Clean Beams at ISOL Facilities

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 at ISOL facilities, in particular
at CERN/ISOLDE.
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Where, Why, What - Motivations
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 the staircase at N82 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
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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 with plasma
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 !
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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
gt105
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Request Selectivity !
UCx target and neutron converter
Proton-beam on neutron converter ? only fission,
avoids p-rich isobars
A. Nolen et al.
7
Request Selectivity !
Isobar separation
in reality, on a good day DM/M 1/4000
Cd 2.000
In 17.000
HRS design DM/M 1/104
8
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
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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.

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

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Request Selectivity !
Hyperfine splitting
Separation of isomers by fine-tuning of laser
frequency