Search results for "Helium"
showing 10 items of 1689 documents
Photon absorption on a proton-proton pair inHe3
1995
We calculate the total cross sections for different multiple transitions in photon absorption in a {sup 1}{ital S}{sub 0} {ital pp} pair embedded in a {sup 3}He nucleus. Employing two models, one involving only nucleon degrees of freedom, the other in addition the {Delta}(1232) isobar, we show that the {Delta} gives important effects even in transitions where the photon cannot couple directly to the isobar.
A large-area transition radiation detector
1990
Abstract The construction and the operation of a large-area transition radiation detector (TRD) for the NA31 experiment at CERN are described. The TRD incorporates several novel features for stabilizing the detector response. The density of the gas mixture (xenon+helium+methane) in the detection chambers is matched to the carbon dioxide gas in the surrounding radiators by tuning the helium concentration to avoid a hydrostatic pressure difference, which would deform the chamber walls. The chamber pressure is continuously regulated by computer control to maintain it to within 1 μbar of the radiator pressure. The gas gain of each of the four chambers is regulated to better than 0.2% by changin…
Isothermal flow of helium II through plane slits
1972
Measurements are reported on the isothermal, gravitational flow of liquid He II through narrow plane slits. The employed superleak consists of ring-shaped Hostaphan foils packed in layers. The distance between the foils is varied by pressure. The performance of the measurements and the observance of the isothermal conditions are described. The velocity of the superfluid component of He II is calculated in dependence on the width of the slits, the temperature and the hydrostatic pressure. Especially the critical velocity is considered.
Roper excitation in alpha-proton scattering
1995
We study the Roper excitation in the $(\alpha,\alpha')$ reaction. We consider all processes which may be relevant in the Roper excitation region, namely, Roper excitation in the target, Roper excitation in the projectile, and double $\Delta$ excitation processes. The theoretical investigation shows that the Roper excitation in the proton target mediated by an isoscalar exchange is the dominant mechanism in the process. We determine an effective isoscalar interaction by means of which the experimental cross section is well reproduced. This should be useful to make predictions in related reactions and is a first step to construct eventually a microscopic $NN \rightarrow NN^*$ transition poten…
Polarization transfer in the 4HeH reaction
2001
Abstract Polarization transfer in the 4He ( e → ,e′ p → ) 3 H reaction at a Q2 of 0.4 (GeV/c)2 was measured at the Mainz Microtron MAMI. The ratio of the transverse to the longitudinal polarization components of the ejected protons was compared with the same ratio for elastic ep scattering. The results are consistent with a recent fully relativistic calculation which includes a predicted medium modification of the proton form factor based on a quark–meson coupling model.
Streuung von 50 keV-Elektronen an flüssigem Helium
1970
The behaviour of liquid helium in relation to electron-scattering has been investigated. Electrons of 50 keV have been scattered by thin targets of liquid helium with temperatures between 1.34 and 2.15 K. The thickness of targets was chosen between 0.047 and 0.62 mm. The angular distribution and the energy loss of the scattered electrons have been measured in dependence on the thickness of layer and on temperature. The extrapolated range of electrons in liquid helium has been determined.
Focusing ions by viscous drag and weak electric fields in an ion guide
1991
Abstract A new special ion guide has one or more grids at low electric potential in the space between nozzle and skimmer. Viscous drag caused by helium flow is used together with weak electric fields for focusing ions through the skimmer. A typical yield through the skimmer is 75% of that through the nozzle. The focusing device is called a “squeezer”. Most properties of the “squeezer ion guide” are similar to those of an ordinary ion guide. Because the kinetic energies are only of the order of 10 eV, however, problems caused by ion scattering are greatly reduced as compared to ordinary ion guides.
Energy distribution of ternaryαparticles in spontaneous fission ofCf252
2008
The energy distribution of the ternary $\ensuremath{\alpha}$ particles in spontaneous fission of $^{252}\mathrm{Cf}$ was measured. For the first time an energy threshold as low as 1 MeV was reached. The experiment used an array of unshielded silicon detectors measuring energy and time-of-flight (TOF) of ternary particles in coincidence with fission fragments. The TOF resolution of the system was sufficient for clear separation of $^{6}\mathrm{He}$ and tritons from $^{4}\mathrm{He}$. The statistics were adequate to extract the $^{6}\mathrm{He}$/$^{4}\mathrm{He}$ yield ratio. For both $^{4}\mathrm{He}$ and $^{6}\mathrm{He}$, an excess in the yield (as compared to a Gaussian shape) was observe…
Towards a magnetic field stabilization at ISOLTRAP for high-accuracy mass measurements on exotic nuclides
2008
Abstract The field stability of a mass spectrometer plays a crucial role in the accuracy of mass measurements. In the case of mass determination of short-lived nuclides with a Penning trap, major causes of fluctuations are temperature variations in the vicinity of the trap and pressure changes in the liquid helium cryostat of the superconducting magnet. Thus systems for the temperature and pressure stabilization of the Penning trap mass spectrometer ISOLTRAP at the ISOLDE facility at CERN have been installed. A reduction of the temperature and pressure fluctuations by at least an order of magnitude down to Δ T ≈ ± 5 mK and Δ p ≈ ± 5 Pa has been achieved, which corresponds to a relative magn…
On the existence of higher waves in a layer of superfluid helium
1973
The two implicit equations that contain the dispersion laws of waves propagating in a He II layer of variable thickness are formally investigated for solutions that go beyond those associated with the layer modifications of first and second sound: A series of symmetric and antisymmetric layer modes are found to exist by calculating the distribution of roots of the dispersion equations in the complex wave number plane as a function of layer thickness and angular frequency. All these modes turn out to be strongly attenuated and can be regarded as layer modifications of the viscous wave. Phase velocities, attenuation coefficients, and velocity profiles of some of them are calculated numericall…