Isospin-forbidden alpha decay of the 15.11-MeV state inC12

Abstract

The branching ratio I /I~, has been measured for the 15.11-MeV 1+ T = 1 state in "C. The result is I /I, "l = (4.1 + 0.9)%, in agreement with one previous measurement and in disagreement with another. The result was obtained by direct observation of the decay a particles from the reaction "C('He, a)"Ca 3a. The data are consistent with a sequential decay via the first excited state of 'Be.The 3a breakup of the 1+ T = 0 state at F. "=12. 71 MeV was also studied. The data in both cases were fitted with a model in which the a-a final state interaction was treated as a simple Breit-signer resonance; order-of-emission interference effects were included. If the (isospin-forbidden) a width is interpreted as resulting from mixing between the two 1+ states via a charge-dependent interaction, a large isospin-violating matrix element is involved.

Three-body decays andR-matrix analyses

“…For the 1 + state in 12 C the effect of this interference is to deepen the minima between the three peaks such that the agreement with the data is significantly improved [5,6].…”

“…This was pointed out for the decay spectrum of the 12.71-MeV state in Ref. [5] where a modified R-matrix expression was given (Eq. (3) in Ref.…”

“…, (3) where = 2P (E)γ 2 with P (E) the penetrability for either the α- 8 Be ( 1 ) or α-α( 2r ) breakup, γ 2 is the reduced width and S is the shift function [5,6]. E 2r is the formal resonance energy in 8 Be.…”

“…Parity conservation prevents the state from decaying sequentially by α emission to the narrow 0 + ground state of 8 Be. In the literature, the three characteristic peaks from the breakup of this state have been described as signatures of both sequential [4][5][6] and direct [3,7] breakup.…”

R-matrix analysis of theβdecays ofN12Hyldegaard1, Alcorta2, Bastin3 et al. 2010Phys. Rev. C627410View full textAdd to dashboardCite

“…One obvious shortcoming is the lack of symmetrization of the final states that consist of three identical bosons. This affects the R-matrix expression for breakup via excited states in 8 Be, as we do not know the order of emission of the 3α particles [28]. It has been shown that symmetrization affects the single-α spectrum for the breakup of the 12 C 1 + state at 12.7 MeV [11], but it is assumed to be less significant in the 3α sum spectrum.…”