HADES is a versatile magnetic spectrometer aimed at studying dielectron production in pion, proton and heavy-ion induced collisions. Its main features include a ring imaging gas Cherenkov detector for electron-hadron discrimination, a tracking system consisting of a set of 6 superconducting coils producing a toroidal field and drift chambers and a multiplicity and electron trigger array for additional electron-hadron discrimination and event characterization. A two-stage trigger system enhances events containing electrons. The physics program is focused on the investigation of hadron properties in nuclei and in the hot and dense hadronic matter. The detector system is characterized by an 85 % azimuthal coverage over a polar angle interval from 18• to 85• , a single electron efficiency of 50 % and a vector meson mass resolution of 2.5 %. Identification of pions, kaons and protons is achieved combining time-of-flight and energy loss measurements over a large momentum range. This paper describes the main features and the performance of the detector system.
Rf-spectroscopic measurements in the atomic ground state of the 1.2 sec-isotope 37K yielded independent values of magnetic moment pi(aTK)--0.20320(6)nm (diamagnetically corrected), hfs separation A W(37K)=240.2672 (7)MHz and the resulting hfs anomaly 37A39= (0.249 + 0.035). 10 -2. The 37K was polarized by spin exchange scattering with optically pumped 87Rb; as a detector for rf transitions the asymmetry of the fl-decay of the polarized 37K nuclei was used. A detailed analysis of the spin exchange mechanism under the conditions of experiments of this type was performed. This led to an explanation for the observed strong enhancement of the resonance signal heights by the spin bath. Furthermore the dependence of the nuclear polarization and relaxation time constants on the strength of the hfs coupling could be demonstrated.
21Na(Tu2=23sec ) and 37K(T1/2=l.25sec) have been produced in gas targets by (d, n) reactions and polarized by means of optical pumping or spin exchange scattering with optically pumped 87Rb. An asymmetry up to 3% in the /?-decay of the polarized nuclei was found, which served to detect rf transitions between hfs levels of the atomic ground states of 21Na and 37K. AF= 0 and AF= 1 resonances have been recorded, yielding spin, hfs separation and magnetic moment, especially I(37K)= 3/2, A W(37K)=240.266 (3) Mc/sec and /~I(37K)=0.02033(6) nm (diamagnetically corrected). For 21Na earlier rf-spectroscopic data have been confirmed.
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