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Multichannel instruments for fluorescent X-ray spectroscopy
SpectrocNmka
Acta, 1965, Vol. 7, pp. 141 to 148.
Pergamon Press Ltd. London
Multichannel instruments for fluorescent X-ray spectroscopy J. W. KEMP, M. F. HASLER, J. L. JONES, and LOUTS ZEITZ Applied Research Laboratories, Glendale, Calif. (Received 5 August 1954) (Accepted Jo7 publication 8 December 1954)
Summary-Two multichannel instruments for fluorescent X-ray spectroscopy are described in detail, the Applied Research Laboratories’ X-ray Research Quantometer and X-ray Industrial Quantomeber. Each instrument uses a 60 kV, full-wave, filtered supply with a Machlett OEG-60 tube. The XRQ has been used with multiple flat-crystal spectrometers; the XIQ with multiple The recording circuits are very similar to those used flat- and curved-crystal spectrometers. in the Applied Research Laboratories’ line of optical emission Quantometers.
Although the basic principles of fluorescent X-ray analysis have been known for a long time, serious attempts to use this method have been m&de only in the last three or four years. A review of the current literature indicates that a number of laboratories have studied this method of analysis [l, 2, 3, 4, 5, 6, 71. Generally, two types of instruments have been used: Commercial diffraction units converted to crystal spectrometers, or special instruments constructed in the various laboratories. Practically all of this equipment provides only single-channel recording. It was felt that a completely new set of instruments was required to meet the needs of the industrial analyst. A study of this problem over a period of several years has led to the design of the instruments to be described here. The X-ray Research Quantometer (XRQ) was the first instrument to be designed on the basis of this study. Experience with this instrument led to the design of t,he X-ray Indust,rial Quantometer (XIQ). The
X-ray tube supply
The basic circuit used is a full-wave rectifier. The usual secondary current meter, flow-sensing switch, master switch, etc., have been used. Since the unit is intended for use with various non-dispersive instruments where accurate voltage control is important, as well as with dispersive units, the supply is filtered and a resistancetype kilovoltmeter in the secondary is used. The specifications for the standard unit are given in Table 1. Since ratios of intensities rather than absolute intensities are measured with the Applied Research Laboratories’ X-ray Quantometers, Table 1. Power
Input . output Rectifier Filter . Ripple
. . . . .
. . . .
. . . . .
Voltmeter resistors Voltmeter accuracy Current control . Voltage control .
* . . .
. . . .
. . . . .
supply
230 V, 50-60~
specificdons
60 kV, 30.5 ma or 37.5 kV, 50 ma max. Two G.E. KR-7 tubes 0.1 ,uF 3.0 per cent at 37.5 kV 50 ma IO.5 per cent at 10 kV 50 ma ~&TWO megohm f 1 per cent pyrolytic carbon resistors 2 per cent Rheostat in filament transformer primary Variable transformer in power transformer primary 141
J. W. KEUE+,M. F. HASLER, J. L. JONES, and LOUIS ZEITZ
extreme regulation of the supply is not required. This was demonstrated, as shown in Table 2, by measurement of the precision of stainless-steel analysis with the X-ray Research Quantometer with O-1 per cent input voltage regulation and with the X-ray Industrial Quantometer with no regulation. Conditions were such that the precision was limited by the maximum intensity allowable with the receivers used, so that other differences between the instruments did not enter into the experiment. Table 2
XXI Element
Iron Nickel Chromium Per cent standard deviation of rtbsolu* Fe intensity
-
XIQ
CoegEcient of
Concentration
vardation’
%
%
71.0 9-12 17.48
Coeficiend of
COT&C6??U?Uk?lvariation* -
*0*5 fl.0 51-l kO.7
%
%
72-Q 8.64 17.14
*to*4 *1*0 hl.1 f 10.6
* The coefficient of variation is the standard deviation of the concentration divided by the concentration times 100. It should be noted that the probable error (P.E.), used by some workers, is 067 times the standard deviation.
The percent standard deviations and the coefficients of variation were determined from twenty runs of one minute each. Although satisfactory results can be obtained with no regulation, tube filaments should be prot,ected by the use of moderate (1 per cent) regulation. Physically, the supply is housed in a bench-height cabinet (Fig. 6) constructed so that various spectrometers can be mounted on top. The high-voltage section, shown in Fig. 1, is completely oil-immersed and features a hermetically sealed unit containing the power and filament transformers. This sealed portion is filled with a specially processed high-dielectric strength oil and is contained within the same water-cooled tank as the remainder of the high-voltage components. These included the voltmeter resistors, rectifiers, and capacitor terminals. In later models, the entire capacitor has been included in the tank. Connection to the X-ray tube is made with standard X-ray cable, so that no unprotected high-voltage points exist in the unit.
The spectrometers Three types of spectrometers have been used: non-dispersive, flat-drystal dispersive, and curved-crystal dispersive. Each type has its field of usefulness, although it appears that curved crystals will eventually replace the flat crystals altogether. The non-,dispersive receivers are made up of Geiger counters or scintillators and filter holders; in other words, they serve as X-ray filter photometers. These can be used in simple analytical problems and in thickness gauging. For example, Table 3 gives the data for zinc-dipped steel plate. Here, iron radiation is excited in the baseplate and measured after absorption by the zinc coating. Precision data are given of dipped plate, for twenty-one runs on a single area. Due to the non-uniformity large areas must be integrated to obtain accuracies approaching this precision. 142
Fig. 1. High-voltage, X-ray tube supply, shown without tank.
Fig. 3.
Motor-driven
flat-crystal
spectrometer.
Fig. 5. X-ray Research Quantometer, showing two dispersive spectrometers (right and left) and three non-dispersive units (centre). Filter and sample drawers are shown open.
Multichannel instruments for fluorescent X-ray spectroscopy
The XRQ was also designed so that samples could be placed in the filter positions and an absorpt,ion analysis carried out using a variety of samples as For all but the simplest analytical problems, dispersive secondary emitters. analyzers are required. Most of the work at the Applied Research Laboratories has been done with flat crystals. The general arrangement used is shown in Fig. 2.
Fig. 2. Schematic drawing of arrangement, of multiple flat-crystal spectrometers.
Since high dispersion is desirable, aluminium* or lithium fluoride single crystals are used. The (200) planes of aluminium are roughly equal in intensity and dispersion Table 3. Zinc p&z& gauging Tube . . . . Power . . . . Fe Channel Filter . . Exposure . . . . Receivers . . . , Precision . Zn thickness’(oz/sq ft)t ’ Coefficient of variation (%)
. . . . . ’
.
.
. . . .
. . . .
‘o-33. 0.4
Machlett AEG-BOT, Tungsten Target 15 kV, 20 ma 0.0030~cm Fe Sheet 16 sec. Scintill~tore 0.55 0.3
065 0.4
10l 0.3
1.22 1.9
1.35 1.7
Generally, 4” collimators are used where to the (200) planes of lithium fluoride. All flat-crystal spectrometers were possible, and )” collimators are available. designed for the l- to 3-angst.rom range, although they can be used down to 0.7 A with some loss in intensity due to vignetting. Siqgle, curved LiF or NaCl crystals have been developed for the XI&-type instrument, whose planes are cylindrically bent parallel to a given radius and the surfaces cylindrically ground to one-half this radius. These crystals are used in focusing spectrometers in the same manner as concave gratings are used in Rowland-type mountings. A 56-cm diameter spectrometer is used from 0.35 to 1-OA, and a 20-cm diameter spectrometer from 1-O to 4.0 8. Some work has also been done above 4 A with a lo-cm diameter helium atmosphere unit. The XRQ is provided with motor- and hand-driven spectrometers (Fig. 3) as well as with only hand-driven units. The XIQ is provided with semi-fixed units A scanning arrangement is provided designed to be set to a particular wavelength. to allow scanning over a few degrees for final. alignment. * Obtainable from Horizons, Inc., Cleveland 4, Ohio. t Note:
1 oz/sq ft = 30.6 m&q
cm. 143
J. IV. KEMP, M. F. HASLER, J. L. JONES, snd LOUIS ZEITZ
The receivers and recording circuits On the basis of sensitivity requirements, Geiger-Miiller tubes were designed int’o the In this work, the usual procedure for measuring integrated original instruments. X-ray intensities has been to use a Geiger tube and to record the number of counts in a given time or the time for a predetermined number of counts. It is well known INTEGRATING CAPACITORS
,
ii_
,__, 4
ZERO
r -
-
-
-
-
-
-i
AND
SENSllIVITY CONTROLS
!__-_---I
ATTENUATORS
Fig. 4. Simplified diagram of receiving snd recording circuits.
that the standard deviation of a total count is the square root of the number of counts. This gives 1 per cent standard deviation for 10,000 counts and 0.3 per cent, for 100,000 counts. This applies, however, only when all factors involved-source, power supply, etc.-are held absolutely constant. Since absolutje constancy of all factors is difficult to attain in practice, it was decided to use the ratio system t’hat has been used so successfully in optical spectroscopy. A simplified circuit diagram of this system is shown in Fig. 4. All of the Geiger pulses are collected by t#he integrating capacitors and the voltages attained by the capacitors at the end of the integration period are read by a standard Applied Research Laboratories’ Electrometer Amplifier. All of the circuitry t’o the right of the dotted line is exact,ly the same as t’hat used in t,he Applied Research Laboratories’ Optical Quantjometers. In this circuit’, the logarithm of the Geiger-tube output is a linea,r function of Geigertube voltage in the “plateau” region, much the same as the characteristic of a multiplier phototube in the medium-voltage range. The high-voltage supply designed for the X-ray instruments consists of an electronically regulated unit of the same design as that used for the phototubes in the latest optical Quantometers. For high pulserates, sensitive, short-dead-time Geiger tubes are required. In order to have reasonable values of integrating capacitance, small average pulse size is desirable. This can be cont,rolled to some extent by attenuat.ion of the Geiger-tube voltage, but only in the plateau region. Capacitance could be reduced by charging t,o higher than the 4-volt maximum used. This, however, is undesirable, since the integrator voltage reduces t,he effective Geiger-t,ube voltage as the charge builds up and contribut,es to nonlinearity of the system. A study of commercially available Geiger tubes indicated t’hat’ approximately 60,000 counts could be integrated over 1 minute, using capacitances of the order of 50 ,uF. This condit’ion is obtained by using Anton 202T tubes at 1375 volts. In 144
Fig. 6. X-ray Industri~l Quantometor, including console.
Fig. 7. Spectrometers and sample handling mechanism, X-ray Industrial Quantometer.
Multichannel instruments
for fluorescent X-ray
spectroscopy
operat#ion, one Geiger tube receives radiation by some means and utilizes it for control just as the internal standard system is utilized for control in optical emission analysis. In a non-dispersive system, this radiation could be from the sample being analyzed or from a standard sample, with appropriate filters in each case. In a multichannel dispersive system, it could be an internal standard line in the sample, or a line from an external standard sample. The output of this Geiger tube Upon reaching a predetermined is int,egrated and recorded dutiing the exposure. value, the recorder disconnects all Geiger tubes. Automatic switching then connects each integrator in turn to the amplifier and recorder, switching in the proper sensitivity and zero controls at the same time. Thus, the ratio of integrated intensity of each channel to the standard channel is recorded. This allows the same recording and calibrating techniques to be used that are used with the optical Quantometers. The primary advantage of this system is its ability to record a number of elements in the time required to record one element with the usual scanning spectrometer. A second advantage is the higher precision obtained under industrial operating conditions, as can be seen from the data given in Table 2. There is one disadvant’age inherent in this system, however. Intensities should be limited to 60,000 counts per minute because of Geiger-tube non-linearity, and for many problems much higher intensities than this are available. The 60,000-count limit means that the maximum precision of the ratio of two channels obtainable in one minute is f0.6 per cent standard deviation. To make full use of the intensities available, a scintillator has been designed consisting of Patterson CB-2 screen material deposit’ed on an RCA 6199 multiplier phototube. Since a non-dispersive head viewing an external standard is generally used for control, there is always sufficient intensity to allow the use of a scintillator in the standard channel. This improves the maximum precision obtainable in one minute to f0.4 per cent standard deviation. When scintillators can be used in both channels, precision improves to IfO*l-0.2 per cent standard deviation. Where scintillators can be used, 2 ,uF rather than 50 ,uF are required for integration.
The X-ray research quantometer The XRQ was the first instrument to be designed, and has been used for much of the ana.lytical research in the authors’ laboratories. It requires a separate base cabinet, but uses the same power supply and console as the XIQ. Eight positions have been provided around an end-windowed X-ray tube in which may be placed any combination of dispersive or non-dispersive receivers, as shown in Fig. 5. (A similar instrument has been described by ADLER and AXELROD, [9].)Each receiver is mounted upon a track, so that it may be positioned above either of two ports. The inner port, in each case, receives the radiation from a central sample position directly below the X-ray tube, while the outer port receives radiation from a secondary position which can be used for a reference standard. The ports are provided with automatic shutters to allow removal or replacement of the receivers without danger to the operator. The unknown and reference.samples are placed in a sliding compartment which is positioned under the X-ray tube. Fig. 5 shows this compartment pulled out to loading position. A safety shield automatically covers the internal X-ray ports when this compartment is drawn out, 145
J. W. KEBW, M. F. HASLER, J. L. JONES,end LOUIS ZEITZ
providing complete protection for the operator. Between the sample compartment and the receiver section is another sliding compartment providing for the use of the various filters and apertures which may be required in any of the receivers. In this compartment m?y also be placed cells for use in absorption work. In conjunction with the sample compartment, these cells may be used in the beam from a secondary emitter. Should it be desirable to have the cells in the primary beam of the X-ray tube, the tube may be placed below the cells and pointing upward. This filter compartment is also provided with an automatic safety shield. The integrating capacitors for the receivers are mounted in the cabinet and the front panel contains switches for selecting the capacitance values for each channel as well as attenuators for controlling the Geiger voltage.
The X-ray industrial quantometer The complete XIQ, shown in Fig 6, provides for eight channels with any combination of dispersive or non-dispersive receivers. The eight receivers are housed in a single enclosure and the entire spectrometer (Fig. 7) mounts directly on top of the source unit. In the centre of the housing is the end-on OEG-60, tungsten target X-ray tube placed so that its window is 5.6 cm from the horizontal surface of the sample, which is held by spring pressure against a flat aperture plate to assure that the A lead aperture in front of the window tube-to-sample distance is constant. of the X-ray tube permits X-rays to fall only upon the usable area of the sample, thus reducing extraneous scattered radiation to a minimum. The sample surface forms part of the walls of a compl_etely enclosed brass chamber in which the collimators or slits for the eight receivers are mounted. The ends of these collimators are 5 cm from the centre of the sample and are spaced at equal angles around the X-ray tube. They are placed at an angle of 47’ from the axis of the tube. For the flat-crystal receivers, the collieators are of the Soller slit type and consist of thin parallel sheets of phosphor bronze held in a square tube 20.4 cm long with sheet spacings to provide various The remainder of the flat-crystal receiver consists of a resolutions as required. simple and rugged assembly providing for a single flat crystal of lithium fluoride, aluminium, or other material, and a Geiger counter. An external vernier control is provided for rotation of the crystal through a simple linkage, so that after the initial rough setting to a given spectral line, the crystal may be oriented accurately during X-ray excitation to set it upon the peak of the line. Means are provided for adjustment of the position of t’he collimator in rotation about its own axis for accurate alignment with the axis of rotation of the crystal, so as to insure maximum resolution. The curved-crystal units designed for use in the XIQ are also of semi-fixed design, with means provided for setting to a particular wavelength. The receiver, shown schematically in Fig. 8, consists of a primary slit, a curved crystal, a secondary slit, and a Geiger counter or scintillator. The primary slit, of fixed design, is-provided in various widths to provide a balance between resolution and intensity requirements, and is held in a fixed relation to the sample at a distance of 5.1 cm. The single crystal, of lithium fluoride, rock salt, or other material, is bent, ground, The and polished accurately to provide maximum intensity and resolution. 146
secondary slit is similar to the primary slit and is also provided in various widths to fit a particular problem. As in the flat-crystal unit, the spectrometer may be scanned through a few degrees for final alignment. The nondispersive receivers utilize a simple tube collimator to prevent the Geiger tube from “seeing” radiation other than that proceeding from the sample. Filter holders for 1.27 cm diameter briquettes or foil discs are integral with the collimators. Scintillators may be used in place of Geiger tubes. CRYSTAL
Fig. 8. One or more positions may be provided for external standard samples for use with either nondispersive or dispersive channels. A specially designed samplehandling mechanism forms an integral part of the spectrometer. This sample changer incorporates a labyrinth path in all directions from the X-ray chamber to insure complete safety for the operator at all times and to avoid the necessity for turning off the X-rays while changing samples. Placement or removal of the sample from its holder requires only a simple one-hand motion to compress the holding spring with the sample. A mirror allows accurate positioning of the sample surface with respect to the X-ray beam. Two positions are provided so that a sample may be placed in its holder while another is being analyzed. It then requires the motion of a single control lever to reverse the positions and allow analysis of the second sample. The high-voltage X-ray cable, X-ray tube water-supply and return, and the Geiger power- and signal-leads are all enclosed within a single sheath which connects the spectrometer to the X-ray source upon which it is mounted. The console containing the electronic supplies and recording circuits is essentially t,he same as the unit used with the Applied Research Laboratories Optical Quantometers. If desirable, a single console can be used to record the outputs of an optical and an X-ray spectrometer. This suggests the use of a combined optical-X-ray laboratory; the Optical Quantometer for low concentrations and the X-ray Quantometer for high concentrations. LidtdiOlM
These instruments are not suitable in their standard form for the analysis of low concentrations of elements lower than atomic number 20. The limits of detectability obtainable for the remaining elements are a function of the base material as well as of the various instrumental factors involved. The limit, defined as that 147
J. W. KEMP, M. F. HASLER, J. L. JONES, and LOUIS ZEITZ
concentration at which coefficient of variation of 25 per cent will be obtained, varies from 1 ppm to several thousand parts per million. Precision may be limited by the intensity available, the intensity limitations of Geiger counters, or the precision of the circuits. In the first case, curved crystals offer a considerable advantage, and in simple systems, non-dispersive techniques can be of help. In the second case, the use of curved crystals or non-dispersive techniques can often provide sufficient intensities to allow the use of scintillators. Where sointillators can be used, coefficients of variation of 0.1 to 0.5 per cent can be expected. The electronics used have-been shown to provide percent standard deviations of intensity ratios of 0.1-0.2 per cent. Accuracy is a function of instrumentation, excellence of standards, and the care with which working curves are constructed. Where the last two points have been properly considered, accuracies approaching the precision of the system are obtainable.
References 111FRIEDMAN, H., BIRRS, L. S., and BROOKS, E. J.: A.S.T.M. Special Technical Publication No. 157, p. 3 (1954) 121 SHERMAN, J. ; A.S.T.M. Special Technical Publication No. 157, p. 27 (1954) [31 BRISSEY, R. M., LIEBHAFS~Y, H. A., and PFEIFFER, H. G.; A.S.T.M. Special Technical Publication No. 157, p. 43 (1954) [41 NOAKES, GORDEN E.; A.S.T.M. Special Technical Publication No. 157, p. 57 (1954) [51 CARL, HOWARD F., and CAMPBELL, WILLIAM J.; A.S.T.M. Special Technical Publication No. 157, p. 63 (1954) [61 DAVIS, ELWIN N., and VAN NORDSTRAND, ROBERT A.; And. Chem. 1954 20 973 [71 MORTIMORE, D. M., ROMANS, P. A., and TEWS, J. L.; Applied Spectroscopy 1954 8 24 PI BIRKS, L. S., BROOKS, E. J., and FRIEDMAN, H.; Anal. Chem. 1953 25 692 191 ADLER, ISIDORE, and AXELROD, JOSEPH M.; J. Opt. Sot. Amer. 1963 48 769
148
Acta, 1965, Vol. 7, pp. 141 to 148.
Pergamon Press Ltd. London
Multichannel instruments for fluorescent X-ray spectroscopy J. W. KEMP, M. F. HASLER, J. L. JONES, and LOUTS ZEITZ Applied Research Laboratories, Glendale, Calif. (Received 5 August 1954) (Accepted Jo7 publication 8 December 1954)
Summary-Two multichannel instruments for fluorescent X-ray spectroscopy are described in detail, the Applied Research Laboratories’ X-ray Research Quantometer and X-ray Industrial Quantomeber. Each instrument uses a 60 kV, full-wave, filtered supply with a Machlett OEG-60 tube. The XRQ has been used with multiple flat-crystal spectrometers; the XIQ with multiple The recording circuits are very similar to those used flat- and curved-crystal spectrometers. in the Applied Research Laboratories’ line of optical emission Quantometers.
Although the basic principles of fluorescent X-ray analysis have been known for a long time, serious attempts to use this method have been m&de only in the last three or four years. A review of the current literature indicates that a number of laboratories have studied this method of analysis [l, 2, 3, 4, 5, 6, 71. Generally, two types of instruments have been used: Commercial diffraction units converted to crystal spectrometers, or special instruments constructed in the various laboratories. Practically all of this equipment provides only single-channel recording. It was felt that a completely new set of instruments was required to meet the needs of the industrial analyst. A study of this problem over a period of several years has led to the design of the instruments to be described here. The X-ray Research Quantometer (XRQ) was the first instrument to be designed on the basis of this study. Experience with this instrument led to the design of t,he X-ray Indust,rial Quantometer (XIQ). The
X-ray tube supply
The basic circuit used is a full-wave rectifier. The usual secondary current meter, flow-sensing switch, master switch, etc., have been used. Since the unit is intended for use with various non-dispersive instruments where accurate voltage control is important, as well as with dispersive units, the supply is filtered and a resistancetype kilovoltmeter in the secondary is used. The specifications for the standard unit are given in Table 1. Since ratios of intensities rather than absolute intensities are measured with the Applied Research Laboratories’ X-ray Quantometers, Table 1. Power
Input . output Rectifier Filter . Ripple
. . . . .
. . . .
. . . . .
Voltmeter resistors Voltmeter accuracy Current control . Voltage control .
* . . .
. . . .
. . . . .
supply
230 V, 50-60~
specificdons
60 kV, 30.5 ma or 37.5 kV, 50 ma max. Two G.E. KR-7 tubes 0.1 ,uF 3.0 per cent at 37.5 kV 50 ma IO.5 per cent at 10 kV 50 ma ~&TWO megohm f 1 per cent pyrolytic carbon resistors 2 per cent Rheostat in filament transformer primary Variable transformer in power transformer primary 141
J. W. KEUE+,M. F. HASLER, J. L. JONES, and LOUIS ZEITZ
extreme regulation of the supply is not required. This was demonstrated, as shown in Table 2, by measurement of the precision of stainless-steel analysis with the X-ray Research Quantometer with O-1 per cent input voltage regulation and with the X-ray Industrial Quantometer with no regulation. Conditions were such that the precision was limited by the maximum intensity allowable with the receivers used, so that other differences between the instruments did not enter into the experiment. Table 2
XXI Element
Iron Nickel Chromium Per cent standard deviation of rtbsolu* Fe intensity
-
XIQ
CoegEcient of
Concentration
vardation’
%
%
71.0 9-12 17.48
Coeficiend of
COT&C6??U?Uk?lvariation* -
*0*5 fl.0 51-l kO.7
%
%
72-Q 8.64 17.14
*to*4 *1*0 hl.1 f 10.6
* The coefficient of variation is the standard deviation of the concentration divided by the concentration times 100. It should be noted that the probable error (P.E.), used by some workers, is 067 times the standard deviation.
The percent standard deviations and the coefficients of variation were determined from twenty runs of one minute each. Although satisfactory results can be obtained with no regulation, tube filaments should be prot,ected by the use of moderate (1 per cent) regulation. Physically, the supply is housed in a bench-height cabinet (Fig. 6) constructed so that various spectrometers can be mounted on top. The high-voltage section, shown in Fig. 1, is completely oil-immersed and features a hermetically sealed unit containing the power and filament transformers. This sealed portion is filled with a specially processed high-dielectric strength oil and is contained within the same water-cooled tank as the remainder of the high-voltage components. These included the voltmeter resistors, rectifiers, and capacitor terminals. In later models, the entire capacitor has been included in the tank. Connection to the X-ray tube is made with standard X-ray cable, so that no unprotected high-voltage points exist in the unit.
The spectrometers Three types of spectrometers have been used: non-dispersive, flat-drystal dispersive, and curved-crystal dispersive. Each type has its field of usefulness, although it appears that curved crystals will eventually replace the flat crystals altogether. The non-,dispersive receivers are made up of Geiger counters or scintillators and filter holders; in other words, they serve as X-ray filter photometers. These can be used in simple analytical problems and in thickness gauging. For example, Table 3 gives the data for zinc-dipped steel plate. Here, iron radiation is excited in the baseplate and measured after absorption by the zinc coating. Precision data are given of dipped plate, for twenty-one runs on a single area. Due to the non-uniformity large areas must be integrated to obtain accuracies approaching this precision. 142
Fig. 1. High-voltage, X-ray tube supply, shown without tank.
Fig. 3.
Motor-driven
flat-crystal
spectrometer.
Fig. 5. X-ray Research Quantometer, showing two dispersive spectrometers (right and left) and three non-dispersive units (centre). Filter and sample drawers are shown open.
Multichannel instruments for fluorescent X-ray spectroscopy
The XRQ was also designed so that samples could be placed in the filter positions and an absorpt,ion analysis carried out using a variety of samples as For all but the simplest analytical problems, dispersive secondary emitters. analyzers are required. Most of the work at the Applied Research Laboratories has been done with flat crystals. The general arrangement used is shown in Fig. 2.
Fig. 2. Schematic drawing of arrangement, of multiple flat-crystal spectrometers.
Since high dispersion is desirable, aluminium* or lithium fluoride single crystals are used. The (200) planes of aluminium are roughly equal in intensity and dispersion Table 3. Zinc p&z& gauging Tube . . . . Power . . . . Fe Channel Filter . . Exposure . . . . Receivers . . . , Precision . Zn thickness’(oz/sq ft)t ’ Coefficient of variation (%)
. . . . . ’
.
.
. . . .
. . . .
‘o-33. 0.4
Machlett AEG-BOT, Tungsten Target 15 kV, 20 ma 0.0030~cm Fe Sheet 16 sec. Scintill~tore 0.55 0.3
065 0.4
10l 0.3
1.22 1.9
1.35 1.7
Generally, 4” collimators are used where to the (200) planes of lithium fluoride. All flat-crystal spectrometers were possible, and )” collimators are available. designed for the l- to 3-angst.rom range, although they can be used down to 0.7 A with some loss in intensity due to vignetting. Siqgle, curved LiF or NaCl crystals have been developed for the XI&-type instrument, whose planes are cylindrically bent parallel to a given radius and the surfaces cylindrically ground to one-half this radius. These crystals are used in focusing spectrometers in the same manner as concave gratings are used in Rowland-type mountings. A 56-cm diameter spectrometer is used from 0.35 to 1-OA, and a 20-cm diameter spectrometer from 1-O to 4.0 8. Some work has also been done above 4 A with a lo-cm diameter helium atmosphere unit. The XRQ is provided with motor- and hand-driven spectrometers (Fig. 3) as well as with only hand-driven units. The XIQ is provided with semi-fixed units A scanning arrangement is provided designed to be set to a particular wavelength. to allow scanning over a few degrees for final. alignment. * Obtainable from Horizons, Inc., Cleveland 4, Ohio. t Note:
1 oz/sq ft = 30.6 m&q
cm. 143
J. IV. KEMP, M. F. HASLER, J. L. JONES, snd LOUIS ZEITZ
The receivers and recording circuits On the basis of sensitivity requirements, Geiger-Miiller tubes were designed int’o the In this work, the usual procedure for measuring integrated original instruments. X-ray intensities has been to use a Geiger tube and to record the number of counts in a given time or the time for a predetermined number of counts. It is well known INTEGRATING CAPACITORS
,
ii_
,__, 4
ZERO
r -
-
-
-
-
-
-i
AND
SENSllIVITY CONTROLS
!__-_---I
ATTENUATORS
Fig. 4. Simplified diagram of receiving snd recording circuits.
that the standard deviation of a total count is the square root of the number of counts. This gives 1 per cent standard deviation for 10,000 counts and 0.3 per cent, for 100,000 counts. This applies, however, only when all factors involved-source, power supply, etc.-are held absolutely constant. Since absolutje constancy of all factors is difficult to attain in practice, it was decided to use the ratio system t’hat has been used so successfully in optical spectroscopy. A simplified circuit diagram of this system is shown in Fig. 4. All of the Geiger pulses are collected by t#he integrating capacitors and the voltages attained by the capacitors at the end of the integration period are read by a standard Applied Research Laboratories’ Electrometer Amplifier. All of the circuitry t’o the right of the dotted line is exact,ly the same as t’hat used in t,he Applied Research Laboratories’ Optical Quantjometers. In this circuit’, the logarithm of the Geiger-tube output is a linea,r function of Geigertube voltage in the “plateau” region, much the same as the characteristic of a multiplier phototube in the medium-voltage range. The high-voltage supply designed for the X-ray instruments consists of an electronically regulated unit of the same design as that used for the phototubes in the latest optical Quantometers. For high pulserates, sensitive, short-dead-time Geiger tubes are required. In order to have reasonable values of integrating capacitance, small average pulse size is desirable. This can be cont,rolled to some extent by attenuat.ion of the Geiger-tube voltage, but only in the plateau region. Capacitance could be reduced by charging t,o higher than the 4-volt maximum used. This, however, is undesirable, since the integrator voltage reduces t,he effective Geiger-t,ube voltage as the charge builds up and contribut,es to nonlinearity of the system. A study of commercially available Geiger tubes indicated t’hat’ approximately 60,000 counts could be integrated over 1 minute, using capacitances of the order of 50 ,uF. This condit’ion is obtained by using Anton 202T tubes at 1375 volts. In 144
Fig. 6. X-ray Industri~l Quantometor, including console.
Fig. 7. Spectrometers and sample handling mechanism, X-ray Industrial Quantometer.
Multichannel instruments
for fluorescent X-ray
spectroscopy
operat#ion, one Geiger tube receives radiation by some means and utilizes it for control just as the internal standard system is utilized for control in optical emission analysis. In a non-dispersive system, this radiation could be from the sample being analyzed or from a standard sample, with appropriate filters in each case. In a multichannel dispersive system, it could be an internal standard line in the sample, or a line from an external standard sample. The output of this Geiger tube Upon reaching a predetermined is int,egrated and recorded dutiing the exposure. value, the recorder disconnects all Geiger tubes. Automatic switching then connects each integrator in turn to the amplifier and recorder, switching in the proper sensitivity and zero controls at the same time. Thus, the ratio of integrated intensity of each channel to the standard channel is recorded. This allows the same recording and calibrating techniques to be used that are used with the optical Quantometers. The primary advantage of this system is its ability to record a number of elements in the time required to record one element with the usual scanning spectrometer. A second advantage is the higher precision obtained under industrial operating conditions, as can be seen from the data given in Table 2. There is one disadvant’age inherent in this system, however. Intensities should be limited to 60,000 counts per minute because of Geiger-tube non-linearity, and for many problems much higher intensities than this are available. The 60,000-count limit means that the maximum precision of the ratio of two channels obtainable in one minute is f0.6 per cent standard deviation. To make full use of the intensities available, a scintillator has been designed consisting of Patterson CB-2 screen material deposit’ed on an RCA 6199 multiplier phototube. Since a non-dispersive head viewing an external standard is generally used for control, there is always sufficient intensity to allow the use of a scintillator in the standard channel. This improves the maximum precision obtainable in one minute to f0.4 per cent standard deviation. When scintillators can be used in both channels, precision improves to IfO*l-0.2 per cent standard deviation. Where scintillators can be used, 2 ,uF rather than 50 ,uF are required for integration.
The X-ray research quantometer The XRQ was the first instrument to be designed, and has been used for much of the ana.lytical research in the authors’ laboratories. It requires a separate base cabinet, but uses the same power supply and console as the XIQ. Eight positions have been provided around an end-windowed X-ray tube in which may be placed any combination of dispersive or non-dispersive receivers, as shown in Fig. 5. (A similar instrument has been described by ADLER and AXELROD, [9].)Each receiver is mounted upon a track, so that it may be positioned above either of two ports. The inner port, in each case, receives the radiation from a central sample position directly below the X-ray tube, while the outer port receives radiation from a secondary position which can be used for a reference standard. The ports are provided with automatic shutters to allow removal or replacement of the receivers without danger to the operator. The unknown and reference.samples are placed in a sliding compartment which is positioned under the X-ray tube. Fig. 5 shows this compartment pulled out to loading position. A safety shield automatically covers the internal X-ray ports when this compartment is drawn out, 145
J. W. KEBW, M. F. HASLER, J. L. JONES,end LOUIS ZEITZ
providing complete protection for the operator. Between the sample compartment and the receiver section is another sliding compartment providing for the use of the various filters and apertures which may be required in any of the receivers. In this compartment m?y also be placed cells for use in absorption work. In conjunction with the sample compartment, these cells may be used in the beam from a secondary emitter. Should it be desirable to have the cells in the primary beam of the X-ray tube, the tube may be placed below the cells and pointing upward. This filter compartment is also provided with an automatic safety shield. The integrating capacitors for the receivers are mounted in the cabinet and the front panel contains switches for selecting the capacitance values for each channel as well as attenuators for controlling the Geiger voltage.
The X-ray industrial quantometer The complete XIQ, shown in Fig 6, provides for eight channels with any combination of dispersive or non-dispersive receivers. The eight receivers are housed in a single enclosure and the entire spectrometer (Fig. 7) mounts directly on top of the source unit. In the centre of the housing is the end-on OEG-60, tungsten target X-ray tube placed so that its window is 5.6 cm from the horizontal surface of the sample, which is held by spring pressure against a flat aperture plate to assure that the A lead aperture in front of the window tube-to-sample distance is constant. of the X-ray tube permits X-rays to fall only upon the usable area of the sample, thus reducing extraneous scattered radiation to a minimum. The sample surface forms part of the walls of a compl_etely enclosed brass chamber in which the collimators or slits for the eight receivers are mounted. The ends of these collimators are 5 cm from the centre of the sample and are spaced at equal angles around the X-ray tube. They are placed at an angle of 47’ from the axis of the tube. For the flat-crystal receivers, the collieators are of the Soller slit type and consist of thin parallel sheets of phosphor bronze held in a square tube 20.4 cm long with sheet spacings to provide various The remainder of the flat-crystal receiver consists of a resolutions as required. simple and rugged assembly providing for a single flat crystal of lithium fluoride, aluminium, or other material, and a Geiger counter. An external vernier control is provided for rotation of the crystal through a simple linkage, so that after the initial rough setting to a given spectral line, the crystal may be oriented accurately during X-ray excitation to set it upon the peak of the line. Means are provided for adjustment of the position of t’he collimator in rotation about its own axis for accurate alignment with the axis of rotation of the crystal, so as to insure maximum resolution. The curved-crystal units designed for use in the XIQ are also of semi-fixed design, with means provided for setting to a particular wavelength. The receiver, shown schematically in Fig. 8, consists of a primary slit, a curved crystal, a secondary slit, and a Geiger counter or scintillator. The primary slit, of fixed design, is-provided in various widths to provide a balance between resolution and intensity requirements, and is held in a fixed relation to the sample at a distance of 5.1 cm. The single crystal, of lithium fluoride, rock salt, or other material, is bent, ground, The and polished accurately to provide maximum intensity and resolution. 146
secondary slit is similar to the primary slit and is also provided in various widths to fit a particular problem. As in the flat-crystal unit, the spectrometer may be scanned through a few degrees for final alignment. The nondispersive receivers utilize a simple tube collimator to prevent the Geiger tube from “seeing” radiation other than that proceeding from the sample. Filter holders for 1.27 cm diameter briquettes or foil discs are integral with the collimators. Scintillators may be used in place of Geiger tubes. CRYSTAL
Fig. 8. One or more positions may be provided for external standard samples for use with either nondispersive or dispersive channels. A specially designed samplehandling mechanism forms an integral part of the spectrometer. This sample changer incorporates a labyrinth path in all directions from the X-ray chamber to insure complete safety for the operator at all times and to avoid the necessity for turning off the X-rays while changing samples. Placement or removal of the sample from its holder requires only a simple one-hand motion to compress the holding spring with the sample. A mirror allows accurate positioning of the sample surface with respect to the X-ray beam. Two positions are provided so that a sample may be placed in its holder while another is being analyzed. It then requires the motion of a single control lever to reverse the positions and allow analysis of the second sample. The high-voltage X-ray cable, X-ray tube water-supply and return, and the Geiger power- and signal-leads are all enclosed within a single sheath which connects the spectrometer to the X-ray source upon which it is mounted. The console containing the electronic supplies and recording circuits is essentially t,he same as the unit used with the Applied Research Laboratories Optical Quantometers. If desirable, a single console can be used to record the outputs of an optical and an X-ray spectrometer. This suggests the use of a combined optical-X-ray laboratory; the Optical Quantometer for low concentrations and the X-ray Quantometer for high concentrations. LidtdiOlM
These instruments are not suitable in their standard form for the analysis of low concentrations of elements lower than atomic number 20. The limits of detectability obtainable for the remaining elements are a function of the base material as well as of the various instrumental factors involved. The limit, defined as that 147
J. W. KEMP, M. F. HASLER, J. L. JONES, and LOUIS ZEITZ
concentration at which coefficient of variation of 25 per cent will be obtained, varies from 1 ppm to several thousand parts per million. Precision may be limited by the intensity available, the intensity limitations of Geiger counters, or the precision of the circuits. In the first case, curved crystals offer a considerable advantage, and in simple systems, non-dispersive techniques can be of help. In the second case, the use of curved crystals or non-dispersive techniques can often provide sufficient intensities to allow the use of scintillators. Where sointillators can be used, coefficients of variation of 0.1 to 0.5 per cent can be expected. The electronics used have-been shown to provide percent standard deviations of intensity ratios of 0.1-0.2 per cent. Accuracy is a function of instrumentation, excellence of standards, and the care with which working curves are constructed. Where the last two points have been properly considered, accuracies approaching the precision of the system are obtainable.
References 111FRIEDMAN, H., BIRRS, L. S., and BROOKS, E. J.: A.S.T.M. Special Technical Publication No. 157, p. 3 (1954) 121 SHERMAN, J. ; A.S.T.M. Special Technical Publication No. 157, p. 27 (1954) [31 BRISSEY, R. M., LIEBHAFS~Y, H. A., and PFEIFFER, H. G.; A.S.T.M. Special Technical Publication No. 157, p. 43 (1954) [41 NOAKES, GORDEN E.; A.S.T.M. Special Technical Publication No. 157, p. 57 (1954) [51 CARL, HOWARD F., and CAMPBELL, WILLIAM J.; A.S.T.M. Special Technical Publication No. 157, p. 63 (1954) [61 DAVIS, ELWIN N., and VAN NORDSTRAND, ROBERT A.; And. Chem. 1954 20 973 [71 MORTIMORE, D. M., ROMANS, P. A., and TEWS, J. L.; Applied Spectroscopy 1954 8 24 PI BIRKS, L. S., BROOKS, E. J., and FRIEDMAN, H.; Anal. Chem. 1953 25 692 191 ADLER, ISIDORE, and AXELROD, JOSEPH M.; J. Opt. Sot. Amer. 1963 48 769
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