Charge Exchange and Weak Strength for Astrophysics

Transcription

1 Charge Exchange and Weak Strength for Astrophysics Sam Austin STANfest-July

2 Charge Exchange and Weak Strength for Astrophysics Interesting phenomena Electron capture strength (GT) (Langanke talk) Pre-supernova evolution (A 45-65) Core collapse (A ) Type Ia Supernovae Forbidden strength (L =1) Neutrino physics (Martinez-Pinedo talk) Role of Experiment Most WI strengths must come from nuclear structure models--many nuclei, excited state transitions important Principal role of experiment validate models used.

3 Weak Interactions in the Neutron Star Crust Explosive H-burning--neutron star surface. Deposits heavy ashes on surface Electron capture processes material to n-rich Si (14) Mg (12) Ne H,He Ar (18) S (16) Ca (20) Cr (24) STAN Ti (22) Fe (26) αp/rp αp/rpprocess process Ni (28) g/cm 3 ) 56 Ni Electron capture capture 10 9 g/cm 3 ) 34 Ne 56 Ar and so on Electron capture capture and and n-emission NSCL Reach Pyconuclear fusion fusion border known masses 68 Ca g/cm 3 )

4 Validation Via Hadronic Charge Exchange Why not study β decay? Most strength not accessible Principle of hadronic approach (p,n), (n,p),. operators similar to β decay operator B(GT) = σ CEX (q = 0)/σ unit σ unit depends on A, calibrated from known transitions Accuracy 5-10% (usually) For E t 120 MeV Seems similar for heavier probes, not fully established σuni t= Taddeuchi, NPA 469,125 Gamow Teller

5 Some Options Probes used Name,E-MeV/A Cases E (kev) Comments (n,p)-triumf 200 MeV ~10 ~1000 Well understood (d, 2 He)-KVI 85 MeV/A ~ Complex analysis, somewhat low E (t, 3 He)-NSCL 120 MeV/A ~ Simple analysis, calibration from ( 3 He,t) Comments: Program at TRIUMF has ended. Program at KVI continues, new program at NSCL.

6 Extant data-comparison with Shell Model Experimental data (n,p) measurements at TRIUMF MeV 58,60,62,64 Ni, 54,56 Fe, 51 V, 55 Mn, 59 Co Resolution 1MeV Compare to Shell Model Results fairly good, not perfect (?) New (d, 2 He) from KVI E= 85 MeV/Nucleon, good resolution (150 kev) Don t always agree with TRIUMF Caurier, et al. NPA 653, 439 (99)????

7 (d, 2 He) at KVI and TRIUMF (n,p) Hageman, et al., PLB 579, 251 (2004) (n,p) Beautiful high resolution data. Not consistent with (n,p). Not known why.

8 Results from KVI Frekers et al./martínez-pinedo, Langanke

9 Experimental situation: Establishing a Systematics Small set of experimental information, ~15 nuclei in the A ~ range, few heavier. Not all results consistent. Results in fairly good agreement with SM calculations, although there are exceptions. No data for radioactive nuclei, one for an odd-odd nucleus, 50 V, the only stable odd-odd. Model results Shell model in the A=45-65 range, effective interaction from fitting energy levels. Shell Model Monte Carlo + RPA for heavier nuclei, not yet compared to data.

10 What Do We Need to Do? Models: Would including experimental information on GT strengths in determination of V eff for SM improve the reliability of B(GT) calculations? Experiment: Data for stable nuclei-(d, 2 He) at KVI, ( 3 He,t) at MSU/NSCL, especially heavier nuclei. Data for radioactive nuclei: No stable target MUST use inverse kinematics--nscl Target (light nucleus) Heavy beam (A, Z) (A, Z-1) recoil (~few MeV)

11 (t, 3 He) Option For Stable Targets First expts Secondary triton beams 10 6 /sec at MSU/NSCL, 120 MeV/A tritons (Sherrill, et al.) Resol: 160 kev achieved Preliminary data on 58 Ni Near Future (Zegers, et al.) 10 7 /sec secondary beams CH2, 24 Mg, 50 V, 53 Cr, 74 Ge, 94 Mo,. Comments MeV MeV kev 230 kev Sherrill, et al 4.5 MeV MeV MeV MeV 1-12 C(t, 3 He) 12 B Θ lab =0 o 1.7 o 12 C(t, 3 He) 12 B Θ = o E(MeV) Pros: Unique beam-spectrometer(s800), simple analysis, calibration available from ( 3 He, t) reaction at Osaka. Con: More beam nice though not essential. C o u n t s lab o

12 Preliminary results on 58 Ni (Zegers, et al., NSCL) Expt l detail or two E t =336 MeV, I = 10 6 /sec. 24 hour run. (Future: >10x more beam) E triton = 3.4 MeV, dispersion matched spectrometer (S800) gives E x 0.25 MeV Multipole decomposition L=0 + L=1 + L 2 (t, 3 He) 336 MeV preliminary L>0 Agrees better with KVI than with TRIUMF (n,p)

13 Inverse Kinematics is Hard Why? Outgoing energy low, => targets extremely thin =>low rates. (d, 2 He) Option(R. Zegers, et al. MSU/NSCL) Detect two protons, remnant; complex detection, analysis Resolution perhaps 1 MeV. d-target 56 Ni 56 Co * S800 Spect. p p 2 He Monte-Carlo Simulations in progress charge-particle detector (HIRA?) need good energy and angle resolution!

14 The (p,n) Option Principle: Matrix elements for exciting T > states in (p,n) and EC are the same (within a Clebsch) (n,p) (p,n) Compare: (n,p) 200 MeV TRIUMF (p,n) 135 MeV MSU, Kent State, IUCF Find: The (p,n) and (n,p) results differ for 60 Ni. Recall earlier problem with 58 Ni

15 (p,n) Experiments Inverse Kinematics Must Use Inverse kinematics 1 H 1 H( 60 Co, n) 60 Ni E x (MeV) 60 Co 60 Ni Unusual kinematics Light particle has low E, few MeV, angle near 90 o. Lab angle => E x Lab E => Θ c.m. With sufficiently granular detectors E x ~ kev seems possible n Elab(MeV) θ lab (deg) Details depend on Q(g.s.), A

16 Nature of spectra: Possible Strategy for (p,n) Excitation of T <,T o, and T > in ratio 1:~1/T o :~1/T o 2 Spectra shown have poorer resolution than expected for inverse kinematics Directly observe main T > strength for N-Z<10(optimistic?) Model Tests: For larger N-Z, test model with T o and T < excitations Need to determine how well this constrains predictions of centroids for T >. T> To+T< T<

17 Role of the GT - Resonance Situation: Normally transitions from the Electron Capture giant resonance dominate in presupernova stars High ρ, T during collapse At very high ρ, T, GTGR thermally excited, EC from GTGR important (p,n) to the GTGR may provide the necessary information: Fuller, Aufderheide, Martínez- Pinedo GTGR (n,p) (p,n)

18 Centroids of Electron Capture Strength -Systematics SM calculations: A=45-65, 95 cases, three curves because of different pairing effects. Systematics of centroids RMS deviations O-O 0.7 MeV Odd 0.6 E-E 0.7 Energy in Daughter (MeV) Centroids of Electron Capture Strength--Langanke, Martínez-Pinedo, Nucl. Phys. A 673 (2000) odd-odd nuclei 10 odd mass nuclei even-even nuclei (N-Z)/A

19 Same, But Add Available Experimental Results SM calculations: A=45-65, 95 cases, three curves because of different pairing effects. Systematics of centroids RMS deviations O-O 0.7 MeV Odd 0.6 E-E 0.7 Adding expt l results Energy in Daughter (MeV) Centroids of Electron Capture Strength--Langanke, Martínez-Pinedo, Nucl. Phys. A 673 (2000) odd-odd odd-odd-kvi nuclei odd-mass KVI odd mass nuclei even-even KVI even-even nuclei odd-mass TRI even-tri (N-Z)/A

20 Some Questions Can a centroid systematics be established? Are larger deviations model issues or physics? Can one use measured GT strengths to improve the determination of V eff and hence to improve predictions of B(GT)? To my knowledge, determinations of the effective interaction for model predictions are now based on describing low lying level properties. Can measurements of (p,n) strength provide relevant information for calibrating the models used to describe presupernova evolution? Can measurements of the GTGR provide more directly the strengths needed for high ρ, T circumstances as in core collapse?