physics.fsu.edu | Experimental Nuclear Physics

Intruder structures and the "Island of Inversion"

The "island of inversion", a region of nuclei with Z near the bottom and N near the top of the sd shell where intruder fp configurations fall below the normal sd ones, has long challenged theoretical understanding. Some of the questions involve the relative importance of a changing neutron shell gap with changing proton number and of the effects of correlations or deformation. We have performed a series of experiments at both the FSU Fox lab and at the NSCL to delineate the limits of the ''island'' and to test theoretical models.

One investigation has focused on the sodium isotopes from ²⁶Na to ³⁰Na [1-5]. The lighter isotopes have been investigated in the Fox lab using long-lived radioactive beams and targets and detecting γ rays in coincidence with charged particles [1,3]. A complementary approach at the NSCL for the heavier isotopes involves a study of the γ rays following the β decay of beams produced by fragmentation of ⁴⁸Ca and separated with the A1900 [2,4,5]. There are no clear deviations from the predictions of the pure sd model in the low-lying structure up through ²⁸Na (N =17). However, at N = 18 (²⁹Na), low-lying states appear that are not described by the pure sd model, but are accounted for in an extended sd-pf model space using the SDPF-M interaction [2,4]. The latter calculation predicts a 50%-50% mixing in the ground state of pure sd configurations and ones involving two particles in the fp shell (2P-2H). Beta-decay strengths confirm the SDPF-M predictions of a 100% 2P-2H configuration in the ground state of N = 19 ³⁰Na where the nearly pure sd structures start at about 1 MeV of excitation [5].

A decreasing N = 20 gap in some models comes from movement upward of the d_3/2 orbital. This causes a new gap at N = 16 to open. Complementary in-beam and β-decay studies of ^24,25Ne have found evidence for such a gap opening in deviations from the predictions of pure sd shell model calculations using the USD interaction [6,7].

Changing shell gaps and correlation energies play a role in the surprising jump in the neutron drip line from N = 17 in the O isotopes to well over 20 in the F nuclei. The recent measurement with the MoNA collaboration of the neutron decay energy of the ground state of ²⁵O provides a critical test of the models which attempt to explain the jump [8]. The experiment also provides some support to the shell gap at N = 16 for ²⁴O.

Further interesting evidence on the systematics of intruder effects in the southeast corner of the Segre chart comes from a comparison of the excitation energies Ex of the lowest 4^- states in the odd-odd nuclei in this region. As shown in the figure, the 4^- energies decrease steadily with both increasing N and also with decreasing Z [3,9] until they reach the ground state. One might say the systematically decreasing energies of the intruder states map out the “underwater shelf of the island of inversion.” Ie, intruder states decrease in energy until they reach the ground state (or rise above sea level in the island analogy) and fully qualify for the term “intruder”. Most interesting are the almost horizontal rows of points indicating that the 4^- energies are almost purely a function of N – Z. Thus even though the most successful models for cross shell interactions treat the Z and N dependence differently, the result of decreasing Z or increasing N seems to be exactly the same.

For two decades the USD interaction has been very successful in describing the structure of the positive-parity states in the sd shell. Because it is an effective interaction, fitted to known structure, the USD may include some of the effects of 2P-2H intruder configurations. Recently, two new interactions, USDA and USDB, were adjusted to better describe the wealth of new nuclear structure in the sd shell exposed in the intervening decades [10]. Since a significant amount of the new information is on more neutron-rich nuclei, the new interactions may implicitely include more intruder effects. We have carefully compared the γ decay patterns as well as excitation energies of states in ²²F [11]and ³⁰Al[9] to the predictions of all 3 interactions. The new interactions do describe the excitation energies better, but there is no significant difference among the predicted branching ratios. Many of the higher states in ³⁰Al are not described properly by the sd shell model with any of the interactions. These states are well described as moderately high spin negative parity intruder states. The 4^- to 6^- states have a similar relative spacing to those in ²⁸Al, but all are shifted downward by about 1200 keV, as shown for the 4^- level in the figure.

Crossing the N = 8 gap at the bottom of the sd shell also appears to be dependent on neutron excess. A number of previously unknown states were observed in ²⁰O in the ¹⁰B(¹⁴C,⁴He) reaction in the Fox lab [12]. In shell model calculations for ²⁰O and other neutron-rich O and F isotopes, it was found necessary to systematically reduce the p-sd gap with increasing N to reproduce experiment.

References

[1] M.W. Cooper, et al., Phys. Rev. C 65, 051302(R) (2002).

[2] Vandana Tripathi, et al., Phys. Rev. Lett. 94, 162501 (2005).

[3] Sangjin Lee, et al., Phys. Rev. C 73, 044321 (2006).

[4] Vandana Tripathi, et al., Phys. Rev. C 73, 054303 (2006).

[5] Vandana Tripathi, et al., Phys. Rev. C {\bf 76}, 021301 (2007).

[6] C.R. Hoffman, et al., Phys. Rev. C 68, 034304 (2003).

[7] S.W. Padgett, et al., Phys. Rev. C 72, 064330 (2005).

[8] C.R. Hoffman, et al., to be published.

[9] T.A. Hinners, et al., to be published.

[10] B.A. Brown and W.W. Richter, Phys. Rev. C 74, 034315 (2006).

[11] Sangjin Lee, et al., Phys. Rev. C (in press).

[12] M. Wiedeking, et al., Phys. Rev. Lett. 94, 132501 (2005).

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	FSU Experimental Nuclear Physics Main			Intruder structures and the "Island of Inversion" The "island of inversion", a region of nuclei with Z near the bottom and N near the top of the sd shell where intruder fp configurations fall below the normal sd ones, has long challenged theoretical understanding. Some of the questions involve the relative importance of a changing neutron shell gap with changing proton number and of the effects of correlations or deformation. We have performed a series of experiments at both the FSU Fox lab and at the NSCL to delineate the limits of the ''island'' and to test theoretical models. One investigation has focused on the sodium isotopes from ²⁶Na to ³⁰Na [1-5]. The lighter isotopes have been investigated in the Fox lab using long-lived radioactive beams and targets and detecting γ rays in coincidence with charged particles [1,3]. A complementary approach at the NSCL for the heavier isotopes involves a study of the γ rays following the β decay of beams produced by fragmentation of ⁴⁸Ca and separated with the A1900 [2,4,5]. There are no clear deviations from the predictions of the pure sd model in the low-lying structure up through ²⁸Na (N =17). However, at N = 18 (²⁹Na), low-lying states appear that are not described by the pure sd model, but are accounted for in an extended sd-pf model space using the SDPF-M interaction [2,4]. The latter calculation predicts a 50%-50% mixing in the ground state of pure sd configurations and ones involving two particles in the fp shell (2P-2H). Beta-decay strengths confirm the SDPF-M predictions of a 100% 2P-2H configuration in the ground state of N = 19 ³⁰Na where the nearly pure sd structures start at about 1 MeV of excitation [5]. A decreasing N = 20 gap in some models comes from movement upward of the d_3/2 orbital. This causes a new gap at N = 16 to open. Complementary in-beam and β-decay studies of ^24,25Ne have found evidence for such a gap opening in deviations from the predictions of pure sd shell model calculations using the USD interaction [6,7]. Changing shell gaps and correlation energies play a role in the surprising jump in the neutron drip line from N = 17 in the O isotopes to well over 20 in the F nuclei. The recent measurement with the MoNA collaboration of the neutron decay energy of the ground state of ²⁵O provides a critical test of the models which attempt to explain the jump [8]. The experiment also provides some support to the shell gap at N = 16 for ²⁴O. Further interesting evidence on the systematics of intruder effects in the southeast corner of the Segre chart comes from a comparison of the excitation energies Ex of the lowest 4^- states in the odd-odd nuclei in this region. As shown in the figure, the 4^- energies decrease steadily with both increasing N and also with decreasing Z [3,9] until they reach the ground state. One might say the systematically decreasing energies of the intruder states map out the “underwater shelf of the island of inversion.” Ie, intruder states decrease in energy until they reach the ground state (or rise above sea level in the island analogy) and fully qualify for the term “intruder”. Most interesting are the almost horizontal rows of points indicating that the 4^- energies are almost purely a function of N – Z. Thus even though the most successful models for cross shell interactions treat the Z and N dependence differently, the result of decreasing Z or increasing N seems to be exactly the same. For two decades the USD interaction has been very successful in describing the structure of the positive-parity states in the sd shell. Because it is an effective interaction, fitted to known structure, the USD may include some of the effects of 2P-2H intruder configurations. Recently, two new interactions, USDA and USDB, were adjusted to better describe the wealth of new nuclear structure in the sd shell exposed in the intervening decades [10]. Since a significant amount of the new information is on more neutron-rich nuclei, the new interactions may implicitely include more intruder effects. We have carefully compared the γ decay patterns as well as excitation energies of states in ²²F [11]and ³⁰Al[9] to the predictions of all 3 interactions. The new interactions do describe the excitation energies better, but there is no significant difference among the predicted branching ratios. Many of the higher states in ³⁰Al are not described properly by the sd shell model with any of the interactions. These states are well described as moderately high spin negative parity intruder states. The 4^- to 6^- states have a similar relative spacing to those in ²⁸Al, but all are shifted downward by about 1200 keV, as shown for the 4^- level in the figure. Crossing the N = 8 gap at the bottom of the sd shell also appears to be dependent on neutron excess. A number of previously unknown states were observed in ²⁰O in the ¹⁰B(¹⁴C,⁴He) reaction in the Fox lab [12]. In shell model calculations for ²⁰O and other neutron-rich O and F isotopes, it was found necessary to systematically reduce the p-sd gap with increasing N to reproduce experiment. References [1] M.W. Cooper, et al., Phys. Rev. C 65, 051302(R) (2002). [2] Vandana Tripathi, et al., Phys. Rev. Lett. 94, 162501 (2005). [3] Sangjin Lee, et al., Phys. Rev. C 73, 044321 (2006). [4] Vandana Tripathi, et al., Phys. Rev. C 73, 054303 (2006). [5] Vandana Tripathi, et al., Phys. Rev. C {\bf 76}, 021301 (2007). [6] C.R. Hoffman, et al., Phys. Rev. C 68, 034304 (2003). [7] S.W. Padgett, et al., Phys. Rev. C 72, 064330 (2005). [8] C.R. Hoffman, et al., to be published. [9] T.A. Hinners, et al., to be published. [10] B.A. Brown and W.W. Richter, Phys. Rev. C 74, 034315 (2006). [11] Sangjin Lee, et al., Phys. Rev. C (in press). [12] M. Wiedeking, et al., Phys. Rev. Lett. 94, 132501 (2005). Back to Dr. Tabor's Research Menu