Transparent lithiated polymer films for thermal neutron detection Andrew N. Mabe a,n , John D. Auxier II a , Matthew J. Urffer b , Dayakar Penumadu c , George K. Schweitzer a , Laurence F. Miller b a Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA b Department of Nuclear Engineering, University of Tennessee, Knoxville, TN 37996, USA c Department of Civil and Environmental Engineering, University of Tennessee, Knoxville, TN 37996, USA article info Article history: Received 12 December 2012 Received in revised form 11 April 2013 Accepted 16 April 2013 Available online 25 April 2013 Keywords: Scintillator Thermal neutron Lithium-loaded scintillator Thermal neutron detector abstract Novel water-soluble 6 Li loaded copolymer scintillation films have been designed and fabricated to detect thermal neutrons. Styrene and maleic anhydride were copolymerized to form an alternating copolymer, then the anhydride functionality was hydrolyzed using 6 Li hydroxide. The resulting poly(styrene-co- lithium maleate) was mixed with salicylic acid as a fluor and cast as a thin film from water. The maximum 6 Li loading obtained that resulted in a transparent film was 4.36% by mass ( 6 Li to polymer). The optimum fluorescence output was obtained for 11.7% salicylic acid by mass, presumably in the form of lithium salicylate, resulting in an optimum film containing 3.85% by mass of 6 Li. A facile and robust synthesis method, film fabrication protocol, photoluminescence results, and scintillation responses are reported herein. & 2013 Elsevier B.V. All rights reserved. 1. Introduction The development of effective thermal neutron detectors is relevant to the fields of nuclear physics, nuclear power generation, imaging, and homeland security. 3 He filled proportional counters are widely used to detect fissile materials by detection of low- energy neutrons in radiation portal monitors and in neutron scattering experiments. Due to the recent expanded use and reduced production of 3 He, it is of interest to national security and basic scientific research to develop a technology to replace 3 He for neutron sensing devices [1]. Conventional research in the area of polymer scintillation detector development usually involves the use of common aryl vinyl polymers such as polystyrene (PS), polyvinyltoluene (PVT), and polyvinylxylene (PVX) or their modifications. The use of these polymers stems from the relatively low cost, facile synthesis, and intrinsic luminescent properties of the aromatic pendant groups. These polymers can be blended with appropriate fluors to improve the overall quantum efficiency of fluorescence and to move the wavelength of maximum emission to regions where common bialkali photomultiplier tubes (PMTs) are most sensitive (390– 450 nm). In order to function as a thermal neutron scintillation detector, a neutron capture nuclide such as 6 Li, 10 B, or 157 Gd must be mixed with an aryl vinyl polymer and a fluor [2–4]. In this study, 6 Li was chosen as the neutron capture nuclide because it has a high absorption cross section (940 b) and a large reaction energy (Q ¼ 4.78 MeV). On capture of a thermal neutron, 6 Li fissions into an alpha particle (2.05 MeV) and a triton (2.78 MeV). These charged particles generate ionizations and excitations in the surrounding matrix which are then collected by appropriate fluors that subse- quently emit photons at longer wavelengths. Scintillation light is generated inside the material and must escape the surface in order to be detected; hence, the ideal scintillation detector is completely transparent such that it does not scatter or absorb its own scintillation light [5]. In our previous works, we investigated poly(2-vinyl naphthalene) (P2VN) contain- ing lithium-6 salicylate ( 6 LiSal) and organic dyes [2] as well as poly (ethylene naphthalate) containing lithium-6 fluoride ( 6 LiF) and organic dyes [3] as potential thermal neutron detectors. 6 LiSal was chosen because it contains 6 Li to capture neutrons and salicylate (Sal - ) to function as a fluor and 6 LiF was chosen because it has a high atom density of 6 Li and is not hygroscopic. However, the resulting composites were not transparent due to phase separa- tion of the organic and inorganic components. There are a few examples of transparent composites that have been reported in the literature. For example, PVT can be loaded with either o-carborane [4] or gadolinium(III) isopropoxide [6] to result in transparent composites. An independent previous report of a transparent lithiated scintillation detector utilized a styrene– lithium methacrylate copolymer [7]. The present work intends to circumvent the issue of phase separation and concomitant reduc- tion in optical clarity by providing chemical bonds between the 6 Li and the polymer matrix. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.04.040 n Correspondence to: 1420 Circle Drive, 552 Buehler Hall, Knoxville, TN 37996, USA. Tel.: +1 865 438 5044. E-mail address: andrew.n.mabe@gmail.com (A.N. Mabe). Nuclear Instruments and Methods in Physics Research A 722 (2013) 29–33