1 / 33

Andrei Andreyev University of the West of Scotland Paisley, Scotland

178. Tl. 150 ms. a. Low-energy fission in ‘Terra Incognita’ (towards detailed studies of the new region of asymmetric fission). 200,202,204 Fr. N/Z~1.25. 192,194,196 At. 186,188 Bi. Andrei Andreyev University of the West of Scotland Paisley, Scotland. 178,180,182 Tl. Z=82.

hazina
Télécharger la présentation

Andrei Andreyev University of the West of Scotland Paisley, Scotland

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 178 Tl 150 ms a Low-energy fission in ‘Terra Incognita’(towards detailed studies of the new region of asymmetric fission) 200,202,204Fr N/Z~1.25 192,194,196At 186,188Bi Andrei Andreyev University of the West of Scotland Paisley, Scotland 178,180,182Tl Z=82 ISOL@MYRRHA Workshop, 24th April 2012

  2. Collaboration Andrei Andreyev Valentina Liberati Joseph Lane Kalvinder Sandhu Victoria Truesdale Xavier Derkx P. Möller, A. Sierk Thomas E. Cocolios RILIS & ISOLDE + many colleagues Yukawa Institute (Kyoto, Japan) Takatoshi Ichikawa Stanislav Antalic Zdenka Kalaninova Katsuhisa Nishio Akira Iwamoto + many colleagues Nick Bree Hilde De Witte Jan Diriken Jytte Elseviers Lars Ghys Mark Huyse Deyan Radulov Elisa Rapisarda Paul Van den Bergh Piet Van Duppen L. Popescu, D. Pauwels, SCKCEN, Mol, Belgium U. Koster (ILL, Grenoble, France) R. Page, OLL, University of Liverpool S. Vermote, C. Wagemans (Gent, Belgium) M. Veselský, M. Venhart (Slovakia) I.Tsekhanovich (CENBG, France) K.T. Flanagan (Manchester, UK)

  3. A Detour:What one can learn with a rate of 1 decay/h?OrWhat can one learn on fission in a month-long experiment?ORWhat can one learn from ~1000 fission events?

  4. Bimodal Fission of 260Md (T1/2~32 d) J. F. Wild et al., Phys. Rev. C4, 640, 1990 • 22Ne+254Es(T1/2=276 d)260Md (a transfer reaction) • 34 irradiations in a 2.5 months irradiation period • Collection of recoils on a foil • Radiochemical separation of ~3000 260Md 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) ~235 (60%) TKE ~200 Mass • Two-humped TKE • Two components in masses • High TKE – low  • Low TKE – high 

  5. Bimodal FissionE. K. Hulet et, Phys. Rev. C40, 770 (1989) Mass components as a function of TKE Masses Total Kinetic Energy (TKE) ~300 ff 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”

  6. Asymmetry-to-Symmetry Transition in SHED.C.Hoffman and M.R.Lane, Radiochim. Acta 70/71, 135, 1995 It is expected that yet un-accessible 264Fm should fission symmetrically with a very narrow mass split in two double-magic 132Sn By approaching A~260 (N~160,Z~100) – dominant mass split in fission fragments in vicinity of doubly-magic 132Sn

  7. Outlook • Many nuclear properties change far fromstability 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 side? • Not simple to answer, as to fission these nuclei at low excitation energy (E*~Bf) is a very challenging task (none fissions from g.s.)

  8. 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? • Earlier work on bDF in Uranium and Lead regions • bDFof 178,180,182Tl and 200,202Fr at ISOLDE (CERN) • Towards bDF of 192,194,196At • Further plans and ideas

  9. Fission Barrier and Mass Distribution in LDM LDM: Macroscopic part ONLY Text-book figure saddle point VS+VC Bfis Potential g.s d V Elongation A1-A2 A1+A2 Fission Mass asymmetry= Mass asymmetry=0, if A1=A2 Pure LDM (no shell corrections): Symmetric Mass Split a huge puzzle in 40’s as experiments showed asymmetry!

  10. Fission Barrier and Mass Distribution in LDM Macroscopic + Microscopic Macroscopic part ONLY V sp gs Symmetric Mass Split elongation Asymmetric Mass Split

  11. Macroscopic and Macro-Microscopic PES for 238U

  12. Experimental information on low-energy fissionNuclei with measured charge/mass split (RIPL-2 + GSI) Heavy Actinides, N/Z~1.56: predominantly asymmetric; spontaneous fission, fission isomers Region of our interest: bDF of nuclei with A~180-200, N/Z~1.22-1.3: Hg,Pb,Po,Rn Z=82 Z=82 Z=82 Z=82 180Hg N/Z=1.25 - particle induced x - e.m. –induced E*~11 MeV Pre-actinides, light Ir-Th, N/Z~1.4-1.5: predominantly symmetric, e.g. FRS(GSI) 187Ir 196Au K.-H. Schmidt et al.

  13. Beta-Delayed FissionDiscovery: 232,234Am (1966, Dubna) • Low-energy fission (E*~3-12 MeV, limited by QEC) • Relatively low angular momentum of the state • 12 cases known before our work (neutron-deficient Uranium region) b+/EC Nb NbDF bDF bDFbranch PbDF= g Nb NbDF g g 180Tl(Z=81,N=99) I=(4,5) QEC 10.44 MeV Bf ~ 10 MeV deformation 180Hg(Z=80 N=100)

  14. 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) neutron-rich side (r-process nuclei): yet to be studied

  15. 242Es (48) PDF 246Es (1) PDF 248Es (4) 180Tl ? 180Tl ? Dubna, ‘89 QEC [MeV] QEC-Bf [MeV] bDFin trans-U region • 10 known bDFcases in trans-Uranium region (all odd-odd!) • Relatively low QEC and Bf values (3-5 MeV) DF P PDF

  16. bDF studies at Mass Separator ISOLDE@CERN Proton beam 1.4 mA, 1.4 GeV from PSB 238UC 50g/cm2 GPS Target Protons Hot cells GPS Magnet HRS Target RILIS HRS Magnets Control Room Experiments

  17. + Laser System 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 HRS@ISOLDE IP= 6.108194 eV Tl a,b, fission and & g-decay station λ2= 535 nm + 6s2 6d2D3/2 J = 3/2 HPS λ1=276.8 nm Distance to Target: 18 m 6s2 6p2P1/2 J = 1/2 Lasers: nrep= 11.000 Hz, tlas~15 ns 3 copper vapor tubes, 2-3 dye lasers with amplifiers, nonlinear crystals BBO: U Target Proton Beam 1.4 GeV, 3·1013 pps 2.4 ms pulse length, 1.2 s period

  18. Detection system for bDF studies at ISOLDE MINIBALL Ge cluster Si Annular Si Annular Si pure 30-60 keVbeam from RILIS+ISOLDE Si a ff 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

  19. bDFof178,180,182Tl isotopes at ISOLDEQEC(Parent)-Bf(Daughter) matters! Masses/QEC and Bf values from FRLDM 178Tl 180Tl 182Tl QEC(178Tl)-Bf(178Hg)=1.82 MeV , bb(178Tl)~50% ~0.8 ions/s ~0.3 ff/h QEC(180Tl)-Bf(180Hg)=0.63 MeV, bb(180Tl)=94(4)% ~200 ions/s ~20 ff/h QEC(182Tl)-Bf(182Hg)=-1.09 MeV, bb(182Tl)>99% ~104 ions/s <1 ff/10 h

  20. Asymmetry-to-Symmetry Transition in SHED.C.Hoffman and M.R.Lane, Radiochim. Acta 70/71, 135, 1995 It is expected that (yet un-accessible) 264Fm should fission symmetrically with a very narrow mass split in two 132Sn 180Hg can be considered as an analogue of 264Fm, with expected split in two semi-magic 90Zr

  21. Energy/Mass distribution of fission fragments from 180Hg Singles 1111 ff ~20 ff/h Eff1-Eff2 coincidences ~330 events ~7 coincident ff/h ASYMMETRIC energy split, thus asymmetric mass distribution The most probable fission fragments: 100Ru (N=56,Z=44) and 80Kr (N=44,Z=36) TKE (330 events) single Gaussian? Still under debate: is there any symmetric fission in two semi-magic 90Zr+90Zr? J. Elseviers et al, to be submitted soon

  22. Why there is no split of 180Hg in two semi-magic 90Zr? viabDF of 180Tl E 80Kr 100Ru 90Zr+90Zr Calculations according to 5D fission model (P. MÖller et al., Nature 409, 785, 2001)

  23. bDF of 178Tl @ISOLDE (2011)V. Liberati et al (draft is ready) QEC(178Tl)=E*max(178Hg)=11.14 MeV QEC(178Tl)-Bf(178Hg)=1.82 MeV 178Hg 8 ff singles ~0.8 ions/s ~0.3 ff/h ~30 hours At this level of statistics: also asymmetric fission of 178Hg, with mass split similar to 180Hg 1 counts 0 Question: do we want more statistics for 178Hg or everything is clear? A case for ISOL@MYRRHA? ~200 ions/s ~20 ff/h ~50 hours 180Hg 1111 ff singles 20 16 12 8 4 E*max(180Hg)=10.44 MeV 0 40 50 60 70 80 Fission Fragments Energy in Si detector [MeV]

  24. Low-energy Electromagnetically-Induced Fission in-flight at FRS(GSI), K.-H. Schmidt et al. • Primary beam 238U at 1 AGeV • 1 g/cm2primary target • Separated RIBs from FRS • Pb secondary target • E*~11-12 MeV (GDR) Identified RIB from FRS e.g. 204Rn Fission Fragments Active Target 204Rn@FRS symmetric (E*~12 MeV) We can study low-energy fission of 200,202,204Rn at ISOLDE via bDF of 200,202,204Fr Work by Lars Ghys (IKS, Leuven)

  25. bDF of 202Fr (2011): Mass/Energy distributions Single events (Energy spectra) 202Rn (βdf of 202Fr) 180Hg Full statistics IS466 (2009) 180Hg Sample from IS466 run (2009) • 202Rn: • Very broad energy distribution (cf to 180Hg) • Significant amount of symmetric fission events (as in 204Rn) • Evidence for asymmetric split? • Two fission modes? (bimodal fission?) • More statistics is definitely needed (how much?) Coincident events (Mass spectra) 202Rn (IS466 III) 180Hg (IS466) Courtesy Lars Ghys

  26. bDF of 202Fr: Total Kinetic Energy (TKE) • 202Rn: • Much broader TKE distribution: real or low statistics? • If real – bimodal fission of 202Rn? • Need more statistics! (ISOLDE/TRIUMF... ISOL@MYRRHA?) • 200Rn: only ISOL@MYRRHA could do 180Hg TKE = 134.6(7) MeV FWHM = 13(2) MeV 202Rn: TKE ≈ 147 MeV FWHM (single Gauss) = 19(3) MeV Lars Ghys

  27. Yields at ISOL@MYRRHA 229Fr • High yields along isotopic chains • Detailed spectroscopy

  28. New Region of Asymmetric Fission Heavy Actinides, N/Z~1.56: predominantly asymmetric; spontaneous fission, fission isomers Z=82 Z=82 Z=82 Z=82 178,180Hg - particle induced x - e.m. –induced E*~11 MeV Pre-actinides, light Ir-Th N/Z~1.4-1.5: predominantly symmetric, e.g. FRS(GSI) ISOLDE 187Ir 196Au

  29. New Region of Asymmetric Fission Heavy Actinides, N/Z~1.56: predominantly asymmetric; spontaneous fission, fission isomers Lightest Hg isotopes with N/Z~1.25: asymmetric Z=82 Z=82 Z=82 Z=82 178,180Hg - particle induced x - e.m. –induced E*~11 MeV Pre-actinides, light Ir-Th N/Z~1.4-1.5: predominantly symmetric, e.g. FRS(GSI) ISOLDE 187Ir 196Au

  30. Transition from asymmetry of 178,180Hg to symmetry in 204Rn (ISOLDE) 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 Z=82 Z=82 Z=82 178,180Hg - particle induced x - e.m. –induced E*~11 MeV Pre-actinides, light Ir-Th N/Z~1.4-1.5: predominantly symmetric, e.g. FRS(GSI) ISOLDE 187Ir 196Au

  31. 2012: Mass distributions of 194,196Po via bDF of 194,196At at ISOLDE Yields with RILIS@ISOLDE Isotope Yield ions/s Fission yield 194At 2 ~5 ff/h 196At 500 ~5 ff/h 205At ~150pA! 2012: Fission mass distributions of 194,196Po are certainly doable The ISOLDE run is scheduled for 9-14 May 2012 2013 on: Fission mass distributions of 186,188Pb via bDFof186,188Bi

  32. Experiment: • Low-energy fission (ISOLDE) from asymmetry of 178,180Hg to symmetry of 202Rn: • Asymmetry of 178,180Hg was measured (via bDF of 178,180Tl) • (A)symmetry of 202Rn was measured (via bDF of 202Fr) • ‘Soon’ (2012), 194,196Po (via bDF of 194,196At) • >2012, 196,198Pb (via bDF of 196,198Bi) Conclusions and wishes (from an experimentalist’s point of view) Theory: Systematic calculations for all above cases, especially for the isotopes from 178,180Hg to 202Rn

  33. Thank you!

More Related