New Phenomena in Gamma-Ray Strength Functions

Transcription

1 Text optional: Institutsname Prof. Dr. Hans Mustermann Mitglied der Leibniz-Gemeinschaft Ronald Schwengner Institut für Strahlenphysik New Phenomena in Gamma-Ray Strength Functions Ronald Schwengner Institute of Radiation Physics, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany Experiments: - Photon scattering using bremsstrahlung at γelbe and quasi-monoenergetic polarized γ rays at HIγS - Data analysis E1 strength in the pygmy region M1 strength in the spin-flip region Low-energy M1 strength

2 Photonuclear and radiative-capture reactions Excitation and deexcitation of the nucleus by γ rays: - Statistical models applied to describe reaction rates at high excitation energy and high level density. - γ-ray strength functions used to describe average transition strengths in a certain energy range. - Understanding of the properties of γ-ray strength functions attracts increasing interest. Application: - Photonuclear and radiative-capture reactions play a central role in the synthesis of the elements in various stellar environments. Cassiopeia A (gallery.spitzer.caltech.edu)

3 Effect of enhanced low-energy dipole strength in the r-process The nucleus goes red. Unexpected upbend of dipole strength toward low energy observed in Fe, Mo, Cd isotopes. Courtesy of M. Guttormsen. Ratios of Maxwellian-averaged (n,γ) reaction rates at T = 10 9 K for isotopic chains up to the neutron drip line using strength functions with and without low-energy upbend of dipole strength. A.C. Larsen, S. Goriely, PRC 82, (2010) Big influence of low-energy strength on neutron-capture rates of very neutron-rich nuclei in the astrophysical r-process.

4 Gamma-ray strength functions Gamma-ray strength functions describe average electromagnetic transition strengths in the quasicontinuum of nuclear states at high excitation energy: f fil (E γ ) = Γ fil ρ(e i,j i ) / E 2L+1 γ E γ = E i E f J i = 0,..., J max Photoabsorption: f L = σ γ / [(2J i +1)/(2J 0 +1) (π hc) 2 E 2L 1 γ ] E γ = E i J i = 1, (2) Brink-Axel hypothesis: The strength function does not depend on the excitation energy. The strength function for excitation is identical with the one for deexcitation.

5 The bremsstrahlung facility at the electron accelerator ELBE dipole quadrupole Accelerator parameters: Maximum electron energy: 18 MeV Maximum average current: 0.8 ma Micro-pulse rate: 13 MHz dipole quadrupoles Be window steerer electronbeam dump radiator dipole quartz window beam hardener collimator polarisation monitor Pb accelerator hall Pb HPGe detector Micro-pulse length: 5 ps target experimental cave photonbeamdump PE Pb 1 m R.S. et al., NIM A 555, 211 (2005) door

6 Detector setup at γelbe Ronald Schwengner Institut für Strahlenphysik

7 3 Ronald Schwengner Institut für Strahlenphysik Highest intensity (γ-rays/s/kev) γ-ray beam in the world Produces γ-rays by Compton backscattering inside the optical cavity of a storage-ring FEL Produces both linearly and circularly polarized beams Electron Accelerator Facility 180 MeV Linac pre-injector 180 MeV 1.2 GeV Booster Injector 250 MeV 1.2 GeV Storage Ring FELs: planar (linear pol.) and helical (circular pol.) For more details see: γ-ray beam parameters Values Energy MeV Linear & circular polarization > 95% Intensity with 5% E γ /E γ > 10 7 γ/s Courtesy of C. Howell (TUNL)

8 Flux spectra at γelbe and HIγS 400 Photon flux 300 Φ γ ((evs) 1 ) 200 γelbe HIγS E γ (kev)

9 Measurements at γelbe and HIγS Kr + steel ELBE E e = 11.2 MeV Counts steel x 2 Identification of transitions in 86 Kr by comparison of spectra measured with the filled and with the empty steel container HIγS E γ = 6.5 MeV E γ (kev) R.S. et al., PRC 87, (2013)

10 Unresolved strength in the quasicontinuum La(γ,γ ) E e kin = 11.5 MeV Pb(γ,γ ) E e kin = 17 MeV Counts / 10 kev atomic background experimental spectrum Counts / 1.6 kev experimental spectrum atomic background E γ E γ Experimental spectra (corrected for room background, detector response and efficiency) and simulated spectra of atomic background. 139 La data: A. Makinaga et al., PRC 82, (2010). 208 Pb data: R.S. et al., PRC 81, (2010).

11 Photon scattering - feeding and branching E e feeding Γ 0 Γ 1 Γ f E, x 0 Γ branching Photon flux HIγS γelbe E γ (kev)

12 Simulations of γ-ray cascades Monte Carlo: G. Rusev et al., PRC 77, (2008) Code γdex: G. Schramm et al., PRC 85, (2012) R. Massarczyk et al., PRC 86, (2012); PRC 87, (2013) Γ 0 Γ 1 Γ 2 Γ E J π Level scheme of J = J 0 ±1,...,5 states in 10 kev energy bins constructed by using: Constant-Temperature Model with level-density parameters from T. v. Egidy, D. Bucurescu, PRC 80, (2009). Parity distribution of level densities according to S.I.Al-Quraishi et al., PRC 67, (2003). Wigner level-spacing distributions. Partial decay widths calculated by using: Photon strength functions approximated by Lorentz curves (www-nds.iaea.org/ripl-2). Porter-Thomas distributions of decay widths. Subtraction of feeding intensities and correction of intensities of g.s. transitions with calculated branching ratios Γ 0 /Γ. Determination of the absorption cross section. 0

13 Simulations of γ-ray cascades Kr Ground state La Ground state I γ (arb. units) 0.5 Branching transitions transitions I γ (arb. units) 0.5 transitions Branching transitions E γ E γ Simulated intensity distribution of transitions depopulating levels in a 100 kev bin around 9 MeV. Subtraction of intensities of branching transitions. 86 Kr data: R.S. et al., PRC 87, (2013). 139 La data: A. Makinaga et al., PRC 82, (2010). Ronald Schwengner Institut für Strahlenphysik

14 Branching ratios in 136 Ba from measurements at HIγS Response-corrected spectra. Simulated spectra of γ rays scattered by atomic processes. Subtracted spectra contain bunches of transitions to the ground state and to excited states. Intensities of branching transitions to low-lying 2 + states deduced. R. Massarczyk et al., PRC 86, (2012).

15 Test of simulated branching ratios - measurements at HIγS b 0 b Xe 134 Xe E x Red diamonds: Branching ratios deduced from measurements with monoenergetic γ rays at HIγS ( E 200 kev). Black lines: Average branching ratios (solid) b 0 = Γ 0 /Γ versus the excitation energy as obtained from simulations of γ-ray cascades and their uncertainties (dashed). R. Massarczyk et al., PRC 90, (2014).

16 Absorption cross section in 86 Kr Kr (γ,γ ) data from γelbe R.S. et al., PRC 87, (2013) σ γ (mb) S n 0 (γ,γ ) peaks E x

17 Absorption cross section in 86 Kr Kr (γ,n) (γ,γ ) data from γelbe R.S. et al., PRC 87, (2013) σ γ (mb) (γ,n) data from HIγS R. Raut et al., PRL 111, (2013) 5 S n 0 (γ,γ ) peaks E x

18 Absorption cross section in 86 Kr Kr (γ,n) (γ,γ ) data from γelbe R.S. et al., PRC 87, (2013) σ γ (mb) (γ,n) data from HIγS R. Raut et al., PRL 111, (2013) 5 S n (γ,γ ) 0 (γ,γ ) peaks E x

19 Absorption cross section in 86 Kr Kr (γ,n) (γ,γ ) data from γelbe R.S. et al., PRC 87, (2013) σ γ (mb) (γ,γ ) corr (γ,n) data from HIγS R. Raut et al., PRL 111, (2013) 5 S n (γ,γ ) 0 (γ,γ ) peaks E x

20 Absorption cross section in 86 Kr 25 σ γ (mb) Kr extra Pygmy strength (γ,γ ) corr = (γ,n) TLO (γ,γ ) data from γelbe R.S. et al., PRC 87, (2013) (γ,n) data from HIγS R. Raut et al., PRL 111, (2013) TLO 5 S n (γ,γ ) 0 (γ,γ ) peaks A.R. Junghans et al., PLB 670, 200 (2008) E x

21 Influence of nuclear properties on the pygmy strength Summed dipole strength in stable even-mass xenon isotopes: 1.5 E x = 6 to 8 MeV 136 Ba Solid lines: QRPA calculations in a deformed basis Σ B(E1) ( e 2 fm 2 ) Mo 124 Xe 100 Mo 134 Xe 138 Ba 140 Ce 136 Xe Dashed lines: TLO with deformation Summed E1 strength increases with the neutron excess Sm 142 Nd N/Z R. Massarczyk et al., PRL 112, (2014) neutron excess

22 Influence of nuclear properties on the pygmy strength Summed dipole strength in stable even-mass xenon isotopes: 1.5 E x = 6 to 8 MeV 136 Ba Solid lines: QRPA calculations in a deformed basis Σ B(E1) ( e 2 fm 2 ) Ba Xe Mo Ce 136 Xe 124 Xe 100 Mo Dashed lines: TLO with deformation Minor influence of the deformation on the summed E1 strength Sm 142 Nd β 2 R. Massarczyk et al., PRL 112, (2014) quadrupole deformation

23 E1 strength in the PDR region and M1 strength in the spin-flip region E1/M1 E1/M1 σ γ (mb) E1 M1 E = 0.3 MeV 10 1 E = 0.3 MeV 128 Xe 134 Xe σ γ (mb) 10 0 E1 M E x E x Solid lines: RIPL. Dashed lines: E. Grosse et al., EPJA 53, 225 (2017). Crosses: deformed QRPA. E1/M1: strength from experiments at γelbe. E1 and M1: strengths from experiments at HIγS. Intensities include the quasicontinuum. R. Massarczyk et al., PRC 90, (2014)

24 Absorption cross section of 74 Ge Ge (γ,γ ) data (HZDR): R. Massarczyk et al. PRC 92, (2015) σ γ (mb) (γ,γ ) 12.5 MeV (γ,γ ) 7.5 MeV E x

25 Absorption cross section of 74 Ge Ge (γ,x) (γ,γ ) data (HZDR): R. Massarczyk et al. PRC 92, (2015) σ γ (mb) (γ,γ ) 12.5 MeV (γ,x) data: P. Carlos et al., NPA 258, 365 (1976) (γ,γ ) 7.5 MeV E x

26 Dipole strength functions in 74 Ge Ge (γ,γ ) data (HZDR): R. Massarczyk et al. PRC 92, (2015) f 1 (10 9 MeV 3 ) 10 1 (γ,γ ) 7 MeV (γ,γ ) 12 MeV E γ

27 Dipole strength functions in 74 Ge Ge ( 3 He, 3 He ) (γ,γ ) data (HZDR): R. Massarczyk et al. PRC 92, (2015) f 1 (10 9 MeV 3 ) 10 1 (γ,γ ) 7 MeV (γ,γ ) 12 MeV ( 3 He, 3 He ) data (OCL): T. Renstrøm et al. PRC 93, (2016) E γ

28 Dipole strength functions in 74 Ge Ge ( 3 He, 3 He ) (γ,γ ) data (HZDR): R. Massarczyk et al. PRC 92, (2015) f 1 (10 9 MeV 3 ) 10 1 = (γ,γ ) 7 MeV (γ,γ ) 12 MeV ( 3 He, 3 He ) data (OCL): T. Renstrøm et al. PRC 93, (2016) 10 0 Low-energy upbend E γ What is the origin of the upbend?

29 Calculations of M1 strength functions Can model calculations describe the low-energy upbend of dipole strength? Shell-model calculations: - 40 levels of each spin from 0 to 10 for each parity. - All possible transitions between these 440 states - about M1 transitions. - Average B(M1) values in bins of E γ. - M1 strength function. Codes: - NuShellX@MSU B.A. Brown and W.D.M. Rae, Nucl. Data Sheets 120, 115 (2014). - RITSSCHIL D. Zwarts, Comput. Phys. Commun. 38, 365 (1985).

30 Shell-model calculations of M1 strength functions Mo f 1 (10 9 MeV 3 ) E1 + M1 E1 ( 3 He, 3 He ) data M1 shell model (γ,n) data E γ ) -3 f M1 (MeV (a) Fe 3 56 ( He,αγ) Fe, Voinov et al. (2004) 56 (p,p'γ) Fe, Larsen et al. (2013) SM calc ) -3 f M1 (MeV (b) Fe 3 3 ( He, (p,p'γ) 57 He'γ) Fe, Voinov et al. (2004) 57 SM calc Fe, Larsen et al. (2013/2014) Low-energy enhancement of M1 radiation RITSSCHIL R.S., S. Frauendorf, A.C. Larsen PRL 111, (2013) NuShellX B.A. Brown, A.C. Larsen PRL 113, (2014) E γ

31 Average M1 strengths in Fe isotopes with N = 30 to ca48mh1, π = + J = 0 i to 10 i, i = 1 to ca48mh1, π = J = 0 i to 10 i, i = 1 to 40 B(M1) (µ N 2 ) Fe Fe Fe 68 Fe B(M1) (µ N 2 ) Fe 60 Fe 64 Fe 68 Fe E γ E γ Enhancement of M1 strength toward very low transition energy. Development of a bump around 3 MeV with increasing neutron number. R.S. et al., PRL 118, (2017)

32 Dipole strength functions in 151 Sm and 153 Sm ) -3 SF (MeV γ (p,d) Sm, present exp Sm(γ, n ), Filipescu Sm(γ, n ), Filipescu 149 Sm(n, γ ), Kopecky, E1 (p,d) data: 10 7 S n GDR A. Simon et al. PRC 93, (2016) ) (p,d) Sm, present exp. 152 Sm(γ, n ), Filipescu 154 Sm(γ, n ), Filipescu 149 Sm(n, γ ), Kopecky, E1 M1 First observation of upbend and scissors resonance in one nuclide. -3 γ SF (MeV 10 7 S n GDR Strength in the scissors region about three times that found in (γ, γ) experiments. M γ-ray energy E γ

33 Strength functions from shell-model calculations f 1 (10 9 MeV 3 ) M1 60 Fe M1 68 Fe E2 60 Fe (x10) E2 68 Fe (x10) E1 RIPL (10 9 MeV 3 ) f 2 E γ 2 Dipole strength function f 1 = 16π/9 ( hc) 3 B(M1) ρ(e x,j) ρ(e x,j) - level density of the shell-model states, includes π = +, π =, all spins from 0 to E γ E γ < 2 MeV 2 E γ 5 MeV Sum 60 Fe Fe Fe R.S. et al., PRL 118, (2017) Strength in the scissors region about three times that found in (γ, γ) experiments. B(M1) tot (µ 2 N )

34 Summary Photon-scattering experiments with broadband bremsstrahlung at γelbe and with quasimonoenergetic γ rays at HIγS up to the neutron-separation energies. Strength in the quasicontinuum of states included. Observed strength corrected for branching and feeding by means of statistical methods. Novel experimental information about the behavior of the full E1 strength in the pygmy region and the M1 strength in the spin-flip region for nuclides around masses 80, 130 and 180. Large-scale shell-model calculations of M1 and E2 strengths. - Description of the experimental low-energy upbend by enhanced M1 strength. - Correlation between the low-energy M1 radiation and the scissors mode in open-shell nuclei. - Explanation of different strengths found in photoexcitation and in light-ion reactions. Modification of phenomenological strength functions used as input of statistical reaction codes needed.

35 Collaborators Data analysis: R. Massarczyk (HZDR/LANL) G. Rusev (HZDR/LANL) Experiments at γelbe (HZDR): D. Bemmerer, R. Beyer, R. Hannaske, A.R. Junghans, T. Kögler, A. Makinaga, K. Schmidt, A. Wagner et al. Experiments at HIγS (TUNL): Calculations: C. Bhatia, M. Bhike, H. Kelley, E. Kwan, M.E. Gooden, R. Raut, A.P. Tonchev, W. Tornow B.A. Brown (NSCL/MSU) F. Dönau (HZDR) S. Frauendorf (Notre Dame)