In recent years, luminescent materials have been proposed as active elements for smart sensors to measure structural damage using triboluminescence (TL) and to monitor radiation fluence. To sense structural damage, these materials are embedded in a composite host structure for various applications, such as crew launch or exploration vehicles. When the damage/fracture occurs in the host composite structure, it will lead to the fracture of the TL crystals resulting in light emission. This will provide a warning, in real time, that structural damage has occurred. The TL emission of the candidate material has to be sufficiently bright, so that the light signal reaching from the point of fracture to the detector, through a fiber optics cable, is strong enough to be detected. The majority of the known TL materials do not emit light with sufficient intensity to allow detection with compact and inexpensive detectors. Over the past 40 years, some materials have been reported that TL with sufficient intensity for the light emission to be easily observed with the naked eye. Out of these TL materials, only few could satisfy the above criterion. Europium tetrakis dibenzoylmethide triethylammonium (EuD₄TEA), one of the brightest known TL materials and is a potential candidate for application to this type of hybrid smart sensor.

Space radiation poses a significant challenge to human and robotic exploration missions to deep space. To be used in space, any intelligent sensor material must also be resistant to ionizing radiation. Sources and types of such radiation include galactic and solar electrons and protons, x-rays, ultraviolet light, and magnetically trapped charged particles in radiation belts. While much research has been devoted to characterizing material degradation due to electron exposure, surprisingly little work seems to have been done on proton damage. Research has shown that proton bombardment in the keV to MeV range, like that occurring in space, reduces the intensity of fluorescence. Thus, proton irradiation will likely reduce the ability of a candidate smart material to emit fluorescence.

Practical space-based sensors based on luminescence will depend heavily upon research investigating the resistance of these materials to ionizing radiation and the ability to anneal or self-heal from damage caused by such radiation. In 1951, Birks and Black showed experimentally that the fluorescence efficiency of anthracene bombarded by alphas varies with total fluence (N) as (I/I₀) = 1/(1 + AN), where I is the luminescence yield, I₀ is the initial luminescence yield, and A is a constant. The half brightness fluence (N_1/2) is a material figure of merit and is defined as the reciprocal of A. Broser and Kallmann developed a similar relationship to the Birks and Black equation for inorganic phosphors irradiated using alpha particles. From 1990 to the present, we have found that the Birks and Black relation describes the reduction in emission yield for every tested luminescent material except lead phosphate glass due to proton irradiation. These results indicate that radiation produced quenching centers compete with emission centers for absorbed energy.

Recently, we measured the N_1/2 for EuD₄TEA irradiated with 3 MeV protons to be about 3 x 10¹⁰ mm^-2. Conversely, ZnS:Mn compounds tested were found to have N_1/2 values for 3 MeV protons that are nearly one thousand times larger (~10¹³ mm^-2). According to Tribble, a spacecraft at 1 AU from the Sun will receive a 1 MeV proton fluence of less than 10¹¹ mm^-2 from a large solar event. Likewise, 1 MeV proton fluences in the Earth’s radiation belts and the Earth-Moon-Sun Lagrange points will be even less than the 10¹¹ mm^-2 value from large solar events. For this reason, EuD₄TEA might be a candidate sensor material to monitor large-scale solar events for astronauts in vehicles flying in near earth space. Since their N_1/2 values are larger, ZnS:Mn compounds are more suited for use as sensors in higher radiation environments or for very long duration space missions. Late this year, a 1U CubeSat payload will be placed in Earth orbit carrying an innovative first generation EuD₄TEA sensor that will be used to monitor radiation fluence. This flight experiment is a natural outgrowth of earlier ground based radiation exposure measurements.

The purpose of this presentation is to present results from research completed in this area from 1990 to present. Particular emphasis will be placed on recent TL and radiation exposure measurements using EuD₄TEA. This groundbreaking research can be used to help determine if EuD₄TEA-based materials can be used as a real-time structural damage and/or radiation sensor for space applications.