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Experiments on Exotic Atom Spectroscopy Exp eriments on exotic atom sp ectroscopy D Gotta Institut fur Kernphysik Forschungszentrum Julic h D Julic h Germany Abstract Results are reviewed from recent exotic atom exp eriments using a fo cussing Bragg sp ectrometer The measured energies of the L Xrays from antiprotonic hydrogen and deuterium and the K transition from pionic deuterium are compared to the transition ener gies as calculated from quantum electro dynamics in order to determine the strong interaction antiprotonnucleon and pionnucleon scattering parameters at threshold A high precision exp eriment to determine the mass of the charged pion to ppm has b een started by investigat ing the transitions in pionic nitrogen where the o ccurance of Coulomb explosion has b een seen now directly from the Doppler broadening In a forthcoming high precision exp eriment of the pionnucleon scatter ing length new calibration metho ds using an ECR source will b e used to connect energy standards b oth from exotic and electronic atoms Such techniques oer the p ossibility of high precision studies in few electron systems Intro duction Atoms in which the electron shell is replaced partly or in total by other elementary particles of negative charge are called exotic atoms Up to now the systems with muons pions kaons antiprotons and sigma hyp erons have b een established Muonic and pionic atoms are b eing investigated since years at the meson factories of the PaulScherrerInstitut PSI TRI UMF or earlier at LAMPF At CERN in the years until the LowEnergyAntiprotonRing LEAR provided highquality lowenergy an tiproton b eams which were esp ecially suited for stop exp eriments and thus for the formation of antiprotonic atoms Normally the observables in the Xray sp ectroscopy of exotic atoms are the energies and the intensities of the radiation which is emitted during the deexcitation of the atomic system From the line intensities conclusions are drawn b oth for the capture pro cess and the interaction with the remaining part of the electron cloud and pro cesses originating from the interaction with neighb ouring target atoms during the rst steps of the atomic cas cade In the lower part of the cascade the exotic atom can b e prepared as a hydrogenlike system when suitable exp erimental conditions are chosen Such hydrogenlike atoms may then serve as a testing ground for the calcula tions of transition energies and to investigate the prop erties of the captured particle itself At smaller distances ie for the lowlying states the in uence of the nite nuclear size on the binding energy increases and for strongly interacting particles the eects of the nuclear forces b ecome more and more visible Of sp ecial interest are the elementary systems formed with the hydrogen isotop es to study the strong pion and antiprotonnucleon in teraction at threshold Thus the Xray sp ectroscopy of exotic atoms oers access to a large variety of problems The precise calculation of the binding energies has to use the relativistic description as given by the Dirac or the KleinGordon formula and in addi tion has to include the contributions of quantum electro dynamics QED relativistic recoil corrections and p olarisation eects Due to the large mass of the orbiting particle the predominant QED contribution in exotic atoms is vacuum p olarisation in contrast to self energy as in the case of electronic atoms Highest energy resolution for Xrays in the few keV range is achievable only with crystal sp ectrometers which however on principle are devices of low eciency Although high uxes are reached in the b eam lines of the meson factories the exp erimental challenge is to achieve a high resolution together with sucient count rates as well as to achieve stable conditions for the nevertheless unavoidable long measuring p erio ds It is essential to use the particle ux provided by the accelerator in an optimal way and to maximize b oth the line yields of the Xray transitions and the detection ecieny up to the theoretical limits In particular for the atoms formed with the hydrogen isotop es high line yields can b e obtained only with gas targets A crystal sp ectrometer however requires a small and bright Xray source which cannot b e achieved with the usual linear setup to stop the b eam Sucient stop densities are available if for degrading and stopping of the b eam the cyclotron trap is used For such exp eriments with low counting rates an ecient background suppression is essential It is achieved by the use of ChargeCoupledDevices CCDs as Xray detectors Figure Formation and atomic cascade of exotic atoms After slowing down to a kinetic energy of a few eV negatively charged par ticles are captured in the Coulomb eld of atoms into highly excited atomic levels by the emission of electrons The depletion of the electron cloud pro ceeds shell by shell by continued emission of Auger electrons as so on as the energy dierence b etween two exotic atom levels exceeds the ionisation energy Thus the lighter exotic atoms in the end b ecome hydrogenlike systems provided no electron relling o ccurs as is the case for dense or solid targets In the lower part of the atomic cascade the deexcitation by emission of Xray quanta dominates even in the presence of electrons The dimensions of exotic atoms are obtained by replacing the electrons mass in the expression for the Rydb erg energy by the reduced mass m of r ed the particlenucleus system During the last steps of the atomic cascade the extensions of exotic atoms are already closer to nuclear dimensions than to the atomic scale There in the case of hadrons the electromagnetic deexci tation then has to comp ete with channels op ened by the strong interaction A detailed discussion of the atomic cascade may b e found in Exotic hydrogen o ccupies a sp ecial p osition regarding the interaction with the target material It is electrically neutral and therefore can easily p enetrate the Coulomb eld of neighb ouring atoms This leads to a drastic reduction of the Xray yields in pionic and antiprotonic hydrogen Day SnowSuchereect b ecause in the high angular momentum states now s and pwave contributions are induced and in hadronic atoms an admixture of s or pwaves leads with high probability to annihilation For atomic levels with large n the Stark mixing prevails all other internal and external deexcitation pro cesses Hence for hydrogen the use of low pressure gas targets is indisp ensible to achieve high Xray yields Exp eriment Cyclotron trap and Xray source A fo cussing Bragg crystal sp ectrometer requires a small and bright Xray source having an extension matched as go o d as p ossible to the extension of the line pattern to b e observed Such conditions are achieved when the cyclotron trap is used a device which was designed to obtain high stop densities with muon pion and antiproton b eams The range curve of the b eam is wound up in a magnetic eld with fo cussing prop erties which is pro duced by two sup erconducting coils Between the coils the target chamb er is installed containing a suitable degrader arrangement and the target gas The particles lo ose kinetic energy rst in the degraders and in the target gas itself thus approaching the center of the magnet on spiral 3 orbits and nally are stopp ed in a volume of a few cm even at low gas pressures Crystal sp ectrometer For Xradiation with energies b elow keV b ecause of the absorption losses reection typ e crystal sp ectrometers are used The Xrays are scattered at the electrons of atoms arranged regularly in lattice planes having the dis tance d According to Braggs law n dsin a coherent sup erp osition B for one wave length o ccurs only at the Bragg angles where n is the B order of reection The width of the angular distribution of a parallel b eam ro cking curve width reected from a plane crystal yields consequently the theoretical limit for the energy resolution It is given only by the intrinsic prop erties of the material and the wave length Usually quartz or silicon crystals are used which b ehave like ideal crystals The intrinsic prop er ties can b e calculated from the dynamical theory to one p er cent or b etter except in the close vicinity of absorption edges For the investigation of broad Xray lines or multiplets horizontally b ent crystals are of great advantage Johann geometry b ecause here an energy interval according to the width of the source can b e recorded simultaneously with twodimensional p ositionsensitive detectors Thus an acceptable duration of measuring p erio ds of a few days p er transition b ecomes p ossible To achieve in addition partial vertical fo cussing spherically b ent crystals were used The reduced height of the image then allows the ecient use of the rather small pixel detectors like CCDs 6 An accuracy of up to is aimed at for the energy measurement in the describ ed exp eriments which requires an energy resolution of the order of 4 Such values are reached in the few keV range with quartz and silicon crystals having b ending radii of ab out m and Bragg angles larger than 4 degrees where the geometrical abb eration accounts at maximum to and hence do es not ab olish the intrinsic resolution of the crystal lattice 6 The overall eciency of such a sp ectrometer is ab out for crystal areas 2 of cm CCDs are the optimal detectors for recording Xrays in a high back ground environment b ecause of their distinguished twodimensional p osi tion resolution together with an excellent energy resolution E eV at keV Because the charge pro duced by one Xray is dep osited in one or two pixels
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