Proofs and hypotheses

On the basis of an analysis of common features and differences between general statements in every day situations, in physics and in mathematics the paper proposes a didactical approach to proof. It is centred around the idea that inventing hypotheses and testing their consequences is more productive for the understanding of the epistemological nature of proof than forming elaborate chains of deductions. Inventing hypotheses is important within and outside of mathematics. In this approach proving and forming models get in close contact. The idea is exemplified by a teaching unit on the angle sum theorem in Euclidean geometry.     

Partial photoionization cross sections and angular distributions for double excitation of helium up to the N = 13 threshold

Partial photoionization cross sections sigmaN(Egamma) and photoelectron angular distributions betaN(Egamma) were measured for the final ionic states He+ (N > 4) in the region between the N = 8 and N = 13 thresholds (Egamma > 78.155 eV) using the cold target recoil ion momentum spectroscopy technique (COLTRIMS). Comparison of the experimental data with two independent sets of theoretical predictions reveals disagreement for the branching ratios to the various HeN(+) states. The angular distributions just below the double ionization threshold suggest an excitation process for highly excited N states similar to the Wannier mechanism for double ionization.     

Unified picture of polariton propagation in bulk GaAs semiconductors

High-resolution amplitude and phase linear spectroscopy of high-quality bulk GaAs are reported. The detailed structure of the observed full complex transmission is consistently explained by polariton effects on the basis of microscopic calculations. The coupled equations for the excitonic polarization and the light field in the slab configuration are evaluated using appropriate boundary conditions for the electromagnetic field and the excitonic wave function without reference to additional boundary conditions for the macroscopic polarization.     

Buchbesprechungen

Ohne Zusammenfassung     

Experimental observation of interatomic coulombic decay in neon dimers

Recently Cederbaum et al. [Phys. Rev. Lett. 79, 4778 (1997)]] predicted a new decay channel of excited atoms and molecules termed interatomic Coulombic decay (ICD). In ICD the deexcitation energy is transferred via virtual photon exchange to a neighboring atom, which releases it by electron emission. We report on an experimental observation of ICD in 2s ionized neon dimers. The process is unambiguously identified by detecting the energy of two Ne1+ fragments and the ICD electron in coincidence, yielding a clean, background free experimental spectral distribution of the ICD electrons.     

Spallation neutron production and the current intra-nuclear cascade and transport codes

A recent renascent interest in energetic proton-induced production of neutrons originates largely from the inception of projects for target stations of intense spallation neutron sources, like the planned European Spallation Source (ESS), accelerator-driven nuclear reactors, nuclear waste transmutation, and also from the application for radioactive beams. In the framework of such a neutron production, of major importance is the search for ways for the most efficient conversion of the primary beam energy into neutron production. Although the issue has been quite successfully addressed experimentally by varying the incident proton energy for various target materials and by covering a huge collection of different target geometries --providing an exhaustive matrix of benchmark data-- the ultimate challenge is to increase the predictive power of transport codes currently on the market. To scrutinize these codes, calculations of reaction cross-sections, hadronic interaction lengths, average neutron multiplicities, neutron multiplicity and energy distributions, and the development of hadronic showers are confronted with recent experimental data of the NESSI collaboration. Program packages like HERMES, LCS or MCNPX master the prevision of reaction cross-sections, hadronic interaction lengths, averaged neutron multiplicities and neutron multiplicity distributions in thick and thin targets for a wide spectrum of incident proton energies, geometrical shapes and materials of the target generally within less than 10% deviation, while production cross-section measurements for light charged particles on thin targets point out that appreciable distinctions exist within these models. Received: 22 May 2001 / Accepted: 3 September 2001