Abstract:
A γ-ray scanner includes a Solid-State Single-Photon Detector (“SSSPD”) and a γ-ray source, which may be a radioisotope such as Americium oxide (Am-241) that may not require certification since it has a low intensity that is safe even over extended periods of exposure to a human body. The γ-ray scanner may be used for monitoring a fixed object such as a pipe and includes an imaging assembly having a stationary annular gantry surrounding the pipe and an armature that fixedly supports the γ-source and the detector in mutual opposed alignment, so that they are constrained to move together. The armature rides around an inside periphery of the gantry, while the armature or the gantry moves laterally in a direction parallel to a rotation axis of the armature so as to move the γ-ray beam around and along the pipe.

Description:
FIELD OF THE INVENTION 
     This invention relates to low-rate digital γ-ray imaging and inspection. 
     BACKGROUND OF THE INVENTION 
     Most X-ray inspection/imaging systems sold on the market are complex, versatile and expensive systems (US$10,000, for instance, would not buy very much, if anything at all). Typical systems also come equipped with a high-intensity X-ray generator that is instantaneously hazardous if exposed to humans. Consequently, current X-ray systems are well beyond the purview of the ordinary consumer, both because of the high price and because of the danger. 
     Lower X-ray intensity compares with less light if seen in analogy with ordinary photography and a very low X-ray intensity could therefore be compared with taking a photo in near darkness. In both photography and X-ray imaging this leads to a longer exposure time. For the most traditional digital X-ray detectors, very long exposure has been a problem that is caused by the inherent properties of those detectors (i.e. integrating-mode type detectors with too high dark-currents for long integrations). A different technique exists however, using detectors formed of solid-state sensor material that are responsive to single photons (named here “Solid-State Single-Photon Detectors” (SSSPD)). Such technology is not new, but has still encountered stiff competition from the integrating-mode type detectors for mainstream usage despite having several advantageous properties. 
     It would desirable and would offer many commercial advantages to provide a portable, low-rate digital imaging/inspection system based on the use of SSSPD and employing a γ-ray source of such low rate that certification is unnecessary. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a portable, low-rate digital γ-ray imaging/inspection. In different implementations, such a system has the following features:
         Light weight   Battery operation possibility (i.e. low power)   (wireless) operation through portable computer (e.g. mobile phone)   Better accessibility for all users by virtue of:
           Low cost   Uses a very low rate γ-ray source that preferably requires no certification   Easy operation/training   
               

     While the system may be portable, the invention does not preclude also permanent installations. 
     One particularly winning property of the SSSPD is its ability to handle ultra-low γ-ray rates. This property of the SSSPD is a feature that is not yet commercially explored and is a key element of the present invention. 
     Another key element of the present invention is to combine the above mentioned SSSPD ultra-low rate quality with a γ-ray generating device that produces γ-rays at a rate that is very low in comparison to conventional X-ray generators. This may be achieved by the use of a radioisotope source rather than an X-ray generator. One particular radioisotope source that may be used is of the kind employed in conventional smoke alarms whose rate is so low that it is commercially available “off-the-shelf” as a part of the smoke alarm without the need for any certification, training or other security measures. 
     Such a smoke alarm radioisotope source consists of approximately 0.2 mg of the material Americium oxide (Am-241) with an α-activity of ˜35 kBq. It is primarily an α-emitting source but the α rays can easily be blocked and the (by-product) γ rays can be used as the “X-ray generator” equivalent. We refer here to such source as a “γ-source”, although for our purposes the term “γ-ray” or “γ-source” is interchangeable with “X-ray” as this is the common term used for imaging. It will be appreciated that γ-rays and X-rays are both photons and are indistinguishable if at same wavelength: their only difference being how they originate. 
     Naturally, the benefit of the concept described above is achieved at the expense of the speed of the imaging/inspection, which will be very slow compared with conventional systems. On the other hand, it will be:
         Possible to produce at very low cost   Possible to render suitable for use without X-ray certification, protection etc.       

     There are numerous applications where the speed of the imaging/inspection is of lesser importance than these benefits. 
     Using a radioisotope as the “X-ray machine” is known per se. The significance of the present invention is that provided its energy falls within a range that permits detection by the sensor, any γ-ray radioisotope can be used and that it is typically chosen based on its wavelength- (energy-) characteristics combined with the type of material to be inspected for achievement of optimal contrast. The higher the rate, or Becquerel- (Bq-) value of the source, the faster the imaging, and if speed is the top priority, a source with too high a Bq-value will be hard to find although it will of course require certification and will therefore not be available for common household use as an “off-the-shelf” item. 
     On the other hand, the above-mentioned smoke alarm radioisotope, Am-241, which has an α-activity of ˜35 kBq and usable γ energies of 13.9 keV and 59.5 keV (with activity similar to the α-activity), has the special and very useful property of being “certified” for common household use and thereby for everyone to handle. A particularly advantageous embodiment of the invention exploits this property and therefore the invention will be described with particular regard to this source. 
     Features and Applications 
     Typically, the features of the system according to the invention will address the following needs:
         where cost is a dominant factor i.e. in many cases since conventional systems are typically out of reach not only for most private household budgets but also for many small workshops and work-departments;   where ease-of-use is a dominant factor, since the invention avoids the usually cumbersome process of certification/regulatory-permission to use conventional high-radiation systems, and also avoids costs relating to training and possible customization;   where portability is a dominant factor, as the system according to the invention is very light compared with conventional systems and also can operate on batteries; and   where there is a need for a permanently installed system for inspection of for instance a stationary object over long time. One example might be looking for occurrence of corrosion or leaks e.g. in a water-pipe, and even more if there is a need for a multitude of such permanently installed systems.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a pictorial representation of a low-cost, low rate γ-ray scanner having a low-rate γ-source according to an embodiment of the invention; 
         FIG. 2  shows schematically a suitable γ-source as a point-source at the centre of a virtual sphere; 
         FIG. 3  is a block diagram showing the principal functionality of a detector for use with the system of  FIG. 1 ; 
         FIG. 4  shows schematically a detail of an Application Specific Integrated Circuit (ASIC) for use with the detector of  FIG. 3 ; 
         FIG. 5  is a timing diagram relating to operation of the ASIC shown in  FIG. 4 ; 
         FIG. 6  shows the active area coverage of the gamma beam as function of the longitudinal position between the source and the object; 
         FIG. 7  shows schematically how pixel binning may be used to vary the scan time of the system; 
         FIG. 8  shows schematically auxiliary detector electronics for read-out and control; and 
         FIGS. 9 to 12   d  show pictorially different applications of the system according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following description of some embodiments, identical components that appear in more than one figure or that share similar functionality will be referenced by identical reference symbols. 
       FIG. 1  is a pictorial representation of a low-cost, low rate γ-ray scanner  10  comprising a low-rate γ-source  11  and a Solid-State Single-Photon Detector (SSSPD)  12 . Arranging a γ-source  11  and an SSSPD  12  as depicted in  FIG. 1 , and scanning both of them together across (or around) an object  13 , permits use of a small area SSSPD. This also creates the potential for a very low cost γ-ray inspection system, since this kind of γ-source is very inexpensive (range ˜US$1). 
       FIG. 2  shows schematically the γ-source  11  as a tiny, i.e. point-source at the centre of a virtual sphere  15 . Unless screened in some directions, the source will radiate in all directions and at any given spherical distance the γ rate will be equal at every point on the spherical surface. 
     The surface (A) of the virtual sphere is:
 
 A= 4π r   2   (1)
 
where r=radius of sphere=distance from source
 
     The γ rate, N, arriving per given defined area of the surface, a, is: 
                   N   =         a   A     ·     N   0       =       a     4   ⁢   π   ⁢           ⁢     r   2         ·     N   0                 (   2   )               
where N 0 =γ emission rate of the source.
 
     For a smoke detector source, N 0  is the emission rate of the γ rays which is almost the same as the specified α-activity (35 kBq) meaning that it emits about 35,000 γ-photons per second. 
     Assuming further that we place a detector with one square-cm surface (a) 10 cm away from the source (r) (depicted also in  FIG. 2 ), and the open air γ rate on the surface of this detector will be: 
                           1   ⁢     (     cm   2     )         4   ⁢     π   ·   100     ⁢     (     cm   2     )         ·   35     ,   000     =   27.9           (   3   )               
i.e., approximately 28 γ-rays per second will hit the surface of the detector when there are no obstacles between the source and the detector.
 
     The Detector 
     A major aspect of the present invention is that such very low rate of rays (X-rays or γ-rays), actually can be utilised for real usage γ-ray imaging/inspection, albeit not with a conventional digital detector (i.e. the integrating-mode type detectors), but with an SSSPD. Obviously, it is a slow process but the image can be of high quality in cases where the object for the imaging/inspection is stationary for the entire (long) duration it will take. 
       FIG. 3  is a block diagram showing the principal functionality of the detector  12  according to an embodiment of the invention, comprising a sensor  20  formed of a single monolithic crystal of Cadmium-Zinc-Telluride (CZT) or Cadmium-Telluride (CdTe), and sub-divided (pixelated) in a 2D matrix of pixels  21 . The size of each pixel is about 1 mm by 1 mm, the number of pixels being typically 10×10, in which case the entire sensor (-crystal) surface is about 10 mm×10 mm. The thickness of the sensor (-crystal) is about 1 mm. Each pixel of the sensor is attached to a respective active readout circuit  22  (shown in  FIG. 4 ), which is separate to the crystal but is coupled thereto by a respective connecting signal line. 
     The individual readout circuits  22  thus total the same number as the total number of pixels and each circuit belongs to, i.e. is connected to, a corresponding unique pixel in the sensor  20 . In such an arrangement there is consequently a total of 100 readout circuits each constituting a discrete electronics channel. A selected number of channels or pixels are grouped into an Application Specific Integrated Circuit (ASIC)  25 , a working example of which is shown schematically in  FIG. 4 . 
     Here, assuming the sensor has 100 pixels, 25 channels are bundled into one (Front-End (FE-) ASIC), and four identical such ASICs are placed one on each side of the sensor and handle 25 pixels each. Typically the 25 pixels form a triangle whose base is an edge and whose apex is the centre of the crystal (these four triangles are illustrated in the drawing with thick lines). 
       FIGS. 5 and 6  depict an electronic implementation that is well-suited for the low γ- (X-ray) rates employed. 
     When a photon (γ-ray/X-ray) strikes a pixel in the sensor, an electronic charge is released that is proportional to the energy of the photon. Each pixel readout circuit  22  comprises a pre-amplifier  26  and a signal-shaper  27  ( FIG. 4 ) that respectively amplifies and shapes the signal into a semi-Gaussian looking pulse in  FIG. 5  (labelled “origin for sig. value”). Further electronics inside each readout circuit  22  include a peak-hold/stretcher device  28 , a discriminator  29  and a ROM cell  30  which operate, in a data-driven manner, to push out the peak-value of the signal, a digital trigger, and a unique in address for the actual pixel. This unique address can be in the form of an analog level, or as a conventional binary code. The readout circuit  22  may be constructed similar to that described in U.S. Pat. No. 6,153,882 in the name of the present inventor and whose contents are fully incorporated herein by reference. 
     These three signals are auto-multiplexed through a data-driven multiplexer  31  that serves all 25 channels in the FE-ASIC  25  so that they appear on three respective common outputs of the ASIC. This occurs with substantially no data-loss caused by coincident events, due to the low rate. In  FIG. 5  there is depicted another channel (#M) which also receives and processes an event, in parallel but at a later time. At the bottom of this figure is depicted the output of both events, pushed out from the ASIC on the same data-lines. 
     Resolution Versus Image Capture Speed 
     The sensor shown by way of example in  FIG. 3  has 10×10 pixels covering 1 mm square each, so that in total it covers 1 cm 2  and has 100 pixels in total (P no =100). Consider the case that an 8-bit image grey scale resolution will suffice for these low-end products and that the desired average number of counts per pixel (cpp) is 200 if performing an open air image (i.e. without any intervening object obstructing the view between the source object and the sensor). With the rate exemplified in expression 3 (i.e. N=28 γ photons per second), the total image capture time (T cap ) would then be: 
                     T   cap     =         cpp   N     ·     P   no       =         200   28     ·   100     =     714   ⁢     (   seconds   )                   (   4   )               
i.e. almost 12 minutes. This is just for generating an image in one detector position. In order to image a larger object as would usually be required, the value of T cap  would need to be multiplied by the number of steps needed to scan the larger area. Also, as seen by the sensor, the γ-photons are emitted within a conical “beam” having a square cross-section as shown in  FIG. 6  that originates from the source. Therefore, the scanned area of any part of the object whose distance from the source is less than that of the detector (i.e. any feature that is intermediate the source and the detector) will be less than the 1 cm 2  coverage of the sensor. As a result, any such features will be overlooked during a single step of the sensor and it is necessary to stepwise move the sensor by smaller steps than 1 cm at a time to fully cover the object.
 
     Thus, in order to step-scan e.g. 10 cm×10 cm, it might be necessary to make 400 steps or more, and for each step, the T cap  is required, and the full image would take 80 hours (˜3 days). There are three ways to reduce this imaging time (easily deduced from expression 4): 
     1. Increasing the activity of the source (i.e. increasing N) 
     2. Reducing the contrast (by reducing the requirement of cpp) 
     3. Reducing the number of pixels (P no ) 
     Option 1 would cause regulatory problems if only the smoke detector source requires no regulation and is therefore not always an option, although doable of course. Option 2 will just reduce the contrast quality and is therefore an option that depends on the user&#39;s requirement and is automatically achieved by increasing the stepping speed. Option 3 is achievable without replacing the sensor by combining (summing) the pixels to form larger ones using software/firmware. 
     Using Option 3 would obviously reduce the spatial resolution if exactly the same geometry relating to the relative positions between the source, the object and the sensor were preserved. However, in practice the imaging system may be designed to compensate for potential reduction in spatial resolution by locating the object closer to the source where the effective intensity is more concentrated owing to the conical shape of the γ-beam. This can actually increase the effective resolution of the detector owing to the reduction of the covered area of the object in proportion to the object&#39;s position between the source and the sensor. This makes reduction of the sensor resolution a sensible thing to do in many cases. In a 10×10 pixel configuration, the binning of pixels can be done logically (i.e. raw-data binning) as shown in  FIG. 7  whereas through software they might be binned at any resolution. As can be seen in this example, binning the sensor into just 4 (binned-) pixels, the capture speed increases by 25 times. If this is combined by a reduction of the required cpp to e.g. 100, the overall speed would increase by 50 times, and the 80 hour (˜3 day) total imaging duration in the example above would be reduced to just about 1.5 hours. 
     Thus, depending on the needs, this γ-ray imaging/inspection system can have imaging capture speeds ranging from extremely slow to tolerably slow. 
     Auxiliary Electronics (Read-Out and Control) 
     The above described detector architecture was chosen for providing high performance at low speed, but it also offers the potential for a low-power, low-cost system. These two qualities are important factors overall to this invention, and these also need to be reflected in the read-out and control of the system electronics, which also need to facilitate two other main features, i.e. lightweight and wireless operation. Thus, the read-out and control needs to ensure low-cost, low-power, lightweight and possible wireless functionality. 
       FIG. 8  shows a system  40  that meets these requirements and comprises a micro-controller unit (MCU)  41  constituted by a “System-on-Chip MCU” IC chip (“SoC IC”)  42 . Several suitable such ICs exist off-the-shelf (produced by e.g. Texas Instruments or Nordic Semiconductor). They are typically inexpensive (a few US$) and the customized FE-ASIC  25  of the detector as described with reference to  FIG. 4  can be developed to directly operate hand-in-hand with the SoC IC. Respective Analog-to-Digital Converter(s), ADC(s)  43  are needed to convert the pixel value and possibly the pixel address signals from the FE-ASIC(s). Several ADC-inputs are typically required, and this can be accomplished by using several ADCs, or a single ADC with sufficient parallel input channels. As depicted in  FIG. 8 , the ADC(s) could be realized by pre-existing components in the SoC IC or by external components shown as  44 . Timing/control of the FE-ASIC(s) and the ADCs is handled by the SoC IC. 
     The SoC IC  42  is also used to control the mechanical scanning movement of the γ-source  11  and the detector  12 , and it may interface to possible expansion RAM memory (not shown) if more is needed than available in the SoC IC itself. Likewise, the permanent storage capacity can be extended by auxiliary Flash memory  45 . 
     Optionally, the SoC IC  42  may also have integrated wireless communication capability  46 , which may be used to enhance the portability feature of the invention, in combination with e.g. Android/Iphone App software to operate the system. This does not preclude the possibility to use a wired communication e.g. to a desktop computer. 
       FIGS. 9 to 11  show pictorially different applications of the system according to the invention. 
       FIG. 9  is an example of a very simple gamma-imager  50  for general purpose 2D imaging use, which employs the same basic concept as depicted in  FIG. 1 . An object shown by way of example as a pipe  51  is adapted to be γ-ray imaged. The γ-source  11  and the detector  12  are fixed in mutual opposed alignment via an integrated imaging assembly  52  comprising a first arm  53  and a second arm  54 . The imaging assembly  52  is connected to the moving element of an x-y table  55 . The x-y table  55  moves the imaging assembly  52  in predetermined steps in the x and y directions that are appropriately sized to acquire for each step a γ-ray image fragment of a size (x-y) that is determined by the cross-section of the utilized γ-beam intersecting the object (as described related to  FIG. 6 ). The x-y steps are small enough to make the γ-beam (and the step image-fragments) cover the entire area of the x-y table  55  (typically minus some edge areas). The pipe  51  being imaged must be stationary during this step-scan. All of the step image-fragments are joined together into a full 2D gamma-ray image of approximately the same size as the x-y table  55 . 
       FIG. 10  shows a similar but slightly more advanced system  60  for imaging an object  61  disposed on a fixed platform  62 . The system comprises a stationary hollow cylindrical gantry  63  and an arcuate armature  64  that fixedly supports the γ-source  11  and the detector  12  in mutual opposed alignment, so that they are constrained to move together as a unitary imaging assembly  65 . The imaging assembly is fixed to a traveler  66  that is configured to ride around the gantry  63  so as to rotate in the φ-direction about a rotation axis x while the imaging assembly  65  is adapted to move linearly back and forth on the traveler  66  parallel to the rotation axis x. Consequently, the γ-ray beam will step circularly (approximately) around center of a circle, as defined by the gantry  63 , and back and forth in the x-direction. γ-ray image fragments are thereby acquired at each step, from which both 2D and 3D images of the volume that the beam covers can be reconstructed. The object  61  depicted to be γ-ray imaged is shown by way of example as a can of Coca Cola, which is a registered trademark of the Coca-Cola Company of Atlanta, Ga., USA. 
     The armature  64  is shown in the figure as arcuate, its radius being sufficiently less than that of the gantry  63  that it is able to rotate completely within the inside periphery of the gantry  63 , while being sufficiently large to sweep around the object  61  without obstruction. It will be understood that the shape and dimension of the armature may be varied so long as these requirements are met. 
     While the system  50  shown in  FIG. 9  is comparable to conventional 2D X-ray imaging, the system  60  in  FIG. 10  is comparable to CT scan imaging and can therefore also be used for 3D image reconstruction. It is in fact very similar to an ordinary CT, which performs stepped image fragments, the main difference being the physical size of the gantry, the γ-source replacing an X-ray generator, and therefore also the size of the γ-beam. The principle is otherwise the same as for a clinical CT machine. 
       FIG. 11  shows an example of a more specialized purpose oriented 2D γ-ray inspection system  70  for imaging a pipe  71  in a region of a bend or joint in the pipe, which may be particularly vulnerable to leaks. To this end, an arcuate wall-mounted rail  72  is fixed in close proximity to the pipe  71  so as to follow its shape around the bend. An imaging assembly  73  in the form of a C-shaped gantry  74  supports the γ-source  11  and the detector  12  in mutual opposed alignment, and is moveably supported on the arcuate rail  72  so as to ride along the rail whereby the pipe is stepwise imaged along its length, depicted by the x-axis. The γ-source  11  is attached to the gantry  74  via an extension tube  75  in order to extend the coverage of the conical beam so that γ-photons will image a larger area of the pipe during each imaging step. Movement of the gantry  74  along the rail  72  may be achieved by means of a motor (not shown) attached to the gantry  74  that rotates a wheel having a rubber rim or tire that is in frictional contact with the wall. Rotation of the wheel thus causes it to ride along the wall and induces counter-movement of the gantry along the rail. Alternatively, the rail can have a toothed track that engages a gear wheel that is rotated by a motor mounted on the gantry. In addition to movement of the gantry  74  along the rail  72 , the γ-source  11  and the detector  12  are both fixedly attached to a traveler  76  that allows them both to move in tandem laterally along the y-axis as shown in the figure. It will be appreciated that the figure is schematic and details of the attachment of the γ-source  11  to the traveler  76  are not shown, it being understood that the manner of attachment is not a feature of the invention, so long as the γ-source  11  and the detector  12  can move together laterally in mutual alignment. One way to do this is to use a split traveler having tied component travelers at the top and bottom of the gantry with the γ-source  11  and the detector  12  each fixed to a respective one of the component travelers. 
       FIGS. 12 a  to 12 d    show an example of a different type of γ-ray imaging system  80  for pipe-inspection. The system  80  is based on a γ-ray imaging assembly  81  depicted in  FIG. 12 a    in the form of a cylindrical hollow gantry  82  around whose inside periphery an arcuate armature  83  is configured to rotate in a manner that is identical in principle to the system  60  shown in  FIG. 10 . The gantry  82  is fixedly mounted around a section of a pipe  84  carrying liquid or gas as depicted in  FIG. 12 b    for constantly monitoring the integrity of the pipe in order to check for emerging cracks, corrosion etc. depending of the pipe&#39;s material and to provide early warning alerts for possible emerging leaks. In order to retain the gantry  82  in position around the pipe, end caps  85  may be provided on both sides of the gantry  82  as shown in  FIG. 12 c    so as to form a generally cylindrical enclosure around the pipe that accommodates the γ-ray imaging assembly  81 . The end caps  84  may also serve as a protective mechanical and environmental shielding of the γ-ray imaging assembly  81 . It is seen in  FIG. 12 d    that the system  80  covers only a small critical section of a long pipe, for example a joint area. However, the same principle may be used to image any length of coverage along a pipe regardless of diameter by increasing the length l of the cylindrical gantry  82 . 
     It should be noted that the systems shown and described in  FIGS. 11 and 12  are permanent fixtures that are constructed around the pipe, typically during installation. Since they employ a very low rate γ-source, can be battery powered and will operate for many years without the need to replace the battery and thus provide constant monitoring of the pipe or of a critical section thereof. This is distinct from known flow-meters that use γ-sources to monitor the density of liquid flowing inside of a pipe and which are fixed in space and do not move relative to the pipe. 
     It should also be noted that in a typical application employing a cylindrical gantry of the type shown in  FIGS. 10 and 12 , the object to be imaged is disposed inside a stationary gantry and the γ-source  11  and the detector  12  are adapted to move together around an inside periphery of the gantry. However, it is also possible to support the γ-source  11  and the detector  12  on a traveler that rides around the outside of the gantry so as to encircle an object to be imaged while still allowing lateral movement of the γ-source  11  and the detector  12 . In either case, the gantry serves as an anchor for supporting the γ-source  11  and the detector  12  and for facilitating 3-D motion thereof around the object.