Patent Publication Number: US-6215851-B1

Title: High current proton beam target

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to gamma ray based nitrogen detection systems, and more particularly to a high current proton beam target which generates gamma emissions and utilizes a stopping layer formed of a refractory metal. 
     BACKGROUND OF THE INVENTION 
     The need to detect contraband, such as drugs and explosives, is well appreciated. Efforts to detect contraband from being smuggled through various ports of entry, such as airports, border crossings and boat docks, has been a focus of attention. Various non-intrusive scanning techniques have been developed in the art which are more accurate than contemporary X-ray scanning techniques. It is known that nitrogen is a common element found in many illicit drugs and explosives. As such, nitrogen detection systems have been developed to detect nitrogen containing contraband. 
     A type of nitrogen detection system utilizes gamma rays. Generally, such a system uses a beam of energetic protons which are focused upon a target. The incident proton beam excites the target material according to well known principles, thereby causing it to produce gamma rays. 
     In this regard, it is known that when about 1.75 MeV protons impinge on a suitable target, e.g., a material coated with  13 C, they have a high probability of producing 9.17 MeV gamma rays by the reaction  13 C(p, y) 14 N. These gamma rays are emitted from the target nonuniformly at all angles. Those gamma rays emitted at about 80.66°, with respect to the direction of the proton beam, have a large probability of being resonantly absorbed by  14 N contained in an object of interest. Detection of such absorption phenomenon is used to analyze the amount of nitrogen in an object of interest in order to detect nitrogen containing contraband. 
     As depicted in FIG. 1, a typical configuration of a prior art proton beam target consists of a thin film of  13 C which is used to produce gamma rays. This gamma reaction layer is formed onto a proton stopping layer via an electron beam (or e-beam) evaporation process. The stopping layer is used to prevent undesirable transmission of energetic protons after they have traversed through the  13 C gamma reaction layer. Because the incident proton beam results in the generation of substantial heat energy within the target, the stopping layer is attached to a cooling support for transferring heat energy away from the gamma reaction and stopping layers. The cooling support is typically formed of Copper or Copper alloys or Beryllium. 
     The stopping layer is formed of a suitable high atomic number (z) material. The high z material is required to effectively prevent the transmission of energetic protons. In this regard, the high z stopping layer is required to be of a minimal thickness necessary to fully stop the proton beam. The stopping layer, however, is also desired to be less than a thickness which substantially attenuates the gamma signal generated by the  13 C gamma reaction layer. The stopping layer must additionally be formed of a material which will not react with the high energy proton beam to produce additional gamma signals which will interfere with the desired  13 C resonant gamma emission. In addition, the stopping layer must survive the operating temperatures of the target. Thus, for example, the prior art has been to use a stopping layer formed of gold (Au) which is electro-plated onto a cooling support to a thickness of roughly 20 microns. 
     The desire to decrease the inspection or scanning time has driven the need to increase the operating current of the proton beam. Previously, due to the limited proton beam operating currents, prior art configurations have typically utilized proton beams operating at currents on the order of 10 micro-amperes. Proton beams have now been developed which are capable of operating at currents of 10 milliamperes (mA), three orders of magnitude greater than prior art devices. Prolonged exposure of the above described prior art targets to such high current protons, however, results in blistering and delamination of the prior art gold stopping layer contained therein, as well as, blistering and delamination of the outer  13 C layer. Blistering describes the phenomenon wherein the incident beam of protons result in implantation of hydrogen atoms into the gold stopping layer. The implanted hydrogen tends to coalesce to form bubbles and causes the stopping layer to blister and delaminate the  13 C coating/stopping layer interface from the stopping layer/cooling layer interface. As a result, the generated gamma rays are of an undesirable quality and nature and of a greatly reduced quantity. Thus, while prior art targets have been effective while used with proton beam currents of 10 micro amperes, such targets are inadequate when used with relatively high current proton beams. 
     It is therefore evident that there exists a need in the art for a proton beam target which is able to withstand exposure to the bombardment of high current protons while producing the desired gamma emissions. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a proton beam target for generating gamma rays which are generated therefrom in response to an impinging proton beam. The proton beam target is provided with a  13 C gamma reaction layer for generating the gamma rays therefrom. The proton beam target is further provided with a stopping layer for mitigating transmission of the proton beam therethrough and thus mitigating the production of undesirable gamma rays. The stopping layer is formed of a refractory metal which has a relatively high hydrogen solubility for dissolving implanted hydrogen atoms therewithin as a result of the impingement of the proton bean and which is also chemically reactive with the  13 C gamma reaction layer for improved chemically bonding therewith. 
     Preferably, the refractory metal is chosen from the group consisting of Tantalum, Zirconium, Niobium and Hafnium. As such, the stopping layer has a hydrogen solubility greater than that of gold. In addition, the  13 C gamma reaction layer is sputter deposited onto the stopping layer. In this regard, sputter deposition ensure that a carbide phase is formed between the  13 C gamma reaction layer and the stopping layer as a result of sputter ion assisted chemical reactions thereat. 
     The proton beam target is preferably provided with a cooling support for dissipating heat energy away from the stopping layer. The stopping layer is attached to the cooling support and the stopping layer through a brazing process and a braze layer is formed therebetween. The braze layer is formed of a Silver based braze alloy. 
     As such, based on the foregoing, the present invention mitigates the inefficiencies and limitations associated with prior art proton beam targets. The present invention is particularly adapted to facilitate impingement of relatively high current proton beams. When exposed to relatively high current protons, prior art targets suffer from blistering due to hydrogen bubble formation. Advantageously, the stopping layer is formed of a refractory metal which has a relatively high hydrogen solubility. In this regard, the target of the present invention mitigates against blistering due to the formation of hydrogen bubbles formed within the stopping layer. This is in comparison to prior art stopping layers which are typically formed of electro-plated gold which has no solubility with regard to hydrogen. In addition, the bond between the gamma reaction layer and the stopping layer is contemplated to be stronger than prior art designs. This is especially the case where gold is used in prior art designs and the target is subjected to relatively high operating temperatures. In particular, because the stopping layer is formed of a refractory metal, it is inherently heat resistant and also capable of reacting with the gamma reaction layer for forming a stable carbide phase therebetween. Such a carbide phase is contemplated to facilitate strong bonding thereat. Furthermore, the gamma reaction layer is preferably sputter deposited which provides a relatively stronger bond with the stopping layer than electron beam evaporation techniques used in the prior art. Further, the sputter depositing process is contemplated to facilitate selective attachment of the gamma reaction layer and thus mitigates against unnecessary material loss in comparison to electron beam techniques. 
     Furthermore, the stopping layer is brazed to a cooling support. Such brazing forms an intermediate braze layer therebetween which facilitates a strong bond thereat and facilitates residual stress relief with respect to the stopping layer. Accordingly, the present invention represents a significant advance in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These, as well as other features of the present invention, will become more apparent upon reference to the drawings wherein: 
     FIG. 1 symbolically illustrates a cross-sectional view of a prior art proton beam target; and 
     FIG. 2 symbolically illustrates a cross-sectional view of the proton beam target of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings wherein the showings are for purposes of illustrating a preferred embodiment of the present invention only, and not for purposes of limiting the same, FIG. 2 illustrates a proton beam target  18  which is constructed in accordance with the present invention for generating gamma rays used in nitrogen containing contraband detection systems. 
     In accordance with the present invention, there is provided a proton beam target  18  for producing gamma rays when impinged upon with a proton beam. The proton beam target  18  is provided with a gamma production layer  20  which is formed of  13 C for producing gamma rays. The gamma production layer  20  generates resonant gamma rays at an energy of 9.17 MeV when subjected to the impingement of a proton beam having an energy of 1.75 MeV by the reaction  13 C(p, y) 14 N. 
     The gamma production layer  20  is attached to a high z stopping layer  22 . As one of ordinary skill in the art will appreciate, the high z stopping layer  22  is formed to be of a minimal thickness necessary to mitigate the transmission of energetic protons therethrough. Furthermore, the high z stopping layer  22  is also formed to be less than a thickness which substantially attenuates the gamma signal generated by the  13 C gamma reaction layer  20 . 
     The high z stopping layer  22  is formed of a refractory metal. In the preferred embodiment of the present invention, the stopping layer  22  is formed of Tantalum (Ta) which has an atomic number of 73. Furthermore, the Tantalum stopping layer  22  is preferably 20 to 130 microns thick and takes the form of a thin foil. Other refractory metals which may be used to form the stopping layer  22  include, for example, Zirconium (Zr, atomic number 40), Niobium (Nb, atomic number 41) and Hafnium (Hf, atomic number 72). It is contemplated that a refractory metal is a metal or alloy that is relatively heat-resistant and, therefore, having a relatively high melting point. Importantly, it is further contemplated that the refractory metal has a relatively high hydrogen solubility, i.e., capable of dissolving hydrogen atoms. 
     Because the stopping layer  22  is formed of a refractory metal, the stopping layer  22  is characterized by being formed of relatively high atomic number or high z material which facilitates the mitigation of energetic protons being transmitted therethrough. Furthermore, the refractory metal stopping layer  22  is substantially non-reactive with high energy protons with respect to any undesirable production of gamma signals which would interfere with the desired resonant gamma emissions from the gamma reaction layer  20 . 
     As mentioned above, in operation, a substantial amount of heat energy is generated within the target  18  as a result of the impingement of the energized protons. In this regard, the refractory metal formed stopping layer  22  is particularly adapted to withstand high operating temperatures when subjected to relatively high current protons. 
     As further mentioned above, blistering may result in the prior art stopping layer  14  of a typical prior art target  10  when subjected to high current proton beams (See, FIG.  1 ). This is due to hydrogen molecules being implanted therein. Referring back to FIG. 2, the stopping layer  22  of the target  18  of the present invention, however, is formed of a refractory metal which has a relatively high hydrogen solubility. In this regard, the stopping layer  22  mitigates against the formation of hydrogen bubbles therein, and therefore mitigates against blistering. As such, is stopping layer  14  is contemplated to be formed of a material which is has a hydrogen solubility greater than that of gold which has typically been used for prior art stopping layers. 
     In the preferred embodiment of the present invention, the  13 C gamma reaction layer  20  is sputter deposited onto the high z stopping layer  22 . Such sputtering deposition is effectuated according to those procedures which are well known to one of ordinary skill in the art. Sputter deposition, is contemplated to facilitate selective placement of the  13 C material onto the stopping layer  22  to enhance the bonding of the  13 C to the refractory metal. In addition to sputter deposition, other fabrication methods may be used and are chosen from those well known to one of ordinary skill in the art. Advantageously, because the high z stopping layer  22  is formed of a refractory metal, such as Tantalum, the  13 C gamma production layer  20  is particularly suited to chemically bond therewith. In particular, a carbide phase may be produced at the interface between the high z stopping layer  22  and the  13 C gamma production layer  20  as a result of chemical reactions thereat. 
     The stopping layer  22  is attached to a cooling support  26 . The cooling support  26  is used to transfer and dissipate heat energy away from the gamma production and stopping layers  20 ,  22 . In addition, the cooling support  26  provides structural support for the relatively thin gamma reaction and stopping layers  20 ,  22 . Preferably, the cooling support  26  is formed of material having a relatively high thermal conductivity such are Cooper (Cu), Beryllium (Be) and alloys formed thereof. It is contemplated that other suitable materials may be used which are chosen from those well known to one of ordinary skill in the art. 
     In the preferred embodiment of the present invention, the stopping layer  22  is attached to the cooling support  26  through a brazing process. Brazing is a joining process which is effectuated at temperatures above 500° C. As such, a braze layer  24  is interposed between the stopping layer  22  and the cooling support  26 . The material used to form the braze layer  24  is one which wets both the interfaces with the stopping layer  22  and the cooling support  26 . It is contemplated that a wetability characteristic encourages adhesion between the interfacing materials. For example, where the stopping layer  22  is formed of Tantalum, Silver based braze alloy is preferably used to form the braze layer  24 . 
     The target  18  of the present invention may be exposed to operating temperatures of approximately 400° C., especially where a 1.75 MeV proton beam is operated at about 10 mA. It is contemplated that attachment via brazing provides an effective bond between the stopping layer  22  and the cooling support  26  within such operating temperatures. Such effective bonding is due to alloying effects at the interfaces between the stopping layer  22 , the braze layer  24  and the cooling support  26 . The melting point of the material used to form the braze layer  24  is considered with respect to the target operating temperatures. Importantly, as a result of the relatively high temperatures resulting from proton bombardment, thermal stresses may develop within the stopping layer  22  with respect to the cooling support  26 . This is due to differences of the coefficients of thermal expansion between the stopping layer  22  and the cooling support  26 . The braze layer  24 , however, provides a medium in which any built-up thermal stress contained within the stopping layer  26  may be gradually released across the braze layer  24 . For example, where Tantalum is used to form the stopping layer  22  and Cooper or Beryllium is used to form the cooling support  26 , Silver based alloy is preferably used to form the braze layer  24 . 
     Additional modifications and improvements of the present invention may also be apparent to those of ordinary skill in the art. Thus, the particular combination of parts described and illustrated herein is intended to represent only one embodiment of the present invention, and is not intended to serve as limitations of alternative devices within the spirit and scope of the invention.