Abstract:
A method of determining structural defects in a component that utilizes neutron activation of a solution having the ability to penetrate small cracks on the surface of a material via capillary absorption that produces a discernable prompt gamma release of a defined energy when exposed to a neutron pulse. The intensity of the gamma rays produced at the desired energy at a user controlled position on the surface is used to determine the crack location, length and depth.

Description:
BACKGROUND 
     1. Field 
     This invention relates in general to the detection of cracks in irradiated surfaces and, more particularly, the nondestructive examination of irradiated components to determine structural flaws. 
     2. Related Art 
     In the event highly radioactive components or the containers of radioactive materials need to be manipulated, it is important to ensure the structural integrity of the components or material containers be assessed to minimize the potential for loss of control and containment of the radioactive material. The structural integrity of radioactive components or containers of radioactive materials resident in high radiation fields is difficult to assess using standard visual and ultrasonic Non-Destructive Examination (NDE) techniques due to the impact the radiation field has on equipment access and operability. A need exists to provide a means to evaluate the structural integrity of radioactive components and containers of radioactive materials using methodology and devices suitable for a high radiation environment. 
     SUMMARY 
     This invention discloses a method of nondestructively detecting structural defects in a surface of an irradiated material comprising the step of applying a liquid crack penetrant, preferably one high in Nitrogen content, or one that is mixed with a chemical species containing a large proportion of an isotope that has a relatively large fast neutron prompt capture gamma emission cross section, such as Scandium, Vanadium, Manganese or Titanium, that emits a prompt gamma release of a defined energy, to a surface of the material. The surface is then irradiated with a neutron pulse generator and a plurality of beta radiation detectors tuned to the defined energy, are positioned in a regular pattern over the surface of the material to which the mixture is applied to; the plurality of beta radiation detectors respectively providing an output indicative of a reception of the prompt gamma release of the defined energy in an area of the surface viewed by a corresponding one of the beta radiation detectors. The reception is employed to map a characteristic of the defect. In one embodiment the characteristic is the location and length of the defect on the surface. In another embodiment, the characteristic is the depth of the defect on the surface. Preferably, the depth of the defect is determined from the strength of the reception. 
     In one such embodiment, the mixture is absorbed into the surface by capillary absorption. Desirably, the neutron pulse generator is a neutristor style Neutron Pulse Generator. Preferably, the tuning of the beta radiation detectors is accomplished by placing a high atomic number sacrificial material between the surface and an active portion of the beta radiation detectors to act as an electron radiator. Desirably, the beta radiation detectors are silicon carbide (SiC) detectors with the thickness, distance from the active detector region, and type of material used in the sacrificial layer of the beta radiation detector elected to ensure that mostly the electrons produced by Photoelectric Absorption by the desired emitted prompt gamma radiation energy are completely stopped inside the active region of the SiC detectors. In one such embodiment the sacrificial material is either platinum or tungsten. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic representation of the device layout of this invention; and 
         FIG. 2  is a schematic layout of the SiC detector and pre-amplifier used in the SiC detector array of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The operational bases for this invention are founded on a novel combination of dye-penetrant crack detection, prompt neutron gamma radiation detection, and planar Computerized Tomography (CT) techniques. The system also uses a novel SiC tuned gamma intensity detection method and a preamplifier for the very small SiC signal output based on Solid State Vacuum Tube technology. The preferred embodiment is as follows: A non-corrosive crack penetrant, preferably one high in Nitrogen content, or a crack penetrant such as Dynaflux Visible Dye Penetrant available from the Dynaflux Quality Products Company, Cartersville, GA, that is mixed with a chemical species containing a large proportion of an isotope that has a relatively large fast neutron prompt capture gamma emission cross section, such as Scandium, Vanadium, Manganese, or Titanium, is applied at a controlled temperature and pressure, as necessary to ensure the penetrant is in a liquid state when it reaches the surface being inspected. The system described herein has the capability to spray the mixture onto the surface to be examined at high pressure to allow the material to be applied without the system hardware actually touching the surface. A “neutristor” style Neutron Pulse Generator (NPG) assembly, developed by Sandia National Laboratory (Sandia National Laboratory, “Innovation Marketplace”, September, 2014, Vol. 1, Issue 3), contained in the measurement assembly is then brought to within an inch of the surface at a radial position known to within 0.1 mm of a fixed surface reference point on the surface by the operator. The NPG assembly is surrounded by an array of specially configured SiC radiation detectors (e.g., 100×100 square of 1 mm 2  detectors, such as described in U.S. patent application Ser. No. 13/769,401, filed Feb. 18, 2013, entitled Solid State Radiation Detector With Enhanced Gamma Radiation Sensitivity) having positions known to within 0.05 mm of a reference point on the NPG assembly that are tuned to measure primarily the intensity of the Photoelectric Absorption generated electrons produced in material between the gamma radiation emitted from the prompt gamma emitting isotope in the crack penetrant mixture and the active region of the SiC detector as schematically shown in  FIG. 1 . 
     The nondestructive examination system  10  of this invention has a spray system  18  that is moveable over the surface  20  of the material to be examined to spray the crack penetrant solution  30  over the surface. The neutron pulse generator  12  emits a stream of pulses that react with the isotope within the crack penetrant solution  30  to emit prompt gamma radiation that is detected by the SiC radiation detectors  24 . The outputs of the SiC radiation detectors are fed to the pre-amplifiers  26  with the outputs of the pre-amplifiers being sent to the processing electronics which interpret the variations in and strength of the received signals to determine the presence of cracks and location, length and depth. The tuning of the SiC beta radiation energy sensitivity is accomplished by placing a high atomic number sacrificial material  36 , such as platinum or tungsten, between the surface  20  and the active portion  40  of the SiC detector  24  to act as an electron radiator. The thickness, distance from the active detector region  40 , and type of material used in the sacrificial layer  36  of the detector is selected to ensure that mostly the electrons produced by Photoelectric Absorption by the desired emitted prompt gamma radiation energy in the sacrificial layer are completely stopped inside the active region of the SiC detectors. This can be accomplished by those skilled in the art by proper adjustment of the distance between the surfa.ce of the electron radiator  36  and the front face of the SiC Aluminum  32  covering the Schottky interface region  34  so that electrons with less than full gamma energy have little probability of reaching and being stopped in the active volume of the SiC detector. This will ensure that the measured detector output is proportional to the amount of penetrant material in the crack seen by each of the detectors, and therefore to the dimensional properties of the crack.  FIG. 2  provides an outline of the SiC detector and associated array geometry. 
     The SiC detectors  24  receive the prompt gamma radiation  38  through the sacrificial material  36  that convert the prompt gamma radiation to the generation of electrons through Photoelectric Absorption with the distance from the face of the Schottky contact ( 34 ) variable to assure all the electrons are captured. The electrons travel through then-doped SiC  40  of approximately 10 μm thick in this example, to the substrate  42  where the electrons are collected by a gold backed ohmic contact  44  and the output current  28  is conveyed to the processing electronics  46  through the pre-amplifiers  26 . The relative measured intensity data obtained from each very small and precisely positioned SiC detector in the detector array after the neutron pulse generator triggers one or more times will be used to determine the dimensional characteristics of the defect. The detector signal output will be input to individual preamplifiers integral to the neutron pulse generator structure utilizing a miniature design like that described in U.S. patent application Ser. No. 14/996,667, filed Jan. 15, 2016, entitled “In-Containment Ex-Core Detector System.” The amplified measured detector current signals are then transmitted to a measurement and analysis system located at a convenient location outside the high radiation area. An application specific correlation between the desired measured crack dimensional properties (e.g., depth, width, length) is determined from the SiC detector array geometry and relative measurements of each detector in the array using in-planar CT algorithms. 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.