Search

This paper reports results on simultaneous measurements of the reaction channels pp $ \rightarrow$ pK $ \Lambda$ and pp $ \rightarrow$ pK $ \Sigma^{0}_{}$ at excess energies of 204, 239, and 284MeV ( $ \Lambda$ and 127, 162, and 207MeV ( $ \Sigma^{0}_{}$ . Total and differential cross-sections are given for both reactions. It is concluded from the measured total cross-sections that the high-energy limit of the cross-section ratio is almost reached at an excess energy of only about 200MeV. From the differential distributions observed in the overall CMS as well as in the Jackson and helicity frames, a significant contribution of interfering nucleon resonances to the $ \Lambda$ production mechanism is concluded while resonant $ \Sigma^{0}_{}$ production seems to be of lesser importance and takes place only through specific partial waves of the entrance channel. The data also indicate that kaon exchange plays a minor role in the case of $ \Lambda$ but an important role for $ \Sigma^{0}_{}$ production. Thus the peculiar energy dependence of the $ \Lambda$/ $ \Sigma^{0}_{}$ cross-section ratio appears in a new light as its explanation requires more than mere differences between the p $ \Lambda$ and the p $ \Sigma^{0}_{}$ final-state interaction. The data provide a benchmark for theoretical models already available or yet to come. [ABSTRACT FROM AUTHOR]

In this article we describe the experimental results of a methane hydrate moderator as well as as mesitylene moderator operated at a temperature around 20K at the JESSICA (Jülich Experimental Spallation Target Set-up In COSY Area) experiment at the Jülich cooler synchrotron COSY. For the first time the cold neutron spectrum of a methane hydrate moderator was experimentally investigated. A comparison with a solid methane and an ice moderator at T = 20 K will be shown. MCNPX simulations with new developed S(α,β) scattering kernels will be compared with experimental data. The applicability of mesitylene and methane hydrate as cold moderators at spallation neutron sources will be discussed. [ABSTRACT FROM AUTHOR]

In this article measured time-of-flight spectra and energy spectra of a water moderator operated at various temperatures between 20K and room temperature are described. These data are used to validate generated and processed neutron scattering law data. It was possible to simulate the leakage spectrum of a water moderator at cryogenic temperatures and compare it with experimental results. [ABSTRACT FROM AUTHOR]

Kinematically complete measurements of the pp → ppπ0 reaction were performed for beam energies in the range 292–298MeV. We detected both protons in coincidence by using the large-acceptance COSY-TOF spectrometer and an external proton beam at COSY-Jülich; thus, we measured total and differential cross-sections and energy distributions. A strong enhancement was observed in the Dalitz plots due to the final-state interaction between the two outgoing protons; the data are well reproduced by Monte Carlo simulations using a standard scattering length of a0 = -7.83 fm and effective range of r0 = 2.8 fm. Our measured total cross-sections are roughly 50% larger than those of recent internal-target experiments at IUCF and CELSIUS. This discrepancy may be due to the final-state interaction pushing some events into the very small-angle region which is almost inaccessible to an internal-target experiment. In the angular distributions we found only very slight deviations from isotropy. The analysis of our momentum distributions yielded a small fraction of 5 ±5% for Ps or Pp wave contributions. [ABSTRACT FROM AUTHOR]

The reaction pp → ppη was measured at excess energies of 15 and 41 MeV at an external target of the Jülich Cooler Synchrotron COSY with the time-of-flight spectrometer. About 25000 events were measured for the excess energy of 15 MeV and about 8000 for 41 MeV. Both protons of the process ppη were detected with an acceptance of nearly 100% and the η was reconstructed by the missing-mass technique. For both excess energies the angular distributions are found to be nearly isotropic. In the invariant-mass distributions strong deviations from the pure phase space distributions are seen. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]