Search results for "Cooling"
showing 10 items of 470 documents
Understanding the glass transition and the amorphous state of matter: can computer simulation solve the challenge?
1999
The glass transition of supercooled fluids is one of the big puzzles of condensed matter physics, because there occurs a dramatic slowing down (the viscosity η can increase from about η = 1 Poise at the melting transition to η 10 13 Poise at the glass transition temperature T g ), but one hardly sees any accompanying change in the static structure. Theoretical concepts are very controversial - e.g., the Gibbs-di Marzio theory attributes glassy freezing to an underlying entropy catastrophe (the entropy of the supercooled fluid would fall below the crystal entropy at the Kauzmann temperature T 0 T g . Computer simulations offer the advantage that atomistically detailed information on structur…
Computer simulation of models for the structural glass transition
2008
In order to test theoretical concepts on the glass transition, we investigate several models of glassy materials by means of Monte Carlo (MC) and Molecular Dynamics (MD) computer simulations. It is shown that also simplified models exhibit a glass transition which is in qualitative agreement with experiment and that thus such models are useful to study this phenomenon. However, the glass transition temperture as well as the structural properties of the frozen-in glassy phase depend strongly on the cooling history, and the extrapolation to the limit of infinitely slow cooling velocity is nontrivial, which makes the identification of the (possible) underlying equilibrium transition very diffi…
Molecular correlations in a supercooled liquid
1998
We present static and dynamic properties of molecular correlation functions S_{lmn,l'm'n'}(q,t) in a simulated supercooled liquid of water molecules, as a preliminary effort in the direction of solving the molecular mode coupling theory (MMCT) equations for supercooled molecular liquids. The temperature and time dependence of various molecular correlation functions, calculated from 250 ns long molecular dynamics simulations, show the characteristic patterns predicted by MMCT and shed light on the driving mechanism responsible for the slowing down of the molecular dynamics. We also discuss the symmetry properties of the molecular correlation functions which can be predicted on the basis of t…
Measurement of the transverse Doppler shift using a stored relativistic7Li+ ion beam
1992
We have performed for the first time precision spectroscopy on a coasting fast7Li+ ion beam in a storage ring. The ion beam moving with 6.4% speed of light was first electron cooled and then merged with two counterpropagating laser beams acting on two different hyperfine transitions sharing a common upper level (λ-system). One laser was frequency locked to thea 3 127J2 hfs frequency component established as a secondary frequency standard at 514 nm. The second laser was tuned over theλ-resonance, which was recorded relative to127J2 hfs components. This experiment is sensitive to the time dilation in fast moving frames and will lead to new limits for the verification of special relatively. Th…
New results with stored exotic nuclei at relativistic energies
2004
Recently, much progress has been made with stored exotic nuclei at relativistic velocities ( v c = 0.7 ) . Fragments of 208Pb and 209Bi projectiles and fission products from 238U ions were produced, separated in flight with the fragment separator FRS, and injected into the storage-cooler ring ESR for precision measurements. 114 new masses of neutron-deficient isotopes in the lead region have been measured with time-resolved Schottky Mass Spectrometry (SMS). A new isospin dependence of the pairing energy was observed due to the improved mass accuracy of typically 1.5×10-7 (30 keV). New masses of short-lived neutron-rich fission fragments have been obtained with Isochronous Mass Spectrometry …
The selection and performance of diamond radiators used in coherent bremsstrahlung experiments
2005
The bremsstrahlung emitted as a result of scattering electrons in thin diamond crystals provides a useful source of high energy photons for use in photonuclear experiments, since the coherent bremsstrahlung produced is linearly polarized. Techniques for selecting the most favorable diamonds have been investigated. These are optical polaroid analysis, X-ray topography and rocking curve measurements. The diamonds are characterized with a view to determining their performance as radiators, and bremsstrahlung spectra from a diamond radiator used at the Mainz MAMI-B facility are presented. The changes caused by high energy electrons to the crystal properties of the diamond and to the resulting c…
Ion beam preparation of 7Li+ for precision experiments at heavy ion storage rings
1997
Abstract Heavy ion storage rings allow for tests of the structure of local space time via the Doppler effect. At the TSR/Heidelberg an experiment with high resolution laser spectroscopy at 7 Li + is performed. To gain the maximum resolution for saturation spectroscopy new methods of relativistic ion beam preparation and diagnostics have been developed. The laser cooling of the beam allows for precision determination of the mean velocity of the ions. A novel phase synchronous detection scheme, ultimately sensitive to single ions, gives insights into the cooling mechanism and dynamics. With an additional synchronous excitation scheme systematic uncertainties of the test experiment can be dras…
Electron and positron cooling of highly charged ions in a cooler Penning trap
2004
Abstract Electron cooling is a well-established technique to increase the phase space density of particle beams in storage rings. In this paper, we discuss the feasibility of electron and positron cooling of ions in a Penning trap. We calculate the cooling times for the cases of trapped bare ions with nuclear charge Z =1 (protons), Z =36 (krypton) and Z =92 (uranium) with the Spitzer formula. Our calculations show that for typical experimental conditions the time for cooling from initial energies of 10 keV per charge down to rest is in the order of a second. We investigate the dependence of the cooling time on the number of ions and electrons, and their charge and mass.
ALICE T0 detector
2005
T0-the fast timing and trigger detector for the ALICE experiment at CERN LHC-is described. Performance of the T0 prototype measured with a mixture of 6 GeV/c negative pions and kaons is given. The best time resolution (28 ps r.m.s.) was reached with a radiator diameter matching that of the photocathode. The results for all the tested radiator sizes are considerably better than 50 ps-the minimum requirement for the ALICE experiment.
Angular distribution of coherent bremsstrahlung
2012
The angular distribution of the linearly polarised photon beam produced by coherent bremsstrahlung from an aligned diamond radiator has been measured at the MAMI A2 tagged photon facility. The measurements were made with a prototype position sensitive photon detector which utilises the pair production process and a double sided silicon strip detector. This polarised photon beam is used for nuclear and hadronic experiments and in their analysis the polarisation is obtained from a calculation, which matches the experimental intensity spectrum. As the polarisation is related to the photon beam angular distribution, the present measurements can be used to test this calculation. The overall agre…