Patent Number: 062158511
Section: description

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 .sup.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 .sup.13 C(p, y).sup.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 .sup.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 .sup.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 .sup.13 C material onto the stopping layer 22 to enhance the bonding of the .sup.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 .sup.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 .sup.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.degree. 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.degree. 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.