Analysis of inclusive nucleon spectra fromp- andα-induced reactions in the frame of the INDEX model,and implications for stopped π−-induced reactions

The previously published INDEX model is tested for nucleon spectra fromp- andα-induced reactions. The results of two alternative versions, the INCLUSIVE INDEX model and the EXCLUSIVE INDEX model, quite well agree with the data. It is found that in the INCLUSIVE INDEX model three preequilibrium stages are sufficient to describe single- and multi-nucleon emission. The model provides an useful first order estimate of the influence of the finite Fermi energy on particle spectra. This effect is very strong for nucleon induced reactions while forα-induced reactions it can be neglected. The deduced mean-free-path multiplier corroborates the long stated discrepancy between models in which excitons interact independently or not. Using preequilibrium parameters similar to those found for nucleon induced reactions the important branching ratio of contributingnp andpp pairs in stoppedπ ^?-absorption can be determined by INDEX model calculations. Deduced values from publishedn- andp-spectra agree reasonably well with those of other experimental analyses but deviate significantly from microscopic model predictions.     

Particle hole yrast states in146Gd and147Gd and the Z=64 shell closure

The level structures of¹⁴⁶Gd and¹⁴⁷Gd have been investigated by in-beamγ-ray ande ^? spectroscopy with (α, xn) reactions on enriched Sm targets. Detailed level schemes up to ～4 MeV, which differ radically from earlier schemes, are reported. The energy levels are characterized as particle-hole excitations using empirical single particle energies and two nucleon interactions. Analysis of pure 1p 1h proton excitations demonstrates that theZ=64 andN=82 energy gaps are about equally large.     

π+/π− Ratio and two particle production in Proton's fragmentation region by Monte Carlo simulation

Hadron jet at lowp _T is generated by Monte Carlo method using several models including our quark cascade model in which the recombination mechanism is taken into account. The predicted results from those models on π⁺/π^? ratio,K ⁺ /K ^? ratio, π^± meson spectra and π⁺/π^? ratio under π^± meson and Λ particle trigger, ratio between invariant two particle cross-section (p→π ⁺ /π ^? )/(p→π ^? /π ^? are compared with the experimental data. The effects of the asymmetric initialu- andd-quark distributions the scaling and its violation of particle spectra and particle ratios under the other particle trigger are discussed. Predictions from our recombination model fit to the experimental data fairly well.     

Analysis of excitation functions from light ion induced reactions in the EXCLUSIVE INDEX model

The previously introduced EXCLUSIVE INDEX model allows to predict the population of 6 residual nuclei including the primary compound nucleus through two stages of the preequilibrium phase. The present version is limited to maximum two-nucleon emission. The preequilibrium ejectiles may reduce the brought-in rotational energy by a model of maximum angular momentum decoupling. Subsequent evaporation of protons, neutrons andα-particles is treated in the frame of the Weisskopf-Ewing ands-wave approximation considering pairing effects only in compound nucleus state densities. The sensitivity of essential preequilibrium parameters on the shape of calculated excitation functions is tested. The model predictions well compare to excitation functions fromp, d,³He and⁴He induced reactions including the large set from the reaction⁹³Nb(⁴He,xn yp) up to 170 MeV bombarding energy. The general importance of two-nucleon preequilibrium emission is accentuated in several examples. The deduced preequilibrium parameters corroborate the results from the INDEX model analysis of nucleon spectra.     

Proton yields from quasielastic pion scattering on 27Al and 208Pb

A singles measurement of the quasielastic scattering of 255 MeV pions by ²⁷Al and ²⁰⁸Pb has been made. The measured values for the ratio R of π⁺ and π^? induced cross section, averaged over all measured angles of 2.2 ± 0.2 and 1.5 ± 0.1 for ²⁷Al and ²⁰⁸Pb respectively are substantially below the classical impulse approximation value of 11. A semiclassical nucleon charge exchange model is described for singles and coincidence measurements and is compared with the measured proton spectra. The comparison indicates that the singles proton spectra are dominated by events other than the quasielastic process whereas the coincidence data reported earlier and compared with the present calculations support the quasielastic interpretation. A strongly decreasing dependence of R on A for coincidence measurements may be interpreted as supporting the view that recoil nucleon charge exchange plays an important role in the quasielastic knockout process.     

Particle plus octupoleM2/E3 isomers and high-spin particle-hole states in147Tb and148Tb

The level structures of theN=82 andN=83 nuclei¹⁴⁷Tb and¹⁴⁸Tb have been studied by means of (α, 8n) and (α,7n) reactions induced by 68 to 110 MeVα particle bombardments of¹⁵¹Eu targets. In-beam conversion electron measurements have established that isomers withT _1/2=4.8(6)ns in¹⁴⁷Tb andT _1/2=22(1)ns in¹⁴⁸Tb decay byM2+E3 transitions to the ground states. The measuredB(E3) values show that the isomeric states arise from the coupling of the valence nucleon(s) to the¹⁴⁶Gd core octupole. Particlephonon coupling in these nuclei and in the one-neutron nucleus¹⁴⁷Gd is discussed and compared with well known cases involving the²⁰⁸Pb core. The higher lying yrast states in the two Tb nuclei are described as shell-model particle-hole excitations using empirical single particle energies and nucleon-nucleon interactions.     

The beta-decay of95Rb and97Rb

In this paper the spectra ofα particles emitted in (p, α) reactions induced by ～24 MeV protons on adjacent target nuclei, onemagic, with a magic neutron or proton shell and the othernear magic, with one more nucleon outside the shell, are measured and compared. The nuclei investigated are^{90, 91}Zr,¹²²Sn,¹²³Sb,^{142, 143, 144}Nd,²⁰⁸Pb and²⁰⁹Bi. The weak population of low energy levels of the residual nucleus from thenear magic target nucleus and the excitation of homologous states in the residual nuclei from neighbouringmagic andnear magic target nuclei convincingly prove that in most of the cases the unpaired nucleon outside the magic shell acts as a spectator in the process. The implications of these experimental findings are discussed.     

Nature of continua in inelastic α- and6Li-scattering at 26 MeV/A

The continua excited by inelastic scattering of 26 MeV/Aα- and⁶Li-particles from²⁸Si have been studied at forward angles by coincident detection ofγ-rays in final nuclei after particle decay. While the coincident γ-ray spectra in⁶Li-induced reactions show only transitions in massA=24, 27 and 28 nuclei, in theα-induced reactionsγ-transitions in mass 20 to 28 nuclei were observed. Inelastic⁶Li-scattering with energy losses between 30 and 80 MeV predominantly leads toγ-transitions in²⁷Si and²⁷Al. We interpret this as due to a dominant direct one-nucleon removal process producing the characteristic “triangular” shapes of the inclusive inelastic⁶Li spectra.⁶Li-induced reactions on²⁷Al show a corresponding behaviour. The typical flatα-spectra result from additional truly inelastic processes which lead to highly excited states in the target nuclei and subsequent evaporation of nucleons andα-particles. This is supported by the measuredγ-multiplicities and by means of Hauser-Feshbach calculations which allow to deduce that part of the inelasticα-continuum which decays by particle evaporation.     

Determination of the internal structure of valons

We determine the internal structure functions of the valons introduced by Hwa and coworkers, using low-p _t data and Regge model constraints. Our calibration reaction is the fragmentation processp→π^? observed in 70 GeV/cK ⁺ p interactions. The fitted valon structure parameters are used to make successful predictions for various other fragmentation processes (p→π⁺ andp→K ⁺). A comparison with experimental data of the DISvW ₂ (x)-values implied by this same fit, indicates that soft processes probe the nucleon typically at aQ ²-value of ～1 GeV². This finding supports the hypothesis that soft hadronic reactions are useful (and often unique) tools to learn about hadron quark structure.     

Pion-production in heavy-ion collisions at SIS energies

We investigate the production of pions in heavyion collisions in the energy range of 1–2 GeV/A. The dynamics of the nucleus-nucleus collisions is described by a set of coupled transport equations of the Boltzmann-Uehling-Uhlenbeck type for baryons and mesons. Besides theN(938) and theΔ(1232) we also take into account nucleon resonances up to masses of 1.95 GeV/c² as well asπ-,η- andρ-mesons. We study in detail the influence of the higher baryonic resonances and the 2π-production channels (NN→NNππ) on the pion spectra in comparison toπ ^? data fromAr+KCl collisions at 1.8 GeV/A andπ ⁰-data forAu+Au at 1.0GeV/A. We, furthermore, present a detailed comparison of differential pion angular distributions with the BEVALAC data forAr+KCl at 1.8 GeV/A. The general agreement obtained indicates that the overall reaction dynamics is well described by our novel transport approach.     

A study of the centrally produced π+π−π0 system formed in the reactionpp→p f (π+π−π0)p s at 300 GeV/c

The reactionpp→p _f (π⁺π^?π⁰)p _s , where the π⁺π^?π⁰ system is centrally produced, has been studied at 300 GeV/c. The π⁺π^?π⁰ mass spectrum shows evidence for the η, ω anda ₂ (1320) as well as an enhancement in thea ₁ (1260) region. A Dalitz plot analysis of the π⁺π^?π⁰ system has been performed. Thea ₁ (1260) parameters coming from the fit of the 1⁺ S wave arem=1208±15 MeV and Γ=430±50 MeV. No evidence is found for theh ₁(1170) orh ₁(1380).     

Study of the γD → ppπ− reaction in the Δ(1236) region for well defined kinematical conditions

The γD → ppπ^? reaction cross section, in the Δ(1236) region, is measured in a counter experiment with high statistical accuracy. Particular emphasis is put on the accurate determination of the complete kinematics. For low values of the undetected nucleon momentum (p_r, ≈ 50 MeV/c), the validity of the spectator nucleon model is experimentally checked and the γn → pπ^? elementary reaction cross section is extracted and compared with other experimental data. When the recoiling nucleon momentum increases (p_r ≈ 150 MeV/c), significant departures from the spectator nucleon model are found. Presumably they are the signature of final state interaction effects.     

Excited states in neutron deficient even-even thorium isotopes (218≦A≦222)

The nuclei^{218, 220, 222}Th were investigated by conversion electron andγ-ray spectroscopy after compound nucleus reactions of¹⁴N with²⁰⁹Bi and^{16, 18}O with²⁰⁸Pb. The intenseγ-background from fission was suppressed by spectroscopy of conversion electrons andγ-rays in coincidence with the evaporation residues or theirα-decay. Level schemes were determined for²¹⁸Th up toI ^π=10⁺ and for²²⁰Th and²²²Th up toI ^π=15^?. The observed structure of²¹⁸Th may be explained in the spherical shell model with residual interaction by two neutron excitation. ForN≧130 completely different level schemes are observed showing very intense, collective electric dipole transitions (B(E1)?10^?2 W.u.) with energies of typically 200 keV. Interpretation in terms of stable octupole deformations of the ground state as well as in anα-cluster picture are discussed.     

Lepton and hadron pair production in two-photon reactions

We have measured the production of charged particle pairs in e⁺e^? initiated two-photon reactions. The observed rates and invariant mass spectra agree with expectations from QED if a resonant π⁺π^? production at the f⁰ mass is added. Assuming dominance of the helicity 2 production we obtain the width Γ(f⁰→γγ) = 2.3 ± 0.5 keV.     

SubthresholdK + production on nuclei byπ + mesons

The inclusiveK ⁺ mesons production inπ ⁺-nucleus reactions in the subthreshold energy regime is analyzed with respect to the one-step (π ⁺ n→K ⁺ Λ) and the two-step (π ⁺ n→ηp ₁,ηp ₂→K ⁺ Λ) incoherent production processes on the basis of an appropriate folding model, which allows one to take into account the various forms of an internal nucleon momentum distribution as well as on- and off-shell propagation of the struck target nucleon. Contrary to proton-nucleus reactions primary reaction channel is found to be significant practically at all considered energies. Detailed predictions for theK ⁺ total and invariant differential cross sections fromπ ⁺C¹²- andπ ⁺Pb²⁰⁸-collisions at subthreshold energies are provided.     

Decays of millisecond isomers in odd-oddN=81 nuclei146Tb,148Ho and150Tm

Following reactions induced by 245 MeV⁶⁰Ni ions, isomers have been identified in the odd-oddN=81 isotones¹⁴⁶Tb,¹⁴⁸Ho and¹⁵⁰Tm with half-lives of 1.18(2), 2.35(4) and 5.2(3) ms, respectively. Their decays have been characterized byγ-ray spectroscopy. The isomers are interpreted as (πh _11/2 νh ₁ ^2/?1 ) 10⁺ states decaying to members ofπh _11/2 νd ₃ ^2/?1 andπh ₁₁ ^2/?1 multiplets. The observed level spectra and M1 branching ratios are found to be in good agreement with shell model predictions based on empirical and Schiffer-Trueπν ^?1 residual interactions.     

A Comparative Analysis of In-Medium Spectral Functions for N(940) and N ∗(1535) in Real-Time Thermal Field Theory

In the real-time thermal field theory, the nucleon self-energy at finite temperature and density is evaluated where an extensive set of pion-baryon (π B) loops are consider. On the other side, the in-medium self-energy of N ^?(1535) for π N and η N loops is also determined in the same framework. The detail branch cut structures for these different π B loops for nucleon N(940) and π N, η N loops for N ^?(1535) are addressed. Using the total self-energy of N(940) and N ^?(1535), which contain the contributions of their corresponding loop diagrams, the complete structures of their in-medium spectral functions have been obtained. The Landau and unitary cut contributions provide two separate peak structures in the nucleon spectral function while N ^?(1535) has a single peak structure in its unitary cuts. At high temperature, the peak structures of both at their individual poles are attenuated while at high density Landau peak structure of nucleon is completely suppressed and its unitary peak structure is tending to be shifted towards the melted peak of N ^?(1535). The non-trivial modifications of these chiral partners may indicate some association of chiral symmetry restoration.     

Emission spectra of the cations of some fluoro-substituted phenols in the gaseous phase

Emission spectra of the cations of 2,5- and 3,5-difluorophenol, of 2,3,4- and 2,4,5-trifluorophenol, of 2,3,5,6-tetrafluorophenol and of 2,3,4,5,6-pentafluorophenol have been obtained in the gas phase using low-energy electron beam excitation. The band systems are assigned to the B?(π^?1) → X?(π^?1) electronic transitions of these cations by reference to photoelectron spectroscopic data. The He(Iα) photoelectron spectra and the ionisation energies of ten fluoro-substituted phenols are reported. The symmetries of the four lowest electronic states of these cations are inferred from the radiative decay studies. The lifetimes of the lowest vibrational levels of the B?(π^?1) state of the six fluoro-substituted phenol cations above have also been measured.     

New limits on D0 (1.865) production in proton-nucleus collisions at 400 GeV/c

We present final results of a sensitive search for new particles in π^±K^? effective mass spectra observed in proton-nucleus collisions at 400 GeV/c. We establish a limit for D⁰ (1.865) production B_π⁺π^? dσ/dy < 360 nb/nucleon at y_cm^≈ ?0.4. For D?⁰ → π^?K⁺ the limit is 290 nb/nucleon.