The $n n$ Scattering Length From the Photon Spectra of the Reactions $\pi^- d \to \gamma n n$ and $\pi^- p \to \gamma n$

A high-statistics photon spectrum from the reaction ..pi../sup -/d ..-->.. ..gamma..nn, measured with a 0.72-meV--resolution pair spectrometer is compared with different theoretical spectra folded with the measured photon line from the reaction ..pi../sup -/p ..-->.. ..gamma..n. A value of a/sub n/n = -18.5 +- 0.5 fm (theoretical error included) is deduced assuming a fixed value of r/sub n/n = 2.8 +- 0.1 fm. The problem of extracting r/sub n/n using the present theories is indicated.