Theoretical calculations of the DDCSs were made for C6+ ions using the continuum distorted wave-eikonal initial state model (CDW-EIS) in its straight-line version of the impact parameter approximation, showing general good agreement with experimental data, except in the intermediate- and high-energy region. On the other hand, the single differential cross section (SDCS), which was obtained by integrating the measured DDCSs over the solid angle, showed fairly good agreement with the CDW-EIS in the low-energy region, while a significant discrepancy between the observed SDCS and the Rudd-model scaling can be seen,suggesting that a simple -scaling law (i.e. first Born approximation) is not applicable for bare projectiles such as C6+ ions. The SDCS of C4+ ions was observed to be smaller than that of C6+ ons by 50% in the low-energy region (due to the screening effect of its bound electrons in C4+ ions, which could be explained quantitatively by taking accountFil: Ohsawa, D.. Kyoto University. Radioisotope Research Center; JapónFil: Tawara, H.. National Institute for Fusion Science; JapónFil: Soga, F.. National Institute of Radiological Sciences; JapónFil: Galassi, Mariel Elisa. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); ArgentinaFil: Rivarola, Roberto Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentin

Galassi, Mariel Elisa

Ohsawa, D.

Rivarola, Roberto Daniel

Soga, F.

Tawara, H.

Physica Scripta

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This content has been downloaded from IOPscience. Please scroll down to see the full text.Download details:IP Address: 168.96.15.5This content was downloaded on 27/01/2014 at 21:17Please note that terms and conditions apply.6.0 MeV u−1 carbon ion (C6+ and C4+)-induced secondary electron emission from watervaporView the table of contents for this issue, or go to the journal homepage for more2013 Phys. Scr. 2013 014039(http://iopscience.iop.org/1402-4896/2013/T156/014039)Home Search Collections Journals About Contact us My IOPscienceIOP PUBLISHING PHYSICA SCRIPTAPhys. Scr. T156 (2013) 014039 (5pp) doi:10.1088/0031-8949/2013/T156/0140396.0 MeV u−1 carbon ion (C6+ andC4+)-induced secondary electron emissionfrom water vaporD Ohsawa1, H Tawara2, F Soga3, M E Galassi4 and R D Rivarola41 Radioisotope Research Center, Kyoto University, Kyoto 606-8502, Japan2 National Institute for Fusion Science, Toki 509-5292, Japan3 National Institute of Radiological Sciences, Chiba 263-8555, Japan4 Instituto de Fı́sica Rosario, CONICET and Universidad Nacional de Rosario, Rosario, ArgentinaE-mail: ohsawa.daisuke.2s@kyoto-u.ac.jpReceived 3 September 2012Accepted for publication 20 January 2013Published 23 September 2013Online at stacks.iop.org/PhysScr/T156/014039AbstractWe have precisely measured absolute double differential cross sections (DDCSs) forsecondary electron emission produced in collisions of 6.0 MeV u−1 C6+ and C4+ ions withwater vapor. Theoretical calculations of the DDCSs were made for C6+ ions using thecontinuum distorted wave-eikonal initial state model (CDW-EIS) in its straight-line version ofthe impact parameter approximation, showing general good agreement with experimental data,except in the intermediate- and high-energy region (>50 eV), particularly at the backwardangles (>110◦). On the other hand, the single differential cross section (SDCS), which wasobtained by integrating the measured DDCSs over the solid angle, showed fairly goodagreement with the CDW-EIS in the low-energy region (<200 eV), while a significantdiscrepancy between the observed SDCS and the Rudd-model scaling (×36) can be seen,suggesting that a simple Z2-scaling law (i.e. first Born approximation) is not applicable forhigh-Z bare projectiles such as C6+ ions. The SDCS of C4+ ions was observed to be smallerthan that of C6+ ions by ∼50% in the low-energy region (<200 eV) due to the screening effectof its bound electrons in C4+ ions, which could be explained quantitatively by taking accountof an effective projectile charge.PACS number: 34.50.Gb1. IntroductionAlthough secondary electron emission from water by ionimpact has been of great interest in fields such as radiobiologyand atomic physics, there are only a few data, particularly forheavy-ion impact with incident energies of several MeV u−1or higher. Motivated by this situation, considerable efforthas been invested into this study since 1994. Our previouspapers [1, 2] reported in detail on an apparatus for measuringsecondary electrons from water vapor, along with thededuction of experimental parameters such as target moleculedensity, energy resolution and transmission of the electronspectrometer and detection efficiency of the electron detector(MCP). The absolute double and single differential crosssection (DDCS and SDCS) data obtained with He2+ ions werealso discussed by comparing them with a simple Z2-scaling ofthe semiempirical Rudd model and the modified Rutherfordcross sections taking into account the binding energy of anelectron ejected from each subshell of a water molecule.In emission angles, the measurements were carried out at20–160◦ by 10◦ steps for both 6.0 and 10.0 MeV u−1 He2+ions, while in electron energies, 7 eV–10 keV for 6.0 MeV u−1and 20 eV–10 keV for 10.0 MeV u−1. Subsequently, themeasurements with 6.0 MeV u−1 C6+ ions were made atelectron energies of 10 eV–1 keV. Theoretical investigationswere also performed for these experimental DDCS and SDCSdata, showing meaningful and significant discrepancies [3,4]. Recent modifications allowed us to measure the DDCSsat electron energies down to 1 eV as well as to measurehigh-energy electrons (>1 keV) with low-background level.In this paper, we present experimental DDCS and SDCSdata for secondary electron emission from water vapor by the0031-8949/13/014039+05$33.00 1 © 2013 The Royal Swedish Academy of Sciences Printed in the UKPhys. Scr. T156 (2013) 014039 D Ohsawa et alimpact of 6.0 MeV u−1 C6+ and C4+ ions. The experimentaldata with C6+ ions are compared with the continuum distortedwave-eikonal initial state model (CDW-EIS). The screeningeffect of the bound electrons in C4+ ions is also discussed.2. ExperimentAn outline of our experiments has been described in detail[1, 2]. In the present measurements with C6+ and C4+ ions,three collimators with apertures of 6–8 mm diameter wereinstalled immediately upstream from the MCP in order toavoid the injection of scattered electrons produced within thespectrometer housing into the MCP. Besides, in the presentmeasurements of low-energy electrons (<100 eV) with C6+ions, the cooling cover was changed to that made of µ-metalin order to suppress the effects of stray magnetic fields. Thetotal systematic errors were estimated to be ±13% for allmeasured angles and energies, while the statistical errors wereof the order of 1% at energies smaller than 100 eV and reachedseveral ten % at ∼10 keV.3. Theoretical modelCalculations of single ionization cross sections for C6+ ionswere made using the CDW-EIS into its straight-line versionof the impact parameter approximation [5]. This modelhas been used with success in the calculation of singleionization cross sections of water molecules by heavy-ionimpact [6–8]. The model takes into account the long-rangeCoulomb interactions between the collision aggregates inthe entry and exit channels. To avoid difficulties associatedwith the treatment of multielectronic targets, the reactionis reduced to a one-active-electron description. Then, inthe entry channel it is considered that the projectile fielddistorts the initial active-electron orbital with a multiplicativecontinuum eikonal phase, while in the exit channel the factthat the emitted electron travels in the combined fields of theresidual target and projectile is taken into account. Thus, thiselectron is described as a double product of a plane wave andtwo Coulomb continuum factors associated with the projectileand residual potentials.The calculation of the ionization cross sectionscorresponding to the different molecular orbitals of thewater molecule is done within the complete neglect of thedifferential overlap (CNDO) method. In this approximation(which takes partly into account the molecular character ofthe target), the ionization cross section for each molecularorbital is calculated by making a linear combination of atomiccross sections, whose weight coefficients are obtained froma population analysis. The characteristics of the molecularorbitals in the gas phase are introduced through theirmolecular binding energies, instead of using separately thoseof the atomic compounds of the target. The oxygen atomicorbitals employed in this linear combination correspond tothe Slater orbitals of the isolated atom.4. Results and discussion4.1. DDCSFigure 1 shows the energy spectra of DDCSs obtained incollisions of 6.0 MeV u−1 C6+ and C4+ ions with water vaporat angles between 20◦ and 160◦ by 10◦ steps. For both C6+and C4+ ions, binary-encounter peaks were clearly observedat the several keV region for angles smaller than 90◦, aswell as the K-LL Auger peak of oxygen at ∼500 eV for allangles. In addition to these peaks, the DDCSs of C4+ ionsshowed peaks of the electron loss to the continuum (ELC)at ∼3.3 keV particularly at the forward angles. In the low-and intermediate-energy region (<400 eV), the DDCSs of C4+ions were observed to be smaller than those of C6+ ions by∼50% due to the screening effect of the bound electronsin C4+ ions. A quantitative analysis for the screening effecttaking account of an effective projectile charge is describedlater.Theoretical calculations of the DDCSs were made forC6+ ions using the CDW-EIS approximation for targetionization into its straight-line version of the impact parameterapproximation. As can be seen in figure 1, fairly goodagreement with measured data is obtained for emissionenergies smaller than 50 eV. However, the observed DDCSsof C6+ ions disagree with the CDW-EIS ones in theintermediate- and high-energy region (>50 eV), mainly dueto the fact that contributions from Auger emission are notincluded in the present calculations. This effect appearsto be more pronounced at the backward angles (>110◦).A similar tendency has also been clearly observed in thepreviously measured DDCSs with 6.0–10.0 MeV u−1 He2+ions [1, 2], which seems to be common in other data oflow-energy H+ (0.3–1.5 MeV) and He2+ (0.3–0.5 MeV u−1)ions on the same target (water vapor) [9]. Two-active-electronCDW-EIS calculations for impact of C4+, which must includeprojectile ELC and simultaneous electron emission from bothaggregates of the collision system, are in progress.4.2. SDCSFigure 2 shows the energy spectra of SDCSs obtained incollisions of 6.0 MeV u−1 C6+ and C4+ ions with watervapor, which were deduced from the measured DDCSs byintegration with respect to the ejected angle. The observedSDCS of 6.0 MeV u−1 C6+ ions was compared with theCDW-EIS and a simple Z2 scaling (×36) of the Ruddmodel. The Rudd model is based on both the molecularpromotion model applied to electron emission by low-energyions and the classical binary-encounter approximation athigh energies, so as to agree with the Bethe theory onionization. Although this model was basically derived fromexperiments with protons having energies of 5–5000 keV,it has been widely accepted to be applicable to other bareprojectiles by Z2 scaling. In the energy range of 1–200 eV,our SDCS agreed well with the CDW-EIS, and both ofthem were smaller than the Rudd-model scaling by ∼22% atmaximum. In contrast, the previously measured SDCSs with6.0–10.0 MeV u−1 He2+ ions were quite consistent with theRudd-model scaling (×4) in the low-energy region (10 eV <E < 100 eV). This suggests that a simple Z2 scaling of the2Phys. Scr. T156 (2013) 014039 D Ohsawa et al(a) (b)(c)(e)(d)Figure 1. (a)–(e) Energy spectra of DDCSs obtained in collisions of 6.0 MeV u−1 C6+ (◦) and C4+(•) ions with water vapor at anglesbetween 20◦ and 160◦ by 10◦ steps. The solid line shows the CDW-EIS calculated for 6.0 MeV u−1 C6+ ions. The contributions of K-LLAuger peaks of oxygen, which were clearly observed at ∼500 eV for all angles, are not included in the present theoretical calculations.3Phys. Scr. T156 (2013) 014039 D Ohsawa et alFigure 2. Energy spectra of SDCSs obtained in collisions of6.0 MeV u−1 C6+(◦) and C4+(•) ions with water vapor, which werededuced from the measured DDCSs by integration with respect tothe ejected angle. The solid and dashed lines show the CDW-EISand a simple Z 2 scaling (×36) of the Rudd model, respectively.Figure 3. SDCS ratio between C4+ and C6+ ions as a function ofelectron energy. The ratio exhibited a nearly constant value around0.5 in the low-energy region of 50–200 eV along with a pronouncedpeak due to the ELC at ∼3.3 keV. The dashed line represents thesquare of the effective-charge ratio between C4+ and C6+ ions.Rudd model is not sufficient to describe the experimental datafor high-Z bare projectiles such as C6+ ions. In the energyrange of 300 eV–7 keV, compared with the CDW-EIS, ourSDCS showed a meaningful reduction by ∼50% at maximumat 2 keV, which was also quite consistent with our previousmeasurements with 6.0–10.0 MeV u−1 He2+ ions.In figure 3, the SDCS ratio between C4+ and C6+ ionswas shown as a function of electron energy in order to discussthe screening effect quantitatively, where the ratio was nearlyconstant around 0.5 in the low-energy region of 50–200 eV.In fast ion–atom collisions, ionization processes occurringwith large impact parameters (small momentum transfer)can be treated based on the well-known perturbation theory.According to the theory, the SDCS in the low-energy regionis expected to be proportional to the square of the projectilecharge. Note here that in the case of the dressed projectile,the effective projectile charge Zeff should be used instead ofthe real charge. The Zeff of the dressed projectile is generallyexpressed asZeff =(n2 Eb13.6)1/2, (1)where n is the principal quantum number of the electron in theprojectile and Eb is its binding energy [10]. Using equation(1), the effective charge of C4+ ions is found to beZC4+ =(22 × 64.513.6)1/2= 4.36. (2)Thus, the square of the effective-charge ratio between C4+and C6+ ions is calculated to be(ZC4+ZC6+)2=(4.366)2= 0.527. (3)As can be seen in figure 3, the SDCS ratio agreed wellwith the square of the effective-charge ratio in the low-energyregion (<200 eV) except at 10 eV within the overall errors,indicating that the approach based on the perturbation theorytaking account of the effective projectile charge is valid.We believe the discrepancy at 10 eV may be attributed toan unwanted increase in the residual magnetic field in themeasurements with C4+ ions.5. ConclusionFurther improving our previous apparatus, we havesuccessfully measured absolute DDCS and SDCS forsecondary electron emission from water vapor by the impactof 6.0 MeV u−1 C6+ and C4+ ions within total systematicerrors of ±13%. Comparison of the observed DDCS of C6+ions with the CDW-EIS showed that experiments and theoryare in good agreement at low–intermediate emission energiesbut also revealed that there were significant discrepanciesconcerning electron emission at the backward angles largerthan 110◦, especially in the high-energy region. Suchdiscrepancies were similar to other data of low-energyH+(0.3–1.5 MeV) and He2+(0.3–0.5 MeV u−1) ions onthe same target (water vapor), suggesting the necessity ofsystematic measurements with other projectiles. Also theconsideration of Auger contributions in the theoretical modelas well as a more complete description of backscatteringprocesses in CDW-EIS appear to be necessary. In the nearfuture, experiments with O and Ne ions will be carried out,after installing a compact 10 GHz ECR ion source to the axialinjection line of the cyclotron. Recently, we also developedan event-by-event Monte-Carlo track structure code for Heand C ions in water, in which our experimental DDCS andSDCS data were used. Details of it will soon be publishedelsewhere.4Phys. Scr. T156 (2013) 014039 D Ohsawa et alAcknowledgmentsWe are grateful to the cyclotron staff at NIRS for providingsteady beams during the experiments. The work waspartially supported by a Grant-in-Aid for Scientific ResearchContract(C) no. 21540408 from the Ministry of Education,Science, Sports and Culture of Japan.References[1] Ohsawa D, Kawauchi H, Hirabayashi M, Okada Y, Honma T,Higashi A, Amano S, Hashimoto Y, Soga F and Sato Y 2005Nucl. Instrum. Methods B 227 431–49[2] Ohsawa D, Sato Y, Okada Y, Shevelko V P and Soga F 2005Phys. Rev. A 72 062710[3] Champion C, Boudrioua O, Cappello C D, Sato Y andOhsawa D 2007 Phys. Rev. A 75 032724[4] Cappello C D, Champion C, Boudrioua O, Lekadir H,Sato Y and Ohsawa D 2009 Nucl. Instrum. Methods B267 781–90[5] Fainstein P D, Ponce V H and Rivarola R D 1988 J. Phys. B:At. Mol. Opt. Phys. 21 287–99[6] Olivera G H, Martinez A E, Rivarola R D and Fainstein P D1995 Radiat. Res. 144 241–47[7] Olivera G H, Fainstein P D and Rivarola R D 1996 Phys. Med.Biol. 41 1633–47[8] Fainstein P D, Olivera G H and Rivarola R D 1996 Nucl.Instrum. Methods B 107 19–26[9] Bernal M A and Liendo J A 2007 Nucl. Instrum. Methods B262 1–6[10] Pal’chikov V G and Shevelko V P 1995 Reference Data onMulticharged Ions (Berlin: Springer)5

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6.0 MeV u-1 carbon ion (C6+ and C4+)-induced secondary electron emission from water vapour

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6.0 MeV u-1 carbon ion (C6+ and C4+)-induced secondary electron emission from water vapour

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