Conversion Electron Spectroscopy in the Second Minimum of Actinides

Transcription

1 Mini Workshop on Future n Beam Conversion Electron Spectroscopy SKP Bonn, 3./4. January 3 Conversion Electron Spectroscopy in the Second Minimum of Actinides P.G. Thirolf, LMU München prompt fission energy isomeric fission 6 4 deformation ntroduction: double humped fission barrier, fission isomers Experiments in the superdeformed. minimum: 4f γ spectroscopy conversion electron spectroscopy Predictions from phenomenological systematics Summary and Outlook

2 . minimum and fission isomers doublehumped fission barrier: 4 : (macroscopic) droplet model Deformation 1. minimum. minimum (microscopic) shell corrections (Strutinsky, 1967) Deformation Energy.8 MeV.4 MeV E isomeric fission Deformation β ~.4 β ~.66 magic neutron number N=146, fission isomer: t 1/ = 3.8 ns pioneer experiment by Specht et al. (197) : 38 4 conversion electron spectroscopy after U( α,n) first identification of fission isomeric ground state rotational band

3 γ spectroscopy in 4f 6 GeCLUSTER of German EUROBALL Collaboration: 4 detectors 38 U(α, n) 4f (t 1/ =3.8 ns), 44 hrs. beamtime delayed coincidence with fission fragments Counts / kev Counts / kev b (1) g 3 3 ā 4 a g 3 g ā g 7 c (1) ā c ā 4 b g kev a g b ( 1) g E γ [kev] single intensive γ line (786.1 kev, 36.6 %) weaker lines regular rotational band structure: starting point for level scheme D. Pansegrau et al., Phys. Lett. B 484 () 1

4 Mini Orange setup for conversion electron spectroscopy Principle: Si(Li) (3x) e 6cm 38 U Target α 6cm 1.8mm MiniOrange (3x) MiniOranges: annular Si detector (delayed fission) wedgeshaped permanent magnets around central Pb absorber toroidal magnetic field configurations: 3 <= E (kev) <=6 e 6 <= E (kev) <=8 e Si(Li)detectors: E~ 3.1 kev total efficiency:.4 % (6 kev)

5 Experimental Setup for Conversion Electron Spectroscopy

6 Conversion electrons from 4f from series of experiments (ca. 7 hrs. beamtime): transmission optimized for 36 kev, 68 kev reaction: 38 U(α,n) 4f E = MeV α electrons in delayed coincidence with fission fragments counts / 1.8 kev 1 1 K L M β gs Klines c b β vibration β gs Llines transmission efficiency / % electron energy [kev] counts / 1.8 kev 1 1 E E/M1 β band: c b (1) c a counts / 1.8 kev β gs K a g E E electron energy [kev]

7 β 36f,38(f) bands in U and 4 1. minimum: (α,α ) 38 U E β band (α,n) 4 β band β band E e [MeV] J.M. Hoogduin et al. Phys. Lett. B 384 (1996) 43. minimum: 38f U 38 38f U(d,pn) U E d = MeV hω = 64 kev 36 36f U(d,pn) U hω = 68 kev 36f U U. Goerlach et al., Phys. Rev. Lett. 48 (198) 116 β g degeneracy removed

8 Combined analysis: γ s conversion electrons 14 counts / 1 kev γ kev ā g 36.6% E γ [kev] E =11.8 kev K counts / 1.8 kev 1 1 e K L M c β gs Klines b β gs Llines electron energy [kev] all strong electron lines are E transitions conversion coefficient of kev transition: first excited βvibrational phonon: kev connecting E transitions between excited rotational bands E1

9 Level scheme of 4f (1816) (173) (161) (114) 986.8(13) 89.4(1) 8.(11) 78.1(11) 769.9(1) (364.) (14) E (.1) 18.(14) 118.7(13) 146.7(6) 141.4(6) (3) 136.9() (13) E (.19) 7.(1) E (.) 78.(11) E (.36) 76.(11) E (.49) 769.9(1) E (.33) (13) 9.7(1) (6) 81.1(4) 4 8.6() 86.(1) β band K π = ~.3 ps, 1.7 % E 46.7(9) E (67) 73.1(1) E (44) 7. E (~) 31.7 E (~1) 44.8 E (~1) 38(1) [1.()U] 4.8() [.7(3)1.Y] M1 (<.1) 3.() [1.(3)] 61.() [1.8(3)] M1 (.1) 3.(3) [1.1(3)] 7.3(4) [3.()] M1 (.) 38.6() [U] 8.9() [1.1(3)] M1 (<.1) 4(1) [.7] 98.() [.8] 4.3(4) [Y] 614.() [.] M1 (<.7) 99.3(13) E (4) 4f E1 8.4() [.3(3)] 78.9() [1.6(3)X] 786.1(1) [36.6(9)] E1 (.) () [1.3] 16.() [.7] a band K π = 41 % K π = 3 % o band 4.7 [<1.] 34.6 [.] 69.6 [>.7] E/ M1 1 ps 3 1 E1 3.7 ns 1341 [.6] 13 [.8] [.14] [.46] (117) (114) 14.9() 998.3(7) 96.7() 918.8(3) 891.(3) 866.(1) 846.8(3) 836.() E1 8.4 [.] E (<.) [.] 836(1) [.(3)] 816(1) [1.7(3)] 14.8(1) [.8] E (.3).4(14) E (.4) 9.(1) E (.4) 43.6(1) E (.3) 6.(1) E (.3) 81.8(1) E (.) 9.1(18) E (.) 68.3(13) E (.) 644.9(14) E (.) 86.7(3) [3.(4)] 846(1) [.6(3)] 799(1) [.9()] 84.4(3) [.6(4)] 8.() [.()] 778.9(3) [1.(3)] 8.8() [1.8(3)] 88.7(3) [.()] 78.9(7) [X] 81.7() [1.7(3)] < 1 ps E c band K π = 1 % b band K π =1 1 % E < 1 ps ~1 ps gsband excited states in. minimum: ca. 98% negative parity D. Gassmann et al., Phys. Lett. B 497 (1) 181

10 Moments of nertia (dynamical) moments of inertia: E = (h / Θ ) ( ( 1)) Θ/ h = ( 1) / (E E ) / h [MeV ] 1 Θ 1 rigid rotor β π band π K = band, "aband" ground state band K = 1 band, "cband" / h [MeV ] π K = 1 band, "bband" odd spins even spins Θ Variation of moments of inertia: in β band from rigid rotor limit (low ) to value of gs band (high ) oddeven staggering in b band known from K = 1 bands in 1. minimum of actinides separately smooth behaviour for even/odd spins in b band

11 E( ) [kev] γ E( ) [kev] β Systematics of collective excitations Z=8 N= VCS: Valence Correlation Scheme : Sum of valence nucleon pairs as ordering scheme Minimum:. Minimum: 9 γ band (N p N n )/ β band 1 16 U Th Ra U Th Ra (N p N n )/ 17 E( ) [kev] β E( ) [kev] γ (N p N n )/ Z=1 N=1 Z=78 N=146 Z= enables prediction of phonon energies in. minimum 4 4 γ 6 band 7 β band exp. determination of new magic numbers in. minimum (N p N n )/ 1 1 4f 38f 36f U 34f U 4f 38f 38f U 36f U 34f U

12 Extension of the Grodzins Systematics Grodzins (/Raman): /3 B(E) E( ) =.6 Z A Actinide region: data plotted as function of quadrupole moment Q actinides,4 No 36f,38f U linear interpolation ( ) B(E) = /16 π e Q (Single shell asymptotic Nilsson model) actinides,4 36f,38f U No linear interpolation Nilsson WoodsSaxon } converted from Sobiczewski et al.

13 Outlook: Study of a fission isomer with odd neutron number measurement of single particle energies γ spectroscopy: measurement of Nilsson orbitals in odd fission isomer MNBALL (new Germanium spectrometer) 37f Conversion electrons: identification of β vibrational bands in 37f Mini Oranges mprovement of models for description of superheavy elements main objective of MAFF project at new research reactor FRM

14 &! ' B B N F A? J H F A H J E A B! % B K E C A = H J E? A J H K? J K H A B H A K J H = A B H = J E B M A, A? = O F H F A H J E A? = $? = $ > > > > L A H E A A? J H F A? J H? F O F H A F = H = J / = H? D E C? =! & 1 C,! C H = O F A? J H? F O 1 1 * ) #! ' % C "!! 1 # 4 8 A >? D A J = D O 4 A L & ' %! & 4 = B = E L E? D A J = 4 " & ' & ' &

15 = = F K = J E B J D A E E K N? E J = J E B K? J E " A 8 4 A =? J E! # 7 =! % K 1 A H E?? H A? J E! % B K " B K! # 7 =! % B K! & 7 = " B K " A 8 " A A = O A = O A = O F H F J A = O F H F J $ & # D H J C 4 8 A >? D A J = D O 4 A L & ' %! & ) 4 K A J = D O 4 A L! ' % # ' A * = H H A J = D O )!! ' & $

16 Summary/Outlook Advantage of fission isomers: low angular momenta, few K mixing clear separation between vibrational and rotational excitations Conversion electron spectroscopy indispensable tool: complementary to γ ray spectroscopy: removal of ambiguities Superdeformed. minimum: identification of superdeformed collective bands determination of Outlook: β detailed level scheme phonon energy predictive power for phonon energies in. minimum exp. determination of new magic numbers in. minimum extension of the Grodzins systematics identification of Nilsson single particle states 37f candidate: with conversion electron, spectroscopy γ (in beam) identification of the fission isomer in 39 U

17 Collaboration: LMU München D. Gassmann D. Habs M.J. Chromik P. Reiter H.J. Maier PGT Univ. Bonn E. Mergel H. Hübel J. Domscheit A. Görgen S. Neumann A. Neusser G. Schönwasser DEBRECEN nst. Nucl. Research, Debrecen/Ungarn A. Krasznahorkay CEA/Saclay K. Hauschild CSNSM Orsay A. Lopez Martens MP Heidelberg D. Pansegrau H. Bauer T. Härtlein F. Köck H. Scheit D. Schwalm