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Structure of 10Li and 11Li
NUCLEAR PHYSICS A ELSEVIER
Nuclear Physics A588 (1995) 1 lc-14c
S t r u c t u r e o f l°Li a n d n L i W. Benenson a ~Dept. of Physics and Astronomy and National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan, USA 48824 Recent two-body nuclear reaction measurements of the structure of l°Li and 11Li are reviewed. The mass of nLi is now well known, but the situtation for l°Li is not so clear. However recent evidence presented at this conference seems to confirm the existence of a slightly unbound s-wave ground state. 1. I N T R O D U C T I O N It has become very important to the understanding of the structure of nLi to determine its mass and binding energy accurately and to characterize the wavefunction of the low lying states of l°Li properly. We have carried out experiments on both of these questions recently at MSU, and a brief description of the results is given below. 2. n L i Mass There have been three previous measurements of the mass of nLi. The results are not in good agreement, and the need for a high precision determination has been known for some time[l]. In the simplest model of nLi, that of a di-neutron outside of a 9Li core, the binding energy determines both extent of the halo in space and the width of the momentum distribution. Therefore, even complex models depend strongly on the binding energy. The three previous measurements are 1) a 1975 direct mass measurement[2], S,~=160 4- 80 keV, 2) a time of flight determination from Los Alamos[3], S~=320 4- 120 keV, and 3) a pion double charge exchange Q-value determination[4], S,,=340 4- 50 keV. In the most recent measurement[5] we used the 14C(nB,nLi)140 reaction at E/A = 32. MeV and zero degrees. Several hundred counts were obtained with both outgoing nuclei in the ground state and hundreds more with 140 in low lying excited states. The spectra from this experiment is shown in Fig. 1. At the same time, at the same field and with the same target, spectra were collected for the 14C(nB,I°Be3+)I~N reaction. This served as an excellent calibration which reduced the sources of uncertainty to the statistics and the beam energy determination. Measurements of the 9Li mass with the same techniques gave an agreement of 10 keV or less with the known value. The result, S,,=295 4- 35 keV, is in good agreement with the previous measurements but improves the uncertainty and the confidence theorists can put in the value appreciably. We therefore conclude that the mass of llLi is now known reliably. 0375-9474/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0375-9474(95)00092-5
12c
W. Benenson / Nuclear Physics A588 (1995) 11c-14c
3. l°Li M a s s
The situation with respect to l°Li is not nearly so straightforward. There are four published determinations of the mass, and they disagree totally on the mass excess and character of the ground state. The table below, which illustrates the situation, also gives our results[6] for the XlB(rLi,SB)I°Li reaction at E / A = 18.6 MeV and 5 degrees. A spectrum from this reaction is given in Fig 2. There is a definite, strong peak near S~= -500 keV. It has the correct shape for a p-wave neutron resonance at S~= -486 -4- 55 keV. There is a second possible peak in the spectrum near S~= 0. A fit that includes it gives substantially better agreement with the data than one which includes the p-wave alone. There is whole series of additional results which support the existence of this low-lying, probably s-wave, ground state for l°Li. They include: • The existence of a peak at S . = -150 keV by Amelin et. al M in ~r- capture in 11B. • The observation[9] of a narrow state in 1°Be which could be the analog of a state in l°Li at S,~= -60 keV. • A neutron experiment at MSU by Kryger[10] in which neutron-gLi coincidences show a peak near zero relative velocity. • The observation by Kobayashi et al [11] of a low energy neutron enhancement in
the decay of nLi into 9Li. • An excess of weak peak in Bohlen et al.[12] in a spectrum near S~ = 0 in the 13C(14C,lrF) reaction. • Shell model calculations[13] which predict an low lying s-wave state for the ground state of l°Li. The neutron spectra from single particle transfer reactions shown by the previous speaker at this conference [15] and their interpretation clearly point to a very low lying s-state. None of these results in themselves prove that the ground state of l°Li is barely unbound with a neutron in an s-wave state as originally suggested by Barker and Hickey[14], but together they make a compelling argument for it. This result has important ramifications on calculations of the 11Li wave function as has been presented at this conference by Thompson[16]. 4. A C K N O W L E D G E M E N T S The reader should recognize that the work presented here was performed by many individuals. Notable among this group is the graduate students whose thesis work was presented, B.M. Young. This work was supported in part by the National Science Foundation under cooperative agreement PHY 92-14992.
W. Benenson /Nuclear Physics A588 (1995) 11c-14c
13c
Table 1 Summary of experimental data on low-lying structure l°Li published to date. The neutron separation energy and width are given for each state. Also given for each state is the identification (if any) claimed by the experimenters. S. (MeV) Flab (MeV) Identification Wilcox et al. [7] -0.80 -4- 0.25 1.2 -4- 0.3 G.S. Amelin et al. [8] -0.15 ± 0.15 _< 0.4 s 1, G.S. Kryger et al. [10] > -0.15 or G.S. -2.5 Bohlen et al. [12] -0.42 -4- 0.05 0.15 i 0.07 p½, G.S. -0.80 + 0.06 0.30 3= 0.10 p½ Present Work[6] > -0.10 < 0.23 G.S. -0.54 -4- 0.06 0.36 3= 0.02 p!2 REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
P.G. Hansen and B. Jonson, Europhys. Lett. 4, 409(1987). C. Thibault et al., Phys. Rev. C12, 644(1975) J. Wouters et al., Z Phys. A331, 229(1988) T. Kobayashi et al., KEK Report No. 91-22, 1991 B.M. Young, et al. private communication and to be published B.M. Young et al., submitted to Phys. Rev. C. K.H. Wilcox et al., Phys. Lett. B59, 142(1975) A.I. Amelin et al., Sov. J. Nucl. Phys. 52, 783(1990) S.N. Abramovich et al., Bull. Acad. Sci. USSR, Phys. Ser. 37, 144(1973) R. Kryger et al., Phys. Rev. C in press T. Kobayashi et al., this conference H.G. Bohlen et al., Z. Phys. A344, 381(19930 E. K. Warburton and B.A. Brown Phys. Rev. C46, 923(1992) F.C. Barker and G.T. Hickey, J. Phys. G3, L23 (1977) P.G. Hansen, previous talk at this conference I. J. Thompson, this confernce.
W. Benenson / Nuclear Physics A588 (1995) 11c-14c
14c
First 4000
'"'1
Second
Run
.... I .... I .... z l'C(11B't°Be~+)16N
3000
....
Run
I ....
I ....
4000
I ....
3000
o
2000
2000
I000
o
:
e.
:I-*°( '"1
15
o
I
i000
oZ ....
I ....
I
....
....
I I I s ' - (MeV)
II
0
....
I]
0
,I .... 100
50
II
i?
z . C ( . B , . L i ) I. 0
0
I .... I .... I'" l'C(11B,ULi)l'O
25
20
[s~
-,o,
,.,
lo I .... I 150 200
...... 50
I,,,, 100
I .... o 150 200
Channel
Figure 1.
S p e c t r a f r o m a r e c e n t 11Li m a s s m e a s u r e m e n t .
The two reactions were measured
simultane-
ously.
20
. . . .
I
. . . .
I
. . . .
I
I
]
. . . .
- S . (MeV)
I 15 - -
3
2
. . . .
I 220
. . . .
[
I
~
I
1
11B('U,sB)I°Li d a t a - p + s wave fit ...... p wave fit
10
0 200
l
0
J~ [] • V
. . . .
240
I 260
I
I
,11
,11 280
ffh,m,,el
Figure 2. Spectra from a recent l°Li mass measurement. T h e s o l i d line is a fit t o t h e s p e c t r u m u n d e r t h e a s s u m p t i o n o f a p - w a v e r e s o n a n c e w i t h a c a l c u l a t e d s h a p e p l u s a l o w l y i n g r e s o n a n c e c l o s e t o S n - - 0.
Nuclear Physics A588 (1995) 1 lc-14c
S t r u c t u r e o f l°Li a n d n L i W. Benenson a ~Dept. of Physics and Astronomy and National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan, USA 48824 Recent two-body nuclear reaction measurements of the structure of l°Li and 11Li are reviewed. The mass of nLi is now well known, but the situtation for l°Li is not so clear. However recent evidence presented at this conference seems to confirm the existence of a slightly unbound s-wave ground state. 1. I N T R O D U C T I O N It has become very important to the understanding of the structure of nLi to determine its mass and binding energy accurately and to characterize the wavefunction of the low lying states of l°Li properly. We have carried out experiments on both of these questions recently at MSU, and a brief description of the results is given below. 2. n L i Mass There have been three previous measurements of the mass of nLi. The results are not in good agreement, and the need for a high precision determination has been known for some time[l]. In the simplest model of nLi, that of a di-neutron outside of a 9Li core, the binding energy determines both extent of the halo in space and the width of the momentum distribution. Therefore, even complex models depend strongly on the binding energy. The three previous measurements are 1) a 1975 direct mass measurement[2], S,~=160 4- 80 keV, 2) a time of flight determination from Los Alamos[3], S~=320 4- 120 keV, and 3) a pion double charge exchange Q-value determination[4], S,,=340 4- 50 keV. In the most recent measurement[5] we used the 14C(nB,nLi)140 reaction at E/A = 32. MeV and zero degrees. Several hundred counts were obtained with both outgoing nuclei in the ground state and hundreds more with 140 in low lying excited states. The spectra from this experiment is shown in Fig. 1. At the same time, at the same field and with the same target, spectra were collected for the 14C(nB,I°Be3+)I~N reaction. This served as an excellent calibration which reduced the sources of uncertainty to the statistics and the beam energy determination. Measurements of the 9Li mass with the same techniques gave an agreement of 10 keV or less with the known value. The result, S,,=295 4- 35 keV, is in good agreement with the previous measurements but improves the uncertainty and the confidence theorists can put in the value appreciably. We therefore conclude that the mass of llLi is now known reliably. 0375-9474/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0375-9474(95)00092-5
12c
W. Benenson / Nuclear Physics A588 (1995) 11c-14c
3. l°Li M a s s
The situation with respect to l°Li is not nearly so straightforward. There are four published determinations of the mass, and they disagree totally on the mass excess and character of the ground state. The table below, which illustrates the situation, also gives our results[6] for the XlB(rLi,SB)I°Li reaction at E / A = 18.6 MeV and 5 degrees. A spectrum from this reaction is given in Fig 2. There is a definite, strong peak near S~= -500 keV. It has the correct shape for a p-wave neutron resonance at S~= -486 -4- 55 keV. There is a second possible peak in the spectrum near S~= 0. A fit that includes it gives substantially better agreement with the data than one which includes the p-wave alone. There is whole series of additional results which support the existence of this low-lying, probably s-wave, ground state for l°Li. They include: • The existence of a peak at S . = -150 keV by Amelin et. al M in ~r- capture in 11B. • The observation[9] of a narrow state in 1°Be which could be the analog of a state in l°Li at S,~= -60 keV. • A neutron experiment at MSU by Kryger[10] in which neutron-gLi coincidences show a peak near zero relative velocity. • The observation by Kobayashi et al [11] of a low energy neutron enhancement in
the decay of nLi into 9Li. • An excess of weak peak in Bohlen et al.[12] in a spectrum near S~ = 0 in the 13C(14C,lrF) reaction. • Shell model calculations[13] which predict an low lying s-wave state for the ground state of l°Li. The neutron spectra from single particle transfer reactions shown by the previous speaker at this conference [15] and their interpretation clearly point to a very low lying s-state. None of these results in themselves prove that the ground state of l°Li is barely unbound with a neutron in an s-wave state as originally suggested by Barker and Hickey[14], but together they make a compelling argument for it. This result has important ramifications on calculations of the 11Li wave function as has been presented at this conference by Thompson[16]. 4. A C K N O W L E D G E M E N T S The reader should recognize that the work presented here was performed by many individuals. Notable among this group is the graduate students whose thesis work was presented, B.M. Young. This work was supported in part by the National Science Foundation under cooperative agreement PHY 92-14992.
W. Benenson /Nuclear Physics A588 (1995) 11c-14c
13c
Table 1 Summary of experimental data on low-lying structure l°Li published to date. The neutron separation energy and width are given for each state. Also given for each state is the identification (if any) claimed by the experimenters. S. (MeV) Flab (MeV) Identification Wilcox et al. [7] -0.80 -4- 0.25 1.2 -4- 0.3 G.S. Amelin et al. [8] -0.15 ± 0.15 _< 0.4 s 1, G.S. Kryger et al. [10] > -0.15 or G.S. -2.5 Bohlen et al. [12] -0.42 -4- 0.05 0.15 i 0.07 p½, G.S. -0.80 + 0.06 0.30 3= 0.10 p½ Present Work[6] > -0.10 < 0.23 G.S. -0.54 -4- 0.06 0.36 3= 0.02 p!2 REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
P.G. Hansen and B. Jonson, Europhys. Lett. 4, 409(1987). C. Thibault et al., Phys. Rev. C12, 644(1975) J. Wouters et al., Z Phys. A331, 229(1988) T. Kobayashi et al., KEK Report No. 91-22, 1991 B.M. Young, et al. private communication and to be published B.M. Young et al., submitted to Phys. Rev. C. K.H. Wilcox et al., Phys. Lett. B59, 142(1975) A.I. Amelin et al., Sov. J. Nucl. Phys. 52, 783(1990) S.N. Abramovich et al., Bull. Acad. Sci. USSR, Phys. Ser. 37, 144(1973) R. Kryger et al., Phys. Rev. C in press T. Kobayashi et al., this conference H.G. Bohlen et al., Z. Phys. A344, 381(19930 E. K. Warburton and B.A. Brown Phys. Rev. C46, 923(1992) F.C. Barker and G.T. Hickey, J. Phys. G3, L23 (1977) P.G. Hansen, previous talk at this conference I. J. Thompson, this confernce.
W. Benenson / Nuclear Physics A588 (1995) 11c-14c
14c
First 4000
'"'1
Second
Run
.... I .... I .... z l'C(11B't°Be~+)16N
3000
....
Run
I ....
I ....
4000
I ....
3000
o
2000
2000
I000
o
:
e.
:I-*°( '"1
15
o
I
i000
oZ ....
I ....
I
....
....
I I I s ' - (MeV)
II
0
....
I]
0
,I .... 100
50
II
i?
z . C ( . B , . L i ) I. 0
0
I .... I .... I'" l'C(11B,ULi)l'O
25
20
[s~
-,o,
,.,
lo I .... I 150 200
...... 50
I,,,, 100
I .... o 150 200
Channel
Figure 1.
S p e c t r a f r o m a r e c e n t 11Li m a s s m e a s u r e m e n t .
The two reactions were measured
simultane-
ously.
20
. . . .
I
. . . .
I
. . . .
I
I
]
. . . .
- S . (MeV)
I 15 - -
3
2
. . . .
I 220
. . . .
[
I
~
I
1
11B('U,sB)I°Li d a t a - p + s wave fit ...... p wave fit
10
0 200
l
0
J~ [] • V
. . . .
240
I 260
I
I
,11
,11 280
ffh,m,,el
Figure 2. Spectra from a recent l°Li mass measurement. T h e s o l i d line is a fit t o t h e s p e c t r u m u n d e r t h e a s s u m p t i o n o f a p - w a v e r e s o n a n c e w i t h a c a l c u l a t e d s h a p e p l u s a l o w l y i n g r e s o n a n c e c l o s e t o S n - - 0.