Measurement of the imaginary part of theI=1N¯N S-wave scattering length

The survival time spectrum of slow antineutrons produced in a liquid-hydrogen target has been measured. From these data the imaginary part of the I=1 spin-averaged S-wave antineutron proton scattering length has been deduced to be Im$a_1$=-0.83±0.07 fm. The result lies within the range of values calculated from current potential models. In addition, by combining $a_1$ with the antiproton-proton scattering length deduced from antiprotonic atoms, the imaginary part of the I=0 spin-averaged N¯N scattering length was calculated to be Im$a_0$=-1.07±0.16 fm.