Subthreshold production of neutral pions with Ar ions of 44 MeV/u

Abstract

The production of neutral pions in nucleus nucleus collisions with ztzt MeV/u Ar projectiles from the accelerator" GANIL has been observed. The angular distributions indicate a collective production process, and the pion spectre reflect properties of the collision dynamics.In proton-proton collisions neutral pions can be produced only above the threshold energy of 280 MeV in the laboratory. In collisions of nuclei a pion production is possible at smaller specific energies either as a consequence of the extra momentum of at least one of the nucleons clue to its intranuclear Fermi motion 1 or alternatively through the cooperative action of several nucleons in the collision, i.e. a collective effect. Previous pion production data ~'3 from subthreshold nucleus-nucleus collisions ranging from 183 down to 85 MeV/u were explained by Fermi motion enhanced N-N-collisions. Subsequently it was demonstrated" that at energies as low as 85 MeV/u and below N-N-contributions to the pion production should be rather small and that they depend strongly on the far tails of the Fermi distribution and on the details of the extrapolation into the region off the energy shell.Additionally the Pauli principle blocks the creation of a pion when the two colliding nucleons do not have enough momentum left to find an empty cell in phase space; this effect was shown 1 to cut off any pion creation below about 50 MeV/u. Pion production at even lower energies thus would strongly indicate the existence of collective effects in the collision dynamics. Pion spectra and angular distributions should thus contain information about the details of the collision dynamics and especially they should help to answer the question, how the energy of the relative motion is converted into internal energy, which eventually is high enough to lead to pion production.The experiments were performed at the "~ beam of the two coupled isochronous cyclotrons (GANIL) at Caen, which came into operation recently, We used a beam of 44 MeV/u at an average intensity of 109 particles/s. The integrated dose, as measured in a Faraday cup with an accuracy of 10%, was about G ~ 1013 projectiles on each of the three targets Ca, Sn and U, which had a thickness of 30 mg/cm 2, determined by weighing to within 5%. The very low pion production rates required a high detection efficiency; we obtained more than 1% for neutral pions by observing the two decay

Neutral pion production in subthreshold heavy ion collision with16O (38 MeV/u) and20Ne (200 MeV/u) beams

“…The observed cross sections and angular distributions agree roughly with an interpretation in terms of various statistical descriptions or by assuming cooperative actions of several nucleons of the target and projectile [7][8][9][10][11][12]. On the other hand data obtained at 44, 35 and 25 MeV/u [13][14] are consistent only with a more collective production mechanisms. Due to experimental difficulties in the detection of very low energy charged pions [15], only rc ~ inclusive data are available at low energy beams (<40 MeV/u).…”

Deep-Subthresholdηandπ0Production Probing Pion Dynamics in the ReactionA<

“…This scaling has been observed between p 0 and h mesons at near-threshold energies ( f ϳ 0.5 1.0) [6,20,21]. However, the h meson multiplicity at 180A MeV ( f h 0.14) is more than 1 order of magnitude lower than the p 0 multiplicity at the corresponding energy (40A MeV, f p 0.14) [23,24].…”

Light charged particles associated with subthreshold neutral pion emission in the16O+27Al reaction

“…For neutral pions, several inclusive experiments carried out at energies 25-100 MeV/nucleon have been reported [4][5][6][7][8][9]. Correlations between light charged particles and charged pions have already been studied around 100 MeV/nucleon, in experiments where the pions were detected by a range telescope and the associated light particles were measured in large area multidetectors [10][11].…”