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OCR for page 163
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
OCR for page 164
164
INTRODUCTION
CONSERVATION OF HISTORIC STONE BUILDINGS
The disintegration and decomposition of stone materials—natural rock
or mineral components and fabricated composites—often 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 carbonation—chiefly 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-
OCR for page 165
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
OCR for page 166
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-
OCR for page 167
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.
OCR for page 168
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
OCR for page 169
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
OCR for page 170
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-
trons—that 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.
OCR for page 171
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
OCR for page 172
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.
OCR for page 173
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 number—e.g., silver, aluminum, sulfur, and phosphorus—as 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
OCR for page 174
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 SPECTROSCOPY—CHEMICAL 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
OCR for page 175
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.
OCR for page 176
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
OCR for page 177
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
oxyhydroxides—e.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 ratios—e.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
OCR for page 178
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.
OCR for page 179
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
OCR for page 180
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.
OCR for page 181
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.
OCR for page 182
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 25°C, 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.
Representative terms from entire chapter:
methods related
