[Sneap] NEC pelletron C stripper foil question

Thu Apr 30 11:36:16 EDT 2009

Oops, I left out the crucial step, passing the ion beam through a gas cell to excite the atomic lines (A guest from CA, stepped into my office as I was composing the email and I rushed things, sorry).  Quentin  
________________________________________
From: sneap-bounces at tunl.duke.edu [sneap-bounces at tunl.duke.edu] On Behalf Of Kessel, Quentin [quentin.kessel at uconn.edu]
Sent: Thursday, April 30, 2009 10:57 AM
To: Symposium of Northeastern Accelerator Personnel
Subject: Re: [Sneap] NEC pelletron C stripper foil question

I'm not in AMS, but I have done some electron-ion and ion-ion  coincidence experiments  and  atomic spectroscopy.  I have often wondered why in AMS one doesn't look at  carbon photons(or whatever the ion of interest is)  in coincidence with the ion in question.  Not only would this lower backgrounds, but it would relax the ion  resolution required. Quentin
________________________________________
From: sneap-bounces at tunl.duke.edu [sneap-bounces at tunl.duke.edu] On Behalf Of Duggan, Jerome [jduggan at unt.edu]
Sent: Thursday, April 30, 2009 10:38 AM
To: Symposium of Northeastern Accelerator Personnel
Subject: Re: [Sneap] NEC pelletron C stripper foil question

Albert thank you for your reply to Lucas.I talked to greg Norton and Luck with your new machineI remember windy Wellington quite well.I hope some day I will be able to return.You guys are the friendliest people in the world.It makes travel to NewZeland a pleasure.Jerry Duggan

-----Original Message-----
From: sneap-bounces at tunl.duke.edu [mailto:sneap-bounces at tunl.duke.edu] On Behalf Of Albert Zondervan
Sent: Wednesday, April 29, 2009 5:16 PM
To: Symposium of Northeastern Accelerator Personnel
Subject: Re: [Sneap] NEC pelletron C stripper foil question

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

        To:     SNEAP <sneap at tunl.duke.edu>
        cc:
        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
(940) 565-3336_______________________________________________
Sneap mailing list
Sneap at daytona.tunl.duke.edu
http://daytona.tunl.duke.edu/mailman/listinfo/sneap

Notice: This email and any attachments are confidential. If received in error please destroy and immediately notify us. Do not copy or disclose the contents.

_______________________________________________
Sneap mailing list
Sneap at daytona.tunl.duke.edu
http://daytona.tunl.duke.edu/mailman/listinfo/sneap
_______________________________________________
Sneap mailing list
Sneap at daytona.tunl.duke.edu
http://daytona.tunl.duke.edu/mailman/listinfo/sneap
_______________________________________________
Sneap mailing list
Sneap at daytona.tunl.duke.edu
http://daytona.tunl.duke.edu/mailman/listinfo/sneap