Dr Barbara Melon (University and INFN Firenze)
We have investigated the ability of AGATA modules to measure the linear polarisation of gamma rays, exploiting the dependence of the Compton scattering differential cross section on the azimuthal angle. To this aim, partially polarised gamma rays have been produced by Coulomb excitation of the first excited state of 104Pd and 108Pd which deexcite to the ground state by emission of gamma rays of 555.8 keV and 433.9 keV, respectively. The position of the Agata array was chosen to select gamma rays at angles not far from 90 degrees to the beam direction. The azimuthal distributions, with respect to the reaction plane, of the first Compton scattering for a properly selected sample of these gamma rays have been evaluated and compared with the corresponding distribution for the unpolarised 661 keV gammas from a 137Cs source. The instrumental distortions in the measured distributions appear to cancel almost exactly in the ratio R_phi of the COULEX data to those of the 661 kev gammas, and a clear signal of linear polarisation becomes apparent. The amplitude of the cos(2 phi) modulation has been compared to the theoretical linear polarisation of gamma rays from Coulomb excitation (calculated with the help of the GOSIA code) to deduce the experimental analysing power, which turns out to be about 45% in both cases. A ``theoretical'' value of the average analysing power has been deduced from the values calculated, for each of the selected events, as a function of the Compton scattering angle, taking into account the experimental uncertainty on the coordinates of the interaction points. A satisfactory agreement between theoretical and experimental values has been found.
Dr Barbara Melon (University and INFN Firenze) Prof. Piergiorgio Bizzeti (University and INFN Firenze) Pietro Sona (University and INFN Firenze)
Adriana Nannini (INFN Firenze) Andrea Gottardo (INFN Legnaro) Prof. Andrea Perego (Università and INFN Firenze) Dr Andres F. Gadea Raga (IFIC CSIC-University of Valencia) Dr Anna Maria Bizzeti - Sona (Univ. and INFN Firenze) Calin Alexandru Ur (INFN Padova) Caterina Michelagnoli (University and INFN Padova) Daniel Ricardo Napoli (INFN Legnaro) Daniele Mengoni (University and INFN Padova) Dino Bazzacco (INFN Padova) Ms Eda Sahin (LNL) Enrico Farnea (INFN Padova) Francesco Recchia (University and INFN Padova) Giacomo De Angelis (INFN Legnaro) Jose' Javier Valiente Dobon (INFN Legnaro) Roberto Menegazzo (INFN Padova) Prof. Santo Lunardi (University and INFN Padova) Prof. Silvia Monica Lenzi (University and INFN Padova)