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Scissors Mode of Excited Nuclei from (n,γ) reaction

Scissors Mode of Excited Nuclei from (n,γ) reaction. Milan Krtička. Outline. Properties of the “ground-state” scissors mode Description of (n, g ) experiments (that allow us to study decays of excited nuclei) – Two-Step Cascade Measurement (using Ge detectors)

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Scissors Mode of Excited Nuclei from (n,γ) reaction

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  1. Scissors Mode of Excited Nuclei from (n,γ) reaction Milan Krtička

  2. Outline • Properties of the “ground-state” scissors mode • Description of (n,g) experiments (that allow us to study decays of excited nuclei)– • Two-Step Cascade Measurement (using Ge detectors) • Multistep Cascade Measurements (using 4p BaF2 detector arrays) • Data processing– statistical model simulations • Results/Conclusions – Scissors mode on excited states

  3. Resonance–like structure in magnetic orbital strength Discovered in (e,e’) experiment in 156Gd D. Bohle et al., Phys. Lett. B137 (1984) 27 Rich data on even-even nuclei measured in NRF experiments Strength concentrated near 3 MeV in several transitions (e-e nuclei) Total strength depends on the square of deformation For well-deformed e-e nuclei SB(M1)  3 mN2 Problems with the strength in odd nuclei due to very high fragmentation Scissors mode build on the ground state How is it on excited states? U. Kneissl, H.H. Pitz, A. Zilges, Prog. Part. Nucl. Phys. 37 (1996) 349

  4. There exists two experiments/reactions which were used to study the scissors mode on excited states Coincident measurement of products from (3He, 3Heg) or (3He, ag) reactions – the method is known as “Oslo method” A. Schiller et al., Phys. Lett. B 633 225 (2006), A. Schiller et al., Nucl. Instrum. Methods Phys. Res. A 447, 498 (2000), … Measurement of spectra g-ray spectra from (n,g) reaction Scissors mode build on excited states

  5. The method of two-step γ-cascades following TNC Geometry: Three-parametric, list-mode Data acquisition: - Energy Eg1 - Energy Eg2 - Detection-time difference

  6. Spectrum of energy sums Bn-Ef Energy sum Eγ1+ Eγ2 TSC spectra Accumulation of the TSC spectrum from, say, detector #1: The contents of the bin, belonging to the energy E1, is incremented by q =ij, where ijis given by the position and the size of the corresponding window in the 2D space “detection time”דenergy sum E1+E2”.  background-free spectrum i=-1 i=0 i=+1 j=-1 Detection-time difference Time response function j=0 j=+1 List-mode data

  7. Spectrum of energy sums Bn-Ef TSC spectrum - taken from only one of the detectors Energy sum Eγ1+ Eγ2 X 5 TSC spectra i=-1 i=0 i=+1 j=-1 Detection-time difference Time response function j=0 j=+1 List-mode data

  8. Measured with (a 4p) array of BaF2 detectors Installed at neutron TOF facilities DANCE @ LANSCE (160 crystals), n_TOF @ CERN (40 crystals), formerly FzK Karlsruhe (40 crystals) The method of multistep-step γ-cascades (MSCs)

  9. Eg1 Eg2 Eg3 Eg4 MSC spectra • We can gate on strong resonances possibly with different spins (DANCE, n_TOF) • unresolved resonance region (Karlsruhe)

  10. TSC and MSC spectra are compared with predictions based on simulations within Extreme statistical model The statistical model was incorporated in the Monte-Carlo code (DICEBOX) – all the fluctuations are taken into account during simulation of g decay Detector response must be applied to simulated cascades – GEANT simulations of 4p BaF2 arrays – simple in the case of TSC spectra – knowledge of detector efficiencies is sufficient Data processing

  11. Main assumptions: For nuclear levels below certain “critical energy” spin, parity and decay properties are known from experiments Energies, spins and parities of the remaining levels are assumed to be a random discretization of an a priori known level-density formula A partial radiation width if(XL), characterizing a decay of a level i to a level f, is a random realization of a chi-square-distributed quantity the expectation value of which is equal to f(XL)(Eγ) Eγ2L+1/(Ei), where f (XL) – g-ray strength functions – and ρ – level density – are also a priori known Selection rules governing the  decay are fully observed Any pair of partial radiation widths if(XL) is statistically uncorrelated Simulation of  cascades

  12. g-ray strength functions are needed not only for transitions to the ground state but also for transitions between excited states –Brink hypothesis (suggested/works for GDER) is usually adopted Is the Brink hypothesis a good approximation for scissors mode? Are the parameters of the scissors mode the same in even-even, odd, and odd-odd nuclei? f(E,T=0) (MeV-3) EGLO SR SF KMF+BA SP Models used - examples g-ray strength functions Level density

  13. Probability 6271 keV ½+ n 162Dy Probability SR 3/2- Eγ1 SR 251 keV 5/2+ G.s. 5/2- 163Dy Eγ1+Eγ2 = 2 ESR Eγ2 Sharpening the peak at the midpoint of the TSC spectrum for the 251 keV final level TSCs in the162Dy(n,γ)163Dy reaction M. Krticka et. al, PRL 92 (2004) 172501 A unique possibility of a sensitive test for presence of SRs built on the levels in the quasicontinuum

  14. TSCs in the162Dy(n,γ)163Dy reaction 1/2 + M1 - + E1 - E1-M1 & M1-E1 + E1-E1 & M1-M1

  15. TSCs in the162Dy(n,γ)163Dy reaction πf = + DICEBOX Simulations πf = - Corridors predicted by simulations reflect fluctuations involved in the decay (mainly Porter-Thomas fluctuations) Entire absence of SRs is assumed

  16. TSCs in the162Dy(n,γ)163Dy reaction A “pygmy E1 resonance” with energy of 3 MeV assumed to be built on all levels

  17. TSCs in the162Dy(n,γ)163Dy reaction SRs assumed to be built only on all levels below 2.5 MeV

  18. Sharpening the 3 MeV peak well reproduced plus a quantitative agreement between the predicted and simulated TSC spectra TSCs in the162Dy(n,γ)163Dy reaction Scissors resonances assumedto be built on all163Dy levels SB(M1)  6 mN2

  19. MSC spectra from the capture of keV n’s in162Dy SRs builton all 163Dy levels …only on levels with Ef <2.5 MeV En = 90-100 keV

  20. The same parameters of f (XL) (i.e. also parameters of the scissors mode) needed for nice reproduction of TSC spectra are used:Lorentzian with E = 3 MeV, G = 0.6 MeV, SB(M1)  6 mN2 MSC spectra from the capture of keV n’s in162Dy SRs builton all 163Dy levels En = 90-100 keV

  21. Scissors mode is build upon all (or at least majority of) level up to neutron separation energy and its parameters (energy, width, total strength) seem to be very stable with excitation energy Preliminary analysis of well-deformed odd Gd isotopes from DANCE measurement (spectra from isolated resonances) indicates the total strength similar to 163Dy, i.e. about 5-7 mN2, is required Similar strength seems to be needed for description of TSC spectra for odd-odd nucleus 160Tb - poster of Jiri Kroll For even-even Gd isotopes the scissors mode needed for reproduction of DANCE spectra is much weaker – at maximum SB(M1)  3 mN2, but we need additional “smooth” M1 strength Analysis of TSC spectra for even-even Gd isotopes should be available soon Additional results – rare-earth nuclei

  22. Additional results – actinides • Several actinides (235U, 237U, 239U, 242Am, 244Am) has been measured during past years at DANCE and n_TOF experiments; more measurements are planned • Experimental data from DANCE look very good and the analysis of the MSC spectra has just started • The analysis of n_TOF data is much more complicated due to the pile-up of detected events

  23. Thank … you for your attention! and my collaborators Prague – F. Bečvář, J. Kroll Karlsruhe– F. Käppeler, F. Voss, K. Wisshak, R. Reifarth DANCE – G.E. Mitchell, M. Jandel, U. Agvaanluvsan, T.A. Bredeweg, R. C. Haight, J. M. O’Donnell, R. S. Rundberg, J.L. Ullmann, D.J. Vieira, J.B. Wilhelmy, J.M. Wouters

  24. Spectra from unresolved region are very similar for different neutron energies in the range En = 10 – 100 keV Almost no contribution of p-wave resonances at the lowest neutron energies while significant contribution (more than 50%) for the highest energies MSC spectra from the capture of keV n’s in162Dy

  25. For even-even Gd isotopes the scissors mode needed for reproduction of MSC spectra (from DANCE) seems to be much weaker – at maximum SB(M1)  3 mN2, but we need additional “smooth” M1 strength Analysis of TSC spectra should be available soon Even-even rare-earth nuclei

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