High energy-resolution experiments with the K600 magnetic spectrometer at intermediate energies

Transcription

1 High energy-resolution experiments with the K600 magnetic spectrometer at intermediate energies Iyabo Usman ithemba Laboratory for Accelerator Based Sciences, South Africa On behalf of itl/wits/uct/rcnp/ikp-tu-darmstadt K600 Group *Supported by the South African NRF and the German DFG under contracts SFB 634, NE 679/2-2 Symposium on Exciting Physics: November 2011

2 Outline K600 magnetic spectrometer Giant Resonances High energy-resolution experiments on giant resonances: => ISGQR and IVGDR Using Light-ion inelastic scattering at finite angles and zero-degree => (p,p ) and (alpha,alpha ) Extraction of energy scales => Wavelet Analysis Comparison with theoretical calculations Extraction of spin and parity dependent Level Densities Experiments on Cluster states: Reaction channels => (p,t) and (p, 3 He)

3 ithemba LABS Cyclotron Facility Polarized ion source Separated-Sector Cyclotron Facility SSC Target vaults Spectrometer SPC2 ECR ion source SPC1 electronics electronics Radioisotope production Proton therapy Neutron therapy beam swinger m

4 K600 Magnetic Spectrometer Dipole magnet 1 Drift Chambers Scattering chamber Plastic scintillators Best resolution = MeV on Au target Best resolution = 9 66 MeV on Pb target Largest angle so far = 87 o Smallest angle = 21 o with external beamstop Smallest angle = 7 o with internal beamstop Zero-degree facility 2 VDC: high accuracy horizontal position determination in focal plane 1 HDC: high accuracy vertical position determination 2 plastic scintillators (BC-408) acting as trigger detectors for particle identification Contact person: neveling@tlabs.ac.za

5 K600 magnetic spectrometer at 0 o Focal plane detectors: 2 multiwire drift chambers (U and X wireplanes) 2 plastic scintillators (p,p') setup: B(D1)/B(D2)=1.5 (p,t) setup: B(D1)/B(D2) = 1 beam of 1 na: 6x10 9 particles/s scattered particles: 10 3 particles/s

6 Giant Resonances Isoscalar Isovector Monopole L = 0 Dipole L = 1 Quadrupole L = 2 Courtesy of P. Adrich T = 0 S = 0 T = 1 S = 0

7 Excitation and Decay of Giant Resonances Γ Γ Γ Direct decay Pre-equilibrium and statistical decay Γ = Γ + Γ + Γ Resonance width Landau damping Escape width Spreading width

8 Fine Structure of Giant Resonances High energy-resolution is crucial Possible probes: electron and hadron scattering

9 Recent high energy-resolution resolution giant resonance experiments at 200 MeV ISGQR: (p,p ) applying dispersion-matching techniques Targets: 58 Ni, 89 Y, 90 Zr, 120 Sn, 166 Er, 208 Pb E = kev ( ) 12 C, 27 Al, 28 Si, 40 Ca E = kev (2007) 142,144,146,148,150 Nd E = kev (2010) Scattering angles: Maximum of L = 2, finite scattering angles IVGDR: (p,p ) Targets: 27 Al, 40 Ca, 56 Fe, 58 Ni, 208 Pb E = kev (2009) 142,144,146,148,150 Nd (Dec. 2011) Scattering angles: Maximum of L = 1, zero-degree measurements

10 Fine structure of the ISGQR Results for Finite angle measurements Excitation energy spectra at angles corresponding to the maximum of the ISGQR It should in particular be noticed that as target mass increases the ISGQR becomes more compact and moves to lower excitation energy

11 Spectra of the (p,p ) reaction at E p = 200 MeV on 58 Ni, 90 Zr, 120 Sn, and 208 Pb with a typical energy resolution E 40 kev FWHM. The scattering angles were chosen to enhance the excitation of the ISGQR.

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13 Comparison of an earlier (p,p ) Experiment on the ISGQR in 90 Zr by Bertrand et al. with high energyresolution data at ithemba LABS under similar kinematics. The resolution of about 1 MeV is insufficient to observe any detailed structure. A double-hump structure deviating from the typical assumption of a single Lorentzian.

14 What is the origin of scales in 40 Ca? The RPA model accounts for Landau damping, which plays an important role in the case of 40 Ca (but not in heavier nuclei).

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16 Fine structure of the IVGDR Counts/10keV Counts/10keV Counts/10keV Al(p,p ) at E =200 MeV p o o 0 θ scat nat 600 Ca(p,p ) at E =200 MeV p o o 0 θ scat IVGDR IVGDR 56 Fe(p,p ) at E =200 MeV p o o 0 θ scat 1.91 IVGDR Results for Zero-degree measurements Energy spectra for inelastic proton scattering at zero degrees from 27 Al, nat Ca, 56 Fe and 208 Pb at E p = 200 MeV. Fine structures in the region of the IVGDR are clearly visible. Counts/10keV IVGDR 208 Pb(p,p ) at E =200 MeV p o o 0 θ scat (MeV) E x

17 Fine Structure of Giant Resonances Global phenomenon have been established in - other nuclei - other resonances Dominant damping mechanisms? Spin- and parity-resolved level densities?

18 Wavelet Analysis

19 Wavelet Analysis

20 Wavelet Analysis

21 Wavelet Analysis

22 Characteristic energy scales Wavelet analysis : powerful tool to extract the magnitude and localization of characteristic scales ISGQR is not mainly concentrated in a well- defined peak in light nuclei but completely fragmented e.g. 40 Ca, 28 Si & 12 C There exists a broad peak in medium and heavy nuclei e.g. 208 Pb, 120 Sn, 90 Zr & 58 Ni Characteristic scales are defined by the collective damping mechanism Excitation Energy (MeV)

23 Evidence of probe independent Darmstadt 1980 s E= 50 kev IUCF K E= 50 kev itl K E< 40 kev Proton scattering data show excellent agreement on a peak-to-peak basis

24 Comparison with theoretical calculations To understand the origin and physical nature of different scales, comparison of experimental results with model calculations is important. Such models include - Quasi-particle Phonon Model (QPM) - Random Phase Approximation (RPA) - Second-RPA (SRPA) - Extended Theory of Finite Fermi Systems (ETFFS) - Extended Time Dependent Hartree-Fock (ETDHF). Characteristic energy scales extracted from experimental spectra could be related to those determined from microscopic model calculations including two-particle two-hole (2p-2h) degrees of freedom.

25 variety of microscopic calculations including 2p2h states at different levels of approximation and using different effective interactions are available. The importance of complex degrees of freedom is stressed by the fact that random phase approximation (RPA) calculations of the ISGQR, i.e, restriction to 1p1h states does not produce any scales in the wavelet analysis at all. A comparison of ISGQR strength functions calculated with SRPA [8], quasiparticle-phonon model (QPM), extended time-dependent Hartee-Fock (ETDHF) [11] and a continuum RPA including 1p-1h phonon configurations [5] to the data is presented in Fig. 3. MICROSCOPIC CALCULATIONS USING 2p-2h 2h STATES, DIFFERENT LEVELS OF APPROXIMATION AND EFFECTIVE INTERACTIONS > Second Random Phase Approximation > Quasi-Particle Phonon Model Best reproduces scales in experimental data > Extended Time-Dependent Hartree-Fock FIG. 3: Comparison of the experimental ISGQR in 208 Pb to microscopic calculations including the coupling to complex configurations: SRPA [8], QPM (see text), ETDHF [11], and CRPA plus 1p-1h phonon configurations [5]. The predictions differ substantially, in particular with respect to higher-lying strength around 14 MeV suggested by the experimental results. However, when analyzed in the framework described above characteristic scales are identified in all cases. A summary of the results is presented in Tab. I. Qualitatively, the features observed in the data are > Continuum RPA + 1p1h x phonon configurations. 5

26 Smoothed E2 Strength Function (a.u.) 208 Pb(p,p ) E o = 200 MeV = 8 o QPM QPM coll. QPM non-coll. Excitation Energy (MeV) Wavelet Power (a.u.) 208 Pb(p,p ) E o = 200 MeV = 8 o QPM QPM coll. QPM non-coll. Wavelet Scale (MeV) FIG. 4: Left: Decomposition of the QPM E2 strength function in 208 Pb into a collective part due to the coupling to low-lying surface vibrations and an non-collective part due to the coupling to the sea of 2p2h states. Right: Corresponding power spectra compared to experiment. The dashed line shows the distribution obtained for the stochastic coupling model described in the text. To conclude, high-resolution proton scattering data on nuclei covering a wide mass range establish the fine structure of the ISGQR, first observed in 208 Pb, as a global phenomenon. A novel method based on wavelet transforms is presented which allows the extraction of scales characterizing the fine structure. These are signatures of the coupling of the collective 1p1h state to low-lying surface vibrations. Thus, the present results provide direct experimental evidence for the doorway mechanism [1] being the dominant contribution - at least in heavy nuclei - to the spreading width. On the other hand, the coupling to the background of complex states leads to a characteristic pattern in the wavelet analysis which is also found in the data. Despite the successful qualitative analysis, problems remain for a quantitative interpretation. A theoretical understanding of the strong model dependence of the predicted scales summarized in Tab. I must be achieved. Another open question is the role of the escape width, whose contributions become important for lighter nuclei. Experimental evidence for scales induced by the next levels of complexity in the hierarchical coupling scheme would be 8 Energy Scales Experimental Class I: 120 kev Class II: 440 and 850 kev Quasi-Particle Phonon Model (QPM) Class I: 115 kev Class II: 510 and 820 kev QPM Collective Coupling low-lying lying surface vibrations. Class I: 115 kev Class II: 510 and 820 kev QPM non-collective Mixing initial 1p1h states with background of many-particle many-hole states A. Shevchenko et. Al. Phys. Rev. Lett. 93, (2004)

27 Extraction of spin- and paritydependent Level densities Level densities of J π = 2 + states extracted from high energy-resolution resolution (p,p') experiments in the region above particle thresholds Continuum background determination using Quasi- free calculations and model-independent method of Discrete Wavelet Transform (DWT) Self consistent procedure to extract level densities based on fluctuation analysis and Autocorrelation functions Level densities as important ingredients use as input for calculations in nuclear astrophysics

28 Level density result Fluctuation Analysis Autocorrelation Experimentally extracted 2 + level densities are in agreement with model calculations. Usman et.al.,accepted accepted for publication in Phys. Rev. C.

29 Experiments on cluster states of 12 C

30 Hoyle state (0 + 2 state of 12 C) α condensate state Importance for 12 C synthesis in stars Hoyle : observed abundance requires accelerating mechanism, J π =0 + (E x =7.65 MeV) excited state in 12 C close to the threshold for 8 Be + 4 He fusion. Predict that Hoyle-like states very likely in low-density states in heavier n-alpha nuclei.

31 Identification of the 2 + excitation of the 12 C Hoyle-state M.Itoh et.al., Submitted to Nature October 2011

32 More experiments Discovering Giant Pairing Vibrations with (p,t) reactions : E. Khan Resonant states in 30 S, 34 Ar and 38 Ca nuclei using the (p,t) reaction and reaction rates in the rp process: G. Berg 58 Ni(p, 3 He) 56 Co experiment using 120 MeV polarized proton with spectrometer angles between 25 o to 60 o : J. J. Van Zyl

33 Further into the Future Spectrometer Plus HPGe Detectors Pygmy Dipole Resonance Experiments relating to Astrophysical applications Forward angle measurements 2 o k600 6 o Mixed-Symmetry states

34 K600 Collaboration ithemba LABS R. Neveling F. D. Smit I. Usman Z. Buthelezi S.V. Förtsch G. Steyn J. Mira C. Swartz F. Nemulodi TU Darmstadt University of the Witwatersrand J. Carter G.R. Cooper E. Sideras-Haddad M. Jingo O. Kureba Stellenbosch University P. Papka J. Mabiala JJ. Van Zyl University of Birmingham M. Freer University of Notre Dame G. Berg University of Cape Town P. von Neumann-Cosel V.Yu. Ponomarev A. Richter A. Shevchenko Y. Kalmykov J. Wambach I. Poltoratska A. M. Krumbholz RCNP/Osaka University H. Fujita T. Adachi Y. Fujita A. Tamii R.W. Fearick Thank You

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36 Ψ ( x) dx = 0 Ψ 1 = δ E ( x ) 2 dx ( δ E ) σ ( E ) C E Wavelet Analysis < Wavelet coefficients E E Ψ δ E x, x de Ψ(x) Ψ(x) x Biorthogonal Morlet x Ψ(x) Ψ(x) x Complex Lorentzian Complex Morlet x position scale spectrum wavelet Morlet: Complex Morlet: Complex Lorentzian: 2 1 x Ψ ( x) = 1 cos( ikx)exp π Ψ ( x) = exp(2 π if c ) exp π f b ( Γ 2) ( ) + 8 x Ψ ( x) = exp 2 2 n= 8 2 f ( x xo + nfc ) + ( Γ 2) 2 x f 2 b 2 b

37 Quasi-free calculations Code THREEDEE: Quasi-free continuum 40 Ca(p,2p) 39 K and 40 Ca(p,pn) 39 Ca Energy levels: 1d 3/2, 2s 1/2 and 1d 5/2

38 Discrete Wavelet Transform DWT background subtraction Decomposition into Approximations and Details Approximation level A10 chosen as background

39 Fluctuation Analysis Measure of cross section fluctuations with respect to a stationary mean value. Assumptions: Γ D E α = α + α w PT Procedure: Background subtraction from the experimental spectrum Smoothing by convolution with a Gaussian function of width larger than E g> ( E x ) Folding with a Gaussian function with a width smaller than E g ( E x ) g ( Ex ) Create a stationary spectrum d ( Ex ) = = g E Γ D E α Mean level width Mean level spacing Energy resolution Sum of normalised variances > ( ) x 1

40 Autocorrelation Function Mean level spacing proportional to the variance of Intensity fluctuations in d E x can be autocorrelated at energies E and E + ε d ( Ex ) d ( Ex + ε ) C ( ε ) = d E d E + ε D ( ) ( ) can be extracted from ε = energy increment C(ε) = Autocorrelation function x D = x 1 ρ ( ) d ( E ) x α D C f E 2 E π ( ε ) 1 = ( ε, )

41 Theoretical Models 1) Back-Shifted Fermi Gas (BSFG) ρ Phenomenological Approach Rauscher et al., Phys. Rev. C 56, 1613 (1997). T. Von Egidy et al., Phys. Rev. C 80, (2009). ( E, J ) with x x δ = ρ( E ) = = ( Z, N ) ( ) ( ) 2 2 exp 2 a E 1 x δ J J + 2σ 2J σ 12 2σ a δ Level density at energy E σ = spin cutoff parameter a = Level density parameter = 5.29MeV δ = Backshift energy = = Pairing energy = 3.63MeV 4.385MeV x ( E ) x 1

42 2) Hartree-Fock-Bogoliubov (HFB) Plus Combinatorial Microscopic Single-Particle Levels Approach S. Hilaire and S. Goriely, Nucl. Phys. A779, 63 (2006). S. Goriely et al., Phys. Rev. C 78, (2008). Using basic nuclear structure properties: Single-particle energies Pairing strength k i Quadrupole deformation parameter Deformation energy For spherical nuclei: For deformed nuclei: E def k ε i β 2 ( E, J, ) = ( E, M = J, ) ( E, M = J + 1, ) ρ π ρ π ρ π sph i i 1 ρdef π ρi rot π δ δ ρi rot π 2 k = J, k 0 J J, k J,0 ( E, J, ) = ( E E, K, ) + ( ) ( ) ( E E,0, Jeven π =+ ) J,0 ( ) ( ) i ( E Erot,0, Jodd π = ) + δ δ ρ π

43 10 data: Cluster state results from 12 C(α,α') Investigate 2 + excitation of the Hoyle statein 12 C with an alpha beam

44 Cluster states in in O with the (p,t) reaction Earlier measurements using (p,p ) or (p,t) reactions with K600: excellent resolution in the focal plane in (30-40 KeV). 0 mode: excellent selectivity of events with low angular momentum transfer na beam of 200 MeV 50 mm X 50 mm DSSSD s: 300 µm Targets composed of combinations of: 6 Li, 7 Li, 12 C, 16 O, 18 O Beam operated in momentum dispersion matched 18 O(p,t) 16 O[4α] reaction: Triton emitted with a max. cross section at 0 leaving breakup nucleus 16 O emitted at 180 w.r.t the beam with E = 4.4 MeV. Complete kinematic reconstruction of the reaction will be performed to reconstruct 16 O * decay channels (α+ 12 C, α+ 12 C *, p+ 15 N, 4α), branching ratios and widths calculated. Initial Measurement of 16 O energy spectrum: K600 events as master trigger

45 Preliminary Results Correlation of light charged particles with tritons 14 O * p+ 13 N binary breakup loci Measured triton energy spectrum Energy resolution of about 45 kev achieved!!!