372596205

11

RIKEN Accel. Próg. Rep. 24 (1990)

111-1-2. Three a Disintegration of ¹²C in the Field of

²⁰⁸Pb Nucleus at 35 MeV/u

T. Motobayashi, S. Kox, C. Perrin, F. Merchez, D. Rebreyend,

S. Kubono, T. Ichihara, and M. Ishihara

NUCLEAR REACTIONS ²⁰⁸Pb(¹²C, 3a)²⁰⁸Pb, E = 35 MeV/u; measured o(0). CsI(Tl) scintillator array, MWPC.

A projectile breakup process was studied for a ¹²C+²⁰⁸Pb reaction by measuring outgoing three a particles in coincidence. A 35 MeV/u carbon beam was provided by the separate sector cyclotron of SARA facility at ISN Grenoble. Complete kinemat-ical conditions of four body finał State (3a and ²⁰⁸Pb) were determined from the energies and scattering angles of the three a particles obtained by a multi-detector system consisting of 3 planes of MWPC and 24 CsI(Tl) scintillators based on the EMRIC device developed at ISN Grenoble.^1}

Figurę 1 shows a spectrum for the relative energy of the three outgoing oc particles (or the excitation energy measured from the 3-a threshold of ¹²C). The prominent peak corresponds to the second 0⁺ State of ¹²C at E_ex = 7.65 MeV. The 3“state at 9.68 MeV is weakly excited. No evi-

dence is seen for possible excitation of the 10.3 MeV State, where the spin assignment is still uncertain and strong effects on the elastic scattering channel is predicted by CDCC calculations.²⁾

(MeV)

Fig. 1. Relative energy spectrum for outgoing three oc particles. The horizontal axis represents the excitation energy from the three a threshold at 7.27 MeV above the ground State of ¹²C. Two peaks correspond to the second 0⁺ State at E_ex = 7.65 MeV and the 3~ State at 9.64 MeV.

E,g (keV)

Fig. 2. Dalitz plot for three a particles decaying from the second O⁴ State of ¹²C. The events are concentrated on three lines showing that two a particles out of three are in the ⁸Be ground State with £_rei = 92 keV.

Figurę 2 shows a Dalitz plot for the decay of the 0* State. The State decays with almost 100% prob-ability to the ⁸Be(0⁺)+a channel and few counts are seen for the decay to the 3-a continuum. This indicates that the decay width of the second 0⁺ State of ¹²C is dominated by the ⁸Be(0⁺)+a channel. That is consistent with the present understand-ing of the helium buming process in stars, where the process ⁸Be(0⁺)+a —> ¹²C(0⁺;7.65 MeV) —> ¹²C(0⁺;0.0 MeV) is assumed to play a dominant role in producing ¹²C nucleus.^3>

References

1)    F. Merchez, S. Kox, C. Perrin, J. Mistretta, J.C. Gon-drand, L.N. Imouk, P. Gretillat, and E. Schwarz: Nuci. Instrum. Methods, A275, 133 (1989).

2)    Y. Sakuragi, M. Kamimura, M. Yahiro, and Y. Fukushima: J. Phys. Soc. Jpn., 54, Suppl. II, p.88 (1985).

3)    C. Rolf and H.P. Trautvetter: Ann. Rev. Nuci. Part. Sci., 28, 115 (1978).