Isoscalar Spin Response in the Continuum Studied via the 12 C(d,d ) - - PowerPoint PPT Presentation

SQS04 symposium, Tokyo Tech., February 19-21 (2004)

Isoscalar Spin Response in the Continuum Studied via the 12C(d,d’) Reaction at 270 MeV

Y.Satou Tokyo Institute of Technology S.Ishida, H.Kato, H.Sakai, A.Tamii, K.Yako, M.Hatano, Y.Maeda University of Tokyo H.Okamura, T.Uesaka, K.Suda Saitama University N.Sakamoto, T.Ohnishi, K.Sekiguchi, T.Ichihara Institute of Physical and Chemical Research (RIKEN) T.Wakasa Research Center for Nuclear Physics

Introduction

• Polarization studies in the (p,p’) and (p,n) reactions

at intermediate energies (100 ～ 1000 MeV)

– Reaction mechanisms – Spin dependent modes of nuclear excitation via Snn (spin-flip probability)

Gamow-Teller (GT) resonance, spin-flip dipole resonance

• Spin-flip study in the (d,d’) reaction

– Selective excitation of isoscalar ΔT=0 transitions – Possible probe of double spin-flip ΔS=2 state

Isoscalar Spin-flip Transitions

・ Much less is known about this mode due to

• the lack of efficient probes
• the weakness of the effective interaction Vσ

・ Information on the isoscalar spin response is useful in elucidating

• the quenching mechanism of spin transitions
• the residual interaction in the isoscalar ΔT=0, spin ΔS=0 channel

Interaction V0 Vτ Vσ Vσ・τ Spin transfer ΔS=0 ΔS=1 Isospin transfer ΔT=0 ΔT=1 ΔT=0 ΔT=1 Resonance GMR(E0) GQR(E2) GDR(E1) IAS GTR, M1 SDR Reaction

(p,p’),(α,α’ ) (p,p’),(p,n ) (d,d’) (p,p’),(p,n)

?

Double Spin-flip Transitions

• Proposed double Gamow-Teller (DGT) ΔS=2 state

– P.Vogel et al., PLB212(88)259 – N.Auerbach et al., Ann. Phys. 192(89) 77

• Experimental information on the DGT strength distribution

– A better understanding of spin-isospin properties of nuclei – An excellent way to calibrate double beta decay matrix elements

Necessary ingredients in extracting the neutrino mass from the double beta decay life time measurements

Observable

Spin-flip probability: fractions of deuterons undergoing spin-flip by 1 and 2 units along an axis normal to the reaction plane (y-axis)

• Single spin-flip probability ΔＳｙ=1
• Double spin-flip probability

ΔＳｙ＝２

A: Analyzing power, P: Polarizing power, K: Polarization transfer coefficient To measure S1 and S2, vector and tensor polarized beams and a vector and tensor polarimeter are needed.

) 2 4 ( 9 1

' ' ' ' 1 y y yy yy y y

K A P S − − − =

) 9 2 2 4 ( 18 1

' ' ' ' ' 2 y y yy y y yy y y

K K A P S + − + + =

y-axis

Δsy=0 Δsy=1 Δsy=2

Objective

• To investigate isoscalar spin-flip transitions

using the deuteron spin-flip measurement as a probe

– focal plane deuteron polarimeter

Principle of the present scheme tested with the first polarization transfer measurement on 12C. New measurement performed up to around 40 MeV, in search for isoscalar single and double spin-flip strengths in highly excited continuum region Spin structure in 12C above 20 MeV in excitation energy

Y.Satou et al., PLB521(01)153

Experimental Condition

Facility RIKEN Accelerator Research Facility Beam Polarized deuteron, 270 MeV, 10 nA Target

12C, 87.2 mg/cm2

Observable dσ/dΩ, Ay, Ayy,Py’,Py’y’,Ky

y’,Ky y’y’,Kyy y’,Kyy y’y’

・Beam line polarimeter ・Ring Cyclotron (K=540) ・AVF Cyclotron (K=70) ・Spectrometer SMART

・Polarization: (py,pyy)=(0,0), (0,2),(2/3,0), (-1/3,1) ・Magnitude ～ 60–70% ・Θ Range: 2.5°～ 7.5 ° ・Ex range: 7～38 MeV

Spectrometer SMART

Deuteron POLarimeter DPOL

・Vector & tensor components

iT11, T20, T21, T22

・Enhanced double scattering efficiencies

12C(d,d0)～ 1.7 % 1H(d,2p) ～ 0.09 %

・Encompass the whole focal plane

80 cm × 16 cm

・Calibrated at Ed=210 ～ 270 MeV

d d p p

HOD CM Scatterer

(plastic + CH2) H : 0.538 g/cm2 C : 4.314 g/cm2

MWDC

1 m

MWDC Scatterer MWDC2 Hodoscope

d d p p

Vector

12C(d,d0)

Tensor

1H(d,2p)

φ θ

σ=σ0(1+2iT11・it11cosφ+T20・t20+ 2T21・t21cosφ+2T22・t22cos2φ)

Calibration Results 12C(d,d0)

270 MeV

・Large vector analyzing power iT11 ・Double scattering efficiency ～ 1.7 % ・Optical model works

250 MeV 230 MeV 210 MeV

σ iT11 T20 T21 T22

Calibration Results 1H(d,2p)

・Large tensor analyzing powers T20 , T22 , T21 ・Double scattering efficiency ～ 0.09 % ・Impulse approximation model works

270 MeV 250 MeV 230 MeV 210 MeV

σ iT11 T20 T21 T22

Excitation Energy Spectrum

12C

0+ 3- 1+ 2- 2-, 18.3 MeV 1+,12.71 MeV 3-, 9.64 MeV 0+, 7.65 MeV 2+, 4.44 MeV

Single Spin-Flip Spectrum

0+3- 1+ 2- ・In the spin-flip spectrum, the non-spin-flip 0+, 3- states vanish, while the ｓpin- flip 1+, 2- states remain. Usefulness of S1 in identifying spin transitions. ・A peak structure at 20.5 MeV （Γ～ 1 MeV）. An isoscalar spin-flip transition

? ?

1+ 2- 1+ 2-

σ S1 σ×S1

Spectra with a smaller angular bin size

・We observe the 20.5 MeV peak in each of the spectra. We can be confident on the presence of this peak. ・Previously reported only in one experiment. B.N.Johnson et al, PLC51(1995)1726

What is the spin-parity for the peak at 20.5 MeV ?

Comparison with Shell Model

1+ 2-

σ×S1

・Assumptions

• Cohen-Kurath w.f. for 1+

states

S.Cohen and D.Kurath, NP73(1965)1

• Millener-Kurath w.f. for 2-,

1-, 0- (L=1) states

D.J.Millener and D.Kurath, NPA255(1975)315

• Distorted wave impulse

approximation (DWIA) for reaction model ・For known 1+ and 2- states, agreement is good. ・Shell model predicts 2-(4th), 2-(5th), 1-(2nd), 0-(2nd) states at around 20 MeV.

Double Spin-Flip Spectrum

0+3- 1+ 2- ・Natural and unnatural parity transitions vanish in the double-spin-flip spectra. ・S2 values are close to zero

• ver the measured excitation

energy range up to 40 MeV. ・No indication of double spin

• flip states could be obtained.

σ S２ σ×S２

Summary

• We measured single and double spin-flip probabilities, S1 and S2,

in inelastic deuteron scattering on 12C at 270 MeV up to 40 MeV in excitation energy.

– a focal plane deuteron polarimeter, DPOL.

• In the spin-flip spectrum, an isoscalar spin-flip transition is

identified at 20.5 MeV (Γ～ 1 MeV）.

• Comparison with shell model suggests a possibility that this peak

consists of several transitions with different J π values, most likely 2-, 1- and 0-.

• Double spin-flip S2 values are close to zero; no indication of

double spin-flip states could be obtained up to 40 MeV.