Abstract:
A method of imaging an object by generating laser pulses with a short-pulse, high-power laser. When the laser pulse strikes a conductive target, bremsstrahlung radiation is generated such that hard ballistic high-energy electrons are formed to penetrate an object. A detector on the opposite side of the object detects these electrons. Since laser pulses are used to form the hard x-rays, multiple pulses can be used to image an object in motion, such as an exploding or compressing object, by using time gated detectors. Furthermore, the laser pulses can be directed down different tubes using mirrors and filters so that each laser pulse will image a different portion of the object.

Description:
This application claims priority to provisional patent application Ser. No. 60/133,053, filed May 6, 1999, titled “Laser Radiography”. 
    
    
     The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods for generating x-rays by using laser driven sources for high-energy radiography. 
     2. Description of the Related Art 
     Referring to FIG. 1, a conventional x-ray machine  10  is shown. The electron accelerator bremsstrahlung source  12 , for example, a flash x-ray (FXR) source, creates high velocity electrons  8  directed toward a target  14 . These electrons have an energy level of 15 MeV. The typical target  14  consists of several layers of materials sandwiched together with a thick radiator layer of, for example, tantalum  22  (1 mm thick). As the high velocity electrons pass through the target  14 , bremsstrahlung x-rays  16  are formed and pass through the object  18  to form an image on detection plate  20 . One disadvantage of the FXR source is that the source needs to be close to the target when generating the high-energy electrons, because these electrons quickly dissipate over short distances. Since these electron accelerator sources are extremely large, there is a limit to the number of multiple axis views that can be performed on an object  18  at one time. If the tests performed on the object are destructive, for example, an impact or explosive experiment, then exposure to only one or two detection plates  20  is possible. 
     In addition, the “burst” of high-energy electrons usually lasts a long period of time, such as tens of nanoseconds, causing a substantial amount of scattered x-rays that will affect the exposure of the detection plates. Also, it may take a long time for the energy fields created by the electron accelerator source to dissipate before another procedure can be performed. Therefore, there is usually inferior spatial and temporal resolution of the imaged object by using conventional electron accelerators. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a method and apparatus for imaging an object by generating laser pulses with a short-pulse, high-power laser. When the laser pulse strike a conductive target, Bremsstrahlung radiation is generated such that hard ballistic high-energy electrons are formed to penetrate an object. A detector located on the opposite side of the object detects these electrons. The detector could be time gated in order to detect specific ballistic high-energy electrons. 
     An object of the invention is to form hard x-rays from the bremsstrahlung radiation to image objects. 
     Another object of -the invention is use multiple laser pulses to image an object in motion, for example, an exploding or imploding object. 
     Another object of the invention is to generate multiple laser pulses that can be directed down different tubes using mirrors and beam splitters so that each laser pulse will image a different portion of the object. 
    
    
     Other objects and advantages of the present invention will become apparent when the apparatus of the present invention is considered in conjunction with the accompanying drawings, specification, and claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention and further features thereof, reference is made to the following detailed description of the invention to be read in connection with the accompanying drawings, wherein: 
     FIG. 1 depicts a prior art x-ray source; 
     FIG. 2 shows a radiography system of the present invention described as the first preferred embodiment; 
     FIG. 3 shows a detail drawing of the high-energy ultra-short pulse generator of the first preferred embodiment; 
     FIG. 4A-4D shows the pulse at several stages passing through the high-energy ultra-short pulse generator of the first preferred embodiment; 
     FIG. 5 depicts the target used in the first preferred embodiment of the present invention; 
     FIG. 6 shows a graph comparing the concentration of electrons at specific energies between the FXR source and the pulsed laser source of the present invention; 
     FIG. 7 shows a graph comparing the photon energy of the prior art with the first preferred embodiment of the present invention; and 
     FIG. 8 depicts a multi-axis x-ray system of the second preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While this invention is described in some detail herein, with specific reference to illustrated embodiments, it is to be understood that there is no intent to be limited to these embodiments. On the contrary, the aim is to cover all modifications, alternatives and equivalents falling within the spirit and scope of the invention as defined by the claims. 
     A radiographic laser  30  of the first preferred embodiment will be described with reference to FIGS. 2-5. In FIG. 2, a high power laser pulse generator  32  generates a high-energy ultra-short pulse  40 . The conduit  34  directs the pulse  40 , which can travel a long distance without attenuation, toward the target  36 . In the first preferred embodiment, the laser pulse has an energy level of 10 21  W/cm 2 . As shown in FIG. 5, the target  36  uses a material  22  (approximately 1 mm thick) with high electrical conductivity such as gold or copper. It is preferable to have target density of at least 5 g/cm 3 . As an example, the remaining layers could be: beryllium  24  (4.4 mm thick), copper  26  (0.7 mm thick), aluminum  28  (3 mm thick), aluminum oxide  30  (1 mm thick) and polyester  32  (3.7 mm thick). However, the one layer of gold without the filler layers would be adequate. The high conductivity in the material is necessary to provide a sufficient return current in the target in order to neutralize the strong space-charge potential created by the rapid depletion of electrons from the target region. 
     Referring to FIG. 3, the high power laser pulse (HPLP) generator  32  will be described in detail. A short-pulse oscillator  50  generates an initial short pulse  90  as shown in FIG.  4 A. For example, the initial short pulse  90  is 0.1 psec long with a power that is only a few milliwatts. If this short pulse  90  were simply amplified, the power amplifiers  70  would be destroyed. Therefore, using a grating system  54  spreads the short pulse  90  out. In this configuration, the beam splitter  52  directs the pulse  92  to mirrors  56 ,  58 , which direct the short pulse  90  to diffraction grating  60  that separates the different wavelengths. In this lo figure, only three colors (red, green, blue as noted by R, G, B respectively) are shown. However, the full spectrum would actually be spread out and directed through lenses  62 ,  64  to the second diffraction grating  66  and the mirror  68 . As the wavelengths are reflected back through the grating system  54 , the light that travels the shortest distance, for example B, would pass through beam splitter  52  first. As shown in FIG. 4B, the pulse  92  is a long low-power pulse with the blue light traveling as the front of the wave. In this example, the pulse would now be 2000 psec long still at the low power of several milliwatts. 
     The pulse  94  is now amplified by passing through power amplifiers  70 . The resulting high-energy pulse  94  is shown in FIG.  4 C. It still has the same color spectrum, but now at the much higher power level of several gigawatts. The high-energy pulse is passed through a reverse grating system  76  by being reflected by mirror  74  toward gratings  78 ,  80  and concave mirror  82 . The resulting high-energy, ultra-short pulse  40  is directed toward the target by a mirror  96 . FIG. 4D shows the high-energy ultra-short pulse is now 0.3 psec, but with a petawatt (10 21  W/cm 2 ) power level. 
     Referring to FIG. 2, the high-energy ultra-short laser pulse  40  is directed toward the target  36 , via the conduit  34 . The advantage of this system is that the source of the pulse can be at any distance from the target  36 . Therefore, the bulky laser pulse generator can be far away from the object to be imaged. As the laser pulse  40  penetrates the target  36 , hard x-rays  44  are produced as Bremsstrahlung radiation from the interaction of electrons with the nuclei of the dense target (Au) atoms. The smaller the area of the target that the laser pulse 40 hits, the more focused the x-ray beam  44  will be when exiting the target  36 . The conventional electron beam sources focus the electron beam to an area about 2 millimeters in diameter. In contrast, the laser pulse can be focused to an area of only 50 microns in diameter. Therefore, the smaller diameter x-ray spot produced by the laser improves imaging resolution. 
     Referring to FIG. 7, the graph shows an initial electron spectrum from FXR (curve  126 ) and petawatt pulse lasers (curve  128 ) interacting with a target of high conductivity. The petawatt electron spectrum is estimated by the following equation: 
     
       
           N ( E ) dE=C ( E   ½   /&lt;E   e &gt; {fraction (3/2)} )Exp[−E/&lt;E e   &gt;]dE   (1) 
       
     
     where C is the normalization constant. The distribution shows that a petawatt pulse laser as the source can produce hard x-rays in the 1-10 MeV range. In contrast, the FXR source produces a well-defined set of electrons around 16 MeV range. The electrons are the source for x-ray production via subsequent bremsstrahlung. Although the electron distributions are different, the x-ray distributions are similar as shown in FIG.  8 . 
     Referring to FIG. 7, the similarities between using an FXR source and a petawatt laser source for hard x-ray production are evident. The distribution of x-rays when calculated from distribution curve  122  for a computer model FXR source. The distribution curve  120  is for the experimental results using an FXR source and distribution curve  124  is for the calculated results using the petawatt laser source. It is clear that use of the petawatt class lasers of the present invention can produce hard x-rays with a spectral distribution similar to that achievable with high-currout induction accelerators despite the very different electron distributions. 
     The present invention can be focused on an extremely small source size such that more sophisticated bremsstrahlung target designs and higher spatial resolution can be performed. Referring to FIG. 8, a multi-axis configuration of the second preferred embodiment of the present invention  130  is shown. Although this shows only two dimensions, it is clear that this can be expanded to a three-dimensional configuration. Pulse lasers  132 ,  134 ,  136  generate laser pulses down conduits  138 ,  144 ,  146  respectively. Conduit  144  is a straight tube that allows the laser pulse to strike the target  142 . However, the conduit  138  has a mirror  140  to reflect the laser pulse to target  142 . The conduit  146  has both a mirror  140  to direct the laser pulse down conduit  152  and a beam splitter  148  to direct part of the laser pulse down conduit  150 . On the opposite side of object  156  are detection plates  154 . 
     The distance that the laser pulse travels determines when the x-rays will penetrate the object  156 . Therefore, either the lasers could be fired at different times in order to have all of the x-rays penetrate the object at the same time. However, the laser pulses could be timed such that x-rays pass through the object  156  at different time intervals. Since the laser pulse is extremely short and the production of x-rays is concentrated at the time the pulse hits the target and dissipate quickly, x-rays produced by one pulse would not interfere with the x-rays from the next pulse. This can be accomplished by using time-gated detection of the detection plates  154 . 
     As an alternative, one laser can be set up with a multi-pulse format. Instead of a beam splitter  148 , a moveable mirror could be used to direct the different pulses down different conduits, the object can be radiographed at several angles and the detection plates would only detect the x-rays for a specific timed pulse. Either of these methods is extremely useful if the object is going through a destructive test and one wants to observe different phases of the objects movement. 
     It is clear that any application requiring time resolved or high image contrast ballistic x-ray radiography is enabled by the present invention. For example, medical x-rays applications can be improved by the use of ballistic imaging enabled by the picosecond duration of the laser source. As another example, time resolved x-ray images of dynamic events such as ordnance interactions or blade failure in a gas turbine engine will be greatly enhanced. 
     Although the foregoing invention has been described in some detail by way of illustration for purposes of clarity of understanding, it will be readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims.