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Experimental study of pionic fusion ^4He(^3He, π^0)^7Be

Pionic fusion reactions where two nuclei fuse into a new one by emitting a pion need a highly coherent mechanism and therefore give an insight into the collective motion of nucleons in nuclei. The reaction ^4He(^3He, π^0)^7Be has been studied experimentally at beam energies about 10 MeV above the coherent production threshold. The experiment was performed at KVI, Groningen using the AGOR accelerator facility. The created nuclei were detected in a phoswich detector array in the focal plane of the Big-Bite Spectrometer in coincidence with the photons from neutral pion decay which were detected in the Plastic Ball detector. The Plastic Ball contains 552 phoswich detectors covering polar laboratory angles 50^o<θ<160^o in almost the full azimuthal range. To improve the photon detection efficiency an active inner shell of 64 CsI(Tl) crystals is put inside the Plastic Ball. The fused nuclei are identified by a pulse-shape analysis and a time-of-flight measurement. They are clearly correlated with two photons detected in the Plastic Ball at mutually large opening angles corresponding to pion decay. The event selection yields approximately 500 background-free pionic fusion events. The pion angular distribution is corrected for the acceptance and the efficiency obtained from Monte Carlo simulations and normalized with the beam current. The Legendre polynomial fit gives the total cross section σ=(53±9) nb.

pionic fusion; subthreshold pion production; CsI(Tl) scintillator; phoswich; magnetic spectrometer

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