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,..)