm thick, single-sided Si-strip E detector followed by a 1.5 mm thick, double-sided Si strip E detector. All Si detectors were 6.4 cm × 6.4 cm in area with the position-sensitive faces divided into 32 strips. The telescopes were positioned in a square arrangement with each telescope offset from its neighbor to produce a small, central, square hole through which the unscattered beam passed. With this arrangement, the angular range from θ = 1.3 to 7.7 ◦ was covered. Signals produced in the telescopes were read out with the HINP16C chip-readout electronics [9]. Energy calibrations were obtained from the p,d,t , and α-particle “punch through” energies. Energy-loss corrections accounting for the traversal of the fragments through half of the target thickness were derived from Ref. [10]. In order to determine the efficiencies for detecting the 2p + 2α events and the resolution of the reconstructed excitation energy, Monte Carlo simulations were performed. As a test of these simulations, we first looked at 12 C states which decay into the 3α channel. These events were formed in the 9 Be( 10 C, 12 C) 7 Be and possibly other more complex reactions. In Fig. 2, the distribution of reconstructed 12 C excitation energy is plotted as the histogram for 3α events detected in this work. Two prominent peaks are observed associated with the known E ∗ = 7.65 MeV (0 + ,Ŵ = 8.5 eV) and E ∗ = 9.64 MeV (3 − ,Ŵ = 34 keV) states. The widths of the peaks are significantly larger than the intrinsic widths, 0556-2813/2007/75(5)/051304(5) 051304-1 ©2007 The American Physical Society