Alpha-radiation tests on the transmission of cryogenically cooled infrared filter materials used in ISOPHOT

摘要

We have studied potential effects of the ISO spacecraft orbital radiation environment on the transmission of infrared interference filters and filter materials. To simulate the critical proton radiation within the earth radiation belts and its influence on the materials at cryogenic temperatures the samples were cooled to LHe temperature and subjected to an Am-241 $alpha@-radiation source (approximately 4.1 MeV) mounted inside a cryostat. The dose per hour absorbed in a 20 micron thick layer, the mean penetration depth for the $alpha@particles, was about 25 krad (Si). The substrates Ge, ZnSe, and CaF$-2$/ and three tested ISOPHOT interference bandpass filters (3.21-3.37 $mu@m and 2-50 $mu@m even after a total dose of approximately 0.5 Mrad (Si), which is more than 100 times the expected total dose for ISOPHOT. The multilayer interference blocking coating on sapphire used on all ISOPHOT far infrared filters to block the wavelength range 1.7-6.7 $mu@m showed no degeneration either. The organic far infrared antireflex coating materials polyethylene deposited on a quartz substrate, and a 15 $mu@m thick parylene foil used as field lens coating, were investigated in the wavelength range of 16-300 $mu@m. Our data suggests a slightly reduced transmission $LS 6% after 350 krad (Si) exposure.