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A compact time-of-flight spectrometer for measurements of fission fragment distributions is described. The start detector consists of a channel plate arrangement with an electrostatic mirror. The time resolution of PPACs, serving as stop detectors, has been optimized. Velocity distributions of 252 Cf fragments and mean velocities are reported. The results are discussed with respect to planned (e, e'f) coincidence experiments at the Mainz microtron.

An arrangement of two-dimensional position sensitive PPACs (parallel plate avalanche counters) covering a large solid angle (> 50% of 4π) for use in a new type of photofission experiments is described. For the first time simultaneous measurements of both the photofission fragment angular and mass distributions are enabled by this device. The mass information is obtained by the double time of flight technique (with modest accuracy) whereas an excellent angular resolution of < 1° is achieved. The detector performances are outlined. First applications in high intensity bremsstrahlung and tagged photon experiments are discussed.

The absolute photofission cross sections for 232 Th and 235, 238 U has been studied from 20 to 110 MeV using the photon tagging technique at the Saclay linear electron accelerator (ALS). The fission fragments were directly detected using a parallel plate avalanche detector (PPAD). The basic experimental results are a set of data giving σ ( E γ , fission). These results are compared with the few other measurements performed with monochromatic photons. From a comparison of the present fission data with the total absorption cross section, the total fission probability was deduced. The behaviour of this probability was discussed in terms of a cascade model for fission-evaporation competition. Quite good agreement is found with a calculation based on the hybrid model of pre-equilibrium reactions.

The construction and performance of several arrangements of parallel plate avalanche detectors are described. The set ups can be used for the measurement of: fragment time of flight, photofission cross sections, out of plane angular distributions, inclusive and exclusive electrofission cross sections. First results are discussed, demonstrating the wide field of applicabilities of these detectors.

Absolute photofission cross sections of 235,238U have been measured in the energy range 40–105 MeV with “tagged” photons, using parallel plate avalanche counters for a direct detection of the fission fragments. The measured cross sections are nearly constant in the investigated energy range and amount to about 17 mb for both isotopes. The results are compared with recent photoneutron data.

Fragment angular and mass distributions have been measured simultaneously for photon energies near the fission barrier of 236 U. Different angular distributions have been observed for particular fragment mass regions. This establishes a clear relationship between the quantum numbers of the transition states and the fragment mass division. The results are discussed within the transition channel concept of the double humped barrier for different fission paths and the more recent model of multi-ecit channels.

Photons from the reaction {sup 16}O+{sup nat}W have been measured using the Darmstadt-Heidelberg crystal ball. The source velocity for photons above 25 MeV deduced from the angular distribution identifies the nucleon-nucleon system as the emitting system consistent with proton-neutron bremsstrahlung. This observation is supported by a comparison of the experimental angle-integrated spectra with statistical model calculations, which suggest that photons below 20 MeV arise from thermal emission, whereas hard photons above 25 MeV cannot be explained by this mechanism.

Half-lives and yields in photonuclear reactions have been measured for shape isomers in U and Pu isotopes by pulsed-beam techniques bombarding 235,238 U and 239,240,242 Pu targets with brems-strahlung. Isomeric fission cross sections have been deduced from the measured isomeric to prompt yield ratios within an evaporation model using absolute prompt fission data. The results are compared with data from particle-induced reactions.

Two experiments establish an evidence for different fission fragment mass asymmetries for specific nuclear excitations: in near barrier photofission of 236U enhanced quadrupole contributions (fission through the Jπ = 2+ , K = 0 transition state) have been observed in the fragment angular distributions for a far asymmetric mass split. On the other hand, the fission decay of the isoscalar giant quadrupole resonance exhibits larger mass symmetric contributions than the E1 and E3 resonances as observed in 235, 238U(e,e'f) coincidence experiments, where the fission width ratios Γs/ΓA have been determined as functions of excitation energy for separated multipole resonances.

Triggered by a recent paper by Thomas, Hicks and Hosaka, we investigate which observables can be used to determine the parity of the Theta(+) from the reaction (p) over right arrow(p) over right arrow --> Sigma(+)Theta(+) near its production threshold. In particular, we show that the sign of the spin correlation coefficient A(xx) for small excess energies yields the negative of the parity of the Theta(+). The argument relies solely on the Pauli principle and parity conservation and is therefore model-independent. (C) 2004 Elsevier B.V. All rights reserved.

An on-line monitor (consisting of two PPACs and a fissile target) to check the position and the profile of photon beams with energies larger than ≈ 7 MeV is described.

Large BaF 2 crystals have been used for hard photon detection. The response to γ-rays with energies up to 130 MeV has been measured by means of tagged photons. Experimental line shapes and resolutions are compared to simulations with the Monte Carlo shower code GEANT3.

The measurement of fragment angular and mass distributions in photofission represents an important tool to investigate the properties of the double humped fission barrier, in particular the spectrum of transition states, which provides direct information on nuclear shapes and possible collective modes. Photoexcitation has the unique feature to be specific to low spin states. The ratio of quadrupole to dipole photofission is directly connected to the relative heights of the bumps of the double humped fission barrier.

Absolute cross sections for electron- and positron-induced fission of 238U have been measured in the energy range 10 – 35 MeV. The cross sections were compared with the corresponding photo-fission cross sections using DWBA virtual photon spectra. The reliability of the virtual photon formalism to extract E2 strength from inclusive electrodis-integration experiments is critically discussed.

The analyzing powers in proton-deuteron elastic and proton-neutron quasi-elastic scattering have been measured at small angles using a polarized proton beam at the COSY storage ring incident on an unpolarized deuterium target. The data were taken at 796MeV and five higher energies from 1600MeV to 2400MeV. The analyzing power in pd elastic scattering was studied by detecting the low energy recoil deuteron in telescopes placed symmetrically in the COSY plane to the left and right of the beam whereas for pn quasi-elastic scattering a low energy proton was registered in one of the telescopes in coincidence with a fast scattered proton measured in the ANKE magnetic spectrometer. Though the experiment explores new domains, the results are consistent with the limited published information.

For future few‐nucleon interaction studies with polarized beams and targets at COSY‐Julich, a polarized internal‐storage cell gas target is currently being developed and will be implemented in the near future at ANKE. The polarized atomic beam source, which will feed the target, provides beam intensities of 7.4 ⋅ 1016 atoms/s in two hyperfine states of hydrogen. The implementation of the target at the internal spectrometer ANKE constitutes a major technological enterprise. The differential pumping system at ANKE has already been installed, as well as a new large target chamber to accomodate storage cells in the future, a new set of small‐aperture horizontal and vertical beam position monitors, and a system of target‐near detectors. In order to determine the nuclear polarization of the target, a Lamb‐Shift polarimeter is currently set up. Tests with prototype storage cells aiming at the identification of cell dimensions suitable for the ANKE target are underway.

The differential cross sections for Compton scattering from the proton have been measured at MAMI with three different detector setups. There is a good agreement among all the results. The theoretical calculation based on dispersion relations shows that there is no drastic change necessary in the parameters of this approach.

The total cross section for the dd -> 4He K+ K- reaction has been measured at a beam momentum of 3.7 GeV/c, corresponding to an excess energy of 39 MeV, which is the maximum possible at the Cooler Synchrotron COSY-J\"{u}lich. A deuterium cluster-jet target and the ANKE forward magnetic spectrometer, placed inside the storage ring, have been employed in this investigation. We find a total cross section of sigma(tot) 4He a0(980) reaction as a means of studying isospin violation.

The production of tensor polarization in an unpolarized deuteron beam after passage through unpolarized carbon targets, which includes the effect of deuteron spin dichroism, has been observed for the first time. Theoretical background, experimental setup, and results of the experiment are presented.

The vector Pz and tensor Pzz polarizations of a deuteron beam have been measured using elastic deuteron‐carbon scattering at 75.6 MeV and deuteron‐proton scattering at 270 MeV. After acceleration to 1170 MeV inside the COSY storage ring, the polarizations of the deuterons were remeasured by studying the analyzing powers of a variety of nuclear reactions at the ANKE magnetic spetrometer. The overall precisions obtained were about 4% for both Pz and Pzz. One of the motivation for the experimental programme is the direct reconstruction of the spin‐dependent amplitudes, including relative phases, of large angle neutron‐proton elastic scattering, through the study of the p(d,2p)n charge‐exchange reaction up to the highest available deuteron energy at COSY (2.3 GeV).

The reaction pp->{pp}_s\gamma, where {pp}_s is a proton pair with an excitation energy E_{pp}d\gamma is on the 10^{-3}-10^{-2} level. The increase of the pp->{pp}_s\gamma cross section with T_p might reflect the influence of the Delta(1232) excitation., Comment: 4 pages + 4 figures

The European physical journal / A 56(8), 211 (2020). doi:10.1140/epja/s10050-020-00215-8, Published by Springer, Berlin ; Heidelberg

Differential cross sections for Compton scattering from the free proton at Theta(gamma)(lab) = 130.7 degrees in the energy region from 200 MeV to 410 MeV and far quasi-free Compton scattering from the proton bound in the deuteron at Theta(gamma)(lab) = 148.8 degrees in the energy region from 200 MeV to 290 MeV have been measured. The free proton data are in agreement with dispersion-theory predictions based on standard parameters. The difference of the proton polarizabilities has been extracted from the quasi-free data. Our result, - = [9.1 +/- 1.7(stat + syst) +/- 1.2(mod)] x 10(-4) fm(3), is in reasonable agreement with the world average of the free proton data if the backward spin polarizability gamma(pi) is taken to be -37.6 x 10(-4) fm(4) as predicted by dispersion theory in agreement with many theoretical calculations. This implies that quasi-free Compton scattering may also be used to determine the electromagnetic polarizabilities of the neutron. No indication has been found of a recently suggested new contribution to gamma(pi).

The comparison of fission fragment angular distributions in electron-and positron-induced fission is proposed as a new method to test virtual photon spectra. The sensitivity of this technique is shown by first experiments on232Th (bombarding energy: 9 and 16 MeV).

The search for electric dipole moments of particles in storage rings requires the development of dedicated electrostatic deflector elements. The JEDI prototype-ring design consists of more than 50 electric deflectors of 1 m length with 60 mm spacing between the plates with electric fields of 10 MV m−1. This paper presents studies of scaled-down uncoated prototype electrodes with 10 mm radius made of stainless steel. The electric field at electrode gap distances from 1 mm to 0.05 mm increased from 15 to 90 MV m−1. In future investigations, we will study different materials and coatings at similar electrode spacings. Preparations are also underway to study large deflector elements.