[Sneap] NEC pelletron C stripper foil question

[Albert Zondervan]

Lucas

Being most familiar with Accelerator Mass Spectrometry I can share some 
information that might help. In AMS, one quintessential parameter is the 
efficiency of counting ions of the rare isotope of interest. The 
acceleration step, which is needed to destroy ion beams that could 
interfer with that counting, involves one component of the overall 
efficiency: the percentage of negative ions that get converted to the 
positive charge state that is selected for further 
momentum/velocity/energy-analysis and ultimately for counting with a 
particle detector. For AMS, one selects and optimises terminal voltage to 
get the highest possible charge-state fraction. (The finer points of 
optimisation do not need mentioning here.) That percentage is a function 
of projectile velocity, its nuclear charge, stripper medium, and stripper 
thickness. The process, due to the stochastic nature of ions travelling 
thru the medium thereby having numerous charge-exchange interactions with 
it, can be described by charge-state distributions. These be found in the 
literature and websites etcetera for the equilibrium situation. 
Equilibrium means here: Adding more thickness does not change the 
distribution. Of course thickness has a strong effect on scattering 
(energy, angular) and thus influences beam optical transport. 

Most AMS systems have recirculating-gas strippers. Some older systems, 
like my own, still have C stripper foils. Quite a few systems out there 
probably have both. The important message others can comment on much 
better is that there are applications where it pays to work with foils 
rather than with gas. But I don't think getting the highest possible 
energy for C ions using a 3 MV tandem is helped by using foils rather than 
gas. I think if you'd experiment with the nitrogen gas thickness at fixed 
terminal voltage, you will see that there is an optimum thickness 
(pressure) at which the beam intensity is maximum. This probably is 
charge-state dependent. Certainly it will depend on how well beam 
transport is taken care of. From that "angle" you can probably argue that 
replacing the gas by foil is not going to make a major difference. 

Pointer: [Shima et al. "Equilibrium charge fractions" Atomic Data and 
Nuclear Data Tables, Vol 51, Number 2, July 1992, page 189]. It shows the 
fraction (%) of C projectiles thru C foil, as function of velocity-squared 
(MeV/u) and for the different charge states (e). To generate the 
bottom-end tail percentage of 1%, you need 0.2 MeV/u which converts to 2.4 
MeV C-12 ions at the terminal. At 3 MeV, F(q=5+) = 3% and F(q=6+) << 1%. 
So, at first glance you might be able to have some 5+ charge state at 3 
MV, even with gas. Careful momentum analysis with the HE-side magnet will 
(dis)prove this. And then the question is if the overall low efficiency 
for this charge state found at your highest possible terminal voltage can 
be compensated by injecting with a stiffer beam, as the Pelletron is 
limited by how much beam can be injected (charge can be dumped at the 
terminal per unit time). If you need for your IBA applications only a few 
nA, then you might get away with injecting one or several uA... Ask NEC 
where the limits are for your Pelletron-system.

Upshot:
1/ A foil stripper might help, but probably not give you a substantial 
gain.
2/ You might be able to get a nA of carbon in the 5+ charge state.
3/ Your terminal voltage is way too low to reach the 6+.

Good luck with the search: both in literature and by experimentation !

cheers
Albert




Lucas Phinney <lucas.phinney at gmail.com>
Sent by: sneap-bounces at tunl.duke.edu
28/04/2009 04:21 a.m.
Please respond to Symposium of Northeastern Accelerator Personnel

 
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        Subject:        [Sneap] NEC pelletron C stripper foil question





Hello,

I am a graduate student at the University of North Texas working in the 
Ion Beam Modification and Analysis Laboratory (IBMAL) with Dr. Jerry 
Duggan.  I am trying to get higher charge states of a C beam (+5 and +6), 
with a 3 MV NEC 9SDH-2 Pelletron.  We are currently using the Nitrogen gas 
stripper and I can only get up to +4 charge state, and I was hoping that 
by going to the solid foil I would be able to get the higher charge 
state.  I was wondering if anyone had experience using C foil strippers in 
the tandem and could give me some pointers.  I was specifically wondering 
what thickness would be best, but any information would be appreciated.

Thanks
-- 
Lucas Phinney
Ion Beam Modification and Analysis Laboratory
Department of Physics
University of North Texas
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