- Email: [email protected]
Determination of the number of ions in the Oxford electron beam ion trap
Nuclear Instruments and Methods in Physics Research B 98 (1995) 562-565
Beam Interactions with Materials lb Atoms
ELSEVIER
Determination of the number of ions in the Oxford electron beam ion trap H.S. Margolis
*, A.J. Varney, R.A. Jarjis, J.D. Silver
Department of Physics, University of Oxford, Oxford OXI 3PU, UK
Abstract In order to determine absolute numbers of ions trapped in the Oxford electron beam ion trap (EBIT), the efficiency of a lithium-drifted silicon X-ray detector has been characterized over the energy range 1.5-15 keV using the Oxford scanning proton microprobe. Preliminary results for the number densities of neon-like barium and highly charged argon ions are presented.
1. Introduction An electron beam ion trap (EBIT) is now operational at the Clarendon Laboratory in Oxford, providing a convenient spectroscopic source of slow, highly-charged ions. This is very similar in design to the original devices at Livermore [I], with a few modifications [2]. Neutral atoms can be introduced into the trap from a gas injector attached to one of the side ports. So far, argon and krypton have been studied in this way. In addition, barium sputtered or evaporated from the cathode of the electron gun has been trapped. A metallic vapour vacuum arc (MEVVA) ion source [3] is being constructed, to allow the study of metallic ions. The factors which determine the charge state balance and ion energies in an EBIT have been considered by Penetrante et al. [4]. Their computer program to model the atomic processes occurring within the trap has been modified at Oxford, where it has been used to predict optimum operating conditions for experiments planned for the Oxford EBIT [2], and to investigate the range of ion species which may be produced. Due to the lack of experimental data for highly charged ions, the cross sections used in the modelling work are calculated using relatively simple scaling formulae, and there are consequently large uncertainties in the resulting predictions. It is therefore clearly desirable to have some means of determining the charge state balance and total number of trapped ions experimentally.
* Corresponding author, address: Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom; tel. +44 865 272377, fax. + 44 865 272400, e-mail: [email protected]. 0168-583X/95/$09.50
Ion extraction measurements [4] suffer from large uncertainties since the ion extraction and detection efficiencies are not well known and may differ for the various ionization stages. Wong et al. measured the intensity ratios of X-ray emission lines and used theoretical cross sections for their excitation to deduce the ion charge state distribution [5]. However, since the absolute detector efficiencies were unknown, the number of trapped ions could not be deduced. Fourier-transform ion cyclotron resonance spectroscopy has been used on the Livermore Super-EBIT, with one of the aims being to determine the absolute number of ions in the trap for each charge state and element [6]. An estimate of 10’ trapped krypton ions was reported, but no uncertainty was quoted. Most recently, the numbers of U91’ and U9*+ ions trapped in the Super-EBIT have been estimated from nominal detector parameters [7]. The object of the present work is to improve the method of Wong et al. by determining the absolute efficiency of a lithium-drifted silicon X-ray detector (Si(Li) detector), in particular for the low energy region below 5 keV.
2. Detector effkiency
The efficiency E of a Si(Li) detector may be accurately described using the model of Cohen [8]:
c=g(eXP[
- S~i~i])f..,[l-exP(-pSiD)l.
Cl)
Here R is the solid angle subtended by the detector crystal at the source. The pi and di are the attenuation coefficients and thicknesses of the absorbing materials in front of the crystal (a beryllium window, a gold contact layer, a silicon dead layer, and often a layer of condensed ice). Losses to the silicon escape peak are accounted for by f,,,,
0 1995 Elsevier Science B.V. All rights reserved
SSDI 0168-583X(95)00012-7
calibration
H.S. Margolis et al. /Nucl.
and the final factor allows for the fraction of incident photons stopped within the sensitive volume of depth D. Most of the parameters which determine our detector efficiency have been measured experimentally using the Oxford scanning proton microprobe facility [9]. The nominal value of D provided by the manufacturer is assumed, but this will result in negligible errors for X-ray energies less than about 15 keV. Since the experience of other authors shows that the crystal position and active area may differ significantly from manufacturers’ specifications [&lo], both parameters have been determined experimentally for our detector. The position of the crystal within the detector housing was deduced by measuring the variation of X-ray intensity as a function of source to detector distance, using a copper target on the microprobe. The crystal active area was determined using .55Fe and “‘Co sources placed at fixed distances from the detector, and measuring the X-ray count rate with a set of apertures of different diameters placed in front of the detector. To determine the thicknesses of the absorbing layers in front of the crystal, the procedure of JakSiE et al. was adopted, which uses proton-induced X-ray emission (PIXE) from samples containing known proportions of a number of light elements [ll]. The attenuation coefficients of beryllium and ice have a very similar energy dependence [12], and so to a good approximation may be treated as a single layer. The nominal beryllium window thickness was therefore assumed, and an effective ice layer thickness determined. An independent measurement of the mean gold contact thickness was made by a fluorescence method similar to that of Pajek et al. [13], using Rb K X-rays produced by bombarding a rubidium chloride sample with a 3 MeV proton beam. From the intensity ratio of the Au Lo fluorescence peak to the RbKo peak in the PIXE spectrum, the gold layer thickness was determined with an accuracy of 10%. The thicknesses of the ice layer and the silicon dead layer were determined from an analysis of PIXE spectra from two different samples of known composition: NBS standard reference material SRM-477 [14] and Corning B glass [15]. The experimentally determined X-ray yields were compared with theoretical thick target yields using
Table 1 Summary
of the parameters
which determine
563
Instr. and Meth. in Phys. Res. B 98 (1995) 562-565 1.0r
0.80 -
30.60.-ii 2 %
0.40 -
0.20 -
0.01 0.00
I
1.00
I
I
I 4.00
2.00
3.00
X-ray
energy
/
I 5.00
, 6.00
keV
Fig. 1. Intrinsic efficiency of our Si(Li) detector for incident X-ray energies below 6 keV. The solid curve represents the measured efficiency, and the dotted curves show the uncertainty. (Above 6 keV the efficiency remains close to 100% up to about 15 keV.)
the analysis program PIXAN [16], and the ice layer and dead layer thicknesses varied until the best overall agreement was obtained. The measured detector parameters are compared with the nominal values in Table 1. In particular, the active area is seen to be 14% lower than the nominal value. The intrinsic efficiency of the detector, calculated according to Eq. (11, is shown in Fig. 1. It has a resolution of about 150 eV at 5.9 keV (for typical pulse processing times).
3. EBIT results Assuming that the majority of the trapped ions are in their ground state, the rate at which ions in a given ionization stage are excited to a state i is given by 2aLuF ReX = ~ 1 e
Td, / 0 j,(r)n(r>r
dr,
where L is the trap length, uiex the electron impact excitation cross section to the state i, e the electron charge, rd, the drift tube radius, j,(r) the electron beam current density and n(r) the number density of ions in the charge state of interest. Corrections may be necessary to allow for cascades from higher-lying states, and for any
the Si(Li) detector efficiency
Parameter
Nominal value
Measured value
Active area of crystal Active depth of crystal Distance of crystal behind edge of housing Beryllium window thickness Ice layer thickness Gold layer thickness Dead layer thickness
10 mm* 3mm
8.6( + 0.3) mm’ _
7.5 pm _ 40Pgcm-*(=21nm) _
10.3( + 0.9) mm _ 0.4( + 0.6; - 0.2) p,m 13.7( + 1.4) nm 0.3(+0.15) pm
5. PRODUCTION/METHODS/APPLICATIONS
564
H.S. Margolis et al. /Nucl.
6
Ins&. and Meth. in Phvs. Res. B 98 (1995) 562-565
The average count rate at the detector from the (2p,k 3d3,?), transition is 16.6 ( + 0.4) s-‘, and the electron impact excitation cross section is calculated from the collision strengths of Zhang and Sampson [20] to be 1.88 X lo-” cm’, with an estimated uncertainty of 15%. It is found that
I
_c
_6.0x1@ ”
d
n(Ba46+)
$4.0x1@2 02.0x10'-
n=3RR
0.0 2.00
4.00
6.00 energy
8.00 /
10.0
keV
Fig. 2. Spectrum from neon-like Ba46+. This was taken using an electron beam energy of 6.7 keV, an electron beam current of 43.5 mA, an axial magnetic field of 2.65 T, and a trapping potential of 55 v.
anisotropy of the radiation intensity resulting from excitation by a directional electron beam. In future work, the form of je(r)n(r) will be inferred by placing a narrow slit close to the beam, and imaging the emission region onto a position-sensitive X-ray detector. However, for these preliminary results, it is assumed that the electron beam dynamics obey Herrmann’s theory [17] and that n(r) is constant. The latter approximation is best justified when the ion escape rates are high, i.e. for continuous gas injection or for ions originating from the electron gun cathode. It will also be best for high axial trapping potentials, which similarly lead to higher ion temperatures. With these assumptions, e p
n= ETbLI,u,,”
‘-)I’
where T is the transmission of any windows on EBIT and any air path between the trapped ions and the detector, b is the branching ratio and RF! j the count rate for the observed X-ray transition, and I, is the electron beam current. Fig. 2 shows a barium spectrum from the Oxford EBIT, obtained using an electron beam energy of 6.7 keV. As predicted by modelling calculations, the majority of the X-rays observed are from neon-like Ba46+. The strongest transitions below the electron beam energy are designated by the upper level involved (all these transitions are to the ground state). At around 10 keV, a broader peak due to radiative recombination (RR) onto the five n = 3 levels of Ba45+ is clearly observed. The density of Ba46+ ions may be estimated from both the (2p$a 3d3,z)t X-ray yield (this level is populated almost entirely by electron impact excitation from the ground state, and has a branching ratio of unity for decay back to the ground state [18]) and, as done by the NIST EBIT group [19], from the n = 3 RR peak. The number of counts in each peak is estimated by least-squares fitting to the spectrum, assuming Gaussian line profiles.
= 7.4( + 1.2) X 10” cmm3,
corresponding to a total of 6.1 (_t 1.0) X lo4 Ba4hf ions within one Herrmann radius of the electron beam (35.9 pm). As yet, no calculations of the polarization of this line have been reported, and so in the light of the experimental cross sections of Marrs et al. [18], no correction has been made for any anisotropy of the emitted radiation. The count rate on the n = 3 RR peak is 3.5 (kO.7) SC’. Interpolating the values of Scofield [21] gives the total differential cross section at 90” as da”/dR = 15.0 b/sr, with an estimated uncertainty of about 6% (including both theoretical and interpolation errors). Hence n(Ba46+)
= l.lO( kO.25)
X 10” cme3,
corresponding to about 9.2 (k2.1) X 10’ Ba46+ ions within one Herrmann radius. The result is somewhat higher than that from the (2p$ 3d3,?)t lme, posstbly due to a shoulder on the low energy side of the RR line (thought to be due to tungsten), RR into the n = 3 shell of lower charge states, or polarization effects. A spectrum from highly charged argon, taken at an electron beam energy of 6.9 keV, is shown in Fig. 3. To prevent too much barium building up in the trapping region, it was emptied after an ion confinement time of three seconds by removing the axial trapping potential and turning off the electron beam. The strongest peak is due to the n = 2-l transitions in helium-like Art”+, with a highenergy shoulder from the n = 2-l transitions in hydrogen-like Ar”+. The R = 3-l transitions in these ions are also seen. In addition, peaks from RR into the n = 2 and n = 1 states are present. The n = 1 RR feature con-
5.0x10'r
7%=2-l
~‘.OXlOJ :
r” 3.0x10' 0
ii
8.00
6.00
energy
/
10.0
12.0
keV
Fig. 3. Spectrum from highly charged argon ions, taken using an electron beam energy of 6.9 keV, an electron beam current of 51.6 mA, an axial magnetic field of 2.38 T, a trapping potential of 300 V, and an ion confinement time of 3 s.
H.S. Margolis et al. / Nucl. Instr. and Meth. in Phys. Res. B 98 (1995) 562-565
sists of two peaks, separated by about 300 eV, due to recombination onto bare and hydrogenic ions. The lines in the 5 keV region are from neon-like barium. To enable a detailed comparison of the experimental charge state balance with modelling predictions, a higher resolution crystal spectrum should ideally be obtained, so that blends in the Si(Li) spectrum can be investigated. As yet there is no suitable spectrometer available for use on the Oxford EBIT. However, preliminary results may be obtained from the present data. The helium-like and hydrogen-like ion densities are estimated from the strengths of the n = 2-l transitions, for which the polarizations have been calculated theoretically [22]. Using the electron impact excitation cross sections of Sampson and Parks [23] gives n(Ar16+)
= 1.32( kO.40)
X 10’” crnm3,
n(Ar”+)
= 0.71( kO.24)
X 10’” cmp3.
The relative densities of bare and hydrogenic ions are estimated by assuming that the RR cross sections scale as predicted by Kim and Pratt [24]:
n(AP+ ) n(Ar”+
)
= 0.133( *0.013).
The angular anisotropy of the emitted radiation is assumed to be approximately the same for each RR line, and the uncertainties quoted are due to fitting errors only. Increasing the electron beam energy was found experimentally to result in an increased fraction of fully-stripped ions, as expected. Most recently, spectra from highly charged krypton ions have been obtained. Spectral lines from helium-like ti34+ were observed at electron beam energies between 15 keV and 20 keV, but the II = 2-1 peaks were asymmetric, with a low energy tail. The line profile contains a number of unresolved components, including satellite lines resulting from inner-shell excitation in lithium-like and lower charge states. To model the lineshape accurately, and thus deduce information about the charge state balance, requires a higher resolution spectrum to be obtained.
4. Conclusions An efficiency-calibrated detector has been used to investigate the charge state balance in the Oxford EBIT. Spectra of barium have been collected under conditions in which the neon-like charge state dominates, and the number of trapped ions estimated. Preliminary measurements of the charge state balance for argon ions at different electron beam energies have been carried out, yielding results which show qualitative agreement with modelling calculations. Using additional information from higher-resolution
565
crystal spectra, and from imaging of the X-ray emission region, more precise results for the number of trapped ions will be obtained. This should enable a detailed investigation of the factors which determine the ionization balance, and provide information about the validity of the assumptions made in the various models of the EBIT developed to date.
Acknowledgements We thank the Oxford EBIT and SPM groups for experimental assistance, and SERC/EPSRC for funding the EBIT project.
References [l] M.A. Levine, R.E. Marrs, J.R. Henderson, D.A. Knapp and M.B. Schneider, Phys. Scripta T22 (1988) 157. 121 J.D. Silver et al., Rev. Sci. Instr. 6.5 (1994) 1072. [3] LG. Brown, J.E. Galvin, R.A. Ma&ill and R.T. Wright, Appl. Phys. Lett. 49 (1986) 1019. 141 B.M. Penetrante, J.N. Bardsley, D. Dewitt, M. Clark and D. Schneider, Phys. Rev. A 43 (1991) 4861. 151 K.L. Wong, P. Beiersdorfer, R.E. Marrs. B.M. Penetrante, K.J. Reed, J.H. Scofield, D.A. Vogel and R. Zasadzinski, Nucl. Instr. and Meth. B 72 (1992) 234. [6] P. Beiersdorfer, B. Beck, S. Elliott, R. Marrs and L. Schweikhard, Rapid Commun. Mass Spectrom. 8 (1994) 141. 171 R.E. Marrs, S.R. Elliott and D.A. Knapp, Phys. Rev. Lett. 72 (1994) 4082. [8] D.D. Cohen, Nucl. Instr. and Meth. 178 (1980) 481. [9] G.W. Grime, M. Dawson, M. Marsh, I.C. McArthur and F. Watt, Nucl. Instr. and Meth. B 54 (19911 52. [lo] J.L. Campbell, R.G. Leigh and W.J. Teesdale, Nucl. Instr. and Meth. B 5 (1984139. [ll] M. JakSiC, G.W. Grime, J. Henderson and F. Watt, Nucl. Instr. and Meth B 54 (1991) 491. 1121 CA. Baker, C.J. Batty and S. Sakamoto, Nucl. Instr. and Meth. A 259 (1987) 501. [13] M. Pajek, A.P. Kobzev. R. Sandrik, R.A. Ilkhamov and S.H. Khusmurodov, Nucl. Instr. and Meth. B 42 (1989) 346. [14] National Bureau of Standards certificate for Standard Reference Material 477 (1983). 1151 R.H. Brill, Proc. 9th Int. Congr. on Glass, L’Institut du Verre. Paris, 1972, Section Bl, p. 93. [16] E. Clayton, P. Duerden and D.D. Cohen, Nucl. Instr. and Meth. B 22 (1987) 64. [17] G. Herrmann, J. Appl. Phys. 29 (19581 127. [18] R.E. Marrs, M.A. Levine, D.A. Knapp and J.R. Henderson, Phys. Rev. Lett. 60 (1988) 1715. [19] J.D. Gillaspy et al., to be published in Phys. Scripta. [20] H.L. Zhang and D.H. Sampson, At. Data Nucl. Data Tables 43 (1989) 1. 1211 J.H. Scofield, Phys. Rev. A 40 (1989) 3054. [22] K.J. Reed and M.H. Chen, Phys. Rev. A 48 (1993) 3644. [23] D.H. Sampson and A.D. Parks, Astrophys. J. Suppl. 28 (1974) 335. [24] Y.S. Kim and R.H. Pratt, Phys. Rev. A 27 (1983) 2913.
5. PRODUCTION/METHODS/APPLICATIONS
Beam Interactions with Materials lb Atoms
ELSEVIER
Determination of the number of ions in the Oxford electron beam ion trap H.S. Margolis
*, A.J. Varney, R.A. Jarjis, J.D. Silver
Department of Physics, University of Oxford, Oxford OXI 3PU, UK
Abstract In order to determine absolute numbers of ions trapped in the Oxford electron beam ion trap (EBIT), the efficiency of a lithium-drifted silicon X-ray detector has been characterized over the energy range 1.5-15 keV using the Oxford scanning proton microprobe. Preliminary results for the number densities of neon-like barium and highly charged argon ions are presented.
1. Introduction An electron beam ion trap (EBIT) is now operational at the Clarendon Laboratory in Oxford, providing a convenient spectroscopic source of slow, highly-charged ions. This is very similar in design to the original devices at Livermore [I], with a few modifications [2]. Neutral atoms can be introduced into the trap from a gas injector attached to one of the side ports. So far, argon and krypton have been studied in this way. In addition, barium sputtered or evaporated from the cathode of the electron gun has been trapped. A metallic vapour vacuum arc (MEVVA) ion source [3] is being constructed, to allow the study of metallic ions. The factors which determine the charge state balance and ion energies in an EBIT have been considered by Penetrante et al. [4]. Their computer program to model the atomic processes occurring within the trap has been modified at Oxford, where it has been used to predict optimum operating conditions for experiments planned for the Oxford EBIT [2], and to investigate the range of ion species which may be produced. Due to the lack of experimental data for highly charged ions, the cross sections used in the modelling work are calculated using relatively simple scaling formulae, and there are consequently large uncertainties in the resulting predictions. It is therefore clearly desirable to have some means of determining the charge state balance and total number of trapped ions experimentally.
* Corresponding author, address: Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom; tel. +44 865 272377, fax. + 44 865 272400, e-mail: [email protected]. 0168-583X/95/$09.50
Ion extraction measurements [4] suffer from large uncertainties since the ion extraction and detection efficiencies are not well known and may differ for the various ionization stages. Wong et al. measured the intensity ratios of X-ray emission lines and used theoretical cross sections for their excitation to deduce the ion charge state distribution [5]. However, since the absolute detector efficiencies were unknown, the number of trapped ions could not be deduced. Fourier-transform ion cyclotron resonance spectroscopy has been used on the Livermore Super-EBIT, with one of the aims being to determine the absolute number of ions in the trap for each charge state and element [6]. An estimate of 10’ trapped krypton ions was reported, but no uncertainty was quoted. Most recently, the numbers of U91’ and U9*+ ions trapped in the Super-EBIT have been estimated from nominal detector parameters [7]. The object of the present work is to improve the method of Wong et al. by determining the absolute efficiency of a lithium-drifted silicon X-ray detector (Si(Li) detector), in particular for the low energy region below 5 keV.
2. Detector effkiency
The efficiency E of a Si(Li) detector may be accurately described using the model of Cohen [8]:
c=g(eXP[
- S~i~i])f..,[l-exP(-pSiD)l.
Cl)
Here R is the solid angle subtended by the detector crystal at the source. The pi and di are the attenuation coefficients and thicknesses of the absorbing materials in front of the crystal (a beryllium window, a gold contact layer, a silicon dead layer, and often a layer of condensed ice). Losses to the silicon escape peak are accounted for by f,,,,
0 1995 Elsevier Science B.V. All rights reserved
SSDI 0168-583X(95)00012-7
calibration
H.S. Margolis et al. /Nucl.
and the final factor allows for the fraction of incident photons stopped within the sensitive volume of depth D. Most of the parameters which determine our detector efficiency have been measured experimentally using the Oxford scanning proton microprobe facility [9]. The nominal value of D provided by the manufacturer is assumed, but this will result in negligible errors for X-ray energies less than about 15 keV. Since the experience of other authors shows that the crystal position and active area may differ significantly from manufacturers’ specifications [&lo], both parameters have been determined experimentally for our detector. The position of the crystal within the detector housing was deduced by measuring the variation of X-ray intensity as a function of source to detector distance, using a copper target on the microprobe. The crystal active area was determined using .55Fe and “‘Co sources placed at fixed distances from the detector, and measuring the X-ray count rate with a set of apertures of different diameters placed in front of the detector. To determine the thicknesses of the absorbing layers in front of the crystal, the procedure of JakSiE et al. was adopted, which uses proton-induced X-ray emission (PIXE) from samples containing known proportions of a number of light elements [ll]. The attenuation coefficients of beryllium and ice have a very similar energy dependence [12], and so to a good approximation may be treated as a single layer. The nominal beryllium window thickness was therefore assumed, and an effective ice layer thickness determined. An independent measurement of the mean gold contact thickness was made by a fluorescence method similar to that of Pajek et al. [13], using Rb K X-rays produced by bombarding a rubidium chloride sample with a 3 MeV proton beam. From the intensity ratio of the Au Lo fluorescence peak to the RbKo peak in the PIXE spectrum, the gold layer thickness was determined with an accuracy of 10%. The thicknesses of the ice layer and the silicon dead layer were determined from an analysis of PIXE spectra from two different samples of known composition: NBS standard reference material SRM-477 [14] and Corning B glass [15]. The experimentally determined X-ray yields were compared with theoretical thick target yields using
Table 1 Summary
of the parameters
which determine
563
Instr. and Meth. in Phys. Res. B 98 (1995) 562-565 1.0r
0.80 -
30.60.-ii 2 %
0.40 -
0.20 -
0.01 0.00
I
1.00
I
I
I 4.00
2.00
3.00
X-ray
energy
/
I 5.00
, 6.00
keV
Fig. 1. Intrinsic efficiency of our Si(Li) detector for incident X-ray energies below 6 keV. The solid curve represents the measured efficiency, and the dotted curves show the uncertainty. (Above 6 keV the efficiency remains close to 100% up to about 15 keV.)
the analysis program PIXAN [16], and the ice layer and dead layer thicknesses varied until the best overall agreement was obtained. The measured detector parameters are compared with the nominal values in Table 1. In particular, the active area is seen to be 14% lower than the nominal value. The intrinsic efficiency of the detector, calculated according to Eq. (11, is shown in Fig. 1. It has a resolution of about 150 eV at 5.9 keV (for typical pulse processing times).
3. EBIT results Assuming that the majority of the trapped ions are in their ground state, the rate at which ions in a given ionization stage are excited to a state i is given by 2aLuF ReX = ~ 1 e
Td, / 0 j,(r)n(r>r
dr,
where L is the trap length, uiex the electron impact excitation cross section to the state i, e the electron charge, rd, the drift tube radius, j,(r) the electron beam current density and n(r) the number density of ions in the charge state of interest. Corrections may be necessary to allow for cascades from higher-lying states, and for any
the Si(Li) detector efficiency
Parameter
Nominal value
Measured value
Active area of crystal Active depth of crystal Distance of crystal behind edge of housing Beryllium window thickness Ice layer thickness Gold layer thickness Dead layer thickness
10 mm* 3mm
8.6( + 0.3) mm’ _
7.5 pm _ 40Pgcm-*(=21nm) _
10.3( + 0.9) mm _ 0.4( + 0.6; - 0.2) p,m 13.7( + 1.4) nm 0.3(+0.15) pm
5. PRODUCTION/METHODS/APPLICATIONS
564
H.S. Margolis et al. /Nucl.
6
Ins&. and Meth. in Phvs. Res. B 98 (1995) 562-565
The average count rate at the detector from the (2p,k 3d3,?), transition is 16.6 ( + 0.4) s-‘, and the electron impact excitation cross section is calculated from the collision strengths of Zhang and Sampson [20] to be 1.88 X lo-” cm’, with an estimated uncertainty of 15%. It is found that
I
_c
_6.0x1@ ”
d
n(Ba46+)
$4.0x1@2 02.0x10'-
n=3RR
0.0 2.00
4.00
6.00 energy
8.00 /
10.0
keV
Fig. 2. Spectrum from neon-like Ba46+. This was taken using an electron beam energy of 6.7 keV, an electron beam current of 43.5 mA, an axial magnetic field of 2.65 T, and a trapping potential of 55 v.
anisotropy of the radiation intensity resulting from excitation by a directional electron beam. In future work, the form of je(r)n(r) will be inferred by placing a narrow slit close to the beam, and imaging the emission region onto a position-sensitive X-ray detector. However, for these preliminary results, it is assumed that the electron beam dynamics obey Herrmann’s theory [17] and that n(r) is constant. The latter approximation is best justified when the ion escape rates are high, i.e. for continuous gas injection or for ions originating from the electron gun cathode. It will also be best for high axial trapping potentials, which similarly lead to higher ion temperatures. With these assumptions, e p
n= ETbLI,u,,”
‘-)I’
where T is the transmission of any windows on EBIT and any air path between the trapped ions and the detector, b is the branching ratio and RF! j the count rate for the observed X-ray transition, and I, is the electron beam current. Fig. 2 shows a barium spectrum from the Oxford EBIT, obtained using an electron beam energy of 6.7 keV. As predicted by modelling calculations, the majority of the X-rays observed are from neon-like Ba46+. The strongest transitions below the electron beam energy are designated by the upper level involved (all these transitions are to the ground state). At around 10 keV, a broader peak due to radiative recombination (RR) onto the five n = 3 levels of Ba45+ is clearly observed. The density of Ba46+ ions may be estimated from both the (2p$a 3d3,z)t X-ray yield (this level is populated almost entirely by electron impact excitation from the ground state, and has a branching ratio of unity for decay back to the ground state [18]) and, as done by the NIST EBIT group [19], from the n = 3 RR peak. The number of counts in each peak is estimated by least-squares fitting to the spectrum, assuming Gaussian line profiles.
= 7.4( + 1.2) X 10” cmm3,
corresponding to a total of 6.1 (_t 1.0) X lo4 Ba4hf ions within one Herrmann radius of the electron beam (35.9 pm). As yet, no calculations of the polarization of this line have been reported, and so in the light of the experimental cross sections of Marrs et al. [18], no correction has been made for any anisotropy of the emitted radiation. The count rate on the n = 3 RR peak is 3.5 (kO.7) SC’. Interpolating the values of Scofield [21] gives the total differential cross section at 90” as da”/dR = 15.0 b/sr, with an estimated uncertainty of about 6% (including both theoretical and interpolation errors). Hence n(Ba46+)
= l.lO( kO.25)
X 10” cme3,
corresponding to about 9.2 (k2.1) X 10’ Ba46+ ions within one Herrmann radius. The result is somewhat higher than that from the (2p$ 3d3,?)t lme, posstbly due to a shoulder on the low energy side of the RR line (thought to be due to tungsten), RR into the n = 3 shell of lower charge states, or polarization effects. A spectrum from highly charged argon, taken at an electron beam energy of 6.9 keV, is shown in Fig. 3. To prevent too much barium building up in the trapping region, it was emptied after an ion confinement time of three seconds by removing the axial trapping potential and turning off the electron beam. The strongest peak is due to the n = 2-l transitions in helium-like Art”+, with a highenergy shoulder from the n = 2-l transitions in hydrogen-like Ar”+. The R = 3-l transitions in these ions are also seen. In addition, peaks from RR into the n = 2 and n = 1 states are present. The n = 1 RR feature con-
5.0x10'r
7%=2-l
~‘.OXlOJ :
r” 3.0x10' 0
ii
8.00
6.00
energy
/
10.0
12.0
keV
Fig. 3. Spectrum from highly charged argon ions, taken using an electron beam energy of 6.9 keV, an electron beam current of 51.6 mA, an axial magnetic field of 2.38 T, a trapping potential of 300 V, and an ion confinement time of 3 s.
H.S. Margolis et al. / Nucl. Instr. and Meth. in Phys. Res. B 98 (1995) 562-565
sists of two peaks, separated by about 300 eV, due to recombination onto bare and hydrogenic ions. The lines in the 5 keV region are from neon-like barium. To enable a detailed comparison of the experimental charge state balance with modelling predictions, a higher resolution crystal spectrum should ideally be obtained, so that blends in the Si(Li) spectrum can be investigated. As yet there is no suitable spectrometer available for use on the Oxford EBIT. However, preliminary results may be obtained from the present data. The helium-like and hydrogen-like ion densities are estimated from the strengths of the n = 2-l transitions, for which the polarizations have been calculated theoretically [22]. Using the electron impact excitation cross sections of Sampson and Parks [23] gives n(Ar16+)
= 1.32( kO.40)
X 10’” crnm3,
n(Ar”+)
= 0.71( kO.24)
X 10’” cmp3.
The relative densities of bare and hydrogenic ions are estimated by assuming that the RR cross sections scale as predicted by Kim and Pratt [24]:
n(AP+ ) n(Ar”+
)
= 0.133( *0.013).
The angular anisotropy of the emitted radiation is assumed to be approximately the same for each RR line, and the uncertainties quoted are due to fitting errors only. Increasing the electron beam energy was found experimentally to result in an increased fraction of fully-stripped ions, as expected. Most recently, spectra from highly charged krypton ions have been obtained. Spectral lines from helium-like ti34+ were observed at electron beam energies between 15 keV and 20 keV, but the II = 2-1 peaks were asymmetric, with a low energy tail. The line profile contains a number of unresolved components, including satellite lines resulting from inner-shell excitation in lithium-like and lower charge states. To model the lineshape accurately, and thus deduce information about the charge state balance, requires a higher resolution spectrum to be obtained.
4. Conclusions An efficiency-calibrated detector has been used to investigate the charge state balance in the Oxford EBIT. Spectra of barium have been collected under conditions in which the neon-like charge state dominates, and the number of trapped ions estimated. Preliminary measurements of the charge state balance for argon ions at different electron beam energies have been carried out, yielding results which show qualitative agreement with modelling calculations. Using additional information from higher-resolution
565
crystal spectra, and from imaging of the X-ray emission region, more precise results for the number of trapped ions will be obtained. This should enable a detailed investigation of the factors which determine the ionization balance, and provide information about the validity of the assumptions made in the various models of the EBIT developed to date.
Acknowledgements We thank the Oxford EBIT and SPM groups for experimental assistance, and SERC/EPSRC for funding the EBIT project.
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5. PRODUCTION/METHODS/APPLICATIONS