High energy gamma-ray production in heavy-ion reactions

The doubly-differential cross section for the production of gamma-rays with energies larger than 20 MeV has been measured in the reaction ⁴⁰Ar+¹⁹⁷ Au at 30 MeV/u incident energy. It is found that the spectra are thermal-like, the emitting source having half the beam velocity and an apparent temperature around 7.3 MeV. The non-observation of a collective component is discussed.     

Prompt light particle emission in the 36Ar + 58Ni reaction at 95 MeV/nucleon

The prompt component at intermediate velocity of light charged particles is investigated. An improved coalescence model coupled to the intra-nuclear cascade code ISABEL is used to obtain light complex particle energy spectra and multiplicities as a function of impact parameter. The results are compared with experimental data from the ³⁶Ar + ⁵⁸Ni experiment at 95 MeV/nucleon, performed with the INDRA 4π detection system. The calculated prompt component is found to rather well reproduce proton spectra. For complex light charged particles the calculated components well populate the high energy part of spectra. Prompt emission can therefore explain the large transverse energies experimentally observed at mid-rapidity. Received: 27 July 2000 / Accepted: 20 November 2000     

New rotational bands in 174Ta

Three new bands in ¹⁷⁴Ta have been identified by using the ¹⁶⁰Gd(¹⁹F,5n) reaction at beam energies of 87 MeV and 96 MeV. Nilsson configurations are assigned to these bands. In the 9/2⁻[514]_p+5/2⁻[512]_n band, the AB neutron crossing occurs at a rotational frequency of 0.30 MeV. This is indicative of the disappearance of the evidence for a reduction in neutron pair correlations. Received: 30 March 1998     

Light charged particle emission induced by neutrons with energies between 25 and 65 MeV on oxygen. I. Protons and deuterons

Double-differential cross sections (energy spectra) for proton and deuteron emission in fast neutron induced reactions on oxygen are reported for nine incident neutron energies between 25 and 65 MeV. Angular distributions were measured at 15 laboratory angles between 20° and 160°. Procedures for data taking and data reduction are presented. Deduced energy-differential, angle-differential and total cross sections are also reported. Experimental cross sections are compared with existing data and with theoretical model calculations. Received: 24 April 1998 / Revised version: 1 July 1998     

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

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 (R_min), thus signaling also a strong dependence on the bombarding energy close to the Coulomb barrier. Received: 22 July 1997 / Revised version: 15 October 1997     

Evidence for a dissipative friction mechanism based on 8Be fragments from the interaction of 12C with 59Co

It is suggested that the spectra of the ⁸Be ground-state (gs) nuclei produced in the interaction of ¹²C with ⁵⁹Co at incident energies varying from 8.3 to 33.3 MeV/amu can be explained by introducing a dissipative friction interaction mechanism preceding projectile break-up. Received: 27 March 2000 / Accepted: 11 April 2000     

The quark condensate in relativistic nucleus-nucleus collisions

We compute the modification of the quark condensate <ˉq q> in relativistic nucleus-nucleus collisions and estimate the 4-volume, where the quark condensate is small (<ˉqq>/<ˉqq>⁰≤ 0.1–0.3) using hadron phase-space distributions obtained with the quark-gluon string model. As a function of the beam energy the 4-volume rises sharply at a beam energy E^lab/A ≃ (2–5) GeV, remains roughly constant up to beam energies ≃ 20 GeV and rises at higher energies. At low energies the reduction of the condensate is mainly due to baryons, while at higher energies the rise of the 4-volume is due to the abundant mesons produced. Based on our results we expect that moderate beam energies on the order of 10 GeV per nucleon are favourable for studying the restoration of chiral symmetry in a baryon-rich environment in nucleus-nucleus collisions. Received: 4 December 1997 / Revised version: 31 July 1998     

First experiment with decelerated bare uranium ions conducted at the ESR storage ring

The deceleration capabilities of the ESR have been used for the first time in a dedicated ground state QED experiment conducted at the gasjet target of the ring. By decelerating bare uranium ions from 358 MeV/u down to energies of as low as 49 MeV/u, X-ray spectra have been obtained which provide an abundant yield of characteristic X-ray transitions. The experiment demonstrates that, by choosing the appropriate beam energy and gasjet target, almost all excited projectile states can be selectively populated. Moreover, the experiment provides the first data for beam lifetimes of stored decelerated high-Z ions. Such data are essential for the design of future experiments dealing with decelerated ion beams far below 50 MeV/u. This revised version was published online in August 2006 with corrections to the Cover Date.     

Search for new isotopes of element 112 by irradiation of 238U with 48Ca

The reaction ⁴⁸Ca+²³⁸U was investigated at the recoil separator VASSILISSA at the Flerov Laboratory of Nuclear Reactions, JINR Dubna, in attempts to synthesize new isotopes of element 112. The experiments were performed at two beam energies resulting in excitation energies of the compound nucleus of 33 and 39 MeV. The collected beam dose was 3.5 × 10¹⁸ and 2.2 × 10¹⁸, respectively. Two spontaneous fission events were observed at the lower beam energy, which tentatively were assigned to the new neutron rich isotope ²⁸³112 produced in the reaction ²³⁸U(⁴⁸Ca,3n)²⁸³112. The measured cross-section is (5.0^+6.3 _−3.2) pb and the half–life is (81⁺¹⁴⁷ ₋₃₂) s. No event was observed at the higher beam energy resulting in the upper cross–section limit of 7.3 pb. Received: 12 October 1998 / Revised version: 12 January 1999     

Electromagnetic probes of nuclear collisions at intermediate energy

We show that emission of high-energy electrons and photons in nuclear collisions at intermediate energies is sensitive to the space-time evolution of the reaction. The electron and photon spectra measure related but complementary quantities connected with the nuclear charge distribution. We show that at 60 MeV/u beam energy production of 50 MeV electrons is predicted to measurable probability.     

Role of cross-shell excitations in the reaction 54Fe($$ \vec d $$, p)55Fe

The reaction ⁵⁴Fe(, p)⁵⁵Fe was studied at the Munich Q3D spectrograph with a 14MeV polarized deuteron beam. Excitation energies, angular distributions and analyzing powers were measured for 39 states up to 4.5MeV excitation energy. Spin and parity assignments were made and spectroscopic factors deduced by comparison to DWBA calculations. The results were compared to predictions by large-scale shell model calculations in the full pf -shell and it was found that reasonable agreement for energies and spectroscopic factors below 2.5MeV could only be obtained if up to 6 particles were allowed to be excited from the f _7/2 orbital into p _3/2 , f _5/2 , and p _1/2 orbitals across the N = 28 gap. For levels above 2.5MeV the experimental strength distribution was found to be significantly more fragmented than predicted by the shell model calculations.     

Observation of K+d correlations from pA collisions

Results of a first experiment on (K^+p) and (K^+d) correlations from proton-carbon (pC) and proton-deuteron (pd) interactions at beam energies above and much below the threshold for elementary kaon production in nucleon-nucleon reactions ( T _NN = 1580 MeV) are discussed. These data, obtained with the ANKE spectrometer at COSY-Jülich, provide first direct evidence for K⁺ production via the two-step mechanism and an indication for a cluster mechanism. It is shown that both processes contribute significantly in pC collisions at 1200 MeV, while they are strongly suppressed at 2300 MeV and also in pd-interactions at 1344 MeV. It is emphasized that the underlying kinematics can be exploited to distinguish between these reaction mechanisms. Received: 26 November 2002 / Accepted: 26 March 2003 / Published online: 20 May 2003     

Continuum electron capture dependence on projectileZ and velocity

Continuum electrons centered in velocity about that of the projectile and ejected near zero degrees in the laboratory frame have been observed for collisions of highly stripped C^q+ and O^q+ ions incident on Argon. For fully stripped projectile ions, the Z dependence and the velocity dependence of the continuum capture cross section have been determined for beam energies of 1.6 to 2.8 MeV/amu. Striking disagreements are noted between some results of this work and present theoretical predictions.     

Target ion source and charge exchange cell development for the EXCYT facility

The EXCYT facility at the INFN-LNS is based on a K-800 superconducting cyclotron delivering stable ion beams on a Target Ion Source (TIS) assembly to produce the required nuclear species, and on a 15 MV Tandem for post-accelerating the radioactive beams. For some ion beams such as for Li, the extraction efficiency from the TIS is higher when obtained by positive ionisation, while the injection into the Tandem is possible only after a charge exchange to obtain negative ions. In this work we present the procedures together with the results of the production of ^6,7,8,9Li beams extracted at EXCYT during the last year. The production of the radioactive elements was performed by sending a ¹³C⁴⁺ primary beam of 45 MeV/u on a graphite target. The ionisation of the production species was achieved by a tungsten positive surface ioniser. The Li⁺ has been extracted from TIS at different energies to cross-check the transmission and the charge exchange efficiency. To perform the conversion from positive to negative ions we employed a Charge Exchange Cell (CEC) containing Cs vapours. The Li beam interacts with the latter in a two-step reaction, thus converting its charge from +1 to –1. The CEC was already characterised during off-line tests; the results obtained at EXCYT confirmed both the isotopic shift effect and the efficiency values at several given extraction energies. Future improvements of the TIS and the CEC are discussed.     

Do extreme conditions of nuclear matter influence particle properties? summary of the HADES project

Heavy ion collisions at beam energies of 1–2 GeV/u (SIS at GSI, Darmstadt) offer a unique possibility to investigate the properties of hot, compressed nuclear matter. Baryon densities of 2–3ρ ₀ and temperature up to 100 MeV can be reached during a collision phase. Due to a high density of the phase new particles are produced in multiple baryon-baryon collisions. Objects decaying into electron-positron pairs play an important role in this context. In contrast to hadrons, dileptons carry away undistorted by final-state interactions information about the hot and dense phase. The two-body decays of the vector mesons ρ, ω and ? are of special interest since width and masses of the resonances are accessible in the dielectron mass spectra. A High Acceptance DiElectron Spectrometer (HADES) has been proposed for the SIS accelerator at GSI in Darmstadt and currently is constructed by the HADES Collaboration in many places of Europe. A Ring Imaging Cerenkov Counter (RICH), blind for hadrons, together with Multiplicity Electron Trigger Array (META) containing Shower detector and Time-of-Flight wall will serve for the electron identification. The momentum of dileptons will be determined in 2 sets of Mini-Drift Chambers (MDC) placed in front of and behind a six-fold super-conducting coil. An expected invariant mass resolution in the ρ/ω region less than 1% (σ) assures a clear separation of the ω and ? from ρ. For the difficult case of the 1 GeV/u Au+Au collision we expect a 10:1 signal-to-background ratio at the ρ/? region. The performance of the spectrometer was studied by detailed simulations using GEANT. Results of simulations and tests of prototypes assure that with its high counting rate capability and large geometrical acceptance HADES will be able to measuree ⁺ e ^? pairs for the heaviest systems at the highest SIS energies. First results are expected in 1999.     

中能区丰中子核11Li的反应总截面的测量

利用HIRFL提供的50MeV/u的¹³C束流轰击Be靶, 通过RIBLL选择出放射性核素¹¹Li. 实验采用透射法测量了25—45MeV/u的¹¹Li在²⁸Si靶上的反应总截面. 采用双参数Gauss密度分布形式, 利用Glauber模型很好地拟合了高能和中能区的¹¹Li实验数据, 并从密度分布中提取了核的物质均方根半径.     

Temperatures and Excitation Energies of Hot Nuclei in Reactions of 40Ar+natAg,209Bi at MeV/u

Coincidence measurements of fission fragment and light charged particle have been perfotrmed for the reactions of ⁴⁰Ar+^natAg,²⁰⁹Bi at E/A=25MeV using 4 PPAC and 11 sets of ΔE-E telescopes.Angular correlations of fission fragment were ploted as a function of the folding angle between the two detected fission fragments.The linear momentum transfer distributions were derived by measuring angular correlations.The backward spectra of light particles detected in coincidence with fission fragments having different average〈LMT〉are analyzed with Maxwell distribution.After some corrections the initial temperatures of the hot nuclei are determined from the energy spectra.The excitation energies corresponding to the different average〈LMT〉are obtained considering the reaction Q values and pre-equilibrum emission.In the central collision of the ⁴⁰Ar+^natAg,²⁰⁹Bi reactions,excitation energies are measured to be about 4.2MeV/u,2.4MeV/u and temperatures about 6.1MeV,5.5MeV,respecdvely.In semi-central collision,excitaionen energies are measured to be about 3.5MeV/u,1.9MeV/u and temperatures about 5.8MeV,4.8MeV respectively.     

Fission fragment anisotropies for 19F +209Bi system

The fission fragment angular distributions have been measured for the system ¹⁹F +²⁰⁹Bi over a range of bombarding energies from 88.0 MeV to 125.6 MeV. The measured fission fragment anisotropies are in agreement with the saddle point statistical model calculations in the above energy range. Combining these data with those available for ¹¹B, ¹²C, ¹⁴N, ¹⁶O and ¹⁸O +²⁰⁹Bi, ²⁰⁸Pb systems, it is concluded that the spherical target plus projectile systems behave “normal” from near- to above – barrier energies. This observation is in contrast to the “anomalous” anisotropies exhibited by the deformed actinide target – projectile systems at near – barrier energies. Received: 5 May 1999 / Revised version: 25 July 1999     

Unnatural-parity states in 44 21Sc23

The neutron-deficient nucleus ⁶⁸As was populated at high spin in two experiments using the reaction ⁴⁰Ca(³²S, 3pn) at beam energies of 105 and 95 MeV. A self-supporting and a gold-backed, highly enriched ⁴⁰Ca target were used. Gamma rays were detected with the EUROBALL array, combined with the charged-particle detector array EUCLIDES and the Neutron Wall. The ⁶⁸As level scheme was considerably extended, especially at negative parity and many previous spin-parity assignments were confirmed or rejected. The total-Routhian-surface (TRS) calculations find shape coexistence and softness for the negative- and positive-parity states, respectively.     

Ionization of iridium ions in the Dresden EBIT studied by X-ray spectroscopy of direct excitation and radiative recombination processes

Ir^q+ ( 41≤q≤64) ions with open-shell configurations have been produced in the electron beam of the room-temperature Dresden Electron Beam Ion Trap (Dresden EBIT) at electron excitation energies from 2 keV to 13 keV. X-ray emission from direct excitation processes and radiative capture in krypton-like to aluminium-like iridium ions is measured with an energy dispersive Si(Li) detector. The detected X-ray lines are analyzed and compared with results from multiconfigurational Dirac-Fock (MCDF) atomic structure calculations. This allows to determine dominant produced ion charge states at different electron energies. The analysis shows that at the realized working gas pressure of 5×10^-9mbar for higher charged ions the maximum ion charge state is not preferently determined by the chosen electron beam energy needed for ionization of certain atomic substates, but by the balance between ionization and charge state reducing processes as charge exchange and radiative recombination. This behaviour is also discussed on the basis of model calculations for the resulting ion charge state distribution. Received 12 July 2001 and Received in final form 10 September 2001