Abstract:
A means and method for distinguishing full energy absorption peaks from background energy in a gamma ray prospecting system. Photopeaks in the received energy spectrum are determined, and boundaries to the peaks are located. The energy within the boundaries assuming a predetermined curve between the boundaries is integrated. The filtered energy within the actual photopeak curve is also integrated, and the two integrated signals are subtracted to provide a signal representative of the net photopeak area or amplitude. This peak area (or amplitude) is directly proportional to intensity of gamma radiation due to uranium, thorium and potassium in the ground. This signal is displayed and continuously updated on a cathode ray tube as a function of time.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an aerial prospecting system, and particularly to a radioactive material prospecting system. 
     Prospecting for radioactive materials usually involves scanning a particular land area from the air with a gamma ray spectrometer for the detection of gamma rays from radioactive material. Systems in common use utilize NaI crystals which release photons of energy upon gamma rays colliding with their atoms. These photons are detected in photomultiplier tubes, and the resulting output signals can be used to indicate the intensity of the gamma ray emissions. 
     Such systems utilize altimeters to record the height of the airplane from the ground, real time clocks which, used in conjunction with the known velocity of the airplane, can provide an indication of the location of any anomaly, etc. 
     Such systems typically group the energy spectrum into energy windows (which, in gamma spectrometry is a group of energy channels, typically over 127) and record on paper by pen chart and/or on magnetic tape the intensity of the gamma rays within each of the energy spectrum windows. 
     Such systems have encountered particular problems. For example gamma rays have been found to be emitted from the earth itself, which are not particularly related to a concentrated source of radioactive material such as uranium. This background radiation which is caused by gamma rays scattered in the ground, air or within the NaI crystal itself is called &#34;Compton background&#34; and is generally taken into account as being a constant proportion of counts in higher energy windows. It is known, however, that the Compton background radiation varies with variations in the amount of matter between the source of gamma rays and the detector. 
     As the spectrometer scans a given area, the geophysicist looks for anomalies in the energy windows of the received spectrum. These anomalies are generally manifested by an increase in amplitude above the Compton background at particular energy windows within the spectrum. The time of persistence, and the peak amplitude or area which is detected above the Compton background allows the geophysicist to estimate the size and concentration of the radioactive anomaly. 
     Particular spectrogram windows have been identified in the past as containing evidence of radioactive material. For instance, uranium, thorium, potassium &#39; , are all natural radioactive elements which emit gamma rays within different window areas of a spectrogram. 
     As the gamma rays collide with the NaI crystal, a certain proportion may be scattered and do not spend all their energy within the NaI crystal. Also some gamma rays have previously interacted with atoms in the rock or soil, thus losing part of their energy and do not enter the crystal with their original energy. The gamma rays which did not lose all their energy within the crystal create Compton background and the number of counts of Compton background increases with decreasing energy level and with the amount of mass between the radioactive source and the detector. 
     Gamma rays which lose all their energy within the crystal manifest themselves in the spectrum as &#34;photopeaks&#34; with the photopeak amplitude (or areas below a graphical plot of the photopeak) being proportional to the intensity of the particular gamma rays at their location in the spectrum corresponding to their energy. 
     Compton background can therefore be considered as &#34;noise&#34;, while photopeaks represent &#34;signal&#34;. One of the most important tasks of airborne gamma spectrometry is to separate photopeak signals from Compton noise. 
     The various channels of energy level which are recorded must therefore be corrected for the everpresent Compton background. This process normally utilizes a correction technique called Compton stripping. 
     It has been found that while previous correction techniques takes Compton background as a constant, that Compton background is actually variable, and, according to prior art techniques, may mask photopeaks of main interest. Accordingly prior art Compton background &#34;stripping &#34; techniques have caused the geophysicist to make, in many instances, erroneous conclusions as to the presence of radioactive anomalies. It has been found that if Compton background correction is treated as a constant as in the prior art, it can create false anomalies or obliterate real ones. 
     The present invention provides a new apparatus and an improved process for obtaining indications of photopeaks which were previously removed during above mentioned Compton stripping technique. 
     BRIEF DESCRIPTION OF THE PRIOR ART 
     Prior art systems attempt to alleviate problems in detection of photopeaks using the following techniques, for example. For low count rate but with high statistical fluctuations, a brute force method of using larger crystal detector volumes is used. From a standard 200 cubic inch crystal mass used during the 1960&#39;s, presently crystal masses of 2000-6000 cubic inches are used, weighing 450-1200 pounds, and costing up to 1/4 million dollars. Of course larger aircraft or helicopters must be used for the survey, increasing the survey cost substantially. However, this approach still does not solve the basic problem of the variability of Compton corrections. 
     A prior art approach for detecting energy anomalies has consisted of the following three steps: 
     (a) all of the photon counts in the energy band (window) which covers the Bi 214  photopeak at 1.76 MeV is summed; 
     (b) some constant portion of counts occurring in the Tl 206  window at 2.615 MeV (which is assumed to be the value of Compton background in the Bi 214  window, and therefore the general Compton correction,) is subtracted from the aforenoted sum. The net Bi 214  window count, i.e., the area of the Bi 214  photopeak is thus assumed to be the result. 
     This operation involves subtraction of two widely fluctuating numbers of similar magnitude, varying in a largely unpredictable manner. It has been found that the signal to be detected is almost always smaller than the corrections which must be applied. A slight imbalance in the detection or signal processing instruments may seriously damage or destroy the data. 
     The problem of variable Compton background appears to be solved for the first time utilizing the present invention. 
     SUMMARY OF THE INVENTION 
     According to the present invention an energy window is defined in a high resolution spectrum, and the values of Compton background at the edges of the window are determined. An integration of the energy defined by the energy levels at the edges of the window is thus made. 
     The entire energy below the photopeak within this same window is integrated, and the results of the two integrated energy values are subtracted. This difference provides an indication of the integrated energy which is higher than the Compton background radiation and below the photopeak window the window. 
     This system utilizes the full energy spectrum, (typically over 127 channels) rather than as in prior art systems, breaking the spectrum down into three or four large windows. With use of the full spectrum, the desired window can easily be defined. 
     To make the spectrum suitable for direct viewing, and to obtain the integrated value of Compton energy, the received signal is passed through a filter having for example the transfer characteristic ##EQU1## where A N  is the received energy level in a particular window, 
     N is the window number and 
     K 0 , K 1 , K 2 , and K 3  are constants. 
     With continuous processing of the received gamma ray signals, the resulting photopeak energy then can be constantly displayed as a bar chart or graph on an energy spectrum forming the time base on a cathode ray tube, and can further recorded on a printer. The entire operating system can be carried by the survey aircraft, and the resulting photopeaks in the filtered spectrum watched on the cathode ray tube by a geophysicist as the survey is made. Since some regions of the country to be scanned are only accessible during a very few months of the year, clearly the ability to separate real anomalies from a large number of statistical and other spurious anomalies, and to return immediately to an intersecting region can save substantial amounts of time and money. 
     In general, the invention is means for distinguishing energy absorption peaks received across an energy band from background energy at a particular physical location comprising: means for receiving a gamma ray energy spectrum of radiation, means for distinguishing the peak energy level with an energy band of said spectrum, and for integrating said energy within said band, means for distinguishing a statistical value of the background gamma radiation within said band, and for integrating the background radiation within said band, means for subtracting the integrated statistical value of background energy from the integrated peak energy, and means for providing an output signal representative of the difference of the integrated peak energy level and said integrated statistical value within said band, to provide an indication of the presence of radiation emissive mineral at said particular location. 
    
    
     INTRODUCTION OF THE DRAWINGS 
     A better understanding of the invention will be obtained by reference to the detailed description below, and to the following drawings, in which: 
     FIG. 1 is a block schematic of the invention; 
     FIG. 2 is a flow chart of operation of the special purpose computer used in this invention; 
     FIG. 3 is a graph of a typical gamma ray energy spectrum; 
     FIG. 4 is a pair of graphs of an unfiltered, and of a corresponding filtered gamma ray energy spectrum; and 
     FIG. 5 is a graph of the photopeaks in a particular window of the gamma ray energy spectrum according to the prior art, and according to the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     A block schematic of a preferred embodiment of the invention is shown in FIG. 1. The apparatus is comprised of a spectrometer which includes a plurality of commercial NaI crystals 1 adapted to receive gamma rays from a radioactive source. Each is coupled to a photomultiplier tube 2. The output of each photomultiplier tube is connected to an input of a summing amplifier 3. The output of the summing amplifier is connected to the input of an analog-to-digital (A/D) converter 4, which has its own output connected to a microcomputer, particularly to the data input of a central processing unit (CPU)5. 
     It is preferred that as high a resolution of the received spectrum should be obtained as possible, and a system utilizing 256 spectra channels has been found to be adequate. 
     The central processing unit 5 is connected to a memory 6, and to a clock 7 in a well known manner. The A/D converter which was used in one successful prototype was Nuclear Data type MD571ADC. A central processing unit which was used in Technico International type TMS9900-SS, and memory which was used is Technico International type 9900-MA. 
     An oscilloscope, such as Hewlett Packard type 1332A should be connected to the output of the central processing unit. It is also desirable to have a printer such as Axiom type EX-800 connected to the output of the central processing unit, and also a magnetic tape recorder. 
     The structure, operation and physical interconnection of the analog-to-digital converter, the central processing unit, the memory, the clock, the oscilloscope, the printer, and the magnetic tape recorder are specified by the equipment manufacturers, and since a repetion of that material would be redundant, and/or are well known to persons skilled in the art, they will not be described in detail. 
     The memory which is preferred to be used with the apparatus is a programmable read-only memory (PROM) which is a permanent memory utilizing a plurality of storage elements which are interconnected using a permanent scheme of metallic conductors or semiconductors which are analogous to a permanent plug board which interconnects the the functional blocks of the CPU. Accordingly the memory forms a physical integral portion of the central processing unit to form a special purpose computer which is unalterable by the particular signals being applied thereto. 
     Since the special purpose nature of the computer is dependent on the structure of the memory, which causes the central processing unit, besides controlling its ancillary apparatus, to operate upon the arrival of electronic signals as particular kinds of functional electronic apparatus, an understanding of the operation of the memory is important to an understanding of the invention and it will be described in more detail. 
     FIG. 2 depicts a flow chart of how the arriving signals from the A/D converter are treated by the CPU-PROM combination, in one embodiment of the invention. The signals are applied to a filter 11. After processing in the filter, the photopeak is detected in photopeak detector 12, and then applied to a lower spectrum window edge detector 13 and an upper spectrum window edge detector 14 (the window edges being CA and DB respectively in FIG. 3). The signal amplitudes at the edges of the window (at C and D in FIG. 3), that is, as defined by the lower and upper window edge detectors are used as area boundaries and after processing in an integrator 15 the Compton background area ACDB of the energy within the window is obtained. 
     With the definition of the window edges the signal 20 including the entire photopeak 21 signal area between the window boundaries CA and DB is integrated in an integrator 16. 
     The signal outputs of integrators 15 and 16 are subtracted in a subtracting means 17 to obtain the net photopeak area, which is the desired signal representative of the photopeak energy above the Compton background. This signal is applied to a displaying means such as a cathode ray tube which preferably has a time base calibrated according to the spectrum scan, and an ordinate which represents the net photopeak energy. In this manner a bar chart of gamma ray energy across the spectrum can be dynamically displayed on the oscilloscope on an on-line basis. 
     Preferably the scan of the spectrometer and output display on the oscilloscope is about once per second (with appropriate refresh cycles to eliminate flicker on the oscilloscope). The geophysicist can thus watch the display of the filtered spectrum as the airplane scans, and short-term anomalies caused by e.g. a small uranium source, can be distinguished from a large number of Compton stripping related anomalies as well as anomalies of statistical and spurious origin, which previously caused serious interference and increased the cost of the exploration program. A closer scan can thus be ordered to examine the anomaly of interest. 
     It is also preferred that at the same time the printer should permanently record the signals applied to the oscilloscope, and the signals themselves should be recorded on the magnetic tape recorder 10. 
     Considering the apparatus described above in more detail, the filter should be of the low-pass type, although bandpass filtering could be used as the nature of the signals dictate. It is preferred that the central processing unit should perform the filtering, utilizing a symmetrical convolution filter, which, in simple form, can be a moving average filter utilizing 5 or 7 points, for example. A recursive filter could also be used, and appears to have particular signal processing advantages. 
     A filter which was found to operate satisfactorily has the transfer characteristic ##EQU2## where A N  is the received energy level in a particular window, 
     N is the window number and 
     K 0  -1.0 
     K 1  =0.9 
     K 2  =0.75 and 
     K 3  =0.5. 
     However, the particular filter which is used has been found to be not very critical. 
     After filtering, the central processing unit should locate the signal photopeaks. The peak maxima are located in the preferred embodiment by finding those input channels where the counts fulfil the conditions ##EQU3## The border channels of the peak, P-l and P+l are determined: ##EQU4## The peaks are located by scanning for groups of channels such that the first derivitive, E i  fulfills the following criteria 
     
         E.sub.i &lt;0; E.sub.i-1 &gt;0; E.sub.i =2&gt;0 
    
     The restriction on channel i-2 assures that a peak will have at least two points on the lower energy side of the spectrum. 
     The CPU then checks whether the number of consecutive negative E values on the higher energy side of the photopeak is less than 4. If so, the peak is discarded. 
     The lower energy boundary is fixed where the first derivative changes sign, and the higher energy boundary is fixed where the first derivative changes sign on that side of the photopeak. 
     Once the boundaries of the spectrum window have been determined, the Compton background energy within the window can be determined in a number of ways. If the change in energy between the two boundary points is considered as being linear, the energy area forms a trapezoid. This is shown as area ACDB in FIG. 3. The energy within the area ACDB is determined by integration. 
     The line CD, however is more accurately a cubic, or even more accurately a smoothed polynomial curve. 
     It should be noted that the entire signal spectrum curve varies in amplitude with time, e.g., sequentially at times T 1 , T 2 , and T 3 , the curves are at varying amplitudes. However it should be noted that the relative anomaly photopeak signal amplitude remains about the same with respect to the Compton background. 
     The entire area under the energy spectrum 20 which has been received within the window AB is also integrated. Following this the integrated signals determinative of the Compton background, and the entire area under the photopeak 21 within the window area are subtracted. The resulting signal is the net photopeak area which is the signal amplitude to be displayed, and is representative of the gamma ray emissions received by the spectrometer. 
     It should be noted that the assumption of a linear decrease in Compton background (line CD in FIG. 3), as noted above, is not strictly correct, since the Compton background curve is nonlinear. The line CD can also be obtained closer to the actual curve by fitting a spectrum amplitude cubic equation to the window points CD. The resulting equation for the net peak area differs from that of a linear base line method by the addition of a nonlinear correction term. The correction is about 1% when the base area is 1400 counts, and 1.5% for 30,000 counts. 
     If the base area (Compton background within the window) is small compared to the total peak area there are no significant differences between the different methods. For a peak of small intensity, it is preferred that a least-square curve be used preferably to a nonlinear base line, which in turn is preferable to a linear base line. 
     The structure of the PROM is best given in the shorthand of the PROM designer, which is in microcode and mnemonic. A listing of the code to fulfill the operation of the invention is attached as Appendix 1, and is specific for the central processing unit and memory noted earlier in this disclosure. 
     The resulting structure of the memory causes the central processing unit to operate as a special purpose computer to filter the input signal using a 5 point weighted average combined with a binary spacial filter. It also provides a five minute potassium photopeak accumulation with a peak position indication for semi-automatic spectrum stabilization. The remaining requirements for operation of the central processing unit, including control of the peripheral units, scanning of the input signals, etc., are available from the aforenoted CPU manufacturer and/or are known to persons skilled in the art, and will not be described in detail. 
     It is preferred that the system clock be manually started in order to provide an indication in real time of time of the scan from start at a particular physical location. 
     Rather than using the microprocessor as described, a person skilled in the art can design an electronic logic circuit using well-known techniques to perform the described functions. 
     FIG. 4 consists of a pair of graphs which illustrate the advantages of the invention. In the top graph of FIG. 4, the unfiltered spectrum is shown, as a series of dots appearing virtually randomly scattered, having various counts for the particular channel numbers. The lower graph illustrates the spectrum after filtering. Peaks are noticeable for the energy region of K 40  , Bi 204  and Ti 208 . Uranium is normally associated with a peak in the Bi 204  window. 
     After filtering, the signals are processed as described above to obtain a net photopeak energy display, and the results for the Bi 204  window for a period of scan over about 16000 feet is shown in FIG. 5. 
     Curve 25 in FIG. 5 shows the signal amplitude after processing using prior art Compton stripping techniques mentioned earlier in this specification. It will be noted that a significant peak 26 is shown at about the 2214 marker point. 
     Utilizing the present invention, a gamma ray peak 27 is clearly shown at about the 2218 market point which was completely ignored by the Compton stripping technique. Further, the peak 26 has been shown to be a false anomaly. 
     Checking the physical locations closer has proven that the procedure which is basic to the invention described herein is significantly more accurate than the prior art Compton stripping technique, and the present apparatus used on an on-line basis saves signficant time and money of the geophysicist in checking false anomalies, and pinpointing more accurately, as the survey proceeds, locations of radioactive ore. 
     A person skilled in the art understanding the description may now conceive of other embodiments or modifications of this invention. All are considered within the sphere and scope of this invention as defined in the claims appended hereto. ##SPC1##