Use of a Multichannel Analyzer for Electron Probe Microanalysis L. S.
BIRKS and A. P. BATT
U. S. Naval Research laboratory, Washingfon 25, D, C.
b A transistorized 400-channel multichannel analyzer attached to the electron probe microanalyzer has increased the versatility and decreased the time required for many analyses. Three types of application are illustrated. The multichannel analyzer replaces a scanning crystal spectrometer for quick identification and semiquantitative analysis of components. Quantitative analysis by mathematical unfolding of the energy spectra is made practical by the fast data collection of standard and unknown spectra; comparison of results by unfolding and from crystal spectrometer data shows differences of less than 4% of the amount present. Quantitative twodimensional topographic display is obtained by multiscaler operation of the 400-channel memory system.
I
s-ray fluorescence and electron probe microanalysis the use of gas proportional or scintillation detectors has become well known in recent years ( I , 9). Each x-ray quantum detected is converted to an electrical pulse whose amplitude is proportional to the x-ray quantum energy. Most s-ray circuits contain .single-channel pulse height analyzers, so that the characteristic quantum energies and hence the chemical elements may be distinguished from one another, although the resolution is orders of magnitude poorer than with crystal spectrometers. This separation according to energy is often called nondispersive analysis, but energy dispersion is a more descript’ive name. One disadvantage of the usual pulseheight analyzers is that, in order to scan the energy spectrum, one must set a narrow energy window and scan through the energy range one energy increment at a time. More recently, transistorized electronics have made possible the construction of compact multichannel analyzers in which thc whole energy spectrum is measured siniultancoualy. The operation is a s N BOTH
r1>11o\vs:
The s-ra.g quanta are detected b y a rc~~riiar y-ray proportional rounter and preamplifier circuit as before. I h l i quantum is converted to an electrical pulse of amplitude corresponding to the 778
ANALYTICAL CHEMISTRY
quantum energy. The input stage to the multichannel analyzer examines the pulse and stores it in the memory core according to its amplitude. Examination and storage of a pulse take approximately 30 pseconds and then the analyzer is ready to accept the next pulse. I n perhaps half a minute the analyzer has collected and stored a sufficient number of pulses to give a very complete picture of the whole x-ray spectrum reaching the detector. Upon command, the analyzer displays the energy spectrum on a cathode-ray tube for visual examination and/or prints out the number of pulses of each amplitude numerically or on a digital tape or plots the spectrum automatically on an X - Y plotter. The possible applications to x-ray spectrochemical analyses are unlimited. Thi- paper describes the initial experiments with a multichannel analyzer attached to the electron probe microanalyzer a t the Kava1 Research Laboratory. Three types of use are discussed : replacement of a scanning crystal spectrometer for quick identification and semiquantitative analysis of unknown precipitates or inclusions; quantitative analysis of multicomponent systems by unfolding of the overlapping energy spectra mathematically or graphically; and quantitative two-dimensional topographic display from beamscanning operation of the electron probe. EQUIPMENT AND OPERATION
The multichannel analyzer used is the T M C Model 404, manufactured by Technical hleasurements Corp.. New Haven, Corm. This is a completely transistorized compact unit with excellent versatility. The memory consists of 400 channels that can be used for detailed coverage of the energy spectrum or divided into four sets of 100 channels each for storage and comparison of four diqtinct spectra. This latter appears t’o be the most useful for the electron probe, because the energy range from, for instance, 1 to 20 k.e.y. can uwallj- be encompassed with sufficient detail in a 100-channel memory. In i w t , it, corresponcis to t,he ticttail :inliie~.ctlwitlt a I-volt, window set’tirig in the standard x-ray pulse-height analyzer circuits. The average storage t,imp pcr p i i l , ~i s 32 ,.ccrnnils a n d t,ot:il wutitiiig rate-; 01 20,000 to 30,000 c.11.“. are casily accommodated. Operation of the cyuipniciit with tlie
electron probe is completely straightforward. Either a sealed-off or a flow proportional counter is used with standard preamplifier to read the full x-ray signal (no crystal spectrometer) from any convenient port in the electron probe. The output signal goes directly to the multichannel analyzer, where the gain is adjusted as desired to position the spectrum within the 100-channel memory. For instance, the F e K a radiation may be peaked at about channel 25 for measurement of the middle range elements. By recording the spectra of several pure elements, relationships between channel number, peak intensity, and atomic number may be obtained for a given set of electron probe operating conditions. Figure 1 shows the results for elements from T i to G a for 26-k.e.v. electrons, 6’ take-off angle, and a mica window, senon-filled proportional counter. With the electron probe striking the desired specimen area, data are collected for some chosen “live” time, usually 1 minute or less, and the energy spectrum is displayed on the cathode ray tube. “Live” time means that the analyzer measures time only during the intervals when i t is ready to accept a pulse. Thus, the inconvenient deadtime corrections necessary with ordinary circuitry are eliminated. The first spectrum may be stored in the memory and the instrument switched to the becond set of 100 channels as the electron probe is moved to another position on the specimen. After four such spectra are collected, they may be displayed simultaneously for visual coniparison as shown in Figure 2 . Another mode of operation possible at the turn of a switch is illustrated below and called niultiscaler operation. Here the input stage of the instrument acts as a single-channel analyzer and allows the operator to select an) energy increment in the spectrum. The memory channels then act as time incrementsthat is, all the pulses in the selected energy increment are collected in the first memory cliannel during the first time increment, all those collected in the second time increment are stored in the second memory channel, etc. The sequencing is automatic, so that oiic iiisy O ~ J > C I Ic tlie di-tiihutioii o f , F e K a with time ah oiie sc‘aiii acr(~55the bpeciiiien continuously.
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Channel
20
30
40
number
Figure 1. Peak energy and intensity of x-ray spectra from Ti to G o plotted against channel number Values for Ti, Cr, Mn, GE, Cu, and Zn measured for 100% rtandords Values for V, Co, Ni, and Ga interpolated on basis of known excitation potentiol
/
\
5052
APPLICATIONS
Replacement of Scanning Spectrometer. One of t h e many uses of t h e electron probe is identification of unknown precipitates or inclusions in metals or minerals. Ordinarily this requires a scanning spectrometer rather t h a n fixed x-ray optics, because one does not know t h e elements i n advance. Even with a rapid scanning rate of 5 degrees 28 per minute for rough identification of major components, i t takes about 15 minutes to cover the usual range of elements from T i to 1290. With the multichannel analyzer a. total counting time of 0.4 minute is afficient for identification of all major constituents simultaneously. It is not that one oannot afford to take 15 minutes for a determination
Figure 2. Cathode-ray tube display of four distinct energy spectra Spectra from four different precipitates in Type
7075 aluminum alloy, showing varying amounts o f Fe, Cu, and Zn
Channel number
Figure 3. Energy spectra from precipitates in aluminum alloys containing gallium See Table I
if he has only a few determinations to make; rather, it is that he can learn so much more if he can make 20 to 25 different identifications in 15 minutes instead of a single determination. To illustrate the identification procedure several commercial aluminum alloys containing 2 to 3% added G a were examined. Table I shows the constituents of the alloys and Figure 3 shows energy spectra for typical precipitates in each alloy. From the calibration curve of Figure 1, one can easily identify the major constituents in each case ( l l g and -11 were not detectable with the operating conditions and detector used, but could be measured with a flow detector) and can make a rough estimate oi relative intensity of the constituents to one another. The 24 ST alloy, which contains over 4'3* Cu, does not +how Cu in tlie precipitates; initead, it sho\vl.s Mn, which is present only at the 0.57, level, along with Ga, which is present a t 2 to 3%. Similarly, the 75 S alloy, which contain, about 5yoZn, uhorv5 110 Zn in the precipitates; indead, it shows Cu along with the Ga. In the
Table 1. Composition of Precipitates in Aluminum Alloys Containing Added Gallium
Alloy
Av . composition,
5% type" 758 Zn 5.5, Mg 1.5, Cu 0.3, Cr 0.3, Ga 2 to 3 24ST Cu 4.5, Mg 1.5, Mn 0.6, Ga 2 to 3 5052 R l g 2.5, Cr 0.25, Ga 2 to 3 Gallium added.
Precipitate composition Ga plus Cu Ga plus Mn Ga or (;a plus Fo
5052 alloy most 1)rccii)itatci sliowctl primarily Ga, although occa~ioiially one was found with F e in addition to G n ; there is no indicated Fe contciit for the alloy. Quantitative Analysis by Unfolding of Overlapping Spectra. I n the spectra shown in Figure 2 , t h e p u 1 ~ amplitude distributions froni iieighboring elements were not completely VOL. 35,
NO. 7,
JUNE 1963
779
Table 11.
Element standard
Intensity Distributions from Fe, Cu, and Zn Standards
Peak intensity" above background
Fractional intensity a t position of element indicated Fe cu Zn 0.003 1.0 0.017 1.0 0.559 0.077 0.506 1.0 o.oti9
Fe 10765 cu 9730 Zn 7845 a Total counts collected in channel corresponding to peak of distribution in 0.4-11ii1I hie.
R,, I?,, etc. Table
Ill.
Intensities from Precipitates in 7075 Aluminum Alloy
(Measured intensity" a t position indicated) Ppt. Fe cu Zn 1 1015 659 680 2 1610 740 630 Total counts collected in indicated channel in 0.4 min.
= true relative intmsitie. froin element i, j , etc., in an unknown composition. These are the desired value< for use in quantitative analysis and are the solution. of the simultaneous equations
To illustrate practical applicatioii of the unfolding procedure we use precipitates in 7075 aluminum alloy (5.8% Zn, 2.3% Mg, 1.7% Cu, 0.2% Fe, 0.2% Cr, balance Al). These contain iron, copper, and zinc, all of which overlap each other. First, individual spectra from 100% standards of the three elements [Figure 4 (top)] weye recorded using the multichannel analyzer and the fractional intensities at the Fe, Cu, and Zn positions were measured (the fractional intensity of an element at its own position is necessarily unity). Table I1 shows the results. Next, the spectra from two unknown precipitates [Figure 4 (bottom)] were recorded and the total intensites were determined a t the Fe, Cu, and Zn positions as shown in Table 111. The desired values to be
Table IV. Unfolding and CrystalSpectrometer Data for Precipitates of Table 111
(Relative x-ray intensities) Crystal lgc- Unfolding, % spectrometer, '/b ment Ppt. 1 Ppt. 2 Ppt. 1 Ppt. 2 11.2 Fe S.6 14.3 8.6 cu 4.3 5.7 4.2 5,s Zn 5.6 4.0 4.9 4.0
resolved (this is always t h e case when proportional or scintillation detectors are used without crystal spectrometers). Thus t h e total intensity a t the copper position is t h e sum of the copper intensity plus the fractional contributions from the iron and zinc pulse-amplitude distributions at t'lie copper position. Dolby ( 2 ) has sho\m that a set of linear simultaneous equations may be written for the intensities a t the positions corresponding to each element. The set of equations for the individual components is then solved by usual algebraic methods. Each of the simultaneous equations takes the form I , = ziiizi
+~R,I~,P~, j
where I , = total intcrisity ( l e s 1)ac.E;ground) a t tlie position of element i as measured in an unknown compcsition Ii, = intensity from 1 0 0 ~ ostandard of element i at the position of element i Zji = intensity from 100% standard of any other elementj at the position of element j P , i = fractional intensity of element j a t the position of element i 780
ANALYTICAL CHEMISTRY
Channel number
Figure 4. Quantitative intensity measurements from Fe, Cu, and Zn standards and from two precipitates in 7075 aluminum alloy Data used in mathematical unfolding process described in text See Tables Il,111, and IV
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Figure 6.
Topographic display of precipitates in
7075 alloy in terms of FeKa radiation Using automatic beam sweep in electron probe Quantitative topographic display of same area using multichannel analyzer in multiscoler mode of operation a.
b.
100
number
Figure 5. Qucmtitative intensity measurements from Cr, Fe, and Ni standards and from sigma phase and matrix in Type 3 17 stainless steel See Table V
found are the relative x-ray intensities, RFe, Rc,, and RE,, for t be three elements in the precipitates. In precipitate 1 they are found from the three simultaneous equations : Iron.
+
1015 = 10765R~, 9730 X 0.077Rcu 7845 X 0.069Rz.
+
ik..
..."
"E 2
+
Copper. 659 = 10765 X 0.017R~. 9730Rcu 7845 X 0.506Rzn Zinc. 6SO = 10765 X 1).003R~,$. 9730 X 0.559Rcu 7845Rzn
+
+
Solution by determinants gives Rre = 8.8%; ReU = 4.3%; Rzn = 5.6%. Similarly, for precipitate 2 in which the right side of the abo-re equations remains the same, the solution gives R p . = 14.3%; Rcu =: 5.7%; Rz. 4.0%. As a check on the unfolding technique, crystal spectrometer data for the three standards and the precipitates were obtained simultaneously with the
mult'ichannel analyzer data. Relative x-ray intensities measured directly with the crystal spectrometers are compared with the unfolding data in Table IV. The agreement is excellent, with the relative standard deviation only &4% of the amount present. As a second example, Type 317 stainless steel (63% Fe,-18%-Cr, 13% Nil 3.2% Mo, 1.5% Mn) was treated to produce sigma-phase precipitates and examined in the same fashion. Here the three overlapping elements were Cr, Fe, and Ni. Figure 5 shows the standard pulse amplitude distributions and those of the sigma phase and matrix. Relative x-ray intensities by unfolding and by crystal spectrometers are shown in Table V. Again the agreement is excellent. On a theoretical basis it would be possible to perform the above experiments with a regular single-channel pulse-height analyzer. One would scan
Table V. Unfolding and CrystalSpectrometer Data for Type 3 17 Stainless Steel
(Relative x-ray intensities) Crystal Vnfolding, % spectrometer, yo Ele- Sigma Sigma ment phase Matrix phase Matrix Cr 20 13.2 19.5 12.5 Fe 29.4 35.8 29.0 35.0 Ni 4.9 7.0 Not measured
first the energy spectrum of each of the standards and then the spectra of the unknowns. The same mathematical treatment would apply. From a practical standpoint, however, such a procedure would hardly be feasible, because each spectrum would require at least 10 minutes (at comparable resolution and statistical precision) even if only 20 one-volt increments were used to cover just the peak of the distributions. This would mean a total of 50 minutes for three standards and two unknowns, and during that length of time the operating conditions of the electron probe would be likely to shift somewhat. VOL. 35,
NO. 7, JUNE 1963
781
With the multichannel analyzer, the whole process of data collection for the same three standards and two unknowns takes about 3 minutes. Unfolding the data from systems of four or more elements is just as straightforward as for three elements, but the number of simultaneous equations increases and is equal to the number of elements. Solution of determinants of more than 3 X 3 terms is tedious, however, except b y computers. From the degree of overlap observed for adjacent elements i t can be said that one would usually not need more than five terms in any equation, because the contribution from third neighbors is negligible unless one of the elements is a major constituent and the others are minor constituents (for a major constituent the tails of the distribution might be strong enough to interfere with minor constituents four or five elements away). Actually, electronic circuits available with the multichannel analyzer allow the operator to simulate an unknown spectrum by adding together arbitrary fractions of any number of component spectra. I n the summation, a separate background spectrum should be used as one of the components. Preliminary tests with the aluminum and stainless steel specimens gave erratic results because the background was not treated separately. Tests with spectra containing similar peaks but no background intensity gave reproducibility about as good as the mathematical unfolding. Quantitative Topographic Display. Most present electron probes have provision for automatic electron-beam sweeping and cathode-ray tube display of the specimen in terms of a selected element or in terms of electron current. Figure 6, a, shows a display of one of t h e precipitates in the 7075 aluminum alloy of the previous section in terms of F e K a radiation. Although t h e whole precipitate is richer in iron than the matrix, there seems to be a variation in iron content within the precipitate. Because of the small total size of the precipitate and the low total concentration of iron, i t is difficult to reach any quantitative conclusions about the variations. The multichannel analyzer allows the display to be placed on a more quantitative basis b y use of the multiscaler mode of operation described in the section on Equipment and Operation. First the input stage was set to select a small energy increment around the F e K a peak. Then the analyzer was set so that each channel would collect all the Fe quanta detected in 1 second.
782
ANALYTICAL CHEMISTRY
The electron beam sweep was arbitrarily set for 30 lines a t 10 seconds per line, giving a total of 300 seconds to cover the desired area and Corresponding to 300 channels in the memory. Readout of the memory is by electric typewriter set to print 10 columns across corresponding to one scan line. Figure 6, b, shows the topographic diqday with contours drawn in to deliniate the Fe-rich regions. Of course, the contours do not represent sharp concentration boundaries because of the resolution of the electron probe. However, they do represent real variations within the precipitate, as may be illustrated by considering the two shaded areas of Figure 6, b, drawn to include six readings each and each area corresponding to about 1 square micron. The sum of the readings in one area is 2540 counts, giving a standard deviation of 3=50 by usual x-ray statistics. The sum of the readings in the other area is 2218 counts, with a standard deviation of &47. The difference between the two sums is 322 counts or a difference of more than 6u, well outside of any possible random fluctuations.
DISCUSSION
One of the shortcomings of both crystal spectrometers and single -channel analyzers is that they measure only a small fraction of the total information available at any one time-that is, they ignore all but one wavelength or one energy increment and effectively waste all of the other radiation. On the other hand, the multichannel analyzer uses all the information reaching the detector. Thus, there is considerable saving in time necessary to collect the same amount of data. From the initial tests run a t KRL, it appears that the greatest advantage of the multichannel analyzer comes from using i t in conjunction with the regular x-ray optics. T h a t is, one port of the electron probe is used for the multichannel analyzer while the others are used for crystal spectrometers with their better resolution of elements of special interest. Thus one has quick quantitative determination of the most important elements from the spectrometers plus the assurance that no other elements of interest are overlooked. Furthermore, the spectra collected by the multichannel analyzer may be placed on a quantitative basis whenever appropriate by the mathematical unfolding described above. It is difficult at this early stage to evaluate all the powihle implications of the quantitative
topographic display, hut it is certain to be of great value. For the low-atomic-number element.. from N a to C, where intensities arc low and suitable crystals are difficult to obtain, the application of the multichannel analyzer and the unfolding technique is probably even more important than for the middle-range elements, but data for light elements are not available as yet for test. The number oi light elements is limited, so that few equations are needed for the unfolding process when only light elements are present. If heavier elements are also present, their L or M spectra may overlap the region of interest and complicate the problem of unfolding. The proper detector is, of course, a flow proportional counter with front and back windows, so that harder radiation will pass through without being detected. dlthough not tested as yet, it seems likely that a sealed-off xenon proportional counter can be used in back of the flow counter to detect the harder radiation that passes through the flow counter. Signals from both detectors would go to the multichannel analyzer for simultaneous display of the full energy spectrum. One question always to be considered in adding new equipment is the cost. Although prices vary considerably, the cost of a curved crystal scanning spectrometer and necessary associated electronics seems to be in the range of $8000 to $12,000. The basic equipment necessary with a multichannel analyzer costs about $15,000 and other desirable equipment brings the cost to about $20,000. For the versatility alone the added expense is well justified. If the time savings are considered, the multichannel analyzer more than pays for itself. ACKNOWLEDGMENT
We thank J. L. Jamison of Q.E.D. Electronic Sales for numerous helpful comments during our initial adaptation of the multichannel analyzer for soft x-ray measurements. LITERATURE CITED
(1) Birks, L. S., “X-Ray Spectrochemical
Analysis,” Interscience, Xew York, 19.59
(2iD&lby, R. M., Proc. Phys. SOC.73, 81 (1959). (3) Liebhafsky, H. A.. Pfeiffer, H. G., Winslow, E. H., Zemany, P. D., “X-Ray Absorption and Emission in Analytical Chemistry,” Wiley, New York, 1960. RECEIVEDfor review February 7, 1963. Accepted March 29, 1963. Midwest Spectroscopy Symposium, Chicago, Ill., May 1963.
