BORON COATED STRAWS FOR NEUTRON DETECTION WITH PIE-SHAPED CROSS-SECTION

Abstract

A boron coated straw detector for use in a neutron detection system is disclosed comprising a boron coated straw having at least one boron-coated septum radially oriented and extending a pre-determined distance towards the center of the straw. Preferably, the straw comprises a plurality of septa comprising a rigid surface, coated on both sides with a boron composition. Preferably, the septa run the length of the straw detector from one end of the straw to the other. The area coated on the septa adds to the area coated on the arc segments offering a significant benefit in sensitivity of the neutron detector.

Claims

1. An improved boron-coated straw detector system comprising a boron-coated straw having a diameter and at least one septum extending radially inward from inner surface of the straw, each septum having a boron-coating on two sides and a length of about 0.25 times the straw diameter or less.

2. The straw detector system of claim 1, wherein the straw comprises a plurality of septa.

3. The straw detector system of claim 1, wherein each septum is within about 5% difference in length.

4. The straw detector system of claim 2 wherein the straw comprises an even number of septa.

5. The straw detector system of claim 2, wherein each septum extends from a first end of the straw to a second opposite end of the straw.

6. The straw detector system of claim 2, wherein the straw comprises 6 to 12 septa.

7. The straw detector system of claim 6, wherein the septa are equally spaced apart around the interior surface of the straw.

8. The straw detector system of claim 7, wherein the length of each septum is within about 5% of the length of an arc length between each septum.

9. The straw detectors system of claim 7, wherein the length of each septum is between about 50% and 100% of an arch length between each septum.

10. A neutron detection system comprising a plurality of boron-coated straws formed into a panel, each boron-coated straw in the panel having a plurality of septa extending radially inward from inner surface of the straw, and each septa having a boron-coating on both sides.

11. The neutron detection system of claim 10 comprising 12 boron-coated straws.

12. The neutron detection syn pastern of claim 10 wherein each straw 12 septa.

13. The neutron detection system of claim 10 wherein each straw has an internal diameter of about 4 mm.

14. The neutron detection system of claim 10 wherein the panel is about 5.0 cm by 10 cm in size.

15. The neutron detection system of claim 10 wherein the panel weights about 0.1 pounds or less.

16. The neutron detection system of claim 10 further comprising a wearable vest to which the panel is attached.

17. The neutron detection system of claim 16 comprising at least ten panels.

18. A process for manufacturing boron-coated straws comprising the steps of: (1) providing a forming station comprising a plurality of forming bars and a roll of foil comprising a boron coating on one side; (2) placing the foil over the forming bars with the coated side of the foil facing the forming bars; and (3) moving at least on forming bars toward another forming bar and allowing foil the foil between the bars to collapse and be compressed together to form a septum having a boron coated two sides.

Description

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

[0034] A better understanding of the invention can be obtained when the detailed description set forth below is reviewed in conjunction with the accompanying drawings, in which:

[0035] FIG. 1 depicts a cross-sectional view of an embodiment of a straw having 12 septa.

[0036] FIG. 2 depicts a 2D electric field map of the embodiment of FIG. 1.

[0037] FIG. 3 depicts a cross-sectional view of an embodiment of a straw having 6 septa with streamlines showing the direction of the electric field along which electrons travel.

[0038] FIG. 4 depicts a graph of electric fields calculated for prior art round cross-sections and two embodiments with septa.

[0039] FIG. 5 depicts an embodiment of an apparatus that can be used to form septa walls on a boron-coated copper foil.

[0040] FIG. 6 depicts a microscope photograph of boron-coated foil following the formation of septa walls but prior to being rolled to form the pie-shaped straw detector.

[0041] FIG. 7 depicts a cross-section of an embodiment having six septa.

[0042] FIG. 8 depicts a cross-section of an embodiment having twelve septa.

[0043] FIG. 9 depicts a graph neutron pulse height spectra collected for a six septa embodiment as compared to a prior are round cross-section straw of the same diameter.

[0044] FIG. 10 depicts a graph neutron pulse height spectra collected for a twelve septa embodiment as compared to a prior are round cross-section straw of the same diameter.

[0045] FIG. 11 depicts a series of steps that can be utilized in a process to manufacture the improved straws.

[0046] FIG. 12 depicts a representation of electrons would travel in an embodiment of a boron-coated straw detector.

[0047] FIG. 13A depicts a panel of neutron detectors including the improved straws suitable for inclusion in a wearable neutron detection system.

[0048] FIG. 13B depicts an embodiment of a wearable neutron detector system utilizing embodiments of straws having a pie-shaped cross-section.

[0049] FIG. 14 depicts a small handheld detector utilizing embodiments of straws having a pie-shaped cross-section.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

[0050] An improved boron coated straw detector is disclosed comprising a boron coated straw having at least one boron-coated septum radially oriented and extending a pre-determined distance towards the center of the straw. Preferably, the straw comprises a plurality of septa comprising a rigid surface, coated on both sides with a boron composition. Preferably, the septa run the length of the straw detector from one end of the straw to the other. The area coated on the septa adds to the area coated on the arc segments offering a significant benefit in sensitivity of the neutron detector.

[0051] In order to improve the neutron detection efficiency of the boron-coated straw detectors, while maintaining manufacturability, the structure of the conventional straw has been modified to provide significant performance benefits. The cross-section of the improved straw design has a pie-like shape as shown in FIG. 1. As will now be recognized by those of skill in the art, the pie-like shape cross-section straws consists of any desired number of arc segments, and an equal number of septa walls, which are radially oriented, and extending a desired distance towards the center of the geometry. Each septum is preferably a segment of the wall that has been collapsed onto itself, resulting in a rigid surface, coated on both sides. Preferably, the septa run the entire length of the straw such that the pie-shaped cross-section can be found anywhere along the length of the straw. This maintains the increased coated area along the entire length and avoids non-uniformities in the electric field that could result from discontinuities in the septa. The area coated on the septa is added to the area coated on the arc segments, thus offering a significant benefit in sensitivity of the neutron detector. For example, compared to a round straw of the same diameter, the coated wall area of the 12-septa pie shape of FIG. 1 is three times larger, assuming the septa are equal in length to each of the arc segments, as may be preferred. Further, a single 0.128 inch ID straw tube with 12 septa can produce the same neutron sensitivity as the same tube filled with 10 atmospheres pressure .sup.3He for traditional detectors, if those detectors were able to function at such small diameters.

[0052] The wall area benefit of the improved boron coated straw design can be calculated analytically, as discussed next. Referring to FIG. 1, the wall perimeter of this pie-like shape can be written as P=D+2N.sub.s, where D is the outer diameter, N is the number of septa (N=12 in the figure), and Ls is the length of the septum wall. Now, assume Ls can be expressed as a fraction of the arc length L.sub.are, such that L.sub.s=f.Math.L.sub.are, and that L.sub.arc=D/N. The fraction f is bounded by 0fN/(2), where this limit ensures that the septum length L.sub.s is less than the circle radius, L.sub.sD/2. As an example, for N=6, f0.95; for N=12, f1.9; and so forth. In practice, LA is preferably not much greater than about D/4, since long septa result in very low electric fields near the outer wall, as discussed in the next section. The ratio of perimeters can now be written as R=P/(D), and substituting for the relations given above, we can simply write R=1+2f. Thus, R is only a function of the factor f, and does not depend on the outer diameter D. The maximum value of R depends on N, and is given by R.sub.max=+N/. For the case of the round straw with no septa, for instance, f=0 and R=1.0. For f=0.5, the ratio is R=2, and for f=1.0, the ratio is R=3.0.

[0053] Pie Straw Geometry Examples:

[00002]$\mathrm{Round}.Math..Math.\mathrm{Straw}.Math..Math.\mathrm{Perimeter}.Math.\text{:}.Math..Math.P_{\mathrm{round}}=.Math.D$$\mathrm{Pie}.Math..Math.\mathrm{Straw}.Math..Math.\mathrm{Perimeter}.Math.\text{:}$$\begin{array}{c}{P}_{\mathrm{pie}}=.Math.D+2.Math.N.Math.L_s\\ =.Math.D+2.Math.N.Math.f.Math.L_{\mathrm{arc}}\\ =.Math.D.Math.\left(I+2.Math.f\right)\end{array}$

Where D=straw diameter, N=number of septa, Ls=length of septa, f=ratio septa/arc length, and L.sub.are=length of arc. Boron Content Ratio: P.sub.pie/P.sub.round, R=1+2.Math.f.
For D=15 mm, N=6, f=0.5, then R=2.
For D=3.25 mm, N=12, f=1.0, then R=3.

[0054] The pie-shaped design should preferably maintain a sufficiently high electric field, to ensure short electron drift times and to prevent loss of ionization due to recombination. As shown in FIG. 2, a 2-dimensional electric field map was calculated for a 12-septa pie-shaped cross-section straw such as shown in FIG. 1, using numerical methods implemented in ELECTRO (Integrated Engineering Software), and assuming a bias of 1000 V, and D=4.43 mm. The field map of FIG. 2 is shown in grey scale that corresponds to different values of the field. The grey scale scaling is logarithmic. The minimum field level in this example is about 3.11 V/cm and occurs in the corners between arc and septa walls (darker grey). Preferably, these corners are rounded as shown (for example with a radius=0.1 mm) in order avoid very low field levels in those areas. The minimum field level required to prevent recombination and provide optimal signal risetime depends on the gas mixture used in the detector but for typical Argon/CO.sub.2 mixtures is 1 Volt per cm.

[0055] In the Electron Field Distribution Example of FIGS. 2 and 4, dimensions chosen to maintain high electric fields and ensure short electron drift times are as follows:

.sub.max<0.35 ms,

HV=1000 V,

Diameter=4.43 mm,

Blue=3.11 V/cm,

Red=2.3.Math.10 V/cm.

[0056] Field drops inside the region between the septa, setting a practical limit to dimensions of about L.sub.s<D/4 or f<N/(4.Math.). As will now be recognized by a person of skill in the art, the longer the septa the better in terms of efficiency, but there is a tradeoff in that longer septa result in a lower electric field, in the region in between septa, which slows down the drift of ionization generated in those regions. This may lead to a reduced detection efficiency. This sets the practical limit on the septa length. There is no minimum septa length in theory. Even very short septa would increase the sensitivity of the straw, but there is extra manufacturing effort to form the septa, even short ones, thus it is best to make then as long as possible, within the limits discussed above.

[0057] FIG. 3 illustrates some of the paths that electrons traverse when liberated at the corresponding locations near the wall in a 6 septa embodiment. Electrons liberated near the septa walls initially travel in a direction normal to the wall, then turn and follow a radial path to the anode wire. FIG. 4 plots the field level calculated along radii of two different pie shapes straws, and includes the analytical field of a round straw with the same diameter (about D=4.43 mm), for comparison. The first pie shaped straw example included 6 septa and an R=2. The second pie shaped straw example included 12 septa and an R=3. All fields are evaluated for a bias of 1000 V. Although the field drops significantly inside the region between the septa (radius>about 0.9 mm) for the pie shapes, it is more or less the same for all shapes in the middle part of the cross section, and at the wire (maximum), meaning that signal amplification, due to avalanching, will be similar in all.

[0058] One method of forming the septa in a pie-shape cross-section straw includes placing a layer of boron-coated foil 20 over a series of spaced precision bar stock 22, 24, as shown in FIG. 5. The foil 20 is preferably of a variety such as manufactured in accordance and/or disclosed in U.S. patent application Ser. No. 14/060,015 filed Oct. 22, 2013, entitled Method and Apparatus for Coating Thin Foil with Boron Coating (Notice of Allowance Received) which is incorporated herein by reference in its entirety for the purpose of disclosing preferred foil parameters. As will now be recognized by a person of skill in the art, the thickness of the bar stock 22, 24 utilized determines the arc length between septa and the spacing between the bar stock 22, 24 determines the septum length. Clamping the assembly together, the foil 20 can be collapsed between the bars 22 and 24, forming the series of extremely straight and stiff walled septa with boron coating on each side. FIG. 6 shows a microscopic side view showing the septa 30 formed on the foil 20 using the method of FIG. 5, before the foil 20 is rolled into a straw. Once removed from the forming station of FIG. 5, the foil 20 can then be rolled and inserted into the appropriate diameter containment tube for wiring, sealing, and gas filling such as is demonstrated, for example, in U.S. Pat. Nos. 7,002,159; 8,569,710, and 9,218,946, each of which is incorporated herein by reference in its entirety for this purpose.

[0059] FIG. 11 further depicts a series of steps that can be utilized in a method to manufacture the improved straws including the steps of: (1) providing a forming station comprising a plurality of forming bars (preferably steel or similar metal), a spacer bar, a cover weight, and a roil of boron covered foil; (2) utilizing the spacer bar to space the forming bars apart approximately 2 times the desired septa length (L.sub.s); (3) placing the foil, coating side down, over the forming bars; (4) place the cover weight over the non-coated side of the foil to hold the foil in place; (5) moving the forming bars toward each other thereby collapsing the intermediate foil and forming a septa having coated walls on both sides; (6) removing the foil with septa from the forming station; (7) rolling the foil into a straw having the septa radiating in the interior of the straw; and (8) inserting the rolled straw into a containment tube.

[0060] One embodiment of the pie-shaped cross section straws tested, as shown in FIG. 7, included a larger 6-point pie shape configuration with R=2 and D=15.0 mm, which fabricated in accord with the septa forming process disclosed above. A second embodiment, a smaller 12-point pie with D=3.25 mm, shown in FIG. 8, was designed to achieve R=3, i.e., three times the boron coating of the round straw of the same diameter. Both embodiments of the detector were tested in the lab, to verify successful generation and readout of signals, and to measure the performance benefit offered by the large increase in boron coated area introduced in the septa. The straws were assembled with a central anode wire, and set up for flow-through operation. A moderated .sup.252Cf source was placed 20 cm away from the detectors. The two detectors were compared against round cross-section straws of the same respective diameter.

[0061] FIG. 9 shows the pulse height spectra collected in the two larger detectors, pie-shaped (darker grey) and round straws (lighter grey) having D=15.0 mm. The two spectra are very similar, indicating that the 6 septa walls have a small effect on the signal shaping. The 6-point pie recorded 1.90 times higher count rate compared to its round counterpart, very close to the theoretical ratio of 2.0.

[0062] FIG. 10 shows the spectra collected in the smaller pie-shaped (darker grey) and round straws (lighter grey) having D=325 mm. Here the two spectra have different shapes, undoubtedly due to signals formed in the tight space enclosed between neighboring septa; still, the pie-12 spectrum shows sufficient resolution to allow successful discrimination of gamma rays, and the resulting count rate is 2.84 times higher than that of its round counterpart.

[0063] Although the embodiments tested utilized six and twelve septa, these are non-limiting examples. A single septum can improve performance but preferably the straws include a plurality (two or more) septa, and more preferably an even number of septa are utilized to maintain symmetry such that the response of a single straw will be uniform regardless of the neutron incidence direction. As will now be recognized by those of skill in the art, the number of septa utilized in a straw detector can be varied depending upon the size of the straw, the manufacturing technique utilized, the sensitivity desired, and other factors which are apparent to those of skill in the art. As will be now be recognized, too many septa will result in low level electric field region which can form in between the septa if they are too close to one another.

[0064] As shown in FIGS. 13A and B, the improved boron-coated straw detectors of the can be formed into panels and utilized as part of a wearable neutron detector system. There is a great unmet demand for a wearable system that can be hidden under clothing that does not encumber activity. The improved straw detectors offer greater sensitivity in the same volume, compared to prior art straws of the same diameter. Utilizing embodiments of the invention, panels can be constructed providing 30% efficiency for thermal neutrons can be reduced from 1.2 cm to 0.4 cm in diameter. For example, the embodiment of FIG. 13A includes a panels of straw detectors weighing about 0.1 pounds utilizing straws having 12 septa such as have been disclosed herein. The straws can be formed into panels of about 5 cm by about 10 cm by about 0.4 cm that can be distributed over the torso in a wearable system such as is the vest shown in FIG. 13B. As used herein, the term vest is intended to include vests, bands, straps, wraps and the like that can be worn and secured around a torso. Preferably 10 to 24 panels are utilized in a wearable system. Preferably, such wearable systems utilize ultra low power electronics such as will now be recognized by those skilled in the art to include on-board HV supply, a single transistor amplifier, utilizing only about 100 A at 3.7 V, capable of operating for 100 plus days on 300 mA/hr AAA batteries, having a layout of about 2 inches by 0.5 inches. Such ultra low power electronics are preferably mass producible without tuning. Utilizing such a wearable system allows a mere 1 pound of detectors to replace a 30-pound backpack using prior art detectors.

[0065] As shown in FIG. 14, the improved boron coated straw detectors of the present invention can also be utilized in a small handheld detector system. The embodiment disclosed in FIG. 14 is a handheld detector having about a 1 diameter containing 19 straws. This detector was tested to assess its sensitivity to a range of neutron energies, its response to gamma rays, and other metrics. A summary of the results is in the Table I below:

TABLE-US-00001 TABLE 1 Efficiency Varying Neutron Fluence Absolute Absolute Intrinsic Intrinsic Efficiency Efficiency Efficiency Efficiency Sources (%) Uncertainty (%) Uncertainty .sup.252Cf 8.9706E05 3.5462E05 7.04E01 0.28 .sup.252Cf 1.1371E04 1.8862E05 8.93E01 0.15 .sup.252Cf 1.2363E04 1.8713E05 1.00E00 0.15 AmBe 1.0059E04 1.5260E05 7.84E01 0.12 AmLi (9976) 1.6616E04 4.2602E05 1.51E00 0.39 AmLi (9975) 1.9727E04 4.6986E05 1.80E00 0.43 WGPu 2.3409E04 7.6719E05 9.47E01 0.31 Efficiency with Varying Moderation HDPE Absolute Absolute Intrinsic Intrinsic FOM Thickness Efficiency Efficiency Efficiency Efficiency (Signal FOM (cm) (%) Uncertainty (%) Uncertainty Noise) Uncertainty 0.0 1.24E04 1.87E05 1.00 0.15 1.61E06 2.43E07 2.0 3.55E04 5.34E05 2.87 0.43 4.61E06 6.94E07 4.0 7.25E04 1.09E04 5.86 0.88 9.42E06 1.41E06 8.0 8.26E04 1.24E04 6.68 1.00 1.07E05 1.61E06 Relative Angular Response Detector Relative Relative Response Orientation Response Uncertainty 0 1.00 0.1027 45 0.94 0.0981 90 1.00 0.1024 135 0.99 0.1018 180 1/10 0.1098 Additional Measurements Net Instrument Response in 30 mR/h Gamma Radiation Field 0.034 cps +/ 0.006 Gamma Absolute Rejection Ratio with Neutron Source 1.01 +/ 0.02 t-Value 6.23 s
Results clearly demonstrate the new pie-like cross-section straw shape is a viable solution for a more sensitive boron-coated straw technology.

[0066] Experimental measurements on first generation prototypes demonstrated 95% of the theoretically predicted gains.

[0067] While the terms used herein are believed to be well-understood by one of ordinary skill in the art, definitions are set forth to facilitate explanation of certain of the presently-disclosed subject matter.

[0068] Following long-standing patent law convention, the terms a, an, and the refer to one or more when used in this application, including the claims. Thus, for example, reference to a window includes a plurality of such windows, and so forth.

[0069] Unless otherwise indicated, all numbers expressing quantities of elements, dimensions such as width and area, and so forth used in the specification and claims are to be understood as being modified in all instances by the term about. Accordingly, unless indicated to the contrary, the numerical parameters set forth in this specification and claims are approximations that can vary depending upon the desired properties sought to be obtained by the presently-disclosed subject matter.

[0070] As used herein, the term about, when referring to a value or to an amount of a dimension, area, percentage, etc., is meant to encompass variations of in some embodiments plus or minus 20%, in some embodiments plus or minus 10%, in some embodiments plus or minus 5%, in some embodiments plus or minus 1%, in some embodiments plus or minus 0.5%, and in some embodiments plus or minus 0.1% from the specified amount, as such variations are appropriate to perform the disclosed methods or employ the disclosed compositions.

[0071] The term comprising, which is synonymous with including containing or characterized by is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. Comprising is a term of art used in claim language which means that the named elements are essential, but other elements can be added and still form a construct within the scope of the claim.

[0072] As used herein, the phrase consisting of excludes any element, step, or ingredient not specified in the claim. When the phrase consists of appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.

[0073] As used herein, the phrase consisting essentially of limits the scope of a claim to the specified materials or steps, plus those that do not materially affect the basic and novel characteristic(s) of the claimed subject matter. With respect to the terms comprising, consisting of, and consisting essentially of, where one of these three terms is used herein, the presently disclosed and claimed subject matter can include the use of either of the other two terms.

[0074] As used herein, the term and/or when used in the context of a listing of entities, refers to the entities being present singly or in combination. Thus, for example, the phrase A, S, C, and/or O includes A, S, C, and O individually, but also includes any and all combinations and subcombinations of A, S, C, and O.

[0075] It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. The foregoing disclosure and description are illustrative and explanatory thereof, and various changes in the details of the illustrated apparatus and construction and method of operation may be made without departing from the spirit in scope of the invention which is described by the following claims.