6LiF:ZnS(Ag) Neutron Detector Performance Optimized Using Waveform Recordings and ROC Curves.

We used Gaussian separation and receiver operating characteristic (ROC) curves to optimize the neutron sensitivity and gamma rejection of an ultra-thin 6LiF:ZnS(Ag)-scintillator-based neutron detector paired with a silicon photomultiplier (SiPM). We recorded the waveforms while operating the detector in a monochromatic cold neutron beam and in the presence of isotopic 137Cs and 60Co gamma sources. We used a two-window charge comparison (CC) pulse-shape discrimination (PSD) technique to distinguish the neutron capture events from other types of signals. By feeding the recorded waveforms through variants of this algorithm, it was possible to optimize the duration of the integration windows [(0-100 ns) for the prompt window and (100-2300 ns)] for the delayed window. We then computed the detector's ROC curve from waveform recordings and compared that with the experimental performance. We also used this procedure to compare a series of detector configurations to select the optimal bias voltage for the SiPM photosensor.

Solid polystyrene and deuterated polystyrene light output response to fast neutrons.

The Neutron Imaging System has proven to be an important diagnostic in studying DT implosion characteristics at the National Ignition Facility. The current system depends on a polystyrene scintillating fiber array, which detects fusion neutrons born in the DT hotspot as well as neutrons that have scattered to lower energies in the surrounding cold fuel. Increasing neutron yields at NIF, as well as a desire to resolve three-dimensional information about the fuel assembly, have provided the impetus to build and install two additional next-generation neutron imaging systems. We are currently investigating a novel neutron imaging system that will utilize a deuterated polystyrene (CD) fiber array instead of standard hydrogen-based polystyrene (CH). Studies of deuterated xylene or deuterated benzene liquid scintillator show an improvement in imaging resolution by a factor of two [L. Disdier et al., Rev. Sci. Instrum. 75, 2134 (2004)], but also a reduction in light output [V. Bildstein et al., Nucl. Instrum. Methods Phys. Res., Sect. A 729, 188 (2013); M. I. Ojaruega, Ph.D. thesis, University of Michigan, 2009; M. T. Febbraro, Ph.D. thesis, University of Michigan, 2014] as compared to standard plastic. Tests of the relative light output of deuterated polystyrene and standard polystyrene were completed using 14 MeV fusion neutrons generated through implosions of deuterium-tritium filled capsules at the OMEGA laser facility. In addition, we collected data of the relative response of these two scintillators to a wide energy range of neutrons (1-800 MeV) at the Weapons Neutrons Research Facility. Results of these measurements are presented.

New water equivalent liquid scintillation solutions for 3D dosimetry.

Despite recent advances in radiochromic film and gel dosimetry techniques, radiation therapy still lacks an efficient, accurate, and convenient dose measurement method capable of measuring the dose simultaneously over a plane or a volume (3D). A possibility for creating such a 3D method based on observing scintillation photons emitted from an irradiated volume was recently reported [A. S. Kirov et al., Med. Phys. 26, 1069 (1999)]. In the present article, we investigate the potential to use a liquid scintillation solution (LS) as a dose sensitive media and, simultaneously, as a water equivalent phantom material which fills the measurement volume. We show that matching water density in addition to energy absorption properties is important for using the LS solution as a phantom. Through a parametric study of the LS attenuation and absorption coefficients as well as Monte Carlo dose calculations and scintillation efficiency measurements we developed novel LS materials. For the new solutions, the calculated dose in LS is within 8% of the dose to water for depths up to 5 cm for photons having energies between 30 keV and 2 MeV. The new LS solutions, which are loaded with a Si containing compound, retain more than 85% of the scintillation efficiency of the unloaded solutions and exhibit high localization of the scintillation process. The new LS solutions are superior with respect to efficiency and water equivalence to plastic scintillator materials used in dosimetry and may be used apart from the mentioned 3D method.

Towards two-dimensional brachytherapy dosimetry using plastic scintillator: new highly efficient water equivalent plastic scintillator materials.

Plastic scintillator (PS) has been proposed for both one- and two-dimensional (1D and 2D) dose measurements for radiation therapy applications. For low-energy photon modalities (e.g., brachytherapy), an efficient water equivalent scintillator is needed. To perform 2D measurements, a high localization of the scintillation process is required. Guided by comparison of the mass energy absorption coefficients as a function of energy and of the dose distribution as a function of distance from the radioactive source, as modeled by Monte Carlo photon transport simulation, a small quantity of medium atomic number (Z) atoms (4% Cl) was incorporated in a polyvinyl toluene (PVT) based PS to approximate closely (within 10%) the radiological properties of water in the 20-662 keV energy range. However, the scintillation efficiency of commercial PS mixtures drops as much as 70% when loaded with high atomic number additives. We developed experimental techniques to assess the scintillation efficiency and locality of 15 new PS mixtures. These mixtures differ by the type of the scintillation dyes and the type of compound containing the medium Z atoms (chlorine). To achieve higher material stability, 4-chlorostyrene was used as a loading compound to ensure polymerization with the PVT base. Two of the new PS materials exhibited scintillation efficiencies within 30% of one of the most efficient commercially available products (BC-400), which is not water equivalent at such low energies. These new scintillator materials are promising candidates for the development of an accurate and efficient radiation dosimetry method not only for brachytherapy, but also for superficial and diagnostic applications.