R. Kirsch

Secondary electron emission from thin carbon foils under hydrogen cluster impact

Abstract The yields of backward and forward electrons resulting from Hn+ cluster (n=1–13) interactions with carbon foils in the 40–120 keV/proton energy range have been measured as a function of the cluster size and of the foil thickness. A strong reduction of both backward and forward yields per proton is observed in comparison to free proton yields, with a saturation for n ⋍ 9 . Surprisingly the forward cluster effect persists for foils as thick as 3000 A.

Axial channeling of relativistic electrons in crystals as a source for positron production

Abstract In the framework of studies of positron sources dedicated to linear colliders, crystals are considered as photon radiators in association with thin amorphous converters or as compact positron sources where photon production and pair creation occur in the same medium. The main features of such sources in comparison with conventional amorphous sources are described. Taking linear collider requirements for such sources, a crystal converter suited for the Japanese Linear Collider (JLC) is considered. Limitations caused by radiation damage are mentioned and indications on critical fluences are given. A proof-of-principle experiment has been undertaken in Orsay, observing radiation enhancement in a tungsten crystal oriented along the 〈111〉 axis and submitted to a 2 GeV electron beam. The main results are reported.

Set-up optimization for an experimental test of a positron source using channeling

Abstract A new experiment is suggested in order to investigate the positron yield at the exit of crystal tungsten targets of different thicknesses submitted to 10 GeV electron beam at one of the SPS CERN transfer lines. The Set-up consists of the beam control counters, targets mounted on the goniometer, positron detector and photon detector. The positron detector is based mainly on a drift chamber inserted between the poles of the magnet. It allows to reconstruct the track of the charged particles, to define the vertex coordinates and initial angle of the particles at the exit of the target, to measure the total and transverse momenta. A Monte Carlo simulation corresponding to the theoretical description of chanelling radiation and also numerical calculation were provided for the estimations of the expected distributions and detector response. The optimization of the Set-up is considered, as well as the status of the project in general.

Photon multiplicity in the hard radiation of 150 GeV electrons in an aligned germanium crystal

Abstract Mean values m of photon multiplicity in the radiation of 150 GeV electrons directed at and near the 〈110〉 axis of a 0.185 mm thick Ge crystal cooled to 100 K have been deduced from the measurements of pair conversion probabilities. Depending on the distribution of multiplicity assumed, values of m ranging from 3.8 to 4.3 are obtained for the previously reported anomalous radiation peak.

Observation of channeling and blocking effect in pair creation in a Ge crystal

Abstract Angular and energy distributions of electrons and positrons created by 83–133 GeV photons in an aligned crystal (strong field pair creation) have been measured. The experiment exhibits the “blocking” effect for the positron and the difference of energy loss between channeled electron and quasi-channeled positron. The data are well reproduced by a Monte Carlo simulation.

Fission lifetime measured by the blocking technique as a function of excitation energy in the 24 A.MeV 238U+28Si reaction

Abstract The blocking technique has been used to infer fission lifetimes as a function of excitation energy for uranium-like nuclei formed in the U+Si reactions at 24 MeV/nucleon. The fission lifetimes are found larger than 10 −19 s for excitation energies up to about 250 MeV.

Influence of intrashell excitation (n=2) on the population of mestastable states of H- and He-like krypton ions in channeling conditions

Abstract We have observed delayed K α photons emitted by 60 A MeV H-like and He-like krypton ions leaving a 37 μm silicon crystal, both for random and 〈110〉 axial alignment of the target. We could extract intrashell (2s→ 2p) excitation probabilities, which are compared with values deduced from PWBA calculations.

Electronic capture and excitation of highly charged channeled ions

Abstract Two aspects of heavy ion channeling are presented. The first aspect is related to the fact that channeled ions interact only with the most loosely bound target electrons. One can take benefit of this feature to study processes such as radiative electron capture (REC) and resonant transfer and excitation (RTE) in a dense quasi-free electron gas. The experimental work, performed at GANIL, devoted to these two processes is described. A possible extension to Nuclear RTE or NEEC (nuclear excitation by electron capture) studies is also described. The second aspect discussed is related to the periodicity of the potential experienced by channeled ions. We show that in a well chosen case this could lead to a significant and detectable coherent excitation of the projectile nucleus.

Search for light neutral objects photoproduced in a crystal strong field and decaying into e+e− pairs

Abstract We describe a search for a light neutral object X0 possibly produced by ≈ 100 GeV photons interacting with the QED strong field encountered along the axis of a cooled Ge crystal. We look at the e+e− decay mode, in short (10−18−10−12 s) as well as long (10−13−10−8 s) lifetime detection conditions. For a mass of 1.8 ±0.2MeV/c2 (“Darmstadton” mass) we find no evidence for X0 production. The upper limit for the ratio between e+e− decay rate and pair creation is given as a function of the X0 lifetime. It reaches ≈ 5 × 10−4 and ≈ 2 × 10−5 in the short and long lifetime mode, respectively, including the crystal absorption contribution. However, for the long-lifetime mode and 108 incident photons, 9 events are compatible with neutral objects having masses ranging from 2.1 to 3.5 MeV c 2 . A Monte Carlo simulation of the background gives only 1 event in the same energy range.

Electron Emission from Swift Hydrogen Cluster Interaction with Thin Carbon Foils

Electron emission yields from solids are a possible indirect measure of the energy loss of charged particles1,2,3. Most of the theoretical models on particle induced kinetic electron emission from solids1 consider the average yield y of electrons per projectile to be proportional to the electronic stopping power Se of the medium, i.e. γ=ΛSe (1)