Measurement of the half brightness fluence for ZnS:Mn due to 20 keV electron irradiation

In early 2005, a series of pressed powder pellets and crystal samples were prepared to determine the radiation hardness due to electron bombardment for candidate phosphors. Several pressed samples composed of a zinc sulfide doped with manganese (ZnS:Mn) powder and a steric acid binder were included in this test series to measure fluorescence efficiency as a function of 20 keV electron fluence. This preliminary "half brightness fluence" (N₁2/) data can be used as a figure of merit to help determine if phosphors like ZnS:Mn can be used in a space environment. The 20 keV electron half brightness fluence measurement was performed using the low energy electron (LEE) system located at the NASA Marshall Space Flight Center (MSFC) in Huntsville, Alabama. Electron current from a Kimball Physics gun was measured using a 0.125 inch diameter Faraday cup. Each test sample (half inch in diameter) was irradiated through a 0.375 inch concentric hole, which provides collimation of the incident electron beam. The ZnS:Mn irradiation data for 20 keV electrons follow the standard Birks and Black expression. The resulting half brightness fluence for 20 keV electron bombardment on ZnS:Mn was measured to be (0.82 ± 0.02) × 10¹⁴ mm^-2, which is similar to the value measured using 3 MeV protons in 2003. It is possible that the pressure used to press the sample tablet damaged some of the ZnS:Mn emission centers, thus reducing N₁2/.