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Measurements of nuclear parameters of high-Z isotopes performed on a high-energy electron beam ion trap
NUCLEAR PHYSICS A ELSEVIER
Nuclear Physics A626 (1997) 357c-364c
M e a s u r e m e n t s of nuclear p a r a m e t e r s of high-Z isotopes p e r f o r m e d on a h i g h - e n e r g y e l e c t r o n b e a m ion t r a p P. Beiersdorfer, ~ S. R. Elliott? J. Crespo Ldpez-Urrutia, a and K. Widmann ~ ~Department of Physics and Space Technology, Lawrence Livermore National Laboratory, Livermore, CA 945,50, USA SDepartment of Physics, University of Washington, P.O.Box 351560, Seattle, WA 98195, USA
Spectral measurements of highly charged ions have been performed on the Livermore high-energy electron beam ion trap (EBIT) for determining various nuclear parameters. A value of # = 4.1267/ZN was inferred for the nuclear magnetic moment from a measurement of the hyperfine transition of 165H066+ in the visible. Values of (5(r2}2aa'2as = -0.432 fm 2 and ~5{r2)235'238 = -0.250 fm ~ were determined for the isotopic variation of the nuclear charge radius from the energy shift of L-shell transitions in 2aaUq+ and 235Uq+ relative to the x-ray line emission from 23sUq+ ions, with q = 86, 87, 88, 89. The accuracy of these measurements equals or exceeds that of earlier measurements relying on different techniques, because the atomic structure of highly charged ions is greatly simplified compared to that of neutral or few-times charged ions. The measurements can be extended to a variety of isotopes from hydrogen to transuranic eleraents. The latter was demonstrated in the first production of highly charged 249Cf ions in the trap. 1. I N T R O D U C T I O N In the following we describe spectroscopic measurements involving highly charged ions that are used to infer nuclear parameters. Our measurements include determinations of the variation of the nuclear charge radii among the uranium isotopes 2aaU, 2asU, and 238U, as well as the determination of the nuclear magnetic moment of 165Ho. These measurements were carried out on the high-energy EBIT facility at the Lawrence Livermore National Laboratory [1]. With this device virtually any ion, including bare uranium, can be produced, trapped, and studied spectroscopically. Moreover, virtually any element can be injected for study. As we show below, this includes radioactive isotopes with half lives of 10: years or longer. Our uses of an EBIT build upon other successful uses of traps for nucear physics studies, such as the trapping of neutral atoms for studying beta decay asymemetries [2,3], or the trapping of low-charge ions for measuring the masses of 0375-9474/98/$19.00 Elsevier Science B.V. PII S0375-9474(97)00558-7
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t? Beiersdorfer et al./Nuclear Physics A626 (1997) 357c 364c
unstable isotopes /4,5]. Our measurements extend the use of traps to the realm of highly charged ions. Because our measurements are performed on highly charged ions, such as hydrogenlike and lithiumlike ions with only one or three electrons remaining, respectively, the atomic physics is greatly simplified. In our measurements, the uncertainty in the atomic physics is u n i m p o r t a n t compared to the statistical uncertainty. This contrasts markedly with spectroscopic measurements involving neutral atoms or few-charge ions, where systematic uncertainties in the atomic physics needed to interpret the d a t a are typically the limiting factors. Moreover, ions in an EBIT device are t r a p p e d and excited by a monoenergetic electron beam. This greatly simplifies spectroscopic measurements when compared to other sources of highly charged ions, such as plasma devices or accelerators. Spectroscopic measurements are free of relativistic Doppler shifts or blending with satellites, and density effects are negligibe because of the low electron density (< 1012 cm -3) of the source [6,7]. Even Doppler broadening can be reduced to levels below that detectable with most spectrometers. By an appropriate choice of the operating parameters, ion thermal temperatures as low as 70 eV were achieved, for example, for Ti 2°+ ions [8,9]. These ion thermal temperatures are so low that the ions can be considered to be completely at rest for spectroscopic purposes. As a consequence, the spectroscopy of trapped, stationary highly charged ions opens tip the possibility to determine various nuclear parameters with unprecedented accuracy. This paper is organized as follows. First, we give a brief description of the EBIT source. The results of the injection of transuranic elements into E B I T are given in Section 3. A discussion of our measurements of the isotopic variation of the nuclear charge radii is given in Section 4. Measurements of nuclear magnetic moments are presented in Section 5. A brief delineation of other possible nuclear physics measurements with t r a p p e d highly charged ions is given in the Conclusion. 2. S O U R C E
CHARACTERISTICS
The high-energy EBIT device, or SuperEBIT [1] uses a monoenergetic electron beam to sequentially ionize and excite ions t r a p p e d in a potential well formed by an externally applied potential and the space charge of the electron beam. The device was designed a.s a spectroscopic source, and six radial ports allow line-of-sight access to the trapping region. The source dimensions are defined by the length of the trap, about 4 cm, and the diameter of the electron beam, about 70 Iml. The dimensions are such that the source may form the entrance slit of a spectrometer. Several different spectrometers have been optimized for such a source configuration. These include x-ray spectrometers in the yon H~mos geometry [10,11], and a prism spectrograph for measurements in the visible, as discussed in more detail in the following sections. A variety of high-purity germanium detectors are available for monitoring the x-ray emission over a broad band of energies. Several different methods are available for injecting ions or neutrals into the trap region. One technique is based on the metal vapor vacuum arc (MeVVA)[12]. Depending on the operation mode, the MeVVA plasma contains ions from either the cathode or the
P Beiersdorfer et al./Nuclear Physics A626 (1997) 357c-364c
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trigger material. The ions are extracted from the plasma and focused into the trap region. A method to introduce neutrals is a gas injector. The pressure in the gas injector can be adjusted between 10 .9 to 10 .6 Torr, which is at least three to six orders of magnitude higher than the pressure in the trap itself. Precise alignment of the gas injector ensures that the atoms or molecules intersect the electron beam, where they get ionized and trapped. A continuous injection of low-Z elements, such as nitrogen or neon, is very important for the evaporative cooling of the collisionally heated highly charged ions [13]. The ionization balance in SuperEBIT is optimized by choosing an appropriate electron beam energy, trap depth, cooling gas pressure, and the time during which the ions are kept in the trap before they are dumped (typically seconds to minutes) and the trap is filled with new ions. For mid-Z elements, almost pure (> 90%) charge states can be obtained for most ion species, especially for ions with a closed-shell configuration. For heavy elements, typically no more than 20 ~ of the ions are in lhe highest charge states. 3. I N J E C T I O N
OF RADIOACTIVE
ISOTOPES
Typically, the EBIT trap is filled with < 106 ions of interest. This is such a small quantity that even radioactive elements can, in principle, be used in EBIT without appreciable contamination of the facility. A method to inject trace amounts of radioactive material into EBIT was described by Elliott and Marrs [14]. It relies on plating a small amount of the desired material on the pointed end of a thin wire. The wire is brought near the electron beam, where sputtering caused by trapped ions surrounding the electron beam slowly transfers the material to the trap. The method was originally demonstrated using a platinum wire plated with 100 ng of gold. In order to demonstrate the utility of the method to inject transuranic ions into EBIT, we employed the coated-wire technique to introduce 249Cf into EBIT. Californium (Z=98) is the highest-Z element with a lifetime of several hundred years. A platinum wire was electrolytically coated with 5 ng (20 nC) of 249Cf, and inserted into the EBIT trap. The trap was operated with a continuous 250-mA electron beam at 150 keV. A minimum of about 1.5 minutes was required before radiation from Cf was seen in the x-ray spectrum. A spectrum collected within 30 minutes after inserting the Cf-coated wire is shown in Fig. l(a). The x-ray spectrum was recorded with a 5-cm diameter, 2-cm thick Ge detector that viewed the EBIT trap through one of its radial ports. The spectrum shows two features, at 110 and 114 keV, attributed to the n=2 to n = l transitions in highly charged Cf. The lower-energy feature corresponds to transitions of an electron in the 2sl/2 or 2pl/2 subshell to the lSl/2 core; the higher-euergy feature corresponds to 2p3/2 --+ lsl/2 transitions. The intensity of the lower-energy feature is clearly higher than that of the higher-energy feature. This intensity ratio is very different from that observed in an x-ray tube where a neutral element is bombarded with an electron beam and the higher-energy feature, the so-called Kal line, is about twice the size as the lower-energy feature, the so-called Kc~ line. Only highly charged ions emit l(c~ features where the lower-energy feature dominates [1,5]. The observed intensity ratio is, thus, a definitive signature that the Cf was highly ionized. For comparison, we show a spectrum of highly charged nranium
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P. Beiersdorfer et al./Nuclear Physics A626 (1997) 357c-364c 700 6OO
r
i
,
(a) _
~
400
J
300
j=l/2
,
i
i
j=l/2 A
i
(b) Uranium
¢n
8
200
200
100
IO0
0
i
600 500
Californium -
5OO
"5
700
i
I
90
i
t
i
I
100 110 Energy (keV)
i
I
120
•
,
130
90
100 110 Energy (keV)
120
130
Figure 1. K a spectra of (a) highly charged 249Cf and (b) highly charged 238U recorded with a high-purity Ge detector. The label refers to the spin-orbit angular momentum of the excited electron. in Fig. l(b). The experiment with 24vCf can be considered a sucessful first step that demonstrates the possibility of introducing transuranic elements into the trap for study. The overall intensity of the Cf emission was relatively weak. In fact, the Cf emission had to compete with the emission from ]ower-Z elements, such as tungsten, barium, or osmium emanating from the electron gun filament and entering the trap. It will be necessary to increase the amount of Cf plated on the wire in order to increase the Cf signal. This is readily possible, as the amount of Cf coated on the wire is still much smaller than radiologically or toxicologically possible. The use of transuranic elements and other radioactive isotopes in an EBIT will undoubtedly increase in the future. 4. M E A S U R E M E N T S
OF THE NUCLEAR
MAGNETIC
MOMENT
The interaction between the magnetic moment of the nucleus and the electron spin results in the splitting of the ls ground level of atomic hydrogen. It also split the 1s level of the appropriate hydrogenlike ions. While line emission arising from the ls hyperfine splitting has been observed from It, D, and He + in radio astronomy, laboratory observation of spontaneous line emission has been precluded by the extremely long lifetime of the upper hyperfine level in such low-Z systems (1.1 × 107 years for H) [16,17]. The lifetime is very much shorter for some of the highly charged hydrogenlike ions. For hydrogenlike 2°gBiS2+ it was measured at the ESR heavy-ion storage ring to be only 0.351(16) ms [18]. Consequently, observation of the spontaneous line emission between the ls hyperfine levels of highly charged ions is readily possible. We have made the first such observation very recently, when we measured the F = 4 -~ F = 3 transition among the ground configuration of hydrogenlike 165tto~+ [19]. We are now in the process of measuring the corresponding transition in XSSRe74+ and 187Re74+. A spectrum of the hyperfine transition in 165t]o66+ is shown in Fig. 2. The wavelength of the hyperfine transition was determined to be 5726.4 ± 1.5 ~(air). This corresponds to a vacuum wavelength of 5727.9 ± 1.5 ~, and a transition energy of 2.1646(6) eV [19].
P. Beiersdorfer et al./Nuclear Physics A626 (1997) 357c-364c
t 1500
,
i
•
J,
i
F---4 - - F = 3
(5726.4
,
i
361c
•
+ 1 . 5 ) ]k
1000
5
-500 L 5500
,
~ 5600
,
, 5700
,
~ 5800
,
, 5900
, 6000
W a v e l e n g t h (/~)
Figure 2. Spectrum of the l s ground-state hyperfine transition in ~6'~Hoe'~+ recorded with a prism spectrograph. From the measured transition energy A E we can infer the vahle of the nuclear magnetic moment #I using the formula [20] A E --
a4Za # I m ¢ ( 2 I + 1)m¢c2[A(1 - 6)(1 - ¢) + ,~~d]. 6 Imp
(1)
Here, c~ is the fine structure constant; me and rap are electron and proton masses, respectively; I is the nuclear spin; A is the relativistic correction factor; 6 is the nuclear charge distribution correction; c is the nuclear magnetization distribution correction; X~d is the radiative correction. Because the values of A and ~aa can be calculated to a very high degree of precision, Eq. (1) can be soved for the nuclear parameters once A E is known. Using the values for the atomic parameters from Ref. [20] we infer #I = 4.1267(11)#N from our measurement of A E . The uncertainty limits reflect solely the uncertainty in the wavelength measurement of the lSSHoS6+ hyperfine transition. Additional, systematic errors arise from uncertainties in the atomic calculations. These have not yet been estimated but are thought to be no larger than a few times the uncertainty from the wavelength measurement alone. We note that the radiative corrections are very small. This is because of a near cancellation of the QED terms arising from the vacuum polarization and the electron self energy [21,22]. In fact, most of the theoretical uncertainties arise from the correct treatment of other nuclear paramenters, i.e., of the distribution of the nuclear charge and of the nuclear magnetization. A comparison of the value of [l I inferred from our measurements with those from earlier measurements is given in Table I. Our value differs significantly from the value [l I 4.160(27)ILN listed in the compilation by Browne et al. [23]. This value was measured using the atomic beam resonance method [24], later recalculated [25], and is cited in the literature as 4.160(27)#N or 4.173(27)#u, depending on the diamagnetic correction employed. Our value agrees better with the newer measurement compiled in the reviews of Peker [26]. The comparison in Table I shows that our method is an accurate alternative for measuring the value of #I, even if the uncertainty from the atomic calculations are taken into =
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P. Beiersdorfer et al./Nuclear Physics A626 (1997) 357c-364c
Table 1. Comparison of the nuclear magnetic moment inferred from the EBIT measurement and those cited in the literature. The uncertainty in the EBIT measurement includes only the experimental uncertainty. The overall uncertainty is no more than a few times larger due to the uncertainty in the atomic calculations. Ref. [23]
Ref. [26]
present measurement
4.1730(270)jUN
4.1320(50)#N
4.1267(11)pN
account. It certainly provides more accurate values in cases where unknown chemical shifts make standard methods of determing the magnetic moment inaccurate beyond the uncertainties commonly cited in the literature, as stressed recently by Persson e* al. [22]. 5. I S O T O P I C
VARIATION
OF THE NUCLEAR
CHARGE
DISTRIBUTION
The effect of the nuclear charge distribution on the ls hyperfine transitions is small because to first order it affects both hyperfine levels with equal amounts. We can maximize the effect and measure it spectroscopically by focusing on transitions between levels that are unequally affected. To demonstrate the technique, we investigated the 2p3/2 --+ 2Sl/2 transitions in highly charged uranium ions [27]. Variations in the nuclear charge distribution affect almost exclusively the 2s configuration and not the 2p configuration, because only the 2s wavefunction has a significant overlap with the nucleus. Spectra of the 2pa/2 -+ 2Sl/2 transitions in lithiumlike through carbonlike uranium, U sg+ to U 86+, are shown in Fig. 3. We studied the three isotopes, 2aaU, 235U, and 2aau. A shift in the x-ray energy of the different lines for different isotopes is clearly seen. The average shift is A E 233'23s = 0.32 eV and A E 2as'2as = 0.18 eV. For comparison, the observed x-ray line width is about 1.0 eV. In order to infer the the variation in the mean-square nuclear charge radius, ~5(r2), we need to know the shift in the Dirac energy, ~SEco~. For the lines studied, ~SEco~l almost exactly equals the observed shift. The reason for this is that to first order none of the atomic energy terms vary for different isotopes except the Dirac energy. In particular, the combined isotopic variation of the specific mass shift, which plays a major role in optical isotope shift measurements of neutral or singly charged uranium, and that of the radiative terms is less than 1 meV and can thus be neglected. A slight variation is found only for the nuclear polarization. This variation is estimated to be less than 4 meV between 2aaU and 238U and even less between easIJ and 238U. Analyzing the shift, of each transition separately and averaging, we find ¢5{r2}233'238 = -0.432(43) fln 2 and a(,,~}235,23s = -0.250(32) fmL The uncertainties include uncertainties from both experiment and atomic theory. However, the uncertainty in the atomic physics is no more than 2 fm 2. Our result can be compared with that of previous studies: neutral-atom optical and K¢* isotope shift measurements [28,29] and muonic-atom x-ray observations [30]. For ~5(r2)233'23s these methods obtained a value of -0.383(44) fm 2 and -0.520(81) fln 2, respec-
P Beiersdorfer et al./Nuclear Physics A626 (1997) 357c--364c
'
1400
-
1200 ta1000"e=800-
'
I
363c
~'
',
L I
i i i
, / f ,2aSUl
i
°
o 600 t
400
B-1,2
2ooL o
i i
'
!o-1 4460
4500
4520
4540
Energy (eV) Figure 3. Spectra of the 2p3/2 --+ 2Sl/2 transitions in the highly charged 233U, 23su, and 2asU isotopes. Individual lines are labeled by the charge state of the emitting ions: C = U s 6 + , B = U s r + , B e = U ss+ a n d L i = U 89+. tively. For 6(r2) 2as'238 these methods obtained a value of -0.246(28) fm ~ and -0.30.5(42) fm 2, respectively. The uncertainties of these measurements are equal or larger than ours and have been subjected to considerable atomic physics corrections. 6. C O N C L U S I O N Precision measurements of the line emission fl'om highly charged ions have focused on providing accurate experimental data for the development of accurate calculational approaches in atomic structure theory. This has lead to significant advances in the accuracy of modern theoretical approaches. In many cases, the uncertainties in atomic theory are now well below the uncertainties in the existing values of nuclear parameters. Spectral measurements involving highly charged ions can thus be used to determine nuclear parameters with an accuracy that may by far exceed that of conventional studies. A systematic measurement of the isotopic variation of the energy of core-level transitions allows a reexamination of much of the nuclear radii data. Measurements of the hyperfine splitting allow direct determinations of nuclear magnetic moments free of chemical effects and without the need for diamagnetic corrections. In the case where the magnetic moment is well known, the measurements provide a way to assess radiative corrections in super strong nuclear fields as well as the distribution of the nuclear magnetization. The study of highly charged ions can thus provide a novel window on nuclear parameters unafforded by conventional methods. This work was performed under the auspices of U.S. Department of Energy' by Lawrence Liverinore National Laboratory under contract No. W-7405-ENG-48.
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P. Beiersdorfer et al./Nuclear Physics A626 (1997) 357c-364c
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
D.A. Knapp, R. E. Marrs, S. R. Elliott, E. W. Magee, and R. Zasadzinski, Nucl. Instrum. Methods A334 (1993) 305. Z.-T. Lu et al., Phys. Rev. Lett. 72 (1994) 3791. G. Gwinner et al., Phys. Rev. Lett. 72 (1994) 3795. H. Stolzenberg et al., Phys. Rev. Lett. 65 (1990) 3104. J. Rink et al., Z. Phys. A 342 (1992) 487. P. Beiersdorfer et al., Phys. Rev. Left. 65 (1990) 1995. P. Beiersdorfer et al., in UV and X-Ray Spectroscopy of Astrophysical and Laboratory Plasmas, ed. by E. Silver and S. Kahn, Cambridge University Press, Cambridge, 1993, p. 59. P. Beiersdorfer, V. Decaux, S. Elliott, K. Widmann, and K. Wong, Rev. Sci. Instrum. 66 (1995) 303. P. Beiersdorfer, V. Decaux, and K. Widmann, Nucl. Instrum. Methods B98 (1995) 566. L. v. Hitmos, Ann. der Physik 17 (1933) 716. P. Beiersdorfer et al., Rev. Sci. Instrum. 61 (1990) 2338. I. G. Brown, J. E. Galvin, R. A. MacGill, and R. T. Wright, Appl. Phys. Lett. 49 (1986) 1019. B.M. Penetrante, M. A. Levine, and J. N. Bardsley, in International Symposium on Electron Beam Ion Sources and their Applications - Upton, NY 1988, AlP Conference Proceedings No. 188, ed. by A. Hershcovitch, AlP, New York, 1989, p. 145; M. B. Schneider et al., ibid, p. 158. S.R. Elliott and R. E. Marrs, Nucl. Instrum. Methods B100 (1995) 529. P. Beiersdorfer et al., Phys. Rev. A 52 (1995) 2693. L. Essen, R. W. Donaldson, M. J. Bangham, and E. G. Hope, Nature 229 (1971) 110. R.J. Gould, Astrophys. J. 423 (1994) 522. I. Klaft et al., Phys. Rev. Lett. 73 (1994) 2425. J.R. Crespo Ldpez-Urrutia, P. Beiersdorfer, D. W. Savin, and K. Widmann, Phys. Rev. Lett. 77 (1996) 826. V.M. Shabaev, J. Phys. B 27 (1994) 5825. S.M. Schneider, W. Greiner, and G. Soft, Phys. Rev. A 50 (1994) 118. H. Persson et al., Phys. Rev. Lett. 76 (1996) 1433. E. Browne et al., in Table of Isotopes, ed. by C. M. Lederer and V. S. Shirley, Wiley, New York, 1978, 7th ed. R.A. Haberstroh, T. I. Moran, and S. Penselin, Z. Phys. 252 (1972) 421. W. Dankwort and J. Ferch, Z. Phys 267 (1974) 239. L.K. Peker, Nucl. Data Sheets 50 (1987) 137. S.R. Elliott, P. Beiersdorfer, and M. H. Chen, Phys. Rev. Lett. 76 (1996) 1031. A. Anastassov et al., Hyperfine Interact. 74 (1992) 31. R . T . Brockmeier, F. Boehm, and E. N. Hatch, Phys. Rev. Lett. 15 (1965) 132. J . D . Zmnbro et al., Phys. Rev. Lett. 53 (1984) 1888.
Nuclear Physics A626 (1997) 357c-364c
M e a s u r e m e n t s of nuclear p a r a m e t e r s of high-Z isotopes p e r f o r m e d on a h i g h - e n e r g y e l e c t r o n b e a m ion t r a p P. Beiersdorfer, ~ S. R. Elliott? J. Crespo Ldpez-Urrutia, a and K. Widmann ~ ~Department of Physics and Space Technology, Lawrence Livermore National Laboratory, Livermore, CA 945,50, USA SDepartment of Physics, University of Washington, P.O.Box 351560, Seattle, WA 98195, USA
Spectral measurements of highly charged ions have been performed on the Livermore high-energy electron beam ion trap (EBIT) for determining various nuclear parameters. A value of # = 4.1267/ZN was inferred for the nuclear magnetic moment from a measurement of the hyperfine transition of 165H066+ in the visible. Values of (5(r2}2aa'2as = -0.432 fm 2 and ~5{r2)235'238 = -0.250 fm ~ were determined for the isotopic variation of the nuclear charge radius from the energy shift of L-shell transitions in 2aaUq+ and 235Uq+ relative to the x-ray line emission from 23sUq+ ions, with q = 86, 87, 88, 89. The accuracy of these measurements equals or exceeds that of earlier measurements relying on different techniques, because the atomic structure of highly charged ions is greatly simplified compared to that of neutral or few-times charged ions. The measurements can be extended to a variety of isotopes from hydrogen to transuranic eleraents. The latter was demonstrated in the first production of highly charged 249Cf ions in the trap. 1. I N T R O D U C T I O N In the following we describe spectroscopic measurements involving highly charged ions that are used to infer nuclear parameters. Our measurements include determinations of the variation of the nuclear charge radii among the uranium isotopes 2aaU, 2asU, and 238U, as well as the determination of the nuclear magnetic moment of 165Ho. These measurements were carried out on the high-energy EBIT facility at the Lawrence Livermore National Laboratory [1]. With this device virtually any ion, including bare uranium, can be produced, trapped, and studied spectroscopically. Moreover, virtually any element can be injected for study. As we show below, this includes radioactive isotopes with half lives of 10: years or longer. Our uses of an EBIT build upon other successful uses of traps for nucear physics studies, such as the trapping of neutral atoms for studying beta decay asymemetries [2,3], or the trapping of low-charge ions for measuring the masses of 0375-9474/98/$19.00 Elsevier Science B.V. PII S0375-9474(97)00558-7
358c
t? Beiersdorfer et al./Nuclear Physics A626 (1997) 357c 364c
unstable isotopes /4,5]. Our measurements extend the use of traps to the realm of highly charged ions. Because our measurements are performed on highly charged ions, such as hydrogenlike and lithiumlike ions with only one or three electrons remaining, respectively, the atomic physics is greatly simplified. In our measurements, the uncertainty in the atomic physics is u n i m p o r t a n t compared to the statistical uncertainty. This contrasts markedly with spectroscopic measurements involving neutral atoms or few-charge ions, where systematic uncertainties in the atomic physics needed to interpret the d a t a are typically the limiting factors. Moreover, ions in an EBIT device are t r a p p e d and excited by a monoenergetic electron beam. This greatly simplifies spectroscopic measurements when compared to other sources of highly charged ions, such as plasma devices or accelerators. Spectroscopic measurements are free of relativistic Doppler shifts or blending with satellites, and density effects are negligibe because of the low electron density (< 1012 cm -3) of the source [6,7]. Even Doppler broadening can be reduced to levels below that detectable with most spectrometers. By an appropriate choice of the operating parameters, ion thermal temperatures as low as 70 eV were achieved, for example, for Ti 2°+ ions [8,9]. These ion thermal temperatures are so low that the ions can be considered to be completely at rest for spectroscopic purposes. As a consequence, the spectroscopy of trapped, stationary highly charged ions opens tip the possibility to determine various nuclear parameters with unprecedented accuracy. This paper is organized as follows. First, we give a brief description of the EBIT source. The results of the injection of transuranic elements into E B I T are given in Section 3. A discussion of our measurements of the isotopic variation of the nuclear charge radii is given in Section 4. Measurements of nuclear magnetic moments are presented in Section 5. A brief delineation of other possible nuclear physics measurements with t r a p p e d highly charged ions is given in the Conclusion. 2. S O U R C E
CHARACTERISTICS
The high-energy EBIT device, or SuperEBIT [1] uses a monoenergetic electron beam to sequentially ionize and excite ions t r a p p e d in a potential well formed by an externally applied potential and the space charge of the electron beam. The device was designed a.s a spectroscopic source, and six radial ports allow line-of-sight access to the trapping region. The source dimensions are defined by the length of the trap, about 4 cm, and the diameter of the electron beam, about 70 Iml. The dimensions are such that the source may form the entrance slit of a spectrometer. Several different spectrometers have been optimized for such a source configuration. These include x-ray spectrometers in the yon H~mos geometry [10,11], and a prism spectrograph for measurements in the visible, as discussed in more detail in the following sections. A variety of high-purity germanium detectors are available for monitoring the x-ray emission over a broad band of energies. Several different methods are available for injecting ions or neutrals into the trap region. One technique is based on the metal vapor vacuum arc (MeVVA)[12]. Depending on the operation mode, the MeVVA plasma contains ions from either the cathode or the
P Beiersdorfer et al./Nuclear Physics A626 (1997) 357c-364c
359c
trigger material. The ions are extracted from the plasma and focused into the trap region. A method to introduce neutrals is a gas injector. The pressure in the gas injector can be adjusted between 10 .9 to 10 .6 Torr, which is at least three to six orders of magnitude higher than the pressure in the trap itself. Precise alignment of the gas injector ensures that the atoms or molecules intersect the electron beam, where they get ionized and trapped. A continuous injection of low-Z elements, such as nitrogen or neon, is very important for the evaporative cooling of the collisionally heated highly charged ions [13]. The ionization balance in SuperEBIT is optimized by choosing an appropriate electron beam energy, trap depth, cooling gas pressure, and the time during which the ions are kept in the trap before they are dumped (typically seconds to minutes) and the trap is filled with new ions. For mid-Z elements, almost pure (> 90%) charge states can be obtained for most ion species, especially for ions with a closed-shell configuration. For heavy elements, typically no more than 20 ~ of the ions are in lhe highest charge states. 3. I N J E C T I O N
OF RADIOACTIVE
ISOTOPES
Typically, the EBIT trap is filled with < 106 ions of interest. This is such a small quantity that even radioactive elements can, in principle, be used in EBIT without appreciable contamination of the facility. A method to inject trace amounts of radioactive material into EBIT was described by Elliott and Marrs [14]. It relies on plating a small amount of the desired material on the pointed end of a thin wire. The wire is brought near the electron beam, where sputtering caused by trapped ions surrounding the electron beam slowly transfers the material to the trap. The method was originally demonstrated using a platinum wire plated with 100 ng of gold. In order to demonstrate the utility of the method to inject transuranic ions into EBIT, we employed the coated-wire technique to introduce 249Cf into EBIT. Californium (Z=98) is the highest-Z element with a lifetime of several hundred years. A platinum wire was electrolytically coated with 5 ng (20 nC) of 249Cf, and inserted into the EBIT trap. The trap was operated with a continuous 250-mA electron beam at 150 keV. A minimum of about 1.5 minutes was required before radiation from Cf was seen in the x-ray spectrum. A spectrum collected within 30 minutes after inserting the Cf-coated wire is shown in Fig. l(a). The x-ray spectrum was recorded with a 5-cm diameter, 2-cm thick Ge detector that viewed the EBIT trap through one of its radial ports. The spectrum shows two features, at 110 and 114 keV, attributed to the n=2 to n = l transitions in highly charged Cf. The lower-energy feature corresponds to transitions of an electron in the 2sl/2 or 2pl/2 subshell to the lSl/2 core; the higher-euergy feature corresponds to 2p3/2 --+ lsl/2 transitions. The intensity of the lower-energy feature is clearly higher than that of the higher-energy feature. This intensity ratio is very different from that observed in an x-ray tube where a neutral element is bombarded with an electron beam and the higher-energy feature, the so-called Kal line, is about twice the size as the lower-energy feature, the so-called Kc~ line. Only highly charged ions emit l(c~ features where the lower-energy feature dominates [1,5]. The observed intensity ratio is, thus, a definitive signature that the Cf was highly ionized. For comparison, we show a spectrum of highly charged nranium
360c
P. Beiersdorfer et al./Nuclear Physics A626 (1997) 357c-364c 700 6OO
r
i
,
(a) _
~
400
J
300
j=l/2
,
i
i
j=l/2 A
i
(b) Uranium
¢n
8
200
200
100
IO0
0
i
600 500
Californium -
5OO
"5
700
i
I
90
i
t
i
I
100 110 Energy (keV)
i
I
120
•
,
130
90
100 110 Energy (keV)
120
130
Figure 1. K a spectra of (a) highly charged 249Cf and (b) highly charged 238U recorded with a high-purity Ge detector. The label refers to the spin-orbit angular momentum of the excited electron. in Fig. l(b). The experiment with 24vCf can be considered a sucessful first step that demonstrates the possibility of introducing transuranic elements into the trap for study. The overall intensity of the Cf emission was relatively weak. In fact, the Cf emission had to compete with the emission from ]ower-Z elements, such as tungsten, barium, or osmium emanating from the electron gun filament and entering the trap. It will be necessary to increase the amount of Cf plated on the wire in order to increase the Cf signal. This is readily possible, as the amount of Cf coated on the wire is still much smaller than radiologically or toxicologically possible. The use of transuranic elements and other radioactive isotopes in an EBIT will undoubtedly increase in the future. 4. M E A S U R E M E N T S
OF THE NUCLEAR
MAGNETIC
MOMENT
The interaction between the magnetic moment of the nucleus and the electron spin results in the splitting of the ls ground level of atomic hydrogen. It also split the 1s level of the appropriate hydrogenlike ions. While line emission arising from the ls hyperfine splitting has been observed from It, D, and He + in radio astronomy, laboratory observation of spontaneous line emission has been precluded by the extremely long lifetime of the upper hyperfine level in such low-Z systems (1.1 × 107 years for H) [16,17]. The lifetime is very much shorter for some of the highly charged hydrogenlike ions. For hydrogenlike 2°gBiS2+ it was measured at the ESR heavy-ion storage ring to be only 0.351(16) ms [18]. Consequently, observation of the spontaneous line emission between the ls hyperfine levels of highly charged ions is readily possible. We have made the first such observation very recently, when we measured the F = 4 -~ F = 3 transition among the ground configuration of hydrogenlike 165tto~+ [19]. We are now in the process of measuring the corresponding transition in XSSRe74+ and 187Re74+. A spectrum of the hyperfine transition in 165t]o66+ is shown in Fig. 2. The wavelength of the hyperfine transition was determined to be 5726.4 ± 1.5 ~(air). This corresponds to a vacuum wavelength of 5727.9 ± 1.5 ~, and a transition energy of 2.1646(6) eV [19].
P. Beiersdorfer et al./Nuclear Physics A626 (1997) 357c-364c
t 1500
,
i
•
J,
i
F---4 - - F = 3
(5726.4
,
i
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+ 1 . 5 ) ]k
1000
5
-500 L 5500
,
~ 5600
,
, 5700
,
~ 5800
,
, 5900
, 6000
W a v e l e n g t h (/~)
Figure 2. Spectrum of the l s ground-state hyperfine transition in ~6'~Hoe'~+ recorded with a prism spectrograph. From the measured transition energy A E we can infer the vahle of the nuclear magnetic moment #I using the formula [20] A E --
a4Za # I m ¢ ( 2 I + 1)m¢c2[A(1 - 6)(1 - ¢) + ,~~d]. 6 Imp
(1)
Here, c~ is the fine structure constant; me and rap are electron and proton masses, respectively; I is the nuclear spin; A is the relativistic correction factor; 6 is the nuclear charge distribution correction; c is the nuclear magnetization distribution correction; X~d is the radiative correction. Because the values of A and ~aa can be calculated to a very high degree of precision, Eq. (1) can be soved for the nuclear parameters once A E is known. Using the values for the atomic parameters from Ref. [20] we infer #I = 4.1267(11)#N from our measurement of A E . The uncertainty limits reflect solely the uncertainty in the wavelength measurement of the lSSHoS6+ hyperfine transition. Additional, systematic errors arise from uncertainties in the atomic calculations. These have not yet been estimated but are thought to be no larger than a few times the uncertainty from the wavelength measurement alone. We note that the radiative corrections are very small. This is because of a near cancellation of the QED terms arising from the vacuum polarization and the electron self energy [21,22]. In fact, most of the theoretical uncertainties arise from the correct treatment of other nuclear paramenters, i.e., of the distribution of the nuclear charge and of the nuclear magnetization. A comparison of the value of [l I inferred from our measurements with those from earlier measurements is given in Table I. Our value differs significantly from the value [l I 4.160(27)ILN listed in the compilation by Browne et al. [23]. This value was measured using the atomic beam resonance method [24], later recalculated [25], and is cited in the literature as 4.160(27)#N or 4.173(27)#u, depending on the diamagnetic correction employed. Our value agrees better with the newer measurement compiled in the reviews of Peker [26]. The comparison in Table I shows that our method is an accurate alternative for measuring the value of #I, even if the uncertainty from the atomic calculations are taken into =
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P. Beiersdorfer et al./Nuclear Physics A626 (1997) 357c-364c
Table 1. Comparison of the nuclear magnetic moment inferred from the EBIT measurement and those cited in the literature. The uncertainty in the EBIT measurement includes only the experimental uncertainty. The overall uncertainty is no more than a few times larger due to the uncertainty in the atomic calculations. Ref. [23]
Ref. [26]
present measurement
4.1730(270)jUN
4.1320(50)#N
4.1267(11)pN
account. It certainly provides more accurate values in cases where unknown chemical shifts make standard methods of determing the magnetic moment inaccurate beyond the uncertainties commonly cited in the literature, as stressed recently by Persson e* al. [22]. 5. I S O T O P I C
VARIATION
OF THE NUCLEAR
CHARGE
DISTRIBUTION
The effect of the nuclear charge distribution on the ls hyperfine transitions is small because to first order it affects both hyperfine levels with equal amounts. We can maximize the effect and measure it spectroscopically by focusing on transitions between levels that are unequally affected. To demonstrate the technique, we investigated the 2p3/2 --+ 2Sl/2 transitions in highly charged uranium ions [27]. Variations in the nuclear charge distribution affect almost exclusively the 2s configuration and not the 2p configuration, because only the 2s wavefunction has a significant overlap with the nucleus. Spectra of the 2pa/2 -+ 2Sl/2 transitions in lithiumlike through carbonlike uranium, U sg+ to U 86+, are shown in Fig. 3. We studied the three isotopes, 2aaU, 235U, and 2aau. A shift in the x-ray energy of the different lines for different isotopes is clearly seen. The average shift is A E 233'23s = 0.32 eV and A E 2as'2as = 0.18 eV. For comparison, the observed x-ray line width is about 1.0 eV. In order to infer the the variation in the mean-square nuclear charge radius, ~5(r2), we need to know the shift in the Dirac energy, ~SEco~. For the lines studied, ~SEco~l almost exactly equals the observed shift. The reason for this is that to first order none of the atomic energy terms vary for different isotopes except the Dirac energy. In particular, the combined isotopic variation of the specific mass shift, which plays a major role in optical isotope shift measurements of neutral or singly charged uranium, and that of the radiative terms is less than 1 meV and can thus be neglected. A slight variation is found only for the nuclear polarization. This variation is estimated to be less than 4 meV between 2aaU and 238U and even less between easIJ and 238U. Analyzing the shift, of each transition separately and averaging, we find ¢5{r2}233'238 = -0.432(43) fln 2 and a(,,~}235,23s = -0.250(32) fmL The uncertainties include uncertainties from both experiment and atomic theory. However, the uncertainty in the atomic physics is no more than 2 fm 2. Our result can be compared with that of previous studies: neutral-atom optical and K¢* isotope shift measurements [28,29] and muonic-atom x-ray observations [30]. For ~5(r2)233'23s these methods obtained a value of -0.383(44) fm 2 and -0.520(81) fln 2, respec-
P Beiersdorfer et al./Nuclear Physics A626 (1997) 357c--364c
'
1400
-
1200 ta1000"e=800-
'
I
363c
~'
',
L I
i i i
, / f ,2aSUl
i
°
o 600 t
400
B-1,2
2ooL o
i i
'
!o-1 4460
4500
4520
4540
Energy (eV) Figure 3. Spectra of the 2p3/2 --+ 2Sl/2 transitions in the highly charged 233U, 23su, and 2asU isotopes. Individual lines are labeled by the charge state of the emitting ions: C = U s 6 + , B = U s r + , B e = U ss+ a n d L i = U 89+. tively. For 6(r2) 2as'238 these methods obtained a value of -0.246(28) fm ~ and -0.30.5(42) fm 2, respectively. The uncertainties of these measurements are equal or larger than ours and have been subjected to considerable atomic physics corrections. 6. C O N C L U S I O N Precision measurements of the line emission fl'om highly charged ions have focused on providing accurate experimental data for the development of accurate calculational approaches in atomic structure theory. This has lead to significant advances in the accuracy of modern theoretical approaches. In many cases, the uncertainties in atomic theory are now well below the uncertainties in the existing values of nuclear parameters. Spectral measurements involving highly charged ions can thus be used to determine nuclear parameters with an accuracy that may by far exceed that of conventional studies. A systematic measurement of the isotopic variation of the energy of core-level transitions allows a reexamination of much of the nuclear radii data. Measurements of the hyperfine splitting allow direct determinations of nuclear magnetic moments free of chemical effects and without the need for diamagnetic corrections. In the case where the magnetic moment is well known, the measurements provide a way to assess radiative corrections in super strong nuclear fields as well as the distribution of the nuclear magnetization. The study of highly charged ions can thus provide a novel window on nuclear parameters unafforded by conventional methods. This work was performed under the auspices of U.S. Department of Energy' by Lawrence Liverinore National Laboratory under contract No. W-7405-ENG-48.
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P. Beiersdorfer et al./Nuclear Physics A626 (1997) 357c-364c
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