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Published: 1962

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Effects due to elementary particle-like collisions within nuclear matter have been observed in several nuclear reactions caused by pions and protons. Simple nuclear reactions of the form ZA(a, an)Z/sup A-1/ and Z/sup A/(a, ap)(Z-1)/ sup A-1/ have excitation functions that are sensitive to changes in the elementary-particle cross sections. The excitation function for the reaction C/ sup 12/(pi /sup -/, pi /sup -/n)C/sup 11/ is measure d from 53 to 1610 Mev by bombarding targets of plastic scintillator with pions. The intensity of the pion beam is monitored with a two-counter telescope and 40 Mc scaling system. The scintillator target is mounted on a phototube and becomes the detector for the carbon-11 positron activity. Corrections are made for muon contamination in the beam, coincidence losses in the monitor system, carbon-11 activity produced by stray background at the accelerator, carbon-1l activity produced by secondaries in the target, and the efficiency of the carbon-11 detection system. The C/sup 12/(pi /sup -/, pi /sup -/n)C/sup 11/ cross sections rise to a peak of abo ut 70 mb at 190 Mev, that corresponds to the resonance in freeparticle pi /sup -/n scattering at 190 Mev. Calculations based on a knock-on'' collision mechanism and sharp-cutoff nuclear density reproduce the shape of the experimental excitation function, but the magnitudes of the calculated values are low by a factor of six, The calculation shows that the C/sup 12/(pi /sup -/, pi /sup -/ n)C/sup 11/ reaction occurs in the nuclear s urface region at all bombarding energies, The contributions to the (pi /sup -/, pi /sup -/n) reaction predominate on the front surface of the nucleus in order to give the pion the maximum probability of escaping. The excitation functions for the reactions ured radiochemically from 400 Mev to 6.2 Bev. The slight increase in the (p,2p) cross sections measured from 400 to 720 Mev is related to the occurrence of a quasi- free-particle pp colliaion within the nucleus. Due to the proton momentum distribution, the increase is not as pronounced as the rise in freeparticle pp total cross sections from 400 to 1000 Mev. From 2.2 to 8.2 Bev, the Zn/sup 68/ (p,2p)Cu/sup 67/ and Fe/sup 67/(p,2p)Mn/sup 5 cross sections are constant at 21 plus or minus 2 mb and 50 plus or minus 8 mb, respectively. The difference in magnitudes of the (p,2p) cross sections is ascribed to the availability of only two protons in zinc-68 and of six protons in iron-57 for this particular reaction. The free-particle effects are not seen in more complex reactions as evidenced by the constant cross sections from 0.72 to 6.2 Bev for the yields of manganese-51 and -52 from the reactions of protons with iron and for the yields of copper-61 and -84 from the reactions of protons with zinc. Cross sections are presented for a few products from pion-induced reactions requiring several nucleons to be emitted. The yields of manganese-58 and -52 and iron-52 from the bombardment of natural copper by negative pi meson, the yields of copper-67, -61, and -64 and manganese-52 and -56 from the negative pi meson bombardment of natural zinc, and the cross section for sodium-24 from negative pi meson + aluminum-27 are compared to the yields for similar reactions induced by protons. At the high energies considered here, interaction of negative pi meson with these targets is shown to give yields that are of the same order of magnitude as the yields from proton reactions. These results are interpreted as experimental evidence that pion processes are very important for energy transfer in high-energy nuclear reactions. 99 references. (auth).