  Frequency gradient analysis (FGA) effectively discriminates neutrons and
gamma rays by examining the frequency-domain features of the photomultiplier
tube anode signal. This approach is insensitive to noise but is inevitably
affected by the baseline drift, similar to other pulse shape discrimination
methods. The baseline drift effect is attributed to the factors such as power
line fluctuation, dark current, noise disturbances, hum, and pulse tail in
front-end electronics. This effect needs to be elucidated and quantified before
the baseline shift can be estimated and removed from the captured signal.
Therefore, the effect of baseline shift on the discrimination performance of
neutrons and gamma rays with organic scintillation detectors using FGA is
investigated in this paper. The relationship between the baseline shift and
discrimination parameters of FGA is derived and verified by an experimental
system consisting of an americium-beryllium source, a BC501A liquid
scintillator detector, and a 5 GSPS 8-bit oscilloscope. Both theoretical and
experimental results show that the estimation of the baseline shift is
necessary, and the removal of baseline drift from the pulse shapes can improve
the discrimination performance of FGA.
