Detection limit for gold by radioisotopic X-ray analysis

International Journal of Applied

Radiation

and Isotopes, 1970 ,Vol. 21, pp. 395-403. PergamonPress. Printed in Northern Ireland

Detection Limit for Gold by Radioisotopic X-Ray Analysis P. G. B U R K H A L T E R a n d H . E. M A R K I I I College Park Metallurgy Research Center, Bureau of Mines, U.S. Department of the Interior, College Park, Maryland, U.S.A.

(Received 21 August 1969) T h e use of a Ge(Li-drifted) semiconductor detector and gamma-rayex citation with radioisotopic sources provides an X-ray analytical method that is advantageous for the determination of h i g h - Z elements in ores. T h e 122-keV g a m m a radiation of57Co was found to be an optimum energy for exciting gold K-radiation. T h e detection limit for gold was determined to be 30-40 p p m using both synthetic standards and analyzed gold ores. With a source strength of 60 mCi, a 21-ppm detection limit is obtainable in 3-mia counting times. T h e sensitivity of the radioisotopic X - r a y method for several other h i g h - Z elements was also determined. Using 57Co for excitation, the detection limits for tungsten, lead, and uranium were 25, 40 and 60 ppm, respectively. LA LIMITE DE D]~TECTION DE L'OR PAR L'ANALYSE RADIOISOTOPIQUE AUX RAYONS X L'emploi d ' u n d4tecteur demiconducteur en Ge (trait6 de Li) et de l'excitation aux rayons g a m m a fournit une m4thode d'analyse aux rayons X qui est avantageuse pour le dosage des dl6ments de haut Z dans les min6rais. Le rayonnement g a m m a de 122 keV du STCo on trouva ~tre de la meilleure 6nergie pour l'excitation du rayonnement K de For. O n trouva 30-40 p p m c o m m e la limite de d4tection de l'or en employant et des 6talons synth4tiques et des min6rais d ' o r pr4alablement dos4s. Avec une source d ' u n e puissance de 60 m C i on peut obtenir une limite de ddtection de 21 p p m avec des p6riodes de comptage de 3 min. O n a dgalement d6termin6 la sensibilitd de la mdthode radioisotopique aux rayons X pour plnsieurs autres 616ments de haut Z. A l'emploi du 57Co pour l'excitation, les limites de d4tection pour le tungst&ne, le plomb et l ' u r a n i u m furent 25, 40 et 60 ppm. IIPE~EJI

OBHAPY?AEHFIH 3OJIOTA HOCPE~CTBOM PA~HO}I3OTOHHOFO P E H T F E H O B C H O F O AHAJI1713A FepMaH~eBLIi~ (C I,i-~pei~oM) noaynpoBoTIHrmOm,Ii~ ~eTeHT0p H Boagym~eHi4e raMMa~laayqeHn~t pa~HoHaOTOHHBIMH HCTOHHHHaMH nMeIOW xopomee npnMeHeHne ~a~t onpeneaeHrt~ B pynax oJIeMeHTOB C BI,ICOHHM-Z(aTOMHBIMHoMepoM). FaMMa-nanyqeHne B 122-HaB OT 57C0 oKaaaaocb onTHMaJihnol~ aHeprHeIi ~:iu BO86yh~enH~I I~-H3JIyqeHHfl 80JIOTa. Hpe~e~ 06HapymeHHn aO~OTa, OT 30 ~0 40 HaCT.]M14JIJI. 6hIJI onpe~eaeH HaK Ha CHHTeTHHecHI4~ cTaH~apTax, TaK n Ha aHazmarlpyeM~x 30JIOTI,IX py~ax. IIp~ cr~ae ~CTOqnriHa B 60 miCi MOh~eT 6BITB noJIyqeH npeae~ B 21 q]r~r*zia. 3a 3 MHHyTBI cqera. B~iaa TaHh~e onpe~eaeHa qyBCTB~TeZn,HOCTr, pa~r~on3oTonHoii penTreHorpadpnr~ ~aH He~oTop~,~X ~pyrnx aaeMeHTOB C BBICOHHM-Z. IIp~MeHeHr~ ~Co ~ I g Boa6y~eHH~ ~aao npe~ea~t o6Hapyr~enng nozn, dppaMa, CBHHI~a H ypaHa 25, 40 ~ 60 q/~na~. COOTBeTCTBeHH0. NACHWEISGRENZE

F O R GOLD

DURCH

RADIOISOTOPEN-RONTGENANALYSE

Die Verwendung eines Ge(Li-Drift) Halbleiterdetektors und einer G a m m a - A n r e g u n g mit Radioisotopenquellea ergibt ein analytisches R6ntgenverfahren, das ftir die Bestimmung von Elementen mit einem h o h e n - Z in Erzen vorteilhaft ist. M a n fand, dass die 122 keV Strahlung von S~Co die optimale Energie zur Anregung yon Gold-K-Strahlung war. Die Nachweisgrenze fiir Gold wurde als 30-40 ppm festgesteUt, wobei sowohl synthetische genormte 2

395

396

P. G. Burkhalter and H. E. Marr III

Proben als auch analysierte Golderze verwendet wurden. Mit einer Q uellenintensit~it yon 60 mCi konnte eine 21-ppm Nachweisgrenze in Z~ihlzeiten yon 3-mitt erhalten werden. Die Empfindlichkeit der RacUoisotopen-Rt~ntgenmethode for andere Elemente mit einem hohen-Z wurde ebenfalls festgesteUt. Bei Verwendung yon 57Co ftir die Artregung waren die Nachweisgrenzen ftir Wolfram, Blei und Uran 25 ppm, bezw. 40 land 60 ppm. INTRODUCTION ONE OF the principal tasks of the Bureau of Mines' Heavy Metals Program is the delineation of gold-ore deposits. The Ge(Lidrifted) semiconductor detector combined with radioisotopic sources provides a possible means for X-ray analysis of gold ores. This proposed method is more adaptable to field usage than conventional X-ray spectrographs. Radioisotopic gamma-ray sources are convenient for excitation of the K-X-ray spectra for high-Z elements in ore samples. Excitation of highenergy X-rays allows analysis of large sample volumes which reduces the problems of variable grain size, surface roughness, and heterogeneity commonly associated with ore analysis by X-rays. For a radioisotopic X-ray analyzer to be a valuable tool, sensitivities for gold below 10 p p m are necessary. The purpose of our investigation was to determine the sensitivities of gold and other high-atomic-number elements by X-ray analysis using a semiconductor detector and gamma rays. Studies were conducted on the effect of excitation energy, source strength, multiple Compton scatter, and electronic discrimination on the peak-tobackground ratio for Au K0~-radiation, in order to obtain maximum sensitivity. After optimization of the experimental parameters, the peakto-background ratios and detection limits were determined from both synthetic gold samples and analyzed gold ores. The detection limits for other high-Z elements were also determined under the same instrumental conditions as for gold. INSTRUMENTATION An O R T E C germanium detector, model 8113-10", was used in the investigation. The detector had a 5-mm-thick by 80-mm 2 germanium crystal, which was calculated to have * Reference to specific models of equipment is made for identification only and does not imply endorsement by the Bureau of Mines.

an absorption efficiency for Au K0t-radiation of 98 per cent. An O R T E C amplifier, model 485 with Gaussian pulse shaping at a fixed time constant of 1.5 #see, was used in unipolar mode of amplification. The pulse-height resolution was measured at low counting rate using a l°~Cd source and also using gold foil excited by the 5~Co source. The pulse-height resolution (FWHM) was 660 eV for Ag Ka (22"2 keV) and 750 eV for Au K0q (68-8 keV). The X-ray spectra were accumulated in a SCI PP 400-channel pulse-height analyzer manufactured by Victoreen. Spectra collected in the pulse-height analyzer were read out on punched paper tape, smoothed in a computer, and plotted by a C A L C O M P X - Y plotter. An O R T E C model-408 biased amplifier allowed expansion of the X-ray spectra such that 400 channels was used for the energy range 56-96 keV. The 57Co source consisted of t w o small copper foils with each having originally 2-0 mCi of activity. The foils were mounted, with appropriate source backing and shielding, into brass cups and fastened onto an annular ring that fits the 7-cm-dia. detector housing. Multi-element shielding, consisting of 1-mm Pb, 3-mm T a and 1-mm Cd, was used as source backing to reduce the 122- and 136-keV gamma radiation from 5~Co to an acceptable level and prevented K-radiation from the highatomic-number elements used for shielding from contributing to the background spectra near the Au K0~-lines. The sources were mounted on opposite sides of the detector as shown in Fig. 1. A large angle source-sample-detector backscatter geometry was used with the sample positioned 2.5 cm from the detector surface. The primary Compton scatter for this arrangement had an average 138-deg scatter angle. The high background on the lower energy side of the Compton peak is the most important factor in limiting sensitivity. The resolution of the Ge(Li-drifted) detector is more than sufficient to resolve the gold

Detection limit for gold by radioisotopic X-ray analysis

397

~le

Primary compton scatt_er

122-keV gamma radiation

-~

Brass /

cup

Cobalt - 57 source

~ . j - Aluminum

collar Ge (Li,drifted) crystal Lead circular aperture

Cryostat housing

FIG. 1. Geometrical arrangement of sample, source, and detector. K-radiation from the primary Compton scatter. The gradual background tailing was determined to be predominately multiple scatter. A gamma ray that is scattered deep inside the sample can undergo one or more small energy losses by scatter and still reach the Ge crystal. Multiple scatter can also arise from largeangle scatter of the primary Compton radiation both from inside the detector housing or from the aluminum ring used as source holder. These multiple-scatter events are illustrated in Fig. 1. A lead disc 1.6-ram thick and 7-cm dia. with a 16-mm-dia. aperture was placed in front of the detector housing. The circular aperture was effective in reducing the background under the Au K~-peaks by 93 per cent. The dashed line in Fig. 1 represents removal of the high angle multiple scatter from the sides of the detector housing by the lead aperture. Synthetic gold standards were made by first blending 0.5-per cent iron oxide powder in

silica powder, average mesh size less than 150, and adding the gold as a chloride solution to form a stock sample at the 5000-ppm gold level. T h e iron oxide powder was necessary to achieve a homogeneity of better than 2 per cent. T h e gold concentration of the stock sample was analyzed by gravimetric and atomic-absorption methods. T h e stock sample was diluted by blending with silica powder and various matrix additives. Four standard gold samples each containing 1000-ppm gold were prepared having matrices of 5-per cent pyrite in silica. T o three of these standards 1-per cent additions of Cu, Te, or Pb were made. These elements commonly are found in gold ores, and their effect on the sensitivity, from changes in X-ray absorption and scatter, was determined. In addition to X-ray measurements from synthetic samples, the sensitivity was determined from several analyzed gold ores. Four ores obtained from C a o w and

398

P. G. Burkhalter and H. E. Mart III

BEMvnsIt(1): Kirkland Lake, Minto, Ashley, and Buffalo Ankerite were used together with two Bureau of Mines reference ores. The gold content, determined by fire-assay, ranged from 0.05 to 2.95 oz/ton (1.0 oz/ton is equivalent to 34-3 ppm). T h e sensitivity for several high-Z elements in addition to gold was measured by the radioisotopic X-ray method with the Ge(Lidrifted) semiconductor detector. Synthetic standards containing 5000-ppm of W, Pb and U were prepared by mechanically blending oxide powders with silica. T h e samples used in this work were pressed pellets of 7.6-cm dia. and 5.5-cm thickness that weighed about 1 lb each. The gold standard with a 5-per cent pyrite in silica matrix had a density of 1.7 g/ cm s. For the radioisotopic source-geometry used, the samples had to have diameters greater than 7 cm, and because of the penetration of gold K-radiation in a light matrix ore, the sample thickness had to be greater than 5 cm in order for the sample volume to be large enough that the gold K-radiation seen by the detector was greater than 95-per cent of the intensity of an infinitely thick and large area ore sample.

expected from previous experience using radioisotopes such as 125I to excite silver samples, t2~ The large energy separation between the 122-keV excitation energy of S7Co and the gold absorption edge at 80.7 keV suggested that another radioisotope with a gamma-ray emission closer to the absorption edge might yield a larger peak-to-background ratio because of better excitation efficiency. 133Ba, 1°9Cd and 153Gd were considered with gamma-ray energies of 81, 88 and 98-103 keV, respectively. 133Ba (Fig. 3) and 1°9Cd were tried but found to be unsatisfactory because the Compton peaks strongly overlap the Au K~-lines in the backscatter geometry. For the latter two radioisotopes the Au K-lines have energies between the Compton peak and the excitation energy. 158Gd has a Compton peak at 75 keV and produces a poorer peak-to-background ratio than 57Co.

X-RAY INTENSITY MEASUREMENTS The peak-to-background ratios were measured with a wide window to include both the Au K0~ and K~ z lines. The total X-ray counts, N~_b, of the gold peaks plus scatter background were determined by summing channel-bychannel intensities of the pulse-height spectra over the energy range from 66.40 to 69.50 keV. EXPERIMENTAL INVESTIGATION 57Co has strong gamma-ray emissions at 122 The background, Nb, under the gold lines was and 136 keV which make it an attractive radio- obtained by linear interpolation of the scatter isotope for high-energy X-ray excitation. The background on both sides of the gold lines. 14.4-keV gamma rays and iron X rays emitted Table 1 lists integrated counts for the four from 57Co can easily be absorbed by covering samples containing 1000-ppm Au for 50-min the source with brass foil and this is desirable accumulations using a 1.7-mCi source strength for reducing the total intensity entering the of S7Co. The number of counts attributable to detector. X-ray intensities were collected from gold K0~2 and K0~1 radiation, N~, is equal to a standard containing 1000-ppm Au in silica N~+b minus N b. A peak-to-background ratio, and from a silica background sample. Figure N J N b , of 0.34 was obtained for the 10002 shows the superimposed spectra over an ppm Au plus 5-per cent pyrite standard. energy range of 56-96 keV from the two The same peak-to-background ratios within samples. The Compton peak from the 122-keV the limit of error were obtained for the 1-per gamma rays occurs at 86 keg, corresponding cent Cu and 1-per cent Te standards. The to a scatter angle of 138 deg. T h e gold X-ray smaller value of 0.22 for the 1-per cent Pb lines, K~ 2 (67-0 keV), K0~1 (68.8 keY), and standard results from X-ray absorption of the Kill (78-0 keV), are shown in addition to lead gold K0~ radiation together with a higher K0c1 and Ka 2 which are stray radiation origin- scatter background. The precision of the peak-to-background ratio was -4-0.014 or about ating from the circular aperture. The peak-to-background ratio for the 1000- 5 per cent of the 0.34 value. The estimate of ppm gold sample was lower than we had error for the peak-to-background ratios was

Detection limit for gold by radioisotopicX-ray analysis

399

20,000

Complon

16,000

g

Cobolt-57

scotler 8 6 keV

source

0

I000

X

Silico bockground

+

ppm gold in silico

12,000

o o l-

z laJ lz

8 000 Gold]

/'

Leod-I K/3'/ . K~, i

ool~ Ka ,i,

4 000

0

Leoo ~

Gold---,. A

~

/

+

~..-,~

~! j

I

I

60

70

I

80 ENERGY, keV

I

90

FIG. 2. Spectrum of 1000-ppm gold in silica excited by 57Co. 4-0"02. This value was determined from measurements of five separate 1000-ppm gold standards prepared from three different gold stock solutions and using both hand-packed samples and pressed pellets. The detection limits given in Table 1 were calculated by linear regression of the X-ray intensities measured from the 1000-ppm gold standards. Since N~_ b and N b were determined independently, the counting statistics must include both measurements. The standard deviation due to Poisson counting statistics of the signal corrected for background, N~_b N~,, is -

= a/(N~+~ + N~).

-

The detection limits calculated from the X-ray intensities are based on a 3ff level so that the minimum concentration of gold detectable by X-rays would correspond to an intensity equal to three standard deviations above the background intensity. T h e detection limits ranged from 28 ppm for a quartz-type ore to 37 ppm for an ore containing up to 1-per cent heavy metal such as lead. I f the accumulation time were doubled, the sum of the peak plus background measurement, N~,+b, and the extrapolated scatter background, Nb, for the 5-per cent pyrite sample would correspond to about 10e counts. A higher accumulation of counts would not yield a better detection limit because

400

P. G. Burkhalter and H. E. Marr III

I0,000 Compton scatter 64. keV

+

8~000

6000 0 u pZ II.Z

Lead

4 000

K;I

Gold

K'! Lead

y - ray 81 keV

2.000

[

Gold

0

Lead K.B I

]

[

~ Lead

l

I

I

I

60

70

80

90

ENERGY. keV

Fxo. 3. Spectrum of 1000-ppm gold in silica excited by aaaBa.

TABLE 1. Total X-ray counts, peak-to-background ratios, and detection limits for 1000-ppm gold in silica samples Sample matrix

N~_b (counts)

Nb (counts)

N~ (counts)

NJN b

Detection limit (ppm)

5% pyrite + 1% Cu + 1% Te + 1% Pb

296,400 277,300 329,400 365,000

220,800 205,400 248,300 298,900

75,600 71,900 81,100 66,100

0.34 -4- 0.02 0.35 0.33 0.22

29 29 28 37

Detection limit for gold by radioisotopicX-ray analysis

401

TCLBLE2. Total X-ray counts for gold ores Gold ore Kirkland Minto Ashley Buffalo Ankerite Bur. Mines ore Bur. Mines ore

Concentration (oz/ton) (ppm) 2.95 0.67 0.64 0.44 0.35 0.05

103 23 22 15 12 1.8

Nv+b (counts) 232,500 288,700 306,800 245,300 299,500 251,000

Nb (counts) 222,400 286,800 305,400 242,700 299,100 250,400

N~ (counts) 10,100 1900 1400 2600 400 600

TABLR 3. Total X-ray counts, peak-to-background ratios, and detection limits for 5000-ppm high-Z elements standards Element

N~q_b (counts)

Nb (counts)

N~ (counts)

Peak-to-backgd. ratio

Detection limit (ppm)

Tungsten Lead Uranium

657,800 871,500 224,400

134,400 444,600 83,700

523,400 426,900 140,700

3.89 0.96 1-68

26 40 59

of the limit imposed by the precision for measuring X-ray intensities with a semiconductor detector. Under this condition, the detection limit would be 21 p p m for gold. X-ray spectra for the six analyzed gold ores were accumulated in the pulse-height analyzer for 50-rain counting intervals and the total X-ray counts are listed in Table 2. A positive value of Nr was obtained from all the ores. T h e number of counts calculated for a 3(r above background for these measurements of N~_~ plus Nb corresponds to between 2000 and 2300 counts. As anticipated, N~ for Kirkland Lake ore (which contains 103-ppm gold) is well above the 3a level. T h e average detection limit for the six ores is 33 ppm which is within the detection limit range determined from the 1000-ppm gold standards. X-ray counts were also measured for the 5000-ppm tungsten, lead, and uranium in silica standards. The X-ray lines used were W K ~ (58.0 keV) plus K~I (59.3 keV), Pb K ~ (72.8 keV) plus K~ 1 (75-0 keV), and U K~ 1 (98.4 keV). T h e U K~ 2 (94-7 keV) line was not included because of overlap with the Compton peak and also the energy separation between the U K~2 and K~ 1 lines was greater than the width of each individual peak. T h e total X-ray counts for 50-rain accumulations, peak-tobackground ratios, and detection limits are given in Table 3. T h e detection limits for

tungsten, lead, and uranium are approximately 25, 40 and 60 ppm, respectively. DISCUSSION T h e selection of the semiconductor detector and the techniques for achieving the best sensitivity for gold involved compromises between pulse-height resolution, source intensity, counting rate, and counting time. T h e 5VCo source strength was 1.7 mCi at the time of intensity measurements, and a total scatter intensity of 2250 cps entered the detector from the 5-per cent pyrite standard. Semiconductor detection systems incorporating pole-zero cancellation and baseline restoration are now usable to as high as 100,000 cps with resolution losses of less than 50 per cent compared with the resolution at low count rate. Increasing the source strength to 60 mCi, would yield a total intensity of 80,000 cps entering the detector. At this higher intensity, the counting time would be reduced to 170 sec. Semiconductor detection systems have excellent gain stability and combined with radioisotopic excitation offer a very stable method of analysis. In previous work ~2) the precision for measuring X-ray intensities with a semiconductor detector was found to have a one-sigma-standard deviation equal to 0.1 per cent when the system

402

P. G. Burkhalter and H. E. Marr III

TABLE 4. Electronic settings vs. sensitivity for gold analysis Electronic discrimination

Gold X-ray lines

Number of channels

Peak-to-backgd. ratio

Detection limit (ppm)

2a-2a

K~a + K~ 1

HM-HM

Ka z ÷ Ka 1

33 27 16 10

0-33 0.36 0.39 0-49

30 31 35 37

2a-2a

Koh

HM-HM

K0c I

was operated several days at constant temperature. This precision of 0.1 per cent would impose a limit for the X - r a y counts, N~+~ -t- Nb, from which the detection limit is calculated corresponding to a total of l0 G counts. Within the limit of instrumental stability, the detection limit would be 21 p p m for gold. Detecting the K-X-ray spectra from high-Z elements with semiconductor detectors presented a situation normally not encountered in X - r a y analysis. The germanium detector has adequate resolution to resolve the Au K0c2 and K0c1 lines, but since both are strong spectral lines, the question arises whether it is better to sacrifice the peak-to-background ratio obtainable with a single line or to have 1.5 times more gold intensity by measuring both lines. Several window settings were tried to determine which yields the best sensitivity for the Au K0c-lines. Table 4 compares the peak-to-background ratio and detection limit for four different levels of electronic discrimination. T h e lowest peak-to-background ratio but best detection limit was achieved with a wide window set at 2a below the center of the Au K0c2 peak on the lower energy side and 2a above the center of the Au K0c1 peak on the upper energy side. Sigma is determined from the half-maximum (HM) intensities of a peak assuming Gaussian shape. T h e detection limit is only slightly poorer if the window is set at H M on the lower energy side of Au K0c2 to H M on the upper energy side of A u K ~ 1. T h e largest peak-to-background ratio is obtained by measuring the single Au K0h-line at H M on both sides of the line, however, to gain this 18-per cent increase in peakto-background ratio resulted in a 36-per cent sacrifice in gold intensity from the widest window setting. I n addition to better sensitivity, a wide window has the advantage that the intensity measurements are less sensitive to

small gain shifts) 2) Also, measuring both the K a 2 and K0c1 lines is advantageous by providing some degree of insensitivity to changes in pulse-height resolution with changing count rate. I t is common in manufacturing semiconductor detectors to use a several-hundred-angstromthick gold layer on the front of the germanium crystal as an electrical contact. Au K0c lines from the crystal were found upon careful examination of the spectra from blank samples. This source of radiation changed the detection limit by less than one per cent and therefore the sensitivity for gold was not seriously affected. Intensity measurements were made in this study with a 400-channel pulse-height analyzer and paper-tape readout. Alternatively, data were measured with three single-channel analyzers and scaler-timer combinations connected in parallel from the amplifier output. T w o of these were used to measure the scatter background above and below the gold lines while the third measured the entire Au K0cI and K0~2 lines. Good agreement in the peak-to-background ratios was found for the two readout methods. An advantage in selecting 5~Co is that the 122-keV radiation can excite X-rays for all high-Z elements including plutonium ( Z equals 94). 57Co is recommended for exciting elements with Z ~ 70. For lower-Z elements the 61keV g a m m a rays from 24aAm would be more efficient. T h e elements hafnium through bismuth would be expected to have detection limits ranging from about 20 to 40 ppm, as have been obtained in this work for W, Au and Pb. T h e only precaution is in selecting the source shielding to avoid X - r a y interferences. 57Co is also a very efficient source for exciting thorium and uranium and the K0~ 1 radiation can be used for analysis of these elements.

Detection limit for gold by radioisotopic X-ray analysis SUMMARY 57Co with its 122-keV gamma-ray emission was found to be the best radioisotope for excitation of gold and most high-Z elements. Even though the primary Compton-scatter peak is completely separated from the Au K0tlines, multiple scatter causes a high background under the gold lines. Shielding consisting of a circular aperture placed in front of the detector, was successful in reducing the multiple scatter from inside the detector housing and resulted in a 23-per cent lower background under the gold lines. T h e excellent pulse-height resolution of the semiconductor detector produces resolved Au K~ 2 and K~ 1 lines. Better detection limits were achieved from X-ray intensity measurements of both lines even though a higher peak-to-background ratio would be achieved using only the K~I line. The peak-to-backgroud ratio, using both lines and the circular-aperture shield, was 0.34 for a 1000-ppm gold in quartz- type-ore standard. Detection limits below 100 ppm were achieved with the low peak-to-background ratios because of the inherent stability of semiconductor

403

detectors and excitation with radioisotopic sources. T h e detection limit for gold ranged from 29 to 37 ppm and was determined from both synthetic standards and analyzed gold ores. By increasing the source strength to 60 mCi, a detection limit of 21 p p m is possible in 3-rain counting intervals. T h e detection limits for other high-Z elements varied from 26 ppm for tungsten to 59 ppm for uranium. In conclusion, the X-ray method had adequate sensitivity for most high-Z elements except for gold ores. A valuable potential application exists as an ore process analyzer. The instrumental requirements are a radioisotopic source of 57Co, a germanium semiconductor detector, and electronic readout incorporating either a pulse-height analyzer or several singlechannel analyzers. REFERENCES 1. CHOW A. and BEAMISH F. E. Talanta 14, 219 (1967). 2. BURKHALTERP. G., Nuclear Techniques and Mineral Resource, STI/PUB/198, IAEA, Vienna, paper SM-112/18, 365 (1969).