H. Paul Martinez

Evaluation of polyvinyl toluene scintillators for fast neutron imaging

We evaluated polyvinyl toluene (PVT) scintillators fabricated with different emitting dopants and scintillator optical configurations for fast neutron imaging. A neutron imaging apparatus was constructed to study scintillators under MeV neutron exposure. PVT with 2% Xylyl Flrpic emitter was identified as the brightest. The addition of a black backing to the scintillator, compared with a specular reflector film backing, improved the resolution of the neutron image obtained with the PVT scintillator by about 2 ×. It is concluded that a PVT imaging screen with 2% Xylyl Flrpic, configured with a black backing, provides the best quality fast neutron image of the scintillators tested.

Characterization of Polyvinyl Toluene (PVT) scintillators for fast neutron imaging

Polyvinyl Toluene (PVT) based plastic scintillators with varying dimensions and fluors have been characterized in terms of relative light output and spatial resolution. Scintillators were exposed to fast neutrons (~2 MeV), and images were obtained with a setup consisting of an EMCCD camera, a mirror and a light-tight apparatus. Among scintillators with 2.0% Flrpic and 10.16 cm (4 inch) diameter, the 10.5-mm thick scintillator featured the highest light output while 3.0- mm provided the best spatial resolution. The deuterated 3.0-mm thick scintillator doped with 2.0% Flrpic showed a worse performance in terms of both light output and spatial resolution compared to that of undeuterated scintillator with the same thickness but doped with 2.0% X-Flrpic. This study reveals the effects of presence of deuterium in PVT, the thickness of scintillator, and the fluor on the light output and spatial resolution of plastic scintillator.

Bismuth-loaded plastic scintillator portal monitors

Plastic scintillators incorporating 8 weight percent elemental Bismuth offer enhanced sensitivity and distinct photopeak spectra in the <1000 keV range typically used in radiation portal monitors. The Bismuth-loaded plastic is based on polyvinyl toluene with standard singlet fluors. It produces ~6,000 photons/MeV with a maximum emission at 430 nm and a ~10 ns decay. Energy resolution of 49% at 59.5 keV and 16% at 662 keV are obtained for a 14 in3 Bismuth plastic scintillator plate. Count rates compared to standard plastic scintillator of the same size reveal a sensitivity improvement of >5x in the <200 keV range. Future spectroscopic radiation portal monitors based on the Bismuth plastic scintillator could provide moderate resolution spectroscopy for radioisotope identification. In addition, the Bismuth plastic offers outstanding environmental stability to weathering effects, in contrast with standard plastic scintillator formulations.

Solution-Grown Rubrene Crystals as Radiation Detecting Devices

There has been increased interest in organic semiconductors over the last decade because of their unique properties. Of these, 5, 6, 11, 12-tetraphenylnaphthacene (rubrene) has generated the most interest because of its high charge carrier mobility. In this work, large single crystals with a volume of ~1 cm3 were grown from solution by a temperature reduction technique. The faceted crystals had flat surfaces and cm-scale, visually defect-free areas suitable for physical characterization. X-ray diffraction analysis indicates that solvent does not incorporate into the crystals and photoluminescence spectra are consistent with pristine, high-crystallinity rubrene. Furthermore, the response curve to pulsed optical illumination indicates that the solution grown crystals are of similar quality to those grown by physical vapor transport, albeit larger. The good quality of these crystals in combination with the improvement of electrical contacts by application of conductive polymer on the graphite electrodes have led to the clear observation of alpha particles with these rubrene detectors. Preliminary results with a 252Cf source generate a small signal with the rubrene detector and may demonstrate that rubrene can also be used for detecting high-energy neutrons.

Plastic scintillators for gamma spectroscopy and neutron radiography (Conference Presentation)

Plastic scintillators are widely deployed for ionizing radiation detection, as they can be fabricated in large sizes, for high detection efficiency. However, commercial plastics are limited in use for gamma spectroscopy, since their photopeak is very weak, due to low Z, and they are also limited in use for neutron detection, since proton recoils are indistinguishable from other ionizing radiation absorption events in standard plastics. We are working on scale up and production of transparent plastic scintillators based on polyvinyltoluene (PVT) loaded bismuth metallorganics for gamma spectroscopy. When activated with standard organic fluors, PVT scintillators containing 8 wt% bismuth provide energy resolution of 11% at 662 keV. When Iridium complex fluors are used, we can load plastics up to 20 wt% bismuth, while obtaining energy resolution of 10% at 662 keV. Another formulation, activated with Ir fluors for use as neutron radiography scintillator may be used for high energy neutron radiography. Acknowledgements This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and has been supported by the US DOE National Nuclear Security Administration, Defense Nuclear Nonproliferation Research and Development under Contract No. DE-AC03-76SF00098

Bismuth- and lithium-loaded plastic scintillators for gamma and neutron detection(Conference Presentation)

Plastic scintillators are widely deployed for ionizing radiation detection, as they can be fabricated in large sizes, for high detection efficiency. However commercial plastics are limited in use for gamma spectroscopy, since their photopeak is too weak, due to low Z, and they are also limited in use for neutron detection, since proton recoils are indistinguishable from other ionizing radiation absorption events in standard plastics. We are working on scale up and production of transparent plastic scintillators based on polystyrene (PS) with high loading of bismuth metallorganics for gamma spectroscopy, and with lithium metallorganics for neutron detection. When activated with standard organic fluors, PS scintillators containing 8 wt% bismuth provide energy resolution of 11% at 662 keV. A PS plastic formulation including 1.3 wt% lithium-6 provides a neutron capture peak at 525 keVee, with 11% resolution for the capture peak and 90% efficiency for thermal neutron capture in 2mm thickness. Acknowledgements This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and has been supported by the US DOE National Nuclear Security Administration, Defense Nuclear Nonproliferation Research and Development under Contract No. DE-AC03-76SF00098