Gamma-ray Spectrometry for by John A. S. Adams, Glenn E. Fryer, and John J. W. Rogers Department of Geology, Rice University, Houston, Texas 77001 G a m m a - r a y spectrometry offers a rapid, low-cost method for airborne exploration of the earth's surface. In the future, such techniques will be important for surveying underdeveloped countries
accusCtomed to the problembeen of making HEMISTS HAVE LONG
analyses of very small amounts of material. Much effort has been directed toward the problem of miniaturizing systems and developing apparatus capable of handling milligram or smaller amounts. Geologists, however, are commonly faced with the reverse problem. Geochemists want to know, for example, the composition of an entire mountain range, or a whole continent, or even the entire earth. The problem of analyzing the entire earth is two-fold. The composition of the earth's surface is largely a sampling problem, whereas the composition of the interior requires inference from surficial data, subsurface geophysical data, and knowledge of many geologic processes. Our efforts in radiometry at Rice have been directed largely toward a partial solution of the first problem, namely a determination of the distribution of radioactive elements on the earth's surface. The three naturally-occurring, long-lived, radioactive nuclides that have interested us are uranium, as the 235 and 238 isotopes, thorium-232, and potassium40. Significant natural gamma radiation is emitted only by potas22 A ·
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sium-40 and members of the thorium and uranium decay series. A number of methods have been used to analyze for thorium, uranium, and potassium. Potassium, which constitutes from one to four percent of most rocks, can be quantitatively determined by wet gravimetric methods or by various physical techniques such as optical and x-ray spectrometry. Uranium can be determined quantitatively by fluorescence after chemical extraction from the rock or mineral in which it occurs. Thorium can be determined colorimetricaily or by total alpha counting and subtraction of the proportion of alpha radiation contributed by uranium. None of these analytical methods, which generally require chemical treatment of the natural material, are as rapid or as efficient as gamma-ray spectrometry (1,2). The uranium and thorium nuclides are parents for well-known decay series culminating in three stable lead isotopes. In the thorium-232 series, the thallium-208 isotope emits a 2.62-MeV gamma ray. In the uranium-238 series, the bismuth-214 isotope emits a 1.76MeV gamma ray. These two gamma rays plus the 1.46-MeV radiation from potassium-40 provide three peaks that can be detected relatively easily by gammaray spectrometry. Assuming that the number of gamma rays emitted is proportional to the abundance of the radionuclides, the concentration of these nuclides can then be determined from measurements of their emitted radiation. In the case of the thallium-208 and bismuth214 gamma rays, conversions to the abundances of thorium and total uranium-238 and uranium-235 are possible on the basis of two as-
sumptions: (1) that a secular radioactive equilibrium exists within the thorium and uranium decay series in such fashion that the ratios of abundances of the measured nuclides and the parent thorium and uranium are directly proportional to the ratios of the half lives of the measured nuclides and the parent elements; and (2) that the uranium-235 and -238 isotopes cannot be fractionated in nature, and that the U-238 over U-235 ratio is a constant 138 in all terrestrial material. No evidence has ever been found that the second assumption is incorrect, but recently formed materials that incorporate either the parent uranium and thorium without daughter products, or the daughter products without supporting parents, may not have had time to attain complete secular equilibrium, and thus gamma-ray spectrometry is not applicable for many very young geologic materials. With the exception, then, of recently deposited sediments, young volcanic rocks, and soils, it is theoretically possible to determine the concentration of thorium, uranium, and potassium by counting gamma-ray emissions at 2.62, 1.76, and 1.46 MeV, respectively. The apparatus used for these determinations is partially shown in Figure 1 (Ref. 3). Here, samples packed in cylindrical plastic containers are fed automatically into a shielded detector system. Shielding from natural gamma radiation is accomplished by placing the assembly in a basement room and by surrounding the actual detectors with 1,000 pounds of mercury. The detector crystals, which emit light upon being struck by gamma radiation, are either thallium-activated
REPORT FOR ANALYTICAL CHEMISTS
Airborne Geochemistry
sodium iodide or cesium iodide. These crystals are optically coupled to photo-multiplier tubes which convert the light t o electrical impulses. The energy conversion processes in the crystal and photomultiplier a r e linear, and the amplitudes of t h e pulses are therefore proportional t o the energies of the gamma radiation striking the crystal. T h e spectrum analyzer now used a t Rice sorts the pulses from the detectors, according t o amplitude, into 128 discrete channels. Each channel corresponds to a narrow range of gamma-ray energies. The number of gamma rays of each energy detected during the analysis interval are stored in a magnetic core memory. These counting data can then be read out on an oscilloscope in the form of a spectrum of the type shown in Figure 2 or in typed numerical form. I n our laboratory the memory can also be read out on a punched tape, which can be fed directly into a computer t h a t performs calculations of t h e abundances of t h e thorium, uranium, and potassium. Basically the instrumentation is the same as t h a t used for gamma radiations produced by neutron activation. For two reasons, the elemental abundances a r e n o t directly proportional to the peak heights shown in Figure 2. First, even through 1,000 pounds of mercury, there is a certain natural g a m m a - r a y background radiation, which increases rapidly toward t h e lower energy parts of the spectrum. This background can be determined easily by counting inactive material such as NaCl, and the resultant values can be subtracted from measured peak heights t o provide values for t h e gamma radiation in each energy spectrum caused by a sample itself.
Figure 1 . Mercury-filled shield around c o u n t i n g chamber w i t h a u t o m a t i c feed of plastic cylinders c o n t a i n i n g rock material. Mr. Elze H e m m e n is i n s t r u m e n t engineer at the Rice Geology Department
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Figure 2. Typical oscilloscope readout o f g a m ma-ray s p e c t r u m f r o m ordinary rock. T h e potassium (1.47-MeV) peak is t h e sharp peak nearly in t h e center o f the illustration. The 1.76-MeV peak in t h e u r a n i u m series is j u s t to t h e right o f t h e pot a s s i u m peak, a n d t h e 2.62-MeV peak in t h e t h o r i u m series is on t h e far right
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A second and more difficult correction t h a t must be made in the computation of elemental abundances is the tendency of monoenergetic gamma rays to dissipate a portion of their energy in secondary radiations of lower energy. This Compton scattering means that, for example, incident 2.62 M e V gamma radiation will produce pulses from the scintillation crystal proportional to the 2.62 MeV radiation, plus a considerable number of pulses of lesser energy. Thus, any sample containing thorium will show not only gamma radiation at the 2.62 MeV level, but also radiation at lower energy levels, particularly and unfortunately in the potassium and uranium channels. Similarly, both thorium and uranium contribute to the radiation in the potassium channels. Fortunately, the ratio of scattered radiation at each lower energy level to the radiation at t h e incident energy level is constant for a fixed sample and geometry. Thus it is possible to calculate the amount of radiation in the uranium channel related to the recorded radiation at the thorium energy level and to calculate the amount of radiation in the potassium channel related to the observed thorium and uranium radiation. An example of such calculations in the case of uranium determinations is shown below. counts per minute in T h channel background per minute in T h channel
5.07 1.28
counts per minute in U channel
7.92
background per minute in U channel
2.79
ratio of cpm to T h content
2.55
ratio of cpm to IT content ratio of cpm to T h content to secondary radiation in U channel
0.67
0.78
p p m T h = 2.55 (5.07-1.28) = 9 . 7 ppmP=0.67 [7.92-2.790.78(3.79)] = 1 . 5 All calculations are based on radiometric determinations of known standards. The uranium standard consists of uraninite (es24 A
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sentially U 0 2 ) diluted with an inert material, such as olivine. The thorium standard is monazite, a rare earth phosphate with a high content of thorium, plus olivine. The potassium standard is pure potassium bromide. Initial analyses of all standards are, of necessity, by methods other t h a n gamma-ray spectrometry. During the counting of any sequence of rocks, cross calibrations are frequently made with known rock standards, such as those supplied by the United States Geological Survey. W i t h such calibration and cross-checking methods, it has been possible to determine thorium and uranium contents routinely within ± 10 per cent in ordin a r y rocks containing thorium in the range of 2 to 20 p p m and uranium in the range of 1 to 5 ppm. Accuracy naturally decreases a t lower concentrations and in rocks with extreme T h / U , K / U , and T h / K ratios. Extreme ratios are rarely encountered. Cross calibration of potassium determinations against x-ray fluorescence and atomic absorption methods are routinely done, and the accuracy of the gamma-ray technique appears to be plus or minus 10 per cent for concentrations in the range of 1 to 5 per cent of the total rock. We are able to count 10 to 20 samples of ordinary granite per day with this accuracy. All of the measurements and calculations described above required a great deal of effort in the collection of samples, transportation from the field location to the university, and preparation of the sample for analysis. We have analyzed some 10,000 samples, weighing in the neighborhood of 350 to 450 grams after preparation for analysis. Since one always sends to the laboratory larger samples t h a n are needed for the direct analysis, we have shipped approximately 20 tons of rock to Rice University. Cores can be mounted directly in the sample counting chamber, b u t irregularly shaped pieces of rock must be ground thoroughly and packed in canisters. This has required uncounted man hours of grinding time, but has provided the employment for a large number of
Figure 3. Portable spectrometer being adjusted by John Adams
undergraduate students. One of our main problems is storage and ultimate disposal of the analyzed rocks. For several reasons it was decided to t r y to take the counter to the rocks rather t h a n the rocks to the counter. T h e first of these efforts resulted in a pack similar to t h a t shown in Figure 3. This is a simple portable spectrometer, consisting of a scintillation crystal, a photomultiplier tube, and an a n a lyzer t h a t counts in one very broad energy level at any _one time. T h e counted energy level can be selected electronically. T h e spectrometer can be placed on any reasonably level rock surface and will detect radiation from approximately one cubic foot of rock, thus providing a sample size considerably larger t h a n the 350 to 450 grams counted in the laboratory. At the high gamma-ray energy levels counted, particularly that of thorium, shielding from background radiation is not particularly necessary for rocks t h a t contain moderate to high concentrations of radioactive elements. Where necessary, how-
Report for Analytical Chemists
ever, a lead shield fitting around the detector crystal can be t r a n s ported by a second person, which has provided considerable employ ment for graduate students. I n studies of loose materials such as beach sand, constant geometry is obtained by completely burying the detector crystal in the sand. Instruments of the type shown in Figure 3 were used in a survey of the Conway Granite of the White Mountains in New Hampshire (4, 5). This survey was conducted to determine the extent of possible thorium resources in the granite. Measurements were made at more than 250 surface stations in moun tainous country during one sum mer, and thorium concentrations were demonstrated to be in the range of 50 to 60 ppm. T h e mainte nance of these concentrations at depth was later demonstrated by analysis of 2800 feet from three cores drilled into the granite. I t should be noted t h a t with hundreds of determinations of thorium on a single Conway granite batholith, it is possible to treat the data as a statistical population or universe in which one determination is not very important, but the statistical mean, mode, skewness, etc., give very high-quality data. I n other words, there is no scientific or eco nomic value in doing more deter minations on the Conway granite
U)· The next logical step from the use of a portable instrument, count
ing a t one station, was the develop ment of a continuously recording instrument t h a t would survey radioactivity as it was moved across the ground. Such an instru ment could be mounted either on a land vehicle and used to survey roads and other areas accessible in such fashion or, for wider use, could be placed in a helicopter. Much of our recent work has been devoted to the development of helicopter-borne surveying methods (Figure 4 ) . T h e instrument cur rently in use (Figure 5) contains a five-by-five inch, thallium-acti vated, sodium iodide crystal at tached to a single multiplier photo tube powered by a battery pack. The analyzer from the phototube is set to count all gamma photons in a preselected energy range re ceived in a given interval of time and to record the number of events on a strip chart. I t has been found empirically t h a t a 0.5-second time interval is most suitable for general radiometric surveying, and thus the strip chart records the number of gammas at a selected energy re ceived within each 0.5-second in terval as the helicopter is in motion. The present instrument records only one energy channel at a time, but we hope to develop multi channel instruments in the near future. T h e measurements made thus far in helicopter exploration are: (1) the total gamma-ray flux in the 0.15- to 3.0-MeV range; (2) energy levels centered around the
Figure 4. Helicopter-borne instrument in Glenn Fryer's laboratory
Figure 5. Gamma spectrometry instru mentation before installation into heli copter. On the lower shelf of the cart is the 5 in. χ 5 in. sodium iodide crys tal (thallium activated) assembly. En cased with the sodium iodide crystal is the high voltage battery pack. The signal wire from the crystal assembly leads into the amplifier and discrimi nator circuits, which are above the specially designed digital rate-meter. In the center of the upper shelf is the recorder. Between Dr. Adams and the recorder, one can see the alternator that provides suitable power for the recorder. Not visible is the 12-volt automobile battery, which is behind the alternator. The entire spectrometer, including these power batteries, is on the cart
1.76-MeV photopeak in the uranium-238 series; (3) energy levels centered around the 2.62-MeV photopeak in the thoriiim-232 series; and (4) a variable energy channel used in p a r t for calibration of the analyzer against a cesium137 standard which emits gamma radiation at the 0.663-MeV level. Several factors are critical in ob taining accurate radiometric mea surements by helicopter surveying methods. One factor is the neces sity of maintaining relatively con stant altitude above ground level. Although gamma rays undergo little absorption in air, there is some attenuation with height. Most of our experiments have been carVOL. 4 1 , NO. 6, MAY 1969
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Figure 7. Record of gamma-ray intensities in the 0.15- to 3.0-MeV interval recorded over Galveston jetty from helicopter flying at 50 feet and a speed of 60 knots. The full scale is 500 counts in a 0.5-second time interval
Figure 6. Helicopter passing over Galveston jetty
ried out with the helicopter at a height of 50 feet above the ground, where the gamma-ray flux is essentially the same as at ground level, and variations of ±10 feet from that elevation were found to have little effect. A second factor is the necessity of maintaining relatively constant air speed. Although this would not be a critical factor in areas of homogeneous distribution of radioactivity, efforts to detect small linear anomalies would be greatly hampered by variable speed. Furthermore, very high air speeds naturally will cause the instrument to miss small anomalies such as planar ore bodies with a linear outcrop on the earth's surface. The ground speed found most suitable for general surveying is 60 knots, and an experienced pilot can maintain this speed quite accurately. At such a speed and a height of approximately 50 feet, linear anomalies approximately 15 to 20 ppm of thorium and uranium above the concentrations in surrounding rocks can be detected if their width is on the order of 10 feet or more. The granite jetty at Galveston Beach has provided a convenient anomaly that is about 10 feet wide and infinitely long for the purposes of helicopter surveys. 26 A ·
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Figure 6 shows the helicopter going over the jetty, and Figure 7 is a full-scale reproduction of the record obtained in the 0.15- to 3.0MeV interval. Calibration of an airborne instrument to provide absolute values of thorium, uranium and finally potassium concentrations poses a considerable experimental problem. The best method is to record gamma-ray spectra over an area of homogeneous rock whose radioactive element contents have been determined by analysis of ground samples. By measuring rocks with a variety of thoriumuranium and potassium-uranium ratios, it is then possible to convert gamma-ray intensities directly into elemental abundance. The presumed absolute accuracy of such measurements is clearly not as high as for measurements made in the laboratory on individual samples. Measured abundances within approximately 20% of the correct value are about as close as can be expected. Some uncertainty is also attached to measured values owing to the fact that soil zones, from which most of the gamma rays are derived, may exhibit some secular disequilibria within the thorium and uranium series. Nevertheless, the fact that a single airborne measurement accumulates radiation from several thousand square feet of ground surface, plus the speed of accumulation of data makes the
approximate methods of helicopter surveying useful for general exploration purposes (6). It should be noted that the cost per data point is very low and the speed of data acquisition very high. Thus the 400 miles of modern Texas beach were flown in five days, during which some 48,000 separate data points were recorded at the rate of one each 0.5 second, which equals one for each 90 feet or so along the beach. Furthermore, the anomalies were examined and sampled directly after they had been detected from the helicopter. Doing the analysis in live time over the geologic anomaly is a most powerful and useful method of investigation. The future of gamma-spectrometric exploration includes multiple-channel analyzers operated in conjunction with other geophysical surveying techniques. Many ore bodies, for example, have associated magnetic anomalies, and simultaneous gamma-ray and magnetic surveying from the same helicopter gives promise of rapid location of unusual mineralization. Such techniques will be of particular importance in surveys of underdeveloped countries, where speed of operation is particularly significant. Acknowledgment
Our work on gamma-ray spectrometry is a portion of our re-
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search on the geochemistry of thorium and uranium. Since 1955 this study has been partially supported by Robert A. Welch Foundation Grant C-009 to John A. S. Adams and John J. W. Rogers. Other organizations and agencies have contributed to various specific studies, and many of them are acknowledged in the cited references. Literature Cited
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(1) P. M. Hurley, Bull. Geol. Soc. Am., 67,395 (1956). (2) Ibid., 405 (1956). (3) J. A. S. Adams, in "The Natural Radiation Environment," J. A. S. Adams and W. M. Lowder, Eds., Univ. Chicago Press, Chicago, 1964, p. 485. (4) J. A. S. Adams, M. C. Kline, K. A. Richardson, and J. J. W. Rogers, Proc. Nat. Acad. Sci. 4 8 , 1898 (1962). (5) J. J. W. Rogers and J. A. S. Adams, Geochim. Cosmochim. Acta, 2 7 , 775 (1963). (6) J. A. S. Adams, in "Use of Nuclear Techniques in the Prospecting and Development of Mineral Resources," Intern. Atomic Energy Comm. Symposium, Buenos Aires, Nov. 5-9, 1968.
JOHN A. S. ADAMS was born in Independence, Missouri, on November 1, 1926. He received all his degrees from the University of Chicago, culminating with the Ph.D. in 1951. During 1949-1951 he studied under T. F. W. Barth in Oslo, Norway. After receiving his doctorate, Dr. Adams was a Project Associate with Dr. Farrington Daniels at the University of Wisconsin. They worked together on an Atomic Energy Commission Research Contract entitled "Geochemistry of Uranium and Its Recovery from Low-Grade Ores." Since 1954 he has been Assistant Professor, Associate Professor, and Professor of Geology at Rice University. He was appointed Chairman of the Department of Geology in 1965. Dr. Adams has served as consultant to Shell Development Company, Humble Oil and Refining Company, Resources for the Future (Ford Foundation), and the Atomic Energy Commission. His research interests have been in geochemistry of thorium and uranium, the determination and distribution of trace elements and K-Ar geochronology.
GLENN E. FRYER was born in Medicine Hat, Alberta, Canada, on August 30, 1930. After receiving his B.Sc. degree from the University of Manitoba in 1952, he began working for McCullough Tool Company as a Project Engineer. From 1955 to 1963 he was Chief Electronic Engineer for Great hakes Petroleum Services. Since 1963 Mr. Fryer has served as Instrumentation Engineer in the Department of Geology at Rice University where Mr. Fryer's work has been in the development and improvement of instruments, especially nuclear devices, used in various methods of physical measurements of geological and geochemical data.
JOHN J. W. ROGERS was born in Chicago, Illinois, on June 27, 1930. He received his B. S. from California Institute of Technology (1952), his M.S. from the University of Minnesota (1952), and his Ph.D. from California Institute of Technology (1955). Since receiving his doctorate he has served on the faculty of the Department of Geology at Rice University. He became a Full Professor in 1963. Dr. Rogers' research has been into the geochemistry of thorium and uranium and other trace elements; igneous petrology; and clastic sedimentation, especially size distributions and other statistical analyses.
