The methods employed and the results obtained from measurements and calculations of the detection efficiency for the neutron detectors used at Triangle Universities Nuclear Laboratory (TUNL) in the simultaneous determination of the 1S0 neutron-neutron and neutron-proton scattering lengths ann and anp, respectively, are described. Typical values for the detector efficiency were 0.3. Very good agreement between the different experimental methods and between data and calculation has been obtained in the neutron energy range below En = 13 MeV. © 2008 Elsevier B.V. All rights reserved.