Like duh. In addition to the pretty Maltese Cross night light, a ~20kV power supply produces 3mR/hour bremsstrahlung from the front of the glass. Try googling 'bremsstrahlung x-rays' for more information. What is the mass of an electron?
(Calculate it from this experiment and references you find from the search, don't just look it up. The answer is 2 x 10^-30 pounds.)