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

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!)

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)

Experimental techniques and setup:

Single stripper with increasing thickness and at varying terminal energy:

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)

2 successive C-strippers (D=9cm):

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)

2 successive C-strippers (D < 0.5mm):

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)

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)

8 9 10 110

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88Sr from SrF3TV=8MV

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7 8 9 10 11 120

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7 8 9 10 11 12 130

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88Sr f rom SrF3TV=12MV

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