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OCR for page 120
6
Scientific ant! Societal
Benefits
Nuclear physics presents a remarkable paradox: its awesome tech-
nological progeny, nuclear power and nuclear weapons, are among the
most well-known and hotly debated topics of our age, yet the physics
of the nucleus itself is possibly the least understood of the basic
sciences. This is all the more puzzling in light of the profound impact
that nuclear physics has had on the development of the other sciences
as well as on countless areas of modern technology. From solid-state
physics to molecular genetics, from food technology to forensic
medicine, from mineral prospecting to cancer therapy, the principles
and techniques of nuclear physics are applied in ways far too numerous
to survey comprehensively in a book of this size.
In this chapter we touch on a few applications of nuclear physics that
reflect its broad impact on science and technology. Although these
applications cannot evoke the cosmic themes of nuclear astrophysics,
discussed in the preceding chapter, the benefits they confer on a
technological society are both more immediate and more tangible-
even if we tend to take many of them for granted. It is noteworthy that
most of these applications are derived from research carried out at
low-energy facilities, which have provided much of the basis for our
present understanding of nuclear physics.
Implicit in our discussion of the impact of nuclear physics, of course,
is the realization that it is a two-way street. Many advances in nuclear
physics, for example, depend critically on state-of-the-art accelerator
120
OCR for page 121
SCIENTIFIC AND SOCIETAL BENEFITS 121
technology, which hinges, in turn, on new developments in solid-state
electronics, physical chemistry, materials science, cryogenic engineer-
ing, and computer-aided design, to name a few. Theoretical nuclear
physics, which contributes much to our understanding of the basic
forces that govern all natural phenomena, likewise benefits greatly
from the development of physical concepts and mathematical methods
in other disciplines as well as of faster, more powerful computers.
CONDENSED-MATTER PHYSICS
The condensed phases of ordinary matter solids and liquids-
exhibit an enormous diversity of form and function, owing in part to the
great variety of the chemical elements and the types of chemical
bonding that they undergo. Atomic and molecular interactions are
purely electromagnetic, which simplifies the description of solids and
liquids compared with that of nuclear matter. In analogy with nuclear
matter, however, there can be a variety of cooperative motions of large
numbers (here, essentially infinity) of interacting particles, whose net
effect superconductivity, for example transcends the underlying
properties of the particles. Much of the richness of solid-state phenom-
ena, in particular, is due to such cooperative effects.
In probing the structure and behavior of ordinary solid matter
(typically, crystals), physicists have found that accelerated nuclear
beams are extremely useful, since nuclei (ions) of almost every element
can be implanted into a chosen crystal lattice to any desired depth. The
value of this ion implantation technique for solid-state physics research
lies in studies of the ensuing hyperfine interactions: subtle interplays
between the electromagnetic properties of the implanted ions and the
electron configuration of the crystal. Such studies can reveal details of
the crystal's vibrational modes and of its microscopic magnetic and
electrostatic properties. One can also study aspects of the crystal
structure, such as the locations and mobilities of impurities, as well as
the radiation damage caused by the implanted ions, the healing of this
damage through heat treatment,.and the eject of the ions on the
crystal's electrical conductivity.
Information obtained by the ion-implantation technique and by other
techniques derived from nuclear-physics research, such as perturbed
angular correlations, is of great value in developing new materials-
magnetic compounds and alloys, for example with properties that are
tailor-made for specific purposes.
Another phenomenon of solid-state physics that makes use of
nuclear physics techniques is the channeling of charged particles in
OCR for page 122
122 NUCLEAR PHYSICS
5W
_
_
_ _-
FIGURE 6.1 Artistes conception of the channeling of a positively charged particle in a
diamondlike crystal lattice. The particle typically follows a spiral path that actually
consists of a series of oblique ricochets from lattice nuclei along the channel, caused by
the repulsive Coulomb force between the particle and the nuclei. The distance traveled
by the particle in one turn of the spiral is of the order of 100 interatomic distances. (From
W. Brandt, Scientific American, March 1968, p. 91; ~ 1968 by Scientific American, Inc.)
crystals. Here the energetic projectiles bombarding the surface of the
crystal are found to be channeled through the tunnels formed by
adjacent rows of atoms in the lattice structure (see Figure 6.11. Studies
of the behavior of charged particles as they are channeled-or some-
times blocked inside crystals have yielded much information on
surface conditions and the locations of impurities, for example. These
studies can reveal a much deeper level of detail than that provided by
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SCIENTIFIC AND SOCIETAL BENEFITS 123
even the best electron microscopes. They are particularly useful in
evaluating the effects of radiation damage in solids.
Research on channeling is now being conducted at many accelera-
tors around the world, including the highest-energy ones. There is
apparently no practical upper limit to the kinetic energy of charged
particles that can be made to channel in crystals. Relativistic effects
associated with extremely high velocities are being exploited in order
to measure ultrashort time intervals, in an effort to determine the
lifetimes down to perhaps 10-20 second or even less-of some
elementary particles. An intriguing offshoot of these experiments was
the discovery that by bending the crystal, even the most highly
relativistic particles at energies of hundreds of GeV-can be made to
follow curved paths; to bend such particle beams through equivalent
deflection angles in an accelerator would require immensely powerful
superconducting magnets.
The positrons emitted by some radioactive elements have been used
for many years as a sensitive probe with which to map the charge and
energy distributions of electrons in solids. In recent years, however,
the intense, high-quality beams of muons, both positive and negative,
developed at nuclear-physics laboratories have proved to be even more
versatile than positrons in the study of solids. Muons are heavy
leptons much heavier than electrons or positrons, but much lighter
than nucleons. This intermediate mass alone makes them a valuable
probe with which to study solid-state phenomena such as particle
diffusion. Their characteristic decay properties are also valuable.
In addition, muon beams have the useful property of being almost
100 percent spin-polarized, i.e., their spins are all oriented in the same
direction. This property provides the basis for the technique of muon
spin rotation, in which the changing direction and gradual degradation
of the spin polarization are monitored after the beam is injected into a
crystal. The rate and degree of these changes provide information
about the muons' local magnetic environment, at any of several kinds
of sites within the crystal lattice.
As a local probe of solid-state structure and dynamics, muon spin
rotation nicely complements several other techniques derived from
nuclear physics, such as nuclear-magnetic-resonance spectroscopy,
Mossbauer spectroscopy, and neutron scattering. These last three are
also used, to varying degrees, by chemists, biologists, geologists, and
others in countless analytical applications. The influence that nuclear
physics exerts in these sciences is far-reaching and beneficial.
OCR for page 124
124 NUCLEAR PHYSICS
ATOMIC PHYSICS
Although every atom contains a nucleus, many of the physical
properties of the atom are determined by its cloud of orbital electrons.
The electrons interact not only with each other (through the repulsive
Coulomb force) but also with the electric and magnetic fields of the
nucleus. As the properties of nuclei vary throughout the periodic table
or through an isotopic sequence of a given element, so, to different
degrees, do the characteristic features of the associated optical spectra
of the atoms, which are determined by the electron energy levels and
the transitions between them. For many years, much information about
nuclear properties has been deduced from analyses of atomic spectra.
Now, however, with nuclear accelerators that can produce ion
beams of precisely controllable energy and ionization state, it is
possible to create exotic atomic species unlike any that exist under
ordinary conditions, and thus to use nuclear beams to study novel
aspects of atomic physics. Such experiments and the corresponding
atomic-structure calculations are interesting in their own right. They
also have a direct bearing on our understanding of the nature of
thermonuclear fusion plasmas both in stellar interiors and in terres-
trial machines such as tokamak fusion reactors.
In collisions between very heavy ions (uranium and curium, for
example, for which the combined Z value is 188), a massive nuclear
system can be created that exists long enough for the electrons of the
two ions to rearrange themselves in a configuration corresponding to
the combined Z value. In the formation of this extremely high-,
pseudo-atom, however, a vacancy is sometimes created in the lowest
electron shell. This shell becomes tightly bound while the nuclei are
close together, and if the vacancy is filled during this period, the effect
is formally equivalent to the creation of a positron. Recently positrons
have, in fact, been detected in heavy-ion collisions at the GSI accel-
erator in Darmstadt, West Germany. Surprisingly, one observes a
discrete structure superimposed on a continuum spectrum. The origin
of the sharp features in this structure is a mystery. Speculations have
arisen as to whether it is due to the formation of relatively long-lived
giant nuclear complexes or to some hitherto unknown physical phe-
nomenon.
In a different kind of atomic-physics experiment, accelerated heavy
ions are stripped of most of their electrons by passing the beam through
thin films or low-pressure gases. Careful stripping can yield heavy
nuclei with only one orbital electron (a hydrogenlike ion) or two
electrons (a heliumlike ionJ. These species thus exhibit a huge imbal
OCR for page 125
SCIENTIFIC AND SOClETA ~ BENEFITS 125
ance between the positive nuclear charge and the surrounding negative
electron charge. Studying their atomic spectra affords a unique oppor-
tunity for making stringent tests of some of the predictions of quantum
electrodynamics (QED), the quantum field theory of the electromag-
netic interaction. One of these predictions concerns a fundamental but
subtle spectroscopic effect called the Lamb shift, which can be
measured with great accuracy. To date, all measurements of the Lamb
shift in hydrogenlike ions (e.g., one-electron chlorine) and helium-
like ions (e.g., two-electron neon) have confirmed the correctness of
QED.
It is also possible to strip all the electrons from an accelerated ion,
leaving a bare nucleus as the projectile. In 1982 the production of
relativistic beams of fully stripped uranium (U92 +) was demonstrated at
the Bevalac accelerator at the Lawrence Berkeley Laboratory. Also
recently, very-low-energy, fully stripped heavy ions have been pro-
duced at the Double Tandem facility at Brookhaven National Labora-
tory. Collisions between these slow nuclei and target atoms produce
relatively long-lived, very heavy pseudo-atoms. The study of x rays
resulting from such collisions is expected to provide a better under-
standing of the processes that are critical for the production of
superheavy atoms and to enable further tests to be made of the
attendant QED phenomena in very heavy atomic species.
The experiments described above illustrate only a few of the ways in
which the techniques of nuclear physics have expanded the boundaries
of atomic physics, thereby both broadening and deepening our under-
standing of this vital subject.
GEOLOGY AND COSMOLOGY
Ancient objects whether man-made or natural, whether of geolog-
ical or cosmological origin- are fascinating to scientists in many fields
because of the invaluable clues they provide about the nature of the
environment in which they were formed. Along with the chemical and,
sometimes, microbiological analyses of such objects, their accurate
dating is clearly of great importance. The familiar technique of radio-
carbon dating (using carbon-14, which has a half-life of 5730 years) was
one of the earliest practical applications of nuclear physics. It has
proved to be of inestimable value in archeology and paleontology,
enabling scientists to date events that occurred as far back as 50,000
years ago. Similar measurements of the decay products of other
long-lived radionuclides have extended the applicability of the tech-
nique.
OCR for page 126
126 NUCLEAR PHYSICS
Another dramatic advance in dating technology has taken place in
the last few years, again as a spinoff of basic nuclear-physics research.
Various heavy-ion accelerators around the world have been modified
for use as ultrasensitive mass spectrometers, in which the atoms of
long-lived radionuclides in the sample of interest are counted directly,
rather than indirectly (and slowly) by detecting the radiations associ-
ated with their decay. The immediate result of this ability to circum-
vent the tedious process of radiation monitoring of specimens has been
a spectacular increase in the sensitivity of dating measurements by a
factor of as much as 10'2! This sensitivity, in turn, allows the use of
much smaller samples (in the range of micrograms to milligrams) than
before.
Thus the technique of accelerator mass spectrometry, still in its
infancy but developing rapidly, has vastly enlarged our scientific
window on the past. Among the growing list of subjects being inves-
tigated with this powerful new tool are atmospheric methane, polar ice,
lake and ocean sediments, manganese nodules, tektites, meteorites,
and long-lived radionuclides produced by cosmic rays.
Geophysicists, paleoclimatologists, cosmologists, and others have
much to gain from such studies, which reveal new information on
changes that have occurred both on the Earth and outside the Earth
over periods ranging from thousands to tens of millions of years.
Already it has been learned, for example, that some manganese
nodules on the ocean floor have grown at a uniform rate (of the order
of a few millimeters per million years) for as long as 10 million years,
whereas others have grown at sharply different rates during different
periods of geologic time. The latter phenomenon suggests that signifi-
cant changes in the manganese and iron content of local undersea
growth environments have occurred at certain times in the past.
Another interesting discovery from the deep is that ocean sediments
at plate-tectonic boundaries are not scraped off during the subduction
process, in which the edge of one crustal plate is bent downward and
very slowly slides beneath the edge of the other. Instead, the sediments
are carried down with the subducting plate, eventually to reappear in
volcanic eruptions in these geologically volatile areas. The radio-
nuclide whose atoms were counted in these studies, as in those of the
manganese nodules, was beryllium-10, which has a half-life of 1.6 x 106
years; it thus allows the dating of events that occurred over the last 10
million to 20 million years. Similarly useful in geochronological studies
over an even greater time scale are the radionuclides manganese-53 and
iodine-129 (half-lives of 3.7 x 106 years and 1.6 x 107 years, respec
OCR for page 127
SCIENTIFIC AND SOCIETAL BENEFITS 127
lively), whereas aluminum-26 (half-life of 7.2 x 105 years) is useful
over a time scale of a few million years.
Of obvious scientific interest are any objects, such as meteorites and
cosmic rays, that reach the Earth from outer space. Until recently, it
was thought that most tektites- strange, glassy objects that have been
found widely distributed on land and under the seas-were of extrater-
restrial origin. However, a careful comparison of their nuclidic com-
positions with those of terrestrial and extraterrestrial rocks, using
accelerator mass spectrometry, has now made it certain that tektites
are terrestrial objects after all. Whatever the ultimate significance of
this fact may prove to be, its discovery exemplifies part of the
excitement of scientific research- knowing that progress will be made,
but never knowing exactly from which quarter the breakthrough will
come.
NUCLEAR AND RADIATION MEDICINE
For many years, nuclear physicists have been collaborating with
physicians, chemists, pharmacologists, and computer scientists in a
highly successful effort to solve some of society's most pernicious
health problems. Their efforts have firmly established nuclear medicine
as a standard part of modern medical practice. While the most widely
applied techniques of nuclear medicine entail the use of radioactive
tracers to diagnose diseases and monitor their treatment, radionuclides
and accelerated particle beams also play important therapeutic roles.
In addition, nuclear physics serves medical science through the devel-
opment of exotic materials for use in prosthetic implants.
In a typical nuclear-medicine examination, of which many millions
are performed annually, a radiopharmaceutical agent is administered
intravenously, and gamma rays emitted by the tracer nuclide are
recorded with an array of radiation detectors positioned about the
patient; this technique is called emission tomography. The tracer
compounds are usually chosen for their selective uptake by a particular
organ or type of tissue so that the detected gamma rays provide a
detailed image of the region of interest. Advances in detector design
and in data acquisition and analysis have led to markedly improved
instruments for emission tomography of both the photon and positron
types (see Figure 6.21. To the trained eye, the images produced can
reveal structural or metabolic abnormalities whose recognition can
lead to a diagnosis that might otherwise be more difficult or even
impossible.
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128 NUCLEAR PHYSICS
FIGURE 6.2 A computerized tomographic (cross-sectional) image of the human brain,
showing regional metabolic demand for oxygen. A few seconds after the subject inhaled
oxygen labeled with the positron-emitting radionuclide i50 (half-life of 122 seconds), the
distribution of oxygen in the brain was revealed (bright areas) by gamma rays resulting
from annihilation of the positrons with electrons in the surrounding tissue. The technique
of positron-emission tomography has become a powerful tool of nuclear medicine.
(Courtesy of R. J. Nickles, University of Wisconsin.)
The recent development of the radionuclide thallium-201 from the
research stage to commercial production for worldwide clinical use
provides an illustration of how progress results from multidisciplinary
investigations. One out of every six Americans is afflicted with
cardiovascular disease, often undiagnosed, and over 70,000 deaths
from heart attacks occur in the United States each year. Until about 10
years ago, the available tracer nuclides for early diagnosis of cardio-
vascular disease were generally unsatisfactory. Following the demon-
stration that after thallium is administered it is rapidly and selectively
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SCIENTIFIC AND SOCIETAL BENEFITS 129
localized in the heart muscle, however, nuclear scientists devised
techniques for producing pure thallium-201 (half-life of 73 hours) in
commercial quantities at affordable cost. As a result, nuclear cardiol-
ogy tests using this nuclide were administered to some 250,000 patients
in 1981.
An even more impressive example of progress in nuclear medicine is
the development of the radionuclide technetium-99m (a metastable
excited state of technetium-99, with a half-life of 6 hours) over the last
two decades. Radiopharmaceuticals incorporating this nuclide have
proved to be invaluable for studying the brain, liver, thyroid, lungs,
skeletal system, kidneys, heart, and hepatobiliary system. About 5
million patient studies using technetium-99m were performed in the
United States in 1981.
Roughly half of the 850,000 new cases of cancer that occur in the
United States each year receive radiation therapy, either alone or in
conjunction with surgery or chemotherapy. The electiveness of radi-
ation therapy can be increased by improving both the dose localization
and the biological effect of the delivered dose. Either of these will
result in proportionally more damage to the tumor and less damage to
normal tissue. Improved dose localization can be achieved by using
accelerated beams of charged particles such as electrons, protons,
heavy ions, and negative pions. Biological electiveness depends in
part on the stopping power of the tissue for the particle in question and
can be increased by using the particle in its characteristic stopping
region.
Nuclear physics contributes in a number of ways to this research. A
thorough understanding of nuclear as well as atomic phenomena is
required not only for determining the optimal type and energy of the
primary beam, the production target material, and the shielding re-
quirements but also for calculating dose distributions. Because of the
slim margin between the responses of tumors and normal tissues,
differences as small as about 5 percent in dose must be carefully
monitored and controlled for proper treatment. In therapeutic radiol-
ogy, as in nuclear medicine, progress depends on close collaboration
among physicists, chemists, and physicians, with the additional re-
quirement of coordinated advances in accelerator physics and instru-
mentation. For example, the improved design of compact, relatively
inexpensive linear accelerators has led to their widespread use in
clinical x-ray and electron-beam radiotherapy.
A final example in this abbreviated survey of ways in which nuclear
physics is contributing to medicine concerns some new work on
surgical alloys used for articulating orthopedic implant devices, such as
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130 NUCLEAR PHYSICS
artificial hip joints. Over 75,000 hipjoint replacement operations are
performed in the United States each year. Unfortunately, with pro-
longed use, these joints are subject to gradual deterioration caused by
the corrosive effects of normal body fluids; the resulting metallic debris
can then poison and inflame the surrounding tissues. This can neces-
sitate a second replacement of the joint an obviously undesirable
prospect.
Recently, however, materials scientists have taken a major step
toward solving this problem. Using ion-source and accelerator tech-
nology originally developed by nuclear physicists for basic research,
they have found that the implantation of nitrogen ions to a concentra-
tion of 20 atom percent to a depth of about 100 nanometers (100 x 10-9
meter) into the surface of a typical surgical alloy reduces the wear
corrosion by a factor of at least 400. The successful clinical application
of these new results could be of great benefit to patients requiring
artificial articulating joints.
MATERIALS MODIFICATION AND ANALYSIS
Armed with ion sources, accelerators, and instruments developed in
low-energy nuclear-physics research, investigators in numerous disci-
plines are using energetic ion beams to modify and study the near-
surface properties of materials in highly selective and often unique
ways. When these beams stop in a solid, ion implantation occurs,
which can alter or even dominate the electrical, mechanical, chemical,
optical, magnetic, or superconducting properties of the material. The
results are often dramatic.
Perhaps the most impressive application of ion implantation arises in
solid-state electronics. Most semiconducting devices require the selec-
tive doping of silicon or germanium crystals with impurity atoms, and
ion implantation has rapidly become the dominant doping technique in
the semiconductor industry. Among its many advantages is that it
permits extreme miniaturization; consequently, most semiconductor
devices and integrated circuits for watches, calculators, computer
chips, and other electronic porducts requiring small components are
fabricated by this method.
Ion implantation has also been exploited in a myriad of other
applications. Controlled ion damage to insulators and semiconductors
is used to alter the index of refraction of such materials for the
fabrication of optical waveguides and mixers and to modify magnetic-
bubble memory devices selectively. Ion implantation holds promise as
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SCIENTIFIC AND SOCIETAL BENEFITS 13 1
a fabrication method for high-temperature superconducting materials,
since these require the formation or stabilization of a metastable phase
that need exist only within a few tens of nanometers of the surface. Ion
bombardment has also recently been discovered to be effective in
bonding thin films to substrates.
Studies of the dynamic behavior of light impurities such as hydrogen
and helium embedded in materials and of the changes in the proper-
ties of materials induced by the presence of these impurities have
been carried out in recent years with new accelerator-based tech
niques. The depth distribution of the impurity can be precisely mapped
by making use of the sharp resonance behavior of nuclear reactions as
a function of the incident beam energy. These reactions, using ion
beams such as lithium-7, boron-11, nitrogen-15, ~9uorine-19, and chlo-
rine-35, have very fine depth resolution (about 5 nanometers) and high
sensitivity (better than 1 part per million). Problems for which this
technique is used include the erosion of thermonuclear fusion reactor
walls, the characterization of amorphous silicon solar cells, the embrit-
tlement of steels and niobium by hydrogen contamination, and the
effects of the solar wind (high-energy hydrogen and helium nuclei
ejected by the solar corona) on moon rocks.
ENERGY TECHNOLOGY
Basic research in nuclear physics has created-and continues to
create a fund of advanced technology that pervades energy-related
research and development. The most familiar examples, of course, are
those of nuclear fission and fusion. Nuclear fission reactors currently
satisfy about 13 percent of the electric power demand in the United
States, and nuclear fusion holds the promise of satisfying the bulk of
this demand in the twenty-first century and beyond.
The impact of nuclear physics on energy technology is also felt,
however, in other, less-well-known areas. Nuclear techniques are used
by the drilling industry to help probe geologic formations and locate
hydrocarbons and other valuable resources that are deep underground.
Passive forms of nuclear well-logging employ gamma-ray detectors to
distinguish regions containing clean sands and carbonates (low natural
radioactivity) from the less productive but more radioactive regions
containing clays or shale rock. More sophisticated well-logging tech-
niques generate neutrons with the aid of miniaturized nuclear acceler-
ators that can be lowered into the test bores, which are typically about
10 cm in diameter. The apparatus produces fast neutrons by bombard-
ing a tritium target with an accelerated, pulsed deuteron beam, and the
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132 NUCLEAR PHYSICS
interactions of the neutrons with the surrounding material provide the
logging information.
In one application, gamma rays following inelastic neutron scattering
are measured, and the well log is inspected for the characteristic yield
that indicates the presence of carbon, the main constituent of oil and
gas. In another application, neutron detectors are used to measure the
duration of the well-defined slow-neutron pulse that results when the
initial fast neutrons from the accelerator encounter hydrogen in the
surrounding material. Rapid disappearance of the slow-neutron pulse
suggests that the hydrogen in the region is accompanied by chlorine,
which has a high efficiency for the capture of slow neutrons, and
indicates the presence of saltwater. A long-lasting pulse shows that
chlorine is not present and provides a good indication of petroleum
deposits. The sensitivity of these and related nuclear techniques helps
identify oil- or gas-bearing regions that might otherwise be overlooked.
Whenever research and development efforts lead to increased effi-
ciencies in existing energy technologies, the result is energy conserva-
tion. Here too, the impact of nuclear physics is felt in various ways.
For example, tracer techniques have been used to study friction and
wear in gasoline engines by incorporating radioactive carbon in steel
piston rings. Inhibiting friction and wear and hence improving effi-
ciency- can often be accomplished by using the ion-implantation
method to modify the surface properties of materials. Wire-drawing
dies that have been ion implanted with nitrogen, at a cost of only a few
dollars per die, can be kept in service about five times longer than
ordinary dies, with consequent savings in tooling costs, plant down-
time, and other tool-replacement costs.
Ion implantation also shows promise for the fabrication of corrosion-
resistant surface alloys, the use of which would conserve rare or
strategic alloying metals such as chromium, platinum, cobalt, and
tungsten. The conservation occurs not only through corrosion reduc-
tion but also because nuclear accelerators permit the implantation of
these scarce elements selectively into the surface of the material-
precisely where they are needed for corrosion resistance.
Intimately intertwined with these ongoing studies are efforts by
metallurgists and other materials scientists to understand the ejects of
intense radiation on the properties of structural materials and to design
new materials for service in advanced fission and fusion reactors.
Examples of the problems that must be investigated are the stability of
waste-containment materials and the embrittlement and damage of
reactor materials due to irradiation by neutrons, protons, and alpha
particles. Such studies have already helped to identify metallurgical
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SCIENTIFIC AND SOCIETAL BENEFITS 133
A: :]
FIGURE 6.3 Saint Rosalie interceding for the Plague-Stricken of Palermo, by
Anthony Van Dyck. A conventional photograph of this oil painting is shown at the top
left. At the top right is an x-ray radiograph, which reveals traces of a hidden painting
underneath. This underlying painting is revealed more clearly in the two neutron
autoradiographs shown at the bottom. The hidden painting turned out to be a self-
portrait. (Courtesy of the Metropolitan Museum of Art, New York, and the Brookhaven
National Laboratory.)
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134 NUCLEAR PHYSICS
techniques for minimizing high-temperature swelling and grain-
boundary embrittlement. They are also being used to look for possible
ways of minimizing radiation damage by annealing the materials, using
either controlled preirradiation or the ambient radiation of the reactor
itself.
THE FINE ARTS
Nuclear techniques based on the use of neutron-induced radioactiv-
ity in art objects have been used for many years as tools for determin-
ing the elemental composition and thus, often, the origin of these
objects. Recently, however, the complete neutron irradiation of paint-
ings, followed by autoradiography, has proved to be a valuable
technique for studying the underlying paint layers, which record the
evolution of paintings by the great (and lesser) masters. The technique
involves making a series of radiographic exposures over periods of
many days following the neutron irradiation. Because of the difference
in half-lives of radioactive nuclides of elements such as manganese,
sodium, copper, arsenic, mercury, and antimony, it is possible to view
selectively the images contained in the many layers of paint present in
a typical oil painting.
In a program conducted by the Metropolitan Museum of Art in New
York, many oil paintings by masters such as Rembrandt, Hats, Van
Dyck, and Vermeer have been examined. Many reveal not just one but
several previously unknown underlying images (see Figure 6.3), which
reveal the compositional evolution of the painting and the thoughts and
moods of the artist.
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III
Burnt Unhip of
clc~ Panics
OCR for page 136
Representative terms from entire chapter:
ion implantation
