Electromagnetic fission of238U at 600 and 1000 MeV per nucleon

Electromagnetic fission of²³⁸U projectiles at E/A =600 and 1000 MeV was studied with the ALADIN spectrometer at the heavy-ion synchrotron SIS. Seven different targets (Be, C, Al, Cu, In, Au and U) were used. By considering only those fission events where the two charges added up to 92, most of the nuclear interactions were excluded. The nuclear contributions to the measured fission cross sections were determined by extrapolating from beryllium to the heavier targets with the concept of factorization. The obtained cross sections for electromagnetic fission are well reproduced by extended Weizsäcker-Williams calculations which include E1 and E2 excitations. The asymmetry of the fission fragments' charge distribution gives evidence for the excitation of the double giant-dipole resonance in uranium.Communicated by V. Metag     

Response of cooled PWO scintillators to low-energy gamma-rays and its importance in studies of the h c →η c + γ transition in charmonium

Suppression of non-resonant background due to emission of π ⁰ increases signal to background ratio in γ-ray emission studies between the discrete states of charmonium. We demonstrate with Monte-Carlo simulations how π ⁰ identification capability increases with decreasing threshold for γ-ray detection in π ⁰ decay. An apparatus has been designed and measurements of the FWHM resolution performed for the three energies 6.13, 12.7 and 16.9 MeV. The trend established in the 50–650 MeV energy range is corroborated.     

Energy dependence of the isotopic effect in the (n,p) reaction on the germanium isotopes

Cross sections for^70,72,73,74Ge(n, p)^70,72,73,74Ga,⁷⁰Ge(n, 2n)⁶⁹Ge,⁷²Ge (n,)⁶⁹Zn ^m and⁷⁴Ge(n, )⁷¹Zn ^m reactions are measured in the energy range from 13.0 to 16.6 MeV by the activation method using Ge(Li) detector-ray spectroscopy and compared with predictions of the reaction model incorporating preequilibrium and equilibrium emission mechanisms to interpret the energy dependence of the isotopic effect occuring in the (n, p) reaction. The fitted single-particle state-density parametersg, determined here for the germaniums are discussed together with theg-values found previously for the Se, Zr and Pd isotopic chains. A validity of the consistency condition between the precompound and compound models, which relatesg to the experimental level-density parametera viaa= ² g/6 is demonstrated.     

Neutron-hole strength distributions in heavy nuclei: (II). The 144, 148, 152Sm(3He, α) 143, 147, 151Sm and the 144, 148, 150, 152, 154Sm(p,d) 143, 147, 149, 151, 153Sm reactions

Valence and deep-lying neutron-hole strengths corresponding to orbits near and well below the Fermi surface have been observed in high-resolution studies of the ^{144, 148, 152}Sm(³He, α) and of the ^{144, 148, 150, 152, 154}Sm(p, d) reactions at 70 and 42 MeV bombarding energy, respectively. The explored excitation energy range was 28 MeV for the (³He, α) experiment and about 12 MeV in the (p, d) study. Complete angular distributions have been measured in both cases and the data was analyzed within the framework of the distorted waves Born approximation theory of direct reactions.For the neutron closed shell target (¹⁴⁴Sm), in addition to the well-known fragmentation of the 2d_{$\text{5}\text{2}$} and 1g_{$\text{7}\text{2}$} valence-hole strength, a new bump observed around 7.6 MeV excitation energy is excited in both reactions. This structure corresponds to the 1g_{$\text{9}\text{2}$} inner-hole strength in ¹⁴³Sm and the analysis of the (³He, α) data suggests that more than 50% of the l = 4 strength can be found between 6 and 12 MeV. When one goes to the heavier Sm isotopes, the energy spacing between valence-hole states located above and just below the N = 82 shell decreases strongly and disappears in ¹⁵¹Sm as a result of increasing deformation.Combining good energy resolution and detailed analysis of the two reactions, rather complete spectroscopic information is obtained for the valence-hole strength distributions. With regard to inner-hole states, the energy spectra exhibit a narrow structure whose centroid energy decreases from 4.4 to 2.9 MeV when the mass number increases from A = 147 to A = 153. The main peak displays an asymmetric shape with an extremely large high-energy tail. The 1h_{$\text{11}\text{2}$} hole strength is split into the Nilsson Orbitals. The narrow bumps are found to carry a large fraction of the l = 5 and l = 2 hole strengths in ^{147,149,151,152}Sm isotopes. In the high-energy tail of the structures one observes overlapping and increasing spreading of the g_{$\text{7}\text{2}$}, 2d_{$\text{5}\text{2}$} and possibly 1g_{$\text{9}\text{2}$} inner-hole strengths due to the disappearance of the N = 82 shell gap between N = 83 and N = 89 neutron numbers. The experimental hole strengths distributions are compared where possible to the predictions of the quasiparticle-phonon nuclear model or to the simple Nilsson model.     

Study of resolution of the PANDA GEM detector with Garfield

The forward GEM tracker of the P?ANDA detector at the future FAIR facility will track the particles produced in antiproton-proton annihilations and emitted in the polar angle range 5° –22°. Position resolution at the level of 100 μ m and good time resolution are critical to work under luminosities up to 2×10³² c m ^?2 s ^?1. The simulations performed with Garfield program compared several detector layouts and determined the optimal granularity of readout electronics. The time resolution for two possible gas mixtures was also estimated.