Polyvinyl Toluene Scintillator Research Articles

Spaceborne COTS-Capsule hodoscope: Detecting and characterizing particle radiation

The COTS-Capsule Spaceborne hodoscope was launched into low Earth orbit aboard the International Space Station and operated from 2021 to 2022. The primary objectives of the payload are measuring and characterizing the radiation environment within the space station, serving as a technology demonstrator for the COTS-Capsule radiation mitigation apparatus, and testing the interaction of high-energy cosmic particles on-orbit with our detectors. The payload features a particle hodoscope equipped with an array of novel detectors based on polyvinyl toluene scintillators and silicon photomultiplier sensors that are used for radiation detection and characterization employing 2D intensity-based triangulation. This paper provides a comprehensive account of the COTS-Capsule payload’s construction, preflight performance, testing, qualification, and on-orbit calibration. The hodoscope, sensitive to ionizing cosmic particles, facilitates impinging particle track reconstruction by multi-detector 2D position estimation, energy deposition estimation, and linear energy transfer estimation.

Dual-panel polyvinyl toluene scintillator application as a radiation portal monitor device for gamma and neutron detection

Detection techniques for radiation portal monitors (RPMs) are being developed at Canadian Nuclear Laboratories. One of the purposes of RPM systems at the border ports of entry is to detect illegal transportation of special nuclear material (SNM), which can be used for radiation dispersal devices or nuclear weapons production.One of the most widely used detection technologies considered for RPMs is the polyvinyl toluene (PVT) scintillator. PVT is the most widely used material due to its low cost and its capability to be formed into different shapes and sizes. However, the primary limitation of PVT scintillators is their poor energy spectrum resolution, which limits the discrimination between neutron and gamma radiation.In the present work, a method that allows for the discrimination of fast neutron events from gamma events in a PVT-based RPM was investigated. This method uses two sets of plastic scintillators separated by an air gap. The gap increases the travel distances between scattering events to more easily distinguish pulses produced by neutron interactions in the scintillator from those produced by gamma rays. The time-of-flight (TOF) values between the coincident events in these two sets of scintillators were measured. The TOF for neutrons is significantly longer than the TOF for gamma rays. Another distinguishing characteristic of neutrons is that the flight time between these two scintillator panels will vary according to their energy.The results show that TOF measurements can distinguish neutron sources with sufficiently different energies. Therefore, this method is capable of identifying radioactive material based on neutron energy.In addition to the experimental work, Monte Carlo simulations were carried out using MCNP6.2. The simulation showed that the detector setup had an efficiency (which is the ratio of the number of counts recorded by the detector to the number of neutrons that are incident on the detector) of approximately 12%. Also, the MCNP results showed that the thickness of the second panel had a minimal effect on the signal and no effect on the gamma/neutron peak separation, and therefore could be any conveniently available thickness. However, increasing the air gap between panels increased the peak separation distance but reduced the signal, with a gap distance between 50 cm and 100 cm producing acceptable results.

An automated fast neutron computed tomography instrument with on-line focusing for non-destructive evaluation.

A fast neutron tomography imaging instrument has been designed, built, and tested at The Ohio State University 500 kW Research Reactor on a fast neutron beamline with a peak neutron flux ≈5.4 × 107 n·cm-2·s-1 at 1.6MeV median neutron energy. The instrument and beamline are also configurable for thermal neutron imaging. The imaging apparatus is composed of a lens coupled, water-cooled Electron Multiplying Charge Coupled Device camera, a front-surface mirror, and a high light yield plastic Polyvinyl toluene scintillator. The instrument sits on a mobile cart. A total of 5 motion-control stages are built into the system for XYZ and rotational degrees of freedom for sample positioning; the fifth stage fine tunes the focal distance between the camera and the scintillator to achieve on-line focusing. A Python code with a user-friendly graphical user interface controls the fully automated image acquisition, not requiring user interaction, yet facilitating tracking of the image acquisition. A complete fast neutron computed tomography dataset with 360 projections requires less than 3 h, with 30s per projection. On-line focusing is accomplished with a commercial, off-the-shelf, dielectrically actuated liquid lens. Finally, tomographic reconstructions are visualized using the Livermore Tomography Tools software package. The effective pixel size (width and height) is ≈0.1058mm, yielding a minimum voxel size of 0.1058 × 0.1058 × 0.1058mm3, and produces a spatial resolution of 231μm when calculated from knife-edge measurements.

Muon Tracker with Unsegmented Plastic Scintillator Panels

A muon tracker was developed using three polyvinyl toluene scintillator panels instrumented with photomultiplier tubes (PMTs) mounted at the corners. Panels are mounted in parallel on an aluminum frame which allows for simple adjustment of angle, orientation and separation distance between the panels. The responses of all PMTs in the system are digitized simultaneously at sub-nanosecond sample spacing. Software was developed to adjust settings and implement event rejection based on the number of panels that detected a scintillation event within a 400-nanosecond record. The relative responses of the PMTs are used to calculate the position of scintillation events within each panel. The direction of the muons through the system can be tracked using the panel strike order. Methods for triangulation by both time-of-flight (TOF) and PMT magnitude response are reported. The time triangulation method is derived and experimentally demonstrated using parallel cables of differing length. The PMTs used in this experiment are only optimized for amplitude discrimination, not for time spread jitter as would be required to implement TOF methods into the scintillator panels. A Gaussian process regression machine learning tool was implemented to learn the relationship between PMT response features and positions from a calibration dataset. Resolution is analyzed using different numbers of PMTs and low-versus-high PMT sensitivities. Muons traveling in forward and reverse directions through the detector system were counted in all six axis orientations. The muon detector was deployed for 28 days in an underground tunnel and vertical muon counts were recorded.

Development of an optical sensor for measuring opacity changes in polyvinyl toluene scintillators

Polyvinyl toluene (PVT) based detectors are used in radiation portal monitors (RPMs) throughout the world to detect the trafficking of illicit nuclear material. PVT scintillators, which are normally optically clear, have been observed to suffer internal fogging throughout their volume due to prolonged exposure to varying environmental conditions. These changes could lead to reduced performance for RPMs that utilize plastic scintillators. In this research, a proof of concept system, consisting of different color light emitting diodes (LEDs) and a single optical sensor (OS), were used to examine the change in light transmission through a PVT scintillator. This Optical Monitoring System (OMS), coupled with an environmentally exposed PVT detector, was tested in an environmental chamber where it was subjected to changes in temperature and humidity ranging from 55 °C at 100% relative humidity to −20 °C at 40% relative humidity. At temperatures below 10 °C, light transmission was reduced by 81% ± 8% for blue LEDs, 84% ± 5% for yellow LEDs, and 49% ± 4% for green LEDs. Similar reductions in detected light were not recorded when the OMS was tested with only air between the LEDs and OS. Therefore, the significant reductions in transmitted light were attributed to changes occurring within the PVT scintillator. These results indicate that a significant reduction in PVT opacity occurs due to wide environmental changes. A device such as the OMS could be used to track these changes and provide users an early indication that a portal monitor is suffering from reduced performance.

Development of mobile scanning system for effective in-situ spatial prediction of radioactive contamination at decommissioning sites

In this study, a new mobile system was developed for in-situ scanning and prediction of spatial radiological contamination at nuclear decommissioning sites. The system analyses γ radioactivity, in addition to gross α and β activity, by scintillation spectrometry. Two NaI(Tl) scintillators are located at the centre of the γ-ray spectrometer and together function as a primary detector to track potential areas of elevated activity. Three polyvinyltoluene scintillators surround the primary detector, forming a guard detector to reduce background counts by suppressing Compton events. To validate the system, experimental and simulated γ-ray spectra were matched with respect to detection geometry and energy resolution. We analysed the performance of the system with surface soil under varying conditions, including homogeneity and vertical location of contamination and distance from the surface. Notably, we developed an effective quantitative method to predict the surface and vertical distributions of radioactivity in soil by correlation of detection efficiencies with peak-to-Compton ratios. Achievable scan minimum detectable concentrations for monitoring large areas were determined in case of moving the system. The results verify that the proposed system and prediction method are suitable for use at decommissioning sites and provide fair information for decision-making.

Monte Carlo Calculations of the Detection Efficiency of Composite Scintillator Arrays for Fast and Moderated Neutrons, and for Gamma-Ray Spectroscopy

Fissionable materials can be distinguished from normal materials by their emission of fast neutrons and neutrons moderated by surrounding materials. Radioisotopes can often be identified by their gamma-ray spectra. In this work, we use the nuclear physics Monte Carlo package MCNP6.2 to compare the detection efficiencies of different detector designs for the triple-mode detection of fast and moderated neutrons, and gamma-ray spectroscopy. For each design, we tabulate the detection efficiencies for neutrons from 0.1 eV to 5 MeV and for the full absorption of gamma rays from 0.1 to 5 MeV. All designs use 100 scintillators 1 cm $\times $ 1 cm $\times $ 5 cm. Of these, 36 are Cs2 LiLaBr6:Ce (90% 6Li) (CLLB) and 64 are polyvinyl toluene (PVT). The first design uses separate arrays of $6\times 6$ CLLB and $8\times 8$ PVT scintillators. Three other designs are $10\times 10$ composite arrays using all 100 scintillators combined in different ways. PVT is a fast scintillator that can be used to detect fast neutrons by time-separated multiple proton recoils and discriminate against gammas. PVT in the three composite designs serves to moderate neutrons of all energies for capture by the 6Li in the CLLB. Gamma rays interact in the PVT and CLLB by Compton scattering and pair production; full energy absorption requires photoelectric absorption in the CLLB. The energy deposited by each gamma ray can be determined from a weighted sum of the CLLB and PVT signals from separate photodetectors. We find that for a fixed number of neutrons/cm2, one of the composite arrays has much higher 6Li detection efficiencies than the design that uses separate CLLB and PVT arrays, and has similar detection efficiencies for fast neutrons and gammas. We also find that in all cases, the average energy of the neutrons at capture by 6Li is well above the 25-meV thermal energy.

Optimisation of the scintillation light collection and uniformity for the SoLid experiment

This paper presents a comprehensive optimisation study to maximise the light collection efficiency of scintillating cube elements used in the SoLid detector. Very short baseline reactor experiments, like SoLid, look for active to sterile neutrino oscillation signatures in the anti-neutrino energy spectrum as a function of the distance to the core and energy. Performing a precise search requires high light yield of the scintillating elements and uniformity of the response in the detector volume. The SoLid experiment uses an innovative hybrid technology with two different scintillators: polyvinyltoluene scintillator cubes and 6LiF:ZnS(Ag) screens. A precision test bench based on a 207Bi calibration source has been developed to study improvements on the energy resolution and uniformity of the prompt scintillation signal of antineutrino interactions. A trigger system selecting the 1 MeV conversion electrons provides a Gaussian energy peak and allows for precise comparisons of the different detector configurations that were considered to improve the SoLid detector light collection. The light collection efficiency is influenced by the choice of wrapping material, the position of the 6LiF:ZnS(Ag) screen, the type of fibre, the number of optical fibres and the type of mirror at the end of the fibre. This study shows that large gains in light collection efficiency are possible compared to the SoLid SM1 prototype. The light yield for the SoLid detector is expected to be at least 52±2 photo-avalanches per MeV per cube, with a relative non-uniformity of 6 %, demonstrating that the required energy resolution of at least 14 % at 1 MeV can be achieved.

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.

Passive neutron detection for interdiction of nuclear material at borders

Radiation portal monitor systems based upon polyvinyl toluene scintillator gamma-ray detectors and pressurized 3He-based neutron detector tubes have been deployed to detect illicit trafficking in radioactive materials at international border crossings. This paper reviews the neutron detection requirements and capabilities of passive, as opposed to active interrogation, detection systems used for screening of high-volume commerce for illicit sources of radiation at international border crossings. Computational results are given for the impact of cargo materials on neutron spectra, for the response of various detector geometries, the effects of backgrounds including “ship effect” neutrons, and for simulation of a large neutron detection array.

Baseline suppression of vehicle portal monitor gamma count profiles: A characterization study

Radiation portal monitor (RPM) systems based upon polyvinyl toluene scintillator (PVT) gamma-ray detectors have been deployed to detect illicit trafficking in radioactive materials at border crossings. This report sets forth a characterization of the baseline suppression effect in gross-count gamma-ray profiles due to shadow shielding by vehicles entering RPMs. Shadow shielding is of interest because it reduces the alarm sensitivity of RPMs. This observational study investigated three types of PVT-based commercial RPM systems currently deployed at selected ports of entry in terms of spatial effects relative to detector panel positioning. Radiation portal monitor sites were characterized by driver versus passenger side, top versus bottom panel, and narrow lanes versus wide lanes as observed for a large number of vehicles. Each portal site appears to have a distinctive baseline suppression signature, based on percent maximum suppression relative to measured background. Results suggest that alarm algorithms based on gross-counts may be further refined through attention to individual site characteristics. In addition, longer vehicle transit times were often correlated with stronger baseline suppression, suggesting that baseline suppression studies should take transit time into account.

The use of organometallic compounds in plastic scintillators for the detection and resolution of gamma rays

Tin-loaded plastic and lead-loaded liquid and plastic scintillators were evaluated for γ-photopeak resolution. Polyvinyltoluene scintillators containing physically incorporated triphenyl-4-ethylphenyltin and copolymerized p-triphenyltinstyrene show well-resolved photoelectric peaks towards Cs 137 and Hg 203 γ-rays. Photoelectric peaks are absent in the spectra of liquid and plastic scintillators containing triphenyl-2,4-dimethylphenyllead. Quenching studies indicate that the scintillation efficiency is approximately independent of whether the organometallic is incorporated by physical means or by copolymerization and is dependent upon the type of fluor. Efficiency is also improved through the use of naphthalene as a secondary solvent.

Quenching of the scintillation process in plastics by oxygen

Spectral evidence has been obtained to indicate that fluorescence quenching by air in polyvinyltoluene scintillators is probably produced by oxidation products containing the carbonyl group. The evidence for the chemical nature of the quencher has been obtained indirectly by demonstrating that analogous carbonyl compounds cause a similar shift in the fluorescence spectra and produce quenching when incorporated in polyvinyltoluence scintillators. The lower order of magnitude of quenching by air in liquid scintillation solvents at room temperature is due in part to the absence of oxidation products.