The T2K experiment studies oscillations of an off-axis muon neutrino beam between the J-PARC accelerator complex and the Super-Kamiokande detector. Special emphasis is placed on measuring the mixing ...angle theta sub(13) by observing v sub(e) appearance via the sub-dominant v sub( mu ) arrow right v sub(e) oscillation and searching for CP violation in the lepton sector. The experiment includes a sophisticated, off-axis, near detector, the ND280, situated 280 m downstream of the neutrino production target in order to measure the properties of the neutrino beam and to understand better neutrino interactions at the energy scale below a few GeV. The data collected with the ND280 are used to study charged- and neutral-current neutrino interaction rates and kinematics prior to oscillation, in order to reduce uncertainties in the oscillation measurements by the far detector. A key element of the near detector is the ND280 electromagnetic calorimeter (ECal), consisting of active scintillator bars sandwiched between lead sheets and read out with multi-pixel photon counters (MPPCs). The ECal is vital to the reconstruction of neutral particles, and the identification of charged particle species. The ECal surrounds the Pi-0 detector (PemptysetD) and the tracking region of the ND280, and is enclosed in the former UA1/NOMAD dipole magnet. This paper describes the design, construction and assembly of the ECal, as well as the materials from which it is composed. The electronic and data acquisition (DAQ) systems are discussed, and performance of the ECal modules, as deduced from measurements with particle beams, cosmic rays, the calibration system, and T2K data, is described.
Direct observation of the survival of 199Au residues after 2n transfer in the He8+Au197 system and the absence of the corresponding 67Cu in the He8+Cu65 system at various energies are reported. The ...measurements of the surprisingly large cross sections for 199Au, coupled with the integral cross sections for the various Au residues, is used to obtain the first model-independent lower limits on the ratio of 2n to 1n transfer cross sections from 8He to a heavy target. A comparison of the transfer cross sections for 6,8He on these targets highlights the differences in the interactions of these Borromean nuclei. These measurements for the most neutron-rich nuclei on different targets highlight the need to probe the reaction mechanism with various targets and represent an experimental advance towards understanding specific features of pairing in the dynamics of dilute nuclear systems.
Shape coexistence in the light krypton isotopes was studied in two low-energy Coulomb excitation experiments using radioactive {sup 74}Kr and {sup 76}Kr beams from the SPIRAL facility at GANIL. The ...ground-state bands in both isotopes were populated up to the 8{sup +} state via multi-step Coulomb excitation, and several non-yrast states were observed. Large sets of matrix elements were extracted for both nuclei from the observed {gamma}-ray yields. Diagonal matrix elements were determined by utilizing the reorientation effect. In both isotopes the spectroscopic quadrupole moments for the ground-state bands and the bands based on excited 0{sub 2}{sup +} states are found to have opposite signs. The experimental data are interpreted within a phenomenological two-band mixing model and model-independent quadrupole invariants are deduced for the relevant 0{sup +} states using the complete sets of matrix elements and the formalism of quadrupole sum rules. Configuration mixing calculations based on triaxial Hartree-Fock-Bogolyubov calculations with the Gogny D1S effective interaction have been performed and are compared both with the experimental results and with recent calculations using the Skyrme SLy6 effective interaction and the full generator-coordinate method restricted to axial shapes.
Measurements of the beta-decay half-lives of neutron-rich nuclei (MgAr) in the vicinity of the N=28 shell closure are reported. Some 22 half-lives have been determined, 12 of which for the first ...time. Particular emphasis is placed on the results for the Si isotopes, the half-lives of which have been extended from N=25 to 28. Comparison with QRPA calculations suggests that 42Si is strongly deformed. This is discussed in the light of a possible weakening of the spin–orbit potential.
The transfer of neutrons onto 24Ne has been measured using a reaccelerated radioactive beam of 24Ne to study the (d,p) reaction in inverse kinematics. The unusual raising of the first 3/2+ level in ...25Ne and its significance in terms of the migration of the neutron magic number from N=20 to N=16 is put on a firm footing by confirmation of this state's identity. The raised 3/2+ level is observed simultaneously with the intruder negative parity 7/2- and 3/2- levels, providing evidence for the reduction in the N=20 gap. The coincident gamma-ray decays allowed the assignment of spins as well as the transferred orbital angular momentum. The excitation energy of the 3/2+ state shows that the established USD shell model breaks down well within the sd model space and requires a revised treatment of the proton-neutron monopole interaction.
Results from a study in inverse kinematics of the $^{24}$Ne(d, pγ)$^{25}$Ne reaction, using a radioactive beam of $^{24}$Ne from the SPIRAL facility at GANIL, are reported. First, a brief overview is ...given of several methods using radioactive beams to study the classic single-nucleon transfer reactions such as (d, p) or (d, t)/(d, $^{3}$He), where the experimental design is strongly influenced by the extreme inverse kinematics. A promising approach to deliver good energy resolution is to combine a high geometrical efficiency for kinematically complete charged particle detection with a high efficiency array for gamma-ray detection. One of the first dedicated set-ups for this type of experiment is the TIARA silicon strip array combined with the EXOGAM segmented germanium array. Together they comprise a highly compact, position-sensitive particle array with 90% of 4π coverage, mounted inside a cubic arrangement of four segmented gamma-ray detectors in very close geometry with 67% of 4π active coverage. Using this setup, the structure of $^{25}$Ne has been studied via the (d, p) reaction. A pure ISOL beam of 10$^{5}$ s$^{-1}$ of $^{24}$Ne at 10 MeV/A was provided by SPIRAL and bombarded a CD$_{2}$ target of 1 mg/cm$^{2}$. The $^{25}$Ne was detected at the focal plane of the VAMOS spectrometer where the direct beam was separated and intercepted. Reaction protons were detected in coincidence with little background. Four resolved peaks were recorded between E$_{x}$ = 0 and 4 MeV. The data confirm and extend the results from a multinucleon transfer study using the ($^{13}$C,$^{14}$O) reaction. Further information has been obtained using the energies of coincident gamma-rays. The reactions $^{24}$Ne(d, dγ)$^{24}$Ne, $^{24}$Ne(d, t)$^{23}$Ne and $^{24}$Ne(d, $^{3}$He)$^{23}$F were recorded simultaneously and analysis of these is also underway.
We have studied the structure of light neutron-rich nuclei around N = 16 by employing the in-beam gamma-ray spectroscopy technique using the fragmentation of secondary beams of Ne-25,Ne-26, ...Na-27,Na-28 and Mg-29,Mg-30 isotopes. This secondary-beam cocktail was obtained by the fragmentation of a 36 S beam at 77.5 MeV.A by the SISSI/GANIL facility. By a second-step fragmentation, we have measured gamma-ray-residue coincidences in C17-20 and O-23,O-24 and described the obtained levels in the framework of the shell model.
Measurements of the helium-cluster breakup and neutron removal cross-sections for neutron-rich Be isotopes 10–12,14Be are presented. These have been studied in the 30 to 42 MeV/nucleon energy range ...where reaction measurements are proposed to be sensitive to the cluster content of the ground-state wave-function. These measurements provide a comprehensive survey of the decay processes of the Be isotopes by which the valence neutrons are removed revealing the underlying α–α core-cluster structure. The measurements indicate that clustering in the Be isotopes remains important up to the drip-line nucleus 14Be and that the dominant helium-cluster structure in the neutron-rich Be isotopes corresponds to α–Xn–α.
PDF
