Beta-delayed fission as a tool to study near and sub-barrier fission in extremely exotic nuclei

Transcription

1 Beta-delayed fission as a tool to study near and sub-barrier fission in extremely exotic nuclei Andrei Andreyev University of York, UK Japan Atomic Energy Agency (JAEA, Tokai, Japan) 1

2 Collaboration A. Andreyev V.Truesdale R. Wadsworth D. Jenkins M. Vermeulen C. Barton M. Bentley CERN, Geneva JAEA Tandem K. Nishio S. Mitsuoka I. Nishinaka H. Makii K. Hirose S. Chiba A.Iwamoto H.Koura Y.Utsuno R.Orlandi S. Ota LBNL(Berkeley): Jorgen Randrup J. Elseviers C. Van Beveren L. Ghys M. Huyse D. Radulov E. Rapisarda P. Van Duppen P. Möller Yukawa Institute (Kyoto, Japan) T. Ichikawa I.Tsekhanovich (CENBG, France) L. Popescu, SCK CEN, Mol, Belgium S. Antalic, Z. Kalaninova, Comenium University, Slovakia

3 Outlook Brief (experimental) review on low-energy fission Low-energy fission in new regions of the Nuclear Chart Beta Delayed Fission (bdf) what it is and why? bdf 194,196 At, 202 Fr at ISOLDE (CERN) bdf 194 At at SHIP (GSI) Further plans

4 Outlook Many nuclear properties change far from stability line (e.g. disappearance of traditional magic numbers; appearance of new shell gaps; halos, skins What happens to fission far from stability, e.g. on the extremely proton-rich or neutron-rich side (relevant for r-process)? Not simple to answer, as to fission these nuclei at low excitation energy (E*~B f ) is a very challenging task as none of them fissions from g.s.

5 B f [MeV] Fission Barrier Calculations for the r-process nuclei Full symbols experimental data Lines calculations (LDM,TF, ETFSI) A. Mamdouh et al. NPA679 (2001), 337 Po ETFSI U ETFSI TF TF LDM LDM Neutron Number Neutron Number Good agreement between B f,cal and B f,exp for nuclei close to stability Large disagreement far of stability (especially on the n-rich sides) 5 Need measured fission data far of stability to tune fission models

6 A Detour: What can one learn with a rate of 1 fission/h? OR What can one learn from ~ fission events?

7 Bimodal Fission of 260 Md (T 1/2 ~32 d) J. F. Wild et al., Phys. Rev. C4, 640, Ne+ 254 Es(T 1/2 =276 d) 260 Md (a transfer reaction) TKE 34 irradiations in a 2.5 months irradiation period Collection of recoils on a foil Radiochemical separation of ~ ~200 Md atoms Deposited on a very thin foil 98 days of counting with Si and neutron detectors 1207 singles fission fragments, 905 coincident (TKE), ~12 ff/day Neutron multiplicity (as a function of TKE) Mass ~235 (60%) Two-humped TKE Two components in masses High TKE low Low TKE high

8 Total Kinetic Energy (TKE) Bimodal Fission E. K. Hulet et, Phys. Rev. C40, 770 (1989) ~300 ff Masses Mass components as a function of TKE 382 ff ~1400 ff ~450 ff ~1400 ff Evidence for bimodal fission: strong deviation of TKE from a single Gaussian Detailed fission fragments energy measurements are A MUST Y. Nagame and H. Nakahara, Radiochim. Acta 100, 605, 2012

9 Experimental information on low-energy fission Nuclei with measured charge/mass split (RIPL-2 + GSI) Heavy Actinides, N/Z~1.56: predominantly asymmetric; spontaneous fission, fission isomers Z= Ir 196 Au - particle induced x - e.m. induced E*~11 MeV K.-H. Schmidt et al.

10 Experimental information on low-energy fission Nuclei with measured charge/mass split (RIPL-2 + GSI) Heavy Actinides, Region N/Z~1.56: of our interest predominantly II: betadelayed fission, fission isomers of nuclei with asymmetric; spontaneous Region of our interest I: betadelayed fission of A~ A~230, N/Z~1.6: Fr, Pa. Ac N/Z~ : Tl,Bi, At, Fr ISOLDE(CERN) Z= Hg N/Z= Ir 196 Au - particle induced x - e.m. induced E*~11 MeV Pre-actinides, light Ir-Th N/Z~ : predominantly symmetric, e.g. FRS(GSI)

11 Beta-Delayed Fission Discovery: 232,234 Am (1966, Dubna) Two step process: b decay followed by fission Low-energy fission (E*~3-12 MeV, limited by Q EC ) e.g. 180 Tl: Q EC =10.4 MeV, B f,calc =9.8 MeV Relatively low angular momentum of the state e.g. 180 Tl: I=4 or 5 (some cases: up to 10) bdf branch P bdf = N bdf N b

12 Mass Separator ISOLDE (CERN) Hot cells HRS Target 50 g/cm 2 GPS Target 50 g/cm 2 Proton beam 1.4 ma, 1.4 GeV from PSB GPS Magnet RILIS HRS Magnets 180 Tl + Control Room Experiments 12

13 Detection system for bdf studies at ISOLDE Annular Si Si MINIBALL Ge cluster pure 180 Tl kev from RILIS+ISOLDE C-foils 20 mg/cm 2 Annular Si beam from ISOLDE ff Si a ff C-foil Si detectors A.Andreyev et al. PRL 105 (2010)

14 Mass distribution of fission fragments from bdf of 180 Tl ASYMMETRIC energy split! Thus asymmetric mass split: M H =100(4) and M L = 80(4) E ff1 -E ff2 coincidences ~330 events Singles 1111 ff ~20 ff/h The most probable fission fragments are 100 Ru (N=56,Z=44) and 80 Kr (N=44,Z=36) 14

15 CLDM (P. MÖller et al., yet unpublished) 180 Hg CLDM: Clay Liquid Drop Model (circa 2008) gs 180 Hg Asymmetric valley PM: Look, it s trivial...

16 New Type of Asymmetric Fission in Proton-Rich Nuclei via bdf of 180 Tl E Q EC ( 180 Tl)=E * max=10.44 MeV Q EC ( 180 Tl)-B f ( 180 Hg)=0.63 MeV P bdf ( 180 Tl)=3.6(7) Kr 100 Ru 90 Zr+ 90 Zr Calculations according to 5D fission model (P. MÖller et al., Nature 409, 785 (2001))

17 180 Hg: More surprises? How does 180 Hg fission at higher excitation energies? 36 Ar+ 144 Sm 180 Hg* E*=34-66 MeV : JAEA,Tokai Beam 36 Ar FF2 MWPC1 FF1 144 Sm MWPC , JAEA 36,40 Ar+ 144,154 Sm Hg* E*= MeV 36,40 Ar+ 142 Nd 178,182 Pt* E*= MeV Approved 90 Zr+ 90 Zr 180 Hg* (E*~15 MeV) Courtesy K. Nishio Even at E*=66 MeV: asymmetric mass split with A 1 ~100, A 2 ~80 Supported by Reimei Foundation (JAEA)

18 Two types of asymmetry: what s the difference? 180 Hg 238 U

19 Self-consistent Scission-Point Model

20 Mean-field HFB+Gogny D1S

21 counts bdf of 178 V. Liberati et al (PRC, 2013, in print) Hg 180 Hg ~0.8 ions/s ~0.3 ff/h ~200 ions/s ~20 ff/h Q EC ( 178 Tl)=E* max ( 178 Hg)=11.14 MeV Q EC ( 178 Tl)-B f ( 178 Hg)=1.82 MeV At this level of statistics: also asymmetric fission of 178 Hg, with mass split similar to 180 Hg Fission Fragments Energy in Si detector [MeV] E* max ( 180 Hg)=10.44 MeV

22 From Asymmetry to Symmetry Heavy Actinides, N/Z~1.56: predominantly asymmetric; spontaneous fission, fission isomers Lightest Hg isotopes with N/Z~1.25: asymmetric Z=82 178,180 Hg ISOLDE 187 Ir 196 Au - particle induced x - e.m. induced E*~11 MeV Pre-actinides, light Ir-Th N/Z~ : predominantly symmetric, e.g. FRS(GSI)

23 Fission of Proton-rich nuclei with A~ Courtesy P. Moller (LANL) and J. Randrup (LBNL), 5 th ASRC workshop on Fission, Tokai 2012 CERN-ISOLDE EC -delayed Fission (Low Energy Fission) 200,202 Fr 194,196 At 186,188 Bi 180,178 Tl JAEA tandem Heavy-ion induced Fission Mercury chain of isotopes (High Excitation Energy)

24 IS534 (ISOLDE), 9-14 May 2012: Mass Distributions Measurements of 194,196 Po via bdf of 194,196 At Singles 1111 ff 180 Hg ~20 ff/h E ff1 -E ff2 coincidences ~330 events ~7 coincident ff/h Fission Fragments Energy

25 Energy in Si2 (MeV) IS534, 9-14 May 2012: Mass Distributions Measurements of 194,196 Po via bdf of 194,196 At Energy in Si2 (MeV) Energy in Si2 (MeV) 196 At 10 ff/h Energy in Si1 (MeV) 180 Hg 20 ff/h 194 At 40 ff/h Clear difference in energy (thus, mass) distribution Energy in Si1 between (MeV) 2-peaked fission of 180 Hg and a broad distribution in 194,196 Po Energy in Si1 (MeV)

26 May and June 2012: Mass Distributions Measurements via bdf of 194,196 At and 200,202 Fr 20 ff/h 40 ff/h 10 ff/h 0.2 ff/h Courtesy L. Ghys (KU Leuven)

27 Experiment vs Theory 196 Po 202 Rn 204 Rn 208 Rn 210 Ra 194 Po Proton num ber Hg 88 Ra 87 Fr 86 Rn 85 At Po Bi Pb Tl Hg Au Neutron number 195 Au 198 Hg 210 Po 201 Tl particle - induced E.M. - induced beta- delayed Calculations: courtesy P. Moller (LANL) and J. Randrup (LBNL)

28 Experiment vs Theory ( 196 At) 40 A B C Q 3 [b 3/2 ] Po D1S C B Q 2 [b] Calculations: courtesy M. Warda (Lublin) A 0

29 Mapping beta-delayed fission: from neutron-deficient to neutron-rich nuclei Reviews of Modern Physics, 85, 1541 (2013)

30 Known Beta-delayed fission nuclei 2013: 230 Am at GARIS(RIKEN)

31 Mapping Terra Incognita in Low-Energy Fission A. N. Andreyev, M. Huyse, P. Van Duppen, Beta-delayed Fission in atomic nuclei, Reviews of Modern Physics, 85, 1541 (2013)

32 Thank you! Strong support from Reimei Program of ASRC (JAEA) is very much appreciated

33 Beta-delayed fission in the neutron-rich Fr nuclei? Earlier Gatchina attempts ISOLDE yields (UC, ThC) ThC targets? ~10-8 branching in 1 day now

34 Beta-delayed fission studies at recoil separators (e.g. at Lanzhou?)

35 Why recoil separators to search for bdf? Unlike lead region, heavy actinides cannot be accessed by projectile fragmentation (as at ISOLDE) The only method at present: complete-fusion reactions

36 An example: bdf of 194 At at SHIP (GSI, Darmstadt) SHIP: Separation time: 1 2 μs Transmission: % Background: Det. E. resolution: Det. Pos. resolution: Dead time: Rotating targets ~ mg/cm Hz kev 150 μm 3 25 μs 31 cm 1 pma of 52 Cr, 58 Ni ~ pps E Proj ~250 MeV M proj = E EVR ~50 MeV M EVR =

37 SHIP Detection System Measure efficiently all possible decays: particle decay (a, b, protons, fission) E= MeV gamma decay E= kev internal conversion electrons E= kev Recoils Esc. a, b,ff e g, X-rays BOX of 6 Si detectors PSSD Ge Clover a ff 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 largevolume Clover detector for x rays or g rays in coincidence with a s VETO detector reduces background 37 A.N. Andreyev et al. Conversion electron and b decay spectroscopy at SHIP, NIM A533, 416 (2004)

38 Identification of bdf in 194 At, SHIP(GSI) A. Andreyev et al, PRC, 87, (2013) a Recoils PSSD total Scattered projectiles 56 Fe full energy 56 Fe+ 141 Pr 194 At+3n I( 56 Fe)~600 pna nuclei 66 fission events (pause) a PSSD pause On 5ms OFF 15 ms Fissions (66 events) E PSSD [MeV] Identification based on: Half-life T 1/2,a ( 194 At)=280(20) ms T 1/2,fiss ( 194 At)=300(60) ms Excitation functions Cross-irradiation ( 56 Fe, 52 Cr) Double fold ff events (BOX) Coincidences with gammas First observed in the 52 Cr+ 144 Sm 194 At+pn reaction (SHIP,2006) 16 fissions in pause 38

39 Counts average or most probable TKE [MeV] Total Kinetic Energy in bdf of 194 At TKE: Add up the energies of 2ff from the PSSD and BOX detectors TKE( 194 At) 159 (7) MeV (corrected for PHD) BOX of 6 Si detectors ff2 PSSD ff Q-value (most probable mass split) TKE, Viola fit Md TKE [MeV] Po 194 Po Rf Fm No Cm Cf Pu U Fissility Z 2 /A 1/3 39

40

41 Brownian Metropolis Shape Motion based on J. Randrup and P. Moller, PRL 106, (2011) Phys. Rev. C 85, (2012)

42 Improved Scission-Point Model asymmetric Inter-fragment distance is not fixed and calculated. values of ~0.5-1 fm result (Wilkins fixed at 1.4 fm) Symmetric! Mass symmetry/asymmetry doesn t change as a function of E* (up to E*~60 MeV) good for future experiments Also, J.-L. Sida et al. private communication

43 Mean-field HFB+Gogny D1S

44

45 Fusion-Fission Reactions in the Lead Region at JAEA s tandem (K.Nishio et al) New experiment at JAEA (March-April 2012) 36 40Ar + 144Sm Hg 36 40Ar + 154Sm Hg 36 40Ar + 142Nd Pt 90Zr + 90Zr 180Hg Ebeam from 160 to 235 MeV New reaction chamber, larger MWPC new electronics (time-stamping) Analysis in progress

46 How exotic is bdf? Neutron-deficient side: Beta-delayed p, d, t, a.., b2p...emission ~160 cases M. Borge and B.Blank (2008) Neutron-rich side: Beta-delayed neutron emission.., b2n.. ~217 cases B. Pfeiffer et al., Prog. Nucl. Ener. 41 (2002) 39 Beta-delayed fission: neutron-deficient side: 12 cases before our studies, U region (up to ~2008) 9 new cases in the Pb region (from ~2008 on) several nuclei on the neutron-rich side (data unclear)

47 RILIS: Resonance Ionization Laser Ion Source at ISOLDE Procedure: decay measurements (a, fission, b-g decays) of Tl ions, resonantly ionized by RILIS and mass-separated by HPS + a,b, fission and & g-decay station + Tl IP= ev λ 2 = 535 nm Distance to Target: 18 m Laser System 6s 2 6d 2 D 3/2 J = 3/2 U Target Lasers: rep = Hz, t las ~15 ns 3 copper vapor tubes, 2-3 dye lasers with amplifiers, nonlinear crystals BBO: Proton Beam 1.4 GeV, pps 2.4 ms pulse length, 1.2 s period 6s 2 6p 2 P 1/2 λ 1 =276.8 nm J = 1/2

48 Three regions to search for bdf Necessary conditions for bdf to occur: Q EC (Parent)~B f (Daughter) [Q EC -B f > -2 MeV] Beta-branching ratio b b >0 P. Moller

49 Beta-Delayed Fission Discovery: 232,234 Am (1966, Dubna) bdf branch P bdf = N bdf N b

50 P bdf Probability: Extraction of fission barriers?! Need to know S b!? (next slide) G f G tot = G f G f+ G g -ratio of the fission and total widths of excited levels in daughter (G n is not important for neutron-deficient nuclei) G g = T 4 exp(e/t) 2pr 2p(B G f = 1 {1+exp[ f -E) ]} -1 2pr hw f, r level density, T - temperature -inverted parabola approximation D.L. Hill and J.A.Wheeler Measurement of P bdf allows to deduce Fission Barrier B f e.g. H.V. Klapdor et al., Z.Phys.A292, 1979,249; D. Habs et. al. Z.Phys. A285 (1978), 50 53

51 P bdf Probability: Extraction of fission barriers?! Need to know S b!? Clearly model-dependent (but we tried many parametrizations) Conclusion: experimental barriers are always lower than calculated 51

52 Development of At beams The Problem: Element At has no stable isotopes, T 1/2 =8 h ( 210 At) Only ~70 mg of At present in the 1 st mile of Earth core Must be produced and studied on-line at an accelerator 2010: Letter of Intent (CERN): Development of astatine ion beams with Resonance Ionization Laser Ion Source (RILIS) Spokespersons: A. Andreyev and V. Fedosseev

53 nm Ion count intensity a.u nm First determination of the Ionization Potential for the radioactive element At a) b) c) nm 532 nm 532 nm d) cm cm cm cm cm -1 3 successful on-line development runs ISOLDE (Nov. 2010, May 2011) TRIUMF, Canada (Dec. 2010) Many new atomic levels found Transition strengths measured Ionization potential measured (scan of ionizing laser: converging Rydberg levels allow precise determination of the IP) 205 At 795 nm cm cm nm 216 nm 0 cm -1 Total photon energy cm -1 53

54 First determination of the Ionization Potential for the radioactive element At