We present a measurement of the K-shell spectrum of He-like through Be-like praseodymium ions trapped in the Livermore SuperEBIT electron beam ion trap using a bismuth absorber pixel on the XRS/EBIT microcalorimeter. This measurement is the first of its kind where the n=2 to n=1 transitions of the various charge states are spectroscopically resolved. The measured transition energies are compared with theoretical calculations from several atomic codes

Beiersdorfer, P

Boyce, K R

Brown, G V

Chen, H

Chen, M H

Clementson, J T

Kelley, R L

Kilbourne, C A

Porter, F S

Thorn, D B

UNT Digital Library

UCRL-JRNL-232475High-Resolution Spectroscopy of K-shellPraseodymium with a High-EnergyCalorimeterD. B. Thorn, G. V. Brown, J. H. T. Clementson, H.Chen, M. H. Chen, P. Beiersdorfer, K. R. Boyce, C. A.Kilbourne, F. S. Porter, R. L. KelleyJuly 3, 2007Canadian Journal of PhysicsDisclaimer   This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes.  High-Resolution Spectroscopy of K-shell Praseodymium with aHigh-Energy Microcalorimeter.Daniel B. Thorn,∗ Greg V. Brown, Joel H. T.Clementson, Hui Chen, Mau Chen, and Peter BeiersdorferLawrence Livermore National Laboratory, Livermore, CA 94550, USAKevin R. Boyce, Caroline A. Kilbourne, F. Scott Porter, and Richard L. KelleyGoddard Space Flight Center, NASA, MD 20771, USAAbstractWe present a measurement of the K-shell spectrum of He-like through Be-like praseodymium ionstrapped in the Livermore SuperEBIT electron beam ion trap using a bismuth absorber pixel onthe XRS/EBIT microcalorimeter. This measurement is the first of its kind where the n=2 to n=1transitions of the various charge states are spectroscopically resolved. The measured transitionenergies are compared with theoretical calculations from several atomic codes.∗also at Department of Applied Science, University of California, Davis, CA 95616, USA1I. INTRODUCTIONThe K-shell spectra of highly ionized few-electron systems have been of great interestfor many years. Early measurements focused on relatively low-Z elements such as sulfur,chorine and argon [1–3] to record the K-shell spectra of helium-like and hydrogen-like ions.This interest in K-shell spectra stems from the desire to measure QED and relativistic effectsin the strong field of a heavy nucleus. The reason is that QED corrections and relativisticeffects have steep scaling laws that increase their magnitude strongly with increasing atomicnumber Z. As a result, precise spectroscopic measurements on high-Z systems provide avaluable probe of strong field QED and higher order relativistic effects.There are, however, two difficulties in obtaining K-shell spectra of heavy highly chargedions. The first, is that high-Z highly charged ions are hard to produce in sufficient quantities.The second, is that measuring the energy of the resulting x-ray radiation to high enoughprecision required to test QED is limited by the availability of suitable detectors.The early lower-Z experiments, like those with argon and chlorine, used accelerators toproduce the ions and crystal spectrometers to record spectra with high energy resolution.Subsequently, higher-Z ions, i.e. krypton and xenon, were investigated. The krypton mea-surements were preformed with crystal spectrometers as in the lower-Z experiments [4, 5].Krypton, however, is about the highest-Z system where traditional crystal spectroscopy canbe used. This is because the reflectivity for higher-energy photons becomes too low to allowmeasurements of sufficient statistical quality given the available photon flux.Conversely, the xenon measurements took a different approach from the lower-Z ions.The earlier measurement by Briand et al. [7], used a high purity germanium detectoron the GANIL heavy-ion accelerator to measure both the helium-like and hydrogen-likespectra. But the low intrinsic resolving power of the germanium detector did not allow foras high a precision result as the crystal spectrometers did for the lower-Z ion measurements.The later measurement by Widmann et al. [6], tried to remedy this problem by using ahybrid instrument of a transmission crystal spectrometer and a germanium detector on theSuperEBIT electron bean ion trap and obtained results of higher precision. However, thehybrid method suffered from a low count rate and was extremely time consuming. Thus formeasurements of the spectra of ions with higher-Z than xenon, only germanium detectorswith their inherently low resolving power for atomic transitions have so far been employed2[8].To make progress in measuring the K-shell spectra from high-Z highly charged ions,detectors with high-resolving power and high-quantum efficiency are needed. Ultra-lowtemperature detectors that work by measuring the heat associated with a photon absorbedin a material, so called microcalorimeters, promise to meet both of these criteria. Theabsorbing material, in principle, can be made of a high-Z element or compound thick enoughto stop even the most energetic photons with a reasonable quantum efficiency. In addition,these detectors have already shown the ability to accurately measure the energy of x-rayand γ-ray photons at energies ≤ 60 keV [9, 10].In this paper we present the first measurement of its kind where the n=2 to n=1 transi-tions of few-electron highly ionized ions with Z ≥ 54 are resolved. Using a microcalorimeterwe were able to measure the transition energies of the resonance and forbidden lines inhelium-like through beryllium-like praseodymium.II. METHOD AND RESULTSThe ions were produced at the Livermore SuperEBIT electron beam ion trap facility.Neutral praseodymium was injected into the trap using a laser ablation injection system[11]. The electron beam current was varied from 140-160 mA and the electron beam energywas set at 116 keV. The beam and trapping conditions created a charge balance of helium-like through beryllium-like praseodymium with a small amount of the hydrogen-like chargestate.Two detectors were used in the measurement. The first was a high purity germaniumdetector. It was used to monitor trapping conditions as its large bandwidth makes it possibleto monitor the radiative recombination (RR) part of the spectrum, which gives informationon the relative abundances of the charge states in the trap, while simultaneously measuringthe direct excitation (DE) part of the spectrum [12]. The second detector was a high-energymicrocalorimeter.The high-energy microcalorimeter used in this measurement was originally designed for x-ray astrophysics and used 8 µm HgTe absorbers attached to silicon thermistors. This choiceof absorber material and thickness allowed for the best quantum efficiency (QE) through7 keV [13–15], while achieving an energy resolution of 6 eV. To achieve a higher QE for3photons above 30 keV the microcalorimeter used in this experiment [10], while of the samedesign as the one used for astrophysics, differs in that four of the HgTe pixels were replacedby bismuth absorbers with a thickness of 30 µm. This change in the absorber material andthickness did affect the energy resolution obtained. However, the achieved FWHM of one ofthe pixels was 80 eV at energies near 37 keV giving a resolving power, E/∆E, of over 450.This is a vast improvement over the Ge detector’s resolving power of approximately 80. Asecond pixel failed to operate properly and the energy resolution of the remaining two pixelswas about 150 eV.Knowing the quantum efficiency of the detector is important when trying to extract cross-sections and other features dependent on the number of events that take place. Because ofthis, a measurement of the QE of the bismuth pixels were made. The measurement wasmade relative to the germanium detector which has a QE of 100 percent at 37 keV. Themeasured QE of the high-energy pixel at a photon energy of 37 keV is close to 40 % asshown in figure 1. This agreed with the theoretical calculation based on the thickness of theabsorbing mass of bismuth. For comparison to theory we used the Mass-Energy AbsorptionCoefficient [16] which takes into account other processes by which x-ray radiation energy islost in a material.The data from the pixel with the highest resolution was collected over a period of 40 hoursas shown in figure 2. There was no external calibration (such as a radioactive source) forthe calorimeter so the strongest lines were used as a relative calibration for the other lines.We took the theoretically accepted values of Drake’s calculations for the He-like system [17]and used the resonance line, w, at 37002.7 eV, and the forbidden line, z, at 36292.4 eV, asenergy references. A linear fit was used to establish an energy scale for the measurement. Atest of the calibration is provided by determining the energy of the intercombination line,y. The intercombination line, according to Drake, has an energy of 36390.4 eV. The valuedetermined by our procedure of 36389.1 ± 6.8 eV agrees with the theoretical value withinthe limits of the experimental uncertainty.The resolving power of the high-energy microcalorimeter pixel can be appreciated whencompared with the spectrum taken with the germanium detector as shown in figure 3. In thegermanium detector spectrum there are only 3 features visible. The first is associated withLyman-α1. The other two features are associated with 2p3/2 → 1s1/2 and 2s1/2, 2p1/2 → 1s1/2type transitions in helium-like through beryllium-like praseodymium. In the calorimeter4spectrum the individual transitions of each charge state can clearly be seen.This increased spectral resolution provided by the calorimeter allows for a more stringenttest of theoretical transition energies of the various charge states. Calculations for thehelium-like through beryllium-like charge states were made by using the general relativisticatomic structure package (GRASP) [18], and the flexible atomic code (FAC) [19]. Table 1shows the experimental values determined for the various spectral features relative to thecalibration lines. As expected the best agreement with theory is seen with the more highlycharged helium-like and lithium-like species. The main source of error in the determinationof the measured energies is due to statistics.III. CONCLUSIONThe spectrum obtained with the single bismuth pixel demonstrates that microcalorimetersare highly promising for high-Z K-shell spectroscopic measurements. A relative measurementof 6.8 eV was obtained for the intercombination line, y, in helium-like praseodymium whilean 8.5 eV measurement was obtained for the lithium-like lines and a measurement of 31 eVwas made for the beryllium-like system. Such a measurement would not have been possibleusing standard germanium detector technology. It is even more striking when consideringthat the pixel was not optimized for the calorimeter and simply represents a first attempt.Thus, in the future these devices working on a similar type of experiment can be expected tohave far superior resolving powers approaching or exceeding a E/∆E of 1000. Furthermore,by making arrays of such pixels it is possible to obtain a very high statistical certainty onthe measurement in question.AcknowledgmentsWe would like to thank John Gygax and Regis Brekosky at NASA/GSFC for their part inthe design and fabrication of the high-energy microcalorimeter and Ed Magee at LLNL forhis help in the operation and maintenance of SuperEBIT. This work was performed underthe auspices of the U.S. Department Of Energy by the University of California Lawrence5Livermore National Laboratory under contract W-7405-Eng-48.[1] J. P. Briand, J. P. Moss, P. Indelicato, et. al., Phys. Rev. A 28, 1413 (1983).[2] L. Schleinkofer, F. Bell, H. -D. Betz, et. al., Phys. Scripta 25, 917 (1983).[3] R. D. Deslattes, R. Schuch, and E. Justiniano, et. al., Phys. Rev. A 32, 1911 (1985).[4] J. P. Briand, M. Tavernier, P. Indelicato, et. al., Z. Phys. A 318, 1 (1984).[5] K. Widmann, P. Beiersdorfer, V. Decaux, et. al., Phys. Rev. A 53, 2200 (1996).[6] K. Widmann, P. Beiersdorfer, G. V. Brown, et a., AIP Conf. Proc. 506, 444 (2000).[7] J. P. Briand, P. Indelicato, A. Simionovici, et. al., Europhys. Lett. 9, 225 (1989).[8] J. P. Briand, P. Chevallier, P. Indelicato, et. al., Phys. Rev. Lett. 65, 2761 (1990).[9] B. R. Beck, J. A. Becker, P. Beiersdorfer, et. al., Phys. Rev. Lett. 98, 142501 (2007).[10] F. S. Porter, G. V. Brown, K. R. Boyce, et. al., Rev. Sci. Instrum. 75, 3772 (2004).[11] A. M. Niles, E. W. Magee, D. B. Thorn, et. al., Rev. Sci. Instrum. 77, 10F106 (2006).[12] J. R. Crespo Lo´pez-Urrutia, P. Beiersdorfer, D. W. Savin, et. al., Phys. Rev. Lett. 77, 826(1996).[13] R. L. Kelley, S. H. Moseley, C. K. Stahle, et. al., Journal of Low Temperature Physics 93 225(1993).[14] C. K. Stahle, C. A. Allen, K. R. Boyce, et. al., Nucl. Instrum. Methods A 520, 466 (2004).[15] R. L. Kelley, K. Mitsuda, C. A. Allen, et. al., Astron. Soc. Japan 59, 77 (2007).[16] S. M. Seltzer, Rad. Res. 136, 147 (1993).[17] G. W. F. Drake, Can. J. Phys. 66, 586 (1988).[18] I. P. Grant, B. J. McKenzie, P. H. Norrington, et al., Comput. Phys. Commun. 21, 207 (1980).[19] M. F. Gu, submitted to Can. J. Phys. (2007).6605550454035302520151050X-ray Absorption Percentage (%)6055504540353025Photon energy (keV) Calculation MeasurementFIG. 1: Quantum efficiency of 30 µm bismuth.7444036322824201612840Counts38.037.537.036.536.035.5Photon energy (keV)pyz§qwFIG. 2: The high resolution spectrum of He-like through Be-like praseodymium using the high-energy microcalorimeter870006000500040003000200010000Counts38.037.537.036.536.035.5 Photon energy (keV)Lyman-α12p3/2 - 1s1/22s1/2 , 2p1/2 - 1s1/2FIG. 3: Spectrum of K-shell praseodymium obtained with the germanium detector.9TABLE I: Spectral Lines from XRS K-shell Praseodymium SpectrumLabel Transition Energy (eV)Measurement Theorya b cwd 1s2p 1P1 → 1s2 1S0 - - - 37002.7q 1s2s2p 2P3/2 → 1s22s 1S1/2 36886.8 ± 8.5 36880.35 36877.39 -β 1s2s22p 1P1 → 1s22s2 1S0 36775 ± 31 36802.71 36798.39 -y 1s2p 3P1 → 1s2 1S0 36389.1 ± 6.8 - - 36390.4zd 1s2p 3S1 → 1s2 1S0 - - - 36292.4p 1s2s2 2S1/2 → 1s22p 2P1/2 36088.2 ± 8.5 36089.01 36085.80 -a Present calculation using GRASP [18]b Present calculation using FAC [19]c [17]d Reference line [17]10

High-Resolution Spectroscopy of K-shell Praseodymium with a High-Energy Calorimeter

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High-Resolution Spectroscopy of K-shell Praseodymium with a High-Energy Calorimeter

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