Large-angle quasi-free scattering in 6Li(p,pd)4He at 670 MeV

The ⁶Li(p, pd)⁴He reaction was investigated at 670 MeV by a coincidence experiment with a large-angle scattering geometry. The energy sharing and angular correlation of the reaction products were measured and the momentum distribution of the recoil nucleus was determined for transitions leading to residual nuclei in the ground and excited states. Results were analysed in terms of a simplified distorted-wave impulse approximation using cluster-model and three-body wave functions. The observed momentum distribution of the p-n pair in the p-shell of ⁶Li is in agreement with three-body calculations, while the spectroscopic factor is larger than predicted by theory. Transitions to breakup states of the α-particle also have the characteristics of quasi-free scattering on deuteron clusters.     

Quasi-free (p,pα) scattering on 24Mg, 28Si, 40Ca and 58Ni at 157 MeV

Sixfold energy spectra have been measured for the (p, pα) reaction at 157 MeV on ²⁴Mg, ²⁸Si, ⁴⁰Ca and ⁵⁸Ni around quasi-free kinematic conditions. For the three s-d shell nuclei the experiment covered a map ranging from 0 to 220 MeV/c in recoil momentum and from 0 to 20 MeV in excitation energy of the final nucleus. Recoil momentum distributions have been obtained for the 0⁺ ground state and the 2⁺ first excited state of ²⁰Ne, ²⁴Mg and ³⁶Ar, and also for the states around 4.4 MeV (mainly 4⁺) of ³⁶Ar. The a spectroscopic factors extracted by a DWIA analysis are about three times larger than those predicted by the SU(3) model; however, they agree quite well in relative magnitude for a number of cases. The disagreement in shape between experiment and theory observed at low recoil momentum for the 2⁺ states might result from another reaction mechanism. The cross sections for ⁵⁸Ni are about a factor often smaller than those for ⁴⁰Ca. The ⁵⁸Ni(p, pα)⁵⁴Fe reaction seems to lead mainly to excited states of the final nucleus.     

αγ angular correlations in the inelastic scattering of α particles on 24Mg at Eα = 30.3 MeV

The results from measuring the angular dependence of differential α-scattering cross sections on ²⁴Mg with final nucleus formation in the ground and three lower excited states are presented, along with the double differential ²⁴Mg(α, αγ)²⁴Mg(2⁺, 1.369 MeV) reaction cross sections at E _α = 30.3 MeV. The spin-tensor components of the density matrix of the ²⁴Mg nucleus in the 2⁺(1.369 MeV) state are reconstructed and its orientation characteristics are determined. The experimental results are compared to theoretical results under the assumption of collective excitation and mechanisms for compound nucleus formation.     

A study of the nucleus 107Ag in the core-excitation model with the (p,t) and (τ, d) reactions

The ¹⁰⁷Ag residual nucleus was studied in the core-excitation model using the (p, t) and (τ, d) reactions. The L, J, π of levels between 0.0 and 2.25 MeV was deduced from the combined reactions. The octupole state observed at 2.19 MeV in other experiments was resolved in (p, t) into a triplet of states at 2.182, 2.203 and 2.229 MeV; octupole strength was observed in (p, t) over a range from 1.144 to 2.229 MeV. Core-excitation wave functions for the quadrupole 2⁺ and 2'⁺ vibration doublets of ¹⁰⁷Ag were constructed using electromagnetic data. These wave functions, combined with data from the ¹⁰⁸Pd(p, t) core reaction, effectively reproduced the ¹⁰⁹Ag(p, t) differential cross sections to these states. The ground-state L = 0 transfer in (p, t) to ¹⁰⁷Ag was only 0.752±0.113 as strong as the corresponding transfer to ¹⁰⁶pd. this is an unexpectedly large blocking effect for an unpaired proton to exert upon a neutron-transfer reaction. An apparent dependence of the (p, t) angular distributions to states of ¹⁰⁷Ag built upon the same core excitation was observed, depending upon the J of the final state.     

Study of annihilation in two mesons in the momentum range between 50 to 400 MeV/c with OBELIX

We measured the in flight annihilation frequencies and cross sections of reactions and K⁺K_S in the antineutron momentum range between 50 and 400 MeV/c. The annihilation frequencies of these channels from the different allowed initial states were calculated and some information about the annihilation dynamics were obtained. The first determination of the D-wave contribution in this momentum range was also obtained.     

Delta-isobar configurations in 12C and 16O nuclei

The differential yield of the reaction ¹⁶O(γ, π ⁺ p) at the maximum photon energy of 450 MeV was measured in the region of high momentum transfers to the residual nuclear system. The experimental results obtained for the ¹⁶O nucleus and the cross section measured earlier for the reaction ¹²C(γ, π ⁺ p) were analyzed on the basis of a model that takes into account the admixture of isobar states in the nuclear wave function. The probabilities for the delta-isobar configurations in the ground states of the carbon and oxygen nuclei per nucleon were estimated empirically at 0.012 ± 0.003 ± 0.002 and 0.019 ± 0.003 ± 0.003, respectively.     

Untersuchung der Reaktion Si28 (γ,p) AI27

A silicon semiconductor detector was irradiated with betatron bremsstrahlung of different end point energies and the energy distributions of the protons originating in the detector itself by the reaction Si²⁸(γ,p) Al²⁷ were measured with good statistical accuracy. The end point energies were varied in 1 MeV steps from 15 to 30 MeV. Using the known spectral distribution of theγ-rays the energy dependence of the total cross section of the studied reaction could be derived from the measured proton spectra. The resulting cross section shows a peak at 20, 5 MeV and a half width of 4 MeV. Approximate values for the branching ratios for transitions to excited states of the residual nucleus could also be obtained.     

Angular momentum dependence of the giant dipole resonance width in excited nuclei of mass

Gamma ray spectra in the energy range of 4–25 MeV were measured in the reaction ²⁸Si + ¹²⁴Sn at E(²⁸Si) 150 MeV in coincidence with low energy γ-multiplicities. The spectra were analysed using a simulated Monte Carlo CASCADE code. The centroid energy and width of the giant dipole resonance were extracted for various multiplicity windows. The average angular momentum and temperature of the final states populated after the giant dipole photon emission range from 25 and 1.5 MeV to 56 and 1.3 MeV, respectively. The extracted widths are almost constant for the lower multiplicity windows and show an increase of 1.4 MeV at the highest window. The nuclei are expected to be near the liquid drop regime and the experimental results are not inconsistent with the liquid drop behaviour.     

Gamow-Teller transitions from 58Ni to discrete states of 58Cu

Under the assumption that isospin is a good quantum number, symmetry is expected for the transitions from the ground states of T = 1, T _z = ±1 nuclei to the common excited states of the T _z = 0 nucleus situated between the two nuclei. The symmetry can be studied by comparing the strengths of Gamow-Teller (GT) transitions obtained from a (p, n)-type charge-exchange reaction on a target nucleus with T _z = 1 with those from the β-decay of the T _z = - 1 nucleus. The A = 58 system is the heaviest for which such a comparison is possible. As a part of the symmetry study, we measured the GT transitions from ⁵⁸Ni (T _z = 1) to ⁵⁸Cu (T _z = 0) by using the zero-degree (³ He, t) reaction at 150 MeV/nucleon. With the achieved resolution of 50 keV, many hitherto unresolved GT states have been identified. The GT transition strengths were obtained for states up to 8 MeV excitation, i.e., near to the Q window limitation ( Q _EC = 9.37 MeV) of the β-decay from ⁵⁸Zn (T _z = - 1) to ⁵⁸Cu. The strength distribution is compared with that from shell-model calculations. Received: 24 November 2001 / Accepted: 30 January 2002     

Measurement of the 24Mg(p, t)22Mg reaction for the states near the 21Na + p threshold

Differential cross-sections of the ²⁴Mg(p, t)²²Mg reaction were measured at 34.68 MeV for the states near the proton threshold at 5.502 MeV in ²²Mg. Among them, the new states at 5.962, 6.046, 6.246 and 6.323 MeV, which were reported previously, have been confirmed. Angular distributions for these states were analyzed by distorted-wave Born-approximation calculations to deduce the spins and parities. The angular distribution for the 5.714 MeV state, which is considered to be most crucial for the stellar reaction ²¹Na(p, γ)²²Mg, has been found to be consistent with J ^π = 2⁺ assignment. The 6.046 MeV state is newly assigned to have J ^π = 0⁺, and the 5.962 MeV state is tentatively assigned to have J ^π = (1^-). These two states will also play an important role for ²²Mg production in novae. Received: 7 March 2002 / Accepted: 7 May 2002     

Study of η -production reaction πp → ηn in the near-threshold region⋆

The differential cross-sections (DCS) of the η -production reaction π^- p → ηn are measured at momenta of incident π^- mesons (700, 710 and 720MeV/c) close to the threshold of this reaction. The experiment is carried out at the pion channel of the PNPI synchrocyclotron (with the momentum spread of the pion beam equal to 1.5%) using the Neutral Meson Spectrometer (NMS) designed and created at the Meson Physics Laboratory for detecting two photons from the decay η → 2γ . This spectrometer consisting of two electromagnetic total absorption calorimeter (24 CsI(Na) crystals in each calorimeter) allowed to obtain the DCS in the whole angular range from 0^° to 180^° in the c.m.s. It is shown that at 700MeV/c the DCS angular dependence is isotropic, indicating on the S -wave character of the η -production process while at higher momenta a significant contribution of the D -wave has led to the anisotropic angular dependence having a bowl-like shape.     

One- and two-neutron transfer reactions on 12C with the weakly bound 6He projectile

Using the framework of the coupled reaction channels (CRC) the one- and two-neutron transfer process initiated by the weakly bound nucleus ⁶He on ¹²C at an energy of E _L = 5.9 MeV is studied. The absolute cross-sections for a few states in ¹⁴C are well reproduced within a factor 2 in second order, using microscopic wave functions of ⁶He and ¹²C. Only a small dependence of the cross-section on details of the ⁶He wave function is observed. Good fits to the data are obtained in a calculation with full coupling (25 iterations) with renormalised optical potential parameters and spectroscopic amplitudes of ⁶He. Received: 13 June 2000 / Accepted: 24 July 2000     

The heavy-ion reaction channels of the system 16O + 16O

The reaction channels of the system ¹⁶O + ¹⁶O with outgoing heavy particles from lithium to magnesium have been measured using a ΔE-E telescope. Excitation functions from 49 to 65 MeV at θ_Lab = 30° and angular distributions from θ_Lab = 10° (20°) to 50° at E_Lab = 51.5 MeV are presented for the strong transitions. The excitation function of the ¹²C-²⁰Ne (4.25 MeV) channel shows a pronounced regular cross structure with peaks at 52 and 60 MeV. A selective excitation of certain states in the inelastic scattering and the ¹²C-²⁰Ne channel is observed; the yields of the other heavy-ion channels being weaker by at least one order of magnitude. An explanation of this phenomenon is given by considering the angular momentum matching between entrance and exit channels. Furthermore it is shown that no strong dependence of the cross sections on the transferred angular momentum or on the nuclear structure of the final states is observed. Possible implications of these results on the reaction mechanism are discussed.     

Analysis of data on proton-proton scattering in the energy range 100–1300 MeV

The results of a partial-wave analysis of data on proton-proton interaction in the energy range 100–1300 MeV are presented. The real parts of phase shifts were found for states of orbital angular momentum up to L = 9, while their imaginary parts were determined for states of orbital angular momentum up to L = 5. The sixth parameter of the S matrix was introduced in order to describe states of total angular momentum J = 2 and 4. The inelasticity thresholds were chosen individually for each state and were found to be substantially different from one another. The resulting solution was characterized by χ ² = 1.155 per point in the case where the number of experimental data was 12 841 and by a large imaginary part of the phase shift in the ³ P ₂-wave state at the edge of the energy range. Special features of the interaction in orbital states are discussed along with the energy dependence of integrated amplitudes and amplitudes of the scattering matrix at zero angle.     

Nuclear spectroscopy near the proton drip line in the lanthanide region: The122La nucleus

Excited states in the very neutron-deficient¹²²La nucleus have been established for the first time using the⁹²Mo(⁴⁰Ca, 2α1p1n) reaction at beam energies of 190 and 200MeV. The observed transitions were grouped in three bands. Configurations have been assigned to the observed bands based on the properties extracted from the experimental data and on cranked shell model calculations.     

Statistical model analysis of the 12C(14N, α) reaction and coherence widths in 26Al

The ¹²C(¹⁴N, α)²²Na reaction was studied at bombarding energies between 22 and 39.2 MeV (lab) and a lab angle of 7°. Average coherence widths of states in the compound nucleus ²⁶Al populated in the reaction were obtained from excitation functions of twenty states or unresolved multiplets measured for states in ²²Na. From these data and Hauser-Feshbach predictions, values were determined for the level density parameters and for the effective moment of inertia of ²⁶Al. The critical angular momentum for the reaction was determined by comparing ratios of cross sections for excited states of ²²Na with Hauser-Feshbach predictions.     

Spectroscopy of <Superscript>17</Superscript>C and (sd )<Superscript>3</Superscript> structures in heavy carbon isotopes

The structure of ¹⁷C has been investigated using the three-neutron transfer reaction (¹²C,⁹C) on a ¹⁴C target at 231MeV incident energy, the reaction Q-value is Q ₀ = - 46.930MeV. Eleven new states up to 16.3MeV excitation energy were identified. The same reaction has also been used on a ¹²C target ( Q ₀ = - 38.787MeV), and excited states in ¹⁵C up to 19MeV were observed. In ¹⁷C the three transferred neutrons populate (sd )³ configurations on the ¹⁴C core. The comparison of levels populated by the (¹²C,⁹C) reaction in ¹⁷C, ¹⁶C and ¹⁵C reveals a strong similarity of their properties. This concerns especially nine states in each of the three carbon isotopes, which show practically the same excitation energies except a constant mean shift of +5.82MeV for ¹⁶C and +6.65MeV for ¹⁵C with respect to ¹⁷C. The triples of states from the three isotopes, which correspond to each other, have also similar widths and cross-section ratios. It is concluded that the same (sd )³ structures are populated in the three carbon isotopes. The observed levels of ¹⁷C are also compared to the levels of ¹⁹O with known assignments and to shell model calculations, and their decay properties are discussed.     

Astrophysical <Emphasis Type="Italic">S</Emphasis>-factor for the radiative-capture reaction <Emphasis Type="Italic">p</Emphasis><Superscript>13</Superscript>C → <Superscript>14</Superscript>N<Emphasis Type="Italic">γ</Emphasis>

The possibility of describing experimental data on the astrophysical S factor for radiative proton capture on a ¹³C nucleus at energies in the range 0.03–0.8 MeV is considered within the potential cluster model involving forbidden states. It is shown that the energy dependence of this astrophysical S factor can be reasonably explained on the basis of the E1 transition to the ³ P ₁-wave bound state of the ¹⁴N nucleus in the p ¹³C channel from the ³ S ₁ wave of p ¹³C scattering in the resonance energy region around 0.55 MeV in the laboratory frame.     

Mechanism of the 12C(11B,15N)8Be reaction and 8Be + 15N optical-model potential

Angular distributions of the ¹²C( ¹¹B, ¹⁵N) ⁸Be reaction were measured at the energy E_lab(¹¹B) = 49 MeV for the transitions to the ground and 2.94 MeV (2⁺) excited state of ⁸Be and to the ground and 5.270 MeV (5/2⁺) + 5.299 MeV (1/2⁺), 6.324 MeV (3/2^-), 7.155 MeV (5/2⁺) + 7.301 MeV (3/2⁺), 7.567 MeV (7/2⁺) excited states of ¹⁵N. The data were analyzed by the coupled-reaction-channel method. The elastic, inelastic scattering and one- and two-step transfers were included in the coupling scheme. The data of the ¹²C( ¹¹B, ⁸Be) ¹⁵N reaction at E_cm = 9.4-17.8 MeV known from the literature, were also included in the analysis. The mechanism of the ¹²C( ¹¹B, ¹⁵N) ⁸Be reaction and the optical-model potential parameters for the ¹⁵N + ⁸Be channel were deduced. The energy dependence of the optical-model parameters for the ¹⁵N + ⁸Be channel was obtained.