The Production of Neutrons by<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>μ</mml:mi></mml:math>-Meson Capture in Lead, Magnesium, and Calcium

Abstract

Measurements are reported of the yield of disintegration neutrons from the capture of stopped negative $\ensuremath{\mu}$-mesons in Pb, Ca, and Mg. A magnetized iron lens is used to separate $\ensuremath{\mu}$-meson capture from other processes giving rise to neutrons associated with telescope anticoincidences. The functioning of the lens is tested experimentally by observation of decay electrons from stopped mesons; as a byproduct, the competition between capture (${\ensuremath{\tau}}_{\ensuremath{-}}=1$ \ensuremath{\mu}sec for Mg) and natural decay (${\ensuremath{\tau}}_{0}=2.15$ \ensuremath{\mu}sec) is confirmed. The telescope coincidence and anticoincidence rates are in excellent agreement with recent absolute determinations of the $\ensuremath{\mu}$-meson spectrum at sea level.With Pb, there is a striking increase in the coherent neutron rate in switching from focusing positive to focusing negative mesons. This confirms the interpretation of earlier experiments without magnetic field as demonstrating neutron emission resulting from $\ensuremath{\mu}$-meson capture. The average number of disintegration neutrons per capture appears to be 1.47\ifmmode\pm\else\textpm\fi{}0.13 (statistical standard error), as compared with the value 2.16\ifmmode\pm\else\textpm\fi{}0.15 (statistical standard error) obtained by Crouch and Sard in an underground experiment. Systematic errors can account for the discrepancy; it is felt that somewhat more weight should be attached to the latter figure. With both Mg and Ca, there is little, if any, neutron production in $\ensuremath{\mu}$-meson capture. The neutron detecting efficiency is the same as for Pb, but the ratio of neutrons detected to mesons captured is (0.6\ifmmode\pm\else\textpm\fi{}0.5) percent for Mg and for Ca, in contrast with (3.1\ifmmode\pm\else\textpm\fi{}0.3) percent for Pb. Despite the poor statistics of these preliminary measurements, it is clear that the average neutron multiplicity from Mg and Ca is considerably smaller than from Pb.