- Email: [email protected]
Search for t+t clustering in 6He
ELSEVIER
Nuclear Physics A738 (2004) 426430 www.elsevier.comilocatelnpe
Search for t+t clustering in ’He L.Giot" *, P. Roussel-Chomaza, S. Pitaat, N. Alamanosb, F. Augerb, M-D Cortina-GiF, Ch-E Demonchya, J. Fernandez ’, C. Jouanneb, A. Gillibertb, V. Lapouxb, L. Nalpasb, E.C. Pollaccob, A. Rodind, A. Pakou", K. Rusek’ H. SaVajols",J-L. Sidab t , S. Stepantsovd, G. Ter-Akopiad, R. Wolskig,d. "GANIL, BP 55027, 14076 C a m Cedex 5, France bCEA/DSM/DAPNIA/SPhN, Saclay, 91191 Gif-sur-Yvette Cedex, France
’Dpto Fisica de Particulas, Universidad Santiago de Compostela, 15706 Santiago de Compostela, Spain Russia FLNlt,JINR, Dubna, P. 0. Box 79, 101 000 MOSCOW, "Department of Physics, The University of Ioannina, 45110 Ioannina, Greece ’Department of Nuclear Reactions, The Andrzej Soltan Institute for Nuclear Studies: Hoza 69, PL-00-681 Warsaw, Poland gDepartment of Nuclear Reactions, The Henryk Niewodniczanski Institute of Nuclear Physics, Radzikowskiego 152, PL-31-342, Cracow, Poland The ’ H ~ ( P , ~ H )reaction ~ H ~ was measured with the SPEG spectrometer and the MUST array. These data complement previous ones obtained at Dubna. They have been analysed with DWBA calculations to extract the spectroscopic factors for a+2n and t+t configurations in the ground state of ’He. 1. Introduction
The ’He nucleus is now currently used as one of the benchmark nuclei to study the halo phenomenon and 3-body correlations [l],especially because the a-core can very well be represented as inert. However, in order to have a complete and detailed description of the 6He wave function, the question arises whether the only contributions are the cigar and di-neutron configurations, where only ’He and 2-n clusters intervene, or if some t+t clustering is also present. Translational Invariant Shell Model calculations [a] predict spectroscopic factors, for the a+2n and t+t configurations, which have very similar values. More recently, microscopic calculations show that the binding energy of ’He is better reproduced by including some t+t clustering in the ground state wave function [3,4]. *Present address: LPC Boulevard Marechal Juin, 14000 C a m , France tpresent address. College de France, 75005 Paris, France t Present address. CEA-DAM, BP12, 91680 Bruykres Ir Chatel, France 0375-9474/S - see front matter 0 2004 Elsevier B.V All rights reserved doi:l0. 1016/j.nuclphysa.2004.04.079
L. Giot et al. /Nuclear Physics A738 (2004) 426-430
427
In the case of 1,i nucleus, it was shown that it was possible to have considerable a+d and 3He+t clustering, and the importance of both configurations was studied by analysing angular distributions of the L ~ ( P , ~ H ~reactions ) ~ H ~ [5]. More precisely the absolute value of the cross section measured at forward and backward) angles allowed to determine the spectroscopic factors for a+ d and 3He+t configurations. Following the same ideas, we measured recently at GANIL the complete angular distribution for the H ~ ( P ; H ) ~ Hwith ~ the SPEG spectrometer and the MUST array, with a special emphasis on the most forward and backward angles which could not be measured in a previous experiment performed at JINR Dubna [6]. 2. Experimental details and results
The 25 A MeV He beam was produced by fragmentation of a 13C primary beam at 60 A MeV on the production target of the SISSI device. The purification of the secondary beam was achieved with an achromatic A1 degrader located between the two dipoles of the a spectrometer. The only contaminant was Be at the level of around 1%. The secondary beam was transported to the SPEG [7] experimental room. The focal plane spectrometer was equiped with its standard detection system (2 drift chambers for trajectory reconstruction, an ionisation chamber and a plastic scintillator for particle identification). The experimental set-up inside the reaction chamber is shown on Fig.].
MUST Si-Si&i)-CsI device
V
Taget
Beam detectors
Figure 1. Experimental set-up inside the SPEG reaction chamber.
The beam tracking before the target was achieved with two drift chambers, able to withstand high coiinting rates (lo5 pps in the present experiment). The target was a thin polypropylene foil, 18 mg/cm2 thick. On each side of the SPEG angular acceptance, we had positioned 4 MUST telescopes [8], arranged in squared geometry, with their angular positions optimised to detect coincidences of a s and tritons originating from He(p,t)a reaction for center of mass angles between 20 and 120 degrees.
428
L. Giot et al./Nuclear Physics A738 (2004) 426-430
The SPEG spectrometer was used to measure elastic scattering angular distribution. Figure 2 presents the elastic scattering data obtained in the present experiment together with results obtained at Dubna in two different runs [6,9]. All three sets of data are in excellent agreement.
id
F
I
.g R. Wdski, PLB 167 (1999) 8 0 S VStepanrwv, PLB542 f202)
35
SPEG
Figure 2. Experimental angular distribution measured in the present work for elastic scattering together with previous data [6,9]. The solid line corresponds to the optical model calculation using the potentiel of ref.[l5]. SPEG was also used to measure the forward and backward center of mass angles of the angular distribution for the He(p,t)cY reaction, by detecting respectively the high energy a and high energy triton at forward laboratory angles. The experimental angular distribution measured in the present experiment is presented on Fig. 3. The data points between 120 and 155 deg. could not be obtained due to lack of statistics in a set of runs with the MUST array positioned for this angular region. In the overlap domain with the previous data, the agreement is quite satisfactory, the main difference arising from the width of the first oscillation which is larger in previous data [lo]. 3. Analysis of the data 3.1. Elastic scattering Previous results obtained on p+He elastic scattering in the same energy range have shown that the nucleon-nucleus optical model potentials used for stable nuclei have to be modified in the case of loosely bound nuclei such as He[11,12]. In particular the real part of the potential should be reduced in order to reproduce the data. This was ascribed to possible excitations to the continuum.
429
L. Giot et al. /Nuclear Physics A738 (2004) 426-430
The elastic scattering data have been analysed with the CDCC approach previously used by K. Rusek et al. [13], where the breakup of ’He into a+2n and couplings to the continuum have been included. Then a local equivalent potential was obtained by an iterative inversion method [14,15]. Fig. 2 shows the quality of the agreement between the experimental data and the calculations. The angular distribution calculated within the CDCC approach or with the dynamical potential of ref. [15] can hardly be distinguished. 3.2. Transfer reaction Several DWBA calculations based on the first Dubna data have already been published [6,13,16,17]. The spectroscopic factors extracted from these analyses varied between 0 and 1.77, thus showing the need for data covering a wider angular range. We have performed DWBA calculations including both 2n and t transfer. The coupling to the continuum was also taken into account, via the dynamical potential described previously, which was used as the entrance channel ’He+p potential. A special care was taken in the choice of the exit channel. Indeed no data exist for a + t elastic scattering in the energy range considered presently. Therefore we used elastic scattering data for the system c ~ + ~ H[18] e to obtain the potential for the exit channel. Several potentials were considered. The potential derived in ref. [13] gave the best simultaneous description of both a+3He elastic scattering and ’He(p,t)a reaction. The data are compared to DWBA calculations on Figure 3.
id
i
- 2" + t vans*, --+ mt trensfwt t,a"sier
h
k
e P
1 I
E 167
16’
+ ++
+++
+
+ , +
1n3 0
20
40
i1
++++
60
+
80
100
120
140
160
180
&m(deg)
Figure 3. Experimental angular distribution measured in the present work for ’He(p,t)a reaction. The dashed line corresponds to the DWBA calculation where only the 2n transfer is taken into account, with a spectroscopic amplitude equal to 1. The crosses correspond
430
L. Giot et al. /Nuclear Physics A738 (2004) 426- 430
to the triton transfer, with a spectroscopic amplitude equal to 0.25. The solid line corresponds to the coherent sum of the two processes with these values of their spectroscopic amplitudes. The value of the spectroscopic factor extracted for the t+t configuration is between 0.06 and 0.09, which is much less than predicted. However it is important to include it in order to reproduce simultaneously the forward and backward angles of the angular distribution. 4. Conclusions The spectroscopic factors for a+2n and t+t configurations in the ground state of 6He were extracted from the analysis of the He(p,t) reaction. The value extracted for t+t configuration is much smaller than theoretical predictions from shell models or microscopic models [ 2 4 ] . It should be noted that the present result does not include several effets that could modify this conclusion. For example, the sequential tranfer of the 2 neutrons or of one proton and of 2n (in the case o f t transfer) was not considered. Also, an attempt to include the transfer from the continuum states in a full Coupled Reaction Channel calculation did not give satisfactory results in the present stage of the analysis. Finally the exit channel potential should also be investigated more deeply, since it was shown to strongly influence the angular distribution. 5. Acknowledgement This work was financially supported by the IN2P3-Poland cooperation agreement 02106.
REFERENCES 1. M. Zhukov et al., Phys. Rep. 231 (1993) 151. 2. Yu. F. Smirnov, Phys. Rev. C15 (1977) 84. 3. A. Csoto; Phys. Rev. C48 (1993) 165. 4. K. Arai et al., Phys. Rev. C 59 (1999) 1432. 5. M. F. Werby et al, Phys. Rev. C 8 (1973) 106. 6. R. Wolski et al., Phys. Lett. B 467 (1999) 8. 7. L. Bianchi et al. NIM A 276 (1989) 509. 8. Y. Blumenfeld et al., NIM A 421 (1999) 471. 9. S.V. Stepantsov et al., Phys. Lett. R 542 (2002) 35. 10. L.Giot, These de 1Universite de Caen, 2003. 11. M.D. Cortina-Gil et al., Phys. Lett. B 371 (96) 14. 12. V. Lapoux et al., Phys. Lett. B 517 (2001) 18. 13. K. Itusek et al., Phys. Rev. C64 (2001) 044602. 14. S.G. Cooper et al., Nucl. Phys. A 677 (2000) 187. 15. R.S. Mackintosh et al., Phys. Rev. C67 (2003) 034607. 16. Yu. Ts. Oganessian et a!., Phys. Rev. Lett. 82 (1999) 4996 17. N. Timofeyuk, Phys. Rev. C63 (2001) 054609. 18. O.F. Nemets et al., Yad. Fiz. 42 (1985) 809.
Nuclear Physics A738 (2004) 426430 www.elsevier.comilocatelnpe
Search for t+t clustering in ’He L.Giot" *, P. Roussel-Chomaza, S. Pitaat, N. Alamanosb, F. Augerb, M-D Cortina-GiF, Ch-E Demonchya, J. Fernandez ’, C. Jouanneb, A. Gillibertb, V. Lapouxb, L. Nalpasb, E.C. Pollaccob, A. Rodind, A. Pakou", K. Rusek’ H. SaVajols",J-L. Sidab t , S. Stepantsovd, G. Ter-Akopiad, R. Wolskig,d. "GANIL, BP 55027, 14076 C a m Cedex 5, France bCEA/DSM/DAPNIA/SPhN, Saclay, 91191 Gif-sur-Yvette Cedex, France
’Dpto Fisica de Particulas, Universidad Santiago de Compostela, 15706 Santiago de Compostela, Spain Russia FLNlt,JINR, Dubna, P. 0. Box 79, 101 000 MOSCOW, "Department of Physics, The University of Ioannina, 45110 Ioannina, Greece ’Department of Nuclear Reactions, The Andrzej Soltan Institute for Nuclear Studies: Hoza 69, PL-00-681 Warsaw, Poland gDepartment of Nuclear Reactions, The Henryk Niewodniczanski Institute of Nuclear Physics, Radzikowskiego 152, PL-31-342, Cracow, Poland The ’ H ~ ( P , ~ H )reaction ~ H ~ was measured with the SPEG spectrometer and the MUST array. These data complement previous ones obtained at Dubna. They have been analysed with DWBA calculations to extract the spectroscopic factors for a+2n and t+t configurations in the ground state of ’He. 1. Introduction
The ’He nucleus is now currently used as one of the benchmark nuclei to study the halo phenomenon and 3-body correlations [l],especially because the a-core can very well be represented as inert. However, in order to have a complete and detailed description of the 6He wave function, the question arises whether the only contributions are the cigar and di-neutron configurations, where only ’He and 2-n clusters intervene, or if some t+t clustering is also present. Translational Invariant Shell Model calculations [a] predict spectroscopic factors, for the a+2n and t+t configurations, which have very similar values. More recently, microscopic calculations show that the binding energy of ’He is better reproduced by including some t+t clustering in the ground state wave function [3,4]. *Present address: LPC Boulevard Marechal Juin, 14000 C a m , France tpresent address. College de France, 75005 Paris, France t Present address. CEA-DAM, BP12, 91680 Bruykres Ir Chatel, France 0375-9474/S - see front matter 0 2004 Elsevier B.V All rights reserved doi:l0. 1016/j.nuclphysa.2004.04.079
L. Giot et al. /Nuclear Physics A738 (2004) 426-430
427
In the case of 1,i nucleus, it was shown that it was possible to have considerable a+d and 3He+t clustering, and the importance of both configurations was studied by analysing angular distributions of the L ~ ( P , ~ H ~reactions ) ~ H ~ [5]. More precisely the absolute value of the cross section measured at forward and backward) angles allowed to determine the spectroscopic factors for a+ d and 3He+t configurations. Following the same ideas, we measured recently at GANIL the complete angular distribution for the H ~ ( P ; H ) ~ Hwith ~ the SPEG spectrometer and the MUST array, with a special emphasis on the most forward and backward angles which could not be measured in a previous experiment performed at JINR Dubna [6]. 2. Experimental details and results
The 25 A MeV He beam was produced by fragmentation of a 13C primary beam at 60 A MeV on the production target of the SISSI device. The purification of the secondary beam was achieved with an achromatic A1 degrader located between the two dipoles of the a spectrometer. The only contaminant was Be at the level of around 1%. The secondary beam was transported to the SPEG [7] experimental room. The focal plane spectrometer was equiped with its standard detection system (2 drift chambers for trajectory reconstruction, an ionisation chamber and a plastic scintillator for particle identification). The experimental set-up inside the reaction chamber is shown on Fig.].
MUST Si-Si&i)-CsI device
V
Taget
Beam detectors
Figure 1. Experimental set-up inside the SPEG reaction chamber.
The beam tracking before the target was achieved with two drift chambers, able to withstand high coiinting rates (lo5 pps in the present experiment). The target was a thin polypropylene foil, 18 mg/cm2 thick. On each side of the SPEG angular acceptance, we had positioned 4 MUST telescopes [8], arranged in squared geometry, with their angular positions optimised to detect coincidences of a s and tritons originating from He(p,t)a reaction for center of mass angles between 20 and 120 degrees.
428
L. Giot et al./Nuclear Physics A738 (2004) 426-430
The SPEG spectrometer was used to measure elastic scattering angular distribution. Figure 2 presents the elastic scattering data obtained in the present experiment together with results obtained at Dubna in two different runs [6,9]. All three sets of data are in excellent agreement.
id
F
I
.g R. Wdski, PLB 167 (1999) 8 0 S VStepanrwv, PLB542 f202)
35
SPEG
Figure 2. Experimental angular distribution measured in the present work for elastic scattering together with previous data [6,9]. The solid line corresponds to the optical model calculation using the potentiel of ref.[l5]. SPEG was also used to measure the forward and backward center of mass angles of the angular distribution for the He(p,t)cY reaction, by detecting respectively the high energy a and high energy triton at forward laboratory angles. The experimental angular distribution measured in the present experiment is presented on Fig. 3. The data points between 120 and 155 deg. could not be obtained due to lack of statistics in a set of runs with the MUST array positioned for this angular region. In the overlap domain with the previous data, the agreement is quite satisfactory, the main difference arising from the width of the first oscillation which is larger in previous data [lo]. 3. Analysis of the data 3.1. Elastic scattering Previous results obtained on p+He elastic scattering in the same energy range have shown that the nucleon-nucleus optical model potentials used for stable nuclei have to be modified in the case of loosely bound nuclei such as He[11,12]. In particular the real part of the potential should be reduced in order to reproduce the data. This was ascribed to possible excitations to the continuum.
429
L. Giot et al. /Nuclear Physics A738 (2004) 426-430
The elastic scattering data have been analysed with the CDCC approach previously used by K. Rusek et al. [13], where the breakup of ’He into a+2n and couplings to the continuum have been included. Then a local equivalent potential was obtained by an iterative inversion method [14,15]. Fig. 2 shows the quality of the agreement between the experimental data and the calculations. The angular distribution calculated within the CDCC approach or with the dynamical potential of ref. [15] can hardly be distinguished. 3.2. Transfer reaction Several DWBA calculations based on the first Dubna data have already been published [6,13,16,17]. The spectroscopic factors extracted from these analyses varied between 0 and 1.77, thus showing the need for data covering a wider angular range. We have performed DWBA calculations including both 2n and t transfer. The coupling to the continuum was also taken into account, via the dynamical potential described previously, which was used as the entrance channel ’He+p potential. A special care was taken in the choice of the exit channel. Indeed no data exist for a + t elastic scattering in the energy range considered presently. Therefore we used elastic scattering data for the system c ~ + ~ H[18] e to obtain the potential for the exit channel. Several potentials were considered. The potential derived in ref. [13] gave the best simultaneous description of both a+3He elastic scattering and ’He(p,t)a reaction. The data are compared to DWBA calculations on Figure 3.
id
i
- 2" + t vans*, --+ mt trensfwt t,a"sier
h
k
e P
1 I
E 167
16’
+ ++
+++
+
+ , +
1n3 0
20
40
i1
++++
60
+
80
100
120
140
160
180
&m(deg)
Figure 3. Experimental angular distribution measured in the present work for ’He(p,t)a reaction. The dashed line corresponds to the DWBA calculation where only the 2n transfer is taken into account, with a spectroscopic amplitude equal to 1. The crosses correspond
430
L. Giot et al. /Nuclear Physics A738 (2004) 426- 430
to the triton transfer, with a spectroscopic amplitude equal to 0.25. The solid line corresponds to the coherent sum of the two processes with these values of their spectroscopic amplitudes. The value of the spectroscopic factor extracted for the t+t configuration is between 0.06 and 0.09, which is much less than predicted. However it is important to include it in order to reproduce simultaneously the forward and backward angles of the angular distribution. 4. Conclusions The spectroscopic factors for a+2n and t+t configurations in the ground state of 6He were extracted from the analysis of the He(p,t) reaction. The value extracted for t+t configuration is much smaller than theoretical predictions from shell models or microscopic models [ 2 4 ] . It should be noted that the present result does not include several effets that could modify this conclusion. For example, the sequential tranfer of the 2 neutrons or of one proton and of 2n (in the case o f t transfer) was not considered. Also, an attempt to include the transfer from the continuum states in a full Coupled Reaction Channel calculation did not give satisfactory results in the present stage of the analysis. Finally the exit channel potential should also be investigated more deeply, since it was shown to strongly influence the angular distribution. 5. Acknowledgement This work was financially supported by the IN2P3-Poland cooperation agreement 02106.
REFERENCES 1. M. Zhukov et al., Phys. Rep. 231 (1993) 151. 2. Yu. F. Smirnov, Phys. Rev. C15 (1977) 84. 3. A. Csoto; Phys. Rev. C48 (1993) 165. 4. K. Arai et al., Phys. Rev. C 59 (1999) 1432. 5. M. F. Werby et al, Phys. Rev. C 8 (1973) 106. 6. R. Wolski et al., Phys. Lett. B 467 (1999) 8. 7. L. Bianchi et al. NIM A 276 (1989) 509. 8. Y. Blumenfeld et al., NIM A 421 (1999) 471. 9. S.V. Stepantsov et al., Phys. Lett. R 542 (2002) 35. 10. L.Giot, These de 1Universite de Caen, 2003. 11. M.D. Cortina-Gil et al., Phys. Lett. B 371 (96) 14. 12. V. Lapoux et al., Phys. Lett. B 517 (2001) 18. 13. K. Itusek et al., Phys. Rev. C64 (2001) 044602. 14. S.G. Cooper et al., Nucl. Phys. A 677 (2000) 187. 15. R.S. Mackintosh et al., Phys. Rev. C67 (2003) 034607. 16. Yu. Ts. Oganessian et a!., Phys. Rev. Lett. 82 (1999) 4996 17. N. Timofeyuk, Phys. Rev. C63 (2001) 054609. 18. O.F. Nemets et al., Yad. Fiz. 42 (1985) 809.