Using Video Photometry to Measure the 3 MeV Proton Half Brightness Fluence for Tetrakis (Dibenzoylmethide) Europium (III) Triethylammonium
Authors: Hollerman William A , Miller John , Williams Stephen 
Published: December 2020

In recent years, luminescent materials have been proposed for use as smart sensors to detect structural damage using triboluminescence, measure incident radiation fluence using radioluminescence, and monitor surface temperature using phosphor thermometry. To sense structural damage using triboluminescence, appropriate materials are embedded in a composite host structure for various applications. When the damage occurs in the host structure, it leads to the fracture of the triboluminescent crystals resulting in light emission. This process provides a real-time warning 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 triboluminescent materials do not emit light with sufficient intensity to allow detection with compact and inexpensive detectors. Over the past forty years, some materials have been reported with TL of sufficient intensity for the light emission to be easily observed with the naked eye. Out of these triboluminescent materials, only few could satisfy the above criterion. Tetrakis (Dibenzoylmethide) Europium (III) Triethylammonium (EuD4TEA), one of the brightest known TL materials and is a potential candidate for application to this type of hybrid smart sensor in extreme environments. Recent measurements by the authors indicate that the decay time for EuD4TEA is bi-exponential and can be used to measure temperatures from -10 to 80 °C.

Ionizing radiation in space poses a significant challenge to future human and robotic exploration missions. Any intelligent sensor material used in space 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 (N1/2) 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.

In 2016, the N1/2 for EuD4TEA irradiated with 3 MeV protons was measured to be about 3 x 1010 mm-2. Conversely, ZnS:Mn compounds tested were found to have N1/2 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 N1/2 values are larger, ZnS:Mn compounds are more suited for use as sensors in higher radiation environments or for very long duration space missions.

The purpose of this presentation is to present preliminary results from a 1U CubeSat payload using a EuD4TEA-based sensor to measure ionizing radiation in Earth orbit. This flight experiment is a natural outgrowth of earlier ground-based radiation exposure measurements that will be used to determine if EuD4TEA-based materials can be used as a real-time sensor for applications in space.