  The model R5912-20MOD photomultiplier tube(PMT) is made for cryogenic
application by Hamamatsu. In this paper, we report on the measurement of
relative quantum efficiency (QE) of this model PMT at liquid argon(LAr)
temperature. Furthermore, a specially designed setup and relevant test method
are introduced. The relative QE is measured in visible wavelengths with the PMT
emerged in high purity nitrogen atmosphere. The results show that the change of
QE at LAr temperature is within about 5% compared with room temperature around
420 nm. However, the QE increases about 10% in the shorter wavelength range and
decreases significantly after 550 nm.
