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Exploring the 1/2- State in 7He: Evidence and Theoretical Implications

This study investigates the 1/2- state of the isotope 7He amidst ongoing debates regarding its nuclear properties. The existence of this state is crucial for understanding the neutron drip line and the limitations of the shell model. Using (n,n) and (p,n) scattering experiments, contrasting results arise from different research groups, indicating potential resonances and implications for theoretical frameworks. We discuss various experimental approaches and their outcomes, emphasizing the significance of these findings for nuclear physics and further explorations of exotic isotopes.

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Exploring the 1/2- State in 7He: Evidence and Theoretical Implications

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  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

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