We report a high-precision determination of the S-1(0) neutron-neutron scat
tering length (a(nn)) using the H-2(pi-,n gamma)n reaction. The value obtai
ned in the present work is - 18.50 +/- 0.05(statistical) +/- 0.44(systemati
c) +/- 0.30(theoretical) fm, which is consistent with the values from, prev
ious measurements. Combining our result with previous measurements reduces
the total uncertainty in the world average of a(nn) to +/-0.4 fm, matching
the accuracy to which the charge-symmetric parameter a(pp) is determined. (
C) 1998 Elsevier Science B.V. All rights reserved.