IEEE Transactions on Nuclear Science, Vol. NS-30, No. 2, April 1983 STUDIES OF FAST BEAM PRODUCED RECOIL IONS IN A PENMIING TRAP W. S. Burns, D. A. Church, R. A. Kenefick Texas A&M University, College Station, Texas and S. B. Elston, R. Hiolmes, D. Taylor, J.-P. Rozet, S. Berry and I. A. Sellin University of Tennessee, Knoxville, Tennessee and Oak Ridge National Laboratory, Oak Ridge, Tennessee Summa ry Low energy recoil ions (Ne+q, 1<q<8) have been produced and contained in a Penning-type ion trap by means of a stripped 35 MeV C1+5 beam from the Oak Ridge National Laboratory EN Tandem Van de Graaff. Initial investigation of the axial energy distribution of the stored recoil ions indicates that mean energies are less than 2q eV. The rate constants for several electron transfer processes were determined by the di- rect measurement of stored ion population decay as a function of pressure. Preliminary results are: k(Ne+3,Ne) + Products = O.5x10-9cm3/s k(Ne+4,Ne) + Products = 3.5x10-9cm3/s k(Ne+5,Ne) + Products = 4x109cm3/s k(Ne+6,Ne) + Products = 8x10-9cm3/s. The relative numbers of the stored low energy recoil- ing Ne+q ions (3<q<8) agree with the results of pro- duction processes initiated by much more highly charg- ed projectiles such as Xe+38 and U+44. Introduction Rate constants and cross sections for charge transfer collisions of multi-charged ions with atoms and molecules at energies from near thermal to keV are of increasing interest due to the importance of such collisions in the evolution of fusion plasmas, for UV laser development, and in many astrophysical proces- ses.1 Relatively few measurements of multi-charged ion reaction rates at near thermal energies have been published. Most studies have involved singly and doubly-charged ion reactions in flow tubes containing a relatively inert buffer gas which is used to both control the ion temperature and to select specific low lying states. Although these moderate vacuum tech- niques are suitable for low charge states, the pres- ence of the buffer gas renders them inappropriate for collision measurements involving highly charged ions.2 In the measurements reported here the ions are created as recoils in heavy ion collisions; they are confined in an ultra high vacuum environment, and have kinetic energies in the laboratory frame of only a few eV. It has been known for several years that in a single collision with a fast highly charged ion pro- jectile, a target atom can, with a large probability, lose several electrons.3 More recently it was found that even in collisions sufficiently energetic to ionize inner shell electrons, the recoil target ion acquired very little translational energy.4,5 It is the very low recoil energy of these multi-charged ions that immediately makes them candidates for various well known ion storage techniques.6'7'8'9 There are several advantages to post-production trapping of these ions. Experimentally, stored ions approach the ideal isolated atomic system in which each ion is constrained to orbit in a small volume of space, for long periods of time - virtually free from outside interactions. Once ions are successfully con- tained within the trap, one can then proceed to pre- pare a specific atomic system for further study. The ion trap potential can be carefully defined and then altered to trap or detect only specific ion species. The trap detection circuit can be used to exchange energy with the stored ions, and because storage times are long, a second reaction might be initiated using stored ions as the target to be interroga- ted1, 10 ,1 1 ,12 ,13. Penning Trap The selection of the particular ion trap most suitable for accelerator-produced low energy recoil ions involved a choice among the three basic ion trap configurations: the dc electrostatic (Kingdon) trap, the radio-frequency quadrupole trap, and the Penning trap.11 A Penning ion trap was selected as best suited for the initial studies since the production cross sections were expected to be low, and the recoil energies uncertain. Although it has the disadvantage of subjecting ions to a strong magnetic field, the ions do experience periodic motion in several differ- ent characteristic modes while they are still inside the Penning trap. External electronic coupling to these oscillating ions allows simultaneous detection and species identification with a high sensitivity po- tentially capable of detecting even a few ions. (The RF trap also has these advantages, but is more complex in operation.) Furthermore, in the Penning trap ions in higher charge states are more strongly bound, es- pecially those recoiling into a large solid angle per- pendicular to the fast ion beam axis, where they are trapped primarily by the strong magnetic potential. While the relatively narrow pole gap of the electro- magnet limits the physical size of the electrodes and consequently the volume overlap of the fast ion beam with the trapping region, one can take advantage of the stability criterion for bounded motion (see equa- tion 2). The electric potential can be chosen to ex- clude trapping of the vastly more numerous singly and doubly charged ions which limit the total storage charge. In an ideal Penning trap having a purely quadru- polar electrostatic field and a uniform magnetic field, the potentials are: + = /4Z02)(r - 2Z2) + V/2} (la) A = B0/2 (-Y,X,O) ions with mass m and charge q in these potentials oscillate with SHM along the Z axis with frequency wz=(V0mZ2 1/2 uz=(qV /MZ02)l/. Investigating the equations of motion for bounded solutions to transverse motion yields the critical criterion for stability q/m > 2V0/B0 22 (lb) (2) Transverse motion is characterized by two angular frequencies, w± = wc/2 ± (wc2/4 - wz /2)l/2, in which wc=(qBo)/m, the usual cyclotron angular frequency. In practice it is usually found that wc>>wz. This condition is true, for example, when Bo is large and justifies the approximation of w+ by a Taylor series expansion. Thus 0018-9499/83/0400- 977$01.00 1983 IEEE 977
Transcript
IEEE Transactions on Nuclear Science, Vol. NS-30, No. 2, April 1983
STUDIES OF FAST BEAM PRODUCED RECOIL IONS IN A PENMIING TRAPW. S. Burns, D. A. Church, R. A. Kenefick
Texas A&M University, College Station, Texasand
S. B. Elston, R. Hiolmes, D. Taylor, J.-P. Rozet, S. Berry and I. A. SellinUniversity of Tennessee, Knoxville, Tennessee and Oak Ridge National Laboratory, Oak Ridge, Tennessee
Summa ry
Low energy recoil ions (Ne+q, 1<q<8) have beenproduced and contained in a Penning-type ion trap bymeans of a stripped 35 MeV C1+5 beam from the OakRidge National Laboratory EN Tandem Van de Graaff.Initial investigation of the axial energy distributionof the stored recoil ions indicates that mean energiesare less than 2q eV. The rate constants for severalelectron transfer processes were determined by the di-rect measurement of stored ion population decay as afunction of pressure. Preliminary results are:
The relative numbers of the stored low energy recoil-ing Ne+q ions (3<q<8) agree with the results of pro-duction processes initiated by much more highly charg-ed projectiles such as Xe+38 and U+44.
Introduction
Rate constants and cross sections for chargetransfer collisions of multi-charged ions with atomsand molecules at energies from near thermal to keV areof increasing interest due to the importance of suchcollisions in the evolution of fusion plasmas, for UVlaser development, and in many astrophysical proces-ses.1 Relatively few measurements of multi-chargedion reaction rates at near thermal energies have beenpublished. Most studies have involved singly anddoubly-charged ion reactions in flow tubes containinga relatively inert buffer gas which is used to bothcontrol the ion temperature and to select specific lowlying states. Although these moderate vacuum tech-niques are suitable for low charge states, the pres-ence of the buffer gas renders them inappropriate forcollision measurements involving highly chargedions.2 In the measurements reported here the ionsare created as recoils in heavy ion collisions; theyare confined in an ultra high vacuum environment, andhave kinetic energies in the laboratory frame of onlya few eV.
It has been known for several years that in asingle collision with a fast highly charged ion pro-jectile, a target atom can, with a large probability,lose several electrons.3 More recently it was foundthat even in collisions sufficiently energetic toionize inner shell electrons, the recoil target ionacquired very little translational energy.4,5 It isthe very low recoil energy of these multi-charged ionsthat immediately makes them candidates for variouswell known ion storage techniques.6'7'8'9
There are several advantages to post-productiontrapping of these ions. Experimentally, stored ionsapproach the ideal isolated atomic system in whicheach ion is constrained to orbit in a small volume ofspace, for long periods of time - virtually free fromoutside interactions. Once ions are successfully con-tained within the trap, one can then proceed to pre-pare a specific atomic system for further study. The
ion trap potential can be carefully defined and thenaltered to trap or detect only specific ion species.The trap detection circuit can be used to exchangeenergy with the stored ions, and because storage timesare long, a second reaction might be initiated usingstored ions as the target to be interroga-ted1, 10 ,1 1 ,12 ,13.
Penning Trap
The selection of the particular ion trap mostsuitable for accelerator-produced low energy recoilions involved a choice among the three basic ion trapconfigurations: the dc electrostatic (Kingdon) trap,the radio-frequency quadrupole trap, and the Penningtrap.11 A Penning ion trap was selected as bestsuited for the initial studies since the productioncross sections were expected to be low, and the recoilenergies uncertain. Although it has the disadvantageof subjecting ions to a strong magnetic field, theions do experience periodic motion in several differ-ent characteristic modes while they are still insidethe Penning trap. External electronic coupling tothese oscillating ions allows simultaneous detectionand species identification with a high sensitivity po-tentially capable of detecting even a few ions. (TheRF trap also has these advantages, but is more complexin operation.) Furthermore, in the Penning trap ionsin higher charge states are more strongly bound, es-pecially those recoiling into a large solid angle per-pendicular to the fast ion beam axis, where they aretrapped primarily by the strong magnetic potential.While the relatively narrow pole gap of the electro-magnet limits the physical size of the electrodes andconsequently the volume overlap of the fast ion beamwith the trapping region, one can take advantage ofthe stability criterion for bounded motion (see equa-tion 2). The electric potential can be chosen to ex-clude trapping of the vastly more numerous singly anddoubly charged ions which limit the total storagecharge.
In an ideal Penning trap having a purely quadru-polar electrostatic field and a uniform magneticfield, the potentials are:
+= /4Z02)(r - 2Z2) + V/2} (la)A = B0/2 (-Y,X,O)
ions with mass m and charge q in these potentialsoscillate with SHM along the Z axis with frequencywz=(V0mZ2 1/2uz=(qV /MZ02)l/. Investigating the equations of
motion for bounded solutions to transverse motionyields the critical criterion for stability
q/m > 2V0/B022
(lb)
(2)
Transverse motion is characterized by two angularfrequencies, w± = wc/2 ± (wc2/4 - wz /2)l/2, in whichwc=(qBo)/m, the usual cyclotron angular frequency.In practice it is usually found that wc>>wz.This condition is true, for example, when Bo islarge and justifies the approximation of w+ by aTaylor series expansion. Thus
0018-9499/83/0400- 977$01.00 1983 IEEE
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+ Uc- Up (3a)W( p = 2/20c (3b)
An ion deeply caught in this kind of electrostatic andmagnetic field experiences three principal modes ofmotion. Parallel to the symmetry axis Z, along themagnetic field Bo, the ions experience a restoringforce toward the center plane of the trap (Z = 0) andoscillate harmonically. In the plane perpendicular tothe B field, the ions follow cyclotron orbits aboutthe magnetic field lines, coupled with a much slowerprecessional drift about the symmetry axis.14
The Penning-type quadrupole ion trap consists ofthree cylindrically symmetric electrodes supporting adc voltage VO which defines the quadrupole poten-tial. While axial confinement of charged particles isprovided by this dc well, radial confinement is ac-complished by means of the uniform magnetic field Bodirected along the symmetry axis. Our trap has twopairs of holes in each of the OFHC copper sphericalsection endcaps. These endcaps approximate the hyper-bolic surfaces of an ideal trap. The axial holes al-low the passage of the high energy projectile beamfrom the accelerator. The second pair of small holesoff axis accommodates an internal electron beam sourcefor test and calibration purposes. Two split ringsare used to approximate the usual hyperbolic ringelectrode; this forms a very open structure with easyaccess (see Figure 1). The advantages of this designare simplified construction, high vacuum conductance,and efficient coupling to the'w+ (cyclotron) mode ofion oscillation. Our magnetic field was typically 0.6tesla produced by an electromagnet having 15 cm diame-ter tapered pole tips 5.5 cm apart with a 0.65 cmaxial hole for the focused heavy ion beam. The homo-geniety over the spatial region occupied by ions wasfound to be 5 parts in 104 by examing the w+ re-sonance width of trapped ions.
the resonant frequency of the tuned circuit (approxi-mately 0.1-1 MHz). This circuit can exchange energywith the axial ion motion at angular frequency wz =(LC)-l/2. Typically the tuned circuit is weakly dri-ven by an external oscillator at its resonant frequen-cy and, as the potential VO is ramped to zero, eachdiferent species of m/q ions successively damps thecircuit '(see Figure 2). It is this amplitude modula-tion with an amplitude proportional to the number ofions N and with its position in time proportional tothe voltage VO which we signal-average over manysweeps for each spectrum. We can easily detect a hun-dred ions with a single sweep. The rf absorption sig-nal power loss Ps is simply related to the averagepower Prf dissipated in the tuned circuit by the re-lation D-A= -
. n IADrs =5 I q n rrfwhere A is a constant which depends upon the parame-ters of the dc voltage sweep and the tuned circuit.The peak areas in each spectrum (see Fig. 3) were nor-malized using a Faraday cup placed downstream from theion trap to integrate charge passing through the trap.In order to insure that the number of trapped ions isproportional to this charge, the fast ion beam pulse(see T1 Figure 2) was wide enough to allow trappedions to reach equilibrium populations. After an equi-librium population produced from the target gas hasbeen formed in the trap, the high energy beam isswitched off by electrostatic deflection plates posi-tioned approximately two meters up-stream.
The absolute number of ions as well as their tem-peratures can be estimated by using the following pow-er relationships.6,14 When ions are trapped, theyproduce a fluctuating noise current while passingthrough resonance which has a mean-squared value pro-portional to the number of ions and to the meanenergy. The power contributed is given by (the equation)
Figure 1. Trap Structure.
Ion Detection
Although several techniques permit sampling ionsin the trap, a resonant detection method is one of themost sensitive and convenient. To detect the ions, weuse the surfaces of the electrodes themselves to pro-vide the capacitance in an LC resonant circuit havinga Q near 300. The frequency of this circuit is suchthat each species can be brought into resonance withthe tuned circuit at some trap potential VO. Con-venient values of the trapping potential VO (on theorder of 1-100 volts) dictate the proper choice for
-Vdc
ion beam delay rampiI ~~~~I
I g~ ~ ~~I- r, --J2---r-2 '- t3@",
-e. generation I --storage-----.4dstct-
Figure 2. Ion detection system and time base.
)t
N k T.P. = 1- (5)1 Td
where Td = (4 m Z02)/(q2R) is the damping con-stant, Ti is the ion temperature, N is the number ofions, k is Boltzman's constant, and Af is the band-width. The thermal noise energy of the electroniccircuit itself in the "bandwidth Af" is given by
P. = 4 k T0A f (6)
where R is the effective resistance of the tuned cir-cuit, and To is the effective temperature of thecircuit in degrees Kelvin. Shot noise produced bycharged particles passing through the trap (by theprojectile beam, for example) also contributes to thenoise power.
Psn = 2 q I R Af (7)
where I is the dc current. Finally, for a resonant LCcircuit with a high Q the following relationshipexi sts
(8)
where f is wz/2n at resonance and we can determineAf, the bandwidth at the half-power points. Evenwithout external rf excitation of axial wz motion,Equation (5) indicates that as the potential VOsweeps the ion oscillation frequency through circuitresonance, they can interact to produce a spectrum.Selected ion species (i.e. those ions with a particu-lar m/q ratio) can be eliminated from the trap bypumping energy from an external oscillator into trans-verse motion at w+ or Wp. Resonance absorption ofrf from the split ring electrodes increases orbital
)I-z
z
CHANNEL NUMBER
Figure 3. Ne+q ion signals, 2<q<6.
979
excursions until ions escape. Hydrogen and heliumions produced by electron excitation using the inter-nal electron gun were easily stored for tens of se-conds at a base pressure less than 1 x 10-9 torr.
Rate Constant Measurements
The number of ions with a given m/q ratio (pro-portional to the peak area in a spectrum) does changein time due to charge transfer collisions with otheratoms. In this case the loss rate of each m/q ratioof the trapped ions is simply proportional to the num-ber of such ions, dN/dt = - knN, where k is called therate constant for charge transfer and n is the numberdensity of the neutral background gas. The rate con-stant k for this kind of reaction is a function of thevelocity distribution for the ions f(v) and a veloci-ty-dependent geometric cross-section o(v).
k = foo f(v)a(v)vdv (9)0
This leads to an exponential decay in the number ofions N still contained after time t
N(t) = N(o)et/T (10)where T = (nk)-1 is the mean lifetime. If we have amixture of background gases in the trap region at var-ious densities ni that interact with the trappedions at different rates ki, then dN = I (-k.n .N).ions~~ ~ ~ ~ '~ dt 1 1
The total time constant T, determined for a simplemixture is
= ( X n j )gi
(11)
If all ki's except for one are known, it is possibleto measure the rate constants for target recoils(Ne+q) reacting with other gases (see Figure 4).
Experimental Arrangements
Neon, a convenient monatomic gas was chosen asthe target atom for its appropriate atomic number rel-ative to that of the fast ion beam. In order to pro-duce recoils with high charge states, even highercharge states in the fast ion beam are required. In acompromise between maximum charge state and beam cur-rent a 35 MeV chlorine +5 beam was carbon foil strip-ped to an equilibrium charge state distribution peakednear +11. In addition to a continuous flow of highpurity target gas (in this case an isotopic mixture.90%2uNe, 10%2 Ne) through the target chamber, afast piezo-electric leak valve could periodically puffgas temporarily to a higher pressure (see Figure 5).With the puffer synchronized to the high energy beamburst, recoil ion production rates are raised (duringT1 in figure 2) for long storage times at very lowcontinuous presure.
Since accurate values are needed for ni as wellas T in order to determine ki several means wereused to control and measure gas pressures. A calibra-ted nude ion gauge and a quadrupole residual gas ana-lyzer continuously measured pressures and gas purity.Both the fast ion beam and the internal electron gunwere used to produce low charge state trapped ions forwhiTh the rate constant k was already known in orderto determine n from equation 10. Direct measurementsof the gas pressure at various points in the vacuumchamber along with calculations of conductance and gastransport rates were used to establish the number den-sity n in the trap region, relative to the nude iongauge.
Results
For the case of a continuous fast ion beam, equi-librium populations of the various recoil ion chargestates are produced in the target gas. The relative
number of stored ions distributed among the different
charge states agreed quantitatively with published re-
sults for ion production using much more highly charg-ed projectiles such as X+38 and U+44.15,16 Us-
ing a continu?us beam trapped ion peaks for chargestates up to 8 were identified (either by w+ ejec-tion or from predicted values of channel number vs
m/q). The apparent absence of charge states 9 and 10
is not surprising given the small cross sections for
ionization of ls electrons already reported16.In order to measure the rate constant for charge
exchange reactions, the fast ion beam is gated and the
recoil ion population measured as a function of bothstorage time and pressure. The linear decay of log(N)vs storage time indicates a simple two body process(see Fig. 5). The rate constants depend on correc-tions for conductance and relative sensitivity to thetotal pressure Pi measured at the nude ion gauge inorder to determine the target gas density n inside thePenning trap. The results quoted in the summary areprel mi nary.
Acknowledgements
We thank R. Doerner, S. Huldt, M. Breinig, G.Glass, L. Anderson, R. Mitchell, and G. Wells for con-tributions to various aspects of the experiment. Thisresearch was supported by the National Science Founda-tion, the U.S. Department of Energy, Division of BasicEnergy Sciences under contract No. W-7405-ENG-26 withthe Union Carbide Corportion; the Texas A&M Center forEnergy and Mineral Resources, an NBS/NSF PrecisionMeasurement Grant, and the Oak Ridge AssociatedUni versities.
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