An equilibrium population of approximately 500 hydrogenlike U '+ ions and ten fully stripped U'-' ions has been produced and trapped in an electron beam ion trap at 198-keV electron energy. The equilibrium ionization balance, determined from the intensities of radiative recombination x rays, gives values of 1.55~0.27 and 2.82~0.35 b for the electron impact ionization cross sections of hydrogenlike and heliumlike uranium, respectively. These values are somewhat larger than relativistic distorted wave calculations, and much smaller than previous values inferred from stripping of accelerator beams. PACS numbers: 34.80.Kw, 32.30.Rj The hydrogenlike isoelectronic sequence of oneelectron ions is the simplest atomic system and the only one in which multielectron interactions are completely absent. For this reason the hydrogenlike ions, and atomic hydrogen itself, have been used for many years to obtain fundamental atomic structure information. Experiments include measurements of Lamb shifts, hyperfine splittings, and collision cross sections. The contributions of relativity and QED to atomic energy levels both scale as Z4, and are therefore most apparent in ions of the highest Z. Hence considerable attention has been given to the measurement of transition energies in hydrogenlike and other few-electron high-Z ions, especially the ions of uranium [1 -5]. The electron impact ionization cross sections for the tightly bound 1s electrons of high-Z elements are also of interest as a test of relativistic interactions in a simple atomic system. In spite of this interest, even the approximate size of the high-Z ionization cross sections for 1s electrons has been uncertain in view of an accelerator stripping measurement that obtained values 3 to 5 times larger than any theory for hydrogenlike and heliumlike uranium [6].Production of hydrogenlike or bare uranium ions is extremely difficult, requiring many ionizing collisions with small cross sections, at least one of which must have a center-of-mass energy above the 130-keV ionization potential of the uranium 1s electrons. These ions have previously been produced only in relativistic (-400 Me V/amu) accelerator beams stripped in foil targets, so x-ray measurements have had to deal with substantial Doppler shift corrections [1 -3]. Recently, high velocity U9'+ and U + ions have been stored in a ring [4], and deceleration to lower velocities has been proposed. Production of hydrogenlike and bare uranium ions at rest has been a long sought goal. %e report the first production of stationary hydrogenlike and bare uranium ions, along with a measurement of the electron impact ionization cross sections for producing them. The experiments were done with a high energy electron beam ion trap (EBIT) [7] which is an upgrade of an apparatus used previously to obtain the first measurements of electron impact excitation cross sections for very highly charged ions [8]. Approximately 5 x 104 highly charged uranium ions were trapped in the space charge potential of a compressed 198-keV electron b...
PEREGRINE is a three-dimensional Monte Carlo dose calculation system written specifically for radiotherapy. This paper describes the implementation and overall dosimetric accuracy of PEREGRINE physics algorithms, beam model, and beam commissioning procedure. Particle-interaction data, tracking geometries, scoring, variance reduction, and statistical analysis are described. The BEAM code system is used to model the treatment-independent accelerator head, resulting in the identification of primary and scattered photon sources and an electron contaminant source. The magnitude of the electron source is increased to improve agreement with measurements in the buildup region in the largest fields. Published measurements provide an estimate of backscatter on monitor chamber response. Commissioning consists of selecting the electron beam energy, determining the scale factor that defines dose per monitor unit, and describing treatment-dependent beam modifiers. We compare calculations with measurements in a water phantom for open fields, wedges, blocks, and a multileaf collimator for 6 and 18 MV Varian Clinac 2100C photon beams. All calculations are reported as dose per monitor unit. Aside from backscatter estimates, no additional, field-specific normalization is included in comparisons with measurements. Maximum discrepancies were less than either 2% of the maximum dose or 1.2 mm in isodose position for all field sizes and beam modifiers.
We report the first measurements of electron-impact-excitation cross sections for very highly charged ions (Ba 46+ ), and introduce a powerful new technique for studying these ions. Approximately 2xl0 4 Ba 46+ ions/cm were trapped inside the space charge of an -120-mA electron beam and their x-ray emission spectra observed with Si (Li) and Bragg-crystal-diffraction spectrometers. Cross sections for several /i™3 levels excited from the neonlike Ba 46+ ground state were measured relative to radiative recombination on the same ions.PACS numbers: 34.80.KwThe interaction of highly charged ions with electrons, as well as their spectroscopic properties, are important for the understanding of atomic structure in the highfield (high Z) limit where relativistic and QED effects are important. At present there are a few measurements of ionization cross sections for charge states up to q ;S + 50, * but direct measurements of electron-impact excitation (IE), dielectronic recombination (DR), and radiative recombination (RR) cross sections have been possible only for q ;S + 6 with use of the available techniques of crossed or merged beams. 2, 3 We have developed a new technique which, for the first time, makes it possible to measure all of these cross sections for highly charged ions. The technique consists of trapping ions inside an electron beam compressed to a density of order 2000 A/cm 2 . Cross sections are determined from x-ray spectroscopy of the trapped ions excited by the electron beam. Because the target ions are prepared in a single charge state and the electron beam is monoenergetic, it is possible to unravel all of the separate cross sections which contribute to x-ray emission.The method of successive ionization of ions trapped in an electron beam is also used in the electron-beam ion sources (EBIS) developed to provide highly charged ions for injection into accelerators. 1 There is one published observation of x rays from an EBIS (from DR on Ar 14+ ). 4 However, the EBIS sources have not proved suitable for x-ray spectroscopy of more highly charged ions. In contrast to the EBIS, our device uses a different and much smaller geometry which is optimized for x-ray spectroscopy. 5 In this Letter we describe our electronbeam ion trap and present measurements of the IE cross sections (relative to RR on the same ions) for several n =2 to n =3 transitions in the neonlike Ba 46+ ion.As shown schematically in Fig. 1, the ion trap consisted of a copper cylinder with an inside diameter of 10 mm in the central trap region and 3 mm at the ends. The electron beam, which follows the central magnetic-field line of the superconducting Helmholtz coils, was injected vertically from a Pierce gun. The beam was adiabatically compressed in the Helmholtz-coil field. The electron currents used ranged from 60 to 120 mA, and the beam radius at the peak magnetic field of 3 T was roughly 35 lira.The electron-ion interaction energy was determined by the output voltage of a precision high-voltage regulator that biased the drift tube. Because of th...
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