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Experimental study of the two-proton correlated emission from the excited states of 17,18Ne and 28,29S
Nuclear Physics A 834 (2010) 450c–453c www.elsevier.com/locate/nuclphysa
Experimental study of the two-proton correlated emission from the excited states of 17,18 Ne and 28,29 S C.J. Lina , X.X. Xua , H.M. Jiaa , F. Yanga , F. Jiaa , Z.D. Wua , S.T. Zhanga , Z.H. Liua , H.Q. Zhanga , H.S. Xub , Z.Y. Sunb , J.S. Wangb , Z.G. Hub , M. Wangb , R.F Chenb , X.Y. Zhangb , C. Lib , X.G. Leib , Z.G. Xub , G.Q. Xiaob , and W.L. Zhanb a
China Institute of Atomic Energy, Beijing 102413, P.R. China
b
Institute of Modern Physics, Chinese Academy of Science, Lanzhou 73000, P.R. China
A series of experiments have been performed by complete kinematics measurements to study two-proton (2p) correlated emission from the excited states of 17,18 Ne and 28,29 S via the Coulomb excitation by bombarding on 197 Au target. 2p and residua coincident events were picked out under strict conditions. Visible p-p correlations were observed. It is shown that 2p can be emitted from the high-lying excited states. 2p halo may lead to 2p emission with large spectroscopy factor for the states close to or beyond the threshold. 1. INTRODUCTION Two-proton (2p) radioactivity predicated nearly fifty years ago [1] becomes currently one of the most exciting topics in the field of RNB physics [2,3]. Such a kind of exotic decay provides i) a good probe to extract the information of nuclear structure for the protonrich nuclei close to or beyond the drip-line, ii) a good tool to study the nucleon-nucleon pair-correlation (p-p correlation in particular) inside a nucleus and the relative topics like BCS-BEC crossover [4], iii) a good way to investigate the astro-nuclear (2p,γ) and (γ,2p) processes [5], and more. As charged-particle decay, the half-life is mainly governed by the height of the Coulomb barrier. Roughly speaking, for the heavier emitters with A ≥ 40, such as 42 Cr, 45 Fe, 48,49 Ni, 54 Zn, etc., 2ps may emit from the ground states with long lifetimes (usually larger than ps) due to the higher Coulomb barriers, allowing us to do offline measurements to reduce the background. But for the light emitters with A ≤ 20, such as 6 Be, 12,14 O, 16−18 Ne, 19 Mg, etc., the lower Coulomb barriers cause 2ps to decay in very short times (typically about keV order) from the ground states or lowlying excited states. Online complete-kinematics measurements are required to identify the effective 2p events. Besides these, 2ps may emit from some intermediate-mass nuclei with 20 < A < 40, such as 22−24 Si, 26−29 S, 31,32 Ar, 34 Ca, etc., not only from the ground states and low-lying excited states, but also possibly from the high-lying excited states like 10 MeV state in 29 S [6]. For these medium emitters, 2p decays have short time but with wide scale (about fs/keV/MeV order), and online measurements are also required. In the past years, we performed a series of experiments to study 2p emission from the proton-rich nuclei close to the drip line. In the 29 S+28 Si experiment, an abnormally large 0375-9474/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2010.01.061
C.J. Lin et al. / Nuclear Physics A 834 (2010) 450c–453c
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total reaction cross section has been detected [7], indicating the possibility of 2p halo/skin structure in 29 S. Moreover, signatures of 2p emission were observed in the 29 S+12 C reaction [8]. Later, we focused on 2p emission from the excited states of 17,18 Ne and 28,29 S via the Coulomb excitation by complete-kinematics measurement. In this talk, some experimental results will be presented to show the 2p correlated emission. 2. EXPERIMENTAL SETUP These experiments have been done at the Institute of Modern Physics, Lanzhou, China. The secondary radioactive beams were produced by the primary beams of 20 Ne and 32 S, respectively, with current of 100 enA, bombarding on the 1589 μm 9 Be target, transported and purified through the RIBLL facility [9] by means of the combined Bρ-ΔE-Bρ method, where 27 Al degraders with suitable thicknesses were employed to get the favorable intensities and purities of the secondary beams. Typical secondary beam intensities are 200 and 800 pps for 17 Ne and 18 Ne with purities of 10% and 40%, respectively, and 30 and 200 pps for 28 S and 29 S with purities of 1% and 3%, respectively. Detector array consisting of parallel plane avalanche counts (PPACs), large area silicon detectors, single-sided silicon strip detectors (SSSDs), and a 6×6 array of CsI crystals coupled to PIN photodiodes (CsI-PINs) was emplaced in the chamber to achieve the complete-kinematics measurement. The secondary beams were identified by ΔE-ToF technique, where ToF was generated by two scintillator detectors upstream. Two PPACs with collimators of 40 and 30 mm in diameter, respectively, were used for beam tracking. The orthogonal Y1- and X1-SSSD detected the ΔE-Es and positions of heavy fragments, while D2, X2- and Y2-SSSD, as well as the CsI-PIN array detected the ΔE-Es and positions of light particles. The whole detector array covered 0.33 sr of forward solid angle with the maximum opening angle of ±13.2◦ . Monte Carlo (MC) simulations showed that the detection efficiency for coincident p-p events was about 57%. 3. DATA ANALYSES AND RESULTS In order to pick out the true p-p coincident events, a strict selection procedure was obeyed, as followings: i) identified the secondary beams on the ΔE-ToF spectrum, ii) chose the double-hit 2p events appeared on the ΔE-E spectrum of light-particle identification, iii) selected the residual nuclei on the ΔE-E spectrum of heavy-fragment identification, iv) made a narrow time window for heavy products to eliminate accidental coincidences and the isotopic contaminations directly from the secondary beams, v) selected the reactions only in the target layer by tracking method. Details can be found in a recent paper [6]. Once the p-p coincident events are determined, the relative momentum (qpp = |p1 − c.m. p2 |/2), opening angle (θpp , in the center of three-body mass system), and relative energy (Epp ) of 2p, as well as the invariant mass of three-body system, etc., can be deduced eventby-event by the relativistic-kinematics reconstruction under the constraint of energy and c.m. momentum conservation. Figures 1 and 2 show the qpp and θpp of 2p emission from the 29 ∼ 7.4 and ∼ 10.0 MeV states of S, respectively, for comparison. The results of MC simulations are also plotted in the figures. A simple schematic model was employed in the simulation, in which the extreme decay modes, such as 2 He cluster emission, threebody phase-space decay, and tow-body sequential decay were taken into account. For the
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C.J. Lin et al. / Nuclear Physics A 834 (2010) 450c–453c
c.m. shows a forward distribution with maximum at ∼ 35◦ and qpp has ∼ 10.0 MeV state, θpp an obvious peak at ∼ 20 MeV/c, indicating the 2 He cluster decay. Associated with MC simulations, the branching ratio of 2 He decay was determined as 29+10 −11 %. But for the 7.4 MeV state, no pronounced 2 He signature was found.
Figure 1. The relative momenta and open- Figure 2. Same as FIG. 1, but from the ing angles of 2p emitted from the states of states of 9.6 ≤ Ex ≤ 10.4 MeV. Features of 2 7.0 ≤ Ex ≤ 7.8 MeV of 29 S. He cluster decay were observed. c.m. Figure 3 shows the spectrum of qpp versus θpp for all the 17 Ne→15 O+p+p coincident c.m. events. A prominent peak appeared at qpp 20 MeV/c and θpp 50◦ indicates the 2 strong p-p correlations, probably He cluster formation. In order to investigate the nature of 2p decay from each state of 17 Ne, the excitation energies were deduced from the invariant mass of the final state of 15 O+p+p system, as shown in Fig. 4. The arrows with numbers denote the known levels [10]. Finally, results show a pronounced peak at qpp 20 MeV/c c.m. c.m. but an isotropic distribution in θpp for the first excited state of 17 Ne, whereas forward θpp distributions were observed for other higher excited states. Some relative topics have been discussed at the NSD2009 conference [11]. For the 18 Ne case, some preliminary results have been published [12] and reproduced Raciti’s results [13]. More detailed analyses and simulation, as well as theoretical calculations are required to make final conclusions.
4. DISCUSSIONS AND SUMMARY It is surprised that 2p can be emission from high-lying excited states like 10 MeV states in 29 S and probably some states in 17 Ne, because the sequential decay channels are opened and may dominate the decay processes. A possible explanation is that 2p halo may form in the high-lying states and result in 2p decay with a large spectroscopy factor. Some similar cases can be found in the β-delayed 2p emission, for instant the 22 Al case. We guess that the BCS-BEC crossover phenomenon [4] might occur in the 2p halo. It is worthy of further investigation.
C.J. Lin et al. / Nuclear Physics A 834 (2010) 450c–453c
453c
Figure 3. The relative momenta versus Figure 4. Excitation energies of 17 Ne reconopening angles for all the coincident events. structed from 15 O+p+p events. In summary, a series of experiments have been performed to study 2p correlated emission from the excited states of 17,18 Ne and 28,29 S via Coulomb excitation by bombarding on 197 Au target. Complete kinematics measurements were achieved to reconstruct the qpp , c.m. θpp , Epp , invariant mass, and so on. 2p and residua coincident events were picked out under strict conditions. The mechanisms of 2p emission were analyzed in a simple schematic model. Associated with the Monte Carlo simulations, results show that 2p emitted from the ∼ 10 MeV state of 29 S exhibit the features of 2 He cluster decay with a branching ratio of 29+10 −11 %. Moreover, visible p-p correlations were also observed in the case of 17,18 Ne. It is shown that 2p can be emitted from the high-lying excited states, and 2p halo may lead to 2p emission with a large spectroscopy factor when the states are close to or beyond the threshold. In the future, we should pay more attention to i) 2p emission from high-lying excited states, ii) the link between 2p halo and 2p emission, and iii) exact theoretical descriptions for the 2p decay processes. This work was supported by the National Natural Science Foundation of China under Grants No. 10675169, 10735100, 10727505, and the Major State Basic Research Developing Program under Grant No. 2007CB815003. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
V.I. Goldansky, Nucl. Phys. 19, 482 (1960). B. Blank and M. Ploszajczak, Rep. Prog. Phys. 71, 046301 (2008). B. Blank and M.J.G. Borge, Prog. Part. Nucl. Phys. 60, 403 (2008). K. Hagino, H. Sagawa et al., Phys. Rev. Lett. 99, 022506 (2007). L.V. Grigorenko, K. Langanke et al., Phys. Lett. B 641, 254 (2006). C.J. Lin, X.X. Xu, H.M. Jia et al., Phys. Rev. C 80, 014310 (2009). Liu Zu-Hua, Ruan Ming, Zhao Yao-Lin et al., Chin. Phys. Lett. 21, 1711 (2004). C.J. Lin, X.X. Xu, G.L. Zhang et al., AIP Conf. Proc. 961, 117 (2007). Z. Sun, W.L. Zhan et al., Nucl. Instr. Meth. Phys. Res. Sect. A 503, 496 (2003). V. Guimaraes, S. Kubono, N. Ikeda et al., Phys. Rev. C 58, 116 (1998). X.X. Xu, C.J. Lin, H.M. Jia et al., AIP Conf. Proc. 1165, 106 (2009). F. Jia, C.J. Lin, H.Q. Zhang et al., Chin. Phys. Lett. 26, 032301 (2009). G. Raciti, G. Cardella, M. De Napoli et al., Phys. Rev. Lett. 100, 192503 (2008).
Experimental study of the two-proton correlated emission from the excited states of 17,18 Ne and 28,29 S C.J. Lina , X.X. Xua , H.M. Jiaa , F. Yanga , F. Jiaa , Z.D. Wua , S.T. Zhanga , Z.H. Liua , H.Q. Zhanga , H.S. Xub , Z.Y. Sunb , J.S. Wangb , Z.G. Hub , M. Wangb , R.F Chenb , X.Y. Zhangb , C. Lib , X.G. Leib , Z.G. Xub , G.Q. Xiaob , and W.L. Zhanb a
China Institute of Atomic Energy, Beijing 102413, P.R. China
b
Institute of Modern Physics, Chinese Academy of Science, Lanzhou 73000, P.R. China
A series of experiments have been performed by complete kinematics measurements to study two-proton (2p) correlated emission from the excited states of 17,18 Ne and 28,29 S via the Coulomb excitation by bombarding on 197 Au target. 2p and residua coincident events were picked out under strict conditions. Visible p-p correlations were observed. It is shown that 2p can be emitted from the high-lying excited states. 2p halo may lead to 2p emission with large spectroscopy factor for the states close to or beyond the threshold. 1. INTRODUCTION Two-proton (2p) radioactivity predicated nearly fifty years ago [1] becomes currently one of the most exciting topics in the field of RNB physics [2,3]. Such a kind of exotic decay provides i) a good probe to extract the information of nuclear structure for the protonrich nuclei close to or beyond the drip-line, ii) a good tool to study the nucleon-nucleon pair-correlation (p-p correlation in particular) inside a nucleus and the relative topics like BCS-BEC crossover [4], iii) a good way to investigate the astro-nuclear (2p,γ) and (γ,2p) processes [5], and more. As charged-particle decay, the half-life is mainly governed by the height of the Coulomb barrier. Roughly speaking, for the heavier emitters with A ≥ 40, such as 42 Cr, 45 Fe, 48,49 Ni, 54 Zn, etc., 2ps may emit from the ground states with long lifetimes (usually larger than ps) due to the higher Coulomb barriers, allowing us to do offline measurements to reduce the background. But for the light emitters with A ≤ 20, such as 6 Be, 12,14 O, 16−18 Ne, 19 Mg, etc., the lower Coulomb barriers cause 2ps to decay in very short times (typically about keV order) from the ground states or lowlying excited states. Online complete-kinematics measurements are required to identify the effective 2p events. Besides these, 2ps may emit from some intermediate-mass nuclei with 20 < A < 40, such as 22−24 Si, 26−29 S, 31,32 Ar, 34 Ca, etc., not only from the ground states and low-lying excited states, but also possibly from the high-lying excited states like 10 MeV state in 29 S [6]. For these medium emitters, 2p decays have short time but with wide scale (about fs/keV/MeV order), and online measurements are also required. In the past years, we performed a series of experiments to study 2p emission from the proton-rich nuclei close to the drip line. In the 29 S+28 Si experiment, an abnormally large 0375-9474/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2010.01.061
C.J. Lin et al. / Nuclear Physics A 834 (2010) 450c–453c
451c
total reaction cross section has been detected [7], indicating the possibility of 2p halo/skin structure in 29 S. Moreover, signatures of 2p emission were observed in the 29 S+12 C reaction [8]. Later, we focused on 2p emission from the excited states of 17,18 Ne and 28,29 S via the Coulomb excitation by complete-kinematics measurement. In this talk, some experimental results will be presented to show the 2p correlated emission. 2. EXPERIMENTAL SETUP These experiments have been done at the Institute of Modern Physics, Lanzhou, China. The secondary radioactive beams were produced by the primary beams of 20 Ne and 32 S, respectively, with current of 100 enA, bombarding on the 1589 μm 9 Be target, transported and purified through the RIBLL facility [9] by means of the combined Bρ-ΔE-Bρ method, where 27 Al degraders with suitable thicknesses were employed to get the favorable intensities and purities of the secondary beams. Typical secondary beam intensities are 200 and 800 pps for 17 Ne and 18 Ne with purities of 10% and 40%, respectively, and 30 and 200 pps for 28 S and 29 S with purities of 1% and 3%, respectively. Detector array consisting of parallel plane avalanche counts (PPACs), large area silicon detectors, single-sided silicon strip detectors (SSSDs), and a 6×6 array of CsI crystals coupled to PIN photodiodes (CsI-PINs) was emplaced in the chamber to achieve the complete-kinematics measurement. The secondary beams were identified by ΔE-ToF technique, where ToF was generated by two scintillator detectors upstream. Two PPACs with collimators of 40 and 30 mm in diameter, respectively, were used for beam tracking. The orthogonal Y1- and X1-SSSD detected the ΔE-Es and positions of heavy fragments, while D2, X2- and Y2-SSSD, as well as the CsI-PIN array detected the ΔE-Es and positions of light particles. The whole detector array covered 0.33 sr of forward solid angle with the maximum opening angle of ±13.2◦ . Monte Carlo (MC) simulations showed that the detection efficiency for coincident p-p events was about 57%. 3. DATA ANALYSES AND RESULTS In order to pick out the true p-p coincident events, a strict selection procedure was obeyed, as followings: i) identified the secondary beams on the ΔE-ToF spectrum, ii) chose the double-hit 2p events appeared on the ΔE-E spectrum of light-particle identification, iii) selected the residual nuclei on the ΔE-E spectrum of heavy-fragment identification, iv) made a narrow time window for heavy products to eliminate accidental coincidences and the isotopic contaminations directly from the secondary beams, v) selected the reactions only in the target layer by tracking method. Details can be found in a recent paper [6]. Once the p-p coincident events are determined, the relative momentum (qpp = |p1 − c.m. p2 |/2), opening angle (θpp , in the center of three-body mass system), and relative energy (Epp ) of 2p, as well as the invariant mass of three-body system, etc., can be deduced eventby-event by the relativistic-kinematics reconstruction under the constraint of energy and c.m. momentum conservation. Figures 1 and 2 show the qpp and θpp of 2p emission from the 29 ∼ 7.4 and ∼ 10.0 MeV states of S, respectively, for comparison. The results of MC simulations are also plotted in the figures. A simple schematic model was employed in the simulation, in which the extreme decay modes, such as 2 He cluster emission, threebody phase-space decay, and tow-body sequential decay were taken into account. For the
452c
C.J. Lin et al. / Nuclear Physics A 834 (2010) 450c–453c
c.m. shows a forward distribution with maximum at ∼ 35◦ and qpp has ∼ 10.0 MeV state, θpp an obvious peak at ∼ 20 MeV/c, indicating the 2 He cluster decay. Associated with MC simulations, the branching ratio of 2 He decay was determined as 29+10 −11 %. But for the 7.4 MeV state, no pronounced 2 He signature was found.
Figure 1. The relative momenta and open- Figure 2. Same as FIG. 1, but from the ing angles of 2p emitted from the states of states of 9.6 ≤ Ex ≤ 10.4 MeV. Features of 2 7.0 ≤ Ex ≤ 7.8 MeV of 29 S. He cluster decay were observed. c.m. Figure 3 shows the spectrum of qpp versus θpp for all the 17 Ne→15 O+p+p coincident c.m. events. A prominent peak appeared at qpp 20 MeV/c and θpp 50◦ indicates the 2 strong p-p correlations, probably He cluster formation. In order to investigate the nature of 2p decay from each state of 17 Ne, the excitation energies were deduced from the invariant mass of the final state of 15 O+p+p system, as shown in Fig. 4. The arrows with numbers denote the known levels [10]. Finally, results show a pronounced peak at qpp 20 MeV/c c.m. c.m. but an isotropic distribution in θpp for the first excited state of 17 Ne, whereas forward θpp distributions were observed for other higher excited states. Some relative topics have been discussed at the NSD2009 conference [11]. For the 18 Ne case, some preliminary results have been published [12] and reproduced Raciti’s results [13]. More detailed analyses and simulation, as well as theoretical calculations are required to make final conclusions.
4. DISCUSSIONS AND SUMMARY It is surprised that 2p can be emission from high-lying excited states like 10 MeV states in 29 S and probably some states in 17 Ne, because the sequential decay channels are opened and may dominate the decay processes. A possible explanation is that 2p halo may form in the high-lying states and result in 2p decay with a large spectroscopy factor. Some similar cases can be found in the β-delayed 2p emission, for instant the 22 Al case. We guess that the BCS-BEC crossover phenomenon [4] might occur in the 2p halo. It is worthy of further investigation.
C.J. Lin et al. / Nuclear Physics A 834 (2010) 450c–453c
453c
Figure 3. The relative momenta versus Figure 4. Excitation energies of 17 Ne reconopening angles for all the coincident events. structed from 15 O+p+p events. In summary, a series of experiments have been performed to study 2p correlated emission from the excited states of 17,18 Ne and 28,29 S via Coulomb excitation by bombarding on 197 Au target. Complete kinematics measurements were achieved to reconstruct the qpp , c.m. θpp , Epp , invariant mass, and so on. 2p and residua coincident events were picked out under strict conditions. The mechanisms of 2p emission were analyzed in a simple schematic model. Associated with the Monte Carlo simulations, results show that 2p emitted from the ∼ 10 MeV state of 29 S exhibit the features of 2 He cluster decay with a branching ratio of 29+10 −11 %. Moreover, visible p-p correlations were also observed in the case of 17,18 Ne. It is shown that 2p can be emitted from the high-lying excited states, and 2p halo may lead to 2p emission with a large spectroscopy factor when the states are close to or beyond the threshold. In the future, we should pay more attention to i) 2p emission from high-lying excited states, ii) the link between 2p halo and 2p emission, and iii) exact theoretical descriptions for the 2p decay processes. This work was supported by the National Natural Science Foundation of China under Grants No. 10675169, 10735100, 10727505, and the Major State Basic Research Developing Program under Grant No. 2007CB815003. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
V.I. Goldansky, Nucl. Phys. 19, 482 (1960). B. Blank and M. Ploszajczak, Rep. Prog. Phys. 71, 046301 (2008). B. Blank and M.J.G. Borge, Prog. Part. Nucl. Phys. 60, 403 (2008). K. Hagino, H. Sagawa et al., Phys. Rev. Lett. 99, 022506 (2007). L.V. Grigorenko, K. Langanke et al., Phys. Lett. B 641, 254 (2006). C.J. Lin, X.X. Xu, H.M. Jia et al., Phys. Rev. C 80, 014310 (2009). Liu Zu-Hua, Ruan Ming, Zhao Yao-Lin et al., Chin. Phys. Lett. 21, 1711 (2004). C.J. Lin, X.X. Xu, G.L. Zhang et al., AIP Conf. Proc. 961, 117 (2007). Z. Sun, W.L. Zhan et al., Nucl. Instr. Meth. Phys. Res. Sect. A 503, 496 (2003). V. Guimaraes, S. Kubono, N. Ikeda et al., Phys. Rev. C 58, 116 (1998). X.X. Xu, C.J. Lin, H.M. Jia et al., AIP Conf. Proc. 1165, 106 (2009). F. Jia, C.J. Lin, H.Q. Zhang et al., Chin. Phys. Lett. 26, 032301 (2009). G. Raciti, G. Cardella, M. De Napoli et al., Phys. Rev. Lett. 100, 192503 (2008).