  We developed a new method for determining the bulk etch rate velocity based
on both cone height and base diameter measurements of the etched tracks. This
method is applied here for the calibration of CR39 and Makrofol nuclear track
detectors exposed to 158 A GeV In^{49+} and Pb^{82+} ions, respectively. For
CR39 the peaks corresponding to indium ions and their different fragments are
well separated from Z/beta = 7 to 49: the detection threshold is at REL ~ 50
MeV cm^2 g^{-1}, corresponding to a nuclear fragment with Z/beta = 7. The
calibration of Makrofol with Pb^{82+} ions has shown all peaks due to lead ions
and their fragments from Z/beta ~ 51 to 83 (charge pickup). The detection
threshold of Makrofol is at REL ~ 2700 MeV cm^2 g^{-1}, corresponding to a
nuclear fragment with Z/beta = 51.
