Patent Number: 054323495
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is directed to an apparatus and method for imaging a radiation intensity distribution of a source of x-ray and/or gamma-ray radiation. 2. Description of the Related Art Several conventional devices and techniques using reflective optics (e.g., mirrors) or Fresnel zone plates exist for x-ray microscopy at radiation energies below two kilo-electron volts (keV). At radiation energies above two keV, the performance of x-ray reflective optics devices and techniques becomes poor because mirror reflectivity is relatively small for photons at these energies, and hence the length of the optical surfaces required to achieve grazing incidence reflection increases prohibitively. As a result, most reflective optics systems are unable to obtain spatial resolution below several tens of micrometers at radiation energies above two keV. The performance of devices and techniques using Fresnel zone plates also degenerates at relatively high radiation energies (i.e., above two keV) because the required thickness of a radiation-opaque material forming such Fresnel zone plates, increases at higher radiation energies while the required spacing of the radiation-opaque material decreases. Therefore, the Fresnel zone plates are required to have relatively thick regions of radiation-opaque material which are spaced relatively close together, a structure which is difficult to manufacture. Also, Fresnel zone plates must be designed for a relatively narrow range of radiation energies, a requirement which can limit the use of devices and techniques which employ Fresnel zone plates. Other conceivable devices and techniques for imaging a source of x-ray or gamma-ray radiation might use coded aperture imaging in which a single mask has multiple apertures or pinholes placed between the source and a position-sensitive radiation detector. The detector records a transform of the source image which can be inverse-transformed to reconstruct an image of the radiation intensity distribution of the source. However, this method will not provide imaging at scales much finer than the finest scale measurable with the position-sensitive radiation detector, so that the position resolution of the source is typically limited to several tens of micrometers. Other conventional devices and techniques for imaging a source of x-ray or gamma-ray radiation have been applied to radio-astronomy applications and use interferometers which include pairs of antennae along various base lines which are used to extract Fourier components. Also, with respect to x-ray radiation, modulation collimators and other designs utilizing arrangements of grids have been employed. However, these telescope arrangements are only suitable for applications in which the source to be imaged is relatively distant from the telescope arrangement. Accordingly, these telescope arrangements are not suitable for an x-ray or gamma-ray radiation microscope. To summarize, the devices and techniques described above are not suitable for microscope applications producing relatively fine spatial resolution (e.g., as low as a few microns) for radiation energies above about one-tenth keV. Therefore, these conventional devices and techniques fail to meet the demands of applications such as inertial confinement fusion (ICF) experiments in which a target compressed to less than 100 microns in size radiates copious x-rays above two keV for a short time. Also, these conventional devices and techniques are inadequate for medical applications requiring relatively fine spatial resolution of sources which emit radiation at energies above two keV. SUMMARY OF THE INVENTION An object of the present invention is to provide an apparatus and/or method for imaging a source of relatively high energy radiation with a relatively fine spatial resolution. Another object of the present invention is to provide an apparatus and/or method for generating a magnified image of a source of relatively high energy radiation with a relatively fine spatial resolution. Another object of the present invention is to provide an apparatus and/or method for imaging a source of x-ray and/or gamma-ray radiation with a relatively fine spatial resolution as low as a few microns. Another object of the present invention is to provide an apparatus and/or method for generating a magnified image of a source radiating x-ray and/or gamma-ray radiation with a relatively fine spatial resolution using a position-sensitive detector with sensing elements which do not resolve more finely than a few hundred microns. Another object of the present invention is to provide an apparatus and/or method for imaging a source radiating x-ray and/or gamma-ray radiation in an ICF experiment with a relatively fine spatial resolution as low as a few microns. Another object of the present invention is to provide an apparatus and/or method for imaging a source radiating x-ray and/or gamma-ray radiation with a relatively fine spatial resolution as low as a few microns in medical applications. Another object of the present invention is to provide an apparatus and/or method for generating an image of a relatively small source of x-ray and/or gamma-ray with a relatively fine spatial resolution using a Fourier transform. The above objects are obtained by the apparatus and method herein disclosed. According to the present invention, there is provided an apparatus for imaging a source of radiation by deriving an image of the radiation intensity distribution of the source in spatial frequency domain, which can be converted using an inverse-Fourier transform into an image in spatial domain. To derive the image in spatial frequency domain, the apparatus uses a first grid arranged in proximity to the source, and a second grid arranged in proximity to the first grid. The first grid has an arrangement of first subgrid elements. Each first subgrid element has an arrangement of a first predetermined number n of approximately equally-spaced, parallel or slightly divergent linear first ribs which are opaque to the radiation of interest. In alternation with the first ribs, first radiation-transparent regions are provided which are transparent to the radiation of interest. The first ribs of each first subgrid element have a particular spacing and orientation relative to reference axes in the plane of the first grid. Approximately speaking, the second grid is an expanded version of the first grid. The second grid has second subgrid elements which have a common field of view with corresponding first subgrid elements. However, each second subgrid element has an arrangement of a second predetermined number n+m (rather than merely the first predetermined number n) of approximately equally-spaced, parallel or slightly divergent linear second ribs which are opaque to the radiation of interest. The second ribs of a given second subgrid element have the same orientation as the first ribs of the corresponding first subgrid element (a first subgrid element and its corresponding second subgrid element are termed a `subgrid system`). Photons of the radiation of interest generated by the source, pass through a given subgrid system and generate a radiation intensity distribution termed a `Moire` or `fringe pattern` for each subgrid system. The Moire or fringe pattern has m maxima (i.e., peak amplitudes) which occur at a particular phase relative to a reference system provided for each subgrid system. Because the spatial frequency of the Fourier component measured by a given subgrid system is predetermined by the spacing of the first ribs of the first subgrid element of that subgrid system, and because the orientation of each subgrid system is also predetermined relative to the reference axes, the Fourier component for each subgrid system is completely defined by measuring the amplitude and phase of the Moire or fringe pattern generated by each subgrid system. These measurements can be performed using a position-sensitive detector such as a photographic film or an array of photodiodes, for example. The array of amplitudes, phases, spatial frequencies and orientations of the Fourier components of all subgrid systems in the apparatus are collectively referred to as the image of the radiation intensity distribution of the source in spatial frequency domain. Using a Fourier transform, the image in spatial frequency domain can be converted to an image in spatial domain by employing a processor. If the position-sensitive detector is realized so that it has an analog output (e.g., as would be the case with a photographic film), the analog output is converted into a digital signal using a digitizer, and the digital signal is provided to the processor coupled to the digitizer. Otherwise, the position-sensitive detector can be coupled directly to the processor, so that the digital signal is provided directly thereto. In any case, using a control program stored in a memory coupled to the processor, the processor can perform a Fourier transform on the digital signal of the image in spatial frequency domain to generate a digital signal of the image in spatial domain. The processor can be coupled to a display unit which displays either or both of the image in spatial frequency domain or the image in spatial domain derived from the respective digital signals. These together with other objects and advantages, which will become subsequently apparent, reside in the details of the construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings, forming a part hereof, wherein like numerals refer to like parts throughout.