Utilizing the Mg(P, 2n) reaction and unique particle telescopes, beta-delayed proton emission from Al proceeding via its isobaric analog state in Mg has been observed. The relevant proton group was detected at 223~20 keV with a proton decay branching ratio of 3.5~1.9 %. The proton width determined for this state is approximately an order of magnitude larger than that predicted by a full-basis 1 s-Od shell model calculation which includes the expected isospin mixing. In addition, the resulting resonance strength (45 25 meV) has important astrophysical implications.PACS number(s): 23.40.Hc, 23.50.+z, 27.30.+t, 95.30.Cq There has been considerable astrophysical interest recently in breakout nuclear reactions from both the hot CNO cycle and the rapid proton capture (rp) process. Most of the important information for the relevant reactions involves difficult measurements of cross sections at subbarrier energies and may require radioactive targets. Additionally, many capture reactions [e.g. , (p, y)] must proceed via high-lying nuclear states for which little information is known. As we discuss below, advances in detector development permit the observation of low-energy protons (~180 keV) in a very high beta particle background. This allows us to use the wellknown technique of beta-delayed proton emission [1] in a new way in studying a reaction of astrophysical interest, namely the destruction of Na in the hot NeNa cycle via proton capture to Mg [2].We wish to report our observation of beta-delayed proton emission from Al proceeding via its isobaric analog stateAluminum-23 is the lightest nucleon-stable member of the A =4n+ 3, T, = -3I2 mass series, and the only member of this series in which delayed proton emission from the IAS is potentially observable. We detected this group at a laboratory energy (E&,t,) of 223~20 keV, representing the lowest-energy identified proton group observed to date. Proton decay from the IAS does not conserve isospin, and can therefore only occur due to isospin mixing. A comparison between the proton width (I") of the Mg IAS determined in the present work and the prediction of a full 1s-Od configuration shell model calculation which includes isospin mixing of the IAS indicates the observed isospin mixing is stronger by a factor of -10 than predicted.Beta-delayed proton emission from Al (T»z --470~30 ms) was first observed using standard Si-Si particle identification telescopes [3]. A proton group with E"t, =830~30keV was assigned to emission from an excited state in Mg *Present address: Oak Ridge National Laboratory, Building 6008, MS 6374, Oak Ridge, TN 37831. 658 keV above the IAS. A search for proton emission from the IAS led to a "crude" estimate of I JI "~50 (i.e., b"~2%) for the competition between the 7.8 MeV Ml gamma decay and the E»i, =206~6 keV proton decay of this state [3].In the present study, a helium-jet system [4] was utilized to collect and transport reaction products to a lowbackground counting area. The activity was deposited onto a collection tape in the center of our...
