Assembly and measurements of the Electromagnetic Calorimeter components for "WASA at COSY" setup

Source: arXiv

ABSTRACT This work describes the tests of the scintillator electromagnetic calorimeter of theWASA detector setup after transferring it from the CELSIUS storage ring at The Svedberg Laboratory(Uppsala, Sweden) to the Cooler Synchrotron COSY at the Institut fur Kernphysik (IKP) of Forschungszentrum Julich, Germany. The tests were performed using gammas of 4.4MeV energy from an AmBe source. The status of the CsI(Na) crystals was determined and the indication of gain factor for the energy calibration was extracted.

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    ABSTRACT: The III-nitride semiconductor material system, which consists of InN, GaN, AlN and their alloys, offers a substantial potential in developing ultra-high efficiency photovoltaics mainly due to its wide range of direct-bandgap (0.7 eV -- 3.4 eV), and other electronic, optical and mechanical properties. However, this novel InGaN material system poses technological challenges which extended into the performance of InGaN devices. The development of wide-band gap p--n InGaN homojunction solar cells with bandgap < 2.4 eV is investigated in the present work. The growth, fabrication and characterization of a 2.7 eV bandgap InGaN solar cell with a 1.73 eV open-circuit voltage is demonstrated. Limited solar cell performance, in terms of short-circuit current and efficiency, is observed. The poor performance of the InGaN solar cell is related to the formation of extended crystalline defects in InGaN epilayers of the solar cell structure. To investigate the influence of extended crystalline defects on InGaN epilayer properties, a few In0.12Ga0.88N epilayers with different thicknesses are grown and characterized for structural properties using high-resolution X-ray diffraction. The structural parameters, modeled as mosaic blocks, indicate deterioration in InGaN crystal quality when the film thickness exceeds a critical layer thickness. An associated increase in density of threading dislocations with deteriorated InGaN crystal quality is observed. The critical layer thickness is determined for a few InGaN compositions in the range of 6 -- 21 % In, and it decreases with increasing InGaN composition. Surface roughening and formation of V-defects are observed on InGaN surface beyond the critical layer thickness. An Urbach tail in optical absorption of InGaN epilayer is observed and it is related to the formation of V-defects. The direct consequence of light absorption via V-defects is a decrease in photoluminescence peak intensity with increasing InGaN epilayer thickness beyond critical layer thickness. Two p-i-n InGaN solar cell structures were designed, with InGaN epilayer thickness in one solar cell greater than the critical layer thickness and the other with a lower thickness, to investigate the influence of V-defects on performance of the solar cells. The photoresponse of the p-i-n InGaN solar cell with thicker InGaN epilayer is poor, while the other solar cell had good photoresponse and external quantum efficiency. Extending this investigation to a p-n InGaN solar cell, a solar cell with total InGaN epilayer less than the critical layer thickness is grown. The photoresponse and external quantum efficiency of the present solar cell is superior compared to the initially designed p-n InGaN homojunction solar cells. Solar cell characteristics without p-GaN capping layer in the above p-n InGaN solar cell are also investigated. Good open-circuit voltage is observed, but the short-circuit current and efficiency are limited by the formation of extended crystalline defects, as observed with other initial solar cell designs. A processing sequence is developed to coat III-nitride sidewalls, created during fabrication to form electrical contacts, with SiO2 to maximize the active device area and minimize accidental damage of solar cell during fabrication. Additionally, deposition of current spreading layers on p-type III-nitride epilayer to reduce the series resistance is evaluated. The III-nitrides are primarily grown on sapphire substrate and in a continued effort they are realized later on silicon substrate. InGaN solar cell structures were grown simultaneously on GaN/sapphire and GaN/silicon templates and their photoresponse is compared.
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    ABSTRACT: Charge conjugation C is one of the fundamental symmetries in nature which transforms particles into antiparticles. This symmetry was studied in weak interaction where it is fully violated, but it is poorly known in the strong and electromagnetic interactions. To this end, in this thesis we investigated eta->pi+pi-pi0 and eta->pi0 e+e- decays, which might violate charge conjugation symmetry. The violation of C symmetry in eta->pi+pi-pi0 process could manifest itself as an asymmetry between energy spectra of charged pions. The eta->pi0 e+e- decay is forbidden by C symmetry in the first order of electromagnetic interaction, with the branching ratio on a level of 10^-8. Therefore, observation of a larger branching ratio could indicate a mechanism involving first order electromagnetic interaction violating charge conjugation. Both decays were investigated by means of the WASA-at-COSY detector operating at the COSY synchrotron at the Forchungszentrum Julich in Germany. The eta meson was produced via pp->pp eta reaction at the proton beam momentum of 2.14GeV/c. In total around 5x10^7 eta mesons were collected. As a result of the analysis conducted in the framework of this thesis a Dalitz Plot distribution for the eta->pi+pi-pi0 decay was obtained. From this distribution we extracted asymmetry parameters sensitive to C symmetry violation for different isospin values of the final state and we have established that all are consistent with zero within the obtained accuracy. For the eta->pi0 e+e- decay we have not observe signal and thus we estimated an upper limit for the branching ratio. The established upper limit amounts to $BR(eta->pi0 e+e-)<3.7x10^-5 at the 90% confidence level. This result is more precise than previously obtained in other experiments.
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    ABSTRACT: One of the objectives of the vast physics programme of the recently comissioned WASA-at-COSY facility is the study of fundamental symmetries via the measurements of the eta and eta-prime mesons decays. Especially interesting are isospin violating hadronic precesses of these mesons into 3pi systems driven by the term of QCD Lagrangian which depends on the mass difference of the u and d quarks. When an eta or an eta-prime meson is created in the hadronic reaction signals from such decays may be significantly obscured by the prompt production of pi mesons. In this thesis we present the estimation of the upper limit of the background due to prompt pion production for the eta-prime ->3pi0 and eta-prime->pi+pi-pi decays. Using the data from proton-proton collisions measured by the COSY-11 group we have extracted differential cross sections for the multimeson production with the invariant mass corresponding to the mass of the eta-prime meson. Based on these results and on parametrizations of the total cross sections for the eta-prime meson as well as parametrization of the upper limit for the prompt pi+pi-pi0 production in the collisions of protons we discuss in details the feasibility of a measurement of the eta-prime meson decay into 3pi channels with the WASA-at-COSY facility. Based on the chiral unitary approach the value of the branching ratio BR(eta-prime->pi+pi-pi0) was recently predicted to be about 1%. We show that the WASA-at-COSY has a potential to verify this result empirically.


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