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Terra incognita

Pair transfer – Direct reactions to probe pairing. D. Beaumel, IPN Orsay. Terra incognita. Pair transfer - link between reaction mechanism and structure - Experimental methodology with RIB Ongoing studies of n-p pairing at GANIL Ongoing studies of n-n pairing at GANIL

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Terra incognita

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  1. Pair transfer – Direct reactions to probe pairing D. Beaumel, IPN Orsay Terra incognita • Pair transfer - link between reaction mechanism and structure - Experimental methodology with RIB • Ongoing studies of n-p pairing at GANIL • Ongoing studies of n-n pairing at GANIL • Conclusion and Outlooks ESNT meeting, October 13-15th, 2008

  2. Probing pairing evolution through pair transfer • 2-neutron (and 2-proton) transfer have been used to probe pairing EX: Early work by Broglia, Hansen, Riedel, 1973 What are the dynamical implications of pairing correlations ? Similarity between pairing field and 2-body transfer operator → use (p,t) and (t,p) reactions (L=0,S=0,T=1 transfer) “pairing model” Shape deformations ↔ Pairing distortions • Pairing rotations and vibrations • Superfluid phase transitions • Particle-pairing coupling • GPV • … Evolution of pairing with Isospin ? • Experimental methodology using beams of exotic nuclei established • GANIL/SPIRAL2 beams well adapted for such studies

  3. a n1 n2 A A B : Distorted waves n1 b b n2 Often used 2-neutron direct transfer model: 1-step DWBA a + A → b + B B = A+n1+n2 a = b+n1+n2 b n1 n2 A Overlap function Range function

  4. A : Can be calculated in MF models To evaluate the overlap , one develops : A Spectroscopic amplitudes | B > = A [ |A> |n1,n2 >] • Unlike one-nucleon transfer case : • DWBA cross-section for 2n transfer involve coherent summation of • spectroscopic amplitudes • No access to definite 2n configuration • Measures the correlation aspects • Multistep effects usually non negligible We need microscopic form-factors ! Close collaboration with theorists important to analyze the data

  5. Einc = 20 MeV ds/dW (mb/sr) 90Zr(t,p)92Zr G.S. 122Sn(t,p)124Sn G.S. Ex=2.153 207Pb(t,p)209Pb QCM (deg.) Characteristics of (t,p) and (p,t) reactions First criterion for useful application of DWBA: Angular shapes well reproduced ! True for strong L=0 (and L=2) transitions • Absolute normalisation : • Not well under control ! • Over-simplified triton WF • use of zero-range approximation • too restricted configuration space for the FF Investigations of Pairing using this method should not rely only on absolute cross-sections • Optical potentials: • triton : elastic scattering data • proton : global formula (Perey)

  6. Exotic Beam + light target (p,d,a,..) • Detect the target- like ejectile (E,Q)LAB(E*, QCM ) light target SPECTRO elastic Beam trackers RI Beam Pickup reactions (p,t),(p,3He) (d,a),(a,6He) Stripping reactions (t,p),(a,d) • Good angular and E resolution • Nice angular distributions ! • Bound and unbound states on the same footing • Large solid angle • Highly segmented Detector array Experimental method using RIBs Other method: active target

  7. The Array Collaboration: IPN Orsay, SPhN/Saclay, GANIL • MUST2 : a major upgrade of previous MUST • 3-stage array • Increase angular coverage • Better efficiency • Measure several reactions in one shot • Increase granularity (multiparticle events) • New electronics to handle the increase of channels

  8. Collaboration: IPNO/SPhN-Saclay/GANIL CsI 4cm Si(Li) 5mm DSSD 10x10cm2 128X+128Y 300mm

  9. MUST2 electronics MUST2 ASIC SACLAY (+IPNO) • 16 channels 28 mW/ch • Energy & Time • Si, Si(Li) and CsI • Multiplexer • I2C interface VXI board (GANIL) 16 ADC14 bits 2.3K parameters 2MHz Slow Control I2C Pedestal substraction DNL correction VACUUM • High linear. pulser • T sensor AIR MOTHER BOARDS (IPNO) 4 analog bus Control signals I2C bus 4 telescopes 1 telescope

  10. AMELIORATIONS SOUHAITEES : • ANALYSE (SUITE) : MUST II 6 telescopes configuration for e.g. measurement of (p,t) and (p,p) reactions Analyse des spectres à poursuivre : 4n, 3n, 2n Autres données à etudier : 9He, 8He, 7He, 7H… = Informations sur interaction nn Intensité du faisceau (?) Cible mince deutérium pure (?)

  11. TIARA BT Det ExoGam VAMOS Today’s particle-gamma detection setup

  12. Neutron-proton pairing In nuclei : • 4 types of Cooper pairs T=0 n-p pairs → new phase of nuclear matter! • Extra binding for N~Z “Wigner energy” np additionnal correlations ? • largest when strong spatial correlation same valence shell Study N=Z nuclei • Several approaches: • Mass differences • Coriolis destruction of pairs • jp jn aligned in T=0 pairs • Deuteron-like transfer reaction : • enhanced transfer probabilities • 0+→ 1+ levels Reactions (p, 3He), (3He,p)DT=0,1 (d,a), (a,d)DT=0 (a, 6Li), (6Li,a)DT=0

  13. Ongoing n-p pairing studies at GANIL • First proposed experiment: 48Cr(p,3He) (d,a) • Experimental setup : MUST2 + EXOGAM + VAMOS • Unfortunately delayed due to SISSI device failure… • Spiral 2 will provide beams of N=Z nuclei with higher rate than SPIRAL • technique : • fusion-evaporation reaction with high intensity stable beams and thick target • re-acceleration using CIME • Plan to propose similar experiment on 28 < N=Z < 50 nuclei (ideally mid g9/2 shell) at SPIRAL2

  14. Neutron-neutron pairing Evolution when going far from stability ? • Possible transfer reactions • (p,t), (t,p) • (a, 6He) • (p,t) reaction recently studied using MUST2 for spectroscopy purpose at GANIL • 8He(p,t) X. Mougeot, PhD. Thesis (SphN CEA) • 14O(p,t) H. Iwasaki (IPNO/Koeln) et al. • No specific experiment on pairing • Development of cryogenic helium adapted to transfer reaction studies Could be used to study the (a,6He) reaction

  15. 2+ states 0+ states QCM (deg) QCM (deg) The (a,6He) reaction Could be complementary to (p,t) Surface-peaked reaction Probe pairing in neutron skin ? Already studied with stable beams 10 ~ 50 MeV/u energy Cross-section much lower than (p,t) Sequential transfer 208Pb(a,6He) at ~ 27 MeV/u

  16. Test experiment @ GANIL: 22Ne + a at 30 MeV/u 22Ne(a ,6He)20O 30 MeV/u 22Ne(a , 6Be)20O Using cryogenic He gas target made for missing mass measurements MUST II p p 20Ne 22Ne beam a 20O Cryogenic He target 6He Si annular telescope Ø 16 mm, 3mm thick Havar windows T = 8.5 K P = 1 bar

  17. 22Ne + aat 30 MeV/u

  18. RESULTS - 22Ne(a,6He)at 30 MeV/u Recoil PID Ejectile PID 22Ne 6He 15 hours counting ~ 106 pps beam 2-,3- 2+,3-,4+ 2+ Ex in 20Ne (MeV)

  19. Pairing vibrations Located near closed-shells Fluctuations of the pairing field → collective oscillations Basic modes : pair addition/removal phonons • Observables : • G.S. energies • G.S. → G.S. transfer s • G.S. → second 0+ transfer s Region around 208Pb (D=0) • Model predictions: • Harmonic spectrum • Stripping N0→N0+2→N0+4… have enhanced GS transitions with 1:2:3… ratios • Same for pickup • 2-phonon at 2xEg.s. • 2-phonon state with GS/P.V. intensity ratio ~ 1 Good agreement with data EXP Harmonic vib model Study near 132Sn at SPIRAL2

  20. Pairing rotations : case of Sn isotopes Sn : large D at mid-shell, strong pairing correlations → Static distortion of the pairing field I ↔ N → Strong transitions between members of superfluid band Comparison expnt / Pairing Model • Observables : • G.S. energies • G.S. → G.S. transfer s EXP • large GS→GS cross-sections • Energies well reproduced by linear + quadratic term • relative s in agreement with the model

  21. Conclusions/questions/outlooks • 2-nucleon transfer can be used to investigate pairingfar from stability • Variety of pairing-related phenomena in 2n transfer reactions • Sensitivity of the observables to pairing changes ? Ambiguities ? • Role of the coupling to continuum ? • Methodology for studying 2n transfer reactions established At SPIRAL2 • d-transfer from g9/2 shell for the study of p-n pairing • Dynamical aspects of nn pairing can be studied in new regions • Implementation of triton target (e.g. ti t2 at Munich)

  22. PROJECT for GAmma SPectroscopy and PArticle Detection Integrated 4p (particle) + 4p (g) • Energy resolution with SPIRAL2 beams Thicker targets • Multireaction capab. • Improve PID for low E particles • Modularity Coupling with other devices (neutron, AGATA,..)

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