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Clean Beams at ISOL Facilities

Clean Beams at ISOL Facilities. O.Arndt , H. Fr å nberg, C.Jost, K.-L. Kratz, U. Köster. GSI Workshop on Astrophysics and Nuclear Structure, January 15-21, 2006 in Hirschegg, Austria. Where, Why, What - Motivations. altogether 52 r-process nuclei.

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Clean Beams at ISOL Facilities

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  1. Clean Beams at ISOL Facilities O.Arndt, H. Frånberg, C.Jost, K.-L. Kratz, U. Köster GSI Workshop on Astrophysics and Nuclear Structure, January 15-21, 2006 in Hirschegg, Austria

  2. Where, Why, What - Motivations altogether 52 r-process nuclei have been measured (at least) via their T1/2, which lie in the process path at freeze-out(nn1020 cm-3). These r-process isotopes range from 68Fe to 139Sb. The large majority of these exotic nuclei was identified atISOL facilities, in particular at CERN/ISOLDE.

  3. Where, Why, What - Motivations 132Sn 50 (n,) 134 135 136 137 133 131 165ms Pn~85% 278ms (n,) 134 135 132 130 162ms 131 132 133 129 46ms(g) 158ms(m) 133In84 49 130 128 131In82 49 130Cd82 48 127 129Ag82 47 r-process path 126 128Pd82 (n,) 46 127Rh82 45 R-abundance peaks and neutron-shell numbers ...still today important r-process properties to be studied experimentally and theoretically. already B²FH (Revs. Mod. Phys. 29; 1957) C.D. Coryell (J. Chem. Educ. 38; 1961) b “climb up thestaircase“ at N=82; major waiting point nuclei; “break-through pair“131In, 133In; K.-L. Kratz (Revs. Mod. Astr. 1; 1988) climb up the N= 82 ladder ... A  130 “bottle neck“ “association with the rising side of major peaks in the abundance curve“

  4. What we knew already in 1986 ... Shell-model (QRPA; Nilsson/BCS) prediction 1+ 1.0 Q = 8.0 MeV T1/2 = 230 ms 1+ IKMz – 155R(1986) 6.0 1+ T1/2 = (195 ± 35) ms 1+ 5.0 1+ 1+ 4.0 1+ K.-L. Kratz et al (Z. Physik A325; 1986) 3.0 g7/2, g9/2 Exp. at old SC-ISOLDE withplasma ion-source anddn counting 1+ 4.1 2.0 Problems: high background from -surface ionized 130In, 130Cs -molecular ions [40Ca90Br]+ T1/2(GT) = 0.3 s 1.0 1- 0 Request:SELECTIVITY !

  5. Sb Te I Xe Sn Cd In Ag Cs Request: Selectivity ! Why ? the Ag “needle” in the Cs “haystack” How? at an ISOL facility • Fast UCx target • Neutron converter • Laser ion-source • Hyperfine splitting • Isobar separation • Repeller • Chemical separation • Multi-coincidence setup 50 800 >105

  6. Request: Selectivity ! UCx target and neutron converter Proton-beam on neutron converter ↷ only fission, avoids p-rich isobars A. Nolen et al.

  7. In Cs Cd Mass scan at HRS (ISOLDE) in 2002; efficiency corrected Request: Selectivity ! Isobar separation in reality,„on a good day…“ DM/M≈ 1/4000 Cd 2.000 In 17.000 HRS designDM/M≥ 1/104

  8. 130Cd 1669 keV 130Sb 1749 keV 130Cd 1732 keV Laser ON Laser OFF Energy [keV] Request: Selectivity ! Laser ion-source (RILIS) Laser ON Comparison of Laser ON to Laser OFF spectra Laser OFF g-singles spectrum Chemically selective, three-step laser ionization of Ag into continuum Properties of the laser system: Efficiency ≈ 10% Selectivity ≈ 103

  9. Chemical yield ~ 30% Request: Selectivity ! Molecular sidebands Separation on higher mass ↷ Suppression of isobaric background • RILIS with standard MK3 target unit used • chemical treatment is performed by adding 32S to the target • other possibilities of chemical treatments for further elements have to be investigated.

  10. 131mSn 103 102 131Sn 798.5 keV 101 100 Request: Selectivity ! Molecular sidebands Separation on higher mass ↷ Suppression of isobaric background 450.0 keV 304.3 keV 1226.0 keV 20 ms collection time per p-pulse 131Sb 933.1 und 943.4 keV

  11. Request: Selectivity ! Surface chemistry in the transfer line between target and ion-source ↷ thermochromatography here: deposition of Zn, Rb, Ag, In, Cd and Cs a quartz tube with a temperature gradient ↷ separation Cd, from Cs, In Prototype UCx target at CERN/ISOLDE with temperature-controlled quartz transfer-line was tested in Oct. 2005 Diploma thesis C. Jost (2005)

  12. Request: Selectivity ! Surface chemistry Thermochromatography target set-up at ISOLDE

  13. Request: Selectivity ! Surface chemistry LASER-off gamma-spectrum of mass 131 3 mC protons onto converter, 12 seconds collection,quartz transfer line at 600°C Total suppression of all surface-ionized species (in particular 131In)! All gamma lines are due to background on the tape! U. Koester, H. Frånberg C. Jost, O.Arndt

  14. Comparison of spatial beam profiles laser ions surface ions Request: Selectivity ! Repeller “Skimmer” with negative voltage ↷ retains positive, surface-ionized species 28 Volt repeller: selection between laser ions (laser & repeller on) and surface ions (laser & repeller off) K. Wendt et al.

  15. outer ring (22 3He counters) middle ring (20 3He counters) inner ring (22 3He counters) central hole for tape station and b- or g- detector paraffine matrix with Cd shielding Request: Selectivity ! Detector Selectivity through b-delayed neutron counting Proton-rich isobars are excluded by the detector itself. b-g-n multifold coincidences are the future detection systems for extreme neutron-rich nuclei.

  16. Conclusion • High resolution mass separation combined with laser ion sources is not sufficient to produce isobaric clean beam for nuclear astrophysics. Additional selectivity is needed for further progress in r-process and nuclear structure investigations far from stability. • Repeller can clean RIBs from unwanted surface-ionized species. • Molecular sidebands are powerful tools to clean up RIBs in the 132Sn region. But molecular sidebands must be investigated separately for each element. • Thermo chromatography in the transfer line is a good possibility to produce pure beams. But still most absorption enthalpies on different surfaces are unknown. • Further investigations on chemical treatments of ion beams are urgently needed. • Additional selectivity can be reached by “intelligent” detector systems like multifold coincidence set-ups.

  17. 127Ag pg9/2 pp1/2 pp1/2 129mAg82 pg9/2 129gAg82 T1/2(g)=(46 ) ms +5 -9 T1/2(m)=(15860) ms Request: Selectivity ! Hyperfine splitting Separation of isomers by fine-tuning of laser frequency

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