Scintillating Metal Organic Frameworks: A New Class of Radiation Detection Materials
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Scintillating Metal Organic Frameworks: A New Class of Radiation Detection Materials M. D. Allendorf, R. J. T. Houk, R. Bhakta, I. M. B. Nielsen, F. P. Doty Sandia National Laboratories Livermore, CA 94551-0969 ABSTRACT The detection and identification of subatomic particles is an important scientific problem with implications for medical devices, radiography, biochemical analysis, particle physics, and astrophysics. In addition, the development of efficient detectors of neutrons generated by fissile material is a pressing need for nuclear nonproliferation and counterterrorism efforts. A critical objective in the field of radiation detection is to develop the physical insight necessary to rationally design new scintillation materials for specific applications. However, none of the material types currently used in has sufficient synthetic versatility to exert systematic control over the factors controlling the light output and its dynamics. Here we describe a spectroscopic investigation of two stilbene-based metal-organic frameworks (MOFs) we synthesized, demonstrating that they emit light in response to ionizing radiation, creating the first completely new class of scintillation materials since the advent of plastic scintillators in 1950. This highly novel and unexpected property of MOFs opens a new route to rational design of radiation detection materials, since the spectroscopy shows that both the luminescence spectrum and its timing can be varied by altering the local environment of the chromophore within the MOF. Therefore, the inherent synthetic flexibility of MOFs, which enables both the chromophore structure and its local environment to be systematically varied, suggests that this class of materials can serve as a controlled “nanolaboratory” for probing a broad range of photophysical and radiation detection phenomena. In this presentation we report on the time-dependent fluorescence and radioluminescence of these MOFs and related structures. Multiple decay characteristics have been observed for some materials under study, including fast (ns) exponential and slow (microsecond) non-exponential components. We interpret the results in terms of the electronic states, crystal structures, intermolecular interactions, and transport effects mediating the luminescence. The potential for particle discrimination schemes and large scale production of MOFs and will be discussed. INTRODUCTION Conventional organic scintillators (liquid, plastic, or crystalline) suffer from low and highly nonlinear light yield for recoil protons below 10 MeV, resulting in low signals relative to electrons. This effect contributes to a tradeoff between sensitivity and an effective threshold for gamma rejection near the fission peak [1,2]. Pulse shape discrimination (PSD) is characterized by a figure of merit M, and as demonstrated by Moszynski et al. M degrades rapidly with increasing volume, which is a shortcoming of PSD materials and techniques.[3] In liquid scintillators (NE213, EJ301), the neutron path length for elastic scatt
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