Nucleon-Nucleon Pairing Correlations probed in the Sn+ Pb...
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Nucleon-Nucleon Pairing Correlations probed in the 118 Sn+ 206 Pb System via Transfer Reactions at far Sub-barrier Energies F. Galtarossa 1 , L. Corradi 1 , S. Szilner 2 , E. Fioretto 1 , P. ˇ Colovi´ c 2 , T. Mijatovi´ c 2 , G. Colucci 3 , A. Goasduff 1 , A. Gottardo 1 , J. Grebosz 4 , A. Illana Sis ´ on 1 , G. Jaworski 1 , M. Jurado Gomez 5 , T. Marchi 1 , G. Montagnoli 3 , D. Nurkic 2 , G. Pollarolo 6 , F. Scarlassara 3 , M. Siciliano 1 , N. Soi´ c 2 , A. M. Stefanini 1 , D. Testov 3 , J. J. Valiente-Dob´ on 1 , N. Vukman 2 1 INFN, Laboratori Nazionali di Legnaro, Legnaro, Italy. 2 Rud er Boˇ skovi´ c Institute Zagreb, Croatia. 3 Dipartimento di Fisica, Universit` a di Padova, and INFN, Padova, Italy. 4 The Henryk Niewodniczanski Institute of Nuclear Physics, Krakow, Poland. 5 Instituto de F´ ısica Corpuscular, Valencia, Spain. 6 Dipartimento di Fisica, Universit` a di Torino and INFN, Torino, Italy. INTRODUCTION In nuclear structure studies the pairing interaction, responsible for the two-nucleon correlation, plays a fundamental role in defining the low-energy spectra of nuclei and the properties of their ground state [1, 2]. Two-nucleon transfer reactions are amongst the best tools to investigate such correlations, in particular, for the neutron-neutron case, measurement of the (p,t) and (t,p) reaction cross sections. Understanding whether and how pair correlations can be probed in heavy ion collisions was the purpose of different experiments performed in the past [3]. In these works the authors searched signatures of nucleon-nucleon correlations via the extraction of enhancement coefficients, defined as ratio of the actual transfer cross section to the prediction of models using uncorrelated states. In more recent years neutron-neutron correlations were investigated in the closed-shell 40 Ca+ 96 Zr [4] and superfluid 60 Ni+ 116 Sn [5] systems from above to below the Coulomb barrier. The data were represented via the transfer probability (P tr ), defined as the ratio of the transfer yield over the quasi-elastic one, plotted as a function of the distance of closest approach (D). The use of the large-acceptance spectrometer PRISMA, combined with the inverse kinematics condition, allowed one to measure P tr down to very large values of D, sufficiently far from the nuclear absorption region. In the well Q-value matched system 60 Ni+ 116 Sn, with the ground-to-ground-state Q values for one- and two-neutron transfers very close to the optimum, the microscopically calculated transfer probabilities for the (1n) and (2n) channels, which incorporate nucleon-nucleon pairing correlation, could reproduce the experimental ones in absolute value and slope (we refer to Refs. [4, 5] for details). An interesting and almost unexplored issue is whether and to what extent the effect of nucleon-nucleon correlations in the evolution of the reaction is modified in the presence of high Coulomb fields. In fact, in collisions amongst very heavy ions, population of final states with high excitation and angular momenta and/or multistep processes may significantly change the transfer strength of the ground-to-ground-state transitions [6]. To make a proper comparison with already studied systems, we chose the 118 Sn+ 206 Pb system, which is the heaviest (asymmetric) semi-magic combination with closed proton shells and open neutron shells, where the ground-to-ground-state Q values for neutron transfer are very close to the optimum (Q opt ∼ 0). We report in Table 1 their values for up to four neutron transfer channels. This system was earlier studied [7] by employing two parallel-plate detectors in coincidence with five Euroball clusters and the Heidelberg-Darmstadt crystal ball but the extracted P tr was limited to small values of D. Table 1. Ground-to-ground-state Q values for up to 4 neutron transfer channels in the 118 Sn+ 206 Pb system. Channel 1n ( 119 Sn) 2n ( 120 Sn) 3n ( 121 Sn) 4n ( 122 Sn) Q g.s. (MeV) -1.60 +0.77 -1.45 +0.44 In the present experiment, by employing the inverse kinematics condition, we could detect the “light” target-like ions in PRISMA with sufficient A, Z and Q-value resolution also at energies below the barrier and corresponding large D values. THE EXPERIMENT For the experiment we used a 2-pnA 206 Pb beam delivered by the PIAVE positive-ion injector followed by the ALPI post-accelerator of LNL, impinging onto a 200 μg/cm 2 (2-mm strip) 118 Sn target. We measured an excitation function at three different bombarding energies, E lab =1200, 1100 and 1035 MeV. For the highest energy PRISMA was initially placed close to the grazing angle at θ lab = 35 ◦ and then at 25 ◦ . Decreasing the bombarding energy the angle of PRISMA was kept fixed at 25 ◦ , since below the barrier the transfer angular distributions are peaked at forward angles in the laboratory frame (backward angles in the center-of-mass reference frame). As shown in Fig. 1, in this configuration target-like recoils entering PRISMA had sufficient kinetic
Transcript of Nucleon-Nucleon Pairing Correlations probed in the Sn+ Pb...
Nucleon-Nucleon Pairing Correlations probed in the 118Sn+206Pb Systemvia Transfer Reactions at far Sub-barrier Energies
F. Galtarossa1, L. Corradi1, S. Szilner2, E. Fioretto1, P. Colovic2, T. Mijatovic2, G. Colucci3, A. Goasduff1, A. Gottardo1,J. Grebosz4, A. Illana Sison1, G. Jaworski1, M. Jurado Gomez5, T. Marchi1, G. Montagnoli3, D. Nurkic2, G. Pollarolo6,
F. Scarlassara3, M. Siciliano1, N. Soic2, A. M. Stefanini1, D. Testov3, J. J. Valiente-Dobon1, N. Vukman2
1INFN, Laboratori Nazionali di Legnaro, Legnaro, Italy.2Ruder Boskovic Institute Zagreb, Croatia.
3Dipartimento di Fisica, Universita di Padova, and INFN, Padova, Italy.4The Henryk Niewodniczanski Institute of Nuclear Physics, Krakow, Poland.
5 Instituto de Fısica Corpuscular, Valencia, Spain.6 Dipartimento di Fisica, Universita di Torino and INFN, Torino, Italy.
INTRODUCTION
In nuclear structure studies the pairing interaction,responsible for the two-nucleon correlation, plays afundamental role in defining the low-energy spectra ofnuclei and the properties of their ground state [1, 2].Two-nucleon transfer reactions are amongst the best toolsto investigate such correlations, in particular, for theneutron-neutron case, measurement of the (p,t) and (t,p)reaction cross sections. Understanding whether and howpair correlations can be probed in heavy ion collisionswas the purpose of different experiments performed inthe past [3]. In these works the authors searchedsignatures of nucleon-nucleon correlations via the extractionof enhancement coefficients, defined as ratio of the actualtransfer cross section to the prediction of models usinguncorrelated states. In more recent years neutron-neutroncorrelations were investigated in the closed-shell 40Ca+96Zr[4] and superfluid 60Ni+116Sn [5] systems from above tobelow the Coulomb barrier. The data were represented viathe transfer probability (Ptr), defined as the ratio of thetransfer yield over the quasi-elastic one, plotted as a functionof the distance of closest approach (D). The use of thelarge-acceptance spectrometer PRISMA, combined with theinverse kinematics condition, allowed one to measure Ptrdown to very large values of D, sufficiently far from thenuclear absorption region. In the well Q-value matchedsystem 60Ni+116Sn, with the ground-to-ground-state Qvalues for one- and two-neutron transfers very closeto the optimum, the microscopically calculated transferprobabilities for the (1n) and (2n) channels, whichincorporate nucleon-nucleon pairing correlation, couldreproduce the experimental ones in absolute value and slope(we refer to Refs. [4, 5] for details).
An interesting and almost unexplored issue is whether andto what extent the effect of nucleon-nucleon correlations inthe evolution of the reaction is modified in the presenceof high Coulomb fields. In fact, in collisions amongstvery heavy ions, population of final states with highexcitation and angular momenta and/or multistep processesmay significantly change the transfer strength of the
ground-to-ground-state transitions [6].To make a proper comparison with already studied
systems, we chose the 118Sn+206Pb system, which isthe heaviest (asymmetric) semi-magic combination withclosed proton shells and open neutron shells, where theground-to-ground-state Q values for neutron transfer arevery close to the optimum (Qopt ∼ 0). We report in Table 1their values for up to four neutron transfer channels.
This system was earlier studied [7] by employing twoparallel-plate detectors in coincidence with five Euroballclusters and the Heidelberg-Darmstadt crystal ball but theextracted Ptr was limited to small values of D.
Table 1. Ground-to-ground-state Q values for up to 4 neutrontransfer channels in the 118Sn+206Pb system.
Channel 1n (119Sn) 2n (120Sn) 3n (121Sn) 4n (122Sn)Qg.s. (MeV) -1.60 +0.77 -1.45 +0.44
In the present experiment, by employing the inversekinematics condition, we could detect the “light” target-likeions in PRISMA with sufficient A, Z and Q-value resolutionalso at energies below the barrier and corresponding large Dvalues.
THE EXPERIMENT
For the experiment we used a 2-pnA 206Pb beam deliveredby the PIAVE positive-ion injector followed by the ALPIpost-accelerator of LNL, impinging onto a 200 µg/cm2
(2-mm strip) 118Sn target. We measured an excitationfunction at three different bombarding energies, Elab=1200,1100 and 1035 MeV. For the highest energy PRISMA wasinitially placed close to the grazing angle at θlab = 35◦ andthen at 25◦. Decreasing the bombarding energy the angle ofPRISMA was kept fixed at 25◦, since below the barrier thetransfer angular distributions are peaked at forward angles inthe laboratory frame (backward angles in the center-of-massreference frame). As shown in Fig. 1, in this configurationtarget-like recoils entering PRISMA had sufficient kinetic
Fig. 1. Kinematics of the reaction for the present experimentat Elab=1200 MeV. The yellow band represents the PRISMAacceptance.
energies to be detected with good resolution, also at lowbombarding energies. The kinematics of the reaction allowsalso part of the beamlike ions to enter the spectrometer,being the limiting angle for Pb close to 35◦. Inside thesliding seal scattering chamber two monitors of siliconsurface barrier type were placed at θlab = 48◦ and 58◦.The monitors were used to detect Rutherford scattered Snions for relative normalization between different runs and tocontrol the beam conditions during the measurements.
Figures 2 and 3 show the matrix of the energy lost bythe detected ions in the first two sections of the PRISMAionization chamber, ∆EIC, versus their total energy EICfor the present experiment for two different bombardingenergies, 1200 MeV and 1100 MeV, respectively, withPRISMA positioned at the same angle θlab = 25◦. At thehigher energy (Fig. 2) many proton transfer channels arepresent and quite easily identified, at the lower bombardingenergy (Fig. 3) their yield rapidly drops off and theiridentification becomes progressively more difficult. In bothmatrixes a significant yield corresponding to fission eventsis visible.
In the analysis, which is still at a very preliminarystage, one will selectively gate on the event distributionscorresponding to the different nuclear charges and constructthe mass spectra for the various isotopes at the measuredbombarding energies and angular settings. This will allow
to measure the evolution of the transfer probability as a
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Fig. 2. Matrix of ∆EIC versus energy EIC of the PRISMA ionizationchamber for the 118Sn+206Pb reaction at Elab=1200 MeV andθlab=25◦.
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Fig. 3. Matrix of ∆EIC versus energy EIC of the PRISMA ionizationchamber for the 118Sn+206Pb reaction at Elab=1100 MeV andθlab=25◦.
function of the distance of closest approach. Particularlyinteresting will be the case of Sn isotopes for the study ofneutron-neutron correlations.
[1] D. M. Brink and R. A. Broglia, Nuclear Superfluidity: Pairingin Finite Systems (Cambridge University Press, 2005).
[2] R. A. Broglia and V. Zelevinsky, Fifty Years of Nuclear BCS -Pairing in Finite Systems (World Scientific, 2013).
[3] W. von Oertzen et al., Z. Phys. A 326, (1987) 463.[4] L. Corradi et al, Phys. Rev. C 84, (2011) 034603.[5] D. Montanari et al., Phys. Rev. Lett. 113, (2014) 052501.[6] L. Corradi and S. Szilner, Proposal to the LNL PAC (2015).[7] W. von Oertzen et al., Eur. Phys. J. A 20, (2004) 153.
