Experimental determination of the production mechanism for noncharacteristic radiation in swift heavy-ion collisions

Abstract

Absolute differential cross sections of non-characteristic radiation emitted in energetic heavy-ion collisions (Ep = 3.92-48 MeV, Z = I 1-22) can only be explained in the framework of quasimolecular radiation. All other processes, particularly bremsstrahlung mechanisms are found to be negligible. For high projectile and photon energies the one-step quasimolecular radiation is dominant in solid targets, whereas the two-step process contributes mainly at low projectile and photon energies.The non-characteristic radiation (NCR) above the K-lines emitted in energetic heavy-ion collisions has been the subject of several investigations [1][2][3][4][5][6][7][8] during the last years. For adiabatic collisions (vp ~ v~, where vp and v~ are the projectile and the K-shell electron velocities, respectively) various experimental results [1][2][3][4] were interpreted in the framework of quasimolecular radiation (MOR). In this process the vacancy in the lsa quasimolecular orbital (MO) can be produced either in the K-shell of the projectile in a previous collision (two-step MOR(MOR II)) or in the same collision (one-step MOR(MOR I)), where the vacancy decays. In less adiabatic collisions (Vp<Ve), however, bremsstrahlung processes (like secondary electron (SEB), radiative ionization (Ri) or nucleusnucleus brcmsstrahlung (NNB)) have been suggested [6, 71 to become important or even dominant in the production mechanism of the observed NCR continua. Even the question whether the observed NCR is produced by a one-or a two-step process is still unsolved [4,8]. The ambigous interpretations [4,8] * Supported by BMFT are based on the comparison of gaseous and solid target x-ray yields normalized to the characteristic K-lines. Different fluorescence yields in gaseous and solid targets, however, make this normalization procedure doubtful as pointed out in [8]. Absolute cross section measurements should give more reliable information on the mechanism of the NCR. The aim of this study has been to measure systematically absolute differential x-ray emission cross sections of the NCR for energetic heavy-ion collisions as a function of the photon energy Ex, the projectile energy Ep, the target nuclear charge Z 2 and the target density and to find out, by comparison of experimental data with theoretical estimates, which processes produce the observed NCR continua. Calculations of the SEB [9], RI [9], NNB [10] and REC (radiative electron capture) [11] yield absolute cross sections lower by three orders of magnitude than the measured values for all systems investigated here.Furthermore the predicted dependences on Z 2 and Ep are in disagreement with the data. Hence, their contributions are considered to the negligible and will be discussed in detail in a more extensive paper [12].

Theory of the angular distribution of molecular orbitalK x rays seen in heavy-ion-atom collisions

“…The Betz formula has been tested for many symmetric and asymmetric encounters, and has been found to be in good agreement with experiment. However, most of the measurements were made with light ions and atoms (Z1, Z2<20 ) and high velocities, where the one-collision M O x-ray yield may be significant [47] The disagreement in the H values may be due to either a small contribution from one-collision MO x rays in the experimental yields, i.e., to a non-constant a~ (t) (Eq. (7)), or to coupling of the 2 p or 3 p MOs to the continuum or to other levels during the collision.…”

A scaling law for molecular orbital X Rays

“…Other experimentors [7] have chosen to integrate the yields from a fixed x-ray energy, which means one must integrate over Ftot(v, ~). This complicates the comparison between theory and experiment unnecessarily.…”

Charge-State Dependence of Molecular —K—X-Ray Production in Nearly Symmetrical Ion-Atom Collisions

“…We have observed a nearly exponential increase of the cross section with charge state which is interpreted as evidence of a strong excitation of lso~ electrons into empty higher-lying bound states, A comparison with the neighboring system Cl + Cl using a solid target establishes the predominance of one-collision processes for molecular-K-x-ray production. The strong charge-state dependence explains previous differences found in gas-and solid-target measure-mentSo 7 " 9 One of us (I.T.) acknowledges the hospitality and support of the Max Planck Institut fur Kernphysik at Heidelberg.…”

“…6 The observed charge-state dependence explains the previous controversial results found in gas-and solid-target measurements. 7 " 9 The experiments were performed at the EN tandem Van de Graaff accelerator of the Max Planck Institut fur Kernphysik at Heidelberg using a 32-MeV 32 S beam with charge states n = 5, 7, 9, 10, 11, 12, 13, and 14. The high-charge states (n ^9) were obtained using a second stripper foil before the switching magnet, at a distance of about 13 m from the target.…”