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Nuclear structure studies in the neutron-rich A = 30 mass region by direct nucleon removal reactions
ELSEVIER
Nuclear
Physics
A734
(2004)
469472 www.elsevietcomilocate/npe
Nuclear Structure Studies Removal Reactions J. R. Terrya,
in the Neutron-Rich
A=30
Mass Region
by Direct
Nucleon
J. L. Lecoueya
aNational Superconducting 164 South Shaw Lane East Lansing, MI 48824,
Cyclotron
Laboratory,
Michigan
State
University,
USA
Abstract: An investigation of the systematic behavior of the d3,2 admixture in the ground state and 3oSi has been performed by means of a direct singleof the N=16 isotones 26Ne, “Mg, neutron removal reaction. Results are in good agreement with shell model calculations, with a sharply diminishing admixture in the more exotic neutron-rich region. The single-neutron removal methodology has also yielded several new spectroscopic results for 25Ne including a firm l/2+ spin-parity assignment for the ground state. Since the development of radioactive ion beams, nuclear stucture studies have become possible far from the valley of stability. A result of great interest in these regions is the observed breakdown of magic numbers. In a study by Ozawa et al, the previously discovered breakdown of the N=8 and N=20 magic numbers is observed from neutron separation energy systematics. Ozawa et al also observe an enhancement of magicity with decreasing proton number for the N=16 isotones. From a simple point of view, this effect is explained by the relative migration of the Od3j2 orbital toward the f-shell. Otsuka et al identify the spin-isospin component of the nucleon-nucleon interaction as the primary source behind this migration [2]. The N=16 isotones between the valley of stability and the neutron dripline clearly lie in The goal of this study is to systematically a highly transient region for nuclear structure. investigate the ground state wave functions of the even-even N=16 isotones “jNe, 28Mg, and 3oSi by use of a direct single neutron removal reaction. Reactions of this type have been shown to be a powerful spectroscopic tool for rather loosely-bound nuclei (61. Given that ‘*Mg and 3oSi are well-bound and well-studied, these cases also serve as a check on the validity of single-neutron removal methodology for well-bound systems. The experiment was performed at the National Superconducting Cyclotron Laboratory at Michigan State University. Radioactive beams were produced by fragmentation of a primary beam on a thick beryllium target. Secondary beams were magnetically analyzed in the A1900 fragment separator [3] and subsequently transported to the S800 high-resolution spectrometer [4]. The results presented in this proceeding are based on three data sets taken with secondary beams of ‘“Ne 28Mg, and 3oSi at an incident mid-target energy of 83, 82, and 66 MeV/u, respecmg/ cm2 self-supporting tively. The selondary beam was impinged on a 375(4) beryllium target positioned at the target position of the S800. Reaction residues were magnetically analyzed in the spectrograph and detected at the focal plane. In all three cases presented here, the spectrograph was operated in dispersion-matched optics mode. The Segmented Germanium Array (SeGA) [5] was positioned around the target position of the S800 to observe coincident gamma radiation. The array was arranged in a two ring configuration with 7 detectors positioned at 37” inclination with respect to the beam axis and 8 detectors positioned at 90” inclination. The inclusive cross section for single-neutron removal from 26Ne was observed to be 97(5) 0375-94746 - see front matter doi: 10.1016/j.nuc1physa.2004.0~,088
0 2004 Published
by Elsevier
B.\!
JR.
470
Terry,
.JL.
Lecouey/Nuclear
Physics
A734
(2004)
469-472
J” (512)’ (5/2)+ 112’
, ..I.
500
I II,.
,...d,,
1000
1500
E, (kev)
2000
2500
9.4
9.5
9.6
9.7
9.8
P,, (GeV/c)
Figure 1. Analysis of 26Ne. The left panel shows a fit to the Doppler-corrected coincident gamma spectrum based on the simplified level scheme shown in the inset. The longitudinal momentum distributions are shown in the right panel. Calculated distributions for 1=0,2 are superimposed on each distribution.
mb. Population fractions to individual excited states are extracted from the coincident gamma spectrum (shown in Figure 1). Due to low statistics, only two dominant gamma lines (shown in the inset of Figure 1) can be fit to the data. To account for distortions arising from the interplay between the Doppler effect and the strong energy dependence of the detection efficiency, the response function for each gamma line was generated in a GEANT simulation. The fit also includes a previously observed continuum gamma distribution. The shape of this distribution was taken from another single-neutron removal experiment, using the same setup, in which little or no gamma strength was expected above 100 keV [7]. The shape was modeled as a double exponential with a short and long component. The empirical background also includes an excess of gamma strength in the vicinity of 2 MeV. This excess was not included in the gamma fit since its origin was not understood. However, an excess does appear in the 25Ne coincident gamma spectrum in the vicinity of 2 MeV and is assumed to be associated with the continuum. The results of the gamma fit are shown in Figure 1, and the extracted partial cross sections are shown in Table 1. Parallel momentum distributions for ground-state and excited-state knockouts have been extracted by means of gating on the coincident gamma spectrum. This distributions are shown in the right panel of Figure 1. To interpret the extracted momentum distributions, theoretical distributions, also shown in Figure 1, have been calculated in an Eikonal model using the black disc approximation. The extracted ground state distribution is in clear agreement with the removal of an 1=0 neutron while the excited state distribution is in good agreement with Z=2. The former result allows for the unambiguous assignment of l/2+ to the ground state of 25Ne while the latter allows for the constraint of the two observed excited states to 3/2+ or 5/2+.
JR.
Table
Terry,
JL.
Lecouey/Nuclear
Physics
A734
(2004)
469-472
471
1
Summary of results for the single-neutron removal from 26Ne (upper), “Mg (middle), and 3oSi (lower). An upper limit has been given for the 3/2+ spectroscopic factor for 26Ne. The presence of negative parity states in the “Mg and 3oSi data is likely a result of indirect feeding from highlying states.
E level 0
J”
1703
1/2+ (5/2+)
3316
';;;I,
0 985
1699 1940
3109 3426 3760 0 1273 2028 2426 3067 3624 4840 4895
1/2f 3/2+ 5/2+ 5/2+ 3/2+ (5,7)/Z+ 7/21/2+ 3/2+ 5/2+ 3/2+ 5/2+ 7/21/2+ 5/2+
O3P
cobs
41.7 21.5 19.3 21.5 23.6 15.7 15.1 14.9 14.1 13.9 14.4 18.4 12.3
51(5) 2l(3)
16.2(16) 3.3(6) 6.6(25)
11.9
4.7(9)
13.9 10.5
3.5(21) 8.2(19)
11.9 11.7 11.3
-
S obs 1.2(l) 1.2(2) H(2) <0:5 1.08(29) 0.79(llj 1.88(22) l.lO(14) 0.20(4) 0.22(7) 0.09(4) 1.70(38) 0.58(20)
25(4)
StlZ
E
1.347
;”
2.346 1.837 0.382 0.758
1779 2971 1687 0
0.965
895
3.202 0.399
1666 1978
0.008
3162
1394
1.36(19)
0.878 0.860 2.050
2123
0.28(6) 0.58(23)
0.018 0.071
2633 3516
0.40(8) 0.25(15) 0.77(20)
0.265 0.808
4915
0
4904
Table 1 includes theoretical level energies and spectroscopic factors obtained from a USD shell model calculation. The calculation predicts that the two most highly populated excited states would be 5/2+ states at 1.779 and 2.971 MeV. Citing agreement with shell model calculations, the observed excited states are both given a tentative assignment of 5/2+. Shell model calculations also predict the population of a 3/2+ state at 1.687 MeV with approximately l/5 the intensity of each of the 5/2+ states. Given the low statistics of the coincident gamma spectrum, a search for such a weakly populated state is not possible. Defining an upper limit for the population ratio of the proposed 3/2+ state, fluctuations in the gamma spectrum around 1700 keV allow for as much as a 10% feeding to exist undetected. A similar analysis was performed for the single-neutron removal from “Mg and 3oSi. Inclusive cross sections of 90(5) mb and 81(4) m b are extracted for the single-neutron removal from “Mg and 3oSi, respectively. Analysis of the coincident gamma spectrum is complicated by the large binding of the reaction residues (S&=6.44,8.47 MeV for 27Mg and 2gSi compared to 4.18 MeV for 25Ne). Such binding energies allow for complex level schemes which include high energy gamma transitions. Given the superior resolution of high-purity germanium, such detectors have been utilized to offset the complexity of the level schemes. This step is taken at the expense of efficiency, especially at high energies. As a result of this limited detection efficiency, partial cross sections extracted for low-lying states include a large uncertainty, associated with feeding from high-lying states through undetected gamma transitions. The results of the analysis on these two well-bound systems are presented in Table 1. To summarize the results of this study, the observed 3/2+ spectroscopic factors are plotted as a function of proton number in Figure 2. For the case of 26Ne, in which no 3/2+ state was observed, only an upper limit can be displayed assuming the state lies near 1700 keV. As is predicted by USD shell model calculations (also shown in Figure 2), the ds,z admixture in the ground state wavefunction of the N=16 isotones drops rapidly for lower proton numbers. The results of this study are in good agreement with the proposed near-zero ds,z admixture in 240,
JR.
412
Terry,
Figure 2. The summed 3/2+ observed spectroscopic factors are plotted for each isotone. The dashed line represents spectroscopic factors obtained from a USD shell model calculation. The analysis can only yield an upper limit for the 3/2+ spectroscopic factor of 26Ne.
JL.
Lecouey/Nuclear
Physics
_- - _
A734
(2004)
469472
USD Calculations
1.20 +c4 2 y 0.80 CA0 w 0.40 : _’: ‘“0
:’ :
: _’.’ _’ : I
I *We
I *‘Mg
I
I “Si
indicative of a closed shell with respect to neutron number. In conclusion, single-nuetron removal methodology has been employed to study the systematics of the da/z admixture in the ground state wave function of several even-even neutron-rich N=16 isotones. In the process of this study, new spectroscopic results have been extracted for 25Ne, including a strong l/2+ assignment to the ground state and weaker assignments to two excited states. Systematics in the ground state wavefunctions are in good agreement with shell model calculations and show an enhancement of magicity in exotic neutron-rich N=16 isotones. REFERENCES 1. 2. 3. 4. 5. 6. 7.
A. Ozawa et al. Phys. Rev. Let. 84 (2000) 5493 T. Otsuka et al. Phys. Rev. Let. 8’7 (2001) 082502 D.J. Morrissey et al. Nucl. Intrum. Meth. B (2003) (in press), D.J. Morrissey et al. Nucl. Instrum. Meth. B126 (1997) 316. D. Bazin et al. Nucl. Instrum. Meth. B204 (2003) 629-633, J. Yurkon et al. Nucl. Instrum. Meth. A422 (1999) 291-295. W.F. Mueller et al. Nulc. Instrum. Meth. A466 (2001) 492. P. G. Hansen and J. A. Tostevin. Ann. Rev. Nut. Part. Sci. 53 (2003) A. Gade. Private communication.
Nuclear
Physics
A734
(2004)
469472 www.elsevietcomilocate/npe
Nuclear Structure Studies Removal Reactions J. R. Terrya,
in the Neutron-Rich
A=30
Mass Region
by Direct
Nucleon
J. L. Lecoueya
aNational Superconducting 164 South Shaw Lane East Lansing, MI 48824,
Cyclotron
Laboratory,
Michigan
State
University,
USA
Abstract: An investigation of the systematic behavior of the d3,2 admixture in the ground state and 3oSi has been performed by means of a direct singleof the N=16 isotones 26Ne, “Mg, neutron removal reaction. Results are in good agreement with shell model calculations, with a sharply diminishing admixture in the more exotic neutron-rich region. The single-neutron removal methodology has also yielded several new spectroscopic results for 25Ne including a firm l/2+ spin-parity assignment for the ground state. Since the development of radioactive ion beams, nuclear stucture studies have become possible far from the valley of stability. A result of great interest in these regions is the observed breakdown of magic numbers. In a study by Ozawa et al, the previously discovered breakdown of the N=8 and N=20 magic numbers is observed from neutron separation energy systematics. Ozawa et al also observe an enhancement of magicity with decreasing proton number for the N=16 isotones. From a simple point of view, this effect is explained by the relative migration of the Od3j2 orbital toward the f-shell. Otsuka et al identify the spin-isospin component of the nucleon-nucleon interaction as the primary source behind this migration [2]. The N=16 isotones between the valley of stability and the neutron dripline clearly lie in The goal of this study is to systematically a highly transient region for nuclear structure. investigate the ground state wave functions of the even-even N=16 isotones “jNe, 28Mg, and 3oSi by use of a direct single neutron removal reaction. Reactions of this type have been shown to be a powerful spectroscopic tool for rather loosely-bound nuclei (61. Given that ‘*Mg and 3oSi are well-bound and well-studied, these cases also serve as a check on the validity of single-neutron removal methodology for well-bound systems. The experiment was performed at the National Superconducting Cyclotron Laboratory at Michigan State University. Radioactive beams were produced by fragmentation of a primary beam on a thick beryllium target. Secondary beams were magnetically analyzed in the A1900 fragment separator [3] and subsequently transported to the S800 high-resolution spectrometer [4]. The results presented in this proceeding are based on three data sets taken with secondary beams of ‘“Ne 28Mg, and 3oSi at an incident mid-target energy of 83, 82, and 66 MeV/u, respecmg/ cm2 self-supporting tively. The selondary beam was impinged on a 375(4) beryllium target positioned at the target position of the S800. Reaction residues were magnetically analyzed in the spectrograph and detected at the focal plane. In all three cases presented here, the spectrograph was operated in dispersion-matched optics mode. The Segmented Germanium Array (SeGA) [5] was positioned around the target position of the S800 to observe coincident gamma radiation. The array was arranged in a two ring configuration with 7 detectors positioned at 37” inclination with respect to the beam axis and 8 detectors positioned at 90” inclination. The inclusive cross section for single-neutron removal from 26Ne was observed to be 97(5) 0375-94746 - see front matter doi: 10.1016/j.nuc1physa.2004.0~,088
0 2004 Published
by Elsevier
B.\!
JR.
470
Terry,
.JL.
Lecouey/Nuclear
Physics
A734
(2004)
469-472
J” (512)’ (5/2)+ 112’
, ..I.
500
I II,.
,...d,,
1000
1500
E, (kev)
2000
2500
9.4
9.5
9.6
9.7
9.8
P,, (GeV/c)
Figure 1. Analysis of 26Ne. The left panel shows a fit to the Doppler-corrected coincident gamma spectrum based on the simplified level scheme shown in the inset. The longitudinal momentum distributions are shown in the right panel. Calculated distributions for 1=0,2 are superimposed on each distribution.
mb. Population fractions to individual excited states are extracted from the coincident gamma spectrum (shown in Figure 1). Due to low statistics, only two dominant gamma lines (shown in the inset of Figure 1) can be fit to the data. To account for distortions arising from the interplay between the Doppler effect and the strong energy dependence of the detection efficiency, the response function for each gamma line was generated in a GEANT simulation. The fit also includes a previously observed continuum gamma distribution. The shape of this distribution was taken from another single-neutron removal experiment, using the same setup, in which little or no gamma strength was expected above 100 keV [7]. The shape was modeled as a double exponential with a short and long component. The empirical background also includes an excess of gamma strength in the vicinity of 2 MeV. This excess was not included in the gamma fit since its origin was not understood. However, an excess does appear in the 25Ne coincident gamma spectrum in the vicinity of 2 MeV and is assumed to be associated with the continuum. The results of the gamma fit are shown in Figure 1, and the extracted partial cross sections are shown in Table 1. Parallel momentum distributions for ground-state and excited-state knockouts have been extracted by means of gating on the coincident gamma spectrum. This distributions are shown in the right panel of Figure 1. To interpret the extracted momentum distributions, theoretical distributions, also shown in Figure 1, have been calculated in an Eikonal model using the black disc approximation. The extracted ground state distribution is in clear agreement with the removal of an 1=0 neutron while the excited state distribution is in good agreement with Z=2. The former result allows for the unambiguous assignment of l/2+ to the ground state of 25Ne while the latter allows for the constraint of the two observed excited states to 3/2+ or 5/2+.
JR.
Table
Terry,
JL.
Lecouey/Nuclear
Physics
A734
(2004)
469-472
471
1
Summary of results for the single-neutron removal from 26Ne (upper), “Mg (middle), and 3oSi (lower). An upper limit has been given for the 3/2+ spectroscopic factor for 26Ne. The presence of negative parity states in the “Mg and 3oSi data is likely a result of indirect feeding from highlying states.
E level 0
J”
1703
1/2+ (5/2+)
3316
';;;I,
0 985
1699 1940
3109 3426 3760 0 1273 2028 2426 3067 3624 4840 4895
1/2f 3/2+ 5/2+ 5/2+ 3/2+ (5,7)/Z+ 7/21/2+ 3/2+ 5/2+ 3/2+ 5/2+ 7/21/2+ 5/2+
O3P
cobs
41.7 21.5 19.3 21.5 23.6 15.7 15.1 14.9 14.1 13.9 14.4 18.4 12.3
51(5) 2l(3)
16.2(16) 3.3(6) 6.6(25)
11.9
4.7(9)
13.9 10.5
3.5(21) 8.2(19)
11.9 11.7 11.3
-
S obs 1.2(l) 1.2(2) H(2) <0:5 1.08(29) 0.79(llj 1.88(22) l.lO(14) 0.20(4) 0.22(7) 0.09(4) 1.70(38) 0.58(20)
25(4)
StlZ
E
1.347
;”
2.346 1.837 0.382 0.758
1779 2971 1687 0
0.965
895
3.202 0.399
1666 1978
0.008
3162
1394
1.36(19)
0.878 0.860 2.050
2123
0.28(6) 0.58(23)
0.018 0.071
2633 3516
0.40(8) 0.25(15) 0.77(20)
0.265 0.808
4915
0
4904
Table 1 includes theoretical level energies and spectroscopic factors obtained from a USD shell model calculation. The calculation predicts that the two most highly populated excited states would be 5/2+ states at 1.779 and 2.971 MeV. Citing agreement with shell model calculations, the observed excited states are both given a tentative assignment of 5/2+. Shell model calculations also predict the population of a 3/2+ state at 1.687 MeV with approximately l/5 the intensity of each of the 5/2+ states. Given the low statistics of the coincident gamma spectrum, a search for such a weakly populated state is not possible. Defining an upper limit for the population ratio of the proposed 3/2+ state, fluctuations in the gamma spectrum around 1700 keV allow for as much as a 10% feeding to exist undetected. A similar analysis was performed for the single-neutron removal from “Mg and 3oSi. Inclusive cross sections of 90(5) mb and 81(4) m b are extracted for the single-neutron removal from “Mg and 3oSi, respectively. Analysis of the coincident gamma spectrum is complicated by the large binding of the reaction residues (S&=6.44,8.47 MeV for 27Mg and 2gSi compared to 4.18 MeV for 25Ne). Such binding energies allow for complex level schemes which include high energy gamma transitions. Given the superior resolution of high-purity germanium, such detectors have been utilized to offset the complexity of the level schemes. This step is taken at the expense of efficiency, especially at high energies. As a result of this limited detection efficiency, partial cross sections extracted for low-lying states include a large uncertainty, associated with feeding from high-lying states through undetected gamma transitions. The results of the analysis on these two well-bound systems are presented in Table 1. To summarize the results of this study, the observed 3/2+ spectroscopic factors are plotted as a function of proton number in Figure 2. For the case of 26Ne, in which no 3/2+ state was observed, only an upper limit can be displayed assuming the state lies near 1700 keV. As is predicted by USD shell model calculations (also shown in Figure 2), the ds,z admixture in the ground state wavefunction of the N=16 isotones drops rapidly for lower proton numbers. The results of this study are in good agreement with the proposed near-zero ds,z admixture in 240,
JR.
412
Terry,
Figure 2. The summed 3/2+ observed spectroscopic factors are plotted for each isotone. The dashed line represents spectroscopic factors obtained from a USD shell model calculation. The analysis can only yield an upper limit for the 3/2+ spectroscopic factor of 26Ne.
JL.
Lecouey/Nuclear
Physics
_- - _
A734
(2004)
469472
USD Calculations
1.20 +c4 2 y 0.80 CA0 w 0.40 : _’: ‘“0
:’ :
: _’.’ _’ : I
I *We
I *‘Mg
I
I “Si
indicative of a closed shell with respect to neutron number. In conclusion, single-nuetron removal methodology has been employed to study the systematics of the da/z admixture in the ground state wave function of several even-even neutron-rich N=16 isotones. In the process of this study, new spectroscopic results have been extracted for 25Ne, including a strong l/2+ assignment to the ground state and weaker assignments to two excited states. Systematics in the ground state wavefunctions are in good agreement with shell model calculations and show an enhancement of magicity in exotic neutron-rich N=16 isotones. REFERENCES 1. 2. 3. 4. 5. 6. 7.
A. Ozawa et al. Phys. Rev. Let. 84 (2000) 5493 T. Otsuka et al. Phys. Rev. Let. 8’7 (2001) 082502 D.J. Morrissey et al. Nucl. Intrum. Meth. B (2003) (in press), D.J. Morrissey et al. Nucl. Instrum. Meth. B126 (1997) 316. D. Bazin et al. Nucl. Instrum. Meth. B204 (2003) 629-633, J. Yurkon et al. Nucl. Instrum. Meth. A422 (1999) 291-295. W.F. Mueller et al. Nulc. Instrum. Meth. A466 (2001) 492. P. G. Hansen and J. A. Tostevin. Ann. Rev. Nut. Part. Sci. 53 (2003) A. Gade. Private communication.