393 research outputs found
Antiparticle cloud temperatures for antihydrogen experiments
No full textAn analysis of positron and antiproton cloud temperatures under condition similar to those found in antihydrogen experiments.Some simple general trends are brought out in the analysis, which includes the effects of cloud expansion for the first time
Enhancement of annihilation cross sections by electric interactions between the antineutron and the field of a large nucleus
No full textData relative to antineutron and antiproton annihilation on large nuclei in the range 75-200MeV/c present two unexpected features: a) antineutron and antiproton cross sections have a similar size, b) the rise of the antineutron cross section at decreasing energy is much steeper than predictable for an inelastic process of purely strong nature at that energy. The observed behavior of (Formula presented.)-nucleus annihilations is similar to what would be expected for (Formula presented.)-nucleus annihilations, where the Coulomb attraction focusses (Formula presented.) trajectories towards the nucleus, enhancing the inelastic cross section by a factor 1/p2with respect to (Formula presented.) on the same target. This results in a 1/p2behavior at small energies. The presence of a similar enhancement in the antineutron case may only be justified by an interaction with a longer range than strong interactions. Excluding a Coulomb force because of the (Formula presented.) neutrality, and taking into account that an intrinsic electric dipole is forbidden for the antineutron, the next choice is an electric dipole that is induced by the nuclear electric field. Recent theoretical works have shown that a nonnegligible electric polarization may be induced in a neutron by QED vacuum polarization. Assuming this as a possibility, we have used a simple model to calculate the polarization strengths that are needed to fit the available data in terms of this effect. These are within the magnitude predicted by the vacuum polarization model. We have also discussed alternative scenarios that could induce an electric polarization of the antineutron as a consequence of the interplay between strong and e.m. interactions
Modelling the behavior of the positron plasma temperature in antihydrogen experimentation
No full textAntihydrogen is now routinely produced at CERN by overlapping clouds of positrons and antiprotons. The mechanisms responsible for antihydrogen formation (radiative capture and the three-body reaction) are both dependent on the temperature of the positrons (T e ), though with a different weight. Here we present a simple model of the behavior of the positron temperature based on the main processes involved during antihydrogen synthesis, namely: antiproton–positron collisions, positron heating due to plasma expansion and cooling via the emission of synchrotron radiation. The time evolution of T e has been simulated by changing the relevant parameters of the mechanisms involved in order to highlight the importance of the different (competing) effects
Hypertension in acute ischemic stroke: a compensatory mechanism or an additional damaging factor?
No full textProblems related to short-term antihypertensive therapy in acute ischemic stroke
No full textHypertension is a common early finding after an acute ischemic stroke, even in previously normotensive patients. But its significance and proper management are a matter of debate, because of the lack of adequately powered randomized clinical trials. A close analysis of observational and interventional trials, published so far, fails to convince that an early antihypertensive therapy is needed and beneficial. During the first 24-48 hr after ischemic stroke, only blood pressure values repeatedly higher than 220/120 mmHg require antihypertensive treatment to keep blood pressure levels in the range of 180-220 mmHg systolic and 100-120 diastolic. Blood pressure reduction should be cautious with the aim of keeping the pressure at relatively high values (180/100-105 in previously hypertensive patients and 160-180/90-100 in previously normotensive patients). The usefulness of increasing blood pressure with vasopressive agents in selected patients with ischemic stroke deserves adequate testing with randomized clinical trials
Antihydrogen synthesis in a double-CUSP trap towards test of the CPT-symmetry
No full textHyperfine Interactions
Volume 237, Issue 1, 1 December 2016, Article number 156
Antihydrogen synthesis in a double-CUSP trap towards test of the CPT-symmetry (Article)
Radics, B.ah , Ishikawa, S.b, Kuroda, N.b, Murtagh, D.J.a, Nagata, Y.a, Tajima, M.b, Van Gorp, S.a, Abo, Y.c, Dupre, P.a, Higashi, Y.b, Kaga, C.c, Leali, M.de, Mascagna, V.de, Venturelli, L.de, Zurlo, N.de, Breuker, H.f, Higaki, H.c, Kanai, Y.a, Rizzini, E.L.de, Matsuda, Y.b, Ulmer, S.g, Yamazaki, Y.a
a Atomic Physics Laboratory, RIKEN, Saitama, Japan
b Institute of Physics, Graduate School of Arts and Sciences, University of Tokyo, Tokyo, Japan
c Graduate School of Advanced Sciences of Matter, Hiroshima University, Hiroshima, Japan
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Abstract
The aim of the ASACUSA-CUSP experiment at CERN is to produce a cold, polarised antihydrogen beam and perform a high precision measurement of the ground-state hyperfine transition frequency of the antihydrogen atom and compare it with that of the hydrogen atom using the same spectroscopic beam line. Towards this goal a significant step was successfully accomplished: synthesised antihydrogen atoms have been produced in a CUSP magnetic configuration and detected at the end of our spectrometer beam line in 2012 [1]. During a long shut down at CERN the ASACUSA-CUSP experiment had been renewed by introducing a new double-CUSP magnetic configuration and a new semi-cylindrical tracking detector (AMT) [2], and by improving the transport feature of low energy antiproton beams. The new tracking detector monitors the antihydrogen synthesis during the mixing cycle of antiprotons and positrons. In this work the latest results and improvements of the antihydrogen synthesis will be presented including highlights from the last beam time
Scintillating bar detector for antiproton annihilations measurements
No full textA detector used in two different experiments of the ASACUSA Collaboration at the CERN Antiproton Decelerator is presented. It consists of several modules, each one made of ~1 m long scintillating bars, which allows the detection of the charged particles produced in the antiproton-nuclei annihilations. The modularity of the detector, its electronics readout and the data acquisition system have been designed to be adapted to different experimental conditions
Spectral and spatial distribution of the scattered radiation of an electron radiotherapy beam
No full textThe characterization of the radiation scattered by the collimating system of a linear accelerator for medical applications is becoming a must given the increase in the survival of the radio-treated patients. This paper describes both the spectral and spatial study of the scattered radiation of an electron beam in the energy range 6-20 MeV, produced by a Varian CLINAC 1800 accelerator at the Radiotherapy Unit of the Sant'Anna Hospital in Como. The detectors used are plastic and fiber scintillators characterized by a timing response of ≈ 25 nsec in order to cope with the high fluxes. The results have been compared with a Montecarlo simulation based on Geant 3.21 and are presented both in terms of spectral and dose distributions
Optical Channeling of Low Energy Antiprotons in Thin Crystal Targets
No full textA relevant aspect of the interactions between charged fermions and crystal targets is coherence, which can exist at both classical and quantum levels. In the case of antiprotons crossing crystal targets, there are theories and measurements of classical-level coherence effects, in particular, channeling effects. For the present study, we assume the existence of a low-energy regime where the electrostatic interactions between an antiproton and the crystal atoms lead to a local loss in the beam flux as their leading effect. We expect this assumption to be well-justified for antiproton (p¯) energies below 100 eV, with a progressive transition to a standard “Rutherford regime” in the energy range 100–1000 eV. Under these conditions, the target can be treated as an optical absorber with a periodical structure, which can be simplified by considering a multi-layer planar structure only. As in standard optics, wave absorption is accompanied by interference and diffraction. Assuming sub-nanometer ranges for the relevant parameters and a realistic angular spread for the antiproton beam, we find narrow-angle focusing effects that reproduce the classical channeling effect at a qualitative level. We also find that diffraction dominates over interference, although this may strongly depend on the target details
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