Boris Grynyov

Abstract Plastic-scintillator detectors are among the most common devices used for the detection of elementary particles. They provide good particle identification combined with excellent time resolution, whilst being inexpensive due to the affordability of plastic materials. Particle tracking is achieved by segmenting the scintillator into smaller, independent, optically-isolated voxels. Enhancing the performance of future particle detectors necessitates larger total volumes, possibly combined with even finer segmentation. However, manufacturing such designs with current production strategies is challenging. These strategies involve a variety of time-consuming and costly fabrication processes, followed by the assembly of millions of individual parts. The difficulty in scaling up such a complex workflow underscores the need for technological advancements, which can be met by additive manufacturing. This method enables the construction of complex, monolithic geometries in a single operation. These geometries consist of fine three-dimensional granular sub-structures and require the integration of multiple types of plastic materials, as well as space to accommodate optical fibers, all within a compact volume of several cubic meters. This article presents the fabrication and performance evaluation of the first additive manufactured single-block plastic scintillator detector. To achieve this, a new method called Fused Injection Modeling has been specially developed. The detector is capable of three-dimensional tracking of elementary particles and accurately measuring their stopping power. Its performance is comparable to the current state of the art of plastic scintillator detectors. This work paves the way towards a new feasible, time and cost-effective process for the production of future scintillator detectors, regardless their size and difficulty in geometry.

Plastic-scintillator detectors are among the most common devices used for the detection of elementary particles. They provide good particle identification combined with excellent time resolution, whilst being inexpensive due to the affordability of plastic materials. Particle tracking is achieved by segmenting the scintillator into smaller, independent, optically-isolated voxels. Enhancing the performance of future particle detectors necessitates larger total volumes, possibly combined with even finer segmentation. However, manufacturing such designs with current production strategies is challenging. These strategies involve a variety of time-consuming and costly fabrication processes, followed by the assembly of millions of individual parts. The difficulty in scaling up such a complex workflow underscores the need for technological advancements, which can be met by additive manufacturing. This method enables the construction of complex, monolithic geometries in a single operation. These geometries consist of fine three-dimensional granular sub-structures and require the integration of multiple types of plastic materials, as well as space to accommodate optical fibers, all within a compact volume of several cubic meters. This article presents the fabrication and performance evaluation of the first additive manufactured single-block plastic scintillator detector. To achieve this, a new method called Fused Injection Modeling has been specially developed. The detector is capable of three-dimensional tracking of elementary particles and accurately measuring their stopping power. Its performance is comparable to the current state of the art of plastic scintillator detectors. This work paves the way towards a new feasible, time and cost-effective process for the production of future scintillator detectors, regardless their size and difficulty in geometry.

The paper analyzes the problems that arise when assessing the energy technical light output by existing methods. A modern alternative method for assessing the energy technical light output of various scintillators produced by the Institute of Scintillation Materials of the National Academy of Sciences of Ukraine is described.
 The possibility of evaluating the technical light output of any scintillator by relative comparison with a reference stilbene-based scintillator with a known technical light output is shown. The resulting ratio of responses is recalculated in ph/MeV by taking into account the technical light output of the reference scintillator, equal to 0.023, and the photon formation energy of a particular scintillator.
 The estimation procedure is described. Expressions are given for calculating the values of the technical light yield of scintillators in stilbene units and in ph/MeV. The radioluminescence spectra of the tested scintillators are compared with the sensitivity spectra of the normalized and laboratory photodetectors.
 The technical light yield of scintillators based on single crystals of NaI(Tl), CsI(Tl), CWO, BGO, p-terphenyl, anthracene, stilbene, and a plastic scintillator has been estimated. The values of the responses amplitudes ratio, the spectral normalization coefficients and the tested scintillators technical light output were obtained in stilbene units and in ph/MeV. To check the adequacy of the method the calculation of the tested inorganic scintillators absolute light output was carried out using the light collection coefficients values given in the literature.
 It is shown that with an increase in the scintillators technical light output, in stilbene units, from 0.26 for BGO to 4.3 for NaI(Tl), their technical light output increases from 2500 ph/MeV to 33100 ph/MeV. A decrease in the scintillation photon energy from 2.988 (l = 415 nm) for NaI(Tl) to 2.214 (l = 560 nm) for CsI(Tl) also increases the technical light output of the latter to 35300 ph/MeV. The performed estimates accuracy of scintillators technical light output was 8%.

The paper is dedicated to the evaluation of statistical uncertainty of simulating the process of light transfer in NaI (Tl) and BGO scintillators by Monte Carlo method. The DETECT2000 program with a unified surface model was used. The process of light transfer (“fate”) by light photons was traced from the moment of appearance in the scintillator to the moment of passing through its exit window. The light collection coefficient was determined as the ratio of the number of photons that passed through the exit window to a given number of emitted photons. Different values of the number of emitted photons, optical transparency coefficients and fractions of the surface diffuse reflection were set. Multiple repetitions of the simulation process for a different set of properties made it possible to evaluate the precision and type A uncertainty of simulating the light collection coefficient for all possible options. It is shown that the uncertainty decreases when the statistics of the emitted photons is increased, and increases when the transparency and diffuse reflection fraction are decreased.

The paper is devoted to the estimation of the characteristic limits (statistical criteria) for the detection of small amounts of ionizing radiation by a measuring device under conditions of a natural radioactivity background of the environment: the decision threshold, the detection limit, the minimum detectable activity and the confidence interval. The assessment procedures were carried out in accordance with the national harmonized standard DSTU ISO 11929-3:2009. The threshold for making a decision on the presence of 137Cs and 60Co radionuclides in objects of the external environment and the limit of their detection using a measuring device equipped with plastic scintillators manufactured by the Institute of Scintillation Materials of the National Academy of Sciences of Ukraine were estimated. The influence of the energy of the detected radiation, the dimensions of the scintillators and the geometry of the irradiation on the estimation of the characteristic limits were investigated. Keywords: scintillator; decision threshold; detection limit; confidence interval; minimum detectable activity.