Neutrons from nuclear capture of negative pions

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Abstract
We measured the energy spectra and yields of neutrons above 1.2 MeV from nuclear capture of negative pions slowing down and stopping in C, N, O, Al, Cu, Ta, and Pb targets. Each neutron spectrum was decomposed into an evaporation portion and a direct portion. The number of direct neutrons per stopped pion is substantially constant for all the targets with a mean value of 1.74±0.28, whereas the yield of evaporation neutrons increases by an order of magnitude from about 0.7 for carbon to about 7 for lead. The total kinetic energy carried away by the neutrons is substantially constant also for all the targets with a mean value of 76.7±2.0 MeV. The kinetic energy carried away by evaporation neutrons increases with mass number with an associated decrease in the kinetic energy carried away by direct neutrons. We obtained the nuclear temperature (in MeV) of the first residual nucleus formed in the evaporation process for each target: Pb (1.4), Ta (1.4), Cu (1.9), A1 (2.3), O (2.2), N (2.2), and C (2.1). The spectrum from a light target displays a shoulder in the region around 60 MeV which is consistent with a two-nucleon absorption mechanism. The direct neutron spectra are characterized by a rising yield as the neutron energy decreases in the region from 30 MeV down to about 10 MeV. The data are compared with previous measurements and with the predictions of intranuclear cascade and preequilibrium calculations.