Patent Number: 049960172
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a neutron generating system. More specifically, the invention relates to a new and improved neutron generator tube especially adapted to traverse the narrow confines of a well or borehole for well logging purposes. 2. Description of the Prior Art Over four decades have passed since F. M. Penning disclosed a neutron generator in U. S. Pat. No. 2,211,668 constructed of a low pressure deuterium-filled envelope containing a cathode and anode with an axially oriented magnetic field ion source, a nuclear reaction producing target and one or more acceleration electrodes. For the last three decades this "Penning" ion source has been employed extensively in various neutron generator tubes for downhole oil and gas well neutron logging. During this period extensive modifications and improvements have been suggested with varying degrees of commercial success, yet specific problems still remain, particularly during well logging in deep wells at high temperatures. It is generally known that the permanent magnetic materials used in the conventional neutron generator tubes tend to lose their magnetic properties when subjected to temperatures such as 400.degree. C. or greater (see U.S. Pat. Nos. 3,546,512 and 3,756,682). Because of the small confines of a well borehole, the neutron generator tube must have extremely high magnetic field capabilities in a relatively high vacuum in order to have significant ion production. In order to prevent or eliminate outgassing within the tube during use in deep, high temperature wells, an ultra high temperature bake out is necessary during fabrication of the tube. This creates the pragmatic dilemma; i.e., if an external magnet or field is employed (separate from the neutron generator tube) the physical dimensions of the resulting well logging tool restricts its utility, and if an internal permanent magnet is employed, the bake out procedure again will either restrict the physical size or deleteriously affect the magnetic field strength. Another historically recognized problem which continues to pragmatically limit the contemporary neutron generator tube is the removal of thermal energy from the target surface of the tube. Thus it is known that the energy of the ion beam striking the target and inducing the desired nuclear reaction, if too intense, will result in high temperature sputtering and thermal failure of the target and thus failure of the neutron generator tube. Various methods of modifying the composition and the thickness of the hydrogen occluding target film have been proposed to compensate for this problem. In a recent U.S. Pat. No. 3,784,824, vapor deposition or sputtering of a non-occluder for hydrogen onto the target during operation was suggested; yet, the problem essentially remains as a critical limitation. It is also generally recognized that shielding or confining the magnetic field to the ion source by encapsulating the permanent magnet and ion source and allowing the ion to escape through an aperture creating a narrow intense ion beam (see U.S. Pat. No. 3,112,401) is a desirable practice. In contrast, U.S. Pat. Nos. 3,141,975 and 3,401,264 employ one or more ion beam grids (with and without variable potential) placed between the ion source and target. In this approach the ion beam optics are manipulated across a relatively large cross-sectional ion beam such that the ion path is completely defocused and linear, thus allowing for a low energy acceleration of the positive ions. However, up to this time the combination of optimum thermal energy removal from the target surface and uniform distribution of the ion beam energy impinging on the target per unit surface area has been beyond contemporary technology. SUMMARY OF THE INVENTION In view of the prior art methods and apparatus and their associated limitations, I have discovered a neutron generating system and an associated improved neutron generator tube comprising: (a) a hermetically sealed housing containing an ionizable gas; (b) an axial recess in one end of the housing to accept a removable magnet; (c) a ring anode axially oriented within the housing adjacent to the recessed end for accepting the magnet; (d) an axially oriented thermal conductor cathode penetrating through the other end of the housing wherein the inner surface of the cathode contains a target; (e) an ion screen near the anode ring and between the anode ring and cathode target wherein the ion screen contains an axially positioned gridded aperture; and (f) an electron shield near the cathode target and between the ion screen and cathode target wherein the electron shield contains an axially positioned aperture. The neutron generating system of the present invention further comprises a removable magnet adapted to fit into the axial recess which in the preferred embodiment is a samarium/cobalt magnet. The invention also provides that the thermal conductor cathode have a cross-sectional area of substantially the same size as the target, and that the end of the cathode containing the target be within the housing such as to remove thermal energy from the target. The present invention further provides that the ion screen be an electrically conductive, grounded surface and in a preferred embodiment the gridded aperture be a screen of etched tungsten of about 0.002 to 0.005 inch thick with the openings representing about 90 percent of the surface area. In an alternative embodiment, a means is provided to vary the electrical potential on the ion screen relative to the potentials of the ring anode and cathode such as to assist in ignition of the ion source. It is a primary object of the present invention to provide a neutron generating system and tube that is compatible with the hostile high temperatures and pressures associated with downhole well logging. It is a further object that the device be compatible with extremely small diameter well casing and borehole diameter of deep wells. It is an object of the invention to provide a removable magnet such that the tube can be baked out under vacuum and hermetically sealed without subjecting the permanent magnet to ultra-high temperatures. It is also an object of the invention to provide a means for removing thermal energy from the target thus reducing thermal effects on the neutron output. And it is an object of the invention to provide a means of producing a broad ion beam of reduced power per unit area impinging on the target. Fulfillment of these objects and the presence and fulfillment of other objects shall be apparent upon complete reading of the specification and claims in conjunction with the attached drawing.