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Cluster fragmentations of deep(1s)-hole states in light nuclei
ELSEVIER
Nuclear Physics A738 (2004) 451454
www.elsevier.com/locatelnpe
Cluster fragmentations of deep(1s)-hole statcs in light nuclei M. Yosoi, H. Akimuneb, I. Daito, H. Ejirid, H. Fujimurac*, M.Fujiwarace, K. Fushiniif, K. Haract, K. Y. Harab, H. Hashimoto, T . Ishikawat, YI. Itoh, Y. Itowg. S. Kishi, T. Kawabata!, K. Kawase, M. Kinoshitab, K. Kobayashig, M. NakainuraT: K. Nakanishi, Y. Nakatsugawa, S. Nakayama, T. Noroh, E. Obayashi, S. Okiimura, H. Sakaguchi, Y. Sakemi, M. Shiozawag, H. TakedaI1, T. Taki, A. Tamii, M.Tanakai, S. Terashima, H. Toyokawad, N. Tsukahara, If.Uchida; T. Yamadaj, T. Yamagatab, Y. Yasudaa, H. P. Yoshida, R. G. T. Zegers** and J. Zenihiro Department of Physics, Kyoto University, Kyoto 606-8502, Japan Department of Physics, Konan University, Kobe 658-8501, Japan Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan dJapan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan Advanced Science Research Center, JAERI, Tokai, Ibaraki 319-1195, Japan fDepartment of Physics, University of Tokushima, Tokushima 770-8502, Japan RInstitute for Cosmic Ray Research, University of Tokyo, Kashiwa, 277-8582, Japan Department of Physics, Kyushu University, Fukuoka 812-8581, Japan Kobe Tokiwa College, Kobe 654-0838, Japan JLaboratory of Physics, Kanto Gakuin University, Yokohama 236-0032, .Japan Decay particles from the s-hole stat,es in B, I5N a.nd 6He have been measured in coincidence with the quasifree I2C(p,2p), I6O(p,2p) and 7Li(p,2p) reactions at 392 MeV incident energy. Triton-decay is found t,o be larger than a-decay for both s-hole states in B and I5N despite its smaller &-value than that, of a-decay. This supports the selection rule predicted by the microscopic SU(3)-model calculations. The relat,ion between the He(s-hole) state and the di-triton cluster structure recently found from the charge exchange reaction is briefly discussed. Present address: School of Physics, Seoul National University, Seoul 151-747, Korea Present address: KEK, Tsukuba, Ibaraki, 305-0801, Japan $Present address: LXS, Tohoku University, Sendai, 982-0826, .Japan §Present address: CNS, University of Tokyo, Bunkyo, Tokyo 113-0033,Japan TPresent address: Wakayama Medical University, Wakayama 641-8509, Japan Present address: FU Beam Science Laboratory, RIKEN, Wako, Saitama 351-0198. Japan **Present address: NSCL, Michigan State University, East Lansing, Michigan 48824, USA - see front matter 8 2004 Published by Elsevier B.V. doi:10.1016/j.nucIpliysa.2004.04.084
0375-94746
M. Yosoi et al. /Nuclear Physics A738 (2004) 451-454
452
1. INTRODUCTION
The spatial SU(3) symmetry plays an important role of the clustering aspects in light nuclei. Based on the fact that the deep(1s)-hole stat,e produced by the quasifree reaction such as a ( p , 2p) should have the same spatial symmetry as the ground state of the t.arget nucleus, Yaniada et al. predict,ed a selection rule for fragmentations of the doorway s-hole state in light nuclei, namely, the fragments of a and heavier particles are forbidden in the two-body decay process[l]. On the other hand, since the Q-values for a-fragments in most light nuclei are larger than those for other cluster decay channels, a-decay is favored in the statistical decay process. Decay properties of hole states produced from light targets can, moreover, provide useful informat,ion on the studies of the hypernuclear productions and on the search of proton decay and neutrino detection with, e.g., water Cerenkov detectors. In the present work, particle decays from the quasifree proton-knockout react.ions on 12C, 160and 7Li are studied to understand the st,ructures and fragmentation mechanisms of the s-hole states in "B, I5N and 6He. 2. EXPERIMENT AND RESULTS
Large Acceptance Spectrometer
O 1 2 3 m
Focal Plane LWcctm
Figure 1. Schematic view of the experimental setup. (a) Dual spectrometer system (Grand Raiden and Large Acceptance Spectrometer) at RCNP. (b) SSD-Ball system for the measurement of decay charged particles.
The experiment was carried out at RCNP. Osaka University, by using a 392 MeV proton beam. The quasifree ( p , 2p) reaction was measured with the dual spect,rorneter system consisting of the high resolution spectrometer Grand Raiden (GR) and the large acceptance spectrometer (LAS). A schematic view of the spectrometer system is shown in Fig. l(a). GR was set at 25.5" and the laboratory angle of LAS and the magnetic fields of two spectrometers were determined to satisfy the zero-recoil momentum condition at the central energy of the lsl/2-knockout bump, where the cross section leading to the s-hole state is maximum. Two multi-wire drift chambers in each focal plane of both
453
M. Yosoi et al. /Nuclear Physics A738 (2004) 451-454
._~_____ .... _ ~~.~ _
,
’-10
0
10
20
30
40
50
Ex(’’B) (MeV)
-10
0
10
Ex(%)
20
30 (MeV)
(Ey 16-35MeV)
40
SO
Figure 2. Excitation energy spectra of (a) 12C(p.2p)’lB’ and (b) I 6 0 ( p . 2p)"N* reactions at Ep = 392 MeV. Decay fragments from the s-hole states (hatched regions) were measured in coincidence with two protons at backward angles.
p
d
t
a
(Ex : 20-40 M e V
Figure 3. Experimental branching ratios of the n-; p-, d-, t- and a-decays from the s-hole states in (a) "B and (b) 15N and calculated ones of the statistical model with the code CASCADE.Decay into the ’2-body decay’ region is indicated with dark area for each decay channel (see text).
spectrometers determined the positions and the incidence angles of particles. Charged particle decay of the s-hole states was measured with sixteen telescopes of AE-E Si solid-state detectors (SSD) in coincidence with the quasifree ( p , 2 p ) reactions (see Fig. l(b)). In order to reducc the energy losses of the emitted particles, we used thin targets (0.5-2 mg/cm2). For the "0 target, a combination of a quartz glass (Si02) and a silicon (Si) target was employed. Neutron decay of the s-hole state was measured in the separate setting using a neutron multi-detector array which consists of thirty liquid scintillators. The neutron-decay measurements were carried out only for the 1 6 0 ( p ,271) reaction with an H a 0 ice target. The experimental details and the results for the decay of the "B(s-hole) and "N(s-hole) states were described in Ref. [2]. Excitation spectra of "B and I5% calculated by summing up the energies of both emitted protons are shown in Fig. 2(a) arid (b). The energy spectra around the s-hole states in both "B and 15Nexhibit some bump-like structures, which can be qualitatively explaincd by recent shell-model calculations for both nuclei [3].
454
M. Yosoi et al. /Nuclear Phssics A738 (2004) 451-454
3. DISCUSSION In Fig. 3, the experimental branching ratios of decay particles from the excit.ation energy regions of the s-hole statJesin "B and 15Fi are shown together with the results of [4].Energy levels known below a stat,istical-model calculation with the code CASCADE 12 MeV excitation energy were explicitly included for all nuclei ( 6 5 A 512) necessary calculation, decay particles above the experimental for the calculation. In the CASCADE detection thresholds were only employed to obtain the branching ratios in Fig. 3. Triton-decay was found to be dominant for the llB(s-hole) state and also found t,o be larger than a-decay for the I5N(s-hole) stat,e despite its smaller Q-value compared to Qdecay. The ’2-body decay’ region in each decay channel of "B [15N]is defined as E,(res) 5 Max(5 MeV [8 MeV] Eth(3-body)), where E,(res) indicates the excitation energy relat8ive to the ground state and Eth(3-body) denotes the threshold energy of particle decay in the residual nucleus. The experimental ’n-,p - , and a-decays in the ’2-body decay’ regions are much reduced, suggesting that contributions of the sequential decay or 3-body decay processes are large in thiese channels. The decay pat,tern of the s-hole state in "B can not be explained by t8hest,atistical-decay model at all. Microscopic cluster-model calculations with SU(3) wave funct,ions[l] explains the experiniental decay character of tjhc llB(shole) state qualitatively. In t’he fragmentation of the s-hole state in 15N, t,he suppression of a-decay is clearly recognized, which supports the selection rule predicted by the SU(3) model. Nearly a half of the total fragmentat,ion of the "N(s-hole) state is concluded to be the direct decay of the doorway s-hole state by comparing the experimental results with both the statistical-model and shell-model calculations [2,3]. In the case of ’He, the mean free path of an s-hole is much larger than the nuclear radius and, therefore, the direct decay process is expected to be dominant. Since t,he .,)~ ’He(s-hole) state excited by 7Li(p,2p) reaction is given as the ( ~ ) ~ ( 1 configuration, the special at,tention is paid to the t+t decay because the s-hole state is considered t o correspond to the one-nucleon knockout, state from an a-cluster in 7Li. Our preliminary result shows that the ’He(s-hole) st,at,edominantly decay to the t+t channel. In the recent measurement of the 6Li(7Li,7Be)reaction, a resonance with large binary triton decay was found at &(’He)= 18 MeV, which suggest.s a di-triton clust.er structure 151. It is, however, not clear if this resonance has the same origin as the s-hole state because the peak energy of t,he resonance is a few MeV higher than the central energy of the s-hole state. The analysis of the experiment for the 7Li(p, 2p)’He* reaction is now in progress to elucidate the relevance bet.ween the di-triton cluster struct,ure and the s-hole state in ’He. ~
REFERENCES 1. T. Yamada, M.Takahashi and K. Ikeda, Phys. Rev. C 53 (1996) 752. 2. M. Yosoi et al., Phys. Lett. B 551 (2003) 255; M. Yosoi. PhD thesis. Kyoto University (2003). 3. T. Yamada, Yucl. Phys. A 687 (2001) 297c; these proceedings. 4. F. Puhlhofer, Nucl. Phys. A 280(1977) 267; M. N.Harakeh, extended version of code CASCADE(1983). 5. H. Akimune et al., Phys. Rev. C 67 (2003) 051302; these proceedings.
Nuclear Physics A738 (2004) 451454
www.elsevier.com/locatelnpe
Cluster fragmentations of deep(1s)-hole statcs in light nuclei M. Yosoi, H. Akimuneb, I. Daito, H. Ejirid, H. Fujimurac*, M.Fujiwarace, K. Fushiniif, K. Haract, K. Y. Harab, H. Hashimoto, T . Ishikawat, YI. Itoh, Y. Itowg. S. Kishi, T. Kawabata!, K. Kawase, M. Kinoshitab, K. Kobayashig, M. NakainuraT: K. Nakanishi, Y. Nakatsugawa, S. Nakayama, T. Noroh, E. Obayashi, S. Okiimura, H. Sakaguchi, Y. Sakemi, M. Shiozawag, H. TakedaI1, T. Taki, A. Tamii, M.Tanakai, S. Terashima, H. Toyokawad, N. Tsukahara, If.Uchida; T. Yamadaj, T. Yamagatab, Y. Yasudaa, H. P. Yoshida, R. G. T. Zegers** and J. Zenihiro Department of Physics, Kyoto University, Kyoto 606-8502, Japan Department of Physics, Konan University, Kobe 658-8501, Japan Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan dJapan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan Advanced Science Research Center, JAERI, Tokai, Ibaraki 319-1195, Japan fDepartment of Physics, University of Tokushima, Tokushima 770-8502, Japan RInstitute for Cosmic Ray Research, University of Tokyo, Kashiwa, 277-8582, Japan Department of Physics, Kyushu University, Fukuoka 812-8581, Japan Kobe Tokiwa College, Kobe 654-0838, Japan JLaboratory of Physics, Kanto Gakuin University, Yokohama 236-0032, .Japan Decay particles from the s-hole stat,es in B, I5N a.nd 6He have been measured in coincidence with the quasifree I2C(p,2p), I6O(p,2p) and 7Li(p,2p) reactions at 392 MeV incident energy. Triton-decay is found t,o be larger than a-decay for both s-hole states in B and I5N despite its smaller &-value than that, of a-decay. This supports the selection rule predicted by the microscopic SU(3)-model calculations. The relat,ion between the He(s-hole) state and the di-triton cluster structure recently found from the charge exchange reaction is briefly discussed. Present address: School of Physics, Seoul National University, Seoul 151-747, Korea Present address: KEK, Tsukuba, Ibaraki, 305-0801, Japan $Present address: LXS, Tohoku University, Sendai, 982-0826, .Japan §Present address: CNS, University of Tokyo, Bunkyo, Tokyo 113-0033,Japan TPresent address: Wakayama Medical University, Wakayama 641-8509, Japan Present address: FU Beam Science Laboratory, RIKEN, Wako, Saitama 351-0198. Japan **Present address: NSCL, Michigan State University, East Lansing, Michigan 48824, USA - see front matter 8 2004 Published by Elsevier B.V. doi:10.1016/j.nucIpliysa.2004.04.084
0375-94746
M. Yosoi et al. /Nuclear Physics A738 (2004) 451-454
452
1. INTRODUCTION
The spatial SU(3) symmetry plays an important role of the clustering aspects in light nuclei. Based on the fact that the deep(1s)-hole stat,e produced by the quasifree reaction such as a ( p , 2p) should have the same spatial symmetry as the ground state of the t.arget nucleus, Yaniada et al. predict,ed a selection rule for fragmentations of the doorway s-hole state in light nuclei, namely, the fragments of a and heavier particles are forbidden in the two-body decay process[l]. On the other hand, since the Q-values for a-fragments in most light nuclei are larger than those for other cluster decay channels, a-decay is favored in the statistical decay process. Decay properties of hole states produced from light targets can, moreover, provide useful informat,ion on the studies of the hypernuclear productions and on the search of proton decay and neutrino detection with, e.g., water Cerenkov detectors. In the present work, particle decays from the quasifree proton-knockout react.ions on 12C, 160and 7Li are studied to understand the st,ructures and fragmentation mechanisms of the s-hole states in "B, I5N and 6He. 2. EXPERIMENT AND RESULTS
Large Acceptance Spectrometer
O 1 2 3 m
Focal Plane LWcctm
Figure 1. Schematic view of the experimental setup. (a) Dual spectrometer system (Grand Raiden and Large Acceptance Spectrometer) at RCNP. (b) SSD-Ball system for the measurement of decay charged particles.
The experiment was carried out at RCNP. Osaka University, by using a 392 MeV proton beam. The quasifree ( p , 2p) reaction was measured with the dual spect,rorneter system consisting of the high resolution spectrometer Grand Raiden (GR) and the large acceptance spectrometer (LAS). A schematic view of the spectrometer system is shown in Fig. l(a). GR was set at 25.5" and the laboratory angle of LAS and the magnetic fields of two spectrometers were determined to satisfy the zero-recoil momentum condition at the central energy of the lsl/2-knockout bump, where the cross section leading to the s-hole state is maximum. Two multi-wire drift chambers in each focal plane of both
453
M. Yosoi et al. /Nuclear Physics A738 (2004) 451-454
._~_____ .... _ ~~.~ _
,
’-10
0
10
20
30
40
50
Ex(’’B) (MeV)
-10
0
10
Ex(%)
20
30 (MeV)
(Ey 16-35MeV)
40
SO
Figure 2. Excitation energy spectra of (a) 12C(p.2p)’lB’ and (b) I 6 0 ( p . 2p)"N* reactions at Ep = 392 MeV. Decay fragments from the s-hole states (hatched regions) were measured in coincidence with two protons at backward angles.
p
d
t
a
(Ex : 20-40 M e V
Figure 3. Experimental branching ratios of the n-; p-, d-, t- and a-decays from the s-hole states in (a) "B and (b) 15N and calculated ones of the statistical model with the code CASCADE.Decay into the ’2-body decay’ region is indicated with dark area for each decay channel (see text).
spectrometers determined the positions and the incidence angles of particles. Charged particle decay of the s-hole states was measured with sixteen telescopes of AE-E Si solid-state detectors (SSD) in coincidence with the quasifree ( p , 2 p ) reactions (see Fig. l(b)). In order to reducc the energy losses of the emitted particles, we used thin targets (0.5-2 mg/cm2). For the "0 target, a combination of a quartz glass (Si02) and a silicon (Si) target was employed. Neutron decay of the s-hole state was measured in the separate setting using a neutron multi-detector array which consists of thirty liquid scintillators. The neutron-decay measurements were carried out only for the 1 6 0 ( p ,271) reaction with an H a 0 ice target. The experimental details and the results for the decay of the "B(s-hole) and "N(s-hole) states were described in Ref. [2]. Excitation spectra of "B and I5% calculated by summing up the energies of both emitted protons are shown in Fig. 2(a) arid (b). The energy spectra around the s-hole states in both "B and 15Nexhibit some bump-like structures, which can be qualitatively explaincd by recent shell-model calculations for both nuclei [3].
454
M. Yosoi et al. /Nuclear Phssics A738 (2004) 451-454
3. DISCUSSION In Fig. 3, the experimental branching ratios of decay particles from the excit.ation energy regions of the s-hole statJesin "B and 15Fi are shown together with the results of [4].Energy levels known below a stat,istical-model calculation with the code CASCADE 12 MeV excitation energy were explicitly included for all nuclei ( 6 5 A 512) necessary calculation, decay particles above the experimental for the calculation. In the CASCADE detection thresholds were only employed to obtain the branching ratios in Fig. 3. Triton-decay was found to be dominant for the llB(s-hole) state and also found t,o be larger than a-decay for the I5N(s-hole) stat,e despite its smaller Q-value compared to Qdecay. The ’2-body decay’ region in each decay channel of "B [15N]is defined as E,(res) 5 Max(5 MeV [8 MeV] Eth(3-body)), where E,(res) indicates the excitation energy relat8ive to the ground state and Eth(3-body) denotes the threshold energy of particle decay in the residual nucleus. The experimental ’n-,p - , and a-decays in the ’2-body decay’ regions are much reduced, suggesting that contributions of the sequential decay or 3-body decay processes are large in thiese channels. The decay pat,tern of the s-hole state in "B can not be explained by t8hest,atistical-decay model at all. Microscopic cluster-model calculations with SU(3) wave funct,ions[l] explains the experiniental decay character of tjhc llB(shole) state qualitatively. In t’he fragmentation of the s-hole state in 15N, t,he suppression of a-decay is clearly recognized, which supports the selection rule predicted by the SU(3) model. Nearly a half of the total fragmentat,ion of the "N(s-hole) state is concluded to be the direct decay of the doorway s-hole state by comparing the experimental results with both the statistical-model and shell-model calculations [2,3]. In the case of ’He, the mean free path of an s-hole is much larger than the nuclear radius and, therefore, the direct decay process is expected to be dominant. Since t,he .,)~ ’He(s-hole) state excited by 7Li(p,2p) reaction is given as the ( ~ ) ~ ( 1 configuration, the special at,tention is paid to the t+t decay because the s-hole state is considered t o correspond to the one-nucleon knockout, state from an a-cluster in 7Li. Our preliminary result shows that the ’He(s-hole) st,at,edominantly decay to the t+t channel. In the recent measurement of the 6Li(7Li,7Be)reaction, a resonance with large binary triton decay was found at &(’He)= 18 MeV, which suggest.s a di-triton clust.er structure 151. It is, however, not clear if this resonance has the same origin as the s-hole state because the peak energy of t,he resonance is a few MeV higher than the central energy of the s-hole state. The analysis of the experiment for the 7Li(p, 2p)’He* reaction is now in progress to elucidate the relevance bet.ween the di-triton cluster struct,ure and the s-hole state in ’He. ~
REFERENCES 1. T. Yamada, M.Takahashi and K. Ikeda, Phys. Rev. C 53 (1996) 752. 2. M. Yosoi et al., Phys. Lett. B 551 (2003) 255; M. Yosoi. PhD thesis. Kyoto University (2003). 3. T. Yamada, Yucl. Phys. A 687 (2001) 297c; these proceedings. 4. F. Puhlhofer, Nucl. Phys. A 280(1977) 267; M. N.Harakeh, extended version of code CASCADE(1983). 5. H. Akimune et al., Phys. Rev. C 67 (2003) 051302; these proceedings.