Measurements of (p,n) and (n,p) reactions and planned studies of exothermic chargeexchange reactions with SHARAQ using unstable beams at RIBF

Transcription

1 ECT*, TRENTO, September 28, 2008 Measurements of (p,n) and (n,p) reactions and planned studies of exothermic chargeexchange reactions with SHARAQ using unstable beams at RIBF H. Sakai University of Tokyo

2 Contents 1. Brief introduction on spin-isospin physics and chargeexchange reactions 2. Nucleon charge-exchange reactions and GT transitions in continuum achievement reliability? 3. Comparison to ( 3 He,t) reaction 4. Exothermic charge-exchange reactions with unstable beams magnetic spectrometer SHARAQ for unstable beams experimental proposals (IVSM search, IVMR) 5. Summary

3 Spin-isospin responses and charge-exchange reactions Spin-isospin Responses Low q(~0 fm -1 ) Gamow-Teller GR Spin Monopole GR(?) Double GTGR(???) Medium q (0.5~1.0 fm -1 ) Spin-dipole GR(?) Spin-quadrupole GR(???) High q (~2m p ~1.3 fm -1 ) Pion Condensation :>r C Precursor : r 0 R L in QES region Charge-exchange reactions Nucleon probes (p,n) / (n,p) : DS=0 and 1 Nuclear probes ( 3 He,t) / (t, 3 He) ( 12 N, 12 C) / ( 12 B, 12 C) : DS=1 many many (HI,HI) Momentum transfer q plays decisive role. Probe choice Fermi-Gas Response + Nuclear correlation How reliable? examine by taking GTGR studies via nucleon probes.

4 Momentum transfer (q) and energy transfer (w) Projectile form factor (p,n) reaction : q changes moderately HICE reaction : q changes rapidly steep angular dependence recoilless condition can be achieved by an exothermic HICE reactions.

5 How reliable nucleon CHEX reactions are? GT strengths in the continuum (J p =1 + ) : need to identify DL=0 and DS=1 Multipole Decomposition Analysis (MDA) Extract DL=0 (interference of DL=2?) DS=1 (?) Proportionality relation s D ˆ L 0( 0 ) sgt F(q, w) B(GT ) sˆgt :GT unit cross section F( q, w ) :correctionfor ( q, w) dependenceof s Reliability depends critically on DWIA calculations! How well they describe the data? Distortion Vst/Vt ratio Tensor interaction V T Structure effects?

6 Measurement & analysis (GT in the continuum) s CHEX measurements DT z =-1 and +1 direction Choice of probe Multipole Decomposition Analysis(MDA) Systematic errors? IVSM proportionality 0 ) sˆ D L 0( GT F(q, w) B(GT ) reaction (thoery) DWIA (DW81) DWBA (FOLD) structure (thoery) Shell model, RPA, etc. Simple 1p-1h (cont.) ŝ GT F(q, w) OBTD needs calibration effective interaction Optical potential 6 absolute B(GT) Must be quantitative!

7 Best energy for the nucleon CHEX reactions 1. Effective interaction favors Spin-flip transitions over Non-Spin-flip ones ( t st / t t ) GT transitions are most clearly seen. 2. Distortion effects are smallest ( t 0 ). analysis with DWIA is reliable. T 3. Tensor interaction is smallest ( t t ). Proportionality relation may be reliable. At 300 MeV reaction mechanism becomes most simple. cross section strength Hope : MDA might work best at 300 MeV. tensor FraneyLove

8 Data and results are based on: 1 90 Zr(p,n) 90 Nb reactions by T. Wakasa et al. PRC 55, 2909 (1997) Zr(n,p) 90 Nb reactions by K. Yako et al. PL B615, 193 (2005) 3 systematic study of GT unit cross sections by M. Sasano PRC 79, (2009) Ca(p,n) 48 Sc and 48 Ti(n,p) 48 Sc reactions by K. Yako et al. PRL 103, (2009). 4 exercises by model calculations by M. Sasano and T. Wakasa

9 Importance of choice of T p Less BG in highly Ex at 300 MeV!

10 Relation between s 1+ and B(GT) s( 0 ) sˆ GT F(q, w) B(GT ) s ( q, w) sˆ s ( q, w) B( GT) GT F(q, w) B(GT) J p 1 J p 1 F(q,w): (q,w)dependence correction F( q, ŝ GT K( Tp, w) 1 ( 2 K T w) exp bq exp z ( w) K( Tp,0) 3 K( T K: Kinematical factor z(w): Polynominal function of w b: parameter for q-dependence 1 2 bq 3 x ( w) Comparison with DWIA x(w)+z(w)=a 1 w+a 2 w 2 +a 3 w 3 +a 4 w 4 : GT unit cross section Systematic study by Sasano, w) p exp x ( w) z ( w) p,0) ŝ GT Same is used for both (p,n) / (n,p). Is this valid?

11 Multipole Decomposition Analysis Extract s 1+ (DL=0) s exp ( c. m., w w0 ) a1s J p 1 a2s p a 0 3s p a 1 4s p J J J 2 a5s J p 3 a6s J p 4 Inputs s 1+ s 4- Obtained in DWIA calculations Outputs DL 0 DL 1 DL 2 DL 3 a 1 a 6 a 1 s 1 : L=0 strength How reliable is the DWIA cal. for the continuum? (transition to unbound state)

12 48 Ca(p,n) data

13 48 Ti(n,p) data angular range 0-12 deg energy resolution 1.2 MeV statistical accuracy 1--3% / 2MeV 1deg systematic uncertainty 4%

14 How well does DWIA reproduce data? Reproduce very well at least low Ex!

15 Decomposed spectra Sasano/Miki

16 B(GT +/- ) distribution MD analysis (p,n) : strength exists beyond GTGR (n,p) : peak at 3 MeV shoulder at 6 MeV bump(?) at 12 MeV IVSM? Integrated strengths (E x < 30 MeV) ΣB(GT - ) = 15.3±2.2 ΣB(GT + ) = 2.8±0.3 E x < 5 MeV consistent with ( 3 He,t ) & (d, 2 He) New IVSM? Contamination of IVSM? isovector spin monopole ΔS=1, Δ L = 0, 2ħω, O = r 2 στ contribution estimated by DWIA: 1.0 for (p,n), 0.9 for (n,p)

17 Contribution to M 2v At 8 MeV < E x < 15 MeV db(gt - )/de : large excess B(GT + ) might have significant contribution on M 2v. matrix element w/o sign : M 2 E db(gt ) / de E ( M i db(gt M f ) / 2 ) / de de Consequence on matrix element w/o sign, unknown. We must rely on theorists.

18 Uncertainties in GT strength by Yako 90Zr(p,n)/(n,p) cases Quenching value = S - (30.6) S + (3.0) S - - S + (27.6) Q (0.92) statistical systematic c.s. data unit c.s MDA* IVSM* * : estimated within the standard DWIA framework optical models: (contribution = 0.2 for S - MDA) phenomenological EDAIZr(Cooper), n-global(shen), folding RH-RIA(Horowitz), DParis(von Geramb) radial w.f.:(0.2) Woods-Saxon, Harmonic Oscillator, wine-bottle shapes effective NN interaction:(0.3) 325 MeV & 270 MeV Y. Yako et al. PL B615, 193 (2005) M. Sasano st al. PRC 79, (2009)

19 The burning question is the establishment of IVSM But still we need to worry about proportionality? unbound transition (shallow binding approx.)? L=0, L=2 and their interference effect? interference between GT and IVSM?

20 proportionality? s 0 ) sˆ D L 0( GT F(q, w) B(GT )

21 Some concern by Amos, Faessler and Rodin Proportionality ds d vs. B(GT) questioned. Exercised by using: 76 Ge-- 76 Se system QRPA calculation DWA

22 Some concern by Amos, Faessler and Rodin 76 Ge(p,n) 76 As : reasonable 76 Se(n,p) 76 As : deviate Proportionality?

23 Unit cross sections in QRPA Transition density: QRPA Amos, Faesler & Rodin, PRC76(2007) Reaction code: DW81 Sasano s calc. Confirmed: The proportionality in the (n,p) channel looks worse. but not so significant still! (p,n) (n,p)

24 Double different cross section (A.U.) unit cross section (A.U.) Proportionality test by shell model Dr. Sasano Exercise by using: 48 Ca-- 48 Ti system Shell model calculations Standard DWIA calc. 48 Ca(p,n) 48 Sc 48 Ti(n,p) 48 Sc Deviations are large for small B(GT) for both sides B(GT - ) B(GT + ) Average could work. exact averaged exact averaged Retain the proportionality (optimistic hope?!). E x (MeV) E x (MeV)

25 Proportionality test-2 by Sasano What is the reason of spread? full int. Exercise by using: 58 Ni system Shell model calculations Standard DWIA calc. with/without tensor interactions Deviations are mainly due to the tensor interactions. wo tensor int.

26 unbound transition (shallow binding approx.)?

27 DWIA cal. for continuum (exercise by Wakasa) continuum unbound transition density 12 C(p,n) 300 MeV Ex=15 MeV customary shallow binding approx. is used in the DWIA calc. (DW81).. Need to check the validity of this approx. crdw code by Ichimura et al. Phys.Rev. C 63, (2001). DWIA + continuum RPA under the orthogonality condition Shallow binding approx. seems to work well.

28 L=0, L=2 and their interference effect? 2 1 T( DL 0, DS 1) DL 2, DS 1) s T(

29 DWIA(crdw) MDA L=0, L=2 and their interference (exercise by Wakasa) MDA vs. DWIA for 208 Pb(p,n) 300 MeV L=0, L=2 and interference L=2 & interference IVSM q=0 J p =1 + response consists of GT + IVSM + interference.

30 L=0, L=2 and their interference (exercise by Wakasa) Subtle interference effect is seen. How large this affects B(GT) is not known.

31 interference between GT and IVSM? s 1 T(GT) T(IVSM) 2

32 GT, IVSM and interference (1) (exercise by Sasano) s s int T o w T 2Re T 2 w * 0 w 2cos T T 2 o w 2 w T 2 w R= pure GT pure IVSM interfere GT : 0s1 0s1 IVSM : 0s1 1s1 : constructive destructive R T 2 2 w / To w 2 constructive case MDA might work. destructive?

33 GT, IVSM and interference (2) (exercise by Sasano) (constructive + destructive)/2 R= pure GT pure IVSM con. + destr.

34 Summary for (p,n)/(n,p) and some DWIA exercises Experimentally (to identify GT states) Choose best bombarding energy (prob. 300 MeV) Proportionality : recover (?) for the continuum Poor DE helps? (average over DE) Remove structure dependence MDA Shallow binding treatment seems to work Exercise interference between L=0 and L=2 interference between GT and IVSM

35 Comparison to ( 3 He,t) reaction merit : high resolution! generally (p,n) spectrum =( 3 He,t) spectrum This was first pointed out by Sasano.

36 Comparison between (p,n) and ( 3 He,t) Sasano GT Ep=300 MeV IAS GT SDR Ep=200 MeV ( 3 He,t) > (p,n) Good by agreement 30 % questioned due to poor energy resolution

37 Difference between (p,n) & ( 3 He,t) on 58 Ni Sasano ± MeV 297MeV ( 3 He,t) > (p,n) by 40 % This discrepancy of B(GT) ratio might cast a serious problem on the proportionality of the (p,n) and/or (3He,t) reaction.

38 Break down of proportionality Cole, PRC 74, (2006) Zegers, PRL 99, (2007) (p,n) MeV Ratio Reference (3He,t) MeV/A Rapaport Sasano Rapaport Sasano Sasano Ratio Reference Fujita Break down of proportionality is obvious! B(GT) gs =0.16 They are partly due to interference between DL=0,DS=1 & DL=2,DS=1 (tensor-t int.) contribution from DL=2,DS=1 (st-int.) distortion Explains about half of discrepancy. To my knowledge, it is not yet undrstood.

39 Comparison between (p,n) and ( 3 He,t) Sasano 100 Mo SDR Good agreement up to GTGR region

40 Summary for (p,n) and comparison to ( 3 He,t) Generally good agreement up to GTGR peak in continuum spectrum But some discrete state breaks proportionality due to interference between DL=0,DS=1 & DL=2,DS=1 components (tensor-t interaction) and DL=2,DS=1 mediated via st - interaction ( 3 He,t) contains not only target DL=2 but also projectile DL=2 excitations. Hope such discrete state not used for A-dep. calibration meas. of ŝ GT Nucleon probe shows much less BG in highly Ex. Region (beyond GTGR peak)

41 4. Exothermic charge-exchange reactions with unstable beams magnetic spectrometer SHARAQ for unstable beams what is new?

42 New idea to use unstable nuclei Courtesy of Shimoura With stable beam With unstable beam w m m g 1 3 q Dm >> 0 EXOTHERMIC Photon Line Dm / g Dm < 0: endothermic T z RI beam is capable to probe the w > q region.

43 Requirement of exothermic CE reactions Energy resolution for spectroscopy DE ~500keV for A=12 E/A=300MeV particle 0.5MeV/(300 12)MeV = corresponds to momentum resolution of p/dp ~15000 Angular resolution momentum transfer resolution ex. 12 N, 300MeV/A 9GeV/c CRITERION: Dq < 0.1 fm -1 D ~ p(beam) Dq D < 2 mrad D = 1 mrad Intense unstable beams + Magnetic spectrometer RIBF at RIKEN + SHARAQ +Dispersion matching BL

44 What is SHARAQ? Spectroscopy with Highresolution Analyzer of Radio Active Quantum beams SHARAQ is a high resolution magnetic spectrometer for unstable (secondary) beam experiments. Installed at RIBF of RIKEN d,., 238 U 350MeV/nucl. RI beam by fragmentation Expected intensity ( 14 N 250 MeV/A) 12 N: cps/pna (max cps) 12 B: cps/pna (max cps)

45 SHARAQ Characteristic target Focal plane QQDQD type Maximum rigidity 6.8 Tm Dp/p = 1/14700 Dispersion matching (x ), ( ) Angular resolution < 1 mrad Solid angle 2.7 msr Rotatable (-2 to 15 degree) Weight 500 tons A/Z=3 220MeV/A beam-line SHARAQ

46 Dispersion matching beam line SHARAQ alone unable to achieve high resolution! Unstable beam : spread in energy and direction produced via projectile fragmentation ~ c/2 12 N projectile target 12 N gather only 12 N to be a beam To achieve high resolution, dispersion matching (lateral and angle) is necessary for beam line and SHARAQ. SRC FP detector SHARAQ Unstable beam production target SHARAQ target

47 Dispersion matching Image at SHARAQ focal plane with 14 N beam of 3.5GeV We have successfully demonstrated the dispersion matching technique both in lateral and angle!

48 Proposed experiments Measurement of the spin isovector monpole resonance via the (t, 3 He) reaction at 900 MeV (approved) Spokesperson: K. Miki Scheduled this November, 2009 Studies of isovector non-spin-flip monopole resonance via the super-allowed Fermi type charge exchange reaction Spokesperson: Y. Sasamoto (approved) Development of Exothermic charge-exchange reaction as a new spin-isospin probe and its application to the observation of the isovector spin monoploe resonance in 90 Nb Spokesperson: S. Noji (approved) Search for IVSM β + direction Nothing known! Search for IVM β - direction Search for IVSM β - direction

49 Merits and difficulties in exothermic CE reactions Merits q=small can be achieved. Excellent spin-isospin selection: S=1, T=1 Difficulties Small luminosity (due to secondary beam). Feeding into excited states of ejectile (Doppler shift). beta-decay in flight.

50 SHARAQ construction team UT(CNS+Physics) T. Uesaka, T. Kawabata, S. Michimasa, S. Ohta, A. Saito, K. Nakanishi, Y. Sasamoto, S. Noji, K. Miki, H. Tokieda, H. Miya, K. Kawase, S. Shimoura and H.S. RIKEN BIGRIPS team Abroad G.P. Berg(Notre Dam), A. Zeller(NSCL), R. Zegers(NSCL), D. Bazin(NSCL), P. Chomaz(GANIL) +

51 Summary 1.Nucleon CE reactions and GT transitions in continuum achievement reliability shallow binding approx., DL=0, DL=2 and their interference, GT, IVSM and their interference comparison to (3e,t) 2. Exothermic CE reactions at SHARAQ magnetic spectrometer under commissioning, dispersion matching partly achieved 3. Remarks - SHARAQ is a versatile high resolution magnetic spectrometer. - Good for also missing mass spectroscopy. - Multipurpose usage is possible. - Any collaborations to make full use of SHARAQ are welcome. Thank you for your attention.