  We have investigated the possibility of calibrating the PMTs of scintillation
detectors, using the primary scintillation produced by X-rays to induce single
photoelectron response of the PMT. The high-energy tail of this response, can
be approximated to an exponential function, under some conditions. In these
cases, it is possible to determine the average gain for each PMT biasing
voltage from the inverse of the exponent of the exponential fit to the tail,
which can be done even if the background and/or noise cover-up most of the
distribution. We have compared our results with those obtained by the commonly
used single electron response (SER) method, which uses a LED to induce a single
photoelectron response of the PMT and determines the peak position of such
response, relative to the pedestal peak (the electronic noise peak, which
corresponds to 0 photoelectrons). The results of the exponential fit method
agree with those obtained by the SER method when the average number of
photoelectrons reaching the first dynode per light/scintillation pulse is
around 1.0. The SER method has higher precision, while the exponential fit
method has the advantage of being useful in situations where the PMT is already
in situ, being difficult or even impossible to apply the SER method, e.g. in
sealed scintillator/PMT devices.
