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Electron-Capture Delayed Fission (ECDF) in the Pb region

178. Tl. 150 ms. a. Electron-Capture Delayed Fission (ECDF) in the Pb region. 200,202 Fr. 192,194, 196 At. Andrei Andreyev. 186,188 Bi. 178, 180, 182 Tl. Z=82. ISOLDE workshop 18th November 2008. Collaboration. Jarno Van De Walle RILIS & ISOLDE. Andrei Andreyev Nick Bree

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Electron-Capture Delayed Fission (ECDF) in the Pb region

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  1. 178 Tl 150 ms a Electron-Capture Delayed Fission (ECDF) in the Pb region 200,202Fr 192,194,196At Andrei Andreyev 186,188Bi 178,180,182Tl Z=82 ISOLDE workshop 18th November 2008

  2. Collaboration Jarno Van De Walle RILIS & ISOLDE Andrei Andreyev Nick Bree Thomas Cocolios Jan Diriken Jytte Elseviers Mark Huyse Paul Van den Bergh Piet Van Duppen Martin Venhart Stanislav Antalic Katsuhisa Nishio Robert Page U. Koster (ILL, Grenoble, France) S. Franchoo (IPN, Orsay, France) S. Vermote, C. Wagemans (University of Gent, Belgium) M. Veselský (Slovak Academy of Sciences, Bratislava, Slovakia) I. Tsekhanovich (Manchester University, UK)

  3. Outlook • Electron-Capture Delayed Fission– what it is and why? • Earlier ECDF studies in the U and Pb regions • ECDF of 192,194At at SHIP: cold fission of 192,194Po • ECDF of 180Tl at ISOLDE: first ever fission study at ISOLDE? • Plans

  4. Electron-Capture Delayed Fission (ECDF, T1/2(ff)=T1/2(EC))Discovery: parent isotopes 232,234Am(1966, Dubna) • 2 step process: EC decay of a parent (A,Z) nucleus populates an excited state in the (A,Z-1) daughter, which then might fission (in competition with the g decay to the g.s.) • Low-energy fission! (E*~5-10 MeV) • 12 cases know so far (neutron-def. Uranium region) EC NEC NECDF ECDF EC-delayed branch PECDF= g NEC NECDF g g A,Z QEC Bf deformation A,Z-1 • PECDF depends strongly on: • QEC of the parent: the higher QEC, the larger the PECDF • Bfis of the daughter: the lower Bfis,the larger the PECDF • Actually, QEC-Bfis is important

  5. ECDF Probability: Feeding Part & Decay Part Gf(E) Gtot(E) Gf Gtot Gf Gf+Gg -ratio of the fission and total widths of excited levels in daughter (Gn is not important for neutron-deficient nuclei) = , r – level density, T - temperature 1 2pr {1+exp[ ]}-1 Gf= -inverted parabola approximation D.L. Hill and J.A.Wheeler 9.710-7 T4 exp(E/T) 2pr Gg= 2p(Bf-E) hwf QEC (QEC-E)2Sb(E) dE NECDF 0 PECDF= = NEC QEC (QEC-E)2Sb(E)dE 0 (QEC-E)2 – Phase factor for EC decay Sb(E) – b-strength function (nuclear matrix element) Measurement of PEDCF allows to deduce Fission Barrier Bf

  6. Trans-Uranium elements? Cowan et al, Phys. Rep. 208 (1991) 267 End point of r-process? I.Panov et al, NPA747 (2005) 633 Where the fission process will stop the r-process? Fission cycling:Fission products can serve as seed for the r-process! I. Panov et al, NPAA747 (2005) 633 r-process to the region with A>250 Fission: bdf n-induced sf r-process r-process Fission: bdf n-induced, sf Fission AA/2 ~125 b-delayed Fission and r-process Nucleosynthesis Need fission data for very heavy exotic nuclei!

  7. A. Mamdouh et al. NPA679 (2001), 337 Region of our interest: A~180-200 N/Z~1.22-1.3 ETFSI Po Bf [MeV] TF LDM • ECDF gives access to nuclei with very exotic N/Z ratios: e.g. N/Z(178Hg)=1.225 • Unexpected properties, e.g. mass-distributions, cold fission…? Neutron Number Why ECDF studies? • Experimentally fission barrier are known only in the vicinity of the b-stability • line (e.g. N/Z(238U)=1.59) • Theoretical models for Bf have been ‘tuned’ by using these data • Large discrepancies between different models for n-def. and n-rich nuclei • Must study isospin dependence of Bf values Available data on fission barriers, Z ≥ 80 (RIPL-2 library http://www-nds.iaea.org/ripl-2/ ) N/Z~1.55-1.59 126 82

  8. Courtesy P. Möller Calculated Energy Window for EC-delayed Fission 100 90 Proton Number Z 238U 80 208Pb 90 100 110 120 130 140 150 Neutron Number N QEC values: P. Möller et al., ADNDT masses (1995), based on FRDM (1992)Bf values: P. Möller et al., submitted to PRC (2008)

  9. 242Es (48) PDF 246Es (1) PDF 17 Tl-Hg • Why odd-odd EC-decaying parents? • Larger QEC in comparison with e-e and o-e neighbors (odd-even effect) • Even-even daughters are more fissile (specialization energy) 16 15 14 248Es (4) 13 Bf(Hg) 12 11 10 9 180Tl ? 8 180Tl ? 7 QEC(Tl) 6 5 4 176 178 180 182 184 186 188 190 192 QEC [MeV] QEC-Bf,shell [MeV] ECDF in trans-U region • 12 known ECDF cases in trans-Uranium region (all odd-odd!) • Relatively low QEC and Bf values (3-5 MeV) DF P PDF

  10. ECDF possible QEC > Bf bEC>1% 14 15 14 13 12 12 11 10 10 9 8 8 7 6 6 5 4 4 3 2 1 2 0 206 208 210 212 214 216 218 220 208 210 212 214 216 QEC vs Bf values in Pb region (look for cases QEC > Bf) 20 17 Pb-Tl Tl-Hg 18 Bi-Pb 16 18 16 15 16 14 14 13 14 Bf(Hg) Bf(Pb) 12 12 12 10 11 10 8 10 9 6 8 8 7 QEC(Bi) 4 QEC(Tl) 6 6 2 5 4 0 4 184 186 188 190 192 194 196 198 180 182 184 186 188 190 192 194 176 178 180 182 184 186 188 190 192 18 18 19 Rn-At 17 At-Po Po-Bi 17 18 16 16 17 15 15 16 14 14 15 13 E [MeV] E [MeV]E [MeV] 13 14 Bf(Po) 12 12 13 11 11 12 10 10 11 9 9 10 8 8 9 7 7 8 6 6 7 QEC(At) 5 5 6 4 4 5 3 4 3 2 190 192 194 196 198 200 202 204 206 186 188 190 192 194 196 198 200 192 194 196 198 200 202 204 206 208 18 Ra-Fr Ac-Ra Fr-Rn 16 14 Bf(Rn) 12 10 8 6 QEC(Fr) 4 2 198 200 202 204 206 208 210 212 214 200 202 204 206 Mass Number Red lines: Thomas-Fermi Fission Barriers, W.D. Myers, W. Swiatecki Phys. Rev. C60 (1999) 014606 Black lines: QEC-values, P. Moller et al. At. Data and Nucl. Data table 59 (1995) 185

  11. Velocity Filter SHIP (GSI, Darmstadt) 31 cm SHIP: Separation time: 1 – 2 μs Transmission: 20 – 50 % Background: 10 – 50 Hz Det. E. resolution: 18 – 25 keV Det. Pos. resolution: 150 μm Dead time: 3 – 25 μs Rotating targets ~400-600 mg/cm2 1 pmA of 52Cr,58Ni ~61012 pps

  12. SHIP Detection System • Measure efficiently all possible decays: • particle decay (a, b, protons, fission) E=0.1-250 MeV • gamma decay E=10-4000 keV • internal conversion electrons E=50-500 keV • 3 Time-Of-Flight detectors • STOP detector – 16 position sensitive Si strips (35×80mm), pos. resolution FWHM=150 μm, energy resolution 14 keV • 6 BOX Si detectors – for b and escaping a particles with a solid angle 80% of 2π • GAMMA detectors – large-volume Clover detector for x rays or g rays in coincidence with a’s • VETO detector – reduces background Esc. a, b ,ff g, X-rays Recoils a ff e BOX of 6 Si detectors PSSD Ge Clover A.N. Andreyev et al. ‘Conversion electron and b decay spectroscopy at SHIP’, NIM A533, 416 (2004)

  13. Unambiguous identification of ECDF in 194At, SHIP(GSI)A.Andreyev et al, paper in preparation 56Fe+141Pr197At* 1000 195At(a) 194At (a) 100 10 193At (a) Cross-section [nb] 194At (ff) 1 0.1 0.01 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0 5.1 OFF On Beam energy [MeV/u] 5ms 15 ms a PSSD total 56Fe+141Pr194At+3n I(56Fe)~600 pnA 86103 nuclei 66 fission events (pause) Recoils 56Fe full energy Scattered projectiles Counts a PSSD pause • Identification based on: • Half-life • T1/2,a(194At)=280(20) ms • T1/2,fiss(194At)=300(60) ms • Excitation functions • Cross-irradiation (56Fe,52Cr) • Double fold ff events (BOX) • Coincidences with gammas Fissions (66 events) 0 45 90 135 180 225 270 EPSSD [MeV] First observed in the 52Cr+144Sm194At+pn reaction (SHIP,2006) 16 fissions in pause

  14. PECDF for192,194At 1 LogP =0.81475(Q -B )-4.2196 192At ECDF f EC -1 10 194At -2 10 242Es -3 ECDF 10 232Am 228Np P 250Md 238Bk 244Es -4 10 246Es 234Am -5 248Es 10 240Bk QEC and Bf values are from the TF model -6 10 -2 -1 0 1 2 3 4 Q -B [MeV] f,Shells EC Largest PECDF values ever obtained!

  15. Q-value (most probable mass split) 260 TKE, Viola fit 252 No 246 Fm 240 252,256 Cf Md 220 246,250 ------Q-value (most probable mass split) Cm average or most probable TKE [MeV] 236,240,244 Pu 200 238 U Rf 192 Po No Fm 180 Cf Cm Pu U 160 194 Po 1200 1300 1400 1500 1600 1700 2 1/3 Fissility Z /A Total Kinetic Energy in the EC-delayed fission of 194At BOX of 6 Si detectors PSSD TKE: Add up the energies of 2ff from the PSSD and BOX detectors ff2 ff1 TKE(194At) ≈ 159 (7) MeV (corrected for PHD) Counts 0 75 125 175 225 TKE [MeV] Cold fission of 192,194Po!?

  16. FIRST FISSION STUDY AT ISOLDE!? December 1938: O. Hahn and F. Strassmann : discovery of nuclear fission From 16th October 1967 on: ISOLDE operation 40 years later…. June 2008: first ever fission study at ISOLDE!? : IS466 experiment

  17. IS466: ECDF of 180Tl isotope at ISOLDE (31 may-6 June 2008) E, MeV 17 Tl-Hg 16 15 14 13 Bf(Hg) 12 11 10 9 8 7 QEC(Tl) 6 5 4 176 178 180 182 184 186 188 190 192 Tl/Hg mass MINIBALL Ge cluster Si Annular Si Annular Si pure 30 keV Tl beam from RILIS+ISOLDE Si a ff 30 keV beam from ISOLDE ff C-foil C-foils 20 mg/cm2 Si detectors • Setup: Si detectors from both sides of the C-foil • Simple setup & DAQ: 4 PIPS (1 of them – annular) • Large geometrical efficiency (up to 80%) • 2 fold fission fragment coincidences • ff-gamma coincidences • Digital electronics (5 DGF modules)

  18. IS466: EC and a decay of 180Tl ECDF Due to negligible direct production of 180Hg PECDF(180Tl)= PECDF(180Tl)=5(1)10-5 (PECDF=310-(7±1) ) Nfission(180Hg) Na(180Hg)bEC(180Hg) 180Tl Si detector 180Hg 60000 EC 50000 a 40000 180Hg 176Au Counts 176Au 30000 a a 176Pt 180Tl 176Pt 20000 a 10000 0 5600 5800 6000 6200 6400 6600 Alpha energy [keV] • Very clean spectra: only 180Tl and its decay products! (first on-line use of SSL) • Negligible direct 180Hg production (bEC(180Tl)) • 100 times statistics than in previous works • New detailed decay scheme of 180Tl • Detailed a-g data (~10 new g lines in 176Au) • Correct low-energy level scheme of 176Au

  19. IS466: ECDF of 180Tl IS466: NO! ASYMMETRICAL energy (thus, mass) split! Eff1-Eff2 coincidences ~330 events Singles 1300 ff Eff2 [MeV] Eff1 [MeV] Before the IS466 experiment: How 180Hg (Z=80, N=100, N/Z=1.25) fissions? SYMMETRICAL mass split in two semi-magic 90Zr(Z=40,N=50, N/Z=1.25)? From comparison of TKE and Q-values  Cold fission of 180Hg!

  20. Future ECDF studies in the Pb region 178 Tl 150 ms a • Identification of new ECDF nuclei and detailed studies (e.g. Bf, TKE…) • ISOLDE: • 180Tl – Beta strength function measurements with TAS • 180Tl – HFS scan with RILIS : search for 2 isomeric states • 180Tl – mass measurement at ISOLTRAP • 178,182Tl – ECDF experiments at ISOLDE • SHIP: • 186,188Bi • 200,202Fr, 192At 200,202Fr 192,194,196At 186,188Bi 178,180,182Tl Z=82

  21. Thank you!

  22. Q-value (most probable mass split) 260 TKE, Viola fit 252 No 246 Fm 240 252,256 Cf Md 220 246,250 ------Q-value (most probable mass split) Cm average or most probable TKE [MeV] 236,240,244 Pu 200 238 U Rf 192 Po No Fm 180 Cf Cm Pu U 160 194 Po 1200 1300 1400 1500 1600 1700 2 1/3 Fissility Z /A Total Kinetic Energy in the EC-delayed fission of 194At BOX of 6 Si detectors PSSD TKE: Add up the energies of 2ff from the PSSD and BOX detectors ff2 ff1 252Cf(sf): Q[252Cf-(142Ba+110Mo)]=219 MeV <TKE>=185 MeV D(Q-TKE)=34 MeV 194Po(ECDF): Q[194Po – (98Mo+96Mo)]=166 MeV <TKE>=157(7) MeV D(Q-TKE)=9(7) MeV (+E* after EC) 192Po(ECDF): Q[192Po – (96Mo+96Mo)]=165 MeV <TKE>=170(10) MeV D(Q-TKE)= - 5(10) MeV (+E* after EC) Cold fission of 192,194Po? (no neutron emission during fission!) TKE(194At) ≈ 159 (7) MeV (corrected for PHD) Counts 0 75 125 175 225 TKE [MeV]

  23. 1 0.1 0.01 1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 -3 -2 -1 0 1 2 3 4 ECDF probability in the Pb region Large QEC and QEC-Bf=3-4 MeV  Possibility to reach nuclei with 10-100% ECDF probability! 242 Es 232 Am 228 Np At-Po 18 238 Bk 16 250 Md 244 14 Es DF 246 Es 12 P 234 Am 10 8 240 248 Bk Es 6 4 E [MeV] E [MeV] 190 192 194 196 198 200 202 204 206 18 180 Fr-Rn Tl ??? 16 14 12 10 8 6 4 2 Q -B [MeV] 198 200 202 204 206 208 210 212 214 EC f,shell Mass Number

  24. Low-Energy ECDF data (232,234Am as examples) H. L. Hall et al., PRC42 (1990) 1480 • Fission barriers and their isospin dependence • Energy distributions of fission fragments, thus TKE • Mass distributions of fission fragments • Gamma multiplicities for fission fragments • Neutron multiplicities for fission fragments ALL THIS FOR VERY EXOTIC NUCLEI WHICH DO NOT FISSION SPONTANEOSLY! K X rays in coincidence with ff

  25. 13 12 11 10 9 8 7 6 5 190 192 194 196 198 200 QEC(At) and Bf(Po) in different models Qec_At_TF Qec_At_MN_FRDM Qec_At_MN_FRLDM Bf_Po_TF_FLDM Bf_Sierk_shells_FLDM QEC Qec_DF or Bf [MeV] Audi2003 Qec_AT_EFTSI EC Q Bf exp QEC * At/Po Mass Number

  26. b-Delayed Fission (bDF, T1/2(ff)=T1/2(b)) • 2 step process: b decay of a parent (A,Z) nucleus populates an excited state in the (A,Z+1) daughter , which then might fission (in competition with the g decay to the g.s. and/or neutron emission) • Low-energy fission! (E*~5-10 MeV) • Neutron-rich nuclei (only 5 candidates known so far) b ff bDF g g g A,Z Qb Bf deformation A,Z+1

  27. N/Z>2-2.2 • For the r-process calculations we need fission data far away from stability: e.g. 260Po or 270U (N/Z>2!) – they might not be accessible in the Lab at all! – Use calculations? Example: Fission barriers – what do we know about them? • Experimentally fission barriers Bf are known only in the vicinity of the • beta stability line (e.g. N/Z(238U)=1.59) • Theoretical models for Bf have been ‘tuned’ by using these data Available data on fission barriers, Z ≥ 80 (RIPL-2 library http://www-nds.iaea.org/ripl-2/ ) N/Z~1.55-1.59 126 82

  28. Fission Barrier Calculations for the r-process nuclei • Good agreement between Bf,cal and Bf,exp for nuclei close to stability • Large disagreement far of stability (both on n-def. and n-rich sides) • Need measured fission data far of stability to ‘tune’ fission models Full symbols – experimental data Lines – calculations (LDM,TF, ETFSI) • Unfortunately, so exotic nuclei are not presently accessible by available techniques! A. Mamdouh et al. NPA679 (2001), 337 U ETFSI Po ETFSI • That is why the underlying mechanisms and properties of beta-delayed fission (and of low-energy fission in general) have to be investigated by using alternative approaches and in other regions of the Nuclear Chart. • According to semi-empirical estimates, the neutron-deficient nuclei in the U and Pb regions provide such a possibility via the ECDF decay Bf [MeV] TF TF LDM LDM Neutron Number Neutron Number

  29. Fission Barrier Calculations for the r-process nuclei A. Mamdouh et al. NPA679 (2001), 337 ETFSI Po Bf [MeV] TF LDM Full symbols – experimental data Lines – calculations (LDM,TF, ETFSI) U ETFSI TF LDM Neutron Number • Good agreement between Bf,cal and Bf,exp for nuclei close to stability • Large disagreement far of stability (both on n-def. and n-rich sides) • Need measured fission data far of stability to ‘tune’ fission models

  30. P. Moller (private communication): Most probably – NOT as a main channel • Most probable mass splits ~90/~90 (90Zr/90Zr N/Z=1.25) • but what about very asymmetrical split at the wings? : Schmidt et al., NPA 665 (2000) 221 (Speculations?) Double-Magic Fission? One of the goals of ISOLDE proposal is ECDF of 178Tl How its daughter 178Hg(N/Z=1.225) would fission? Would it give ‘double-magic’ fission – two double-magic ff’s? very neutron-rich 78Ni (Z=28,N=50 N/Z=1.79) very neutron-deficient 100Sn (Z=50,N=50 N/Z=1)

  31. ECDF Probability: Fission/gamma competition QEC Gf(E,Bf) Gtot(E) (QEC-E)2Sb(E) dE NECDF 0 ~ PECDF= QEC NEC (QEC-E)2Sb(E)dE 0 Gf Gtot Gf Gf+Gg -ratio of the fission and total widths of excited levels in daughter (Gn is not important for neutron-deficient nuclei) = , r – level density, T - temperature 1 2pr {1+exp[ ]}-1 Gf= -inverted parabola approximation D.L. Hill and J.A.Wheeler 9.710-7 T4 exp(E/T) 2pr Gg= 2p(Bf-E) hwf Measurement of PEDCF allows to deduce Fission Barrier Bf e.g . H.V. Klapdor et al., Z.Phys.A292, 1979,249; D. Habs et. al. Z.Phys. A285 (1978), 53

  32. 17 Tl-Hg 16 15 14 13 Bf(Hg) 12 11 10 9 8 QEC(Tl) 7 6 5 4 176 178 180 182 184 186 188 190 192 PDF PDF 180Tl ? 180Tl ? QEC [MeV] QEC-Bf,shell [MeV] Previous Studies in the Pb region (Dubna)Yu. A. Lazarev et al. Europhys. Lett. 4 (1987) 893; and Inst. Phys. Conf. Ser. No132 (1992) 739 • “Most probable” candidates: 180Tl (PECDF=310-(7±1)), 188Bi,196At (no PECDF data) • Irradiations inside the cyclotron (no A,Z selection for products) • Rotating wheel system, thick effective targets (2 mg/cm2) • Cross-irradiations, apparent sfis~15-50 pb • Mica detectors (fission tracks only) • Never confirmed (not in the Tables) • No continuation with these studies so far

  33. An example: Fission Barrier of 232PuD. Habs et. al. Z.Phys. A285 (1978), 53 QEC Gf(E) Gtot(E) (QEC-E)2Sb(E) dE 0 PECDF~ QEC (QEC-E)2Sb(E)dE 0 1 2pr {1+exp[ ]}-1 Gf= +4 Measured PECDf (232Am)=(1.3 )10-2  Bfis(232Pu)=5.3(4) MeV Assuming uncertainty of: QEC=±200 keV hwf =±100 keV a factor of 3 in PECDF -0.8 2p(Bf-E) hwf Bf precision of ~7.5% - well comparable to direct methods!

  34. Region of our interest: A~186-200 N/Z~1.26-1.3 Experimental information on fission - Low energy N/Z~1.55-1.59 - particle induced x - e.m. –induced E*~11 MeV

  35. - particle induced+SF x-e.m. -induced Electromagnetically-Induced Fission In-flight (FRS, GSI)A. Grewe et al. NPA614 (1997), 400 Fission from E*~12 MeV (E1 GDR) For comparison: Bf(234U) ~ 6 MeV Bf(220Th) ~ 7.5 MeV Bf(218Ac) ~ 7.5 MeV Thus, still fission from quite above the barrier! In contrast, bdf is near (or sub)-barrier effect!  -EM fission Sierk (FRLDM) Pashkevich • Primary beam 238U at 1 AGeV • 1 g/cm2 Cu primary target • Separated RIBs from FRS • Pb secondary target • s(El.fission)~2.1 b (234U) Identified Secondary Beam E~620 AMeV Fission Fragments Active Target

  36. Q-value (most probable mass split) 260 TKE, Viola fit 252 No 246 Fm 240 252,256 Cf Md 220 246,250 Cm average or most probable TKE [MeV] 236,240,244 Pu 200 238 U Rf 192 Po No Fm 180 Cf Cm Pu U 160 194 Po 1200 1300 1400 1500 1600 1700 2 1/3 Fissility Z /A

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