The Search for the 1/2 - state in 7 He

1. The Search for the 1/2- statein 7He Andrew Yue

2. Properties of 7He / Isobars • Member of neutron drip line • Stable on nuclear time scale (~10-21s), but not in comparison to isotopes 6He and 8He (importance of pairing) • Stable core + orbiting neutron (6He + n) • Expect isobars (e.g. 7Li) to have similar nuclear single-particle energy levels • (n,n) and (p,n) scattering experiments used to determine spectrum • Experiments agree on existence of 3/2- ground state for 7He. What about excitations?

3. Evidence For • Meister, et al. • 8He beam incident on12C target – 12C(8He,n)7He • Can be thought of as6He(n,n)6He • Looking for coincidenceof n and 6He • Resonance cannot bedescribed well by singleBreit-Wigner peak • Suggests two resonances present! • 1/2- state “found” at 1.0(1) MeV • Location of 1/2- state disagrees with shell model

4. Evidence Against • Rogachev, et al. • 6He(p,n)6Li(0+, T=1) • Measure reaction at 180º • Detection of n,g coincidence • Analog to 7He 3/2- detected,no sign of a low-lying excitation • Consistent with shell modelcalculations

5. A Potential Resolution • Halderson • The existence of low-lying 1/2- 7He state would demonstrate failure of shell model for neutron drip line • Recoil Corrected Continuum Shell Model demonstrates decent agreement with both sets of data • RCCSM makes predictions contradicting the conclusions of Rogachev, et al

6. Rogachev, et al argue that collecting data at 180º is sufficient • RCCSM predicts cross section of 6He(p,n)6Li is dominated by ground state resonance at 180º - 1/2- is merely suppressed • 1/2- contribution visible at 90º - simulations demonstrate this

7. References [1] D. Halderson, Phys. Rev. C 70, 041603 [2] M. Meister et al., Phys. Rev. Lett. 88, 102501 [3] G. V. Rogachev, et al., Phys. Rev. Lett. 92, 232502 [4] D. Halderson, Nucl. Phys. A707, 65 [5] M. Zinser, et al., Nucl. Phys. A619, 151