W. Koenig

Subthreshold antiproton andK− production in heavy ion collisions

Subthreshold ¯p andK− and energeticπ− production was studied in Ne + NaF, Cu, Sn and Bi, and in Ni + Ni collisions with incident energies between 1.6 and 2 GeV/u. The measured cross sections indicate a dominant contribution of baryonic resonances. This is also consistent with a generalized scaling behaviour of the cross sections with the energy available in the collision and the energy necessary to produce particles as observed with Ne induced reactions. Deviations from scaling especially pronounced in the Ni-Ni system will be discussed in terms of absorption effects. The flat slope of the excitation function for ¯p production can be related to a reduced production threshold caused by a reduction of the antiproton mass in the dense and heated medium by about 100—150 MeV/c2. A similar in-medium mass reduction is also indicated forK− mesons. An increased ¯p reabsorption probability for the heavier systems is concluded from the comparison of the ¯p yields in Ne + NaF, Ne + Sn and Ni + Ni collisions.

Positron lines from subcritical heavy ion atom collisions

Positron lines were observed in heavy ion-atom collisions at bombarding energies close to the Coulomb barrier in subcritical systems with the sum of the atomic numbers of the colliding nucleiZu=Z1+Z2 being smaller thanZu=172. Each collision system, studied,208Pb +208Pb(Zu=164),238U+181Ta(Zu=165), and238Au(Zu=171), exhibits the emission of two positron lines withZu-independent c.m. energies of ∼ 258 keV and ∼ 340 keV, and with widths of about 30 keV, superimposed on continuous positron spectra from nuclear pair decay and pair emission induced by the time changing Coulomb field of the collision. The production cross section of thee+-lines rises with a high power ofZu(ocZu22), which is comparable to theZu20-dependence for the collision induced positrons.

Weak e+e− lines from internal pair conversion observed in collisions of 238U with heavy nuclei

Abstract. We present the results of a Doppler-shift correction to the measured e+e−–sum-energy spectra obtained from e+e−–coincidence measurements in 238U +206Pb and 238U +181Ta collisions at beam energies close to the Coulomb barrier, using an improved experimental setup at the double-Orange spectrometer of GSI. Internal-Pair-Conversion (IPC) e+e− pairs from discrete nuclear transitions of a moving emitter have been observed following Coulomb excitation of the 1.844 MeV (E1) transition in 206Pb and neutron transfer to the 1.770 MeV (M1) transition in 207Pb. In the collision system 238U +181Ta, IPC transitions were observed from the Ta-like as well as from the U-like nuclei. In all systems the Doppler-shift corrected e+e−–sum-energy spectra show weak lines at the energies expected from the corresponding γ–ray spectra with cross sections being consistent with the measured excitation cross sections of the γ lines and the theoretically predicted IPC coefficients. No other than IPC e+e−–sum-energy lines were found in the measured spectra. The transfer cross sections show a strong dependence on the distance of closest approach (Rmin), thus signaling also a strong dependence on the bombarding energy close to the Coulomb barrier.

Systematic studies of positron production in heavy-ion collisions near the Coulomb barrier

We present the results obtained from systematic studies of positron creation for a series of heavy-collision systems, with united chargeZu=Z1+Z2 ranging fromZu=164 (Pb + Pb) toZu=184 (U+U) at bombarding energies close to the Coulomb barrier, using the Orangeβ-spectrometer at GSI. For each collision system studied, the dominating continuous distributions due to quasiatomic and nuclear positron emission are determined accurately. This is essential in obtaining the characteristics of the still unexplained monoenergetic positron lines which appear in the energy range between 200 keV and 400 keV. Our results are compared with coupled-channels calculations for quasi-atomic positron creation. The latter describe quite well the global features of the measured spectra, but overestimate systematically their absolute values. From the comparison, a common normalization factor of about 0.75 can be established for the calculated spectra. In particular, the dependence onZu of the measured emission probabilities was found to follow a power law (∝Zu195±1), in fair agreement with the theoretical prediction.

Investigations of correlated ρ+ e− emission in heavy-ion collisions near the Coulomb barrier

We present the results obtained from a series of ρ+ρ−-coincidence measurements in heavy-ion collisions using the double-Orangeβ-spectrometer at GSI. The collision systems U+U, U+Pb, and U+Ta were investigated at bombarding energies close to and slightly above (U+Ta) the Coulomb barrier. For all systems studied, very narrow (FWHM−20 keV) ρ+ρ− lines were observed in the sum-energy spectra, with kinetic energies ranging from ∼555 keV to ∼810 keV, superimposed on a continuous distribution mainly due to uncorrelated ρ+ρ− emission. Particularly in the U+Ta system, a pronounced sum-energy line appears at ∼634 keV, predominantly in deep-inelastic collisions. In some cases (e.g. U+Pb) the line characteristics is consistent with a two-body decay mode of an emitter which moves with the c.m. velocity of the colliding ions. However, other lines, and in particular the 634 keV line (U+Ta), exhibit a rather isotropical opening-angle distribution whereas their energy is unequally shared between positrons and electrons, thus being in clear disagreement with this scenario. In general, the data preclude an emission from the separated (moving) nuclei, and, in the latter cases, provide evidence that the e+e−-pair decay occurs in the vicinity of the Coulomb field of a third heavy (positively charged) partner having only a small transverse velocity (|v|⊥<0.02c)

Multiple positron structures observed from U - U collisions at 5.6-MeV/u and 5.9-MeV/u

High resolution positron spectroscopy in U-U collisions at bombarding energies of 5.6 MeV/u, the lowest energy studied until now as well as of 5.9 MeV/u, reveals similar multiple structures between 220 keV and 385 keV superimposed on the induced and nuclear continua.

Positron spectra from internal pair conversion observed in 238U + 181Ta collisions

Abstract: We present new results from measurements and simulations of positron spectra, originating from 238U + 181Ta collisions at beam energies close to the Coulomb barrier. The measurements were performed using an improved experimental setup at the double-Orange spectrometer of GSI. Particular emphasis is put on the signature of positrons from Internal-Pair-Conversion (IPC) processes in the measured e+-energy spectra, following the de-excitation of electromagnetic transitions in the moving Ta-like nucleus. It is shown by Monte Carlo simulations that, for the chosen current sweeping procedure used in the present experiments, positron emission from discrete IPC transitions can lead to rather narrow line structures in the measured energy spectra. The measured positron spectra do not show evidence for line structures within the statistical accuracy achieved, although expected from the intensities of the observed γ-transitions ( Eγ∼ 1250-1600 keV) and theoretical conversion coefficients. This is due to the reduced detection efficiency for IPC positrons, caused by the limited spatial and momentum acceptance of the spectrometer. A comparison with previous results, in which lines have been observed, is presented and the implications are discussed.

On the production mechanism of the narrow positron lines observed in heavy-ion collisions

Following a suggestion of Bang et al. we deduce that the total energy transfer associated with the narrow positron lines observed in heavy-ion collisions is given approximately by the sum of the rest masses of the positron and the accompanied electron plus their kinetic energies. Since the same behaviour has been established previously for the production process of the dynamically induced positrons, our observation represents a strong hint that the origin of the e+ -lines may also be connected with the strong Coulomb field occuring in heavy-ion collisions.

First Energy and Angle differential Measurements of e + e − -pairs emitted by Internal Pair Conversion of excited Heavy Nuclei

Abstract: We present the first energy and angle resolved measurements of e+e−-pairs emitted from heavy nuclei (Z≥ 40) at rest by internal pair conversion (IPC) of transitions with energies of less than 2 MeV as well as recent theoretical results using the DWBA method, which takes full account of relativistic effects, magnetic substates and finite size of the nucleus. The 1.76 MeV E0 transition in 90Zr (90Sr source) and the 1.77 MeV M1 transition in 207Pb (207Bi source) have been investigated experimentally using the essentially improved setup at the double-ORANGE β-spectrometer of GSI. The measurements prove the capability of the setup to cleanly identify the IPC pairs in the presence of five orders of magnitude higher β− and γ background from the same source and to yield essentially background-free sum spectra despite the large background. Using the ability of the ORANGE setup to directly determine the opening angle of the e+e−–pairs (Θe+e−), the angular correlation of the emitted pairs was measured within the range covered experimentally (40°≤Θe+e−≤ 180°). In the 90Zr case the correlation could be deduced for a wide range of energy differences EΔ of the pairs (−530 keV ≤EΔ≤ 530 keV). The 90Zr results are in good agreement with recent theory. The angular correlation deduced for the M1 transition in 207Pb is in strong disagreement with theoretical predictions derived within the Born approximation and shows almost isotropic character. This is again in agreement with the new theoretical results.