Rose, P H

Ryding, G

Wittkower, A

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Particle Accelerators, 1971, Vol. "2, pp. 13-16 © GORDON AND BREACH, SCIENCE PUBLISHERS LTD. Printed in Glasgow, ScotlandEQUILIBRIUM FRACTION MEASUREMENTS OF HEAVY IONS INFLUOROCARBON STRIPPERSG. RYDING, A. WITTKOWER AND P. H. ROSER. J. Van de GraaffLaboratory, High Voltage Engineering Corporation, Burlington, Massachusetts, U.S.A.Equilibrium fractions of bromine ions at 12 MeV and iodine ions at 3 and 12 MeV were measured in fluorocarbons.The mean charge was found to be appreciably higher it;l these vapors than in gases such as oxygen-but not as highas in a carbon foil. The use of fluorocarbons as the terminal stripper material for a large tandem accelerator willallow particles to be accelerated at a higher mean charge thereby increasing the useful energy range of thesemachines.The energy of the .beams available from atandem accelerator operating at a voltage V isdetermined by i, the number of electrons removedfrom the projectile in the terminal stripper. Thebeam energy in the terminal is simply eV electronvolts and the final beam energy E is given byE = (i + 1) eV electron voltsThe charge of a particular ion traversing the stripperfluctuates as a result of electron loss and capture incollisions with the atoms of the target. After asufficient number of collisions, an equilibriumI distribution of charge states is establjshed whichI may be described by the fraction of the beam Fi ineach chalge state i. The values'of Pi for a particularprojectile and stripper combination, therefore,determine the final energy and intensity of theaccelerated beams. These charge state distributionsare strongly dependent on the ion being acceleratedand on the stripper material used, which may be gas,vapor or thin solid foil. Consequently, in order toobtain the greatest intensity at a particular energy,it is important to be able to select the optimumcharge changing material.The strong dependence on stripper material isclearly illustrated in Fig. 1, which shows chargestate distributions for 12 MeV iodine ions travers-ing a number of gas targets(l) and gold and carbonfoiIs(2) of thickness 10 f-Lg cm-2. It can be-seen, forexample, that if a carbon Joil were used in theterminal of a 12 MV tandem .~ 20 per cent of thebeam would emerge with an energy of 144 MeV,whereas for an oxygen stripper, this fraction wouldbe just over 1 per cent and for a hydrogen stripperthe amount would be negligibly small.Unfortunately, the advantages of foil stripperscannot always be exploited when high energy beamsare required. Excessive multiple scattering in thefoil results in beam attenuation and for heavy ionbeam intensities of ;<: 1 f-LA, lifetimes of the foils areoften only a few minutes. The precise destructionmechanism is not yet fully understood, and it ispossible that improved manufacturing techniquesmay lead to more rugged foils. Multiple foil-holders may, of course, be employed to increase theeffective target lifetime; even so the use of foils isgenerally limited to the lighter projectiles at lowbeam currents.However, the action of foil in producing highercharge states is relatively well understood. (3) Thedifference L1 [between the mean charge in a solidcompared with a gas stripper was explained byBohr and Lindhard(4) in a pioneering paper of 1954,from which we quote below. In this generallyaccepted model, those collisions of the ions with thetarget atoms which do not produce ionizationresult in excitation of the ion's most loosely boundelectron. In a dilute gas stripper, de-excitationback to the ion's ground state takes place beforethe next collision occurs. In a solid, however, thecollision rate is too high for such radiative de-excitation to be important. Rather the collisionsconstantly increase the excitation of the outermostelectron until that electron is lost. This reasoningapplies .consecutively to inner electrons and, con-sequently, the charge of the ion will steadily in-crease until the capture cross section becomes largeenough to match the probability for loss of theexcited electron. A new equilibrium value of themean ionization is finally maintained by directcompetition between capture and loss. Mainly oneor, at most, a very few electrons will be in states offairly high excitation and 'an electron capturedinto an excited state will be lost from the same state.'Thus, 'suppression of readjustment of ion excita-tion' is expected to be the principal mechanism for14 G. RYDING, A. WITTKOWER, AND P. H. ROSE171615141312II+_+<x__xx +" ""'x+""~+\\+\+'T7 8 9 10CHARGE STATE65432100 r-----r----r--~---r--__r-___r--r_-...__-_,__-_r_-____r-_.--._-_._-__._-__r-_,• H2.02)( CARBON FOIL+ GOLD FOILo FLUOROCARBON VAPOR10~IZ0~(,)«IE~«wmI86420.10FIG. 1. Equilibrium fractions of 12 MeV iodine ions in H2 and O2 gases, fluorocarbon vapor and gold andcarbon foils.increasing the charge inside solids to the valuesobserved after the ions passed through the target,though Bohr and Lindhard also state that 'the highexcitation of the ions in solids may result in asubsequent emission of electrons from the ionsimmediately after their escape into vacuum, whichincreases the mean charge to a certain extent.'This latter view is now thought to be the' correctexplanation. (3)For a number of years efforts have been directedtowards finding a replacement for foils, one whichwould produce higher mean charge than the gases,yet have a longer lifetime than a foil. Perhaps theearliest of several schemes was suggested to theauthors and tried by us in 1963. Diffusion pumpoil, composed of large hydrocarbon molecules, wasproposed as a possible material. In order to test itsproperties, the beam was passed through thecenter of an operating diffusion pump. Howeverbefore tests with this elementary target could bestarted the diffusion pump oil had transferred itselffrom the pump, through the beam ports and intothe vacuum system. Van de Graaff, (5) in 1965,suggested another route-one which was equallyunsuccessful in practice, although several versionswere tried out. Here carbon dust, each particleonly 100 A diameter, was used as the strippingmedium.. It was found, however, that the dustclustered into large blqbs of 200-300 particles, heldtogether by electrostatic forces. These clusterswere large enough that most 10 MeV ions wereeither scattered at large angles or stopped withinthe cluster.Another attempt was made by one of the presentauthors (A.W.) in 1965, whentwo relatively heavyhydrocarbons, butane (C4H10) and 2,2-dimethyl-propane (C5H12) were used as the charge-changingmedium. (6) The results of these experiments wereREAVY ION STRIPPERSTABLE ISome chemical and physical properties of Fluorocarbons7Chemical Molecular BoilingName Description Weight PointPerfluoromethylcyclohexane C7F14 350 76°C[Flutec 2]Perfluorodimethylcyclohexane CSF16 400[Flutec 3]Perfluorodecalin ClOF 1S 462 142°C[Flutec 5]TABLE IICharge state fractions of Iodine and Bromine beams in FluorocarbonsIon Beam I I I I BrEnergy (MeV) 3 12 12 12 12Target F 3 F 3 F 5 F 2 F 3(CSF16) (CSF16) (C10F 1S) (C7F14) (CSF16)Mean Charge 3.79 7.59 7.62 7.47 6.81iDistribution Width 1.81 2.28 2.29 2.29 1.81aDistribution Skewness 0.61 0.46 0.44 0.47 0.40y0 0.411 6.182 20.4 0.12 0.13 0.14 0.123 23.2 1.35 1.37 1.68 1.58"E; 4 17.7 5.36 5:39 6.18 7.16(1) 5 14.3 11.7 11.4 12.6 15.6u$-I 6 8.90 15.8 15.0 15.8 20.4Q)0- 7 5.63 18.7 18.6 18.2 23.0~ 8 2.23 15.2 15.7 15.2 15.4l 9 0.81 12.2 12.2 11.5 8.88~ 10 0.24 8.38 8.57 8.0 4.76.9U 11 5.48 5.51 5.13 2.09C'i 12 3.0 3.30 2.98 7.7 10-1$-t~ 13 1.5 1.61 1.48 3.1 10-1Q)~ 14 7.5 10-1 0.80 0.70 5.3 10-2r/1 15 3.3 10-1 0.33 0.30 6.0 10-3Q) 16 1.4 10-1 6.0 10-4~$-tC'i 17 5.0 10-2 3.6 10-5..c::U 18 1.3 10-219 3.1 10-320 6.7 10-421 1.1 10-422 1.2 10-51516 G. RYDING, A. WITTKOWER, AND P. H. ROSEdisappointing but the reason for the lack of successwas not appreciated until the recent data presentedin Figure 1 were obtained. It was then apparent thatthe hydrogen in the hydrocarbons, a producer oflow charge states, negated any benefit which mighthave accrued from the use of a large molecule. Thispaper presents measurements of equilibrium frac-tions obtained using a number of fluorocarboncompounds, containing no hydrogen atoms, as thecharge-changing medium.EXPERIMENTAL DETAILSThe experimental method has been described indetail in an earlier publication. (1) For equilibriumfractions Fi greater than 1'0./1 per cent the totalestimated probable error -is less than ± 5 per centwhereas, for fractions less than 1'0./1 per cent theprobable error may be as large as ± 10 per cent.Fluorocarbons used as target materials are listedin Table I. Although fluorocarbons are availablewith higher molecular weights, it has not yet beenpossible to find one with suitable vapor pressurecharacteristics.RESULTS AND DISCUSSIONThe results are shown in Table I and in Fig. 1.Although the fluorocarbons are not as effective as acarbon foil for producing highly stripped ions, theyare appreciably more so than the best. of the'standard' gases, which in this case was oxygenand almost as effective as a gold foil. It is alsoworth noting that due to the large size of eachfluorocarbon molecule, very little vapor wasrequired to produce charge state equilibrium;because of this losses due to scattering are noworse than with the more standard stripping gasesand appreciably better than in foils.Fluorocarbon vapors have not yet been used in atandem terminal, and neither the short nor longterm effects on the voltage holding ability or life-time of accelerator tubes are known. However,the number of fluorocarbon atoms entering theacceleration tubes can be kept to a minimum by theaddition of cooled vapor-condensing baffles ateither end of the terminal stripper assembly. Ifthese baffles are installed, it is felt that the influenceof these heavy molecules on acceleration tubeperformance willI be slight. The use of fluoro-carbons as the terminal stripper material for a largetandem will then allow particles to be acceleratedat a higher mean charge thereby increasing the use-ful energy range of these machines.REFERENCES1. G. Ryding, A. Wittkower and P. H. Rose, Phys. Rev.,185, 129 (1969).2. G. Ryding and A. Wittkower. To be published.3. See H. D. Betz and L. Grodzins, Phys. Rev. Letters, 25,211 (1970), from which this entire paragraph hasbeen quoted by permission of the authors.4. N. Jlohr and J. Lindhard, Kgl. Dan. Vidensk. Selsk.Mat-Fys. Medd. 28, No.7 (1954).5. R. J. Van de Graaff. Private communication.6. A. Wittkower, P. H. Rose, R. P. Bastide, N. B. Brooksand L. Hopwood, Can. J. Phys., 43, 404 (1965).7. Available from Peninsular Chern. Research, CalgonCorp., P.O. Box 1466, Gainsville, Florida 52601.Received 31 August 1970

Equilibrium fraction measurements of heavy ions in fluorocarbon strippers

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