physics.fsu.edu | Experimental Nuclear Physics

NEUTRON-RICH NUCLEI BEYOND ⁴⁸Ca

Dr. Samuel L. Tabor

There is growing evidence that neutron excess also changes the single-particle shell structure in the nuclei above ⁴⁸Ca with Z > 20 and N > 28, i.e., with valence particles in different shells. Complementary experimental approaches have been used. Beta decay following projectile fragmentation provides clean γ spectra from low-lying, low-spin levels. In-beam spectra show much more of the level schemes, but are very complex, because many different reaction products are present. Both γ coincidences in GAMMASPHERE based on lines previously seen in β decay and separation of the desired nuclei in the Fragment Mass Analyzer after GAMMASPHERE have been used to isolate these γ spectra.

A major issue in this region is movement of the neutron f_5/2 orbital with the filling of the proton spin-orbit partner f_7/2. There is clear evidence of a shell gap at N = 32 for nuclei near Ca due to filling of the p_3/2 orbital. Calculations using the GXPF1 interaction which predicts this gap also predict another gap at N = 34 after the p_1/2 orbital is filled and implying that the f_5/2 orbital lies above p_1/2. However, the lowest 2⁺ level in ⁵⁶Ti was observed to lie too low to support an N = 34 gap [1]. Further investigation of ^54,55,56Ti and a comparison with neighboring nuclei confirms a gap between the p_3/2 and f_5/2 orbitals and suggests that p_1/2 lies close to f_5/2 , slightly above for less neutron excess and slightly below with increasing neutron excess [2]. Beta-decay studies of ⁵⁷V, ⁵⁹Cr, and ⁶⁰Mn have shown general agreement with the GXPF1 calculations, but indications of a low lying 9/2⁺ isomer indicative of the g_9/2 orbital in ⁵⁹Cr [3,4].

Shell model calculations using a modified interaction called GXPF1A now reproduce the experimental observations of a gap at N = 32, but none at N = 34. It has been tested further against experimental observations of the structure of the neutron-rich Cr isotopes [5,6]. The low lying states generally agree with the predictions of GXPF1A, but there is evidence that the effects of the higher g_9/2 orbital (not included in the GXPF1A model space) become more important at lower energies with increasing neutron number.

References

[1] S.N. Liddick et al., Phys. Rev. Lett. 92, 072502 (2004).

[2] S.N. Liddick et al., Phys. Rev. C 70, 064303 (2004).

[3] S.N. Liddick et al., Phys. Rev. C 72, 054321 (2005).

[4]S.N. Liddick et al., Phys. Rev. C 73, 044322 (2006).

[5] A.N. Deacon et al., Phys. Lett. B 622, 151 (2005).

[6] S. Zhu et al., Phys. Rev. C 74, 064315 (2006).

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	FSU Experimental Nuclear Physics Main			NEUTRON-RICH NUCLEI BEYOND ⁴⁸Ca Dr. Samuel L. Tabor There is growing evidence that neutron excess also changes the single-particle shell structure in the nuclei above ⁴⁸Ca with Z > 20 and N > 28, i.e., with valence particles in different shells. Complementary experimental approaches have been used. Beta decay following projectile fragmentation provides clean γ spectra from low-lying, low-spin levels. In-beam spectra show much more of the level schemes, but are very complex, because many different reaction products are present. Both γ coincidences in GAMMASPHERE based on lines previously seen in β decay and separation of the desired nuclei in the Fragment Mass Analyzer after GAMMASPHERE have been used to isolate these γ spectra. A major issue in this region is movement of the neutron f_5/2 orbital with the filling of the proton spin-orbit partner f_7/2. There is clear evidence of a shell gap at N = 32 for nuclei near Ca due to filling of the p_3/2 orbital. Calculations using the GXPF1 interaction which predicts this gap also predict another gap at N = 34 after the p_1/2 orbital is filled and implying that the f_5/2 orbital lies above p_1/2. However, the lowest 2⁺ level in ⁵⁶Ti was observed to lie too low to support an N = 34 gap [1]. Further investigation of ^54,55,56Ti and a comparison with neighboring nuclei confirms a gap between the p_3/2 and f_5/2 orbitals and suggests that p_1/2 lies close to f_5/2 , slightly above for less neutron excess and slightly below with increasing neutron excess [2]. Beta-decay studies of ⁵⁷V, ⁵⁹Cr, and ⁶⁰Mn have shown general agreement with the GXPF1 calculations, but indications of a low lying 9/2⁺ isomer indicative of the g_9/2 orbital in ⁵⁹Cr [3,4]. Shell model calculations using a modified interaction called GXPF1A now reproduce the experimental observations of a gap at N = 32, but none at N = 34. It has been tested further against experimental observations of the structure of the neutron-rich Cr isotopes [5,6]. The low lying states generally agree with the predictions of GXPF1A, but there is evidence that the effects of the higher g_9/2 orbital (not included in the GXPF1A model space) become more important at lower energies with increasing neutron number. References [1] S.N. Liddick et al., Phys. Rev. Lett. 92, 072502 (2004). [2] S.N. Liddick et al., Phys. Rev. C 70, 064303 (2004). [3] S.N. Liddick et al., Phys. Rev. C 72, 054321 (2005). [4]S.N. Liddick et al., Phys. Rev. C 73, 044322 (2006). [5] A.N. Deacon et al., Phys. Lett. B 622, 151 (2005). [6] S. Zhu et al., Phys. Rev. C 74, 064315 (2006). Back to Dr. Tabor's Research Menu