stripping molecular beams to higher charge states by means ... · advantages of molecular beams:...
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Stripping molecular beams to higher charge states by means of two successive carbon foils
Outline:● Advantages of molecular beams and double strippers● Experimental techniques and setup● Results of the charge state distribution measurements ● Conclusions
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Advantages of molecular beams:
●Low or no anion yield at the source (eg. Mn or N)→ injection of molecular anions into the accelerator
●Suppression of isobaric interference in AMS e.g.: 7Be →BeF3(-1) 7Li → LiF3(-1) not stable!
Superhalogens: very high electron affinity (> 6eV), favourable molecule formation
Advantages of double strippers for molecular beams:
●Breaking molecular bonds part of the stripping process
●Double stripper → separation of breaking molecular bonds and charging up → reduced Coulomb explosion (“soft stripping”)
●Suppression of molecular background
●Higher yields at higher charge states possible by double stripping (that's new!)
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Hypothesis:
●Element bound in a molecule doesn't accept high charge states → interference between element of interest and constituents of former molecule ●Spatial separation of molecular fragments insufficient in single foil(coulomb explosion)
●Gap in between stripper foils → complete isolation of ion of interest→ no interference with molecular fragments during striping process
Measurements:
●Charge state distribution measurements for molecular beam stripped by single foil (increasing thickness and increasing terminal energy)→ equilibrium charge state of molecular beam
●Comparative charge state distribution measurements of different molecular beams stripped with single and double strippers (increasing terminal energy)
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Experimental techniques and setup:
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Single stripper with increasing thickness and at varying terminal energy:
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Single stripper with increasing thickness and at varying terminal energy:
Results:
●Yield obtained with 8, 12, 16 μg/cm2 never significantly exceeds yield obtained for 4μg/cm2 C-stripper
→ equilibrium charge state distribution for molecular beam established already at4μg/cm2 C-stripper (at energies up to 12 MeV)
●Low yield at 9+ attributed to electron shell effect(e-configuration of Sr: Ar 4s2 p6 5s2)
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2 successive C-strippers (D=9cm):
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2 successive C-strippers (D=9cm):
Results:
● Increased current for charge states higher than most probable one (in case of single stripper) obtained with 2 successive strippers in all investigated cases
● Also shift of most probable charge state observable in most cases
● Gain in current more pronounced for higher charge states
(exponential decrease of charge state distribution)
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2 successive C-strippers (D < 0.5mm):
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2 successive C-strippers (D < 0.5mm):
Results:
●Results comparable with the ones obtained for the double stripper with a distance of 9cm
- increased beam current at high charge states- shift of most probable charge state by at least one unit
► Distance between stripper foils not critical (above the range of 100μm)
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Conclusions:
Charge state distribution measurements of 88Sr (single stripper with increasing thickness, increasing terminal voltage)
→ equilibrium charge state distribution already established for molecular beam after passage through 4μg/cm2 Carbon
Comparison of charge state distribution obtained by double stripper with the one obtained by single stripper →higher yield at charge states higher than most probable one (in case of single foil) observed for all investigated molecular beams at all energies
→shift of most probable charge state observed in most cases
Reduction of distance between stripper foils to less than 5mm
→distance between stripper foils not a critical parameter
Importance of gap between strippers also indicated by comparison of single 8μg/cm2 with double 4μg/cm2 stripper (unfortunately no comparative measurement made)
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8 9 10 110
0,2
0,4
0,6
0,8
1
1,2
88Sr from SrF3TV=8MV
charge state
arbi
trary
uni
ts
7 8 9 10 11 120
0,2
0,4
0,6
0,8
1
1,2
88Sr from SrF3Tv=10MV
charge state
arbi
trary
uni
ts
7 8 9 10 11 12 130
0,2
0,4
0,6
0,8
1
1,2
88Sr f rom SrF3TV=12MV
charge state
arbi
trary
uni
ts
Appendix: