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Comparison between PIXE and XRF for applications in art and archaeology
86
Nuclear
Section IV. Complementarity COMPARISON
between PIXE
Instruments
and Methods
in Physics Research B14 (1986) 86-92 North-Holland, Amsterdam
and XRF and use of SR
BETWEEN PIXE AND XRF FOR APPLICATIONS
IN ART AND ARCHAEOLOGY
Klas G. MALMQVIST Department of Nuclear Phystcs, Lund Institute of Science und Technology, Sijloegatan 14, S-223 62 Lund, Sweden
The properties of X-ray fluorescence and particle-induced X-ray emission have been compared with special reference to applications within art and archaeology. The two techniques have each been found to have several specific merits useful in the analysis of various obiects in these fields. Together they offer the museum scientist and archaeologist excellent complementary analytical tools for nondestructive multielemental analysis.
1. Introduction The respective merits of X-ray fluorescence and PIXE have been avidly debated in many fields of application ever, since the introduction of PIXE in the early seventies. This discussion has not always been very fruitful, but in recent years a general agreement seems to have been reached that the two modes of excitation of characteristic X-rays are indeed complementary rather than competitive. However, it may be of value to potential users within arts and archaeology to make an objective comparison of various aspects of the two techniques. The development of new types of analytical facilities over the last years is also a good reason for comparing the two techniques. In many fields of application the analyst has to consider not only sensitivity, speed and other obvious factors but also, the economical aspects. These normally limit the choice of analytical method and can therefore not be neglected in a discussion of the merits of various analytical techniques.
2. Analytical facilities 2. I. X-ray fluorescence For the production of photons, X-ray tubes have traditionally been used. Electrons are accelerated towards a tungsten anode and the characteristic X-rays and the bremsstrahlung irradiate the sample directly or through a suitable filter or irradiate a secondary target, the radiation from which is used to irradiate the sample [l-3]. Due to the convenience of use and their low cost radioactive sources are also widely used. The intensity of the radiation emitted by the source is, however, normally lower than that of the tubes; hence, the detection limits are higher than for the tube excitation. Another possible way of producing X-rays for XRF is 0168-583X/86/$03.50 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
to use PIXE. A cooled single-element target is irradiated with intense MeV-particle currents and the characteristic X-rays produced are used for XRF analysis [4]. In this manner the energy of the exciting radiation can be selected rather easily and thus the analytical parameters for a particular element optimized. One very significant disadvantage of using traditional X-ray sources with isotropic X-ray production is the re2-dependence of the intensity, which makes it very difficult to design an apparatus which allows a high lateral resolution. However, there are tube-excited systems which allow about 1 mm resolution [5] but the extremely tight geometry limits the usefulness of the system in the analysis of very small objects. In the last few years the development of synchrotron radiation facilities has also provided new prospects for XRF analysis. The intense electromagnetic radiation, which is a nuisance to the high-energy physicist when trying to increase the speed of electrons to the ultimate speed of light, has been recognized as being a very important tool within many different fields, e.g. material studies, biology, etc. [6]. When electrons are deflected while travelling at a speed very close to the speed of light they emit radiation in the forward (tangential) direction. The energy spectrum of this radiation is dependent on the energy of the electrons (typically 1-6 GeV) and on the bending radius of the electron path. The divergence in the vertical plane is very small (mrad) and the radiation is linearly polarized in the horizontal plane. The low divergence offers a high photon flux at the target, and the polarized radiation reduces the background in the X-ray spectrum caused by scattered primary radiation. The need for a storage ring naturally puts severe restrictions on the use of this new X-ray source but nevertheless it is an important tool, which with the development of multiuser facilities [7], will probably be used for synchrotron-radiation-induced XRF-analysis (SXRF) to a significant extent in the near future. Less costly sources of
K.G.Malmquist / Comparison between PIXE and XRF intense hard X-rays of low divergence, e.g. channeling radiation [8], will make this kind of sophisticated analysis more realistic and economical for an “ordinary” user. 2.2. Particle-induced
X-ray emission
The most common source of charged particles is electrostatic accelerators e.g. Van de Graaff machines. The natural choice when purchasing a dedicated PIXE machine would be a “compact” tandem accelerator [9,10]. Although they may not be quite as suitable, cyclotrons are also used in extensive PIXE programmes in the field discussed here [ll]. For most PIXE groups the availability of an accelerator has determined the choice of apparatus. It is often argued that the need for expensive equipment disqualifies this analytical method for use by most analysts. Such a statement, however, is not always justified since an analytical problem should be solved in the best way possible without any prejudices. Furthermore a cost-benefit analysis may very well reveal that the most expensive technique provides so much more information that it is better value for money. The conclusion to be drawn from this is that if you have a choice you should try to focus on the analytical demands rather than on the facilities required. A special, commercially available [12] technique is the use of a radioactive source which emits alpha particles. The very short range of alpha particles in matter makes the technique suitable for analysis of very thin (a few micrometres) surface layers. This method is particularly suitable for analysis of light elements since the cross section for ionization is high and the characteristic X-rays from low-Z elements can only penetrate a very thin sample layer thus matching the low penetration depth of the alpha particles.
3. X-ray fluorescence The use of photons to create inner-shell vacancies in the atoms of a sample has, for many years, been a well established technique. The probability of creating a vacancy is highest for an element with the binding energy of the particular electron shell just below the excitation energy, and then falls off quite rapidly with atomic number. Consequently, to maximize the sensitivity it is advantageous to use several different excitation energies over the whole range of Z. When a tunable source is available, e.g. synchrotron radiation with a monochromator, the energy can be set just above the absorption edge of the element of particular interest. Although the ionization probability is high the energy deposition by photons, in contrast to charged particles, is low [13]. If available, a very high photon flux can
87
therefore be used without any risk of damage due to heating of the sample. This is a fact of particular significance for applications in arts and archaeology where one often has to deal with very delicate and precious objects. The traditional XRF technique is to use wavelengthdispersive detection systems. The energy resolution of a crystal spectrometer is superior to that of the energydispersive solid-state detector which makes it still a very common technique, particularly within geology and metallurgy. The scanning crystal spectrometer covers the interesting elemental range by varying the detected X-ray energy and measuring one X-ray line at a time. This need for line-by-line analysis and the low detection efficiency makes it a time-consuming technique. However, multicrystal X-ray spectrometers are also commercially available and are very useful in many applications where the elements to be determined are well known [14]. In SXRF the wavelength-dispersive systems can also prove to be very useful due to the high intensities available. In energy-dispersive XRF analysis the choice of solid-state detector is very critical since the detection limits are set primarily by the low-energy tail from the peaks of scattered primary radiation. Poor chargecollection characteristics increase this tail significantly and consequently the general background level in the spectrum will be higher. Since the individual detectors are different in this respect test of a new detector should include an estimate of the magnitude of this tail. Use of collimation to mask the edges of the detector crystal or the expensive guard-ring detector [15] reduces this problem. The latter technique significantly improves the analytical sensitivity of XRF. Another important factor for the background level is the intensity of scattered radiation reaching the detector. The use of a tri-axial detection system reduces the scattered peaks (see fig. la) [16], and for special samples a total reflection system can detect very small masses deposited on a special highly polished crystal backing which significantly reduces the scattered radiation [17]. A serious drawback with XRF is the limited spatial resolution (SXRF not included) but through compact design a resolution of about a few millimetres is possible and should be very useful in many applications in art and archaeology. For genuine microprobe or semimicroprobe work a spatial resolution of 1 to 50 pm is required. To obtain this, the photons have to be focused. This is in practice only possible in combination with high-intensity synchrotron radiation. Preliminary experiments and calculations indicate that a spatial resolution of about 20-30 pm should be possible with present focusing technology [18]. This is about an order of magnitude worse than the particle microprobes but still useful in many fields of interest, and the combination of wigglers, producing very high photon intensities in the IV. PIXE/XRF/SR
K.G.Malmqoist
88
/ Comparison between PIXE and
XRF
MO
Fe scatter IO4
i!
Fe
peaks
cu
,.a’:. .- ._ . ..
-:;
1
I
I
IO0
300
200 CHANNEL
I
400
I
500
NUMBER
Rb
n. Rb
Ir
:
20
15 ENERGY
(KEV)
Fig. 1. X-ray spectra from analysis in air of a homogeneous pellet of typical geological material with XRF (a) and PIXE (b). For XRF a secondary target of MO was used, and for PIXE 2.55 MeV protons. The total number of counts in each spectrum is 500ooO [32].
89
K.G. Malmqvist / Comparrson between PIXE and XRF
Table 1 Calculated effective depth of analysis in X-ray spectrometry of three matrices. The values given values are are the thickness in mg/cm2, Matrix
ax, from which 75% of the detected characteristic X-rays originate [31]. Element
Exciting radiation XRF
PIXE
Cellulose
Fluorapatite
Stainless steel
1 MeV
2.5 MeV
5 MeV
Ba MO Sr Ni Ti Ca P
0.63 0.75 0.76 0.77 0.83 0.85 0.83
3.9 4.1 4.2 4.5 4.7 4.5 2.5
14.5 15.8 16.4 17.3 15.5 12.5 2.9
Ba MO Sr Ni Ti Ca P
1.00 1.19 1.20 1.17 1.07 1.22 1.06
5.4 5.6 5.6 5.0 2.6 4.0 1.9
18.7 19.4 18.7 10.3 2.9 5.2 2.1
Ba MO Sr Ni Ti Ca P
1.43 1.70 1.70 1.41 1.70 1.59 0.70
7.1 6.7 6.4 3.1 5.0 3.6 0.71
22.5 18.0 14.0 3.4 6.6 3.9 0.85
X-ray region, and suitable focusing elements, e.g. bent gratings, should be able to give a very intense semimicroprobe not limited by heating effects, as is the particle microprobe. For applications in art and archaeology such a probe may prove to be very useful since the irradiation chambers can be designed to hold rather large objects and the damage to the object will be negligible. The detection limits of XRF are normally of the order of pg/g [19] but are dependent on the degree of optimization of the irradiation conditions. The requirement on sample size is not an important restriction in most applications of art and archaeology but XRF analysis is preferably carried out on samples of tens of mg and larger. The effective depth of analysis in XRF is determined primarily by the elements to be analysed since the attenuation of characteristic X-rays in the sample (self absorption) sets the limits of detection. This means that the effective depth of analysis may vary from a few micrometers to several millimetres (see table 1). When analysing objects with only very thin corrosion layers the relatively large analysis depth for heavier elements compared to PIXE represents an advantage in bulk analysis. On the other hand, if the surface layer is of particular interest, e.g. thin layers of glazing on pottery, it may be difficult to avoid contributions from
Ti
11.5 2.7
2.1 1.3
2.8 0.76
MO
Sm
590 135 32 17.6 2.9
2570 1240 810 144 32 17.6 2.9
51 11.9 3.2 5.8 2.1
510 131 75 12.8 3.2 6.0 2.1
13.2 3.6 6.2 4.0 0.83
136 33 19.7 4.0 7.4 4.4 0.84
lower layers, at least for high-Z elements. The problems of calculating the attenuation of the incident X-rays when the matrix composition is not accurately known renders accurate quantitative XRF analysis of thick samples, based on purely fundamental parameters, very difficult. Quantification is therefore normally based on a comparison with standards with approximately the same matrix composition as the sample. This may cause problems in applications where the sample composition is largely unknown.
4. Particle-induced
X-ray emission
The widespread use of PIXE since its introduction in the early seventies [20] is probably due to the fact that many small accelerators were “out of work“ at nuclear physics laboratories. Many laboratories have tried PIXE on a rather small scale while others have started vast programmes in various fields of application. One purely commercial laboratory is in active operation [21], and at least one more is being planned at present [22]. The necessity of using an accelerator naturally makes it difficult to install PIXE systems in all laboratories, but in large industrial establishments and national analytical laboratories (e.g. Laboratoire de Recherches des IV. PIXE/XRF/SR
Mu&es de France) it is quite possible to realize such a facility. In fact, the investments are of the same order as for a modern mass spectrometer. The properties of PIXE can be summarized as: high sensitivity in small samples, high speed, surface analysis, genuinely multielemental and quantitative, partly nondestructive, possible to combine simultaneously with other ion-beam techniques and microprobes. This may sound entirely positive but this is, in fact, not so. Many mistakes have been made in trying to analyse “anything and everything” with PIXE, and its specific properties should be recognized and thorou~ly understood before selection of suitable samples. However, in many different fields of application the number of samples well suited for PIXE analysis are indeed very large. The use of protons or alpha particles for the production of inner-she11 vacancies combines a high ionization cross section with low X-ray background. The background in the region of low-2 elements is determined by bremsstrahlung from secondary electrons while at higher X-ray energies the background is normally determined by gamma rays produced in the target and Compton scattered in the detector crystal (see fig. lb). Selection of various X-ray absorbers can improve the sensitivity for heavier elements thus smoothing the sensitivity over the whole elemental range [23]. Although the ionization cross section also increases for light elements with increasing particle energy up to rather high energies, the variation in background radiation leads to the lowest general detection limits being obtained for 1.5-3.5 MeV protons. While the absolute detection limits may be as low as below 0.1 ng the concentration detection limits in thick samples of low-Z elements are normally in the interval 0.1 to 10 pg/g [24]. A serious problem in the use of PIXE in art and archaeology is the energy deposition which may damage the surface of the object. The combination of analysis in vacuum and local heating of nonconducting materials creates serious problems. The minimizing of beam flux (e.g. by on-demand beam) and non-vacuum analysis can solve these problems. The extraction of the ion beam through a thin plastic window maintaining the vacuum in the beam tube makes it possible to run PIXE analyses in air or preferably in a helium atmosphere [25]. Using a flow of cooling helium gas directed towards the window, very high currents can be extracted over long periods of time without any window breakdown [26]. Although the temperature of the irradiated spot of the sample may be kept low some radiation damage during ion bombardment is inevitable. It is, however, worth noticing that although PIXE analysis in vacuum may partly damage a precious sample there are examples of very delicate samples which have been successfully analysed in vacuum, e.g. valuable stamps [27]. A very promising application of PIXE technique is
the use of particle microprobes 1281. The combination of high sensitivity and high lateral resolution makes this a unique means of analysing microstructures at trace element level. Due to the extensive electron scattering in electron-microprobe analysis the lateral resolution of the particle microprobe is as good as, or even superior to the electron microprobe when analysing bulk samples. In several particle probes rapid beam scanning is used during analysis since the sample would otherwise be damaged by the beam.
5. XRF versus PIXE The points discussed above are not intended to form a full description of the two techniques. In a comparison of the two methods the earlier-mentioned economical aspects are important. The development of an upto-date XRF laboratory requires access to qualified personnel but normally not very much continuous maintenance. The staff can thus be kept small. The running of a PIXE laboratory puts larger demands on the technical staff. More complex equipment is involved and the larger investment makes it necessary to maintain a high av~I~ability (e.g. three shifts). When large numbers of samples are processed the rapidity of routine PIXE analysis (normally only one irradiation is required) makes it possible to keep the cost per sample at a very competitive level, in fact, calculated as cost per element analysed, among the lowest available. In samples from art and archaeology it is desirable that many elements be analysed since this will give stronger support for interpretations regarding, e.g. geographical origin, manufacturing technique etc. Both XRF and PIXE fulfil this demand although PIXE, at least for single-irradiation analysis, is the more genuine multielement~ technique (see figs. la and lb). In PIXE the simultaneous detection of scattered projectiles and gamma rays [23] offers a means of further extending the elemental range in a single irradiation. The large number of elements that can be detected in PIXE analysis is very useful in modern statistical analyses, e.g. factor and clustering analysis. However, should high-2 elements be of particular interest in a specific analytical situation than XRF with a high-Z secondary target or even a radioactive source [29] is superior to PIXE. The problem of representativity of a surface-oriented analysis is often relevant in the analysis of very old objects and objects found in excavations, possibly with a high degree of corrosion. This aspect is most significant for PIXE with its very small ion penetration depth but it is also important for XRF, especially for lighter elements. Table 1 shows a comparison of effective analysis depths for PIXE and XRF at different X-ray energies and for different matrices. It may therefore be important to try to find a position on an object where a
91
KG. Mulmquist/ ComparisonbetweenPIXE and XRF very tiny amount of material may be removed for analysis. When large sample masses are available the general recommendation is to use XRF. This is also true in cases where the object cannot be moved and one has to go out into the field with the equipment. In such cases, portable XRF apparatus, based on excitation with radioactive sources, is very useful while it will be some time before a portable accelerator is constructed. When large, bulky objects, e.g. paintings or large archaeological objects, are to be analysed in situ, XRF or in-air PIXE can normally both be applied without any difficulties. Special facilities have been developed for PIXE analysis of, for example documents, in which laser alignment is used for accurate positioning of the particle beam Ill]. The advantages of using PIXE in comparison with XRF are the simpler quantification calculations in unknown matrices and the better lateral resolution, while the simpler apparatus and better representativity of XRF also have to be considered. When very small objects are to be analysed the advantages of PIXE become obvious. Without any special microbeam facility a spatial resolution of a few tenths of a millimetre can be realized even in air, allowing details of ornaments, jewelry etc. to be analysed [30]. 5. Conclusions In the multielemental analysis of objects of art and archaeotogical finds the X-ray spectrometric methods XRF and PIXE both offer high analytical potentials. The following is an attempt to summarize the particular advantages of the two techniques with respect to the particular fields of application: X-ray fluorescence: 9 %mple, highly reliable and commercially available systems at a relatively low cost. ii) Equipment based on radioactive sources easily made mobile, e.g. in a museum, in the field. iii) Very slight sample damage. iv) High sensitivity for heavier elements. v) Larger depth of analysis for heavier elements. the vi) By chasing the energy of the exciting radiation, sensitivity can be optimized for a particular elemental region of interest. vii) Suitable for large sample areas. Particle-induced X-ray emission : 9 High absolute sensitivity, well suited for analysis of small samples. ii) More genuinely multielemental. iii) High lateral resolution. iv) Moderate sample damage. v) Suitable for anatysis of thin layers and for analysis of light elements.
vi)
Relatively
straightforward
quantification
in bulk
samples.
can be used to optimize the sensitivity to a particular elemental region of interest. viii) Can be combined with complementary, simultaneous nuclear techniques for analysis of the lightests elements (Such as PIGME). vii)
Absorbers
It is apparent from the features listed above that the two techniques are indeed complementary. References 111 B. Gonsior
and M. Roth, Talanta,
30 (1983) 385.
PI R. Jenkins, Anal. Chem. 56 (1984) 1099A. and M.R. [31 J.M. Jaklevic, W.R. Wrench, T.W. Clarkson Greenwood, Adv. X-ray Anal. 21 (1978) 171. [41 M. Peisach, W. Maenhaut, P. Van Espen and L. De Rue, Nucl. Instr. and Meth. B3 (1984) 253. Pl T.Y. Toribara, D.A. Jackson, W.R. Wrench, A.C. Thompson and J.M. Jaklevic, Anal. Chem. 54 (1982) 1844. (61 J.R. Chen, B.M. Gordon, A.L. Hansen, K.W. Jones, H.W. Kramer. E.C.I. Chao and J.A. Minkin, Scanning Electr. Microscopy 1984/1V p. 1483. Radiation Facility, Suppl. 1, eds. 171 European Synchrotron Y. Farge and P.J. Duke (ESF. Strasbourg;l979). PI B.L. Berman and S.D. Bloom, Energy and Technology Review,. Lawrence Livermore National Laboratory (1981). P.O. Box 3001, 19 Graf Road, 191 General Ionex Corporation, Newbury, MA 01950, USA. Electrostatic Corporation, Graber Road, Box IlO1 National 117, Middleton, WI 53562, USA. 1111 R.A. Eldred, B.H. Kusko and T.A. Cahill, Nucl. Instr. and Meth. 83 (1984) 579. 1101 Chess Drive, Foster City. CA WI Kevex Corporation, 944 04, USA. Radiation Research, eds., H. P31 C.J. Sparks, in: Synchrotron Winick and S. Doniah (Plenum, New York, 1980) ch. 14. R.L. Bennett and K.T. Knapp. in: X-ray P41 J. Wagman, Fluorescence Analysis of Environmental Samples, ed., T.G. Dzubay (Ann Ahor Sci., Ann Arbor, 1977) ch. 3. [1-v J.M. Jaklevic and F.S. Goulding, Trans. Nucl. Sci. NS-19 (1972) 384. and E. Selin, Nucl. Instr. and Meth. 165 1161 P. Standzenieks (1979) 63. f171 H. Schwenke and J. Knoth, Nucl. fnstr. and Meth. 193 (1982) 239. U81 M. Prim, J.M. K&per and M.P.A. Viegers, Nucl. Instr. and Meth. B3 (1984) 246. El91 F.S. Goulding and J.M. Jaklevic, Nucl. Instr. and Meth. 142 (1977) 323. WI T.B. Johansson, R. Akselsson and S.A.E. Johansson. Nucl. Instr. and Meth. 84 (1970) 141. Element Analysis Corporation, 1696 Capital Circle SW., PI Tallahassee, FL 32304, USA. 1221 Studsvik Energiteknik AB, NykGping, Sweden. and K.J.R. Akselsson, Nucl. Instr. and 1231 L.-E. Car&on Me&h. 181 (1981) 531. 1241 F. Folkmann, J. Phys. E8 (1975) 429. v51 E.T. Williams, Nucl. Instr. and Meth. B3 (1984) 211. IV. PIXE,‘XRF/SR
1261 J. RBislnen and A. Anttila, Nucl. lnstr. and Meth. 196 (1982) 489. f27] E.M. Johansson, S.A.E. Johansson, KG. Malmqvist and I.M.B. Wiman, these Proceedings (IBA3 Workshop) Nucl. Instr. and Meth. B14 (1986) 45. (281 R. Nobiling, these Proceedings (IBA” Workshop) Nucl. Instr. and Meth. B14 (1986) 142. [29] C. Heitz, G. Lagarde, A. Pape, T. Tenorio. C. Zarate. M. Menu, L. Scotee, A. Jaidar, R. Acosla, R. Alviso, D.
Gonzalez. and V. Gonzalez, these Proceedings (IBA” Workshop) Nucl. Instr. and Meth. B14 (1986) 93. 1301 G. Demortier, these Proceedings (IBA3 Workshop) Nucl. Instr. and Meth. B14 (1986) 152. 1311 M.S. Ahlberg, Nucl. Instr. and Meth. 146 (1977) 465. [32] L.-E. Carlsson and K.R. Akselsson, Adv. X-ray Analysis 24 (1981) 313.
Nuclear
Section IV. Complementarity COMPARISON
between PIXE
Instruments
and Methods
in Physics Research B14 (1986) 86-92 North-Holland, Amsterdam
and XRF and use of SR
BETWEEN PIXE AND XRF FOR APPLICATIONS
IN ART AND ARCHAEOLOGY
Klas G. MALMQVIST Department of Nuclear Phystcs, Lund Institute of Science und Technology, Sijloegatan 14, S-223 62 Lund, Sweden
The properties of X-ray fluorescence and particle-induced X-ray emission have been compared with special reference to applications within art and archaeology. The two techniques have each been found to have several specific merits useful in the analysis of various obiects in these fields. Together they offer the museum scientist and archaeologist excellent complementary analytical tools for nondestructive multielemental analysis.
1. Introduction The respective merits of X-ray fluorescence and PIXE have been avidly debated in many fields of application ever, since the introduction of PIXE in the early seventies. This discussion has not always been very fruitful, but in recent years a general agreement seems to have been reached that the two modes of excitation of characteristic X-rays are indeed complementary rather than competitive. However, it may be of value to potential users within arts and archaeology to make an objective comparison of various aspects of the two techniques. The development of new types of analytical facilities over the last years is also a good reason for comparing the two techniques. In many fields of application the analyst has to consider not only sensitivity, speed and other obvious factors but also, the economical aspects. These normally limit the choice of analytical method and can therefore not be neglected in a discussion of the merits of various analytical techniques.
2. Analytical facilities 2. I. X-ray fluorescence For the production of photons, X-ray tubes have traditionally been used. Electrons are accelerated towards a tungsten anode and the characteristic X-rays and the bremsstrahlung irradiate the sample directly or through a suitable filter or irradiate a secondary target, the radiation from which is used to irradiate the sample [l-3]. Due to the convenience of use and their low cost radioactive sources are also widely used. The intensity of the radiation emitted by the source is, however, normally lower than that of the tubes; hence, the detection limits are higher than for the tube excitation. Another possible way of producing X-rays for XRF is 0168-583X/86/$03.50 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
to use PIXE. A cooled single-element target is irradiated with intense MeV-particle currents and the characteristic X-rays produced are used for XRF analysis [4]. In this manner the energy of the exciting radiation can be selected rather easily and thus the analytical parameters for a particular element optimized. One very significant disadvantage of using traditional X-ray sources with isotropic X-ray production is the re2-dependence of the intensity, which makes it very difficult to design an apparatus which allows a high lateral resolution. However, there are tube-excited systems which allow about 1 mm resolution [5] but the extremely tight geometry limits the usefulness of the system in the analysis of very small objects. In the last few years the development of synchrotron radiation facilities has also provided new prospects for XRF analysis. The intense electromagnetic radiation, which is a nuisance to the high-energy physicist when trying to increase the speed of electrons to the ultimate speed of light, has been recognized as being a very important tool within many different fields, e.g. material studies, biology, etc. [6]. When electrons are deflected while travelling at a speed very close to the speed of light they emit radiation in the forward (tangential) direction. The energy spectrum of this radiation is dependent on the energy of the electrons (typically 1-6 GeV) and on the bending radius of the electron path. The divergence in the vertical plane is very small (mrad) and the radiation is linearly polarized in the horizontal plane. The low divergence offers a high photon flux at the target, and the polarized radiation reduces the background in the X-ray spectrum caused by scattered primary radiation. The need for a storage ring naturally puts severe restrictions on the use of this new X-ray source but nevertheless it is an important tool, which with the development of multiuser facilities [7], will probably be used for synchrotron-radiation-induced XRF-analysis (SXRF) to a significant extent in the near future. Less costly sources of
K.G.Malmquist / Comparison between PIXE and XRF intense hard X-rays of low divergence, e.g. channeling radiation [8], will make this kind of sophisticated analysis more realistic and economical for an “ordinary” user. 2.2. Particle-induced
X-ray emission
The most common source of charged particles is electrostatic accelerators e.g. Van de Graaff machines. The natural choice when purchasing a dedicated PIXE machine would be a “compact” tandem accelerator [9,10]. Although they may not be quite as suitable, cyclotrons are also used in extensive PIXE programmes in the field discussed here [ll]. For most PIXE groups the availability of an accelerator has determined the choice of apparatus. It is often argued that the need for expensive equipment disqualifies this analytical method for use by most analysts. Such a statement, however, is not always justified since an analytical problem should be solved in the best way possible without any prejudices. Furthermore a cost-benefit analysis may very well reveal that the most expensive technique provides so much more information that it is better value for money. The conclusion to be drawn from this is that if you have a choice you should try to focus on the analytical demands rather than on the facilities required. A special, commercially available [12] technique is the use of a radioactive source which emits alpha particles. The very short range of alpha particles in matter makes the technique suitable for analysis of very thin (a few micrometres) surface layers. This method is particularly suitable for analysis of light elements since the cross section for ionization is high and the characteristic X-rays from low-Z elements can only penetrate a very thin sample layer thus matching the low penetration depth of the alpha particles.
3. X-ray fluorescence The use of photons to create inner-shell vacancies in the atoms of a sample has, for many years, been a well established technique. The probability of creating a vacancy is highest for an element with the binding energy of the particular electron shell just below the excitation energy, and then falls off quite rapidly with atomic number. Consequently, to maximize the sensitivity it is advantageous to use several different excitation energies over the whole range of Z. When a tunable source is available, e.g. synchrotron radiation with a monochromator, the energy can be set just above the absorption edge of the element of particular interest. Although the ionization probability is high the energy deposition by photons, in contrast to charged particles, is low [13]. If available, a very high photon flux can
87
therefore be used without any risk of damage due to heating of the sample. This is a fact of particular significance for applications in arts and archaeology where one often has to deal with very delicate and precious objects. The traditional XRF technique is to use wavelengthdispersive detection systems. The energy resolution of a crystal spectrometer is superior to that of the energydispersive solid-state detector which makes it still a very common technique, particularly within geology and metallurgy. The scanning crystal spectrometer covers the interesting elemental range by varying the detected X-ray energy and measuring one X-ray line at a time. This need for line-by-line analysis and the low detection efficiency makes it a time-consuming technique. However, multicrystal X-ray spectrometers are also commercially available and are very useful in many applications where the elements to be determined are well known [14]. In SXRF the wavelength-dispersive systems can also prove to be very useful due to the high intensities available. In energy-dispersive XRF analysis the choice of solid-state detector is very critical since the detection limits are set primarily by the low-energy tail from the peaks of scattered primary radiation. Poor chargecollection characteristics increase this tail significantly and consequently the general background level in the spectrum will be higher. Since the individual detectors are different in this respect test of a new detector should include an estimate of the magnitude of this tail. Use of collimation to mask the edges of the detector crystal or the expensive guard-ring detector [15] reduces this problem. The latter technique significantly improves the analytical sensitivity of XRF. Another important factor for the background level is the intensity of scattered radiation reaching the detector. The use of a tri-axial detection system reduces the scattered peaks (see fig. la) [16], and for special samples a total reflection system can detect very small masses deposited on a special highly polished crystal backing which significantly reduces the scattered radiation [17]. A serious drawback with XRF is the limited spatial resolution (SXRF not included) but through compact design a resolution of about a few millimetres is possible and should be very useful in many applications in art and archaeology. For genuine microprobe or semimicroprobe work a spatial resolution of 1 to 50 pm is required. To obtain this, the photons have to be focused. This is in practice only possible in combination with high-intensity synchrotron radiation. Preliminary experiments and calculations indicate that a spatial resolution of about 20-30 pm should be possible with present focusing technology [18]. This is about an order of magnitude worse than the particle microprobes but still useful in many fields of interest, and the combination of wigglers, producing very high photon intensities in the IV. PIXE/XRF/SR
K.G.Malmqoist
88
/ Comparison between PIXE and
XRF
MO
Fe scatter IO4
i!
Fe
peaks
cu
,.a’:. .- ._ . ..
-:;
1
I
I
IO0
300
200 CHANNEL
I
400
I
500
NUMBER
Rb
n. Rb
Ir
:
20
15 ENERGY
(KEV)
Fig. 1. X-ray spectra from analysis in air of a homogeneous pellet of typical geological material with XRF (a) and PIXE (b). For XRF a secondary target of MO was used, and for PIXE 2.55 MeV protons. The total number of counts in each spectrum is 500ooO [32].
89
K.G. Malmqvist / Comparrson between PIXE and XRF
Table 1 Calculated effective depth of analysis in X-ray spectrometry of three matrices. The values given values are are the thickness in mg/cm2, Matrix
ax, from which 75% of the detected characteristic X-rays originate [31]. Element
Exciting radiation XRF
PIXE
Cellulose
Fluorapatite
Stainless steel
1 MeV
2.5 MeV
5 MeV
Ba MO Sr Ni Ti Ca P
0.63 0.75 0.76 0.77 0.83 0.85 0.83
3.9 4.1 4.2 4.5 4.7 4.5 2.5
14.5 15.8 16.4 17.3 15.5 12.5 2.9
Ba MO Sr Ni Ti Ca P
1.00 1.19 1.20 1.17 1.07 1.22 1.06
5.4 5.6 5.6 5.0 2.6 4.0 1.9
18.7 19.4 18.7 10.3 2.9 5.2 2.1
Ba MO Sr Ni Ti Ca P
1.43 1.70 1.70 1.41 1.70 1.59 0.70
7.1 6.7 6.4 3.1 5.0 3.6 0.71
22.5 18.0 14.0 3.4 6.6 3.9 0.85
X-ray region, and suitable focusing elements, e.g. bent gratings, should be able to give a very intense semimicroprobe not limited by heating effects, as is the particle microprobe. For applications in art and archaeology such a probe may prove to be very useful since the irradiation chambers can be designed to hold rather large objects and the damage to the object will be negligible. The detection limits of XRF are normally of the order of pg/g [19] but are dependent on the degree of optimization of the irradiation conditions. The requirement on sample size is not an important restriction in most applications of art and archaeology but XRF analysis is preferably carried out on samples of tens of mg and larger. The effective depth of analysis in XRF is determined primarily by the elements to be analysed since the attenuation of characteristic X-rays in the sample (self absorption) sets the limits of detection. This means that the effective depth of analysis may vary from a few micrometers to several millimetres (see table 1). When analysing objects with only very thin corrosion layers the relatively large analysis depth for heavier elements compared to PIXE represents an advantage in bulk analysis. On the other hand, if the surface layer is of particular interest, e.g. thin layers of glazing on pottery, it may be difficult to avoid contributions from
Ti
11.5 2.7
2.1 1.3
2.8 0.76
MO
Sm
590 135 32 17.6 2.9
2570 1240 810 144 32 17.6 2.9
51 11.9 3.2 5.8 2.1
510 131 75 12.8 3.2 6.0 2.1
13.2 3.6 6.2 4.0 0.83
136 33 19.7 4.0 7.4 4.4 0.84
lower layers, at least for high-Z elements. The problems of calculating the attenuation of the incident X-rays when the matrix composition is not accurately known renders accurate quantitative XRF analysis of thick samples, based on purely fundamental parameters, very difficult. Quantification is therefore normally based on a comparison with standards with approximately the same matrix composition as the sample. This may cause problems in applications where the sample composition is largely unknown.
4. Particle-induced
X-ray emission
The widespread use of PIXE since its introduction in the early seventies [20] is probably due to the fact that many small accelerators were “out of work“ at nuclear physics laboratories. Many laboratories have tried PIXE on a rather small scale while others have started vast programmes in various fields of application. One purely commercial laboratory is in active operation [21], and at least one more is being planned at present [22]. The necessity of using an accelerator naturally makes it difficult to install PIXE systems in all laboratories, but in large industrial establishments and national analytical laboratories (e.g. Laboratoire de Recherches des IV. PIXE/XRF/SR
Mu&es de France) it is quite possible to realize such a facility. In fact, the investments are of the same order as for a modern mass spectrometer. The properties of PIXE can be summarized as: high sensitivity in small samples, high speed, surface analysis, genuinely multielemental and quantitative, partly nondestructive, possible to combine simultaneously with other ion-beam techniques and microprobes. This may sound entirely positive but this is, in fact, not so. Many mistakes have been made in trying to analyse “anything and everything” with PIXE, and its specific properties should be recognized and thorou~ly understood before selection of suitable samples. However, in many different fields of application the number of samples well suited for PIXE analysis are indeed very large. The use of protons or alpha particles for the production of inner-she11 vacancies combines a high ionization cross section with low X-ray background. The background in the region of low-2 elements is determined by bremsstrahlung from secondary electrons while at higher X-ray energies the background is normally determined by gamma rays produced in the target and Compton scattered in the detector crystal (see fig. lb). Selection of various X-ray absorbers can improve the sensitivity for heavier elements thus smoothing the sensitivity over the whole elemental range [23]. Although the ionization cross section also increases for light elements with increasing particle energy up to rather high energies, the variation in background radiation leads to the lowest general detection limits being obtained for 1.5-3.5 MeV protons. While the absolute detection limits may be as low as below 0.1 ng the concentration detection limits in thick samples of low-Z elements are normally in the interval 0.1 to 10 pg/g [24]. A serious problem in the use of PIXE in art and archaeology is the energy deposition which may damage the surface of the object. The combination of analysis in vacuum and local heating of nonconducting materials creates serious problems. The minimizing of beam flux (e.g. by on-demand beam) and non-vacuum analysis can solve these problems. The extraction of the ion beam through a thin plastic window maintaining the vacuum in the beam tube makes it possible to run PIXE analyses in air or preferably in a helium atmosphere [25]. Using a flow of cooling helium gas directed towards the window, very high currents can be extracted over long periods of time without any window breakdown [26]. Although the temperature of the irradiated spot of the sample may be kept low some radiation damage during ion bombardment is inevitable. It is, however, worth noticing that although PIXE analysis in vacuum may partly damage a precious sample there are examples of very delicate samples which have been successfully analysed in vacuum, e.g. valuable stamps [27]. A very promising application of PIXE technique is
the use of particle microprobes 1281. The combination of high sensitivity and high lateral resolution makes this a unique means of analysing microstructures at trace element level. Due to the extensive electron scattering in electron-microprobe analysis the lateral resolution of the particle microprobe is as good as, or even superior to the electron microprobe when analysing bulk samples. In several particle probes rapid beam scanning is used during analysis since the sample would otherwise be damaged by the beam.
5. XRF versus PIXE The points discussed above are not intended to form a full description of the two techniques. In a comparison of the two methods the earlier-mentioned economical aspects are important. The development of an upto-date XRF laboratory requires access to qualified personnel but normally not very much continuous maintenance. The staff can thus be kept small. The running of a PIXE laboratory puts larger demands on the technical staff. More complex equipment is involved and the larger investment makes it necessary to maintain a high av~I~ability (e.g. three shifts). When large numbers of samples are processed the rapidity of routine PIXE analysis (normally only one irradiation is required) makes it possible to keep the cost per sample at a very competitive level, in fact, calculated as cost per element analysed, among the lowest available. In samples from art and archaeology it is desirable that many elements be analysed since this will give stronger support for interpretations regarding, e.g. geographical origin, manufacturing technique etc. Both XRF and PIXE fulfil this demand although PIXE, at least for single-irradiation analysis, is the more genuine multielement~ technique (see figs. la and lb). In PIXE the simultaneous detection of scattered projectiles and gamma rays [23] offers a means of further extending the elemental range in a single irradiation. The large number of elements that can be detected in PIXE analysis is very useful in modern statistical analyses, e.g. factor and clustering analysis. However, should high-2 elements be of particular interest in a specific analytical situation than XRF with a high-Z secondary target or even a radioactive source [29] is superior to PIXE. The problem of representativity of a surface-oriented analysis is often relevant in the analysis of very old objects and objects found in excavations, possibly with a high degree of corrosion. This aspect is most significant for PIXE with its very small ion penetration depth but it is also important for XRF, especially for lighter elements. Table 1 shows a comparison of effective analysis depths for PIXE and XRF at different X-ray energies and for different matrices. It may therefore be important to try to find a position on an object where a
91
KG. Mulmquist/ ComparisonbetweenPIXE and XRF very tiny amount of material may be removed for analysis. When large sample masses are available the general recommendation is to use XRF. This is also true in cases where the object cannot be moved and one has to go out into the field with the equipment. In such cases, portable XRF apparatus, based on excitation with radioactive sources, is very useful while it will be some time before a portable accelerator is constructed. When large, bulky objects, e.g. paintings or large archaeological objects, are to be analysed in situ, XRF or in-air PIXE can normally both be applied without any difficulties. Special facilities have been developed for PIXE analysis of, for example documents, in which laser alignment is used for accurate positioning of the particle beam Ill]. The advantages of using PIXE in comparison with XRF are the simpler quantification calculations in unknown matrices and the better lateral resolution, while the simpler apparatus and better representativity of XRF also have to be considered. When very small objects are to be analysed the advantages of PIXE become obvious. Without any special microbeam facility a spatial resolution of a few tenths of a millimetre can be realized even in air, allowing details of ornaments, jewelry etc. to be analysed [30]. 5. Conclusions In the multielemental analysis of objects of art and archaeotogical finds the X-ray spectrometric methods XRF and PIXE both offer high analytical potentials. The following is an attempt to summarize the particular advantages of the two techniques with respect to the particular fields of application: X-ray fluorescence: 9 %mple, highly reliable and commercially available systems at a relatively low cost. ii) Equipment based on radioactive sources easily made mobile, e.g. in a museum, in the field. iii) Very slight sample damage. iv) High sensitivity for heavier elements. v) Larger depth of analysis for heavier elements. the vi) By chasing the energy of the exciting radiation, sensitivity can be optimized for a particular elemental region of interest. vii) Suitable for large sample areas. Particle-induced X-ray emission : 9 High absolute sensitivity, well suited for analysis of small samples. ii) More genuinely multielemental. iii) High lateral resolution. iv) Moderate sample damage. v) Suitable for anatysis of thin layers and for analysis of light elements.
vi)
Relatively
straightforward
quantification
in bulk
samples.
can be used to optimize the sensitivity to a particular elemental region of interest. viii) Can be combined with complementary, simultaneous nuclear techniques for analysis of the lightests elements (Such as PIGME). vii)
Absorbers
It is apparent from the features listed above that the two techniques are indeed complementary. References 111 B. Gonsior
and M. Roth, Talanta,
30 (1983) 385.
PI R. Jenkins, Anal. Chem. 56 (1984) 1099A. and M.R. [31 J.M. Jaklevic, W.R. Wrench, T.W. Clarkson Greenwood, Adv. X-ray Anal. 21 (1978) 171. [41 M. Peisach, W. Maenhaut, P. Van Espen and L. De Rue, Nucl. Instr. and Meth. B3 (1984) 253. Pl T.Y. Toribara, D.A. Jackson, W.R. Wrench, A.C. Thompson and J.M. Jaklevic, Anal. Chem. 54 (1982) 1844. (61 J.R. Chen, B.M. Gordon, A.L. Hansen, K.W. Jones, H.W. Kramer. E.C.I. Chao and J.A. Minkin, Scanning Electr. Microscopy 1984/1V p. 1483. Radiation Facility, Suppl. 1, eds. 171 European Synchrotron Y. Farge and P.J. Duke (ESF. Strasbourg;l979). PI B.L. Berman and S.D. Bloom, Energy and Technology Review,. Lawrence Livermore National Laboratory (1981). P.O. Box 3001, 19 Graf Road, 191 General Ionex Corporation, Newbury, MA 01950, USA. Electrostatic Corporation, Graber Road, Box IlO1 National 117, Middleton, WI 53562, USA. 1111 R.A. Eldred, B.H. Kusko and T.A. Cahill, Nucl. Instr. and Meth. 83 (1984) 579. 1101 Chess Drive, Foster City. CA WI Kevex Corporation, 944 04, USA. Radiation Research, eds., H. P31 C.J. Sparks, in: Synchrotron Winick and S. Doniah (Plenum, New York, 1980) ch. 14. R.L. Bennett and K.T. Knapp. in: X-ray P41 J. Wagman, Fluorescence Analysis of Environmental Samples, ed., T.G. Dzubay (Ann Ahor Sci., Ann Arbor, 1977) ch. 3. [1-v J.M. Jaklevic and F.S. Goulding, Trans. Nucl. Sci. NS-19 (1972) 384. and E. Selin, Nucl. Instr. and Meth. 165 1161 P. Standzenieks (1979) 63. f171 H. Schwenke and J. Knoth, Nucl. fnstr. and Meth. 193 (1982) 239. U81 M. Prim, J.M. K&per and M.P.A. Viegers, Nucl. Instr. and Meth. B3 (1984) 246. El91 F.S. Goulding and J.M. Jaklevic, Nucl. Instr. and Meth. 142 (1977) 323. WI T.B. Johansson, R. Akselsson and S.A.E. Johansson. Nucl. Instr. and Meth. 84 (1970) 141. Element Analysis Corporation, 1696 Capital Circle SW., PI Tallahassee, FL 32304, USA. 1221 Studsvik Energiteknik AB, NykGping, Sweden. and K.J.R. Akselsson, Nucl. Instr. and 1231 L.-E. Car&on Me&h. 181 (1981) 531. 1241 F. Folkmann, J. Phys. E8 (1975) 429. v51 E.T. Williams, Nucl. Instr. and Meth. B3 (1984) 211. IV. PIXE,‘XRF/SR
1261 J. RBislnen and A. Anttila, Nucl. lnstr. and Meth. 196 (1982) 489. f27] E.M. Johansson, S.A.E. Johansson, KG. Malmqvist and I.M.B. Wiman, these Proceedings (IBA3 Workshop) Nucl. Instr. and Meth. B14 (1986) 45. (281 R. Nobiling, these Proceedings (IBA” Workshop) Nucl. Instr. and Meth. B14 (1986) 142. [29] C. Heitz, G. Lagarde, A. Pape, T. Tenorio. C. Zarate. M. Menu, L. Scotee, A. Jaidar, R. Acosla, R. Alviso, D.
Gonzalez. and V. Gonzalez, these Proceedings (IBA” Workshop) Nucl. Instr. and Meth. B14 (1986) 93. 1301 G. Demortier, these Proceedings (IBA3 Workshop) Nucl. Instr. and Meth. B14 (1986) 152. 1311 M.S. Ahlberg, Nucl. Instr. and Meth. 146 (1977) 465. [32] L.-E. Carlsson and K.R. Akselsson, Adv. X-ray Analysis 24 (1981) 313.