Boron trifluoride as a quenching gas in neutron ratio counters

Abstract

A neutron proportional counter is provided. The proportional counter may include a chamber and a gas mixture. The chamber includes an anode and a cathode. The gas mixture is contained within the chamber and includes at least one neutron-sensitive fill gas and a quench gas including BF3. In certain embodiments, the neutron-sensitive fill gas may be configured to detect thermal neutrons (e.g., He-3), fast neutrons (e.g., He-4, H2), or both (e.g., UF6).

Description

Background Art

[0001] Ionizing radiation is energy released by atoms during the disintegration of their nuclei in the form of electromagnetic waves (e.g., gamma rays, X-rays) or particles (e.g., neutrons, alpha particles, beta particles). Sources of ionizing radiation exist naturally (e.g., radioactive materials in soil, water, air, cosmic rays) and from man-made sources (e.g., nuclear power generation, medical radiation). Summary of the Invention

[0002] Gas ionization detectors are radiation detection instruments used in particle physics to detect the presence of ionizing radiation particles and in radiation protection to measure ionizing radiation. Proportional counters are a type of gas ionization detector and are often used when it is necessary to distinguish between radiation types (e.g., alpha particles vs. beta particles) and when prior electrical signal amplification, improving the signal-to-noise ratio, and / or enhancing noise discrimination are required.

[0003] Embodiments of the present disclosure provide an improved proportional counter and corresponding method. As discussed in detail below, the proportional counter includes a novel gas mixture that provides improved detection sensitivity (eg, sensitivity to thermal neutrons) compared to proportional counters employing conventional gas mixtures.

[0004] In one embodiment, a neutron proportional counter is provided. The proportional counter may include a chamber and a gas mixture. The chamber may include an anode and a cathode. The gas mixture may be contained within the chamber and may include at least one neutron-sensitive fill gas and a quench gas including BF3.

[0005] In another embodiment, the fill gas may have a thermal neutron absorption cross section in the range of about 100 barns to 5600 barns.

[0006] In another embodiment, the fill gas may have a total fast neutron cross section in the range of about 1 barn to 8 barns.

[0007] In another embodiment, the fill gas may be He-3. The He-3 may be present in the gas mixture in an amount sufficient to provide a partial pressure in the range of about 1.5 psia to about 150 psia.

[0008] In another embodiment, the fill gas may be at least one of He-4, H2, or UF6.

[0009] In another embodiment, the gas mixture may further include at least one stop gas configured to reduce the mean free path of the primary ions within the gas mixture. In another embodiment, the stop gas may be at least one of Ar, Kr, or Xe.

[0010] In another embodiment, BF3 may be present in the gas mixture in an amount sufficient to provide a partial pressure in the range of about 0.002 psia to about 3.9 psia.

[0011] In one embodiment, a method for preparing a neutron proportional counter is provided. The method may include providing a neutron proportional counter. The proportional counter may include a chamber comprising an anode and a cathode, and the chamber is filled with a gas mixture. The gas mixture may include at least one neutron-sensitive fill gas and a quenching gas comprising BF3.

[0012] In another embodiment, the fill gas may have a thermal neutron absorption cross section in the range of about 100 barns to 5600 barns.

[0013] In another embodiment, the fill gas may have a total fast neutron cross section in the range of about 1 barn to 8 barns.

[0014] In another embodiment, the fill gas may be He-3. The He-3 may be present in the gas mixture in an amount sufficient to provide a partial pressure in the range of about 1.5 psia to about 150 psia.

[0015] In another embodiment, the fill gas may be at least one of He-4, H2, or UF6.

[0016] In another embodiment, the gas mixture may further include at least one stop gas configured to reduce the mean free path of the primary ions within the gas mixture.

[0017] In another embodiment, the stop gas may be at least one of Ar, Kr, or Xe.

[0018] In another embodiment, BF3 may be present in the gas mixture in an amount sufficient to provide a partial pressure in the range of about 0.002 psia to about 3.9 psia. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] These and other features will be more readily understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0020] Figure 1 is a diagram illustrating an exemplary embodiment of a proportional counter including a gas mixture comprising a He-3 filler gas and a BF3 quenching gas; and

[0021] Figure 2 It shows the preparation Figure 1 Flowchart of an exemplary embodiment of a method of a proportional counter.

[0022] It should be noted that the drawings are not necessarily drawn to scale.The drawings are intended to depict only typical aspects of the subject matter disclosed herein, and therefore should not be considered as limiting the scope of the disclosure. DETAILED DESCRIPTION

[0023] A proportional counter for detecting radiation (e.g., neutrons) is provided that includes a novel gas mixture. As discussed in detail below, the proportional counter can include a gas mixture comprising a neutron-sensitive fill gas and a quench gas comprising BF3. In one aspect, the gas mixture provides improved sensitivity for neutron detection compared to a gas mixture without BF3. In another aspect, decomposition products of the BF3 quench gas are capable of recombination, thereby providing the quench gas with an effectively infinite lifetime.

[0024] Figure 1 An exemplary embodiment of a proportional counter 100 is shown. As shown, the proportional counter 100 includes a chamber 102 having two electrodes (e.g., a cathode 104 and an anode 106) separated from each other. The chamber 102 may also contain or be filled to contain a gas mixture 110 under pressure. That is, the chamber 102 may include at least one port (not shown) configured to be coupled to a gas source for receiving the gas mixture 110 or a component thereof. The chamber 102 may also include one or more seals (not shown) configured to ensure that the chamber 102 is substantially fluid-tight, thereby inhibiting the outflow of the pressurized gas mixture 110.

[0025] In one embodiment, the gas mixture 110 may include a fill gas 112 (eg, a neutron sensitive gas) and a quench gas 114. The fill gas 112 may be configured to interact with or be ionized by incident radiation 116. The quench gas 114 may be configured to terminate the pulsed discharge.

[0026] In use, a voltage ΔV is applied between electrodes 104 and 106 to generate an electric field E therebetween. Incident radiation 116 (e.g., neutrons) entering proportional counter 100 can collide with molecules of fill gas 112 and ionize them to produce electrons (primary electrons) and positively charged atoms or molecules (+), collectively referred to as ion pairs 120, also referred to herein as primary ions. Voltage ΔV is sufficient so that conditions within chamber 102 correspond to the proportional region of counter 100. In one aspect, the electric field strength is sufficiently high to prevent recombination of ion pairs, thereby causing positive ions to drift toward the cathode and electrons to drift toward the anode. Near the anode, the field strength is also sufficiently high to accelerate the primary electrons, thereby causing ionization of additional atoms of fill gas 112, thereby generating additional ion pairs (including secondary electrons). The electrons (-) collected at anode 106 form the output of proportional detector 100 (e.g., ion current I) and can be measured by an ammeter A or other current measuring device. Advantageously, in the proportional region, each ionized particle generates only one avalanche. Thus, a proportional relationship is provided between the number of events (ionized particles) and the total ion current I. Additionally, the charge amplification provided by avalanche improves the signal-to-noise ratio of the proportional detector 100 and reduces the amount of subsequent signal amplification required.

[0027] Embodiments of the present disclosure provide a gas mixture 110 that includes a neutron-sensitive gas (e.g., helium-3) as the fill gas 112 and BF3 as the quench gas 114. As discussed in detail below, this combination is contrary to conventional wisdom as understood by those skilled in the art and provides advantages over using an organic gas as the quench gas 114.

[0028] Some early ratiometric detectors used boron trifluoride (BF3) as the fill gas. Boron-10 exhibits an excellent thermal neutron capture cross section, making it suitable for measuring thermal neutron flux. In the thermal neutron boron reaction:

[0029] 10 B+ 1 n→[ 11 B]→ 4 He+ 7 Li+2.8MeV

[0030] Boron-10( 10 B) Absorbing neutrons ( 1 n) to obtain boron-11 ( 11 B). Boron-11 then decays into the reaction product helium-4 ( 4 He) and lithium-7( 7 The short range of the reaction products means that the total energy can be collected over a relatively short distance. This allows the lower energy gamma rays to be distinguished from the thermal neutron signal in a relatively small package.

[0031] However, the use of BF3 as a fill gas in ratiometric detectors has been largely replaced by helium-3. On the one hand, helium-3 exhibits a significantly higher capture cross section than BF3. On the other hand, extreme caution may be required when handling BF3, as it is hazardous (e.g., toxic upon inhalation) and corrosive, forming highly corrosive hydrofluoric acid when dissolved in water. Therefore, for many detection tasks, neutron detection is more easily performed using helium-3.

[0032] The quenching gas used in conjunction with helium-3 is typically an organic gas (e.g., carbon dioxide (CO2), methane (CH4), carbon tetrafluoride (CF4). Quenching generally describes the dissipation of energy from a pulse through the quenching gas. This dissipation occurs when the molecules of the quenching gas use the energy of the incident radiation to break down (or dissociate) into their individual elements, rather than ionizing and re-emit another photon (as would be the case with other gases in the gas mixture).

[0033] While organic gases are suitable quenching agents for many proportional counter applications, they can present some drawbacks. For one thing, these organic gases are consumed over the life of the detector due to their dissociative nature during quenching. That is, once these molecules have been fragmented, they cannot reform. Furthermore, in some cases, the fragmented molecules can cause carbon deposits on the internal components of the proportional counter, degrading its performance.

[0034] On the other hand, it should be understood that these organic gases act only as quenching agents. That is, they represent a portion of the total gas in the detector that is insensitive to thermal neutrons. For detector designs limited by the maximum pressure of the gas mixture 110, the amount of quenching gas 114 must be carefully balanced with the sensitivity required for the desired detection application.

[0035] Therefore, there is a need for a gas mixture comprising different quenching gases that exhibit improved performance compared to the above-mentioned organic gases. Embodiments of the present disclosure provide a gas mixture 110 that includes a neutron-sensitive gas (e.g., helium-3) as a fill gas 112 and BF3 as a quenching gas 114. In certain embodiments, the only quenching gas is BF3. As discussed further below, the use of BF3 as a quenching gas is unconventional given its well-known shortcomings as a fill gas. However, when used as a quenching gas 114 in combination with the fill gas 112, BF3 offers compelling advantages.

[0036] Generally speaking, BF3 is an electronegative gas. Too much BF3 will drive the voltage required for operation too high and unusable (e.g., out of scale). That is, due to the quenching function of BF3, the more it is added, the more it will be used as alpha particles (- 4 For this reason, the higher the voltage ΔV required for the detector operation, the higher the voltage ΔV required for the detector operation. 3- When used unmixed in a proportional counter, its operating pressure is generally limited to about 1 atmosphere.

[0037] On the other hand, too much BF3 can slow the counter's response. For example, the speed at which a proportional counter can collect all the charge from a given event is determined by the drift velocity of the electrons in the fill gas, and this drift velocity varies for a given gas mixture. Because BF3 isn't a gas known for relatively fast drift velocity, too much of it in the gas mixture can slow charge collection and, therefore, limit the speed at which the proportional counter can resolve individual events.

[0038] On the other hand, too much BF3 can potentially change the shape or number of pulses from a single event. Without being bound by theory, we believe that the combination of the low drift velocity and electronegativity of BF3 results in an inability to capture the full energy of the pulse in the same time period as the rest of the pulse.

[0039] In other respects, as discussed above, helium-3 has a higher thermal neutron capture cross section than BF3 and can be filled to higher pressures. Therefore, in situations where the gamma field is low enough to allow the use of helium-3, BF3 does not offer any advantage if used as a fill gas to absorb thermal neutrons instead of helium-3.

[0040] However, using BF 3 as the quench gas 114 in the gas mixture 110 has various advantages compared to conventional organic gases, as discussed in detail below.

[0041] In one aspect, because BF3 is a halogen quencher, its decomposition products are able to recombine. Therefore, the amount of BF3 consumed during quenching is negligible, giving it an effectively infinite lifetime even in a high-flux environment.

[0042] On the other hand, since BF 3 is sensitive to at least thermal neutrons, its use as the quench gas 114 can be used to increase at least the overall thermal neutron sensitivity of the ratiometric detector 100 .

[0043] On the other hand, because boron-10 deposits more energy per neutron interaction than helium-3, the presence of BF3 can help mitigate the sensitivity loss experienced by helium-3 proportional counters when used in moderate gamma fields (e.g., about 10 R / hour to about 1000 R / hour). At these gamma radiation levels, the pulse height discrimination level should be increased to remove gamma interference. Thus, some helium-3 pulses can be distinguished, but higher energy pulses from boron-10 cannot be distinguished.

[0044] BF3 can be present in gas mixture 110 in an amount sufficient to provide a partial pressure within a range of approximately 0.002 psia to approximately 3.9 psia. The lower limit represents the minimum partial pressure of BF3 sufficient for quenching. This pressure corresponds to the pressure typically used for halogen quenching in Geiger–Müller tubes. The upper limit represents the maximum partial pressure of BF3 before BF3 effectively dominates fill gas 112. In other words, before the operating characteristics of gas mixture 110 approach those of pure BF3. It should be understood that these partial pressures are provided as examples, and that other minimum / maximum partial pressures of BF3 may be employed depending on detector size, detector configuration, desired sensitivity, and maximum pressure.

[0045] Embodiments of the gas mixture may employ a variety of neutron-sensitive gases as the fill gas 112 in combination with BF3 as the quench gas 114. In certain embodiments, the neutron-sensitive gas may be helium-3. As an example, helium-3 may be used to detect relatively slow-moving thermal neutrons (e.g., neutrons having energies of approximately 0.025 eV). The helium-3 may be present in an amount sufficient to provide a partial pressure in the range of approximately 1.5 psia to approximately 150 psia. The minimum partial pressure represents an estimate of the minimum amount of helium-3 required for viable operation of the proportional counter 100. Conversely, above a maximum partial pressure, the increase in sensitivity for a given increase in helium-3 partial pressure decreases dramatically. Therefore, it is not economically desirable to fill above this maximum partial pressure.

[0046] In other embodiments, the neutron sensitive gas may be helium-4 ( 4 At least one of He, hydrogen H2, or uranium hexafluoride UF6. As an example, helium-4 and hydrogen can be used to detect fast neutrons (e.g., neutrons with energies in the range of about 1 MeV to about 20 MeV), in which case helium-3 is not suitable. Uranium hexafluoride can be used to detect thermal or fast neutrons. In other embodiments, the at least one neutron-sensitive gas has a thermal neutron absorption cross section in the range of about 100 barns to about 5600 barns. In other embodiments, the at least one neutron-sensitive gas has a total fast neutron cross section in the range of about 1 barn to about 8 barns.

[0047] The gas mixture 110 may also include one or more stop gases. The stop gas is distinct from the quench gas 114. Notably, the quench gas 114 is configured to terminate the pulse, while the stop gas has an ionization potential that allows for gas multiplication of primary ions. Thus, in this context, stop refers to the stop gas's ability to reduce the mean free path of primary ions (e.g., either or both of the ion pairs 120) within the gas mixture 110. Examples of such stop gases may include, but are not limited to, at least one of argon (Ar), krypton (Kr), or xenon (Xe).

[0048] In other embodiments, a method 200 for preparing a proportional counter is provided. Figure 2 2 is a flow chart illustrating an exemplary embodiment of method 200. As shown, method 200 may include operations 202 to 204. However, it should be understood that in alternative embodiments, the method may include operations 202 to 204. Figure 2 More or fewer operations are shown in the , and these operations can be performed in the same Figure 2 The execution order is different as shown in .

[0049] In operation 202, a proportional counter is provided. In one embodiment, the proportional counter may be Figure 1 The proportional counter 100 includes a chamber 102 having an anode and a cathode.

[0050] In operation 204, the chamber 102 may be filled with a gas mixture 110. The gas mixture 110 may include a fill gas 112 (e.g., at least one neutron-sensitive gas) and a quench gas 114 (e.g., BF-3). Examples of the at least one neutron-sensitive gas may include, but are not limited to, He-3, He-4, H-2, or UF6. In one embodiment, the partial pressure of the fill gas 112 within the gas mixture 110 may be provided in a range of about 1.5 psia to about 150 psia. In another embodiment, the partial pressure of BF3 within the gas mixture 110 may be provided in a range of about 0.002 psia to about 3.9 psia.

[0051] In yet another embodiment, the at least one neutron-sensitive gas can be configured to detect thermal neutrons, fast neutrons, and / or combinations thereof. By way of example, in one example, the at least one neutron-sensitive gas can have a thermal neutron absorption cross section in a range of approximately 100 barns to 5,600 barns. By way of another example, the at least one neutron-sensitive gas can have a total fast neutron cross section in a range of approximately 1 barn to 8 barns.

[0052] In an embodiment, the gas mixture 110 may further include at least one stop gas. The at least one stop gas may be configured to reduce the mean free path of primary ions within the gas mixture. The primary ions may be electrons or molecules of the fill gas 112 that are ionized by the incident radiation 116. Examples of stop gases may include, but are not limited to, Ar, Kr, or Xe.

[0053] By way of non-limiting example, exemplary technical effects of the methods, systems, and apparatus described herein include a proportional counter comprising a novel gas mixture comprising a neutron-sensitive gas and BF3 as a quenching gas. In one aspect, the gas mixture provides improved sensitivity for neutron detection compared to a gas mixture without BF3. In another aspect, decomposition products of the BF3 quenching gas are capable of recombination, thereby providing the quenching gas with an effectively infinite lifetime.

[0054] Certain exemplary embodiments are described to provide a comprehensive understanding of the principles of structure, function, manufacture, and use of the systems, apparatus, and methods disclosed herein. One or more examples of these embodiments have been illustrated in the accompanying drawings. It will be understood by those skilled in the art that the systems, apparatus, and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments, and that the scope of the invention is limited solely by the claims. Features shown or described in conjunction with one exemplary embodiment may be combined with features of other embodiments. Such modifications and variations are intended to be included within the scope of this disclosure. In addition, in this disclosure, similarly named components of an embodiment generally have similar features, and therefore, within a specific embodiment, it is not necessary to fully set forth every feature of every similarly named component.

[0055] As used herein throughout the specification and claims, approximating language may be applied to modify any quantitative representation that could vary without resulting in a change in the basic function to which it is related.

[0056] Or "about" can include values ​​that fall within 1% of the value in either direction (greater than or less than the value), or in some embodiments, fall within 5% of the value, or in some embodiments, fall within 10% of the value, unless otherwise stated or obvious from the context (unless such values ​​would impermissibly exceed 100% of the possible values). Therefore, values ​​modified by one or more terms such as "about", "approximately" and "substantially" should not be limited to the precise values ​​specified. In at least some cases, approximate language can correspond to the precision of the instrument used to measure the value. Here and throughout the specification and claims, range limitations can be combined and / or interchanged, and unless the context or language indicates otherwise, such ranges are identified and include all subranges contained therein.

[0057] In the specification and claims of this article, phrases such as "at least one" or "one or more" may appear after the combination list of elements or features. The term "and / or" may also appear in a list of two or more elements or features. Unless otherwise implied in the context in which the phrase is used or explicitly contradicted by the context, such phrases are intended to represent any one of the elements or features listed individually or any one of the listed elements or features with any one of the elements or features listed elsewhere. For example, the phrases "at least one of A and B," "one or more of A and B," and "A and / or B" are each intended to represent "A alone, B alone, or A and B together." A similar explanation is also intended to be used for a list comprising three or more items. For example, the phrases "at least one of A, B, and C," "one or more of A, B, and C," and "A, B, and / or C" are each intended to represent "A alone, B alone, C alone, A and B together, A and C together, B and C together, or A and B and C together." Furthermore, the term "based on" as used above and in the claims is intended to mean "based at least in part on" such that non-recited features or elements are also allowable.

[0058] Based on the above embodiments, those skilled in the art will appreciate other features and advantages of the present invention. Therefore, except as indicated by the appended claims, this application is not limited by the contents specifically shown and described. All publications and references cited herein are expressly incorporated by reference in their entirety.

Claims

1. A neutron ratio counter, comprising: a chamber comprising an anode and a cathode; and a gas mixture contained within the chamber, the gas mixture comprising: at least one neutron-sensitive fill gas; and A quenching gas comprising BF3.

2. The proportional counter of claim 1 , wherein the fill gas has a thermal neutron absorption cross section in the range of 100 barns to 5600 barns.

3. The proportional counter of claim 1 , wherein the fill gas has a total fast neutron cross section in the range of 1 barn to 8 barns. The proportional counter according to claim 1 , wherein the filling gas comprises He-3.

5. The proportional counter of claim 4, wherein He-3 is present in the gas mixture in an amount sufficient to provide a partial pressure in the range of 1.5 Psia to 150 Psia. 6 . The proportional counter according to claim 1 , wherein the filling gas comprises at least one of He-4, H 2 , or UF 6 .

7. The proportional counter of claim 1, wherein the gas mixture further comprises at least one stop gas configured to reduce the mean free path of primary ions within the gas mixture.

8. The proportional counter according to claim 7, wherein the stop gas comprises at least one of Ar, Kr, or Xe.

9. The proportional counter of claim 1, wherein BF3 is present in the gas mixture in an amount sufficient to provide a partial pressure in the range of 0.002 psia to 3.9 psia.

10. A method for preparing a neutron proportional counter, the method comprising: Providing a neutron ratio counter comprising a chamber including an anode and a cathode; and The chamber is filled with a gas mixture comprising at least one neutron sensitive fill gas and a quench gas comprising BF3.

11. The method of claim 10, wherein the fill gas has a thermal neutron absorption cross section in the range of 100 barns to 5600 barns.

12. The method of claim 11, wherein the fill gas has a total fast neutron cross section in the range of 1 barn to 8 barns. The method of claim 11 , wherein the fill gas comprises He-3.

14. The method of claim 13, wherein He-3 is present in the gas mixture in an amount sufficient to provide a partial pressure in the range of 1.5 psia to 150 psia.

15. The method of claim 11, wherein the fill gas comprises at least one of He-4, H2, or UF6.

16. The method of claim 11, wherein the gas mixture further comprises at least one stop gas configured to reduce a mean free path of primary ions within the gas mixture. The method of claim 16 , wherein the stop gas comprises at least one of Ar, Kr, or Xe.

18. The method of claim 11, wherein BF3 is present in the gas mixture in an amount sufficient to provide a partial pressure in the range of 0.002 psia to 3.9 psia.

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