Measurements of double photoelectron emission ( DPE ) probabilities as a function of wavelength are reported for Hamamatsu R8778 , R8520 , and R11410 VUV-sensitive photomultiplier tubes ( PMTs ) .
In DPE , a single photon strikes the PMT photocathode and produces two photoelectrons instead of a single one .
It was found that the fraction of detected photons that result in DPE emission is a function of the incident photon wavelength , and manifests itself below \sim 250 nm .
For the xenon scintillation wavelength of 175 nm , a DPE probability of 18–24 % was measured depending on the tube and measurement method .
This wavelength-dependent single photon response has implications for the energy calibration and photon counting of current and future liquid xenon detectors such as LUX , LZ , XENON100/1T , Panda-X and XMASS .