This study involves the upgrade of a high-resolution position-sensitive detector (HR-PSD) installed on the small-angle neutron scattering spectrometer (SANS-U) at the Japan Atomic Energy Agency. By using both neutron lenses and the HR-PSD, the accessible low-Qlimit can be extended to the order of 10−4 Å−1[Qis the magnitude of the scattering vector defined byQ= (4π/λ)sinθ, where λ and 2θ are the wavelength and the scattering angle, respectively]. The HR-PSD consists of a cross-wired position-sensitive photomultiplier tube (PSPMT) and a commercial ZnS/6LiF scintillator. To improve the experimental efficiency of focusing small-angle neutron scattering (FSANS) experiments, a high-performance ZnS/6LiF scintillator developed at the Japan Atomic Energy Agency has been utilized. For the PSPMT and data-acquisition system installed on SANS-U, the thickness of the ZnS/6LiF scintillator was optimized by measuring the thickness dependence of the pulse-height spectra. Under the experimental conditions of SANS-U, the optimum thickness of the ZnS/6LiF scintillator (ZnS:6LiF = 2:1) was determined to be 0.433 mm by measuring the total counts and peak positions of the pulse-height spectra. Installation of the optimized ZnS/6LiF scintillator improved detection efficiency by 1.39 times over that of a commercial scintillator at the same level of background counts andQresolution in FSANS experiments.