Inelastic Proton Scattering on the N=28 Nucleus ⁴⁶Ar

L.A. Riley, M.A. Abdelqader, M.J. Bojazi and T.P. Spencer
Ursinus College

D. Bazin, B.A. Brown, C.M. Campbell, J.A. Church, D.-C. Dinca, J. Enders, A. Gade, T. Glasmacher, Z. Hu, W.F. Mueller, H. Olliver, B.C. Perry, B.M. Sherrill, and J.R. Terry
NSCL/MSU

P.D. Cottle, K.W. Kemper and B.T. Roeder
FSU

M. Honma

University of Aizu

S. Horibe
Earlham College

T. Otsuka
University of Tokyo and RIKEN

There is a great deal of debate about whether the N=28 shell closure, which is certainly robust in the stable nucleus ⁴⁸Ca, remains or collapses in the neutron-rich nuclei near ⁴²Si (for example, see [1-3]). The N=28 isotope ⁴⁶Ar is two protons removed from ⁴⁸Ca, and the B(E2;0_gs⁺→2₁⁺) electromagnetic matrix element (measured via intermediate energy Coulomb excitation) is smaller than those in the lighter open-neutron shell Ar isotopes, suggesting that the shell closure remains strong in this nucleus. However, the B(E2;0_gs⁺→2₁⁺) value results from the proton transition density, and does not directly involve the dynamics of the neutrons.

To determine the neutron matrix element for the 0_gs⁺→2₁⁺transition, we measured inelastic proton scattering for this transition in inverse kinematics using thick-target technique developed by Iwasaki et al. [4] that involves detecting the g-rays from the deexcitations. The experiment was performed at the National Superconducting Cyclotron Laboratory. Inelastic proton scattering is sensitive to both proton and neutron contributions to the transition. A comparison of the electromagnetic and proton scattering result yields a ratio of neutron and proton matrix elements (M_n/M_p) that is equal to N/Z for an isoscalar collective excitation and generally deviates from this value for closed shell nuclei. The present results were reported in Ref. [5].

The M_n/M_pratio determined in the present experiment was 1.19(25)N/Z; that is, the present results are consistent with the value expected for an isoscalar collective excitation. This is quite different than the corresponding ratios for the N=28 isotopes of elements heavier than ⁴⁸Ca, for which N=28 is known to be a good closed shell and for which M_n/M_p is significantly smaller than N/Z. This indicates a greater degree of collectivity in ⁴⁶Ar than in heavier N=28 isotones, and may signal erosion of the N=28 shell closure in neutron-rich nuclei.

In addition to the 0_gs⁺→2₁⁺matrix element, the experiment resulted in the tentative identification of an octupole excitation at 4982 keV.

[1] J. Fridmann et al., Nature 435, 922 (2005).

[2] J. Fridmann et al., Phys. Rev. C 74, 034313 (2006).

[3] B. Bastin et al., Phys. Rev. Lett. 99, 022503 (2007).

[4] H. Iwasaki et al., Phys. Lett. B481, 7 (2000).

[5] L.A. Riley et al., Phys. Rev. C 72, 024311 (2005).




	FSU Experimental Nuclear Physics Main			*Inelastic Proton Scattering on the N=28 Nucleus ⁴⁶Ar* *L.A.* *Riley, M.A. Abdelqader, M.J. Bojazi and T.P. Spencer* *Ursinus* *College* D. Bazin, B.A. Brown, C.M. Campbell, J.A. Church, D.-C. Dinca, J. Enders, A. Gade, T. Glasmacher, Z. Hu, W.F. Mueller, H. Olliver, B.C. Perry, B.M. Sherrill, and J.R. Terry NSCL/MSU P.D. Cottle, K.W. Kemper and B.T. Roeder FSU M. Honma *University* *of Aizu* *S. Horibe* *Earlham* *College* *T. Otsuka* *University* *of Tokyo* *and RIKEN* There is a great deal of debate about whether the N=28 shell closure, which is certainly robust in the stable nucleus ⁴⁸Ca, remains or collapses in the neutron-rich nuclei near ⁴²Si (for example, see [1-3]). The N=28 isotope ⁴⁶Ar is two protons removed from ⁴⁸Ca, and the B(E2;0_gs⁺→2₁⁺) electromagnetic matrix element (measured via intermediate energy Coulomb excitation) is smaller than those in the lighter open-neutron shell Ar isotopes, suggesting that the shell closure remains strong in this nucleus. However, the B(E2;0_gs⁺→2₁⁺) value results from the proton transition density, and does not directly involve the dynamics of the neutrons. To determine the neutron matrix element for the 0_gs⁺→2₁⁺transition, we measured inelastic proton scattering for this transition in inverse kinematics using thick-target technique developed by Iwasaki et al. [4] that involves detecting the g-rays from the deexcitations. The experiment was performed at the National Superconducting Cyclotron Laboratory. Inelastic proton scattering is sensitive to both proton and neutron contributions to the transition. A comparison of the electromagnetic and proton scattering result yields a ratio of neutron and proton matrix elements (M_n/M_p) that is equal to N/Z for an isoscalar collective excitation and generally deviates from this value for closed shell nuclei. The present results were reported in Ref. [5]. The M_n/M_pratio determined in the present experiment was 1.19(25)N/Z; that is, the present results are consistent with the value expected for an isoscalar collective excitation. This is quite different than the corresponding ratios for the N=28 isotopes of elements heavier than ⁴⁸Ca, for which N=28 is known to be a good closed shell and for which M_n/M_p is significantly smaller than N/Z. This indicates a greater degree of collectivity in ⁴⁶Ar than in heavier N=28 isotones, and may signal erosion of the N=28 shell closure in neutron-rich nuclei. In addition to the 0_gs⁺→2₁⁺matrix element, the experiment resulted in the tentative identification of an octupole excitation at 4982 keV. [1] J. Fridmann et al., Nature 435, 922 (2005). [2] J. Fridmann et al., Phys. Rev. C 74, 034313 (2006). [3] B. Bastin et al., Phys. Rev. Lett. 99, 022503 (2007). [4] H. Iwasaki et al., Phys. Lett. B481, 7 (2000). [5] L.A. Riley et al., Phys. Rev. C 72, 024311 (2005). Back to Dr. Cottle's Research Menu ^{Back to Dr. Kemper's Research Menu}