X-ray or gamma ray systems or devices – Source
Reexamination Certificate
1998-07-22
2001-04-10
Arroyo, Teresa M. (Department: 2881)
X-ray or gamma ray systems or devices
Source
C378S120000, C378S143000, C376S157000
Reexamination Certificate
active
06215851
ABSTRACT:
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 wh
Meilunas Raymond John
Melnychuk Stephan Taras
Zimmerman, Jr. Frederick F.
Arroyo Teresa M.
Inzirillo Gioacchino
Northrop Grumman Corporation
Scully Scott Murphy & Presser
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