Multi-Model Particle Detection Using Pulse Shape Discrimination with Chemically Modified Silicone Matrices

Abstract

Chemically-modified silicone-based matrices for use in radiation detection. The base matrix is capable of multi-modal particle detection via pulse shape discrimination (PSD), relying in differences in the interaction mechanics between various types of radiation and the matrix itself to produce light with characteristic properties dependent on the incident particle type and energy. The materials, radiation detection devices using the materials, and methods of using the materials as radiation detectors.

Claims

1. A scintillator having pulse shape discrimination (PSD) capabilities resulting in a Figure-of-Merit (FOM) value of greater than 1.5 at a light output of less than ADDCCH 1600.

2. A scintillator having PSD capabilities resulting in a FOM value of greater than 1.7 at a light output of less than ADDCCH 4000.

3. A scintillator having PSD capabilities resulting in a FOM value of greater than 2.0.

4. A silicone-based scintillator having PSD capabilities resulting in a FOM value of greater than 1.5 at a light output of less than ADDCCH 1600.

5. A silicone-based scintillator having PSD capabilities resulting in a FOM value of greater than 1.7 at a light output of less than ADDCCH 4000.

6. A silicone-based scintillator having PSD capabilities resulting in a FOM value of greater than 2.0.

7. A silicone-based scintillator matrix with a chemical composition:
[O-M-(R)(R)] wherein M is selected from the group consisting of Si, B, Ge, Ti, Sn, Pb, Bi, Sb, Zn, and W; and wherein R and R are each selected from the group consisting of hydrogen, vinyl, methyl, phenyl, naphthyl, and other alkyl and aromatic substituents.

8. A silicone-based scintillator comprising: silicone-based scintillator matrix of claim 7; and dopant; wherein the total dopant concentration is less than approximately 20 mass % with respect to the matrix mass.

9. The silicone-based scintillator of claim 8, configured to have PSD capabilities resulting in a FOM value of greater than 1.5 at a light output of less than ADDCCH 1600.

10. The silicone-based scintillator of claim 8, configured to have PSD capabilities resulting in a FOM value of greater than 1.7 at a light output of less than ADDCCH 4000.

11. The silicone-based scintillator of claim 8, configured to have PSD capabilities resulting in a FOM value of greater than 2.0.

12. The silicone-based scintillator of claim 8, wherein the total dopant concentration is from approximately 3 to approximately 5 mass % with respect to the matrix mass.

13. A silicone-based scintillator comprising: silicone-based scintillator matrix of claim 7; and a primary dopant in a concentration of between approximately 1 to approximately 30 mass % with respect to the matrix mass.

14. The silicone-based scintillator of claim 13, wherein the primary dopant is selected from the group consisting of PPO, 9,9-dimethyl-2-phenyl-9H-fluorene (PhF), and alkylated soluble terphenyl derivatives.

15. The silicone-based scintillator of claim 13, wherein the primary dopant is selected from the group consisting of reactive fluorescent dopants with functional groups that allow their chemical incorporation into the silicone matrix.

16. The silicone-based scintillator of claim 13 further comprising a secondary dopant different than the first dopant.

17. The silicone-based scintillator of claim 16, wherein the secondary dopant concentration is between approximately 0.1 to approximately 5 mass % with respect to the primary dopant.

18. The silicone-based scintillator of claim 16, wherein the secondary dopant is selected from the group consisting of 1,4-bis(5-phenyloxazol-2-yl) benzene (POPOP), and 9,9-dimethyl-2,7-di((E)-styryl)-9H-fluorene (SFS).

19. The silicone-based scintillator of claim 16, wherein the secondary dopant is selected from the group consisting of reactive fluorescent dopants with functional groups that allow their chemical incorporation into the silicone matrix.

20. The silicone-based scintillator of claim 13, wherein the silicone-based scintillator matrix comprises one or more isotopes to create sensitivity to thermal neutrons via nuclear reactions which produce charged particles producing distinct pulse shapes.

21. The silicone-based scintillator of claim 13, wherein the silicone-based scintillator matrix comprises an optically-bonded scintillator material different from the base scintillator matrix.

22. A silicone-based scintillator comprising: silicone-based scintillator matrix; and dopant; wherein the total dopant concentration is less than approximately 20 mass % with respect to the matrix mass; and wherein the silicone-based scintillator has a processing time of less than one day.

23. A silicone-based scintillator comprising: a base material of a chemically modified silicone material that has a tunable phenyl (aromatic) group content; and dopant; wherein the total dopant concentration is less than approximately 20 mass % with respect to the base material mass.

24. A silicone-based scintillator matrix with a chemical composition:
[O-M-(R)(R)] wherein M is Si; wherein R is methyl; and wherein R is phenyl.

25. A silicone-based scintillator comprising: silicone-based scintillator matrix of claim 24; and dopant; wherein the total dopant concentration is less than approximately 20 mass % with respect to the matrix mass.

26. The silicone-based scintillator of claim 25, configured to have an FOM of greater than 1.5 at a light output of less than ADDCCH 1600.

27. The silicone-based scintillator of claim 25, configured to have an FOM of greater than 1.7 at a light output of less than ADDCCH 4000.

28. The silicone-based scintillator of claim 25, configured to have an FOM of greater than 2.0.

29. The silicone-based scintillator of claim 25, wherein the total dopant concentration is from approximately 3 to approximately 5 mass % with respect to the matrix mass.

30. A silicone-based scintillator comprising: silicone-based scintillator matrix of claim 24; and a primary dopant in a concentration of between approximately 1 to approximately 30 mass % with respect to the matrix mass.

31. The silicone-based scintillator of claim 30, wherein the primary dopant is selected from the group consisting of PPO, 9,9-dimethyl-2-phenyl-9H-fluorene (PhF), and alkylated soluble terphenyl derivatives.

32. The silicone-based scintillator of claim 30, wherein the primary dopant is selected from the group consisting of reactive fluorescent dopants with functional groups that allow their chemical incorporation into the silicone matrix.

33. The silicone-based scintillator of claim 30 further comprising a secondary dopant different than the first dopant.

34. The silicone-based scintillator of claim 33, wherein the secondary dopant concentration is between approximately 0.1 to approximately 5 mass % with respect to the primary dopant.

35. The silicone-based scintillator of claim 33, wherein the secondary dopant is selected from the group consisting of 1,4-bis(5-phenyloxazol-2-yl) benzene (POPOP), and 9,9-dimethyl-2,7-di((E)-styryl)-9H-fluorene (SFS).

36. The silicone-based scintillator of claim 33, wherein the secondary dopant is selected from the group consisting of reactive fluorescent dopants with functional groups that allow their chemical incorporation into the silicone matrix.

37. The silicone-based scintillator of claim 30, wherein the silicone-based scintillator matrix comprises one or more isotopes to create sensitivity to thermal neutrons via nuclear reactions which produce charged particles producing distinct pulse shapes.

38. The silicone-based scintillator of claim 30, wherein the silicone-based scintillator matrix comprises an optically-bonded scintillator material different from the base scintillator matrix.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] FIG. 1 is a graph of optical photon emission from excited atomic electrons as they return to the ground state through various pathways as detected by a photosensor.

[0052] FIG. 2 is the PSD from the present silicone-based scintillator with 4% PPO using a PuBe source.

[0053] FIG. 3 is the PSD from an industry leading PVT material containing approximately 30% PPO using a PuBe source.

[0054] FIG. 4 illustrates the definition of FOM (reproduced from N. Zaitseva et al., Pulse Shape Discrimination in Impure and Mixed Single-Crystal Organic Scintillators, IEEE T.N.S. 58:6, 3411 (2011)).

[0055] FIG. 5 is an example of a market leading liquid scintillator doped with thermal neutron sensitive compounds in attempt to separate thermal and fast neutrons. Thermal neutron events appear in the middle of the fast neutron and gamma ray lobes, centered at ADCCH 800 and PSP 0.2.

[0056] FIG. 6 is an example of the present invention capable of gamma ray (lobe centered at PSP 0.1) as well as thermal (all events with PSP>0.5) and fast neutron (PSP 0.25) separation.

[0057] FIG. 7 illustrates separation FOM from FIG. 6 at a slice in the energy domain from ADCCH 1000 to 3000 (roughly 400 keVee average) showing the separation capability of thermal events (PSP>0.5) from fast neutrons and gamma rays.

[0058] FIG. 8 illustrates a silicone-based scintillator without thermal neutron dopants when exposed to a PuBe source.

[0059] FIG. 9 shows the PSD FOM as a function of energy for the experiment in FIG. 8 where the circles are the calculated FOM from the silicone-based material, the diamonds are the calculated FOM from the industry leading PVT, and the dotted line is the FOM=1.27 threshold.

[0060] FIG. 10 is a FOM vs ADC comparison of exemplary embodiments of the present invention having different representative dopant concentration and types.

[0061] FIG. 11 illustrates the light output of exemplary embodiments of FIG. 10.

DETAIL DESCRIPTION OF THE INVENTION

[0062] To facilitate an understanding of the principles and features of the various embodiments of the invention, various illustrative embodiments are explained below. Although exemplary embodiments of the invention are explained in detail, it is to be understood that other embodiments are contemplated. Accordingly, it is not intended that the invention is limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, in describing the exemplary embodiments, specific terminology will be resorted to for the sake of clarity.

[0063] It must also be noted that, as used in the specification and the appended claims, the singular forms a, an and the include plural references unless the context clearly dictates otherwise. For example, reference to a component is intended also to include composition of a plurality of components. References to a composition containing a constituent is intended to include other constituents in addition to the one named.

[0064] Also, in describing the exemplary embodiments, terminology will be resorted to for the sake of clarity. It is intended that each term contemplates its broadest meaning as understood by those skilled in the art and includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.

[0065] Ranges may be expressed herein as from about or approximately or substantially one particular value and/or to about or approximately or substantially another particular value. When such a range is expressed, other exemplary embodiments include from the one particular value and/or to the other particular value.

[0066] Similarly, as used herein, substantially free of something, or substantially pure, and like characterizations, can include both being at least substantially free of something, or at least substantially pure, and being completely free of something, or completely pure.

[0067] By comprising or containing or including is meant that at least the named compound, element, particle, or method step is present in the composition or article or method, but does not exclude the presence of other compounds, materials, particles, method steps, even if the other such compounds, material, particles, method steps have the same function as what is named.

[0068] It is also to be understood that the mention of one or more method steps does not preclude the presence of additional method steps or intervening method steps between those steps expressly identified. Similarly, it is also to be understood that the mention of one or more components in a composition does not preclude the presence of additional components than those expressly identified.

[0069] The materials described as making up the various elements of the invention are intended to be illustrative and not restrictive. Many suitable materials that would perform the same or a similar function as the materials described herein are intended to be embraced within the scope of the invention. Such other materials not described herein can include, but are not limited to, for example, materials that are developed after the time of the development of the invention.

[0070] Scintillation materials are used in the detection and measurement of radiation. Scintillators are composed of substances which are capable of absorbing energy given off by ionization radiation, e.g., the fission fragments emitted by radioactive elements. The absorbed energy excites fluorescent materials (fluors) contained in the scintillator, such that the fluorescent materials give off light. Such scintillators are useful in many different applications, e.g., the detection of radioactive mineral deposits, and the detection and measurement of radioactive contamination.

[0071] When radiations comprising electrically charged particles such as -rays and -rays penetrate a certain substance, they ionize, excite or dissociate atoms or molecules of the substance at the cost of their energy. On the other hand, the energy thus lost by the radiation and accumulated in the substance is either converted into energy in the form of thermal movement or merely emitted in the form of electromagnetic waves. Where the substance penetrated by the radiation is fluorescent or phosphorescent or contains a fluor, a fair portion of the energy produced is converted and emitted in the form of light, usually of a wavelength in the visible zone. This phenomenon of conversion of energy produced by irradiation with ionizing radiation and light emission is termed scintillation. In the case of radiation comprising gamma-rays and neutron rays which are devoid of electric charge, a similar phenomenon is induced by the action of secondary charged particles which are produced when the radiations interact with a substance. Generally, therefore, this phenomenon is widely utilized for the detection and measurement of ionizing radiation.

[0072] Substances capable of causing the scintillation are generally called scintillators. Examples of scintillators are inorganic crystals, e.g., sodium iodide activated by thallium, organic crystals, e.g., anthracene, stilbene, organic solutions, e.g., xylene solution of terphenyl and plastic scintillators, e.g., terphenylpolystyrene. These substances are extensively used as luminous bodies for the detection of ionizing radiation. Plastic scintillators are easy to handle and are readily moldable in desired large shapes and, owing to these merits, have come to find utility as indispensable devices in the field of research on cosmic rays and research on high-energy physics by use of particle accelerators. In recent years in the field of high-energy physics, development of large particle accelerators has increased demand for a great quantity of large plastic scintillators. Of the properties required of efficient plastic scintillators, high processibility is important in addition to those basic properties of scintillators in general, e.g., amount of emission and transparency, etc.

[0073] Pulse shape discrimination takes advantage of the optical photon emission from excited atomic electrons as they return to the ground state through various pathways. See FIG. 1. The mobility of singlet and triplet excited states leads to some singlets decaying producing prompt light emission and the triplet mobility can lead to triplet-triplet annihilation which leads to delayed light emission. Neutron interactions induce charged particles with more dE/dX leading to higher ionization density, which increases the chance for the delayed light emission. A ratio of the prompt vs. delayed emission determines if the event was from a neutron of gamma ray.

[00001]$\begin{array}{cc}P.Math.S.Math.P=\frac{Q_{l.Math.o.Math.n.Math.g}-Q_{s.Math.h.Math.o.Math.r.Math.t}}{Q_{l.Math.o.Math.n.Math.g}}& \left(1\right)\end{array}$

[0074] Silicone-based, PSD capable, organic scintillators have been proposed before, but this is the first actual demonstration of successful PSD comparable to that of other materials. The present invention uses a charge collection ratio Q.sub.ratio the tail to total pulse, Q.sub.tail/Q.sub.total, to define the pulse shape parameter (PSP). The Q.sub.ratio is the charge in the tail of the pulse to the total charge in the pulse. The present matrix shows similar PSD capability to the industry leading PVT, EJ-299, as shown in FIGS. 2-3, while using approximately seven times less PPO dopant.

[0075] The PSD techniques used to distinguish between the pulses from neutrons and the pulses from gamma rays rely on the differences in the pulse shapes produced. The pulses generated by neutrons will have a longer tail than the pulses generated by gamma rays, as the neutron pulses are the result of triplet state interactions (delayed fluorescence) and the pulses produced by gamma rays are the result of singlet state de-excitation (prompt fluorescence). Thus, the difference in the ratio of the charge in the tail of the pulse to the total charge in the pulse (the Q.sub.ratio) can be calculated and used to discern which type of radiation generated the pulse.

[0076] The Q.sub.ratio for neutron pulses should be larger than the Q.sub.ratio for gamma ray pulses for the same total charge deposited (FIG. 4). The FOM is calculated from the histogram of the Q.sub.ratio versus peak height data.

[0077] FOM can be used to determine which particular scintillator compounds and corresponding temperature ranges can be used for the dual mode application in conjunction with pulse shape discrimination.

[0078] Figure of Merit may be used to determine whether a particular scintillator compound may be useful at a particular temperature and still have sufficiently different outputs between neutrons and gamma radiation to allow for pulse shape discrimination. In the description that follows, a particular composition is provided to allow for better understanding of the concepts regarding determining whether a scintillator composition will be good for neutron-gamma pulse shape discrimination.

[0079] As used herein, FOM is defined as (note that this definition assumes that the pulse distributions are Gaussian):

[00002]$\begin{array}{cc}F.Math.O.Math.M=\frac{s}{{}_{n.Math.e.Math.u.Math.t.Math.r.Math.o.Math.n}+{}_{g.Math.a.Math.m.Math.m.Math.a}}& \left(2\right)\end{array}$

[0080] where Sthe distance between the gamma ray and neutron peaks, and the FWHM of the peaks. See, FIG. 4.

[0081] The definition of the FOM illustrates that the larger the FOM the better the performance of the detector for gamma ray discrimination. A baseline performance requirement can be established by starting with the definition that for two peaks to be considered well separated S>3(.sub.neutron+.sub.gamma), where is the standard deviation. For a Gaussian distribution the FWHM=2.36 . Substituting these definitions into the equation (2) yields the result:

[00003]$\begin{array}{cc}F.Math.O.Math.M\frac{3.Math.\left({}_{n.Math.e.Math.u.Math.t.Math.r.Math.o.Math.n}+{}_{g.Math.a.Math.m.Math.m.Math.a}\right)}{2.3.Math.6.Math.\left({}_{n.Math.e.Math.u.Math.t.Math.r.Math.o.Math.n}+{}_{g.Math.a.Math.m.Math.m.Math.a}\right)}=1.2.Math.7& \left(3\right)\end{array}$

[0082] Thus, any detector with a FOM above 1.27 can be considered to have adequate PSD for fast neutron detection in the presence of gamma rays.

[0083] This present invention is well suited for incorporating materials beyond scintillator, for example .sup.10B or .sup.6Li, to provide thermal neutron detection abilities. These isotopes undergo nuclear reactions, which result in generation of charged particles in the material such as tritons and/or -particles. Some commercially available liquid scintillators, for example the EJ-309B from Eljen Technologies, also incorporate these thermal neutron sensitive isotopes. Theoretically, these detectors should display a thermal neutron reaction signal that is distinguishable from the proton recoil events resulting from neutrons interacting with hydrogen. However, these thermal neutron events are usually buried in the neutron and gamma ray lobes, as seen in FIG. 5, making them useful in only thermal neutron fields.

[0084] In the EJ-309B example, the thermal neutron events present in the middle of the neutron and gamma lobes in the low energy region, around ADC channel 800 in this case. In an exemplary embodiment of the present invention, enriched .sup.6Li doped compound is combined into the matrixnot possible with the liquid scintillator materials. The present material easily incorporates this compound which yields thermal neutron signal through two separate neutron interactions. Both of these interactions produce a pulse shape significantly different than those of the neutron induce proton recoil events and the photon induced electron events to produce a PSD plot with four lobes as seen in FIG. 6.

[0085] As shown in FIG. 6, there is no thermalization of the neutron from the PuBe source, allowing visualization of the fast and thermal neutrons as well as the gamma ray events. All events above a PSP of 0.5 are due to thermal neutron events only. This opens the door to a viable replacement for .sup.3He-based thermal neutron detectors.

[0086] The FOM for separation of neutron and gamma events in PSD capable materials is a widely accepted standard of FOM=1.27. If a slice of events from FIG. 6 is taken in the energy axis from ADC channel 1000-3000 (average of about 400 keVee), three separation FOMs can be calculated from the gamma ray events to each of the neutron-induced lobes as seen in FIG. 7.

[0087] The thermal neutron FOMs are not part of the original intent behind the defined FOM calculation method as it was designed to determine the acceptability of a material for fast neutron detection. Considering the thermal neutron identification differences between the EJ-309B sample and the present invention, a new FOM method may be necessary to evaluate PSD-capable materials for the validity of thermal neutron separation.

[0088] The present invention also may not include the thermal neutron sensitive compound, while still exhibiting PSD rival and sometimes even exceeding that of the industry leading PVT material. One example can be seen in FIG. 8 when the sample is exposed to a PuBe source.

[0089] Here, a sample with custom scintillator dopants is shown. These dopants produce higher light output than PPO in some cases depending on how they are chemically modified to polymerize them into the silicone matrix. The amount of dopant used is 5 wt. % that is roughly six times lower than what is traditionally used in PVT-based scintillators. The total FOM for this exemplary sample is calculated to be 1.98 without applying a low energy cutoff. FIG. 9 illustrates the average FOM as a function of energy cutoff.

[0090] FIG. 10 is a FOM vs ADC comparison of exemplary embodiments of the present invention having different representative dopant concentration and types, and FIG. 11 illustrates the light output of exemplary embodiments of FIG. 10. In FIG. 11, *** denotes custom additives for increased incorporation of dopant into the matrix. This illustrates that the inventive material is able to retain PPO at that concentrationas evident in the difference between 1AS037s8 and 1 AS037s5

[0091] The present material exhibits PSD FOMs similar to the industry leading PVT and liquid scintillators while containing a fraction of the PPO as compared to PVT. The silicone matrix does not suffer from many of the common problems of other organic scintillators; the material is completely inert to most environmental conditions such as temperature and impact.

[0092] Liquid scintillators are generally toxic and flammable limiting the potential applications, especially in mission critical areas or where the detector may be subjected to impacts. The present silicone matrix is completely inert and nonhazardous. It is also incredibly robust to impacts as it can be cured to be slightly elastic.

[0093] The present material also does not suffer from hazing or yellowing like PVT plastics do. The present cured silicone detectors can withstand temperatures up to 225 C. without melting or losing mass. As is known by those of skill in the art, these temperatures would cause most liquid scintillators to combust, all plastic scintillators to melt, and dopants to be leached out and/or aggregate to cause failure.

[0094] Experiments were carried out using a CAEN DT5730 14-bit digitizer operating at 500 MS/s. The DPP-PSD firmware was used with CAEN CoMPASS software. This well-known and validated software, firmware, and hardware combination was chosen for its excellent PSD capability and because it is available to the general public making the experiments highly reproducible by other researchers. Some groups have reported significantly higher FOMs for the PVT material used in our control sample, but those experiments are not reproducible without their custom experimental setup.

[0095] The radiation sources used were a 1-Ci PuBe source for PSD measurements and a 273-Ci .sup.137Cs source for light output and spectroscopic comparisons. No collimation or neutron/gamma attenuators were used, also done for reproducibility. A structural holder was assembled from a chemistry lab stand and multiple clamps. The dimensions of the setup were held constant through all experiments. There was a dedicated holder for each source and the source to detector distance was fixed when the EJ-299 control sample detected 2000 count per second.

[0096] The silicone and PVT samples were all 1.9 cm in diameter and 1.9 cm long weighing 6 g each. These were wrapped in PTFE taps as a reflector and connected to the Hamamatsu R6095 pocket multichannel analyzer (PMT) using Bicron BC-480 optical grease. A 3D printed dome of was designed with 1-mm walls and fabricated to act as a light-proof vessel for the sample. The inside and outside of the PLA printed vessel was spray painted black to ensure no light entered the sample/PMT junction.

[0097] Numerous characteristics and advantages have been set forth in the foregoing description, together with details of structure and function. While the invention has been disclosed in several forms, it will be apparent to those skilled in the art that many modifications, additions, and deletions, especially in matters of shape, size, and arrangement of parts, can be made therein without departing from the spirit and scope of the invention and its equivalents as set forth in the following claims. Therefore, other modifications or embodiments as may be suggested by the teachings herein are particularly reserved as they fall within the breadth and scope of the claims here appended.