R. Fearick

ALICE HLT High Speed Tracking on GPU

The on-line event reconstruction in ALICE is performed by the High Level Trigger, which should process up to 2000 events per second in proton-proton collisions and up to 300 central events per second in heavy-ion collisions, corresponding to an input data stream of 30 GB/s. In order to fulfill the time requirements, a fast on-line tracker has been developed. The algorithm combines a Cellular Automaton method being used for a fast pattern recognition and the Kalman Filter method for fitting of found trajectories and for the final track selection. The tracker was adapted to run on Graphics Processing Units (GPU) using the NVIDIA Compute Unified Device Architecture (CUDA) framework. The implementation of the algorithm had to be adjusted at many points to allow for an efficient usage of the graphics cards. In particular, achieving a good overall workload for many processor cores, efficient transfer to and from the GPU, as well as optimized utilization of the different memories the GPU offers turned out to be critical. To cope with these problems a dynamic scheduler was introduced, which redistributes the workload among the processor cores. Additionally a pipeline was implemented so that the tracking on the GPU, the initialization and the output processed by the CPU, as well as the DMA transfer can overlap. The GPU tracking algorithm significantly outperforms the CPU version for large events while it entirely maintains its efficiency.

Comparison of channeling radiation from diamonds with and without platelets

Channeling‐radiation spectra produced by planar‐channeled relativistic positrons and electrons in Type‐Ia and Type‐IIa diamonds have been measured. Because of the presence of platelets in the Type‐Ia diamond, some of the spectra measured for this crystal differ markedly from their counterparts for the Type‐IIa diamond. These striking differences illustrate the potential applications of channeling radiation as a diagnostic tool for studies of impurities or defects in crystals.

Electron and positron channeling radiation from type-Ia and type-IIa diamonds☆

Abstract Planar channeling radiation spectra have been obtained from type-IIa and type-Ia diamonds with 54.5 and 30.5 MeV electrons and with 54.4 MeV positrons. Type-IIa diamonds are relatively free of impurities while Type-Ia diamonds contain “platelet defects” [N atoms bonded in the (100) plane]. For platelet free (type-IIa) diamond, theoretical calculations are found to fit experimental results quite well for electron channeling radiation. However, significant discrepancies (~ 5%) exist between the theoretical predictions and the positron data. Measurements on a type-Ia diamond show large differences in the planar channeling radiation spectra which are anisotropic and which depend upon whether electrons or positrons are used as a probe.

Channeling in diamond at high depth resolution

Abstract Backscattering spectra of MeV He+ ions channeled along various axial and planar directions in diamond have been measured under conditions chosen to enhance the depth resolution, in order to study the oscillatory dependence of the yield. The planar spectra have been fitted with a simple model and the wavelength and stopping power of the channeled ions have been determined. The stopping power is found to be close to the random value, and the wavelength does not obey a (d p ) 1 2 dependence expected from simple theoretical estimates. Spectra from the axial to planar transition region indicate that the axis does not affect the wavelength to a large extent.

Positron and electron channeling radiation from germanium

Abstract Channeling-radiation spectra have been obtained from a germanium crystal with ≈54.3 MeV positrons and electrons and with 16.9 MeV electrons. Theoretical calculations were performed and are found to agree reasonably well with the experimental results. When the emission spectra for (110) planar channeling in diamond, silicon, and germanium are compared, the corresponding lines for both positrons and electrons are found to be lowest in energy for silicon.

Electron and positron planar channeling radiation from diamond

Abstract Channeling-radiation spectra have been obtained from a diamond with ≈ 54.5 MeV electrons and positrons. Theoretical calculations are found to fit the experimental results well for electrons, but significant discrepancies exist between theoretical predictions and the positron data.

Elastic and inelastic scattering in the 6Li, 9Be + 12C systems: excitation of unbound excited states and cluster transfers

Abstract For the system 9Be + 12C, angular distributions at Ec.m. = 10.86 MeV and excitation functions in the c.m. energy range 3–16 MeV have been measured for the elastic scattering and inelastic scattering leading to the unbound state 9 Be( 5 2 − , 2.43 MeV ) and to the state 12C(2+, 4.44 MeV). For the system 6Li + 12C, angular distributions at Ec.m. = 13.33 MeV and excitation functions in the c.m. energy range 2–16 MeV have been measured for the elastic scattering and inelastic scattering to the unbound state 6Li(3+, 2.186 MeV) and to the state 12C(+, 4.44 MeV). The wide angular range of the inelastic-scattering angular distributions, 20° ≲θc.m.≲ 170°, is obtained by changing appropriately the roles of the projectile and target nuclei. The 9Be + 12C data are analyzed using first-order EFR-DWBA calculations that include 3He-cluster transfers in order to describe the enhancement in the elastic and inelastic scattering at backward scattering angles. In addition to the 3He transfers, compound-nucleus contributions are found also to be significant at the backward scattering angles. Reorientation and coupling effects in the 9Be + 12C system are studied using an optical potential with a double-folded real part, resulting in a good description of the 9Be( 5 2 − , 2.43 MeV) angular distribution without the need for a renormalisation of the M3Y nucleon-nucleon interaction, but with the elastic 3He transfer still required to account for the backward-angle enhancement in the elastic-scattering angular distribution. The 6Li + 12C data are analyzed using coupled-channels calculations; here the inclusion of compound-nucleus contributions is found to be significant for inelastic scattering at backward scattering angles.

Inelastic scattering in the 6Li + 9Be system leading to unbound excited states of either nucleus

Abstract Complete angular distributions for elastic scattering and inelastic scattering to the first excited state (3 + , 2.186 MeV) of 6 Li and the second excited state ( 5 2 − , 2.43 MeV) of 9 Be in the system 6 Li + 9 Be are presented for energies E cm. = 7, 10 and 12 MeV. The inelastic-scattering cross sections for these particle-unbound excited states are measured by detecting the corresponding recoil nucleus. The wide range of the inelastic-scattering angular distributions (30° ≲ θ c.m. ≲ 170°) is achieved by interchanging appropriately the role of the projectile and target nuclei. Excitation functions for the studied elastic and inelastic scattering covering the energy range 4 ≲ E c.m. ≲ 12 MeV are reported also. In the analysis, the channels with the nucleus 6 Li excited and with the nucleus 9 Be excited are coupled to the ground states in one coupled-channels calculation, using an optical potential with a real double-folded part and a Woods-Saxon imaginary part. The inelastic-scattering data are fitted using deformation lengths that exhibit some energy dependence and that approach at lower c.m. energies more closely those deduced from the B (E2) values. Compound-nucleus contributions to the observed cross sections are calculated using the Hauser-Feshbach formalism, but are found to be rather small in this system, except at backward scattering angles. In addition, the effect of elastic and inelastic triton transfers is examined by performing finite-range DWBA calculations and such contributions are found also to be unimportant.

The Faure cyclotron neutron source and a particle spectrometer for neutron induced emission of charged particles at energies between 60 and 200 MeV

Abstract An inexpensive particle spectrometer has been developed for the study of charge-exchange reactions using incident neutron energies between 60 and 200 MeV. The neutron source consists of lithium metal pressed between Havar foils and bombarded by up to 5 μA protons. The particle spectrometer is placed about 7 m from the neutron source to provide for beam collimation and time-of-flight gating which restricts collected data to the monoenergetic peak from the 7 Li(p, n) reaction. Timing resolution of between 0.5 and 1.5 ns (FWHM) is achieved with the Faure cyclotron. The particle spectrometer consists of a) a sandwich of several small multi-wire proportional counters which fix the origin of emitted charged particles, provide background suppression, and enable simultaneous measurement from several target samples, b) a curved plastic strip scintillator which gives a ΔE signal and, at the same time, provides angle measurements and c) four NaI(Tl) detectors with thin entrance windows to determine total energy of the detected particles. The system achieves good particle identification, energy resolution of about 1.2 MeV (FWHM) at all energies, angle resolution of about ±3°, good signal to background ratios and simultaneous measurement over an angular range of > 70°.

Fine structure of the isoscalar giant quadrupole resonance in 40Ca due to Landau damping

Abstract The fragmentation of the Isoscalar Giant Quadrupole Resonance (ISGQR) in 40Ca has been investigated in high energy-resolution experiments using proton inelastic scattering at E p = 200 MeV . Fine structure is observed in the region of the ISGQR and its characteristic energy scales are extracted from the experimental data by means of a wavelet analysis. The experimental scales are well described by Random Phase Approximation (RPA) and second-RPA calculations with an effective interaction derived from a realistic nucleon–nucleon interaction by the Unitary Correlation Operator Method (UCOM). In these results characteristic scales are already present at the mean-field level pointing to their origination in Landau damping, in contrast to the findings in heavier nuclei and also to SRPA calculations for 40Ca based on phenomenological effective interactions, where fine structure is explained by the coupling to two-particle–two-hole (2p–2h) states.