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Analytical Methods Related to Building and Monument Preservation ISIDORE ADLER, SHELDON E. SOMMER, RAPHAEL GERSHON, and JACOB I. TROMBRA The most visible products of the weathering of stone materials are a conse- quence of the fragmentation and disintegration of mineral components. Some- what less obvious are the dissolution of these minerals and subsequent for- mation of new compounds, frequently in the interstices, as a result of the action of chemical and biological agents. An early phenomenon that lends itself to study is the disruption of chemical bonds during physical and chemical disintegration and the formation of highly reactive surfaces. These reactions may include oxidation-reduction, disordering of the mineral structure, and ion-exchange processes, with the eventual formation of microlayers of poorly crystalline materials and Microsystems of cracks and fractures with precipi- tated coatings, cements, and possible phase transformations as complicating factors. The examination of these veneers presents problems that are well matched by the techniques utilized in bulk characterization, such as atomic absorption, X-ray fluorescence, and optical emission spectroscopy. With regard to surfaces and near surfaces Defined as 10 A to a few micrometers in depth, one may consider a variety of techniques, some of which can be utilized in situ, offering the advantage of rapid and nondestructive analysis. The use of neutron-gamma techniques and reflection spectrophotometry are described as examples. Other techniques applied in the laboratory and that also require minimal sampling are electron spectroscopy, electron microprobe analysis, electron microscopy, and X-ray diffraction analysis. This paper examines the use of a number of these techniques, pointing out where a given method or combination of meth- ods is most applicable and the way in which the results may be related to the weathering processes that are occurnng. Isidore Adler is Professor, Departments of Chemistry and Geology, University of Mary- land, College Park. Sheldon E. Sommer is Associate Professor of Geology, University of Maryland, College Park. Raphael Gershon is Student Assistant, Departments of Chemistry and Geology, University of Maryland, College Park. Jacob I. Trombka is Senior Scientist, Goddard Space Flight Center, Greenbelt, Maryland. 163

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164 INTRODUCTION CONSERVATION OF HISTORIC STONE BUILDINGS The disintegration and decomposition of stone materialsnatural rock or mineral components and fabricated compositesoften result in the formation of a veneer that differs from the original material in com- position and texture. The new minerals produced by this weathering process are the result of physical, chemical, and biological reactions of carbonates, silicates, sulfides, or oxicles with water and atmospheric gases. The typical products are often hydrated phases, such as clay minerals and iron and aluminum oxyhydroxides. In addition to this process, termed "hydrolysis," the oxidation of ferrous iron to ferric iron, and carbonationchiefly the dissolution of limestone and marble by acidic waters are major agents of rock weathering. These actions, coupled with ion exchange and physical and biological alteration, pro- duce a marked change immediately below the stone-atmosphere in- terface. Although the differential stability of components in stone materials depends on the complex interaction of various ambient materials with the primary and subsequently formed substances, general understand- ing of mineral degradation may be derived from a study of relative bond strengths. The removal of alkali and alkaline earth elements, resulting in the residual buildup of layers rich in silica, aluminum, and titanium, appears to be, for silicate rocks, a representation of the relative cation-oxygen bond strength. The various surface layers are rendered less stable by progressive bond rupture, and fragments of the mineral's structural framework are liberated in solution or otherwise altered. The weathering of many stone materials is so complex that there is little agreement on the mechanisms at work or on the meth- odologies best suited to such study. The weathering of feldspar minerals, a major component of silicate- rich stone, has been a very active area of research in recent years. There are at least four different models of feldspar decomposition, i.e., for just one component of typical stone building material. The models include: (a) the straightforward dissolution of the material, with the solubility controlled by the concentration of silica and alumina; (b) the production of a leached layer by the exchange of cations upward and downward through the interior of grains, in addition to solution at the interface; (c) the production of an amorphous precipitate rich in aluminum and silicon that is rate controlled and dependent on pH; and (~) the production of a crystalline phase dependent on solution composition and parent solid. This brief summary of the possible analytical context strongly sug-

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Analytical Methods Related to Preservation gests that the proper methodology for the study of stone degradation is one that is capable of: ja) characterizing the surface or near-surface Tens of angstroms to hundreds of micrometers); (b) identifying amor- phous and crystalline materials; {c) determining spatial changes in composition, i.e., chemical analyses for materials heterogeneous on a micrometer level; and jU) detailing the relative bond strengths as a function of physical and chemical alteration. The analytical methods described below have been selected based on the above criteria. In addition, techniques are discussed that offer the advantage of in situ study for the characterization of alterations. These techniques may be used prior to, or perhaps in place of, destruc- tive sampling of historic materials. We shall briefly list the principles and some examples of application. 165 X-RAY FLUORESCENCE SPECTROSCOPY Various uses of X-ray fluorescence spectroscopy have been describe. Any process that produces inner-shell vacancies (i.e., ionization of an atom in its inner shelll will in turn produce characteristic X-rays. To create holes in an atom it is necessary to overcome in some fashion the binding energy of an electron in its particular shell. There are several ways of doing this. A target can be bombarded with electrons, energetic protons, alpha particles, or X-rays. As a case in point, if an X-ray photon has energy in excess of the binding energy of an electron in its shell, it will expel the electron from the atom by the photoelectric process, producing a vacancy and as a consequence an excited atom. The filling of this vacancy by outer-shell electrons as the atom returns to its ground state results in part in the emission of X-rays. Further, as the electrons from outer shells drop into the vacancies in the inner shells, new vacancies are produced and an electron cascade ensues. Electron transitions that end at the K shell produce a K spectrum. One can also expect to see L spectra, M spectra, etc. Examples of the possible transitions are shown in Figure 1. Note that transitions to outer shells produce a correspondingly increasing number of lines because of the greater number of possible transitions. Any particular transition results in a line whose energy, he, is the difference between the binding ener- gies of the two levels. These emitted lines are characteristic of the element. Not every ionization results in the emission of a characteristic X- ray photon, however. There is in fact a very high probability of a radiationless transition in which the atom returns to its ground state by the emission of an electron known as the Auger electron. The

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166 CONSERVATION OF HISTORIC STONE BUILDINGS _ _ _ - - \ ionization Limit ~ ~ ~ WN \\\\\ \ \ \~w it\\ \ \~\\\\~ \ \ Nucleus 1~ ~ K / // / / l ~ l ~ l ~ ~ ~ L M N O FIGURE 1 Possible electron transitions. \ \ \ ! probability of this type of event increases markedly as the atomic number decreases. The Auger electrons also have characteristic ener- gies, as we will see in the section Electron Spectroscopy-Chemical Analysis. Further, the photoelectron ejected initially also carries chem- ical information, since its maximum energy is equal to the difference between the energy of the exciting X-rays and the binding energy. Thus, in summary, bombardment by X-rays produces secondary X-rays, photoelectrons, and Auger electrons, all of which can yield information enabling us to identify an element and to determine its concentration and its chemical state. _. . . . .. The instrumentation used in the practice of X-ray fluorescence spec- troscopy fails into two broad types, described as "wavelength-disper-

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Analytical Methods Related to Preservation 167 sive" or "energy-dispersive." In the wavelength-dispersive mode the various wavelengths produced in the sample are separated for mea- surement by diffraction from a large single crystal and then detected by a proportional, scintillation, or solid-state detector. In the energy- dispersive mode all the wavelengths are seen simultaneously by an energy-sensitive detector. The detector then produces pulses propor- tional in size to the incident energies. The pulses are then sorted on the basis of their heights by an electronic, window-type discriminator. Both modes have particular advantages. Wavelength-dispersive sys- tems have the virtue of superior energy resolution, but the instru- mentation is more complex mechanically, involving a precise crystal monochromator. The energy-dispersive systems are simpler and more efficient but are inferior in inherent energy resolution. The latter mode requires that the energy-separation problems be resolved by sophisti- cated software/computer methods. Figure 2 shows both types of devices. Figure 2a is a plain view of wavelength-dispersive instrumentation. It consists of an exciting source (X-ray tuber, collimators, an analyzing crystal, and a detector. The analyzer is based on Bragg's law, no = 2~1 sin 0, where n is the dif- fraction order, A is the wavelength, ~ is the distance between the planes in the crystal, and ~ is the angle of incidence or the diffraction angle. The expression shows that a given wavelength will diffract at a given angle depending on the `1 spacing of the crystal. In practice, the detector is made to rotate at twice the angular speed of the crystal. A given wavelength (corresponding to a particular element) will be detected as it satisfies Bragg's law. It has also been well established that, to a first order, the intensity of a line is proportional to concentration. Thus, we have the basis of an analytical method. Figure 2b presents a line representation of an energy-dispersive sys- tem. As indicated above, such instrumentation is at least mechanically simpler then the wavelength-dispersive equipment, but it is electron- ically more complex. The output pulses of the detector are processed by a preamplifier and amplifier. These pulses are then sorted by a multichannel analyzer, which not only sorts the pulses by size but also delivers a number that is the sum of the pulses of a given size. Calibration involves relating pulse size to element. In the modern energy-dispersive analyzer, software programs in a dedicated computer identify the elements during the data-reduction phase. Of particular significance is the way in which this latter mode lends itself to in situ devices. There are in fact portable instruments commercially available for in situ analysis; they use radioactive sources to produce the X-rays.

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168 CONSERVATION OF HISTORIC STONE BUILDINGS ad Sample ~ ~ fir X-ray Tube \ - 2a - ,; l Analyzer Crystal // Sample Changer Collimator ~ ~ Filter Automatic Si (L;) Filter Changer T Detector Excitation Source 2b / Detector sol I imator Secondary Target Changer FIGURE 2 Representation of wavelength-dispersive and energy-dispersive equipment for X-ray fluorescence spectroscopy. X-RAY DIFFRACTION The power of X-ray fluorescence lies in its use for elemental analysis, whereas the analysis of crystalline phases falls within the province of X-ray diffraction. If one refers again to the Bragg expression for dif- fraction, no = 2d sin 0, the difference between the two techniques becomes clear. In the X-ray fluorescence mode the known values are the lattice dimensions of the crystal, 3, and the Bragg angle, 0; A, the unknown, is then simply determined and related to the element. In

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Analytical Methods Related to Preservation 169 the diffraction case, a known X-ray wavelength is employed, and the diffraction angle, 0, is measured. These are then combined to determine the lattice parameters of the crystalline material that makes up the sample. Interpretations are drawn based on the values of the Bragg angles and the relative intensities of the various lines. The instrumental arrangement is shown in Figure 3. The basic com- ponents consist of a source of X-radiation monochromatized by ap- propriate X-ray fitters, the diffracting specimen, a radiation detector, a rate meter, and a recorder synchronized to the motion of the gon- iometer. In a general way, any crystalline powder will produce a characteristic pattern. Such patterns are used for qualitative analysis, leading to the identification of the phase or compound. Specific identifications are usually made by reference to data in the Powder Diffraction File main- tained by the American Society for Testing and Materials (ASTM). Given a mixture of crystalline materials, the resulting diffraction patterns will consist of superimposed patterns of the individual components. Interpretation is somewhat complicated, but X-ray diffraction is never- theless useful for analyzing mixtures. It should be apparent that the use of techniques for a preliminary elemental analysis can be of great Counter ~ Goniometer ArcO to 165 - / Center of Focusing Circle ~~ / ma/ _, Anode Take-off Angle \ .1~1 _ ~' Receiving Slit I \, Line Focus' `` \ l / l Specimen ~ 900 ~ FIGURE 3 Representation of X-ray diffraction equipment. I Focusing I Circle

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170 CONSERVATION OF HISTORIC STONE BUILDINGS value in supplying clues to the nature of the compounds. Finally, as in X-ray fluorescence, X-ray diffraction is nondestructive. ELECTRON MICROPROBE AND SCANNING ELECTRON MICROSCOPE The utilization of an electron beam focused on a small cross-sectional area of a sample allows for the spatial probing of composition and topography. The interaction of primary electrons with a sample pro- duces signals for example, X-rays, cathodoluIIiinescence, back-scat- tered electrons, Auger electrons, and transmitted or absorbed elec- tronsthat are related to elemental composition. There are also signals related to the topography of the surface, such as secondary electrons and, to a lesser degree, back-scattered electrons {Figure 41. The version of an electron column instrument that has as its primary function the utilization of characteristic X-radiation produced by elec- tron bombardment is termed an electron microprobe. This X-radiation yields compositional information from a spot as small as 1 Em in diameter and so may be used to determine variation in elemental content both in area distribution and within a surface layer whose depth approximates the diameter of the spot. A similar instrument is the scanning electron microscope (SEM).5 6 Its primary function is to utilize the variation in secondary electron emission (electrons scattered Incident Electron Probe X rays Cathode-luminescence >\ Secondary ~ Electrons / Backscattered / r Electrons Electromotive At\ /: Auger Electrons r 1 1 Transmitted Electrons FIGURE 4 A beam of primary electrons, focused on a small cross-sectional area of a sample, produces a variety of signals related to the elemental composition of the sample.

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Analytical Methods Related to Preservation K~ A ~1~ o - ._ Q o o UJ .- Specimen ~ .. Evacuation Device 171 Electron Filament Supply X-ray Spectrometer, Electron Energy Analyzer, etc. as Signal Selector Scanning ~ 1 ~1 Display & Recording Device Tape-puncher Printer Recorder Detector FIGURE 5 Typical layout of an electron microprobe analyzer. by the surface with Toss in energy) that occurs because of differences in surface topography as the electron beam sweeps in a raster TV-type scan) across the sample surface. The electron microprobe and the SEM were developed as separate instruments. Their similarities have been merged in modern instru- ments capable of performing both functions. A modern microprobe usually utilizes a crystal or wavelength spectrometer for X-ray iden- tification, while an SEM utilizes an energy-dispersive {solid state) ~na- lyzer for X-ray identification. (See the section on X-ray fluorescence spectroscopy.) These are operational distinctions, because an instru- ment may be outfitted with either X-ray system. A microprobe is usually devoted to the highest quality X-ray analyses and is equipped to do optical microscopy concurrently with chemical analyses. An example of a typical microprobe layout is shown in Figure 5. A con- ventional SEM Will differ: (a) in the type and number of electromagnetic lenses for focusing the electron beam, (b) in the absence of an optical microscope, and (c) in the use of an altemate X-ray detection system

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172 CONSERVATION OF HISTORIC STONE BUILDINGS If available). Samples must be growing and polished to a flat surface for quantitative analyses by the SEM. The SEM is best utilized as a topographical analyzer of rough fracture surfaces, coupled with semiquantitative or qualitative elemental anal- yses. The electron microprobe is capable of determining all elements from boron through uranium, although analysis is usually limited to all elements above oxygen. The SEM most often analyzes elements above sodium, although the analyses typically are less accurate than with the microprobe. Either of these instruments is capable of resolving 50-100 A in the secondary electron mode. An important distinction should be noted between the electron spot size for electron resolution) and the volume from which X-rays or other signals are being produced or detected See Figure 61. Auger electrons typically are obtained from dimensions of tens of A, secondary elec- trons from 50 to 250 A, and X-rays from 1000 A to a micron or more. This range in spatial resolution results in some signals Secondary electron and Auger) that have the same resolution as the primary probe and others X-rays and back-scattered) that have poorer resolutions. Thus the location of elements in the sample as viewed by electrons does not coincide exactly with the source of the X-ray production. The manner in which the X-rays are processed and converted to intensities is similar to the procedure detailed in the discussion of X- ray methods. The complex compositions of many building materials, Incident Electron Probe / : Volume of ^\\~ Backscattered Electron Emission Specimen Surface Volume of Secondary Electron . . -mlsslon Volume of ~ Characteristic \ X-ray Emission ~ ' FIGURE 6 The size of the spot on which the electron beam is focused differs from the volumes from which the various signals are produced, with consequent differences in resolu- tion.

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Analytical Methods Related to Preservation especially silicates like granites or schists, require that a data-correc- tion procedure be utilized to compensate for interelement and matrix effects arising from differential X-ray absorption and enhancement processes. These procedures are usually handed by an on-line com- puter to allow the operator to evaluate the data within minutes of the analyses. The SEM iS most suitable for studies where details of particle ori- entation and size as well as textural details, such as packing density, void space, recrystallization, or reprecipitation features, are beyond the reach of light microscopy. The SEM offers a 100-fold increase in depth of field over the light microscope, an increase in magnification of 50- 100 x, and a corresponding improvement in resolving power of 100 x . The added attraction of performing energy-dispersive X-ray analyses on the rough sample is that qualitative elemental composition can then be used as an aid in detailing the characterization. The electron microprobe is best suited to detailed quantitative anal- yses of flat surfaces, where no surface irregularities exist. The very powerful data reduction-correction procedures may then be utilized for a micrometer-level characterization. Both types of instruments al- Tow for an X-ray map format, on which elemental distribution is dis- played as a white-black dot matrix on a cathode-ray tube. Alterna- tively, a line scan may be used for the quantitative distribution of an element in a predetermined direction. This display, visual or printed, is well suited to chemical analyses along a transverse line from the outer portions of a stone sample to its interior. Two lesser-known techniques in detailing the form and composition of stone materials are cathodoluminescence analyses and wavelength- shift effect. Cathodoluminescence (et) refers to the light produced upon electron bombardment of certain materials, especially when activator ions such as Mn+2 are present. Many of these materials, including carbonates, silicates, and other building-stone materials, produce Cal in sufficient quantity that details of fractures, recrystallization, and alteration invisible to optical or electron viewing are visible to the eye or to a suitable detector. The wavelength-shift effect refers to the slight shift in wavelength or energy of the X-ray emission of elements of low atomic numbere.g., silver, aluminum, sulfur, and phosphorusas a function of their chemical or mineralogical environment. A sample of aluminum as Al2O3 has a measurable difference in shift of wavelength from that of aluminum in an aluminosilicate. This effect is of major use in the study of mortars and weathered surfaces where noncrys- talline To X-ray diffraction) or amorphous coatings defy phase char- 173

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74 CONSERVATION OF HISTORIC-STONE BUILDINGS acterization. The shifts can be related empirically to minerals whose shifts have been studied. This chemical-environment parameter yields information analogous to that obtained by ESCA (see following section). ELECTRON SPECTROSCOPYCHEMICAL ANALYSIS Among the various instrumental techniques, one of the fastest growing is electron spectroscopy-chemical analysis ~ESCA).7 The method is built on the study of the energy distribution among the electrons ejected from a target material that is being irradiated by X-rays, ultraviolet radiation, or electrons. A convenient method for distinguishing the various kinds of electron spectroscopies is by the mode of excitation. The categories are X-ray photoelectron spectroscopy (xPs); ESCA ultra- violet photoelectron spectroscopy {ups); or Auger spectroscopy, in which electron excitation is employed. Of the three types, ESCA has been perhaps the most used for chemical studies. The power of ESCA lies in its extraordinary sensitivity to surface chemistry. The method is sen- sitive to monolayers and involves distances of the order of angstroms. Further, the emerging electrons carry important information about such parameters as binding energies, charges, and valence states. A unique quality of ESCA iS that it permits direct probing of the valence and core electrons. Figure 7 is a schematic illustration of the production of primary photoelectrons and Auger (secondary) electrons in an atom. The probability of photoelectron absorption depends on the energy of the incident photon and the atomic number of the element being irradiated. To a first approximation the kinetic energy of the photoelectron is given by: Ep = he - En, where Ep is the kinetic energy of the photoelectron, ho is the energy of the incident photon, and Eb is the binding energy of the electron in its particular shell. Thus, if the incident photons are "monoenergetic," the photoelec- trons ejected from a given atomic shell will also be monoenergetic. For a given incident energy of the photons, the photoelectron spectrum will be characteristic, reflecting the various occupied electronic levels and bands in the material. It is necessary to emphasize, however, that the photoelectrons possess the characteristic energies as they leave the atom but that only a relatively small fraction of them emerge from a

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Analytical Methods Related to Preservation Photoelectron Production M2 3P1 /2 2p3l2 - K or 1s `~. Photoelectron K ~ 1S1/2 175 Auger Electron Production KL2L3 Auger Qua FIGURE 7 Schematic illustration of the production of primary photoelectrons and secondary (Auger) electrons in an atom. target material with their energies undisturbed. This follows from the fact that electrons lose energy by a variety of processes as they leave a sample. A typical arrangement for performing ESCA is shown in Figure 8. The necessary components include an X-ray excitation source (usually a X-ray tube containing a magnesium or aluniinum target), the sample, an electron energy analyzer, and the appropriate electronics for pulse counting. Such instrumentation is available today in various com- mercial forms and in various degrees of sophistication. To summarize, ESCA is among the most powerful of the laboratory tools for the ex- amination of surfaces.

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176 CONSERVATION OF HISTORIC STONE BUILDINGS Vacuum System Sample ~ Photoelectrons he Electron Energy Analyzer \\ X-ray Source Electron In Multiplier detector) | Amplifier | Readout System FIGURE 8 Typical instrumental arrangement for electron spectroscopy-chemical analysis. IN SITU ANALYSES BY VISIBLE AND NEAR-INFRARED REFLECTANCE RADIOMETRY The use of reflectance spectroscopy to study geological materials is well established.7 This technique utilizes radiation reflected from a surface illuminated by the sun or an artificial source to record elec- tronic (atomic) and vibrational Molecular) interactions at the surfaces of materials. The electronic processes associated with iron are of special interest because of this element's association with weathering and because the reflection features produced by the interaction of iron's d- shell electrons with its surroundings are within the detection capa- bilities of field instruments. It is important to note that these in situ devices have been designed to operate only in the spectral regions accessible to sensors used by satellite end aircraft remote-sensing systems and so are limited to spectral bands not absorbed by the atmosphere.8 However, these sys- tems may be adapted to other spectral bands by utilizing other fitters and detectors. In this manner it may be possible to measure spectral

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Analytical Methods Related to Preservation 177 features assigned to the presence of silica or carbonate. The currently available systems allow us to distinguish some of the iron oxides and oxyhydroxidese.g., goethite and hematite. Because the reflection spectra are often affected by particle size, we may be able to establish the relative size distribution of minerals as well. Of more immediate use is the radiometer's ability to detect the vibrational modes of the hy- droxy} (OH) group, which is a major component of clay minerals and so may be used to identify various clays as well as some sulfate min- erals. Recent developments in the application of reflectance spectra to the weathering once alteration of rocks and minerals indicate that the ratios of various band intensities in the visible (electronic) and near-infrared (molecular) spectra allow for semiquantitative determination of var- ious clay minerals, calcite {limestone/marble!, iron minerals, and sul- fates.9 The spectra of clay minerals illustrated in Figure 9 have been divided into characteristic ratiose.g., 2.0/2.20 ,um and 2.20/2.35 ,um enabling mineral differentiation. The hand-held radiometer is nearly the size of a small suitcase and is fully portable. The device may be set to a dual-beam mode for readout in a ratio format or to read in a multichannel mode of ~20 different wavelengths. The capabilities of this type of instrument allow for nondestructive, field or laboratory measurement of the major minerals produced during the weathering/alteration of stone materials. Fur- the'~ore, the measurement is limited to near-surface penetrations i.e., the weathered zone. An example of the change in the optical spectra caused by the thickness of the absorbing layer is illustrated in Figure 10. This change in reflectance/absorption may be utilized to determine the depth of alteration, leaching, or weathering. As a func- tion of mineralogy {i.e., wavelength!, the spectra are representative of the upper 2() 50 Am of the snmple.9 NEUTRON-GAMMA TECHNIQUES The use of prompt neutron-gamma techniques is well established in geochemical exploration and environmental monitoring. Prompt neu- tron techniques have been proposed and used for such applications as borehole logging and the detection of pollutants in river and sea beds. The prompt neutron-gamma method involves the measure- ment of gamma rays that result from the interactions of neutrons with the material under analysis. Fast neutrons (energy >1 MeV) can in- teract by scattering inelastically from a nucleus, thereby producing a nucleus in an excited state. Subsequent deexcitation results in the

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78 CONSERVATION OF HISTORIC STONE BUILDINGS ~ I I I I -I I ~ ~ I ~ I T- Phlogopite _ o . o > '$: UJ cr 111 c: at Cal UJ LL LL of Cal ~ /: Kaolinite Montmorillonite M uscovite lo, \ l - 1 1 1 1 1 1 it 1 1 1 1 1 1 l 1.-0 1.5 2.0 2.5 WAVE LENGTH I N. MICROMETERS FIGURE 9 Spectra of clay minerals determined by reflectance ra- diometry. Spectra are superimposed in this figure with indicated spacings of 10 percent reflectance to MgO. Source: G. R. Hunt in Remote Sensing in Geology, B. S. Siegal and A. R. Gillespie, eds. John Wiley: New York, 1980.

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Analytical Methods Related to Preservation 90 o o ~ 70 al LL c: 6 50 cat IL LL] be 30 LU lo: UJ 10 , ~ i_ Sample Thickness (,um) 2 3 O 5 7 8 9 10 8 16 20 22 25 37 45 60 60 o 1.0 WAVELENGTH IN MICROMETERS 1.5 FIGURE 10 Reflectance spectra of 25 percent goethite and 75 percent kaolinite as a function of the thickness of the absorbing layer. 179 emission of characteristic gamma rays. Thermal neutrons can be cap- tured by a nucleus, which increases its atomic mass by one and in the process is left in an excited state. The return to the ground state is accompanied by the emission of a characteristic gamma ray. Me gamma- ray flux measured by a detector depends on the spatial and energy distribution of the neutrons and the location of the detector relative to the source. Figure 11 summarizes both modes of analysis. A possible instrumental configuration is shown in Figure 12. An example of the great amount of data available in a spectrum

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180 Neutron: Neutron Nucleus 0~ CONSERVATION OF HISTORIC STONE BUILDINGS Gamma Ray ~ ~_~L o Jo De-excitea Excited Nucleus Nucleus BEFORE INTERMEDIATE AFTER I N E LAST I C SCATTE R I N G Neutron N ucleus O TO ~ O New Isotope (Excited) New Isotope BEFORE INTERMEDIATE AFTER RADIATIVE CAPTURE Capture Gamma Ray Gamma Ray N uclear Particle ,' Neutron Nucleus ,: ~ 0 ~0 ~~~ ORadioisotope Stable Isotope ACTIVATION FIGURE 11 Neutron-gamma methods of analysis involve detection of characteristic gamma rays emitted as a result of inelastic scattering of fast neutrons or radiative capture of thermal neutrons.

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Analytical Methods Related to Preservation Pulsed 14-MeV Neutron Generator Attenuator Gamma-ray Detector Detector ~~ ; Lunar Surface Neutron Path FIGURE 12 Possible instrumental configuration for analysis by the neutron-gamma method. 181 accumulated in about an hour is shown in Figure 13. This is a spectrum taken of wet soil in an anticoincidence mode. It was possible to identify hydrogen, oxygen, silicon, iron, aluminum, titanium, sodium, calcium, and potassium. Most of the lines were due to neutron capture. It was also possible to identify lines resulting from neutron activation. While this method has not been applied specifically to the problems being discussed at this conference, it appears to hold promise. The major constraints to be considered are that the methods provide strictly el- Albuquerque Spectrum Anti~oincidence Mode Wet Soil Go N S - (D tO i~ ~533 - ~11 ., _ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0~11~1 1 1000 1400 1800 2200 2600 3000 3400 3800 200 600 FIGURE 13 The extensive spectral data shown in the figure were obtained in about an hour using the neutron-gamma method on a sample of wet soil.

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182 CONSERVATION OF HISTORIC STONE BUILDINGS emental data, and the analysis represents a bulk sample. One important positive factor is that the methods are capable of yielding information about the hydrogen content. REFERENCES AND NOTES 1. See, for example, the following recent studies: Petrovic et al. {1976) Rate control in dissolution of alkali feldspars I. Study of residual feldspar grains by X-ray photoelectron spectroscopy, 40~5), 537; Busenberg {1978) The products of the interaction of feldspars with aqueous solutions at 25C, 42~111), 1679; Holdren and Berner {1979) Mechanism of feldspar weathering I. Experimental studies, 43 A), 1161-1187; Tsuzuki and Suzuki t 1980) Experimental study of the alteration of labradorite in acid hydrothermal solutions, 44~5), 673. All articles are from Geochemica Acta. 2. Adler, I. {1966), X-Ray Emission Spectrography in Geology, Elsevier, N.Y. 3. Liebhafsky, H.A., Pfeiffer, H.D., Winslow, F.H. and Zemany, D.D. t1960J X-ray Absorption and Emission in Analytical Chemistry, John Wiley, N.Y. 4. Auger, D. {1925) Secondary ,B-rays produced in O gas by X-rays. Compt. Rend. 180:65-68. 5. Smith, D.G.W. {1976) Short Course in Microbeam Techniques, Mineralogical Association of Canada, Toronto, Ontario. 6. Goldstein, J.I., et al. { 1975~ Practical Scanning Electron Microscopy, Plenum Press, N.Y. 7. Yin Lo I, and Adler, I. {1978i Electron Spectroscopy, Instrumental Analysis, 418- 442, Allyn and Bacon, N.Y. 8. Tucker, C.J., et al. t1980J NASA Tech. Mem. 80641, NASA~SFC. Barringer Company Sales Literature, Denver, Colorado. 9. Hunt, G.R. {1961) Spectral studies of particulate minerals in the visible and near infrared, Geophysics, 42~3), 501. 10. Hertzog, R.C., Plasek, R.E. {1979~ Neutron excited gamma-ray spectrometry for well logging, IKE Trans. Nucl. Sci., NS-26, p. 1558. 11. Johnson, R.G., Evans, L.G., Trombka, J.I. {1979) Neutron-gamma techniques for planetary exploration, IKE Trans. Nucl. Sci., NS-26, p. 1574.