The present invention relates to energy loss radiographic imaging of objects, particularly to energy loss radiographic imaging with charged particle beams, more particularly to energy loss radiographic imaging of the charged particle beams onto Cerenkov radiation media such that the energy loss of the beam may be spatially calculated for each xy pixel perpendicular to the beam axis, resulting in an XY radiograph related to the line integrated atomic density of the object being radiographed.
Cerenkov radiation (also referred to herein as Cerenkov light) is a coherent electromagnetic effect whereby a charged particle emits radiation while passing through a medium. The condition for Cerenkov radiation is that the velocity v of the particle must exceed the speed of light in the medium, or v greater than c/n , where c is the speed of light in a vacuum and n is the index of refraction within the medium. This defines the threshold velocity for Cerenkov radiation. The radiation is emitted in a cone with respect to the direction of flight of the particle, and the opening angle of the cone is given by the Cerenkov angle, cos(xcex8c) (nxcex2)xe2x88x921, where xcex2=v/c. The number of Cerenkov photons radiated per unit path length through the medium between wavelengths xcex1 and xcex2 is given by dN/dx=2xcfx80xcex1 sin2(xcex8c)(xcex1xe2x88x921-xcex2xe2x88x921), and may be approximated for the optical range (400-700 nm) as dN/dxxcx9c500 sin2(xcex8c) cmxe2x88x921.
Automobile engines are an example of difficult objects which are resistant to traditional X-ray imaging, generating no information after the source beam has passed through several extinction depths of the material being viewed, i.e. all of the beam is absorbed.
Radiography using charged-particle beams has been suggested as an alternative to x-ray radiography with such target materials. High mass charged particles such as protons with energies of 800 MeV to 50 GeV/c have high penetration powers through such traditionally difficult to image objects, and have been used to image these types of objects. Even when proton imaging is used, however, only the flux is typically measured, giving limited information about the composition and density of the object being examined.
In conventional x-ray or proton radiography the number of probe particles removed from the beam by absorption or scattering in the target is the fundamental measurement used to infer the local column density of the target under study. The beam of probe particles passes through the target where scattering or absorption occurs, and a two dimensional image of the number of surviving beam particles is then constructed utilizing a suitable sensor such as a film or a scintillator coupled to a CCD camera.
However, radiography using charge-particle beams provides information on target composition through three mechanisms: attenuation, multiple scattering and energy loss. Unlike x-ray probes which suffer only absorption or scattering in the target, beams of protons, or other massive charged particles also suffer energy loss as they traverse a target through electromagnetic interactions between the charge of the particles and the electrons in the target material. Thus simply counting the number of surviving charged beam particles does not provide all of the available information since the velocity of the surviving beam particles is not measured. Measurement of the energy loss of the surviving beam particles will provide another fundamental measurement related to the local column density of the target.
Because each mechanism has a different dependence on atomic weight and number, imaging techniques that use any combination of the three mechanisms can provide more information on target composition than techniques that use only one.
Because the particle velocity is directly related to the energy loss suffered by the particle in passing through the target, which in turn depends on the composition and density of the target, the column density can be determined from a measurement of the Cerenkov light intensity produced by the portion of the beam that passes through a particular column of the target.
It is another object of the present invention to provide radiographic images utilizing charged particles such as protons, electrons, muons, pions, kaons, or other charged particles for penetrating objects which are resistant to traditional X-ray radiography.
It is another object of the present invention to measure the energy loss of a charged particle beam passing through a target.
It is a further object of the present invention to provide an image whose intensity is proportional to the velocity of the charged particles that have passed through the target.
It is still another object of the present invention to provide a Cerenkov camera to measure the energy loss of a charged particle beam in passing through a target by imaging Cerenkov radiation produced by said beam.
It is still another object of the invention to use pulsed bursts of charged particles onto a gated or switched energy loss Cerenkov camera in such a way as to produce movies of the internal motions of objects to be examined.
Briefly, these and other objects are provided by the present invention in which the energy loss of a charged particle beam passing through an object is imaged in Cerenkov radiation. The energy loss of the beam is determined by using a correlation between Cerenkov radiation which is produced by the high-speed charged particles passing through a radiator medium and the beam energy (beam flux) passing through the object, Cerenkov radiation is emitted from the radiator medium when a particle traverses the radiator medium when the speed of light in the radiator material lower than the speed of the traversing particle. Since the threshold velocity for Cerenkov light emission depends on the index of refraction of the radiator, the radiator material is chosen to provide a threshold just below the minimum velocity of the unattenuated particle beam (i.e., the lowest anticipated energy of particles emerging form the object).
Both the flux and speed of the particle determine how much Cerenkov radiation is emitted in the medium. Since we are interested in the additional information of the energy lost in transmission through the object, a scintillator is used to determine the particle flux. By dividing the Cerenkov radiation imaged on a given pixel by the corresponding particle flux determined by a scintillator, the energy loss of a particular pixel may be determined. When this procedure is applied throughout an array of pixels (for example in a CCD or photodiode array), an image of the energy loss through an object being viewed may be obtained. In simple terms, an image may be formed which represents the line integrated density of the object the beam has traversed.
By rotating the object to be viewed, sequential displays of line integrated density may be observed. Such information may then be used as input data into a Computer Aided Tomography (CAT) scanning algorithm, whereby the densities of regions of the device or object being viewed may be produced. By applying this technique to medical applications, it has been calculated that CAT scans of human biological samples may be produced with approximately 14 times less radiation exposure. Additionally, sequential series of bursts of protons (easily produced on such equipment) can be used to produce movies of dynamic machinery or processes. Dynamic events may be recorded using images from several synchronized CCD or photodiode arrays.
Other objects, advantages and features of the present invention will become apparent from the following description when considered in conjunction with the accompanying drawings wherein like of similar reference characters illustrate similar elements in the several views.