Experimental study of pion capture by hydrogen bound in molecules

An experiment was performed at TRIUMF to study the formation of pionic hydrogen atoms and molecules in solids, particularly in groups of organic molecules of slightly different structure in order to help further clarify the problem. The nuclear capture of pions by hydrogen was measured using the charge exchange of stopped pions. The coincident photons emitted by the decaying {pi}{sup 0} mesons were detected by TRIUMF's two large NaI spectrometers. Experimental results were obtained for the capture probability of stopped {pi}{sup {minus}} mesons in the nuclei of hydrogen atoms, chemically bound in molecules of some simple hydrides, acid anhydrides, and sugar isomers. A possible correlation was found between pion capture in hydrogen and the melting point of sugar isomers. The pion-capture probability in acid anhydrides is fairly well described by a simple atomic-capture model in which the capture probability on the hydrogen dramatically increases as the hydrogen atom is separated from the strongly electronegative C{sub 2}O{sub 3} group. Both effects are consistent with a correlation between pion capture and electron density on hydrogen atoms.