nuclear physics in poland 1996 – 2006

ANTIPROTONIC ATOMST. Czosnyka 1 , K. Gulda 2 , J. Iwanicki 1 , J. Jastrzębski 1 , M. Kisieliński 1 , B. Kłos 3 ,W. Kurcewicz 2 , P. Lubiński 1 , P. Napiorkowski 1 , L. Pieńkowski 1 , R. Smolańczuk 4 ,A. Trzcińska 1 , S. Wycech 41 Heavy Ion Laboratory, Warsaw University, Warszawa2 Institute of Experimental Physics, Warsaw University, Warszawa3 Institute of Physics, University of Silesia, Katowice4 A. Sołtan Institute for Nuclear Studies, WarszawaExperimental facility: Low Energy Antiproton Ring (LEAR) at CERNThe antiproton-nucleon interaction is verystrong. Therefore antiprotons interacting withatomic nuclei are absorbed and annihilate alreadyat the nuclear periphery, where the nucleondensity is significantly smaller than the centralnuclear density. For sufficiently slow antiprotonsthe annihilation takes place after the antiprotonicatom has been formed. In this case the spatialdistribution of the antiproton wave function iswell determined and one can imagine that theannihilation “signals” (whatever they are) couldperhaps be used to test the extent and thecomposition of the nuclear surface.Beginning more than ten years ago wehave performed an experimental study of themedium-heavy antiprotonic atoms using the slowantiproton beam from Low Energy AntiprotonRing (LEAR) at CERN. The main objective of ourprogram was to obtain information on theneutron distributions at the nuclear periphery andto provide data useful in deducing the antiprotonnucleusoptical potential parameters.Two experimental methods wereemployed. First, using the so called“radiochemical method” we have investigated [1-4] the ratios of peripheral neutron to protondensities at distances around 2.5 fm larger thanthe nuclear charge half-density radius [5]. Themethod consisted in measuring the yield ofradioactive nuclei having one proton or oneneutron less than the target nucleus, producedafter antiproton capture, cascade and annihilationin the target antiprotonic atom. The experimentyielded 19 density ratios (determining the socalled “halo factors”, see Ref. [1] for definition)subsequently employed to deduced the shape ofthe peripheral neutron distribution.The second method consisted inmeasurements of the antiprotonic atom levelwidths and shifts due to the strong antiprotonnucleusinteraction. These observables aresensitive to the interaction potential whichcontains, in its simplest form, a term dependingon the sum of the neutron and proton densities.The level widths and in a number of cases also thelevel shifts were measured for 34 antiprotonicatoms (in some cases for different isotopes of thesame element).The rich harvest of the two employedmethods, sensitive to the neutron and protondensity ratio and the sum of these densities hasallowed to derive a number of systematicconclusions on the neutron distributions in nuclei[6-10]. Moreover, our data [11-18] were used todetermine the antiproton-nucleus optical modelparameters through global fits of p X-rays andhalo factors with a substantially larger and moreprecise database than employed in previousapproaches (see Nucl. Phys. A761, 283 (2005) andRef. [10]).Figure 1 presents the deduced neutronprotonrms radii difference obtained from theanalysis of the antiprotonic atom data.Fig. 1. Difference ∆r np between the rms radii of the neutron andproton distributions as deduced from the antiprotonic atom X-raydata, as a function of δ=(N-Z)/A.47