Abstract:
An energy cell, employed as a passive energy source or a detector for sensing high energy photons, takes advantage of the differing electrical properties of metals to produce an induced electromagnetic force charge when exposed to dosages of high energy photons such as x-ray or gamma rays.

Description:
FIELD OF THE INVENTION 
   This invention is directed to devices operative in response to impingement by high energy photons, and, more particularly, to a device which functions as a photon detector or a passive energy source by exploiting the electrical characteristics of metals having different atomic numbers when exposed to a dosage of high energy photons such as x-rays or gamma rays. 
   BACKGROUND OF THE INVENTION 
   One of the challenges for many electronics programs is how to reduce the size and weight of electronics components. In larger systems, such as aircraft and spacecraft, every savings in size and weight increases payload and mission capabilities. It is equally important in miniature systems, such as certain types of medical devices, where each reduction can open a range of applications previously inaccessible at larger scales. 
   One of the key constraints in any electronics system is the demand for some form of local energy to provide power for the system components. Typical solutions include one or more forms of fuel-based power generation, such as fossil or nuclear fuels, or an energy storage device, e.g. a battery. Other applications employ “passive” energy sources such as photovoltaic panels which are commonly used in spacecraft and other equipment in which the energy source cannot be readily replaced. Solar panels, for example, tend to require a substantial amount of surface area to create useful amounts of electrical energy, adding unwanted size and weight. Further, solar panels must be directed toward the sun to operate efficiently and it can difficult to maintain the appropriate attitude of the panels to maximize exposure to the sun. 
   SUMMARY OF THE INVENTION 
   This invention is directed to an energy cell employed as a passive energy source, or a detector/instrument for sensing the dosage of high energy photons, which, when exposed to dosages of high energy photons such as x-rays or gamma rays, produces an induced electromagnetic force charge. 
   It has long been known that every metal ejects electrons from its surface in response to the impingement by photons of a sufficient energy level. The linear absorption coefficient of a particular metal is the sum of different phenomenon, including Thomson scattering, photoelectric absorption, Compton scattering, pair production and photodisintegration. Thomson scattering occurs when high energy photons, such as x-ray photons, scatter after impingement with the metal and there is no change in energy to either the atom of the metal or the x-ray photon. Photoelectric absorption occurs when the atom of a metal absorbs the x-ray photon, resulting in electrons being ejected from the outer shell of the atom and the ionization of the atom. Compton scattering occurs when an x-ray photon ejects an electron from the metal atom, and an x-ray of lower energy is scattered from the atom. At the energy levels of x-ray photons, pair production and photodisintegration have little or no effect on the linear absorption coefficient. 
   In the past, the ejection of electrons from the surface of metals as a result of impingement by x-ray photons or other high energy photons such as gamma rays, had adverse effects on electronic systems of all types. The ejected electrons can damage certain electrical components, interfere with the transmission and receipt of data and cause other problems. As a result, efforts were undertaken to shroud such metal surfaces from impingement by photons to protect electrical components, circuits, instrumentation and the like from damage. 
   This invention is predicated on the concept of using the phenomena described above to create a detector or instrument for sensing the dosage of high energy photons, or to create a passive source of electrical energy, which is inexpensive, reliable and sensitive, exhibits long life, can operate in extreme environmental conditions, requires little or no maintenance and can be integrated in a wide variety of applications and structures. In the presently preferred embodiment, a structure in the form of an energy cell is provided comprising one metal element having a high atomic number, a second metal element with a comparatively low atomic number and a section of dielectric material located between the first and second metal elements. Such “metal elements” may be plates, a wire and sheath or essentially any other configuration in which metal layers are separated by dielectric material. 
   In one example, the energy cell may include a plate formed of gold and another plate formed of aluminum separated by a composite layer. In response to dosage of the plates with x-rays, both the gold plate and aluminum plate eject electrons. But because the gold plate has a comparatively higher atomic number, more electrons are ejected from it than the aluminum plate. This creates an electrical potential across the plates such that when a load is connected to them electrical energy, e.g. an induced electromagnetic force (“IEMF”) charge, flows from the plates to the load. The energy cell of this invention can be scaled in the sense that the physical size of the metal elements can be altered, as desired, and more than one energy cell may be connected together in series or parallel to increase the overall amount of electrical energy produced depending upon the requirement of a particular application. 
   In one embodiment, the energy cell of this invention can be utilized as a source of electrical energy to drive electrical circuits or individual electronic components. Alternatively, the energy cell is employed with a sensing device to detect and provide an indication of the dosage of high energy photons to which it has been exposed. One preferred form of sensing device is a micro-electro-mechanical system (“MEMS”) switch which is coupled to the energy cell and receives the IEMF charge. The switch may be pre-charged using a digital to analog converter to change sensitivity range. At a selected charge level, the switch discharges. The rate which the switch cycles is proportional to the high energy photon dosage. 
   There are a myriad of applications within which the energy cell or capacitor of this invention may be utilized, at both the macro and micro level. It may be applied at a macro level to the housing or chassis of an electronic device, or to the cables, connectors, cable harnesses etc. of same. At a micro level, the energy cell or capacitor herein may be embedded in a printed wiring board, affixed as a device on a circuit board, laminated on the surface of chips, embedded within the chip circuitry as well as other options. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The structure, operation and advantages of the presently preferred embodiment of this invention will become further apparent upon consideration of the following description, taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is schematic cross sectional view of one embodiment of the energy cell of this invention, shown connected to a switch and load; 
       FIG. 2  is an elevational view of the switch shown in  FIG. 1   
       FIG. 3  is a view of an alternative embodiment of the energy cell herein; 
       FIG. 4  is a schematic, plan view of a circuit board employing multiple energy cells; 
       FIG. 5  is a cross sectional view of a stack of printed circuit boards in which energy cells of this invention are embedded at different layers. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIG. 1 , a schematic view of one embodiment of an energy cell  10  according to this invention is depicted. In this embodiment, the energy cell  10  comprises a first plate  12  and a second plate  14  separated by a layer  16  of dielectric material such as a composite material. The plate  12  is formed of a material having a relatively high atomic number, such as gold, whereas the plate  14  is formed of a material having a comparatively low atomic number such as aluminum. The energy cell  10  is subjected to a dose of high energy photons, such as x-rays or gamma rays, as schematically shown by the brackets  17  in  FIG. 1 . 
   As noted above, all metals eject electrons when impinged by photons of sufficient energy. Materials with higher atomic numbers eject a larger quantity of electrons than those with lower atomic numbers, assuming they are exposed to the same dosage of high energy photons, and therefore a potential difference is produced across the plates  12 ,  14  which is represented by a “−” sign associated with the gold plate  12  and a “+” sign associated with the aluminum plate  14 . While both plates  12 ,  14  actually exhibit a negative charge, the charge on the gold plate  12  is more negative than that of the aluminum plate  14 . 
   The gold plate  12  is shown connected by a lead  18  to a switch  20 , which in turn, is connected to a load  22 . The aluminum plate  14  is connected by lead  24  to the load  22 . The term “load” as used herein is intended to broadly encompass a variety of circuits or devices which may be connected to the energy cell  10 . In one aspect of this invention, the energy cell  10  is used as a passive energy source which provides electrical energy to essentially any number of different types of electrical circuits or devices which can be operated at voltage and current levels produced by the energy cell  10 , as discussed below. Further, a suitable threshold circuit and driver circuit (not shown) may be interposed between the energy cell  10  and load  22  which collectively function to store electrical energy produced by the energy cell  10  and then discharge it to a circuit or device when it reaches a predetermined level. It should be noted that electrons are ejected by the plates  12 ,  14  only when they are impinged by the high energy photons, and in the absence of such photons the ejected electrons dissipate. As such, a storage device such as a conventional capacitor or threshold circuit may be employed to capture the electrical energy produced when the energy cell  10  is dosed with X-rays or other high energy photons. 
   Referring now to  FIG. 2 , the switch  20  is preferable an MEMS switch which operates on the same principal as the energy cell  10 . In the presently preferred embodiment, the MEMS switch  20  comprises a lower plate  26  mounted in a fixed position at opposite ends to pedestals  28  and  30 , respectively. The lower plate  26  is preferably formed of gold or other material having a relatively high atomic number. An upper plate  32 , made of aluminum or other material of comparatively low atomic number, is mounted to a section  34  of dielectric material. The section  34  and upper plate  32  are suspended above the lower plate  34  by a pair of springs  36  and  38 , or other suspension devices, each of which is connected between one end of the section  34  and the top of a pedestal  40  and  42 , respectively. These springs  36 ,  38  allow the upper plate  32  to move toward and away from the fixed, lower plate  26 . A contact  44 , schematically depicted as located within the lead  18 , is located immediately above the upper plate  32 . For purposes of this discussion, the terms “upper,” “lower” and “top” refer to the orientation of the plates  26  and  32  as they are depicted in  FIG. 2 . 
   The plates  26  and  32  behave in the same fashion as plates  12  and  14  discussed above when exposed to a dosage of high energy photons. Because the lower plate  26  is formed of gold or similar relatively high atomic number material, a greater number of electrons are produced on its surface than on the surface of the upper plate  32  formed of aluminum or the like. As such, the fixed lower plate  26  has a relatively higher negative charge than the movable upper plate  32 . The electrons resident on the plates  26  and  32  repel one another, and, since the upper plate  32  is movable with respect to the lower plate  26  due to its connection to the suspension devices or springs  36 ,  38 , the upper plate  32  moves away from the lower plate  26  and into engagement with the contact  44  when a sufficient potential difference is created between the plates  26 ,  32 . Upon engagement with the contact  44 , the switch  20  “closes” and thus completes the circuit from the energy cell  10  to the load  22 . 
   Various techniques may be employed to control the sensitivity range of the switch  20 , and different devices may be used to record the discharge cycles of the switch  22 . For example, the switch  20  may be pre-charged using a digital to analog converter (not shown) to alter the sensitivity range, as desired. Further, the rate at which the switch  20  discharges is proportional to the photon dosage, and this can be recorded with any suitable sensing device schematically represented as the “load”  22  in  FIG. 1 . 
   While the MEMS switch  20  is particularly useful with the energy cell  10  to provide a sensing device for detecting the presence and dosage of high energy photons, it should be understood that the switch  20  may be utilized as a form of a threshold circuit as noted above. In this mode of operation, the switch  20  effectively stores the IEMF charge received from the energy cell  10  until such time as it reaches a sufficient level to cause the upper plate  32  to move to the discharge position in engagement with the contact  44 . The switch  20  could be configured, in this application, to discharge only when a predetermined level of IEMF charge is present as required, for example, to operate a particular electrical component or circuit. 
     FIG. 1  depicts one example of an energy cell according to this invention. It should be understood that other configurations of metal structures having different atomic numbers, separated by a dielectric material, can form an energy cell which is considered within the scope of this invention. For example, in  FIG. 3  an energy cell  46  is shown which consists of an insulated wire  48  surrounded by a sheath  50 . The insulated wire  48  has a core  52  of aluminum or a similar material with a relatively low atomic number surrounded by a rubber insulator  54 , and the sheath  50  is preferably formed of gold or other material with a comparatively high atomic number. The energy cell  46  of this embodiment functions in the same manner as energy cell  10 , and may be used in the same types of applications, as desired. 
   Referring now to  FIGS. 4 and 5 , the energy cell  10  is shown in two specific applications for purposes of illustration. In  FIG. 4 , two energy cells  10 A and  10 B are mounted to the surface of a printed circuit board  56  having a variety of electrical components contained in discrete circuits  38  and  60 . The circuit  38  is schematically shown as being connected to and powered by the energy cell  10 A, whereas circuit  60  is powered by energy cell  10 B. 
   In  FIG. 5 , a printed wiring board  62  is shown having a number of layers  64  stacked one on top of the other and multiple ground vias  66 . A number of discrete energy cells  10  are embedded at selected locations throughout the thickness of the board  62  to provide power for various electrical components carried by the board  62 . Lower energy x-ray bands charge the upper layers  64  of the stack, and higher energy x-ray bands penetrate to charge the lower layers  64 . It is contemplated that the higher energy x-ray bands will be partially absorbed by the upper layers  64 , which reduces their band energy and therefore increases the IEMF charge on the lower layers  64  of the stack. One circuit  68  is shown at the top layer  70  of the board  62  connected by a lead  72  to one or more energy cells  10 . A number of independent circuits or individual electrical components (not shown) may be located within a housing  74  which is schematically depicted at the base of the board  62 . A separate lead  76  may be extended between each of such components or circuits and discrete energy cells  10  embedded in the board  62 . 
   As noted above, factors such as the physical size of the plates  12 ,  14  (or wire  26  and sheath  28 ), the duration of their exposure to high energy photons and whether more than one energy cell  10  or  24  are connected together can affect the total electrical energy produced. In one example, a 1 mil thick gold plate having length and width dimensions of 1 inch by 1 inch, and a 1 mil thick aluminum plate with the same dimensions were placed on either side of a 1.5 mil thick section of fiberglass dielectric material and irradiated with X-rays. A 42 Rad (Si) dose of X-rays applied to this test sample for a period of 0.5 minutes resulted in a voltage of about 0.57 volts, a 168 Rad (Si) dose applied to such sample for a period of 2 minutes produced a voltage of about 0.8 volts, and, a 294 Rad (Si) dose applied to the sample for a period of about 3.5 minutes produced a voltage of about 1.18 volts. Testing and software simulations indicate that about 39% of the x-ray energy applied to the energy cell example noted above was “harvested” in the form of an IEMF charge. It is contemplated that levels of electrical energy suitable for a wide variety of applications can be produced by the energy cells  10  or  24  of this invention, when used either as a source of energy or a detector of the presence of high energy photon irradiation. 
   While the invention has been described with reference to a preferred embodiment, it should be understood by those skilled in the art that various changes may be made and equivalents substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.