Time-resolved luminescence measurements on upconversion phosphors for electron beam sterilization monitoring

We present our recent investigations on time-resolved measurements of alterations in the temporal luminescence decay of upconversion phosphors induced by electron beam treatment. The latter is a promising alternative to low-temperature and dry sterilization of surfaces for sensitive packaging materials. Especially in the food and medical sector regulations concerning sterility are increasingly tightened. For this, a secure proof for electron-beam-assisted sterilization is required. However, no non-destructive and in situ method exists up to now. Our approach to provide a secure proof of sterilization is to place a suitable marker material based on rare-earth-doped phosphors inside or on top of the packaging material of the respective product. Upon electron irradiation the marker material changes its luminescent properties as a function of applied energy dose. We verified the energy dependence by means of time-resolved measurements of the luminescent decay of different upconversion materials. In our experimental realization short laser pulses in the near-infrared range excite the marker material. The emitted light is spectrally resolved in a monochromator, collected via a silicon photo diode, and analyzed with an oscilloscope. As the main results we observe a reduction of luminescence lifetime due to electron beam treatment dependent on the emission wavelength. Hence, the electron beam induces changes in the particles' up- and down-conversion properties from which the applied energy dose can be derived.