3Helium gas proportional counter

Abstract

A .sup.3Helium gas counter comprising polyethylene slabs, a rectangular gas tube within the polyethylene slabs, and a mixture of .sup.3Helium and Xenon. A .sup.3Helium gas counter comprising polyethylene slabs, a rectangular gas tube within the polyethylene slabs, and a mixture of .sup.3Helium and Krypton. A method of making a .sup.3Helium gas counter comprising providing polyethylene slabs, placing a rectangular gas tube within the polyethylene slabs, and depositing a mixture of .sup.3Helium and Xenon into the rectangular gas tube.

Claims

1. A .sup.3Helium gas counter comprising: polyethylene slabs; a rectangular gas tube within the polyethylene slabs; and a mixture of .sup.3Helium and Xenon.

2. The .sup.3Helium gas counter of claim 1 wherein the .sup.3Helium and Xenon mixture is at a pressure of 11 mg/cm.sup.2 in the rectangular tube.

3. The .sup.3Helium gas counter of claim 1 wherein the pressure of the .sup.3Helium is lower than 2 atm.

4. The .sup.3Helium gas counter of claim 1 wherein the rectangular tube within the polyethylene slabs is such that there are no air gaps or other gaps between the rectangular tube and the polyethylene slabs.

5. A .sup.3Helium gas counter comprising: polyethylene slabs; a rectangular gas tube within the polyethylene slabs; and a mixture of .sup.3Helium and Krypton.

6. A method of making a .sup.3Helium gas counter comprising: providing polyethylene slabs; placing a rectangular gas tube within the polyethylene slabs; and depositing a mixture of .sup.3Helium and Xenon into the rectangular gas tube.

7. The method of making the .sup.3Helium gas counter of claim 6 wherein the .sup.3Helium and Xenon mixture is deposited resulting in a pressure of 11 mg/cm.sup.3 in the rectangular tube.

8. The method of making the .sup.3Helium gas counter of claim 7 further including the step of maintaining the .sup.3Helium pressure lower than 2 atm.

9. The method of making the .sup.3Helium gas counter of claim 6 further including the step of placing the rectangular tube within the polyethylene slabs such that there are no air gaps or other gaps between the rectangular tube and the polyethylene slabs.

10. The method of making the .sup.3Helium gas counter of claim 6 further including the step of placing the rectangular tube within the polyethylene slabs such that the rectangular tube is flush with the polyethylene slabs and in continuous contact therewith.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic illustrating the cross-sections of a conventional, low-intercepting, undermoderated .sup.3He gas counter system that uses cylindrical tubes; and an exemplary, high-intercepting, well-moderated .sup.3He/Xe gas counter system with a significantly improved detector efficiency.

DETAILED DESCRIPTION

(2) FIG. 1 illustrates some of the main differences between the operation of conventional .sup.3He gas counters and the .sup.3He gas counter of this invention.

(3) Conventional .sup.3He gas counter systems are under-efficient and permanently under-moderated. The geometry of the polyethylene moderator and gas tube are not optimally matched, such that the interaction of fast neutrons with the detector system is often minimal and the detection of subsequent thermalized neutrons is not optimized.

(4) The invention provides for replacement of the cylindrical tubes with a single gas tube with a rectangular cross section. The rectangular cross-section will eliminate the air gaps in the moderator that surrounds the gas tube because the polyethylene slabs will readily mate to the faces of the rectangular gas tube. The improvement in the moderation will reduce losses of diffusing thermalized neutrons and improve the detector efficiency. In addition, the total energy response can be readily tailored to potential targets.

(5) Conventional .sup.3He gas counters require operation at relatively high .sup.3He pressures, up to 4 atmospheres, in order to achieve an overall gas density that is high enough to ensure efficient collection of reaction products within the gas volume. In large volume detectors, the use of high .sup.3He pressure means that large quantities of .sup.3He, a gas that is becoming increasingly scarce and expensive, are needed for each detector.

(6) This invention provides for the use of significantly lower .sup.3He pressures.

(7) In order to ensure efficient collection of reaction products, the reduction in the .sup.3He pressure must be offset by introducing another gas, preferably a high density gas, such as xenon, that ensures a high overall gas density. In fact, the use of a high density gas mixture can significantly improve the efficiency of the detector because it is possible to independently adjust the pressures of the component gases to optimize the efficiency of the detector. The pressure of the high density inert gas can be independently adjusted to optimize the overall gas density in the tube to permit efficient collection of reaction products and reduction of wall losses.

(8) The pressure of the .sup.3He can be independently adjusted to optimize neutron absorption throughout the gas tube.

(9) Conventional .sup.3He gas counters that operate at relatively high .sup.3He gas pressure are not optimized for efficient neutron capture throughout the entire volume of gas in the tube. When the .sup.3He pressure is set such that the density of the .sup.3He is high enough to ensure efficient collection of reaction products, the pressure is much higher than what is needed for optimal neutron absorption. This increases the probability that neutron capture will occur near the tube wall. Neutron capture near the tube wall leads to an increased probability of reaction product energy loss to the wall.

(10) As explained above, this invention solves a long-standing problem.

(11) This invention provides for the use of a gas mixture that permits the use of lower .sup.3He gas pressures. The .sup.3He pressure can be independently adjusted (lowered) to an optimal value that provides for more uniform neutron capture throughout the entire volume of the tube. Independent optimization of the pressure of the .sup.3He and the xenon gases will provide the gas counter of this invention with significantly improved detector efficiency.

(12) Conventional .sup.3He gas counters use cylindrical gas tubes. However, the cylindrical geometry is not optimum with respect to minimization of the wall effect because the cylindrical geometry has a relatively large differential element of volume near the tube surface. This invention teaches the use of a gas tube with a rectangular geometry. The rectangular geometry reduces the differential gas volume near the tube wall compared to the cylindrical geometry.

(13) The innovations in the design of .sup.3He gas proportional counters taught in this disclosure will result in improved detector efficiency, reduced consumption of .sup.3He, and reduced cost per detector. In view of current concerns about high demands for an increasingly scarce commodity, the innovations herein will have a significant impact on verification activities by enabling the manufacture of more large-area neutron gas counters at lower cost per counter.

Example 1—Prior Art

(14) For example, a currently deployed counter system has a hollow cavity with rectangular dimensions of 36″×15″×2″ and uses two tubes, each 2″ diameter by 36″ long with a combined volume of about 3.7 l. The gas pressure is 2 atm.

Example 2

(15) A rectangular replacement counter of about the same volume would have dimensions of 15″×36″×1 cm. This tube will be in a cavity with no air gaps, and optimized polyethylene thickness. MCNP calculations show that a two-fold or greater reduction in the pressure of the .sup.3He occurs.

(16) In addition, the calculations show that a two-fold reduction in the area of the gas tube (7.5″×36″) is achieved.

(17) The engineering innovations taught in this invention will achieve an overall four-fold reduction in the consumption of .sup.3He and still achieve improved detector efficiency.

(18) Given that currently fielded portal monitors (e.g., the TSA VM-250AGN) use two counters per pillar and two pillars per system, the total cost savings could be over $20K-$30K for each portal monitor system, depending on the price of the .sup.3He. If the cost of .sup.3He continues to rise, the savings will be greater.

(19) The specific design improvements taught in this invention, including such parameters as the size, shape, volume and pressure, can be varied for each detector application. State of the art computational tools, such as MCNP calculations, can be performed for each application.

(20) Other high density inert gases, such as Kr, could be used in place of Xe.

(21) Many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any reference to claim elements in the singular, e.g., using the articles “a,” “an,” “the,” or “said” is not construed as limiting the element to the singular.