# Patent

## Title
Lithium-containing scintillators for thermal neutron, fast neutron, and gamma detection

## Abstract

In one embodiment, a scintillator includes a scintillator material; a primary fluor, and a Li-containing compound, where the Li-containing compound is soluble in the primary fluor, and where the scintillator exhibits an optical response signature for thermal neutrons that is different than an optical response signature for fast neutrons and gamma rays.

## Background
RELATED APPLICATIONS
This application claims priority to Provisional U.S. Application No. 61/859,074 filed on Jul. 26, 2013, which is herein incorporated by reference.


The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC for the operation of Lawrence Livermore National Laboratory.


FIELD OF THE INVENTION
The present invention relates to radiation detection, and more particularly to Li-containing scintillators for thermal neutron, fast neutron and gamma detection, and methods of making and using the same.
BACKGROUND
Radioactive materials are often detected and identified by measuring gamma-rays and/or neutrons emitted from the materials. The energy of gamma-rays is specific to that particular material and acts as a “finger print” to identify the material. Similarly, neutron energy is particular to the material, and may be used to identify the material. Of very high value are detectors capable of identifying the distinctive time-correlated signatures corresponding to neutrons and gamma rays, or “gammas” emitted by fissioning material from within a background of uncorrelated natural radiation. A detector capable of distinguishing neutrons from gammas, as well as offering a fast response time typically has better capability for detecting the distinctive time-correlated events indicative of the presence of fissioning nuclei.
The ability to detect gamma rays and/or neutrons is a vital tool for many areas of research. For example, gamma-ray/neutron detectors allow scientists to study celestial phenomena and diagnose medical diseases. Additionally, these detectors are important tools for homeland security, helping the nation confront new security challenges. The nuclear non-proliferation mission requires detectors capable of identifying diversion or smuggling of nuclear materials. Government agencies need detectors for scenarios in which a terrorist might use radioactive materials to fashion a destructive device targeted against civilians, structures, or national events. To better detect and prevent nuclear incidents, the Department of Energy (DOE) and the Department of Homeland Security (DHS) are funding projects to develop a suite of detection systems that can search for radioactive sources in different environments.
One particularly useful type of radiation detection, pulse shape discrimination (PSD) provides means for high-energy neutron detection in the presence of gamma radiation background by utilizing the difference in the shapes of scintillation pulses excited by neutrons (recoil protons) and gamma (γ)-rays in organic scintillators. PSD phenomena are based on the existence of two-decay component fluorescence, in which, in addition to the main component decaying exponentially (prompt fluorescence), there is usually a slower emission that has the same wavelength, but longer decay time (delayed emission). According to a commonly accepted mechanism shown in FIG. 1, the fast component results from the direct radiative de-excitation of excited singlet states (S1), while the slow component originates from the collisional interaction of pairs of molecules (or excitons) in the lowest excited n-triplet states (T1).
Since the triplet is known to be mobile in some compounds, the energy migrates until the collision of two triplets collide and experience a process, shown as Equation 1:

T1+T1→S0+S1  Equation 1

In Equation 1, T1 is a triplet, S0 is the ground state, and S1 is a first excited state. Finally, the delayed singlet emission occurs with a decay rate characteristic of the migration rate and concentration of the triplet population, and is represented by Equation 2:

S1→S0+hv  Equation 2

In Equation 2, hv is fluorescence, while S0 is the ground state and S1 is a first excited state. The lifetime of the delayed emission is determined by the lifetime of T1 and the rate of T1T1 collisions. The short range of the energetic protons produced from neutron collisions yields a high concentration of triplets, compared to the longer range of the electrons from the gamma interactions, leading to the enhanced level of delayed emission with longer decay times in neutron-induced pulses in comparison to those produced by the gamma excitation. The observation of PSD in organics with phenyl groups is believed to be, in part, related to the aromatic ring structure, allowing for the migration of triplet energy.
FIG. 2A shows a plot of average waveforms for a stilbene test crystal indicating different levels of delayed light in neutron and gamma scintillation pulses. As can be seen from the plot, some light is produced by the crystal almost immediately, referred to as prompt light, and other light is produced by the crystal over a period of time, referred to as delayed light. Generally, the plot for each type of radiation will have a steep component 202 and a tail component 204. The upper line in the plot represents neutron light decay, while the lower line represents gamma (γ) light decay. As shown in FIG. 2A, the shape for the neutron response has a large tail component 204, which is much smaller or almost negligible for gammas. Thus, stilbene is able to differentiate between the neutron and gamma light decays, and produces noticeably different lines for each radiation type. However, not every compound has this ability to separate between gamma and neutron light decay; therefore compounds with such ability are very useful for PSD.
Modern high-speed waveform digitizers allow for easy separation of neutron and gamma pulses, enabling rapid characterization of PSD properties, as shown in FIG. 2B. The waveforms may be numerically integrated over two time intervals: ΔTotal and a subinterval ΔTail (e.g., for >50 nsec), corresponding to the total charge and the delayed component of the signal, respectively. The value of the ratio of charge R=QTail/QTotal for the two time intervals indicates whether the considered event was likely produced by a neutron (high R value) or a gamma ray (small R value). The plot shown in FIG. 2B reveals the presence of both neutrons (upper scatter points) and gammas (lower scatter points) in a plot of the ratio of charge (QTail/QTotal) versus the pulse height.
FIG. 2C illustrates one approach where the neutron/gamma delayed light separation, S, in the stilbene test crystal is used for calculation of the PSD figure of merit (FOM). The PSD separation, S, refers to the gap between the mean ratio of charge (QTail/QTotal) for gamma rays and the mean ratio of charge (QTail/QTotal) for neutrons taken over an extended period of time. The larger the separation, S, the better the organic crystal is at PSD for distinguishing gammas and neutrons.
The PSD technique is most frequently utilized for discrimination between fast neutrons (recoil protons) and gamma-rays (Compton electrons) using liquid scintillators and a few organic crystals. Recent developments broadened the group of PSD materials to include scintillating plastics. However, because the composition of organic scintillators is currently comprised of mostly hydrocarbons, traditional PSD materials can be used only for detection of fast neutrons, leaving undetected the large fraction of low-energy and thermal neutrons that do not generate enough light in elastic scatter interaction.
Present techniques for detection of thermal neutrons are typically based on 3He detectors. However, due to the imminent shortage of 3He, other neutron detection technologies utilizing 10B- and 6Li-loaded scintillating materials have been considered as possible replacements for 3He detectors. The neutron detection properties of 10B- and 6Li-containing scintillators are based on known capture reactions:

10B+no=7Li+α+2.79 MeV

6Li+no=3He+α+4.78 MeV.

Current scintillators utilizing the thermal neutron detection capabilities of 10B generally use boron in a gaseous form, as BF3, or in a solid form consisting of pure boron or a compound mixture (e.g. boron carbide) present in a matrix. Moreover, current scintillators utilizing the thermal neutron detection capabilities of 6Li include inorganic single crystals (e.g. LiI, LiF, Li-aluminate, Cs2LiYCl6:Ce (CLYC), 6Li-lanthanide borate, etc.), 6Li-loaded glass scintillators, and composite materials including dispersions of nano- or micro-particles of different Li-containing compounds (e.g. nano- or microscale Li-containing crystals) in liquid or plastic matrices.
However, there are several disadvantages associated with existing 10B and 6Li based detectors. For example, single crystal detectors are limited by the size of the crystals that can be grown and the high cost of doing so. Additionally, difficulties that may arise from use of 6Li-loaded and/or 10B-loaded glass scintillators include long decay times and high sensitivity to gamma rays due to the presence of relatively heavy constituents in their compositions. Further, a drawback with composite materials comprising dispersions of small scintillating compounds, e.g. Li-containing crystals, in polymer and/or liquid matrices includes elevated levels of light scatter due to inhomogeneous composition and structure, as well as limitations in effective neutron detecting due to insufficient optical clarity in the scintillating wavelength region.
SUMMARY
According to one embodiment, a thermal neutron capture reagent includes a lithium-containing compound selected from a group consisting of: Li-3-phenylsalicylate, Li-3,5-di-tert-butylsalicylate, Li-acetylsalicylic acid, and combinations thereof, where the lithium-containing compound is soluble in a fluor, and where the thermal neutron capture reagent exhibits an optical response signature for thermal neutrons.
According to another embodiment, a scintillator includes a scintillator material; a primary fluor, and a Li-containing compound, where the Li-containing compound is soluble in the primary fluor, and where the scintillator exhibits an optical response signature for thermal neutrons that is different than an optical response signature for fast neutrons and gamma rays.
According to yet another embodiment, a method for fabricating a scintillator includes forming a precursor mixture, and heating the precursor mixture until a polymerization process is complete. The precursor mixture includes a monomer, one or more fluors, a coordinating solvent, and a Li-containing compound selected from a group consisting of: Li-3-phenylsalicylate, Li—Li-3,5-di-tert-butylsalicylate. Li-acetylsalicylic acid, and combinations thereof, where the Li-containing compound is soluble in at least one of the one or more fluors.
Other aspects and advantages of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the nature and advantages of the present invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings.
FIG. 1 shows a mechanism for delayed photoluminescence according to the prior art.
FIG. 2A shows a plot of average waveforms for stilbene indicating different levels of delayed light in neutron and gamma scintillation pulses according to one embodiment.
FIG. 2B shows a PSD pattern of a stilbene crystal obtained by digitized separation of neutron and gamma pulses according to one embodiment.
FIG. 2C shows PSD profiles of stilbene used for calculation of the PSD figure of merit (FOM) according to one embodiment.
FIG. 3 shows a simplified layout of an instrument according to one embodiment.
FIG. 4 shows a flowchart of a method of fabricating a scintillator according to one embodiment.
FIG. 5A shows the fraction of delayed light (Qs) in a total scintillation pulse (Qr) for a 6Li-containing non-PSD plastics scintillator measured using a moderated 252Cf source.
FIG. 5B shows a pulse height spectrum corresponding to the neutron signatures obtained with a 6Li-containing non-PSD plastics scintillator using a moderated 252Cf source.
FIG. 5C shows the fraction of delayed light (QS) in a total scintillation pulse (QT) for a 6Li-containing PSD plastics scintillator measured using a moderated 252Cf source.
FIG. 5D shows separated signatures of neutrons and gammas in a 6Li-containing PSD plastics scintillator.
FIG. 6 illustrates optical transparent, homogeneous 6Li-loaded PSD plastics containing PPO (30%) and Li-3-PSA (5-7.5%) dissolved in polystyrene, and 0.2 wt % of secondary dyes (e.g. DPA, Bis-MSB, or POPOP).
FIG. 7A shows PSD patterns for a PSD plastic scintillator comprising 5 wt % of 6Li-3-PSA measured using a 252Cf source without moderation.
FIG. 7B shows PSD patterns for a PSD plastic scintillator comprising 7.5 wt % of 6Li-3-PSA measured using a 252Cf source with 10 cm of HDPE moderation.
FIG. 7C shows PSD patterns for a PSD plastic scintillator comprising 5 wt % of 6Li-3-PSA measured using a 252Cf source with 5 cm of HDPE moderation.
FIG. 7D shows 252Cf and 137Cs spectra corresponding to the PSD pattern in 7B, illustrating a sharply pronounced thermal neutron peak.
FIG. 8A shows PSD profiles obtained with 2.5 cm×2.5 cm thick PSD plastics containing 30 wt % of PPO, 5 wt % of Li-3-PSA in a PS matrix, and 0.2 wt % DPA measured using a 252Cf source with 10 cm of HDPE moderation. FOMs are calculated in near-thermal-neutron-spot energy range of by Gaussian approximation of individual peaks for gamma, fast neutrons and thermal neutrons.
FIG. 8B shows PSD profiles obtained with 2.5 cm×2.5 cm thick PSD plastics containing 30 wt % of PPO, 5 wt % of Li-3-PSA in a PS matrix, and 0.2 wt % Bis-MSB measured using a 252Cf source with 2.5 cm of HDPE moderation. FOMs are calculated in near-thermal-neutron-spot energy range of by Gaussian approximation of individual peaks for gamma, fast neutrons and thermal neutrons.
FIG. 9A shows energy-calibrated PSD patterns for a liquid scintillator prepared as a mixture of 30% PPO in xylene loaded with 5% Li-3-PSA measured using a 252Cf source with 5 cm of HDPE moderation.
FIG. 9B shows the PSD profile corresponding to FIG. 9A in the near-thermal-neutron-spot energy range showing no thermal/fast neutron separation.
FIG. 10A shows energy-calibrated PSD patters for a liquid scintillator prepared as a mixture of 1% DPA in xylene loaded with 5% Li-3-PSA measured using a 252Cf source with 5 cm HDPE moderation.
FIG. 10B shows the PSD profile corresponding to FIG. 10A in the near-thermal-neutron-spot energy range showing the high degree of separation between three types of radiation.
FIG. 11A shows the emission spectrum of a 6Li-loaded liquid scintillator comprising a mixture of 30% PPO and 5% Li-3-PSA dissolved in p-xylene, where the spectrum corresponds to pure Li-3-PSA emission.
FIG. 11B shows Li-3-PSA excitation and PPO emission spectra showing large overlap that leads to the absorption of PPO emission by Li-3-PSA molecules.
FIG. 11C shows the emission spectrum of a mixture containing 1% DPA and 5% Li-3-PSA, where the final emission is produced by DPA molecules.
FIG. 11D shows Li-3-PSA excitation and DPA emission spectra with no spectral overlap.
FIG. 12A shows average 252Cf waveforms corresponding to the normalized scintillation pulses recorded in the electron-equivalent range of 351-492 KeVee with unloaded polystyrene-based PSD plastic containing 30% of PPO and 0.2% DPA.
FIG. 12B shows average 252Cf waveforms corresponding to the normalized scintillation pulses recorded in the electron-equivalent range of 351-492 KeVee with unloaded polystyrene-based PSD plastic containing 30% of PPO, 0.2% DPA and 5% 6Li-3-PSA.



DETAILED DESCRIPTION
The following description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations.
Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc.
It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless otherwise specified.
Further, as used herein, all percentage values are to be understood as percentage by weight (wt %), unless otherwise noted. Moreover, all percentages by weight are to be understood as disclosed in an amount relative to the bulk weight of an organic plastic scintillator material or a liquid scintillator material, in various approaches.
As also used herein, the term “about” when combined with a value refers to plus and minus 10% of the reference value. For example, a length of about 10 nm refers to a length of 10 nm±1 nm, a temperature of about 50° C. refers to a temperature of 50° C.±5° C., etc.
As additionally used herein, a material that is “optical transparent” refers to a material that is substantially free (e.g. >99% free, preferably >99.5% free) of grain boundaries, such that the material is homogenous (e.g. comprises one-phase). Moreover, optically transparent materials are those through which light propagates uniformly.
The description herein is presented to enable any person skilled in the art to make and use the invention and is provided in the context of particular applications of the invention and their requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art upon reading the present disclosure, including combining features from various embodiment to create additional and/or alternative embodiments thereof.
Moreover, the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.
The following description describes several embodiments relating to the use and fabrication of Li-containing scintillator materials with distinctively different scintillation pulse shapes resulting from neutron and gamma excitation, respectively. Moreover, various embodiments of the present invention describe the fabrication of Li-containing scintillator materials capable of simultaneous detection of thermal neutrons and fast neutrons discriminated from the gamma radiation background.
Robust neutron radiation detectors with a large sensitive area/volume, high detecting efficiency and a low cost means of making/using are important for the detection of neutrons in many areas such as nuclear nonproliferation, international safeguards, national security, scientific research, etc. In particular, for nuclear nonproliferation, fast and robust methods for the identification of special nuclear materials (SNM) are needed.
According to their energy, neutrons are typically divided in two major groups: thermal (low-energy) neutrons and fast (high-energy) neutrons. Detection of both types requires the separation of the neutron signatures from the always-present strong gamma radiation background. In common radiation detection practice, identification of both thermal and fast neutrons requires simultaneous use of two different types of detectors, one of which is based on mostly hydrocarbon-comprised scintillators (for fast neutron detection), and the other including scintillating materials loaded with thermal neutron capture reagents.
Conventional detection of thermal neutrons utilizes 3He detectors, the availability of which is sharply decreasing due to the diminishing supply of 3He obtained as a side product of tritium production. Accordingly, 10B- and 6Li-loaded scintillating materials have been considered as possible replacements for 3He detectors. For example, some 10B-loaded liquid scintillators have been shown to discriminate signals from thermal neutrons, fast neutrons and gamma-rays.
6Li possesses many desirable qualities as a neutron target. Among its important advantages are a reasonable capture cross section, relatively high photon yield from charged particles, and absence of gamma-rays in the final products resulting from a capture reaction 6Li+no=3H+α+4.8 MeV. The majority of scintillators utilizing the thermal neutron detection properties of 6Li are inorganic single crystals or scintillation glasses. However, single crystal detectors are limited by the size of the crystals that can be grown and high costs associated with fabrication. Additionally, drawbacks of 6Li-loaded glass scintillators include long decay times and high sensitivity to gamma rays due to the presence of relatively heavy constituents in their compositions.
In some approaches, plastic scintillators offer a good alternative with wide field applications. Being comprised of low-Z materials, hydrocarbon-based plastics are less sensitive to gamma radiation. Additional advantages may be introduced in more approaches by the use of Pulse Shape Discrimination (PSD) formulations that enable separation of neutron signatures from gamma-radiation background. Moreover, plastics may be easily manufactured in different configurations and sizes, and are inexpensive. Further, unlike the handling constraints associated with liquid scintillators, plastic scintillators are easy to handle, thus enabling their use in portal monitoring, environmental radiation protection, and in field conditions with sharply changing temperatures, etc.
10B-containing compounds, such as ortho- and meta-carboranes, may be easily introduced into the composition of plastic scintillators in various approaches. However, the use of such detectors may present difficulties because of the high cost required for isotopical enrichment of carboranes from  ̃19% of 10B in natural abundance to over 90% needed for the neutron capture probability.
The principle drawback that prevents the use of 6Li-loaded plastic scintillators relates to the difficulties in their preparation due to the lack of solubility of highly polar Li-containing compounds in non-polar aromatic matrices needed for efficient scintillation. Thus, the majority of current 6Li-containing organic scintillators are typically prepared as non-PSD dispersions of nano- or micro-particles of different Li-compounds in liquid or plastic matrices that often have insufficient optical clarity and chemical instability due to inhomogeneous composition and structure.
Embodiments disclosed herein overcome the aforementioned drawbacks by providing Li-containing compounds that may be homogeneously dissolved in an organic scintillator material (e.g. a polymer matrix, a liquid, etc.) such that the scintillator material is substantially (e.g. >99%, preferably >99.5%) free of grain boundaries (e.g. comprises a single phase, is homogenous, etc.) and is thus optically transparent. For example, it has been surprisingly found in some approaches that Li-salts of carboxylic aids with coordinating solvents may diminish the polarity of Li, thereby making the resulting Li-compound more soluble in non-polar aromatic matrices.
Following are several examples of general and specific embodiments of Li-containing scintillators configured for thermal neutron, fast neutron and gamma detection, and/or related systems and methods.
In one general embodiment, a thermal neutron capture reagent includes a lithium-containing compound selected from a group consisting of: Li-3-phenylsalicylate, Li-3,5-di-tert-butylsalicylate, Li-acetylsalicylic acid, and combinations thereof, where the lithium-containing compound is soluble in a fluor, and where the thermal neutron capture reagent exhibits an optical response signature for thermal neutrons.
In another general embodiment, a scintillator includes a scintillator material; a primary fluor, and a Li-containing compound, where the Li-containing compound is soluble in the primary fluor, and where the scintillator exhibits an optical response signature for thermal neutrons that is different than an optical response signature for fast neutrons and gamma rays.
In yet another general embodiment, a method for fabricating a scintillator includes forming a precursor mixture, and heating the precursor mixture until a polymerization process is complete. The precursor mixture includes a monomer, one or more fluors, a coordinating solvent, and a Li-containing compound selected from a group consisting of: Li-3-phenylsalicylate, Li—Li-3,5-di-tert-butylsalicylate, Li-acetylsalicylic acid, and combinations thereof, where the Li-containing compound is soluble in at least one of the one or more fluors.
General Scintillator-Based Radiation Detector System
FIG. 3 depicts a simplified spectroscopy system according to one embodiment. The system 300 comprises a scintillator material 302, such as of a type described herein, and which is referred to herein interchangeably as a scintillator. The system 300 also includes a photodetector 304, such as a photomultiplier tube or other device known in the art, which can detect light emitted from the scintillator 302, and detect the response of the material to at least one of neutron and gamma ray irradiation.
The scintillator 302 produces light pulses upon occurrence of an event, such as a neutron, a gamma ray, or other radiation engaging the scintillator 302. As the gamma ray, for example, traverses the scintillator 302, photons are released, appearing as light pulses emitted from the scintillator 302. The light pulses are detected by the photodetector 304 and transduced into electrical signals that correspond to the pulses. The type of radiation can then be determined by analyzing the integral of the light pulses and thereby identifying the gamma ray energy absorbed by the scintillator.
In some embodiments, the system 300 may be, further comprise, or be coupleable/coupled to, a preamplifier and/or digitizer (not shown in FIG. 3). Additionally, a processing device 306 for processing pulse traces output by the photodetector 304. In other embodiments, the system 300 may include a processing device that receives data from a photodetector that is not permanently coupled to the processing device. Illustrative processing devices include microprocessors, field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), computers, etc.
The result of the processing may be output and/or stored. For example, the result may be displayed on a display device 308 in any form, such as in a histogram or derivative thereof.
The program environment in which one embodiment of the invention may be executed illustratively incorporates one or more general-purpose computers or special-purpose devices such hand-held computers. Details of such devices (e.g., processor, memory, data storage, input and output devices) are well known and are omitted for the sake of clarity.
It should also be understood that the techniques of the present invention might be implemented using a variety of technologies. For example, the methods described herein may be implemented in software running on a computer system, or implemented in hardware utilizing one or more processors and logic (hardware and/or software) for performing operations of the method, application specific integrated circuits, programmable logic devices such as Field Programmable Gate Arrays (FPGAs), and/or various combinations thereof. In particular, methods described herein may be implemented by a series of computer-executable instructions residing on a storage medium such as a physical (e.g., non-transitory) computer-readable medium. In addition, although specific embodiments of the invention may employ object-oriented software programming concepts, the invention is not so limited and is easily adapted to employ other forms of directing the operation of a computer.
Portions of the invention can also be provided in the form of a computer program product comprising a physical computer readable medium having computer code thereon. A computer readable medium can include any physical medium capable of storing computer code thereon for use by a computer, including optical media such as read only and writeable CD and DVD, magnetic memory or medium (e.g., hard disk drive), semiconductor memory (e.g., FLASH memory and other portable memory cards, etc.), etc.
Polymers
The organic, plastic scintillators as described in the embodiments disclosed herein may include any suitable polymer matrix as the plastic component. Particularly suitable plastic scintillators are those that include rigid, durable, transparent plastics that possess aromatic or non-aromatic structures and are capable of supporting high concentrations of fluors (e.g. primary fluors, secondary fluors, tertiary fluors, etc.) therein. In some embodiments, suitable plastic scintillators may be capable of supporting a total concentration of the fluors in a range from about 3-75 wt % fluor.
In one embodiment, the polymer matrix may comprise polyvinyltoluene (PVT). Similar polymers may be utilized in other embodiments, such as polystyrene (PS), polymethylmethacrylate (PMMA), polyvinyl xylene (PVX), polymethyl, 2,4-dimethyl, 2,4,5-trimethyl styrenes, polyvinyl diphenyl, polyvinyl naphthalene, polyvinyl tetrahydronaphthalene polymers, other complex aromatic polymers, and certain non-aromatic polymers capable of solubilizing different scintillating fluors, etc. as would be understood by one having ordinary skill in the art upon reading the present disclosure.
Suitable polymers, as used herein in preferred approaches, may be at least 50% light transmissive in a wavelength of interest, e.g. a wavelength emitted by one or more fluors present in the organic plastic scintillator system.
In various embodiments, the organic, plastic scintillator material may comprises a colorless, optically transparent non-PSD plastic material for detection of thermal neutrons via pulse height measurements. In further embodiments, the organic, plastic scintillator may comprise a colorless, optically transparent PSD plastic for simultaneous detection of thermal and fast neutrons discriminated from gamma radiation background via PSD measurement techniques.
In more embodiments, the polymer may be provided as a liquid polymer matrix, a non-liquid polymer matrix, a dry powder, etc. as would be understood by one having ordinary skill in the art upon reading the present descriptions. Moreover, in some approaches the polymer matrix may include aromatic functional groups, such as phenyl groups, among others.
In yet more embodiments, the organic, plastic scintillator material may be fabricated and comprise the polymers as described in U.S. patent application Ser. No. 13/437,836, Apr. 4, 2012, and U.S. patent application Ser. No. 13/471,259, filed May 14, 2012, which are both incorporated herein by reference in their entirety.
Liquids
The organic, liquid scintillators as described in the embodiments disclosed herein may include any suitable liquid scintillator material. For example, a suitable liquid scintillator material may be any commercially available liquid scintillator material comprising known flammable and non-flammable formulations. Suitable liquid scintillator materials may include but are not limited to EJ-301, EJ-309, EJ-315, BC-501A, NE-213, etc.
In another approach the liquid scintillators disclosed herein may include aromatic solvents such as xylene, toluene, PXE [1,2-dimethyl-4-(1-phenylethyl)-benzene], LAB (linear alkyl benzene), DIPN (diisopropylnaphthalene), etc. In yet another approach, the liquid scintillators may include non-aromatic solvents such as methanol, ethanol, dioxanes, DME (dimetoxyethane), etc. In more approaches, the liquid scintillators may include mixed organic-inorganic solvents combined with solubilized scintillating fluors, as would be understood by one having ordinary skill in the art upon reading the present disclosure.
In still more approaches, the liquid scintillator material may be any commercial scintillator material without PSD.
In other approaches, the liquid scintillator material may be any commercial scintillator material with PSD. For instance, in one approach, the liquid scintillator materials disclosed herein may comprise any organic, inorganic or mixed solvents capable of dissolving one or more fluors (e.g. primary fluors, secondary fluors, tertiary fluors, etc.) therein. In further approaches, the concentrations of the one or more fluors in the liquid scintillator material may be tailored to achieve a sufficient and/or a desired amount of scintillation light yield (LY) and/or PSD production. In even more approaches, the concentration of the one or more fluors in the liquid scintillator material that is sufficient to achieve PSD production may be about 2 to about 3 orders of magnitude lower than the concentration of the same one or more fluors needed in plastic scintillators to achieve the same level of PSD production.
Fluors/Dyes
Primary fluors suitable for use in the presently disclosed scintillator systems include any fluor that may be known in the art and capable of exhibiting characteristics for scintillation as described herein. In some embodiments, the primary fluor may be present in various concentrations, e.g. about 0.005-5 wt % or more. In more embodiments, the primary dye may be present in an amount ranging from about 5% to 20 wt % or more. In even more embodiments, the primary dye may be present in an amount ranging from about 20 wt % to about 75 wt %, or an amount ranging from about 30 wt % to about 75 wt %. As disclosed herein in various embodiments, the concentrations of fluors are described relative to a weight of the bulk scintillator material.
In one particular embodiment, a scintillator system may include a primary fluor disposed in a polymer matrix. In some approaches, the primary fluor may be a fluorescent dye present in an amount of 3-5 wt % or more, where such fluorescent dye results in the scintillator material exhibiting an optical response signature for neutrons that is different than an optical response signature for gamma rays.
In approaches where one or more primary fluors are present in high concentration in the exemplary organic plastic scintillator systems, the solubility of the fluor(s) in the polymer matrix is preferably high. For example, in one embodiment, the polymer matrix may be characterized as having a solubility of about 3-5 wt % or more with respect to a particular fluor.
The primary fluor may be incorporated into the polymer matrix according to any suitable mechanism. For example, in some approaches, the primary fluor may be suspended/dispersed in the polymer matrix. In preferred approaches, there may be a substantially uniform distribution of the primary fluor in the polymer matrix. In more approaches, the primary fluor may be crosslinked to the polymer matrix. In still more approaches, the primary fluor may be copolymerized with the polymer matrix, and/or with another component of the polymer matrix, etc. as would be understood by one having ordinary skill in the art upon reading the present descriptions. Of course, other arrangements of fluor and polymer matrix may be utilized without departing from the scope of the present descriptions.
In another particular embodiment, a scintillator system may include a primary fluor disposed in a liquid scintillator material. In some approaches, the primary fluor may be a fluorescent dye present in an amount of 0.005% wt % or more, where such fluorescent dye results in the scintillator material exhibiting an optical response signature for neutrons that is different than an optical response signature for gamma rays.
In yet another embodiment, the primary fluor disposed in an organic plastic scintillator material and/or a liquid scintillator material may be 2,5-diphenyl oxazole (PPO). In other embodiments, the primary fluor may include other dyes that produce scintillation response upon the excitation by any types of radiation. In still other approaches, the primary fluor may include multiple fluorescent dyes.
In a further embodiment, a scintillator system may include a secondary fluor disposed therein. For instance, in one approach, a suitable secondary fluor disposed in the exemplary plastic scintillator systems and/or the exemplary liquid scintillator systems disclosed herein may be characterized by wavelength-shifting qualities. In some approaches, a plastic and/or liquid scintillator system having a secondary fluor in the presence of another fluor, particularly a primary fluor, may exhibit scintillation light yield (LY) or PSD characteristics that are superior to the LY or PSD characteristics of a scintillator system having exclusively the same primary fluor (and no secondary fluor), under otherwise identical conditions.
Suitable secondary fluors may include any fluor characterized by wavelength-shifting such as diphenyl anthracene (DPA), tetraphenyl butadiene (TPB) 1,1,4,4-tetraphenyl-1,3-butadiene, 1,4-Bis(5-phenyl-2-oxazolyl)benzene (POPOP), p-bis(o-methylstyryl)benzene, 1,4-bis-2-(4-methyl-5-phenyloxazolyl)benzene, 2,2′-p-phenylenebis(5-phenoxazole), diphenylstilbene, 1,3,5-triaryl-2-pyrazolines, 4-(n-butylamino)-2-(4-methoxyphenyl)benzo[b]pyrylium perchlorate, sodium salicylate, 1,4-bis(2-methylstyryl)benzene (Bis-MSB), 7-dimethylamino-4-methyl-2-quinoline, 7-amino-4-methylcoumarin, 4,6-dimethyl-7-ethylamino coumarin, etc., or other such suitable second fluors as would be understood by one having ordinary skill in the art upon reading the present descriptions. Particularly preferred secondary fluors may include DPA, TPB, POPOP, and Bis-MSB according to various embodiments.
According to some approaches, the exemplary organic plastic scintillators and/or organic liquid scintillators may include a secondary fluor in a low concentration in order to maximize the beneficial wavelength-shifting effects for PSD performance. For example, the secondary fluor may be present in an amount of about 2 wt % or less, in one embodiment.
Li-Containing Compound
The scintillators disclosed herein (e.g. organic plastic scintillators; organic liquid scintillators) may include a Li-containing compound. In approaches including organic, plastic scintillator systems, suitable Li-containing compounds are those which may be homogenously dissolved in a polymer matrix (e.g. an aromatic polymer matrix, a non-aromatic polymer matrix, etc.), such that the scintillator material is substantially free (e.g. >99% free, preferably >99.5% free) of grain boundaries, comprises one phase, and is optically transparent, in some embodiments. In approaches including liquid scintillator systems, suitable Li-containing compounds are those which may be homogenously dissolved in a liquid scintillator material (e.g. an aromatic liquid, a non-aromatic liquid), such that the scintillator comprises one phase and is optically transparent.
In one embodiment, the Li-containing compound may be present in an amount ranging from about 0.01 wt % to about 20 wt %. In another embodiment, the Li-containing compound may be present in an amount ranging from about 0.01 wt % to about 1.5 wt %.
In additional embodiments, the Li-containing compound may be a Li-salt of a carboxylic acid. For example, a suitable Li-containing compound may be Li-3-phenylsalicylate, an exemplary formation of which is reproduced below:




While the above process shows purified 3-phenylsalicylic acid reacted with a 6LiOH reagent, other Li-containing reagents may be used such as a Li-metal, Li2CO3, LiCl, etc. Moreover, the formation of suitable Li-containing compounds may include combining any suitable carboxylic acid with a Li-containing reagent, as would be understood by one having skill in the art upon reading the present disclosure.

In more embodiments, the Li-containing compounds may include, but are not limited to, Li-acetylsalicylic acid, Li-3,5-di-tert-butylsalicylate, other Li-containing derivatives of salicylic and benzoic acids, lithium benzoate, Li-oxazolecarboxylates, and other such Li-containing compounds as would be understood by one having skill in the art upon reading the present disclosure.
In yet more embodiments, the Li-containing compounds may include substituents that promote hydrogen bonding and/or stability with a coordinating solvent (examples of which are provided below). In further embodiments, the Li-containing compounds may also include non-polar groups to promote solubility in the aromatic polymer matrix of the plastic scintillators and/or in the aromatic liquid scintillator materials of the liquid scintillators disclosed herein.
In particular embodiments, the Li-containing compound may be soluble in a primary fluor present within the plastic scintillators and/or liquid scintillators described herein. For instance, in some approaches, the Li-containing compound may be soluble in a primary fluor, such as biphenyl, naphthalene, PPO and/or other derivatives of oxazole, etc. In more approaches, the Li-containing compound may be soluble in a primary fluor present at ≦1 wt %. In still more approaches, the Li-containing compound may be soluble in a primary fluor present at about 0.5-10 wt % or more, an amount ranging from about 10 wt % to about 30 wt %, an amount ranging from about 30 wt % to about 75% wt %, etc.
Coordinating Solvents
In some embodiments, the organic plastic scintillators and/or the organic liquid scintillators disclosed herein may comprise a Li-containing compound, such as those described above, and a coordinating solvent, where the Li-containing compound is soluble in the coordinating solvent.
In one embodiment, the coordinating solvent may be 1,2-dimethoxyethane (DME). Other polar aprotic solvents may be employed in other embodiments, e.g. to facilitate the introduction of Li-containing compounds into polymer matrices and/or liquid scintillator materials. Exemplary polar aprotic solvents may include but are not limited tetrahydrofuran (THF), acetonitrile (MeCN), triethylamine (TEA), acetone, ethers, nitriles, amines, ketones, etc., and other known highly coordinating solvents.
In more embodiments, the coordinating solvent may be a polar protic solvent, such as methanol or isopropanol. However, as compared to polar protic coordinating solvents, polar aprotic coordinating solvents may have a higher affinity for the scintillating dyes and therefore result in plastic and/or liquid scintillators that are more stable.
In yet more embodiments, the coordinating solvent may be miscible with aromatic (e.g. non-polar and polar) and non-aromatic (e.g. non-polar and polar) compounds comprising the polymer matrix and/or the liquid scintillator materials described herein.
In further embodiments, the coordinating solvent may be present in an amount ranging from about 0.01 wt % to about 50 wt %.
In other embodiments, dissolution of the Li-containing compound in the coordinating solvent may result in the formation of an amorphous gel. For example, in one approach, 6Li-3-phenylsalicylate may be dissolved in excess 1,2-dimethoxyethane (DME) and the resulting solution left to evaporate slowly, thereby a forming a non-polar amorphous gel, as shown below:




The resulting amorphous gel may be soluble in known aromatic and nonaromatic monomer-based solution mixtures used for the preparation of organic plastic scintillators, such as those disclosed herein. Similarly, the resulting amorphous gel may also be soluble in known aromatic and nonaromatic liquid scintillator materials used for preparation of liquid scintillators, such as those disclosed herein.

Initiators and Cross-Linkers
In some embodiments, the scintillators disclosed herein may also comprise an polymerization initiator and/or a cross-linker. Suitable polymerization initiators may include, but are not limited to, organic peroxides, Luperox 231, etc. or other such initiators as would be understood by one having skill in the art upon reading the present disclosure. Suitable cross-linkers may include, but are not limited to, aromatic cross-linkers such divinyl benzene, divinyl toluene, trivinyl benzene, divinyl naphthalene, etc.; aliphatic cross-linking monomers such as di- and polyacrylates and methacrylates, etc; and other such cross-linkers as would be understood by one having skill in the art upon reading the present disclosure.
In additional embodiments, the initiator may be present in an amount ranging from about 0.001 wt % to about 1 wt %. In further embodiments, the cross-linker may be present in an amount ranging from about 0.05 wt % to about 5 wt %.
SCINTILLATOR FABRICATION EXAMPLES
Several illustrative examples for fabricating non-PSD scintillators capable of detecting thermal neutrons, and PSD scintillators capable of simultaneously detecting thermal and fast neutrons discriminated from gamma radiation background, are described below. It is important to note that these illustrative fabrication methods are in no way limiting, and are provided by way of example only.
Example 1
General Method for Fabricating Plastic and/or Liquid Scintillators
FIG. 4 depicts a method 400 of fabricating a plastic scintillator according to one embodiment. As an option, the present method 400 may be implemented to form the scintillators disclosed herein, such as those shown in the other FIGS. Further, the method 400 may be carried out in any desired environment. Moreover, more or less operations than those shown in FIG. 4 may be included in method 400, according to various embodiments. It should also be noted that any of the aforementioned features may be used in any of the embodiments described in accordance with this method and others disclosed herein.
As shown in FIG. 4, a scintillator precursor mixture may be placed in a heating vessel, and subsequently heated until a polymerization process has completed. See operations 402 and 404, respectively.
In one embodiment, the precursor mixture may include a monomer, one or more fluors, a coordinating solvent, and a Li-containing compound, wherein the Li-containing compound is soluble in the coordinating solvent and/or at least one of the one or more fluors.
In some approaches, the monomer may be capable of polymerizing and solvating the one or more fluors. In one particular approach, the monomer may comprise one or more aromatic groups. In another approach, the monomer may be nonaromatic. In yet another approach, the monomer may be non-polar. In a further approach, the monomer may be polar. In particular approaches, the monomer may comprise at least one of a 4-methylstrene, a vinyltoluene, a styrene, a methylmethacrylate precursor, and any other such monomer as would be understood by one having skill in the art upon reading the present disclosure.
In various approaches, the one or more fluors may comprise a primary fluor and a secondary fluor, where the secondary fluor has a longer wavelength than the primary fluor. In one particular approach, the primary fluor may be PPO. In another approach, the secondary fluor may be at least one of 9,10-diphenylanthracene, and p-bis-(o-methylstyryl)-benzene. In yet another approach, the primary and second fluor may be any suitable flour as described herein.
In more approaches, the Li-containing compound may be a Li-salt of carboxylic acid. In other approaches, the Li-containing compound may include at least one of Li-3-phenylsalicylate, Li-3,5-di-tert-butylsalicylate, Li-acetylsalicylic acid, derivatives of lithium benzoate, derivatives of Li-oxazolecarboxylate, etc.
In still more approaches, the coordinating solvent may be 1,2-dimethoxymethane or any suitable coordinating solvent as described herein. In preferred approaches, there may be a homogenous distribution of the Li-containing compound in the resulting scintillator.
In even more approaches, the precursor mixture may further include at least one of an initiator (e.g. Luperox 231) and a cross-linker (e.g. divinylbenzene).
After the scintillator precursor mixture has been placed in the heating vessel, and subsequently heated until a polymerization process has completed (e.g. operations 402 and 404), the resulting scintillator may exhibit an optical response signature for thermal neutrons that is different than an optical response signature for fast neutrons and gamma rays in various approaches. Moreover, the resulting scintillator may be substantially free of grain boundaries (e.g. it is homogenous) and optically transparent in preferred approaches.
Fabrication of Li-loaded liquid scintillators may be formed utilizing a method similar to method 400. For example, in some approaches. Li-containing complexes of carboxylic acids with coordinating solvents may be used for introduction of Li into the composition of liquid scintillators materials. In approaches where a liquid scintillator material with traditional fast neutron/gamma discrimination is used, incorporation of Li may add additional functionality for detection of thermal neutrons discriminated from gamma radiation and fast neutrons (e.g. as shown in FIG. 10B). With the use of a liquid scintillator material without PSD, thermal neutrons may be detected from a pulse height spectrum. Accordingly, utilizing Li-containing complexes of carboxylic acids with coordinating solvents to introduce Li into the composition of liquid scintillators material, may be used to commercially produce liquid scintillators with and without PSD having both traditional and non-flammable formulations.
Example 2
Non-PSD Plastic Scintillators
In one embodiment, colorless, optically transparent non-PSD plastic scintillators were fabricated from:

    
    
        about 0.001-1 wt % Luperox 231 (initiator);
        about 0.5-5 wt % 2,5-diphenyloxazole (PPO), as a primary fluor;
        about 0.1-2.0 wt % of a longer-wavelength compound, such as 9,10-diphenylanthracene (DPA), or p-bis-(o-methylstyryl)-benzene (BisMSB), as a secondary fluor;
        about 0.01-5 wt % 6Li-3-phenylsalicylate, as a thermal neutron capture reagent;
        about 0.01-5 wt % 1,2-dimethoxyethane (DME), for dissolution of the polar lithium salt into the non-polar plastic matrix;
        about 0.05-5 wt % divinylbenzene, as a cross-linker for added stability; and
        balance 4-methylstrene, styrene, or methylmethacrylate precursor.
    
    


The process of forming the above non-PSD plastic scintillators was conducted under oxygen-free conditions in a nitrogen-filled or argon-filled glovebox or in tightly sealed vessels. This process included combining the 6Li-3-PSA and DME to form a first solution; and combining the monomer, the primary fluor, and the second fluor to form a second solution. The first and second solutions were both gently heated until the first and second solutions were completely dissolved.
The first and second solutions were next combined to form a third solution, followed by addition of the initiator and crosslinker to the third solution. The third solution was added to a scintillation vial or glass jar that was degassed prior to entrance into the glovebox. Subsequently, the vessel was sealed and placed in an oven at about 50-55° C. under inert gas flow. The sealed vessel was allowed to cure undisturbed for about 144 hours, followed by a final curing step at about 70-75° C. overnight; after which, the resulting plastic scintillator was cooled to room temperature and removed from the vessel. The resulting plastic scintillator was rigid, homogenous transparent and exhibited scintillation detection properties for thermal neutron by at least pulse-height measurements.
FIGS. 5A and 5B illustrate neutron signatures obtained with the above 6Li-containing non-PSD plastics scintillators using moderated 252Cf source. Specifically, FIG. 5A shows the fraction of delayed light (QS) in a total scintillation pulse (QT) for the non-PSD plastic; whereas FIG. 5B shows the pulse height spectrum of the non-PSD plastic with a peak corresponding to the presence of neutrons.
Example 3
PSD Plastic Scintillators
In one embodiment, a colorless, optically transparent PSD plastic scintillator were fabricated from:

    
    
        about 0.001-1 wt % Luperox 231 (initiator);
        about 25-30 wt % 2,5-diphenyloxazole (PPO), as a primary fluor;
        about 0.1-2.0 wt % of a longer-wavelength compound, such as 9,10-diphenylanthracene (DPA), or p-bis-(o-methylstyryl)-benzene (BisMSB), as a secondary fluor;
        about 0.01-10 wt % 6Li-3-phenylsalicylate, as a thermal neutron capture reagent;
        about 0.01-5 wt % 1,2-dimethoxyethane, for dissolution of the polar lithium salt into the non-polar plastic matrix;
        about 0.05-5 wt % divinylbenzene, as a cross-linker for added stability; and
        balance 4-methylstrene, styrene, or methylmethacrylate precursor.
    
    


The process of forming the above PSD plastic scintillators was conducted under oxygen-free conditions in a nitrogen-filled or argon-filled glovebox or in tightly sealed vessels. This process included combining the 6Li-3-PSA and DME to form a first solution; and combining the monomer, the primary fluor, the second fluor to form a second solution. The first and second solutions were both gently heated until the first and second solutions were completely dissolved.
The first and second solutions were next combined to form a third solution, followed by addition of the initiator and crosslinker to the third solution. The third solution was added to a scintillation vial or glass jar that was degassed prior to entrance into the glovebox. Subsequently, the vessel was sealed and placed in an oven at about 50-55° C. under inert gas flow. The sealed vessel was allowed to cure undisturbed for about 144 hours, followed by a final curing step at about 70-75° C. overnight; after which, the resulting plastic scintillator was cooled to room temperature and removed from the vessel. The resulting plastic scintillator was rigid, homogenous, and transparent and exhibited scintillation detection properties for thermal neutrons and fast neutron discriminated from gamma radiation background by at least pulse shape discrimination measurements.
FIGS. 5C and 5D illustrate neutron signatures obtained with the above 6Li-containing PSD plastics scintillators using moderated 252Cf source. Specifically, FIG. 5C shows the fraction of delayed light (Qs) in a total scintillation pulse (Qr) for the PSD plastic; whereas FIG. 5D shows the pulse height spectrum of the non-PSD plastic with separated signatures of neutrons and gammas.
Example 4
Non-PSD Liquid Scintillators
In one embodiment, non-PSD liquid scintillators were fabricated from:

    
    
        about 0.5 wt % 2,5-diphenyloxazole (PPO), as a primary fluor;
        about 0.01 wt % of a longer-wavelength compound, such as 9,10-diphenylanthracene (DPA), or p-bis-(o-methylstyryl)-benzene (BisMSB), as a secondary fluor;
        about 0.01-10 wt % 6Li-3-phenylsalicylate, as a thermal neutron capture reagent;
        about 0.01-5 wt % 1,2-dimethoxyethane, for dissolution of the polar lithium salt into the non-polar plastic matrix.
    
    


In another embodiment, non-PSD liquid scintillator were fabricated from:

    
    
        any commercial liquid scintillator without PSD;
        about 0.01-10 wt % 6Li-3-phenylsalicylate, as a thermal neutron capture reagent;
        about 0.01-5 wt % 1,2-dimethoxyethane, for dissolution of the polar lithium salt into the non-polar plastic matrix.
    
    


The process of forming the above non-PSD liquid scintillators was conducted under oxygen-free conditions in a nitrogen-filled or argon-filled glovebox or in tightly sealed vessels. This process included combining the 6Li-3-PSA and DME to form a first solution; and combining the liquid scintillator material, the primary fluor, the second fluor to form a second solution. The first and second solutions were both gently heated until the first and second solutions were completely dissolved. The first and second solutions were next combined to form a third solution, followed by addition of the third solution to a scintillation vial or glass jar that was degassed prior to entrance into the glovebox. The resulting liquid scintillator was transparent, homogenous and exhibited scintillation detection properties for thermal neutrons by at least pulse height measurements.
Example 5
PSD Liquid Scintillators
In one embodiment, PSD liquid scintillators were fabricated from:

    
    
        about 20 wt % 2,5-diphenyloxazole (PPO), as primary fluor;
        about 0.1-2.0 wt % of a longer-wavelength compound, such as 9,10-diphenylanthracene (DPA), or p-bis-(o-methylstyryl)-benzene (BisMSB), as secondary fluor;
        about 0.01-10 wt % 6Li-3-phenylsalicylate, as a thermal neutron capture reagent;
        about 0.01-5 wt % 1,2-dimethoxyethane, for dissolution of the polar lithium salt into the non-polar plastic matrix.
    
    


In another embodiment. PSD liquid scintillators were fabricated from:

    
    
        any commercial liquid scintillator with PSD (e.g. EJ-301, EJ-309, etc.);
        0.01-10% 6Li-3-phenylsalicylate, as a thermal neutron capture reagent;
        0.01-5% 1,2-dimethoxyethane, for dissolution of the polar lithium salt into the non-polar plastic matrix.
    
    


Forming the above PSD liquid scintillators was conducted under oxygen-free conditions in a nitrogen-filled or argon-filled glovebox or in tightly sealed vessels. This process included combining the 6Li-3-PSA and DME to form a first solution; and combining the liquid scintillator material, the primary fluor, the second fluor to form a second solution. The first and second solutions were both gently heated until the first and second solutions were completely dissolved. The first and second solutions were next combined to form a third solution, followed by addition of the third solution to a scintillation vial or glass jar that was degassed prior to entrance into the glovebox. The resulting liquid scintillator was transparent, homogenous and exhibited scintillation detection properties for thermal neutrons and fast neutron discriminated from gamma radiation background by at least pulse shape discrimination measurements.
It is important to note that the above fabrication methodologies in Examples 1-5 are illustrative and non-limiting. For instance, organic plastic scintillator systems comprising a polymer matrix other than poly4-methylstrene, polystyrene, or poly(methylmethacrylate); fluors other than DPA/PPO; Li-containing compounds other than 6Li-3-phenylsalicylate; and/or coordinating solvents other than DME; and/or initiators and cross-linkers other than Luperox 231 and divinylbenzene, respectively, may be fabricated under similar conditions, while taking account for slight variations (e.g. to temperature, incubation time, amount of respective components, etc.) as would be understood by a skilled artisan reading the present descriptions. Likewise, the organic liquid scintillator systems may comprise fluors other than DPA/PPO; Li-containing compounds other than 6Li-3-phenylsalicylate; and/or coordinating solvents other than DME may be fabricated under similar conditions while also taking account for slight variations.
Experimental Results
All experiments described below were conducted with the Li-salt of 3-phenylsalicylic acid (Li-3-PSA). A choice of this compound was made based on the properties that allowed for its dissolution in both liquid and plastic aromatic matrices.
Li-3-PSA may be synthesized in variety of ways using 3-phenylsalicylic acid (3-PSA) and different Li-containing compounds, such as Li-metal, Li2CO3, LiCl, etc. In the experiments described below, Li-3-PSA was obtained as a product of a neutralization reaction conducted in MeOH between 3-PSA (TCI America) and lithium hydroxide, LiOH (Aldrich):




Liquid scintillators were prepared in p-xylene solutions, while polystyrene (PS) was used as a matrix for the preparation of plastics. All procedures with liquid or plastic preparations were conducted under oxygen-free conditions in an Ar-filled glove box or in tightly sealed vessels. To facilitate efficient PSD, high concentration of the primary dye (30 weight % of 2,5-diphenyloxazole, PPO) was used in combination with 0.2% of a secondary dye (9,10-diphenylanthacene, DPA or 1,4-Bis-2-methylstyryl-benzene, Bis-MSB), in both liquid and plastic mixtures. Li-3-PSA, dissolved in a small amount of 1,2-dimethoxyethane (DME), was added to liquid scintillator mixtures in the amount from 5% to 10% to the total weight of the scintillator. The same fraction of Li-3-PSA was introduced into the styrene-monomer mixture prior to the polymerization conducted according to a modified procedure used for the standard PSD plastic production.
Photoluminescence (PL) spectra were measured under UV excitation using a commercial Spex Fluoromax-2 spectrometer. Neutron detection properties of samples were studied using a 252Cf source shielded with 5.1 cm of lead, to irradiate samples coupled to Hamamatsu R6231-100-SEL photomultiplier tube (PMT). Gamma-equivalent energy calibration was made based on the position of the Compton edge in the 137Cs spectra, in which 500 keVee was defined by 50% of the Compton edge peak. High density polyethylene (HDPE) moderation was used to obtain a flux of thermal neutrons from the 252Cf source. The signals collected at the PMT anode were recorded using a 14-bit high-resolution waveform CompuScope 14200 digitizer with a sampling rate of 200 MS/s, for offline analysis.
The waveforms were numerically integrated over two time intervals: ΔtTotal and a subinterval ΔtTail, corresponding to the total charge (QTotal) and the delayed component (QTail) of the signal, respectively. The value of the ratio of charge R=QTail/QTotal for the two time intervals indicated whether the considered event was likely produced by a neutron (high R value) or a ganmma ray (small R value). Quantitative evaluation of PSD was made using Figures of Merit FOM=S/(δ1+δ2), where S is the separation between the peaks corresponding to different types of radiation, and δ1 and δ2 are full width at half maxima (FWHM) of the corresponding peaks. The time gates used for PSD measurements were optimized to maximize figures of merit for each composition with different decay characteristics. It should be noted that, since the reported scintillators exhibited triple signatures corresponding to thermal neutrons, fast neutrons, and gammas, corresponding FOMs were calculated not only for the total neutron/gamma discrimination, but considered separately for fast-neutron/gamma, thermal-neutron/gamma, and thermal/fast neutron discrimination using multiple Gaussian approximation from a fitted function of the data corresponding to each type of radiation.
The decay characteristics were measured using time correlated single photon counting based on digitized signals from a tightly coupled, i.e. “start time”, PMT (H3378-50) which responds to a significant fraction of the photons and a loosely coupled PMT (H5783P) which responds to, on average, much fewer than 1 photon per Compton scatter. Signals were digitized at 2.5 GS/s using a PicoScope 6404B USB Oscilloscope. The difference in peak times, as determined with subsample interpolation, for each start/stop pair was added to a histogram. The prompt region of the resulting decay, after the resolution-smeared peak, was fitted with an exponential to measure the prompt decay characteristics.
Thermal neutron capture efficiency was calculated by MCNPX simulation using neutron capture ion algorithm (NCIA). Scintillator of specific composition was exposed to thermal neutron flux (25 meV) incident on detector with the original direction along the detector axis. Pulse height tally (f8) for one of the reaction fragments, tritium, was used to account for a neutron being captured and tritium depositing its energy to the scintillator.
Experimental Results
Plastic Scintillators
Li-3-PSA is a colorless, stable in air compound that melts at about 137-138° C. Similarly to many commercially produced or lab-synthesized Li-organic compounds, it does not dissolve in any aromatic solvents, including toluene, xylene, or monomers, such as vinyltoluene or styrene used for liquid or plastic preparation. However, in the scintillator systems discloses herein, Li-3-PSA has been found to freely dissolve in many coordinating solvents, acetone, methanol, or DME miscible with pure aromatics. It is imporant to note, that simple solubility in coordinating solvents may not be a sufficient condition for the dissolution of Li-salts in aromatic matrices in various cases. Rather, without wishing to be bound by any theory, it is thought that Li-3-PSA reacts with some coordinating solvents to produce stable complexes with a diminished polarity, the diminished polarity helping its incorporation into organic matrices. This is supported by results obtained with the combination of Li-3-PSA and DME dissolved in polymer matrices containing high loading of PPO, which allows for current production of stable, optically transparent plastics with the Li-3-PSA concentrations of 5-10 wt. % (as shown in FIG. 6). In the case of a fully enriched 6Li single-isotope compound, this range of concentrations corresponds to 0.135-0.27 wt. % of 6Li loads. While higher content of 6Li may be utilized in some approaches, estimations show that Li-3-PSA concentrations of 5-10 wt. % are sufficient to provide thermal neutron capture efficiency up to 50-60% at a scintillator thickness of about 3 cm, thereby realizing new plastics useful for practical applications.
With respect to physical properties, such as hardness or transparency, the new Li-containing plastic scintillators disclosed herein are similar to regular plastic scintillators. The new Li-containing plastic scintillators do not evidence any obvious signs of physical instability or scintillation performance degradation. Measurements of PL spectra and decay characteristics showed that new Li-containing plastic scintillators have luminescence and prompt lifetimes typical for organic scintillators, in which the final emission generally occurs from the molecules of the secondary dyes (wavelength shifters). For the Li-containing plastic with DPA and Bis-MSB fluors, the lifetimes were measured as 6 ns and 3.4 ns, respectively. Examples of waveform averages obtained with the PSD plastics containing DPA as a secondary dye are shown in FIGS. 12A-12B. As follows from the comparison, all measured pulse averages have a shape consistent with having the same prompt component determined by the final emission from the DPA molecules. The relative intensities of the delayed emission, on the other hand, are different in pulses induced by different types of radiation. A regular hydrocarbon-based plastic (FIG. 12A) shows a scintillation response typical for organic scintillators with PSD, in which pulses corresponding to fast neutrons have noticeably higher relative intensity of the delayed component. The addition of 6Li to the composition of this plastic adds one more signature with a respective waveform containing the largest fraction of the delayed emission produced by the heavier alpha and triton particles originating in the capture reaction (FIG. 12B).
FIGS. 7A-7B shows experimental PSD patterns obtained, with selected samples of Li-loaded plastics measured with varying degree of HDPE moderation. The first pattern shown in FIG. 7A obtained with a plastic loaded with natural abundance of NatLi ( ̃7.5% of 6Li) and a low degree of moderation provided by a 9 mm-thick cylinder containing the 252Cf source is similar to that typical for unloaded PSD plastics irradiated by a 252Cf source that emits only fast neutrons and gammas, with the exception of the small count increase corresponding to thermalized neutrons always present in the environment (shown by the arrow). The intensity of the thermal neutron peak increases with the increase of 6Li load and the use of moderation that leads to the fast-to-thermal neutron conversion (as shown in FIGS. 7B and 7D). For the cases corresponding to an intermediate amount of 252Cf moderation or to realistic conditions when both thermal and fast neutrons are present, the “triple” signature of two types of neutrons and gamma-rays can be obtained (as shown in FIG. 7C). It should be noted that at increasing size additional moderation effects may be introduced by the hydrogen-rich environment of the detector itself. Due to the monoenergetic nature of the alpha and triton particles produced by the capture reaction, the thermal neutron pulses may be concentrated in a narrow energy range leading to the appearance of a sharp peak with energy resolution (calculated as FWHM/mean) of 13% to 20%, depending on the composition and size of measured plastics.
The ability to simultaneously detect both thermal and fast neutrons discriminated from gamma rays is an important feature of the new Li-containing plastic scintillators. This unique property brings a substantial advantage of lithiated plastics over their regular unloaded counterparts, which can be used only for detection of fast neutrons above the low-energy threshold for neutron/gamma separation. For most known organic scintillators with PSD, the separation threshold falls into the energy range of about 50-100 keVee, which, depending on the type of materials with different quenching factors, may correspond to the neutron energy of 300-700 KeV. This means that a large fraction of less energetic neutrons cannot be detected using regular PSD materials. The fact that particles resulting from capture reaction produce scintillation pulses far above the discrimination threshold allows for the detection of thermal neutrons in the energy range where their signature can be clearly separated from the gamma radiation background.
FIGS. 8A and 8B show 252Cf PSD profiles obtained with 2.5 cm×2.5 cm thick plastics containing 30% of PPO and 5% of Li-3-PSA in PS matrix with 0.2% additions of secondary dyes. Specifically, the PSD plastic shown in FIG. 8A comprises the secondary dye DPA, and was measured with 10 cm of HDPE moderation. The PSD plastic shown in FIG. 8B comprises the secondary dye B—Bis-MSB, and was measured with 2.5 cm of HDPE moderation. FOMs are calculated in near-thermal-neutron-spot energy range of by Gaussian approximation of individual peaks for gamma, fast neutrons and thermal neutrons. The PSD FOMs with mean values of 3.45 and 2.96 calculated for the first examples of 6Li-loaded plastics in the near-thermal-neutron energy range (FIGS. 8A and 8B, respectively) correspond to efficient thermal neutron/gamma discrimination. At the same time, as shown in FIGS. 8A and 8B, the sensitivity to thermal neutrons does not diminish the ability of the new materials to detect fast neutrons (FOMs equal to 2.48 and 2.33, respectively).
Simultaneous detection of thermal and fast neutrons increases the total efficiency of neutron detection which, combined with a good degree of discrimination from gamma-rays, makes these novel 6Li-loaded plastics attractive for application in currently deployed detectors. There is, however, one more property of the new materials never utilized before in organic scintillators. As shown by the results of FIGS. 8A and 8B, in addition to the traditional neutron/gamma separation, there is one more type of discrimination between the signatures of thermal neutrons (alpha and triton particles) and fast neutrons (recoiled protons). The PSD FOMs corresponding to this type of discrimination are 0.759 and 0.63 as shown in FIGS. 8A and 8B, respectively.
Experimental Results
Liquid Scintillators
The experimental results for the liquid scintillators disclosed herein show that the Li compounds used for the preparation of plastic scintillators may also be introduced into compositions of liquids, e.g. aromatic liquids. For example, similar concentrations of Li-3-PSA may be used to obtain efficient PSD in xylene-based mixtures containing different primary and secondary dyes. Preliminary measurements made with traditional liquid scintillators (EJ-309) indicate that the method may be promising for commercial production of 6Li-loaded liquid scintillators with PSD.
Liquid scintillators were studied because of the advantage of easy and inexpensive preparation of efficient PSD mixtures by dissolution of only one scintillation dye in an aromatic solvent. FIG. 9A shows an energy-calibrated PSD pattern obtained with a liquid scintillator prepared with 30 wt. % of PPO dissolved in p-xylene, without any secondary dyes. Similar to plastic scintillators, addition of 5% Li-3-PSA to PPO-xylene solution leads to the appearance of a spot, corresponding to the thermal neutron signature. However, a surprising result is that this spot is practically embedded into the points corresponding to the fast neutron pulses, leading to a PSD profile with only one neutron band which includes both fast and thermal neutron counts separated from gammas (FOM=1.67 shown in FIG. 9B). The result may indicate that there is no detectable difference in the fraction of the delayed light produced by neutron capture products and protons in the scintillation material, making it similar to that reported in the past for a 6Li-loaded organic scintillators, where no discrimination between fast and thermal neutrons was observed.
Without wishing to be bound by any theory, it is thought that due to the much lower concentration threshold in liquids, efficient PSD may be obtained with a larger variety of low-soluble scintillation dyes (e.g., DPA, Bis-MSB, POPOP, etc.), which can be used for comparative analysis. FIGS. 10A and 10B shows the results obtained with one such dye, DPA, which when dissolved as a primary dye only at 1% concentration in xylene, produces PSD comparable to the best organic crystals and commercial liquids. As illustrated by FIGS. 10A and 10B, when 5% of Li-3-PSA is added to the DPA-xylene solution, the resulting mixture not only preserves the initial high degree of fast neutron/gamma discrimination, but also exhibits clearly pronounced separation between the signatures of two kinds of neutrons, with additional improvement of thermal neutron peak resolution calculated for the case of the DPA liquids as 9.5%.
Again not wishing to be bound by any theory, it is thought that one of the possible explanations of the differences in the performance of the liquids represented by FIGS. 9A/9B and 10A/10B may relate to the relation between the component band gaps, reflected by their PL spectra (as shown in FIGS. 11A-11D). Despite the fact that the first solution (FIG. 11A) contained a very high concentration of PPO (30%), its final spectrum is not of PPO but of typical Li-3-PSA emission. On the contrary, the second solution (FIG. 11C), which has only 1% of DPA mixed with 5% of Li-3-PSA, produces PL emission with the spectrum corresponding to pure DPA dissolved in xylene. The spectral differences may be understood based on the well-studied energy transfer phenomena in organic scintillation mixtures. The fact that the excitation spectrum of Li-3-PSA coincides with the emission spectrum of PPO (FIG. 11B) leads to the absorption of PPO emission by the molecules of Li-3-PSA that produces the major fraction of the final scintillation light. No such absorption is possible in the mixture of DPA and Li-3-PSA, in which, due to the lack of the spectral overlap (FIG. 11D), light from DPA cannot be absorbed by Li-3-PSA. The result is that in the first case the final emission is determined by Li-3-PSA molecules that presumably have significantly lower quantum yield in comparison to PPO, while in the second case, the final light output is produced mainly by very efficient DPA molecules. The major fraction of the light yield measured in both PL and scintillation experiments results from the de-excitation of the excited singlet states. However, similar processes of energy transfer may take place with the triplet states, migration and annihilation of which will be influenced by the combination of the bandgaps, leading to the differences in the light yield and PSD efficiency.
Applications and Uses
Embodiments of the present invention may be used in a wide variety of applications, and potentially any application in which high light yield and/or pulse shape discrimination between gammas, fast and thermal neutrons, charged particles, etc. is useful.
Illustrative uses of various embodiments of the present invention include, but are not limited to, applications requiring radiation detection. Detection, surveillance and monitoring of radioactive materials, including identification of special nuclear materials (SNM), are a few such examples. Various embodiments can also be used in the nuclear fuel cycle, homeland security applications, nuclear non-proliferation, medical imaging, special nuclear material, high energy physics facilities, etc. Moreover, the figure of merit (FOM) performance metric is already sufficient to distinguish neutrons from gammas down to the few hundred keV/gamma equivalent regime, and will be very useful for non-proliferation, homeland security and safeguards applications.
Yet other uses include detectors for use in treaty inspections that can monitor the location of nuclear missile warheads in a nonintrusive manner. Further uses include implementation in detectors on buoys for customs agents at U.S. maritime ports, cargo interrogation systems, and instruments that emergency response personnel can use to detect or search for a clandestine nuclear device. Assessment of radiological dispersal devices is another application.
Further applications include radiography, dosimetry, and scientific research.
Any of the methods, systems, devices, etc. described above, taken individually or in combination, in whole or in part, may be included in or used to make one or more systems, structures, etc. In addition, any of the features presented herein may be combined in any combination to create various embodiments, any of which fall within the scope of the present invention. Following are several examples of general and specific embodiments.
For example, according to one embodiment a thermal neutron capture reagent, comprises a lithium-containing compound selected from a group consisting of: Li-3-phenylsalicylate, Li-3,5-di-tert-butylsalicylate, and Li-acetylsalicylic acid, wherein the compound is soluble in a fluor. In some approaches, the thermal neutron capture reagent may exhibit an optical response signature for thermal neutrons.
According to another embodiment, a plastic scintillator comprises a polymer matrix, a primary fluor, and a Li-containing compound, where the Li-containing compound is soluble in the primary fluor. The plastic scintillator may exhibit an optical response signature for thermal neutrons that is different than an optical response signature for fast neutrons and gamma rays in one approach. The scintillator may be substantially free of grain boundaries (e.g. comprise a homogenous composition) and be optically transparent in another approach.
In some approaches, the polymer matrix present in the plastic scintillator may include one or more aromatic groups. In other approaches, the polymer matrix present in the plastic scintillator may be selected from a group consisting of: polystyrene, polyvinyltoluene, and poly(methylmethacrylate). In yet other approaches, an initiator and a cross-linker may be present in the polymer matrix.
In more approaches, the primary fluor deposed in the plastic scintillator may be present in an amount of 0.5 wt % or more. In one approach, the primary fluor may be 2,5-diphenyloxazole (PPO). In another approach, the primary fluor may be crosslinked to the polymer matrix.
In even more approaches, the Li-containing compound present in the plastic scintillator may be a Li-salt of a carboxylic acid. In a preferred approach, the Li-containing compound may be selected from a group consisting of: Li-3-phenylsalicylate, Li-3,5-di-tert-butylsalicylate, and Li-acetylsalicylic acid.
In additional approaches, the plastic scintillator may further include a coordinating solvent. In various approaches, the Li-containing compound may be soluble in the coordinating solvent. In exemplary approaches, the coordinating solvent may be selected from the group consisting of: acetone, methanol, and dimethoxyethane.
In further approaches, the plastic scintillator may further comprise a secondary fluor, where the secondary fluor has a longer wavelength than the primary fluor. For instance, in one approach, the secondary dye may be present in an amount of less than 2 wt %. In another approach, the secondary fluor may be selected from the group consisting of: 9,10-diphenylanthracene and p-bis-(o-methylstyryl)-benzene.
In yet further approaches, a system may include the plastic scintillator described above, and a photodetector for detecting the response of the scintillator material to at least one or neutron and gamma ray irradiation.
According to another embodiment, a liquid scintillator includes a liquid scintillator material, a primary fluor that may be present in an amount of 0.01 wt % or more, and a Li-containing compound that is soluble in the primary fluor. In some approaches, the liquid scintillator may exhibit an optical response signature for thermal neutrons that is different than an optical response signature for fast neutrons and gamma rays. The liquid scintillator may be comprise a homogenous composition and be optically transparent in another approach.
In some approaches, the liquid scintillator material may be any commercially available liquid scintillator material with and/or without PSD properties. In other approaches, the liquid scintillator material may include one or more aromatic groups. In yet other approaches, the liquid scintillator material may include a xylene-based liquid.
In more approaches, the primary fluor deposed in the liquid scintillator may be present in an amount of 0.01 wt % or more. In one approach, the primary fluor may be 2,5-diphenyloxazole (PPO). In another approach, the primary fluor may be crosslinked to the polymer matrix.
In even more approaches, the Li-containing compound present in the liquid scintillator may be a Li-salt of a carboxylic acid. In a preferred approach, the Li-containing compound may be selected from a group consisting of: Li-3-phenylsalicylate, Li-3,5-di-tert-butylsalicylate, and Li-acetylsalicylic acid.
In additional approaches, the liquid scintillator may further include a coordinating solvent. In various approaches, the Li-containing compound may be soluble in the coordinating solvent. In exemplary approaches, the coordinating solvent may be selected from the group consisting of: acetone, methanol, and dimethoxyethane.
In further approaches, the liquid scintillator may further comprise a secondary fluor, where the secondary fluor has a longer wavelength than the primary fluor. For instance, in one approach, the secondary dye may present in an amount of less than 2 wt %. In another approach, the secondary fluor may be selected from the group consisting of: 9,10-diphenylanthracene and p-bis-(o-methylstyryl)-benzene.
In yet further approaches, a system may include the liquid scintillator described above, and a photodetector for detecting the response of the scintillator material to at least one or neutron and gamma ray irradiation.
While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

## Claims

1. A thermal neutron capture reagent, comprising:
a lithium-containing compound selected from a group consisting of: Li-3-phenylsalicylate, Li-3,5-di-tert-butylsalicylate, Li-acetylsalicylic acid, and combinations thereof,
wherein the lithium-containing compound is soluble in a fluor,
wherein the thermal neutron capture reagent exhibits an optical response signature for thermal neutrons.

2. A scintillator, comprising:
a scintillator material;
a primary fluor; and
a Li-containing compound,
wherein the Li-containing compound is soluble in the primary fluor,
wherein the scintillator exhibits an optical response signature for thermal neutrons that is different than an optical response signature for fast neutrons and gamma rays,
wherein the Li-containing compound is selected from a group consisting of: Li-3-phenylsalicylate, Li-3,5-di-tert-butylsalicylate, Li-acetylsalicylic acid, and combinations thereof.

3. The scintillator as recited in claim 2, wherein the scintillator material is optically transparent.

4. The scintillator as recited in claim 2, wherein the primary fluor is 2,5-diphenyloxazole (PPO).

5. The scintillator as recited in claim 2, further comprising a coordinating solvent, wherein the Li-containing compound is soluble in the coordinating solvent.

6. The scintillator as recited in claim 5, wherein the coordinating solvent is selected from a group consisting of: acetone, methanol, and dimethoxyethane.

7. The scintillator as recited in claim 2, further comprising a secondary fluor, wherein the secondary fluor has a longer wavelength than the primary fluor.

8. The scintillator as recited in claim 7, wherein the secondary fluor is present in an amount of less than 2 wt %.

9. The scintillator as recited in claim 7, wherein the secondary fluor is selected from a group consisting of: 9,10-diphenylanthracene and p-bis-(o-methylstyryl)-benzene.

10. The scintillator as recited in claim 2, wherein the scintillator material comprises a polymer matrix.

11. The scintillator as recited in claim 10, wherein the polymer matrix includes one or more aromatic groups.

12. The scintillator as recited in claim 10, wherein the polymer matrix is selected from a group consisting of: polystyrene, polyvinyltoluene, and poly(methylmethacrylate).

13. The scintillator as recited in claim 10, wherein the polymer matrix comprises an initiator and a cross-linker, wherein the initiator is present in an amount ranging from about 0.001 wt % to about 1 wt %, and wherein the cross-linker is present in an amount ranging from about 0.05 wt % to about 5 wt %.

14. The scintillator as recited in claim 2, wherein the scintillator material comprises a liquid scintillator material.

15. The scintillator as recited in claim 14, wherein the liquid scintillator material includes one or more aromatic groups.

16. The scintillator as recited in claim 14, wherein the liquid scintillator material includes a xylene-based liquid.

17. A system comprising:
the scintillator of claim 2; and
a photodetector for detecting the response of the scintillator to at least one or neutron and gamma ray irradiation.

18. A method for fabricating a scintillator, the method comprising:
forming a precursor mixture; and
heating the precursor mixture until a polymerization process is complete,
wherein the precursor mixture comprises:
a monomer;
one or more fluors;
a coordinating solvent; and
a Li-containing compound selected from a group consisting of: Li-3-phenylsalicylate, Li-3,5-di-tert-butylsalicylate, Li-acetylsalicylic acid, and combinations thereof,
wherein the Li-containing compound is soluble in at least one of the one or more fluors.