Pairing studies by two-nucleon transfer

Transcription

1 Pairing studies by two-nucleon transfer D.Beaumel, IPN Orsay Introduction nn pairing study at and far from stability Case of np pairing Tools and experimental challenges the GASPARD-TRACE array 2nd International workshop and 12 th RIBF discussion on neutron-proton pairing Hong Kong, July 6-9,

2 Approach to pairing by 2 nucleon transfer TWO-NUCLEON TRANSFER IS A PROBE FOR PAIRING CORRELATION Early work by Broglia, Hansen, Riedel, Adv. Nucl. Phys. 6 (1973) 287 study of (p,t) and (t,p) cross-sections Importance of pairing for the cross-sections 2n transfer as a probe of dynamical implications of pairing correlation (pairing rotations and vibrations, particle-pairing vib coupling, etc ) Recent efforts to clarify the deep nature of the pairing interaction Attempts to try to constrain the pairing interaction from pairing vibration (E.Khan, M.Grasso, J.Margueron, PRC 80 (2009)) Pairing far from stability Importance for neutron-rich nuclei pairing at low densities NB: The methodology to study pair-transfer for both addition and removal modes is established Neutron stars

3 Pairing and neutron stars Pairing and neutron stars Composition of the crust: Lattice of nuclear clusters unbound superfluid neutrons Superfluid properties specific heat of unbound n Time evolution of the temperature inside the crust No pairing Weak pairing Strong pairing Atmosphere Outer Crus Inner Crust Outer Core Inner Core r [km] Cooling is much faster when the intensity of pairing is stronger M. Fortin, F. Grill, J.Margueron, D.Page, and N. Sandulescu, PRC 82 (2010)

4 SC Skyrme HFB + QRPA in the pp channel (0+ transitions) Our approach Use up-to-date structure calculations as input for cross-section calculations Form-Factors Excitation modes related to pair transfer are expected to be strongly sensitive to the specific features of the pairing interaction Von Oertzen and Vitturi, Rep. Progr. Pys. 64 (2001) Probe sensitivity of cross-sections to nature of pairing Compare stable and neutron-rich cases 2n transfer DWBA Cross-sections Zero-range, density dependent phenomenological pairing interaction is used: The x parameter allows to tune the surface/volume character Application: 124 Sn(p,t) 122 Sn and 136 Sn(p,t) 134 Sn E.Pllumbi, M.Grasso, D.Beaumel, E.Khan, J.Margueron and J.Van de Wiele, PRC 83 (2011)

5 Pairing interaction In Skyrme-HFB models, the surface/volume mixing can be tuned : Values for x : 0.35, 0.65 mixed interactions 1.0 surface interaction V 0 adjusted to 2-neutron Separation energies in Sn Can we constrain the value of x?

6 Form-factors Two cases for Tin isotopes: stable neutron-rich (surface and low-density pairing features enhanced) 124 Sn(p,t) 122 Sn 136 Sn(p,t) 134 Sn form-factors associated to two mixed interactions are quite similar and different from x=1 case

7 Cross-sections Zero-range DWBA using DWUCK4 code Optical potentials from global formulae 124 Sn(p,t) 122 Sn

8 Cross-sections 136 Sn(p,t) 134 Sn Differences are observed at large angles

9 Ratio of cross-sections 136 Sn(p,t) 134 Sn Ratio of gs gs and gs cross-sections Proton incident energy (MeV) Differences are observed for most peripheral collisions First indication of a measurable effect related to the nature of the pairing int. Need further investigations E.Pllumbi, M.Grasso, D.Beaumel, E.Khan, J.Margueron and J.Van de Wiele, PRC 83 (2011)

10 Case of neutron-proton pairing Efforts are ongoing at IPNO to develop an HFB code including np pairing (Y.Peng, M.Grasso) Response for S z =0 and S z =±1 Check ratios of cross-sections with different coupling constants g 0 & g 1 Need improve the reaction model Next step : QRPA METHODOLOGY np pair transfer on N=Z nuclei (rapid quenching of np pairs away from N=Z) REACTIONS: (p, 3 He), ( 3 He,p) T=0,1 (d,α), (α,d) T=0 (α, 6 Li), ( 6 Li,α) T=0 For A > 40, N=Z nuclei are unstable Inverse kinematics experiments

11 Inverse kinematics 48 Cr Beam θ CM = 30 θ CM = 10 θ CM= 20 θ CM = 10 θ CM = 20 Pickup reactions (p, 3 He) (d,α) Easier to perform Solid H targets Reaction products fwd Possibly more background Need large solid angle High resolution Stripping reactions: Helium targets (require cryogenics, limited solid angle, windows) 6 Li difficult to prepare Powerful device

12 GASPARD TRACE A new array for high resolution reaction studies AGATA collaboration meeting, Feb th,

13 Direct reactions A great tool to investigate Exotic Nuclei and astrophysics processes Case of transfer reactions: d transfer p transfer np pairing r-process n transfer rp process Haloes, Clusters Skin, Pygmys Shell Evolution Cluster transfer Terra incognita Pairing changes 2n transfer 1(2)-nucleon transfer Good energy regime : few MeV/u few tenths of MeV/u Methodology : Radioactive Ion Beam Light target (H,He ) Detect the recoil particle with high accuracy Silicon technology

14 Landscape of Si detectors for DR studies Light Beams Fission fragments ~ MUST coll.: IPN,CEA-Saclay, DAM Cooling Connectors MUST2 coll.: IPN,CEA-Saclay, GANIL GASPARD coll.: IPN, INFN, BARC Irfu,Huelva,STFC,Surrey,GANIL Detectors Preamplifiers Particle spectrocopy E x resolution: ~500keV Particle-Gamma Spectroscopy E x resol.: ~5keV (AGATA case)

15 A new Si array for reaction studies 4π, fully integrable in PARIS/AGATA/EXOGAM2 GASPARD-TRACE design Beam Layers of Silicon 300(500) µm DSSD pitch < 1mm 1(or 2) x [1.5 mm DSSD pitch~1mm] 2 main shapes : square & trapezoid, large area Option: Annular detectors Electronics : ~ channels (Digital) high transparency to γ-rays Big integration challenge Other features: State of the art for PID Pulse Shape Discrimination Special targets (pure H,D) Portable device

16 R&D on Pulse Shape Discrimination for light particles 7 Li+ 12 C 35MeV ALTO facility of IPNO M.Assié et al., EPJA 2015 Light particle discrimination : Z=1 : good FoM down to 2.5 MeV Z=2 : (preliminary) seems ok Best observables: Raw data : maximum of the currentsignal (Imax) Filtered data : Haar filter + Time over Threshold Bias of the detector : Option 1 : at depletion + Imax Option 2 : over-depleted + Haar-ToT Sampling rate of ADC: > 200 MHz Next: - separation for Z=2,3 - effect of radiation damage

17 Silicon developments Final design (approved) Silicon detectors plan (short term):.1 st layer (trapez.): 2 prototypes ordered (Micron) in 2013 (IPNO) 1 st layer(square) : 2 prototypes ordered (Micron) in 2014 (INFN-Padova).2 nd layer (thick square) & 2 nd layer (thick trapez): BARC-IPNO Specifications -large area, 6 wafers, ntd, 500um thick - 128X+128Y (pitch~700 um) - <100> random cut (8deg) -Thin frame / Kapton readout at 90deg /High density connectors Test bench Test of uniformity In resistivity Leakage current /strip, capacitance/strip, interstrip resistance Trapezoid under commissioning

18 Trapezoid DSSD prototype Received June 23, 2015

19 FEE architecture (preliminary)

20 ASIC internal architecture ONGOING DEVELOPMENTS

21 The CHyMENE H/D windowless target Cible d HYdrogène Mince pour l Etude des Noyaux Exotiques System providing continuous extrusion of 1 H or 2 H through a rectangular extruder nozzle defining the target-film thickness CHyMENE collaboration : Pure H or D film thickness 50~300 microns CEA/IRFU Saclay project coordinator: A. Gillibert CEA/DAM Bruyères IPN Orsay Grant from ANR ~550k Now being commissioned

22 Cooled Helium gas target Developed at IPNO/ Accelerator division Ø 16 mm, 3mm thick Havar windows, 3.8 microns T = 8.5 K P = 1 bar