High-spin states of odd-odd156Ho

High spin states in the nucleus¹⁵⁶Ho have been studied via the fusion-evaporation reaction¹⁴⁰Ce(¹⁹F,3n)¹⁵⁶Ho at the beam energy of 82 MeV.- coincidences, E, I and excitation functions have been measured. With two rotationally-aligned bands identified as [h_11/2] [i_13/2], we found two new bands with a signature splitting, whose structure is believed to arise from either [h_9/2] or [f_7/2] coupled to [h_11/2]. Our placement of rays belonging to these new bands suggests that the previously known level scheme should be corrected. We also found many new low-energy rays in the low excitation energy. As compared to¹⁵⁴Ho, the present nucleus with two more neutrons becomes more collective, and shows well-observed rotationally-aligned bands with a clear signature splitting.Communicated by B. Herskind     

Comparison ofJ/ψ-suppression in photon,hadron and nucleus-nucleus collisions: Where is the quark-gluon plasma?

The data ofJ/-production cross sections from photon, hadron and nucleus-nucleus collisions are plotted against the length of thec final state trajectory in nuclear matter. A value for the absorption cross section per nucleon of _abs ^N =(6.2±0.3) mb is deduced from the baryon and photon induced reactions and _abs ^N =(6.9±1.0) mb from the nucleus-nucles collisions. The equality of cross sections implies that additional suppression effects from a quark-gluon plasma are not visible.     

Maximum excitation energy in fusion-evaporation reactions from light particle multiplicity measurements in32S+58Ni at 25 MeV/A

By using two 4 detector systems for charged particles and neutrons, we measured the evaporated light particles emitted in coincidence with evaporation residues (ER) produced in the reaction³²S+⁵⁸Ni atE(³²S) 820 MeV. From the analysis of the light particle multiplicities as a function of the ER velocity, we extracted the maximum excitation energy reached in fusion evaporation reactions for the studied system.Supported by the European Community Programme Human Capital and Mobility     

Excitation functions and isomeric ratios in the reaction130Te (α,xpyn) for α-particle energies between 15 and 37 MeV

The¹³⁰Te(,n)¹³³Xe,¹³⁰Te(,p)¹³³I,¹³⁰Te(,pn)¹³²I and¹³⁰Te(, n)¹²⁹Te excitation functions for alpha particle energies between 14.7 and 36.5 MeV are presented. Excitation functions for the last three reactions are measured for the first time and more consistent results for (, n) are obtained. The isomeric ratios for the ( n), (, pn) and (, n) reactions were deduced. Calculations based on different theoretical models of equilibrium and preequilibrium decay are compared with the experimental results and discussed. Acceptable presentation of the energy behavior of the measured isomeric ratios was obtained when higher angular momentum depletion for preequilibrium emitted particles was included. The absolute values of the isomeric ratios however remain overestimated.We want to express our gratitude to Prof. Ts. Vylov and Dr. V. Brudanin (Dubna) who supplied us with measurement equipment, to Dr. Chr. Necheva for taking part in the experiments, to the U-200 cyclotron staff and to the direction of the Laboratory of Nuclear Reactions (JINR) for making the irradiation possible and to Dr. E. Dobreva (Sofia) for fruitful discussion of the experimental method. We wish to express our gratitude also to Prof. M. Uhl and the NEA data bank for making the computer code STAPRE available to us and to Prof. J. Ernst (Bonn) for fruitful discussions. This project has been completed with the financial support of the Committee for Science at the Council of Ministers under contract No. 644.     

Isotonic and isobaric dependencies of nuclear charge radii for rare-earth nuclei

Isotonic and isobaric dependencies of nuclear charge radii in the region between Z=54 and Z=70 are obtained from the analysis of isotope shift data r² and muonic and electron scattering data on r². They are compared with the predictions of the droplet model and the Hartree-Fock calculations. The isobaric dependencies of r² have proven to be especially sensitive to the choice of an effective nucleon-nucleon interaction.     

Photofission of235U and238U at intermediate energies: absolute cross sections and fragment mass distributions

The total photofission cross section _,F for²³⁵U and²³⁸U has been measured in the energy range 50E 800 MeV at the 855 MeV Mainz Microtron MAMI using energy and time tagged photons (Glasgow Tagger) and a 4 arrangement of position sensitive fragment detectors. Besides the absolute photofission cross section _F , which almost completely exhausts the total photon absorption cross section for these nuclei, fragment mass distributions in this energy domain were determined via time of flight techniques (TOF). The results for the total photofission cross sections _,F normalized to the atomic numberA for both isotopes coincide, and agree in the-resonance region, within the systematic errors, with the socalledUniversal Curve _,T /A of the total photon absorption cross section _,T . At higher energies the cross sections exhibit a smooth behaviour. In particular, it is shown for the first time that there isno resonance-like shape near the D₁₃ resonance (at 710 MeV) as observed for the free proton. This complete suppression of the D₁₃ resonance in complex nuclei is not yet understood on a microscopic level. The fragment mass distributions show a predominantly mass symmetric fission. However, contributions from mass asymmetric fission at some photon energies may give a hint of an increased mass asymmetric fission after the onset of the pion and two pion channels.Excerpt from D26 Supported by the Deutsche Forschungsgemeinschaft, SFB 201 Mainz     

Hard form factors for meson-baryon strong couplings as derived from deep inelastic lepton scattering

As stimulated by earlier attempts for obtaining theNN andN form factors from the deep inelastic lepton scattering data, we extend the analysis by taking into account effects of additional mesons including, , ,K, andK ^*, with the coupling constants fixed by the lowenergy nucleon-nucleon and hyperon-nucleon scattering data. Contrary to an earlier claim that the NN andN form factor must be very soft (e.g., with the cutoff mass less than 500 MeV in the monopole form), we find, for example, that with all form factors parametrized in the dipole form, a universal cutoff mass of 1150 MeV in the/N sector and 1400 MeV in the/ sector yields predictions in excellent agreement with recently published neutrino data on the momentum fractions carried by thes, , and¯d quarks, as well as consistent with the sea-to-valence ratio extracted from the CDHS data and the Femilab E615 experiment. Similar results can also be obtained by using exponential cutoffs for all couplings, or by using monopole forms for some vertices while retaining dipole forms for the rest. The success of the mesonexchange picture in generating the strangeness content in a proton suggests an alternative understanding of the origin of sea quarks in the proton.We wish to thank G. Garvey, K. Holinde, L.-C. Liu, M. B. Johnson, M. Strikman, A. W. Thomas and Jochen Wambach for helpful conversations. W-Y. P. Hwang wishes to acknowledge the Alexander von Humboldt Foundation for a fellowship to visit Jülich for conducting research. His research works was also supported in part by the National Science Council of the Republic of China. The work of G. E. Brown is supported in part by a Humboldt award, in part by NATO Grant RG85/0093, and in part by the U.S. Department of Energy.     

Deeply bound pionic states with the (Σ −,Λ) reaction

We study the reaction ^– +A + (A ^–) with the ^– bound in the nucleus, as a means of producing deeply bound pionic states in nuclei, so far unobserved. The reaction is similar to the (n, p) reaction but, because of the ^–, mass difference, it allows the reaction to occur with smaller momentum transfer, thus increasing the transition probability and reducing the effects of distortion. The ratios of signal to background are one to two orders of magnitude better than in the (n, p) reaction.We would like to thank C. Dover and G. Tamas who helped us to get a perspective of the present and future possibilities of this and other reactions.One of us, J. Nieves wishes to acknowledge a fellowship from the Ministerio de Educatión y Ciencia. This work is partially supported by the CICYT. All the calculations have been done in the Centro Informático de la Universidad de Valencia.     

Preequilibrium emission in (p,x n) reactions on palladium isotopes

Energy spectra of neutrons from the^{104, 105, 106, 108, 110}pd(p, xn) reactions atE _p=26.1 MeV have been measured using the time-of-flight technique with good overall energy resolution (< 250=" kev).=" the=" hard=" part=" of=" angle-integrated=" spectra=" corresponding=" to=" low=" residual=" excitations=">U6MeV) exhibits pronounced structure varying from isotope to isotope. The observed structure can be reasonably well described by preequilibrium-model calculations with two-quasiparticle state densities for proton-particle neutron-hole configurations based on Nilsson-model single-particle levels instead of equidistant ones. The influence of nuclear deformation and pairing on preequilibrium particle emission is explored.Supported in part by the exchange program of the Universities of Hamburg and Zagreb, Internationales Büro — KFA Jülich, and by the BMFT (Contract 06 HH 142)     

Excited levels of111In and113In

Levels and transitions in¹¹¹In and¹¹³In have been studied in the¹¹¹Cd(p, n)¹¹¹In and¹¹³Cd(p, n)¹¹³In reactions. By means of- coincidences,-angular distributions, relative excitation functions and conversion electron measurements more than 20 levels below 2 MeV have been established in each nucleus. Several negative-parity states were found above 1 MeV. Two low-spin positive-parity states with features similar to the possible rotational band in the heavier In nuclei are observed below 1,400 keV in both nuclei.     

Determination of an upper limit for the mass of the τ-neutrino at LEP

An upper limit for the mass is determined through the kinematic reconstruction of the decay5 ^± in the OPAL detector at LEP. The limit is obtained using a new method based on the comparison of the two-dimensional distribution of energy and invariant mass of the five-pion system with expectations from different neutrino mass hypotheses. From a sample of five events surviving the selection criteria we obtain an upper limit of 74 MeV at 95% confidence level. It is the first measurement at LEP energies, where the larger average multiplicity of events makes the suppression of this background more robust compared to lower energies.     

The structure of28Si above 10 MeV excitation energy I: gamma-decay modes and radiative widths of levels

The systematics of-decay modes and radiative widths of highly excited states in²⁸Si has been extended by taking-ray spectra on 60 resonances of the²⁷Al(p,) reaction in the range 1097 keVE _p 4492 keV (12643 keVE _x 15915 keV) and on the²⁴Mg(,) resonances atE =3355, 3431, 4003 keV (E _x =12860, 12925, 13 415 keV). The-decay modes of levels in the sub-resonance region (E _x =10–12.5 MeV) were studied with both the²⁷ A1(p, ) reaction and the²⁷ A1(d, n) reaction at E_d=4,5,6 MeV. Information on radiative widths of levels was obtained by measuring the resonance strengthsS =(2I+1) _p / for 52 resonances of the²⁷Al(p, ) reaction with an emphasis on prospective T=1 states.     

Search for an enhanced production of low energy pions in C(p, π+)X,Cu(p, π+)X,and Cu(p, π∘)X reactions for 300 MeV≦Tp≦400 MeV

The observation of an enhancement in the production of low energy pions in the Cu(p, ⁺)X reaction is confirmed. The width of the bump is about 5 MeV around 350 MeV. A similar enhancement appears at 330 MeV for C(p, ⁺)X but the width has not been measured. An attempt to observe this behaviour in Cu(p, )X is described. From the data one cannot conclude about the existence of a structure around 350 MeV proton energy. The lack of resolution in the spectrometer may have washed out the small effect expected from ⁺ measurements.     

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

We present the results obtained from a series of ^+–-coincidence measurements in heavy-ion collisions using the double-Orange-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) ^+– 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 ^+– 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|<>Dedicated to Prof. B. Povh on the occasion of his 60th birthday     

The branching ratio ofβ-delayed two-proton emission

The decay of the ⁺-fed isobaric analog state by one- and two-proton emission is calculated in the formalism of compound-nucleus decay. Support is given to the uncorrelated character of the emitted protons in the2p process. The ratio of2p top probabilities is obtained for the isotopes predicted to be2p emitters up to Z=30, i.e.²²Al,^22,23Si,²⁶P,²⁷S,³¹Ar,³⁵Ca,³⁹Ti,⁴³Cr,⁴⁶Mn,^46,47Fe,^49–51Ni, and⁵⁵Zn.     

A mini transmissionγ-ray Compton polarimeter and measurements of the sign of magnetic moments of radioactive nuclei

A mini transmission-ray Compton polarimeter was developed for the measurement of the sign of magnetic moments of radioactive nuclei with low-temperature nuclear orientation. The signs of the magnetic moments of the following isotopes were determined:¹⁹³Os [=+0.7297(16) _N];^191m Ir [=+6.20(9) _N];¹⁹²Ir [=+1.924(10) _N];¹⁹⁴Ir [=+0.39(1) _N];^195m Pt [=–0.605(15) _N].Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday     

Experimental study of π− π+ system at low invariant-masses

We report on results of an experiment to study the reaction ^– p ^{– +} in the incident pion momentum region between 295 and 450 MeV/c. A departure from phase space was observed in invariant-mass spectra and angular distributions which cannot be explained with and resonance production. It indicates aI=0,J ^P = 0^{+ – +} final-state interaction.Deceased     

Coulomb corrections to Delbrück scattering

The emission of high-energy-rays in the deexcitation of hot⁴⁰Ca and³⁹K nuclei formed in heavy-ion fusion reactions at excitation energyE _x 90 MeV has been studied. The high energy-rays were measured in coincidence with evaporation residues or light charged particles. The spectrum from the self-conjugated compound nucleus⁴⁰Ca shows an appreciable yield suppression in the Giant Dipole Resonance (GDR) energy region with respect to the³⁹K, due to isospin selection rules in the dipole-decay. The spectral line shapes of the spectra are well reproduced by using a statistical code which treats explicitly the isospin quantum number in evaluating level densities and transmission coefficients. The GDR parameters determined from the present coincidence measurements are in good agreement with the systematic in theA 40 mass region at lower bombarding energy based on the analysis of inclusive spectra.We thank M. Caldogno for technical support in the development of evaporation residues detectors. We acknowledge the participation of M. Anghinolfi, P. Corvisiero, M. Taiuti and A. Zucchiatti in the early stages of this work. Thanks are due to M. Kicinska-Habior for providing the isospin-dependent code. Discussions with B. Fornal are also gratefully acknowledged.