Terra incognita

Pair transfer – Direct reactions to probe pairing

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

D. Beaumel, IPN Orsay

ESNT meeting, October 13-15th, 2008

2-neutron (and 2-proton) transfer have been used to probe pairingEX: 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

Probing pairing evolution through pair transfer

a + A → b + BB = A+n1+n2a = b+n1+n2

)()( , fi : Distorted waves

Overlap function

aVb )(

Range function

),( )( ),( ~ )()(* RaAVbBRdRdT ifDWBA

n1 n2

b

n1 n2

A B

a

A

b

A

n1

n2

b

R

Often used 2-neutron direct transfer model:1-step DWBA

AB

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

),,( 1 JT2A : Can be calculated in MF models

BTBJ

BNBM21JT21

2n1nATAJ

A2ATAJJT21

21B rrJTrr

''1

'',,, ),( . )( ),,( ),,(

A

ABTo evaluate the overlap , one develops :

Spectroscopic amplitudes

| B > = A [ |A> |n1,n2 >]

We need microscopic form-factors ! Close collaboration with theorists important to analyze the data

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

Characteristics of (t,p) and (p,t) reactions

Optical potentials:• triton : elastic scattering data• proton : global formula (Perey)

Investigations of Pairing using this method should not rely only on absolute cross-sections

Einc = 20 MeV

d/d(mb/sr)

CM (deg.)

90Zr(t,p)92Zr

122Sn(t,p)124Sn

Ex=2.153207Pb(t,p)209Pb

G.S.

G.S.

• Exotic Beam + light target (p,d,• Detect the target- like ejectile (E,)LAB (E*, CM )

lighttarget

Beam trackers SPECTRORI

Beam

Pickup reactions(p,t),(p,3He)(d,),(,6He)

Stripping reactions(t,p),(,d)

elastic

Experimental method using RIBs

Good angular and E resolution Nice angular distributions ! Bound and unbound states on the same footing

Large solid angle Highly segmented

Detector array

Other method: active target

The Array

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

Collaboration: IPN Orsay, SPhN/Saclay, GANIL

DSSD10x10cm2

128X+128Y300m

Collaboration: IPNO/SPhN-Saclay/GANIL

Si(Li) 5mmCsI 4cm

1 telescope 4 telescopes

4 analog bus

Control signals

I2C bus

MUST2 ASIC SACLAY (+IPNO) 16 channels 28 mW/ch Energy & Time Si, Si(Li) and CsI Multiplexer I2C interface

VA

CU

UM

AI

R

VXI board (GANIL)16 ADC14 bits2.3K parameters2MHzSlow Control I2CPedestal substractionDNL correction

High linear. pulser T sensor

MOTHER BOARDS (IPNO)

MUST2 electronics

• AMELIORATIONS SOUHAITEES : Intensité du faisceau (?)

Cible mince deutérium pure (?)

• ANALYSE (SUITE) : Analyse des spectres à poursuivre : 4n, 3n, 2n

Autres données à etudier :

9He, 8He, 7He, 7H…

= Informations sur interaction nn

MUST II

6 telescopes configurationfor e.g. measurement of (p,t) and (p,p) reactions

VAMOS

ExoGamExoGam

BT Det

TIARA

Today’s particle-gamma detection setup

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

Reactions(p, 3He), (3He,p) T=0,1(d,), (,d) T=0(, 6Li), (6Li,) T=0

• Deuteron-like transfer reaction : enhanced transfer probabilities 0+ → 1+ levels

Neutron-proton pairing

Ongoing n-p pairing studies at GANIL

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

First proposed experiment: 48Cr(p,3He) (d,) Experimental setup : MUST2 + EXOGAM + VAMOS Unfortunately delayed due to SISSI device failure…

Plan to propose similar experiment on 28 < N=Z < 50 nuclei (ideally mid g9/2 shell) at SPIRAL2

(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 (,6He) reaction

Evolution when going far from stability ?

Neutron-neutron pairing

Possible transfer reactions• (p,t), (t,p) • (, 6He)

0+ states

CM (deg)

2+ states

CM (deg)

208Pb(,6He) at ~ 27 MeV/u

The (,6He) reaction

Could be complementary to (p,t)

Surface-peaked reactionProbe pairing in neutron skin ?

Already studied with stable beams10 ~ 50 MeV/u energy

Cross-section much lower than (p,t)

Sequential transfer

p p 22Ne beam

CryogenicHe target

MUST II

Si annulartelescope

Test experiment @ GANIL: 22Ne + at 30 MeV/u

22Ne(,6He)20O 30 MeV/u22Ne(, 6Be)20O

Using cryogenic He gas target made for missing mass measurements

He 20O

20Ne

Ø 16 mm, 3mm thickHavar windowsT = 8.5 KP = 1 bar

22Ne + at 30 MeV/u

RESULTS - 22Ne(6He) at 30 MeV/u

Recoil PID Ejectile PID

2+

2-,3- 2+,3-,4+

Ex in 20Ne (MeV)

15 hours counting~ 106 pps beam

6He

22Ne

Located near closed-shellsFluctuations of the pairing field → collective oscillationsBasic modes : pair addition/removal phonons

Region around 208Pb (=0)

Pairing vibrations

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

Observables : G.S. energies G.S. → G.S. transfer G.S. → second 0+ transfer

Study near 132Sn at SPIRAL2

EXP

Harmonic vib model

large GS→GS cross-sections Energies well reproduced by linear + quadratic term relative in agreement with the model

Comparison expnt / Pairing Model

Sn : large at mid-shell, strong pairing correlations→ Static distortion of the pairing field

I ↔ N → Strong transitions between members of superfluid band

Pairing rotations : case of Sn isotopes

Observables : G.S. energies G.S. → G.S. transfer

EXP

Conclusions/questions/outlooks

2-nucleon transfer can be used to investigate pairing far 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)

GAmma SPectroscopy and PArticle DetectionPROJECT for

Integrated 4 (particle) + 4 ()

Energy resolution with SPIRAL2 beams Thicker targets

Multireaction capab.

Improve PID for low E particles

Modularity Coupling with other devices (neutron, AGATA,..)