Sealed Boron Coated Straw Detectors

Abstract

The present invention is a method and apparatus for operating boron-coated straw detectors in sealed mode, without the need for a continuous flow of gas. Sealed-mode operation is necessary when using the boron-coated straw detectors in the field, where access to a continuous flow of the required gas mixture is not practical. Also, sealed-mode operation is necessary when the straw detectors are used as portable instruments, that must be moved from one location to the next swiftly, or that must be operated while in motion.

Description

STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to radiation detection. More particularly, the invention relates to a method and apparatus for passive detection of neutron emitting materials with applications in homeland security and nuclear safeguards.

2. Description of the Related Art

US government plans to equip major seaports with large area neutron detectors, in an effort to intercept the smuggling of nuclear materials, have precipitated a critical shortage of ³He gas. It is estimated that the annual demand of ³He for US security applications alone is 22 kiloliters, more than the worldwide supply. This is strongly limiting the prospects of neutron science, safeguards, and other applications that rely heavily on ³He-based detectors. Clearly, alternate neutron detection technologies that can support large sensitive areas, and have low gamma sensitivity and low cost must be developed.

The applicant has previously developed and patented a technology based on close-packed arrays of long aluminum or copper tubes (straws), 4 mm in diameter, coated on the inside with a thin layer of ¹⁰B-enriched boron carbide (¹⁰B₄C). In addition to the high abundance of boron on Earth and low cost of ¹⁰B enrichment, the boron-coated straw (BCS) detector offers distinct advantages over conventional ³He-based detectors, including faster signals, short recovery time (ion drift), low weight, safety for portable use (no pressurization), and low production cost.

The background to the present invention and related art is best understood by reference to Applicant's own prior work, including in particularly, U.S. Pat. No. 7,002,159 B2 (the '159) entitled “Boron Coated Straw Neutron Detector” which issued Feb. 21, 2006. The '159 is hereby incorporated by reference in its entirety, for all purposes, including, but not limited to, supplying background and enabling those skilled in the art to understand, make and use in Applicant's present invention.

The background to the present invention and related art is best understood by reference to Applicant's own work. Applicant's issued patents and pending applications that may be relevant, including; (1) U.S. Pat. No. 5,573,747 entitled, “Method for Preparing a Physiological Isotonic Pet Radiopharmaceutical of ⁶²CU; (2) U.S. Pat. No. 6,078,039 entitled, “Segmental Tube Array High Pressure Gas Proportional Detector for Nuclear Medicine Imaging”; (3) U.S. Pat. No. 6,264,597 entitled, “Intravascular Radiotherapy Employing a Safe Liquid Suspended Short-Lived Source”; (4) U.S. Pat. No. 6,483,114 D1 entitled, “Positron Camera”; (5) U.S. Pat. No. 6,486,468 entitled, “High Resolution, High Pressure Xenon Gamma Rays Spectroscopy Using Primary and Stimulated Light Emissions”; (6) U.S. Pat. No. 7,002,159 B2 (the '159) entitled “Boron Coated Straw Neutron Detector”; (7) U.S. Pat. No. 7,078,704 entitled, “Cylindrical Ionization Detector with a Resistive Cathode and External Readout”; (8) U.S. patent application Ser. No. 10/571,202, entitled, “Miniaturized ⁶²Zn/⁶²CU Generator for High Concentration and Clinical Deliveries of ⁶²CU Kit Formulation for the Facile Preparation of Radiolabeled Cu-bis(thiosemicarbazone) Compound”; (9) U.S. patent application Ser. No. 12/483,771 entitled “Long Range Neutron-Gamma Point Source Detection and Imaging Using Rotating Detector”; (10) U.S. Patent Application No. 61/183,106 entitled “Optimized Detection of Fission Neutrons Using Boron Coated Straw Detectors Distributed in Moderator Material”; (11) U.S. Patent Application No. 61/333,990 entitled “Neutron Detectors for Active Interrogation”; and (12) U.S. Patent Application No. 61/334,015 entitled “Nanogenerator.” Each of these listed patents are hereby incorporated by reference in their entirety for all purposes, including, but not limited to, supplying background and enabling those skilled in the art to understand, make and use in Applicant's present invention.

BRIEF SUMMARY OF THE INVENTION

The present invention is a method and apparatus for operating boron-coated straw detectors in sealed mode, without the need for a continuous flow of gas. Boron-coated straw detectors are described in Applicant's prior patent U.S. Pat. No. 7,002,159 B2 entitled “Boron Coated Straw Neutron Detector” which issued Feb. 21, 2006.

The gas contained within the straw detectors is a specified gas mixture, of high purity and specified pressure, and it is critical to the successful operation of the straw detectors. Straw detectors can operate either with a continuous flow of the specified gas mixture, or in sealed mode as presented here. When operated in sealed mode, proper sealing of the straw detectors is crucial for stable operation.

Sealed-mode operation is necessary when using the boron-coated straw detectors in the field, where access to a continuous flow of the required gas mixture is not practical.

Also, sealed-mode operation is necessary when the straw detectors are used as portable instruments that must be moved from one location to the next swiftly or that must be operated while in motion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of the present invention.

FIG. 2 shows the variation of gas gain over time, measured in prototype detectors that were sealed according to the present invention.

FIG. 3 shows the variation of gas gain over temperature, measured in prototype detectors that were sealed according to the present invention.

FIG. 4 a through FIG. 4h shows design examples of boron-coated straw detectors grouped together to form bundles that are sealed inside a single external tube.

FIG. 5 is the predicted thermal neutron sensitivity (per unit length) of boron-coated straw bundles as a function of the number of straws making the bundle.

FIG. 6 shows the variation of gas gain over temperature, measured in a 7-straw bundle that was sealed according to the present invention.

FIG. 7 illustrates the readout circuit for seven 7-straw bundles (49 straws).

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the apparatus comprises combining a thin walled aluminum or stainless steel (or similar material) tube 2, and a fitting 3, at either end of the tube. The boron-coated straw 1 fits entirely within the tube and is secured in place with the two end fittings. The end fittings incorporate a central hole through which a ceramic feed-through tube 4 is positioned. A crimping tube 5 is positioned inside the ceramic tube 4. A thin metallic wire 6 passes through the tube 5. The wire 6 is tensioned, then crimped in place. A gold-plated pin 8 connects to the crimp tube 5. The wire forms the anode electrode that connects, through the crimp tube 5 and pin 8, to a high voltage supply, and to a preamplifier through a coupling capacitor. A plastic collar 7 is used to provide additional electrical insulation between the anode and fitting 3. Tube 9 serves as a gas port, used to purge the volume inside tube 2, and to fill the volume inside tube 2 with a specified gas mixture. A grounding collar 10 connects the tube 2 (cathode) to an electrical ground. The ceramic tube 4, the crimping tube 5, the plastic collar 7, the gas port tube 9 and the end fittings 3 are fixed with epoxy.

Several boron-coated straw detectors were sealed using the present invention. Initially, the gas port 9 of the sealed detector was connected to a supply of a gas mixture of argon/CO2. The detector was then purged with a continuous flow of this gas mixture, while heated to 60° C. for a period of 18-24 hours. Using valves, the gas flow was stopped, then the detector was allowed to cool to room temperature. The detector was then connected to a vacuum pump, and evacuated to a pressure of 0.7 atm. The gas port 9 was then crimp sealed.

In order to gage the seal quality and the resulting gas purity, the amplitude of signals corresponding to a single radiation energy were tracked in the sealed detectors over a period of time. Gas purity is essential to maintaining stable operation and an adequate signal-to-noise ratio. Gas contamination over long periods of time (due to materials outgassing, for instance) may alter the amplitude of signals, which in turn will affect the performance of the detector.

A pulse height spectrum was collected using a ²⁴¹Am gamma ray source. Photons emitted by this isotope, primarily with an energy of 60 keV, interact with the copper walls of the straw detector. At this energy, most interactions in copper are of the photoelectric kind, resulting in the absorption of the incident photon, and prompt emission of a characteristic 8 keV X-ray photon. This 8 keV X-ray photon may subsequently escape into the gas volume, and interact with argon atoms, depositing all of its energy. As a result, an 8 keV energy peak appears in the pulse-height spectrum.

The location of the characteristic X-ray peak in the gamma energy spectrum was used to track gas purity as shown in FIG. 2.

Temperature cycling tests were also carried out to evaluate the ability of the sealed straw detector to maintain stable operation at extreme environments. FIG. 3 shows the measured variation in the neutron counts recorded in a sealed straw detector during operation inside a chamber, where the temperature was varied from +60 C to −40 C.

EXAMPLES

The proposed invention is illustrated in FIGS. 4a to 4h. Each figure shows a stand-alone detector. The detector is a close-packed bundle of straws, where each straw detector is 4 mm in diameter and of length equal to the bundle length. The bundle is housed inside a sealed aluminum or stainless steel tube fitted with a fitting of appropriate design. Depending on the number of straws bundled, the dimensions and neutron sensitivity of the tube will vary, as shown in Table 1.

The anode electrodes of all BCS detectors within the bundle are connected together and read out with a single amplifier, using common electronics typically used to read out ³He tubes. Although the overall capacitance presented to the amplifier will be higher than that presented by a single tube of large diameter, the signals generated in the straw detectors are several times larger than those generated in ³He tubes, and thus, the signal-to-noise ratio is not affected by the larger capacitance.

The detection efficiencies of the straw bundles were estimated in Monte Carlo simulations implemented in MCNP5 and are listed in Table 1. A parallel beam of monoenergetic neutrons was directed normally over the entire side of the bundle. The computed sensitivity (per unit length) is also plotted in FIG. 5 as a function of the number of straws. In all cases, a ¹⁰B₄C coating thickness of 1 μm was assumed.

The thermal neutron sensitivity of a ³He tube, with a 5.08 cm diameter (2 inches), pressurized to 2.5 atm, is ˜3.4 cps/nv/cm, equivalent to that obtained with the 187-straw bundle, whose diameter is only slightly larger at 6.36 cm. The sensitivity of the BCS bundle can be further improved by optimizing the thickness of the ¹⁰B₄C coating.

The gain stability of the 7-straw bundle was also measured over the course of 255 days, as shown in FIG. 6. The gain variation was less than ±4%.

Readout. When several straw detectors are grouped together in a bundle, reading them out separately would require a number of pre-amplifiers equal to the number of straws. Significant savings can be achieved with a readout scheme based on delay lines, offering the capability to decode the identity of the firing straw with only 2 pre-amplifiers. FIG. 7 illustrates the readout circuit for seven 7-straw bundles (49 straws). On one end of the bundles, all straws with the same index across different bundles are connected together, then to a different tap on delay line 1. On the other end of the bundles, all straws within the same bundle, are connected together, then to a tap on delay line 2. In this scheme, delay line 1 identifies the straw index within a single bundle, and delay line 2 identifies the specific bundle among the 7 bundles.

TABLE 1
Boron-coated straw bundle dimensions
and thermal neutron sensitivity.
	Detection	Thermal neutron
	Bundle	efficiency	sensitivity
Number of straws	Diameter	for thermal	per unit length
in bundle	(cm)	neutrons (%)	[(cps/nv)/cm]
1	(FIG. 4a)	0.4	9.0	0.036
7	(FIG. 4b)	1.27	18.4	0.234
19	(FIG. 4c)	2.12	26.6	0.564
37	(FIG. 4d)	2.97	33.3	0.989
61	(FIG. 4e)	3.82	38.8	1.48
91	(FIG. 4f)	4.67	43.3	2.02
127	(FIG. 4g)	5.52	47.0	2.59
187	(FIG. 4h)	6.36	50.0	3.4

Claims

1. An apparatus for detecting radiation comprising: a thin wall tube containing a boron-coated straw tube;wherein said thin wall tube is sealed with a gas mixture.

2. The apparatus of claim 1 wherein the thin wall tube is composed of aluminum or other material which minimizes scattering of low energy neutrons.

3. The apparatus of claim 1 wherein the thin wall tube is composed of low Z material to minimize the sensitivity for gamma ray interactions.

4. The apparatus of claim 1 wherein the length of the thin wall tube is approximately equal to the length of the boron-coated tube contained within; and where such length may vary from a few centimeters to several meters.

5. The apparatus of claim 1 wherein the diameter of the thin wall tube is large enough only to accommodate a single boron-coated tube or multiple boron-coated tubes; and wherein the multiple boron-coated tubes are arranged in close-packed, hexagonal configurations with the following number of tubes

6. The apparatus of claim 1 wherein the gas mixture is Ar/CO2 with CO2 content in the range 1% to 20%.

7. The apparatus of claim 1 wherein the gas mixture is Ar/CH4 with CH4 content in the range 1% to 20%.

8. The apparatus of claim 1 wherein the gas mixture is Xe/CO2 with CO2 content in the range 1% to 20%.

9. The apparatus of claim 1 wherein the gas mixture is Xe/CH4 with CH4 content in the range 1% to 20%.

10. The apparatus of claim 1 wherein the selected gas mixture can be maintained at an absolute pressure less than 2 atm.

11. The apparatus of claim 1 further comprising a fitting used to seal either end of said thin wall tube; wherein said fitting is capable of receiving and positioning the boron-coated tube centrally within said thin wall tube and further capable of receiving and positioning a wire centrally within said thin wall tube and accommodating a gas port used to purge and fill the volume to be sealed.

12. The apparatus of claim 11 wherein said fitting is composed of aluminum or another metal, that is easy to machine and bonds well with other materials that attach to it.

13. The apparatus of claim 11 wherein said fitting has a central hole, through which a feed-through insulating tube is fitted.

14. The apparatus of claim 11 wherein said fitting has an off-center hole, through which a gas port tube is fitted.

15. The apparatus of claim 13 wherein said feed-through insulating tube is composed of ceramic or another electrical insulator material that fits inside the central hole of said fitting and can accommodate a crimping tube.

16. The apparatus of claim 15 wherein said crimping tube is composed of copper and fits inside said feed-through insulating tube, with an inner diameter large enough to accommodate a thin metallic wire up to 50 um in diameter, and capable of crimping around said wire and securely retaining high tension in the wire.

17. The apparatus of claim 11 wherein said gas port fits inside the off-center hole of said fitting and can be connected to an external vacuum and gas filling system, and composed of a ductile metal such as copper, stainless steel, nickel, or aluminum and capable of being sealed using pinch off technique.

18. The apparatus of claim 13 further comprising multiple boron-coated tubes arranged in close-packed, hexagonal configurations; and wherein end fittings are provided with accurately positioned insulating feed-throughs capable of receiving and positioning all associated wires.

19. The apparatus of claim 11 wherein said fitting has multiple holes through which feed-through insulating tubes are fitted.

20. The apparatus of claim 1 wherein the diameter of the thin wall tube is large enough for 187 boron-coated tubes; and wherein the 187 boron-coated tubes are arranged in close-packed, configuration.

21. The apparatus of claim 1 wherein the diameter of the thin wall tube is large enough only to accommodate multiple boron-coated tubes; and wherein the multiple boron-coated tubes are arranged in close-packed, configuration with 187 tubes.

22. The apparatus of claim 1 wherein the diameter of the thin wall tube is large enough only to accommodate a specific number of boron-coated tubes; wherein boron-coated tubes are arranged in close-packed, configuration.

23. An apparatus for detecting radiation comprising: a thin wall tube containing multiple boron-coated straw tubes;wherein said thin wall tube is sealed with a gas mixture;further comprising electronic components with means to decode the individual straw tubes to indicate in which straw each neutron interactions occurs; andfurther comprising electronic components capable of obtaining accurate determination of the longitudinal position of each neutron interaction along the length of the tube in which it occurs.

24. The apparatus of claim 1 wherein the gas mixture is He/CH4 with CH4 content in the range 1% to 20%.

25. The apparatus of claim 1 wherein the gas mixture is He/CO2 with CO2 content in the range 1% to 20%.

26. The apparatus of claim 1 wherein the gas mixture is composed of any noble gas combined with a quench gas whose purpose is to absorb photon emissions and to increase the drift velocity of electrons including CO2, CH4, CF4, C2H6, N2, H2, H2O.