Pt L X-RAYS PRODUCTION CROSS SECTIONS BY 12 C, 16 O, 32 S AND 48 Ti ION-BEAMS IN THE MeV/u ENERGY RANGE *

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1 Pt L X-RAYS PRODUCTION CROSS SECTIONS BY 12 C, 16 O, 32 S AND 48 Ti ION-BEAMS IN THE MeV/u ENERGY RANGE * M.M. GUGIU, C. CIORTEA, D.E. DUMITRIU, D. FLUERAŞU, A. ENULESCU, I. PITICU, A.C. SCAFEŞ, M.D. PENA National Institute for Nuclear Physics and Nuclear Engineering Horia Hulubei (IFIN-HH), P.O.Box MG-6, RO Bucharest-Măgurele, Romania, gmarius@tandem.nipne.ro Received September 14, 2009 By using MeV/u 12 C and 16 O, and MeV/u 32 S and 48 Ti ion beams, delivered by the 9 MV tandem accelerator of IFIN-HH, Pt L X-ray production cross sections have been determined. The comparison with model predictions for direct ionization (SCA and ) has put in evidence the dominance of the ionization from atomic states in the case of 12 C, 16 O + Pt collisions, and the molecular orbital (MO) mechanisms in the case of 48 Ti + Pt collision. A possible contribution of MO mechanisms to the target L-shell ionization in the collision system 32 S + Pt is discussed. The effect of vacancy sharing between the L-subshells, included in the first order in the calculations, is small for the total X-ray yields reported here. Possible solid-state effects on the X-ray transitions involving outer-shell electrons are noted. Key words: L-shell X-ray yields, MeV/u heavy ions. 1. INTRODUCTION The excitation of the X-rays in the light ion atom collisions has been intensively studied in the last decades in order to develop and test theoretical approaches, as well as to build a database of X-ray production cross sections for applications, like the particle induced X-ray emission (PIXE). Therefore, a rather good understanding of X-ray production by light ion impact (protons, alpha particles etc.) has been obtained [1]. The situation is less satisfactory for heavier ions, with less and disparate data. Direct ionization of the inner (K and L) shells by light ions can be reasonably described by first-order treatments based on the plane wave Born () and semiclassical (SCA) approximations. These theoretical approaches have been further extended to include higher order effects, like the energy loss (E), coulomb deflection (C) and relativistic (R) effects within the perturbed stationary state (PSS) approach, * Paper presented at the 10 th International Balkan Workshop on Applied Physics, July 6 8, 2009, Constanţa, Romania. Rom. Journ. Phys., Vol. 56, Nos. 1 2, P , Bucharest, 2011

2 72 M.M. Gugiu et al. 2 which takes into account electron binding or polarization effects, resulting in the so-called model [2] (for calculations see e.g. the ISICS or ERCS08 computer programs [3]). In the SCA [4], the electron binding effect is included in two extreme cases, the united-atom (UA) and separated-atom (SA) limits. In the case of the L-subshell ionization, it is worth to mention the vacancy sharing effect [5,6] due to the intra-shell coupling between the L-vacancy states, which modifies substantially the ionization cross sections of the L-subshell, especially the L 2 -vacancy production at low energies. In slow collisions with heavier projectiles, molecular-orbital (MO) excitation mechanisms [7] may come into play. As shown before for slow collisions in the asymmetric collision system 48 Ti + Pt [8], the inner-shell ionization could not be explained by only direct ionization from atomic states; furthermore, the MO mechanisms that are specific for the K-L level matching region [9, 10] are dominating. It is worth mentioning the multiple ionization of the outer-shells associated with the L-shell vacancy production by energetic heavy ions, which is evidenced by energy and yield shifts [11] of the X-ray lines as compared to tabulated values [12 14] (see also ref. [15] and refs therein). These multiple ionization effects could be used to estimate the mean number of spectator vacancies in the outer-shells during the inner-shell vacancy deexcitation, as well as corrections on the atomic decay parameters [11]. These multiple ionization effects were not further taken into account here. In the present paper, the difference between 48 Ti and other lower-z projectiles ( 12 C, 16 O, and 32 S) excitation of the Pt L X-rays is further documented by new experimental evidence. We report here total Pt Lα, β, and γ X-ray production cross sections for the collision systems 12 C, 16 O, 32 S, 48 Ti + Pt in the MeV/u energy range. By determining the Pt L X-ray yields produced by projectiles with increasing Z in the same collision energy range, and comparing with theoretical predictions of direct ionization models, we intended to put in evidence the triggering of the MO ionization mechanisms in the K-L level matching region. 2. EXPERIMENT The Pt L X-ray production cross sections in the collisions of 12 C and 16 O ions of MeV/u energy and of 32 S and 48 Ti ions of MeV/u energy have been measured at the 9 MV Van de Graaff tandem accelerator of IFIN-HH. The collimated ion beams, charge state and energy selected by a 90 0 analyzing magnet, bombarded a thin (100 µg/cm 2 ) selfsupported Pt target, tilted at 45 0 to the beam direction. The emitted X-ray spectra have been measured with a Ge HP detector, placed at 90 0 to the beam direction. The scattered projectiles were measured by using a thin plastic scintillator foil (110 µm thichness) placed at 90 0 or at a forward angle (5.7 0 to the beam, used in the case of 48 Ti + Pt collision). Measuring the Coulomb scattered particles simultaneously at 90 0 and the forward angle, and using

3 3 Pt L x-rays production cross sections by 12 C, 16 O, 32 S and 48 Ti ion-beams 73 the angular dependence given by the Rutherford cross section, after correcting for detector solid angle, allowed us to determine the forward scattering angle. The efficiency and the solid angle of the X-ray and particle detectors have been measured using calibrated X-ray and alpha radioactive sources ( 241 Am). By normalizing to the Rutherford cross sections for Coulomb scattered projectiles, production cross sections for the total Lα, β and γ X-rays as well as some other lines (L and Lβ 5,7 ) are reported here. An experimental uncertainty of ±25% was estimated, being mainly due to X-ray detector efficiency and solid angle (±10%), particle detector angle and solid angle (±20%), detector deadtime (±10%), as well as the counting statistics and background subtraction (±5%). 3. RESULTS AND DISCUSSION 3.1. THE 12 C AND 16 O + Pt COLLISIONS In the Figs. 1 3 below, the experimental data of production cross sections for Pt Lα, β, and γ X-rays in dependence of the bombarding energy in the range of MeV/u are given. The lines in the figures give predictions of direct ionization model calculations. First order estimations of the vacancy sharing effect [6] have been included in the calculations. As calculations show (see figs. below), the vacancy sharing effect has only a small influence on the total X-ray yields. Pt Lα X-ray production cross sections (k 12 C + Pt Pt Lα X-rays Exp. data + v.s. Lα X-ray production cross sections (k 16 O + Pt Pt Lα X-rays + v.s. 5,0 Fig. 1. Pt Lα X-ray production cross section in the collision with 12 C and 16 O ions in dependence of the bombardment energy. The lines represent direct ionization model predictions and the notations are explained in the figures. The line noted + v.s. includes in the first order corrections for the vacancy sharing effect [6]. The theoretical calculations use tabulated fluorescence and Coster-Kronig yields [16] as well as partial radiative yields [13, 14]. The model calculations, which take into account effects like binding/ polarization, projectile Coulomb deflection and electronic relativistic effects, give the best description of the experimental data, including the energy dependence. It is

4 74 M.M. Gugiu et al. 4 worth to note that for heavy ions at low energies, the calculations are lower than. This is a known overbinding behaviour of the model (see e.g. [15]), that increase with projectile Z. At the highest energies in the present measuring range, the predictions of the two models are reversed. As mentioned before, it was not taken into account here the effect of multiple ionization in the outer-shells, which is affecting the fluorescence yields (which generally are increased compared to single vacancy configuration) and the relative partial widths. Therefore, the rather good agreement between experiment and theoretical predictions for Lα (L 3 -M 4, 5 ) X-ray production sections (Fig. 1) could be only fortuitous. If we consider also the data for Lβ and Lγ X-rays (Fig. 2 and 3, respectively), it could be appreciated that in the collisions 12 C and 16 O + Pt, slightly underestimate the ionization of the Pt L-shell. Pt Lβ X-ray production cross sections (k 12 C + Pt Pt Lβ total X-rays + v.s. Lβ X-ray production cross sections (k 16 O + Pt Fig. 2. The same as Fig. 1 for Pt Lβ X-rays. Pt Lβ X-rays + v.s. The X-ray yields of some other lines, L and the Lβ 5,7, both associated with the ionization of the L 3 -subshell, are also presented in the Figs. 4 and 5, respectively. Pt Lγ X-ray production cross sections (k 12 C + Pt Pt Lγ total X-rays + v.s. Lγ X-ray production cross sections (k 16 O + Pt Fig. 3. The same as Fig. 1 for Pt Lγ X-rays. Pt Lγ X-rays + v.s.

5 5 Pt L x-rays production cross sections by 12 C, 16 O, 32 S and 48 Ti ion-beams 75 The small difference in comparison with theory between the Lα (L 3 -M 4,5 ) and L (L 3 -M 1 ) X-rays, both associated with an initial L 3 vacancy, can be understood as due to the presence of a variable number of spectator vacancies in the M-subshells, namely relatively more spectator vacancies in M 4, 5 than in M 1. L X-ray production cross sections (k O + Pt Pt L X-rays + v.s. Fig. 4. Pt L X-ray production cross section in the collision with 16 O ions in dependence of the bombardment energy. The lines represent direct ionization model predictions and the notations are explained in the figures. The line noted + v.s. includes in the first order corrections for the vacancy sharing effect [6]. The observed significant increase of the experiment compared to the theory for X-ray lines which deexcite weakly bound electronic states (O-shell) (see (L 3 -O) X-rays in Fig. 5) could not be explained by effects like vacancy sharing and multiple ionization. This result may be tentatively attributed to the contribution of an additional solid-state effect, due to the neighboring atoms in the solid target whose electrons could participate in these transitions. X-ray production cross sections (k 12 C + Pt 1x 1x10-5 X-rays + v.s. L β 5,7 X-ray production cross sections (k 1x 1x O + Pt X-rays + v.s. Fig. 5. The same as Fig. 1 for X-rays.

6 76 M.M. Gugiu et al. 6 Pt Lα X-ray production cross section (k S + Pt + v.s. Pt Lβ X-ray production cross section (k S + Pt + v.s. Pt Lγ X-ray production cross section (k 32 S + Pt 10-5 c) + v.s. Fig. 6. Pt Lα X-ray production cross section produced in the collision 32 S + Pt in dependence of the bombarding energy. The same for Pt Lβ X-rays and c) Lγ X-rays. The lines represent theoretical predictions; the notations are given in the figures. The line noted + v.s. includes in the first order corrections for the vacancy sharing effect [6] THE 32 S + Pt COLLISION The present results for Pt Lα, Lβ, and Lγ X-ray production cross sections in the collision 32 S + Pt (see Fig. 6) are significantly larger than the predictions, including the vacancy sharing effect [6]. As said before, this was expected for, where the binding effect is overestimated for heavier ions [15]. Instead, the predictions are close to experiment at lower energies in the measured range, but significantly underestimate the experimental data at higher energies. This behaviour at higher energies could be due to less comply to the adiabaticity condition assumed by the model. Because the possible contribution of the MO excitation mechanisms is expected to be more important at lower energies, where the predictions are close to experiment, from the present data we cannot conclude about a possible contribution of the MO excitation mechanisms. The projectile K X-ray production

7 7 Pt L x-rays production cross sections by 12 C, 16 O, 32 S and 48 Ti ion-beams 77 could say more in this respect, however measuring it by using X-ray spectroscopy is difficult due to the overlap with the M X-rays of the target atom. Pt L X-ray production cross section (k 32 S + Pt 1x 1x10-5 Pt L X-rays + v.s. X-ray production cross section (k 1x 1x S + Pt X-rays + v.s. Fig. 7. Pt L X-ray production cross section produced in the collision 32 S + Pt in dependence of the bombarding energy. The same for Lβ 5,7 X-rays. The lines represent theoretical predictions, which are explained in the figures. The line noted +v.s. includes in the first order corrections for the vacancy sharing effect [6].3.3 The 48 Ti + Pt collision. 48 Ti + Pt 48 Ti + Pt Pt Lα X-ray production cross section (k v.s. Pt Lγ X-ray production cross section (k c) + v.s. 0,5 1,0 1,5 2,0 0,5 1,0 1,5 2,0 Pt Lβ X-ray production cross section (k Ti + Pt + v.s. 0,5 1,0 1,5 2,0 Fig. 8. Pt Lα X-ray production cross section produced in the collision 48 Ti + Pt in dependence of the bombarding energy. The same for Pt Lβ X-rays and c) Lγ X-rays. The lines represent the theoretical predictions of the SCA (UA and SA) and models. The line noted + v.s. includes in the first order corrections for the vacancy sharing effect [6].

8 78 M.M. Gugiu et al. 8 The less intense X-ray yields of L and Lβ 5,7 lines are also shown in Fig. 7. The same comments as before on these X-ray yields are valid also for the case of 32 S + Pt collision. In the case of 48 Ti + Pt collision, the Pt Lα, Lβ, and Lγ X-ray production cross sections are much larger than the predictions of the direct ionization models and, including the vacancy sharing effect, as seen in Fig. 8. These data confirm the earlier results [8] that in this collision system the quasimolecular mechanisms [7], specific to K-L level matching region, are present. That means, the 3dσ MO ionization at small internuclear distances [9] followed by vacancy-sharing between the 3dσ and 2p 3/2 σ MOs at larger distances [10] are the main MO mechanisms of inner- (projectile K- and target L-) shell vacancy production. 4. CONCLUSIONS In the present paper, Pt L α, β, and γ total X-ray production cross sections by MeV/u 12 C, 16 O, and MeV/u 32 S, 48 Ti ion beams are reported; some other X-ray yields (for L and Lβ 5,7 transitions) are given too. The comparison of the present data with direct ionization model predictions (SCA and ) has put in evidence the dominance of the ionization mechanism from atomic states for the 12 C, 16 O + Pt collisions, while ionization from molecular orbitals (namely 3dσ MO) dominate the 48 Ti + Pt collision. Despite a worse match between experiment and theory for the collision system 32 S + Pt, due to the uncertainties in the applicability of the theoretical approaches used here, we cannot conclude about a possible contribution for this collision system of MO mechanisms. The effect of vacancy sharing between the L-subshells, included in the first order in calculations, is not important for the total X-ray yields reported here. Possible solid-state effects on the X-ray transitions involving outer-shell electrons are noted. REFERENCES 1. H. Paul and J. Muhr, Phys. Rep. 135, 47 (1986). 2. W. Brandt and G. Lapicki, Phys. Rev. A 23, 1717 (1981). 3. Z. Liu and S.J. Cipolla, Computer Physics Communications, 97, 315 (1996); V. Horvat, Computer Physics Communications 180, 995 (2009). 4. D. Trautmann and F. Rösel, Nucl. Instrum. Methods 169, 259 (1980). 5. L. Sarkadi and T. Mukoyama, J. Phys. B 14, L255 (1981). 6. K. Finck, W. Jitschin, and H.O. Lutz, J. Phys. B 16, L409 (1983). 7. U. Fano and W. Lichten, Phys. Rev. Lett. 14, 627 (1965).

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