Kinematic Coincidence Research Articles

η-PHYSICS AT GEM

Some experimental studies of η–production and η–interactions performed or presently under way by the GEM collaboration at COSY Jülich are reviewed. The Germanium Wall, a stack of annular detectors made of high purity Germanium, was used to measure a series of differential and total cross sections for η–production with proton and deuteron beams on light nuclei. The unique combination of a high resolution magnetic spectrograph (BIG KARL) and 7an electron cooled beam delivered by COSY is ideally suited to perform high precision experiments like the η–mass determination. In a self calibrating experiment three particles from two reactions were detected simultaneously. Kinematical coincidence of 3H and π+ from p + d → 3H + π+ reaction allowed to calibrate the spectrograph and determine the beam momentum with great accuracy. The η–meson was seen clearly as a sharp missing-mass peak on a slowly varying background in the p + d → 3He + X reaction. A new value for the η-mass has been derived with extremely small error bars.

Binary fission and coplanar cluster decay of 60Zn compound nuclei at high angular momentum

Abstract Fission decays with two heavy fragments in coincidence have been measured from 60 Ni compound nuclei, formed in the 36 Ar + 24 Mg reaction at E lab ( Ar 36 ) = 195 MeV (5.4 MeV/ A ). The experiment was performed with a unique kinematic coincidence set-up consisting of two large area position sensitive ( x , y ) gas detector telescopes with Bragg-ionisation chambers. Very narrow out-of-plane correlations are observed for two heavy fragments emitted in either purely binary events or in events with a missing mass consisting of 2, 3 and 4 α -particles. The broad out-of-plane distributions correspond to binary fission with the evaporation of α -particles or nucleons from excited fragments. The narrow correlations are interpreted as ternary coplanar cluster decay from compound nuclei at high angular momenta through elongated shapes with large moments of inertia, and the lighter mass remains with very low momentum in the centre of mass frame. It is shown in calculations of shapes, that the large moments of inertia of the hyper-deformed configurations expected in these nuclei, will lower the ternary fission barrier so as to make a competition with binary fission possible.

Competition between direct and dissipative processes in the binary channels of the 16O + 12C and 18O + 12C reactions

The two reactions, 16O + 12C and 18O + 12C were studied in the energy range between 5 and 10 MeV per nucleon. The binary fragments were identified at the large scattering angles using the kinematic coincidence method. The energy distributions of the dominant binary channels, and the angular distributions of the strongest populated states are presented. The differences between the two entrance channels are discussed. The evolution of the reaction mechanism from quasi-elastic to fully damped processes are presented as a function of bombarding energy, scattering angle, excitation energy and number of transferred nucleons.

AstrophysicalS17(0) factor from a measurement of theH2(Be7,B8)nreaction atEc.m.=4.5MeV

Angular distribution measurements of $^2$H($^7$Be,$^7$Be)$^2$H and $^2$H($^7$Be,$^8$B)$n$ reactions at $E_{c.m.}\sim$~4.5 MeV were performed to extract the astrophysical $S_{17}(0)$ factor using the asymptotic normalization coefficient (ANC) method. For this purpose a pure, low emittance $^7$Be beam was separated from the primary $^7$Li beam by a recoil mass spectrometer operated in a novel mode. A beam stopper at 0$^{\circ}$ allowed the use of a higher $^7$Be beam intensity. Measurement of the elastic scattering in the entrance channel using kinematic coincidence, facilitated the determination of the optical model parameters needed for the analysis of the transfer data. The present measurement significantly reduces errors in the extracted $^7$Be(p,$\gamma$) cross section using the ANC method. We get $S_{17}$~(0)~=~20.7~$\pm$~2.4 eV~b.

Investigation of scattering between mirror nucleiBe7andLi7

Angular distribution for $^{7}\mathrm{Be}+^{7}\mathrm{Li}$ elastic scattering has been measured using a $^{7}\mathrm{Be}$ radioactive beam at the Nuclear Science Center, New Delhi, at ${E}_{c.m.}=9.87$ and 8.87 MeV. A compact and highly efficient detector system in kinematic coincidence mode and an in-vacuum target transfer system have been developed to minimize contributions from unwanted channels. The angular ranges covered were ${\ensuremath{\theta}}_{c.m.}={42}^{\ifmmode^\circ\else\textdegree\fi{}}--{66}^{\ifmmode^\circ\else\textdegree\fi{}}$ and ${\ensuremath{\theta}}_{c.m.}={114}^{\ifmmode^\circ\else\textdegree\fi{}}--{138}^{\ifmmode^\circ\else\textdegree\fi{}}$. At the backward angles at both energies, the experimental angular distribution shows higher cross sections than the theoretical predictions for the direct elastic channel alone. The experimental data could be fitted with an isospin-dependent complex potential that is analogous to the Lane potential. A coupled-channel calculation with ground-state reorientation could also fit the data.

Study of elastic scattering of mirror nuclei 7Be + 7Li

The angular distribution of 7 Be + 7 Li elastic scattering has been measured using 7 Be from the Radioactive beam at Nuclear Science Centre, New Delhi at E cm = 9.87 MeV. A compact and highly efficient detector system in kinematic coincidence mode and an in-vacuum target transfer system have been developed to minimise contributions from unwanted channels. The angular ranges covered were θ cm = 42°- 66°and θ cm = 114°- 138°. The experimental angular distribution shows higher cross sections than the theoretical predictions at the backward angles. The experimental data could be fitted with an isospin dependent complex potential that is analogous to the Lane potential.

Population of nuclei far from stability via multinucleon transfer: theNi+Pb case

Multinucleon transfer reactions in 58 Ni + 208 Pb were studied at E LAB =328.4 MeV. Light products were identified with a time-of-flight magnetic spectrometer and heavy fragments with a multiwire parallel plate detector. From the kinematic coincidence the survival probability of the heavy fragments against fission was derived. Data are compared with semiclassical model calculations.

Population of nuclei far from stability via multinucleon transfer: theNi+Pb case

Multinucleon transfer reactions in58Ni +208Pb were studied at ELAB=328.4 MeV. Light products were identified with a time-of-flight magnetic spectrometer and heavy fragments with a multiwire parallel plate detector. From the kinematic coincidence the survival probability of the heavy fragments against fission was derived. Data are compared with semiclassical model calculations.

Population of nuclei far from stability via multinucleon transfer: the 58Ni+ 208Pb case

Multinucleon transfer reactions in 58Ni + 208Pb were studied at E LAB =328.4 MeV. Light products were identified with a time-of-flight magnetic spectrometer and heavy fragments with a multiwire parallel plate detector. From the kinematic coincidence the survival probability of the heavy fragments against fission was derived. Data are compared with semiclassical model calculations.

Light and heavy transfer products in58Ni+208Pbat the Coulomb barrier

Multinucleon transfer reactions in Ni+Pb have been studied at ELAB=328.4 MeV, corresponding to ' 3% above the Coulomb barrier. Light reaction products have been detected with the time-of-flight spectrometer PISOLO [1], while the associated heavy partners produced in the binary reaction have been detected in kinematic coincidence using a transmission-type multiwire parallel-plate avalanche counter [2]. The main motivation of the experiment was to simultaneously detect both light and heavy nuclei produced in multinucleon transfer reactions, for a suitable projectile and target combination where large number of neutrons and protons could be transferred at energies close to the Coulomb barrier. The Ni+Pb system is in fact a good candidate, since its Q-value matching conditions, close to optimum up to 8 proton stripping, allows one to investigate in detail the mass and charge partitions of the light reaction products and, at the same time, to see if the same transfer mechanism is suited for producing a significant yield of the corresponding high Z heavy partners [3,4]. A first report on the experiment is given in Ref. [5] while the final work including the comparison between data and theory is reported in Ref. [6]. We refer to the cited references (and references therein) for the details and for the discussion of the physics involved.

Performance of the KVI in-beam polarimeter

A beam polarimeter for medium-energy protons and deuterons has been constructed. Its operation is based on the elastic-scattering reactions p → + p or d → + p . The outgoing particles are detected in kinematical coincidence in four independent reaction planes, each equipped with two pairs of phoswich detectors.

A Molecular Shell with Star Formation toward the Supernova Remnant G349.7+0.2

A field of about 38' × 38' around the supernova remnant (SNR) G349.7+0.2 has been surveyed in the CO J = 1–0 transition with the 12 m telescope of the NRAO, using the on-the-fly technique. The resolution of the observations is 54''. We have found that this remnant is interacting with a small CO cloud, which, in turn, is part of a much larger molecular complex, which we call the CO shell. The large CO shell has a diameter of about 100 pc, an H2 mass of 9.3 × 105 M⊙, and a density of 35 cm-3. We investigate the origin of this structure and suggest as a suitable hypothesis that an old supernova explosion occurred about 4 × 106 yr ago. Analyzing the interaction between G349.7+0.2 and the large CO shell, it is possible to determine that the shock front currently driven into the molecular gas is a nondissociative shock (C-type), in agreement with the presence of OH 1720 MHz masers. The positional and kinematic coincidence of one of the CO clouds that constitute the large CO shell, an IRAS pointlike source, and an ultracompact H II region indicates the presence of a recently formed star. We suggest that the formation of this star was triggered during the expansion of the large CO shell and suggest the possibility that the same expansion also created the progenitor star of G349.7+0.2. The large CO shell would then be one of the few observational examples of supernova-induced star formation.

Role of multi-step processes in 16O(11B,12C)15N at 41.25 MeV

The 16O(11B,12C)15N reaction at 41.25 MeV has been investigated using the kinematical coincidence method. Polarization tensors t20 and t40 of 12C[2+ 1] for the quantization axis taken along the direction of propagation have been measured at center-of-mass angles (Θc.m.) between 48° and 62° by analyzing the energy spectrum of 12C[2+ 1] modulated by the effect of γ-ray emission. The cross-sections of the transfer reactions leading to the 12C[g.s.]+15N[g.s.], 12C[2+ 1]+15N[g.s.] and 12C[g.s.]+15N[3/2- 1] final states have also been measured in the range 48°≤Θc.m.≤ 120°. The polarization tensor terms of 12C[2+ 1] largely deviating from zero have been observed, contrary to the prediction by the distorted-wave Born approximation (DWBA). The one-step DWBA calculation also fails in describing the transfer reaction cross-sections. It is shown that the coupled channel model calculation including excitations and reorientations in 11B and 12C satisfactorily reproduces both the tensor terms and the cross-sections of the transfer reactions. The multi-step processes passing through the excited states of 11B are found to significantly contribute to the reaction.

Tensor polarization of 12C[2+ 1] in the 16O(13C,12C)17O reaction at 50 MeV

The 16O(13C,12C)17O reaction at 50 MeV has been investigated using the kinematical coincidence method. Polarization tensors t20 and t40 of 12C[2+1] for the quantization axis taken along the direction of propagation have been measured by analyzing the energy spectrum of 12C[2+1], modulated by the effect of γ ray emission. The deduced t40 values significantly deviate from zero, contrary to the prediction of the distorted-wave Born approximation theory based on one-step p shell neutron stripping without spin-dependent interactions. The phenomenological spin–orbit interaction necessary to reproduce the magnitude of measured t40 is found to be much larger than the folding model prediction. It is shown that the experimental polarization tensors as well as the cross sections can be reproduced by introducing multi-step processes involving excitations in 12C and 13C without introducing spin-dependent interactions.

Measurement of the ground state 2n pickup probability for 28Si+68Zn and its role in sub-barrier fusion enhancement

The ground state and excited state transfer yields for the 2-neutron pickup channel in the 28Si+68Zn system have been measured explicitly. The recoil mass separator at the nuclear Science Centre, New Delhi was used for the measurement. A NaI(T1) detector was used for detecting the deexcitation γ’s from the transfer products. The kinematic coincidence technique was employed for the transfer measurement. Simplified coupled channels calculations show that out of all transfer channels the major contribution to the sub-barrier enhancement comes from the ground state 2 neutron pickup channel with a ground state Q-value of+1.83 MeV.

Refractive effects in the elastic scattering of light heavy ions between 5 and 10 MeV/n: the 16O + 16O reaction

Complete angular distributions of the 16 O + 16 O elastic channel have been measured at the 9 following bombarding energies: E lab = 75.0, 80.6, 87.2, 92.4, 94.8, 98.6, 103.1, 115.9 and 124 MeV using at forward angles ( θ lab = 5° to 20°) the Q3D spectrometer and at backward angles ( θ lab = 20° to 50°) two Position Sensitive Silicon detectors in kinematical coincidence. Refractive effects giving rise to nuclear rainbows and the associated Airy structures have been observed for this system. An optical model analysis has been done where the real part of the potential has been determined with no ambiguity over the whole energy range whereas the absorption (imaginary part) changed drastically around E lab = 90 MeV.

In-beam γ-ray spectroscopy with binary reactions using the HMI binary reaction trigger spectrometer

Test experiments are reported illustrating the potential of the HMI Binary Reaction Spectrometer (BRS) as a reaction channel filter for γ-ray spectroscopy with binary exit channels. The BRS, developed as an ancillary detector for EUROBALL, was tested with the smaller γ-detector array OSIRIS. For correlated detection of two heavy ejectiles the BRS combines two large-area, fourfold segmented gas detector telescopes in a kinematic coincidence set-up.

Atomic effects in heavy-ion elastic scattering

Angular distributions of a ${}^{208}$Pb beam on ${}^{208}$Pb thin targets at very small angles were measured. The shape of the experimental distribution is not Gaussian, indicating a plural scattering regime rather than multiple scattering. The distribution was well reproduced by a Monte Carlo simulation of the straggling process. For the first time we observed and quantified the influence of the production of electrons during the scattering. This effect was observed in the angular correlation of the scattered ${}^{208}$Pb nuclei detected in kinematic coincidence and also in the limiting angle of the ${}^{208}$Pb scattered in a ${}^{107,109}$Ag target. This effect introduces a spread in the scattered angle which is of the same order of magnitude as the angular straggling.

Sub-barrier few-nucleon transfer reaction and channel coupling effects in heavy ion fusion

Measurements of sub-barrier transfer reactions are reported for the systems , and using the recoil mass spectrometer HIRA in kinematic coincidence mode. The problem related to M/Q-ambiguity in measurements with mass separators has been resolved. Excellent mass resultion with a large solid angle is obtained by correction of the aberrations. Simplified coupled-channels calculations are carried out for these systems with transfer form factors extracted from the measured transfer probabilities. The surface vibration coupling is treated up to two phonon states with second-order coupling terms.

Resonances and fluctuations in the 16O + 15N and 12C + 19F collisions

The search and study of quasi-molecular resonances in the 31P composite system populated via two entrance channels are performed with two different experimental techniques. The 16O + 15N reaction products have been studied by the γ-ray detection method at cm. energies ranging from 15.5 MeV to 36.1 MeV. Binary channels of the 16O + 15N and 12C + 19F collisions have been studied by using the kinematical coincidence method at 26 incident energies ranging from Ec.m. = 20.6MeV to 33.5MeV for the first system, and at energies corresponding to the same excitation energies of the composite system for the second system. The 16O + 15N reaction exhibits two prominent gross structures in the large angle elastic scattering excitation function correlated with the resonant structures observed in inelastic channel γ-ray yield measurements. Spin assignments were tentatively made for the two resonances. On the contrary, no such structures can be clearly established in the 12C + 19F system where only indications of non-correlated structures in various channels have been observed.