Chapter 17. Information from Isotopes

The applications of nuclear processes can be divided into three basic classes—military, power, and radiation. In a conference^[†] shortly after the end of World War II the famous physicist Enrico Fermi discussed potential applications of radioisotopes. He then said, “It would not be very surprising if the stimulus that these new techniques will give to science were to have an outcome more spectacular than an economic and convenient energy source or the fearful destructiveness of the atomic bomb.”

^† Enrico Fermi, “Atomic Energy for Power,” in Science and Civilization, The Future of Atomic Energy, McGraw-Hill, New York, 1946.

Perhaps Fermi would be surprised to see the extent to which radioisotopes have become a part of research, medicine, and industry, as described in the following sections.

Many important economic and social benefits are derived from the use of isotopes and radiation. The discoveries of modern nuclear physics have led to new ways to observe and measure physical, chemical, and biological processes, providing the strengthened understanding so necessary for human survival and progress. The ability to isolate and identify isotopes gives additional versatility, supplementing techniques involving electrical, optical, and mechanical devices.

Radioisotopes have become even more prominent in the wake of the tragic events of September 11, 2001. Detection of potential hazards has become a high priority of the United States Department of Homeland Security.

Special isotopes of an element are distinguishable and thus traceable by virtue of their unique weight or their radioactivity, while essentially behaving chemically as do the other isotopes of the element. Thus it is possible to measure amounts of the element or its compounds and trace movement and reactions.

When one considers the thousands of stable and radioactive isotopes available and the many fields of science and technology that require knowledge of process details, it is clear that a catalog of possible isotope uses would be voluminous. Here we will only be able to compare the merits of stable and radioactive species, to describe some of the special techniques, and to mention a few interesting or important applications of isotopes.

17.1. Stable and Radioactive Isotopes

Stable isotopes, as their name suggests, do not undergo radioactive decay. Most of the isotopes found in nature are in this category and appear in the element as a mixture. The principal methods of separation, according to isotopic mass, that have been used are electromagnetic, as in the large-scale mass spectrograph; and thermal-mechanical, as in the distillation or gaseous diffusion processes. Important examples are isotopes of elements involved in biological processes (e.g., deuterium and oxygen-18). The main advantages of stable isotopes are the absence of radiation effects in the specimens under study, the availability of an isotope of a chemical for which a radioactive species would not be suitable, and freedom from necessity for speed in making measurements, because the isotope does not decay in time. Their disadvantage is the difficulty of detection.

Radioactive isotopes, or radioisotopes, are available with a great variety of half-lives, types of radiation, and energy. They come from three main sources—charged particle reactions in an accelerator, neutron bombardment in a reactor, and separated fission products. Among the principal sources of stable and longer lived isotopes are the United States Department of Energy (DOE) (see References), MDS Nordion of Canada, and Russia. A number of cyclotrons that generate radioisotopes are located at hospitals. The main advantages of the use of radioisotopes are ease of detection of their presence through the emanations and the uniqueness of identifying the half-lives and radiation properties. Potential shortage is a perennial problem for users of radioisotopes. The number of reactor sources is limited and some are being shut down. In an American Nuclear Society position statement (see References) a strong recommendation is made, stating, “There is no present U.S. policy for the purpose of maintaining reliable sources of radioisotope supplies crucial for both medical and industrial applications.”

17.2. Tracer Techniques

We will now describe several special methods involving radioisotopes and illustrate their use. The tracer method consists of the introduction of a small amount of an isotope and the observation of its progress as time goes on. For instance, the best way to apply fertilizer containing phosphorus to a plant may be found by including minute amounts of the radioisotope phosphorus-32, half-life 14.28 days, emitting 1.7 MeV beta particles. Measurements of the radiation at various times and locations in the plant by a detector or photographic film provides accurate information on the rate of phosphorus intake and deposition. Similarly, circulation of blood in the human body can be traced by the injection of a harmless solution of radioactive sodium, Na-24, 14.96-h half-life. For purposes of medical diagnosis, it is desirable to administer enough radioactive material to provide the needed data, but not so much that the patient is harmed.

The flow rate of many materials can be found by watching the passage of admixed radioisotopes. The concept is the same for flows as diverse as blood in the body, oil in a pipeline, or pollution discharged into a river. As sketched in Figure 17.1, a small amount of radioactive material is injected at a point, it is carried along by the stream, and its passage at a distance d away at time t is noted. In the simplest situation, the average fluid speed is d/t. It is clear that the half-life of the tracer must be long enough for detectable amounts to be present at the point of observation but not so long that the fluid remains contaminated by radioactive material.

Figure 17.1. Tracer measurment of flow rate.

In many tracer measurements for biological or engineering purposes, the effect of removing the isotope by other means besides radioactive decay must be considered. Suppose, as in Figure 17.2, that liquid flows in and out of a tank of volume V (cm³) at a rate υ (cm³/s). A tracer of initial amount N⁰ atoms is injected and assumed to be uniformly mixed with the contents. Each second, the fraction of fluid (and isotope) removed from the tank is υ/V, which serves as a flow decay constant λ^f for the isotope. If radioactive decay were small, the counting rate from a detector would decrease with time as exp(−λ^ft). From this trend, one can deduce either the speed of flow or volume of fluid, if the other quantity is known. If both radioactive decay (λ) and flow decay (λ^f) occur, the exponential formula may also be used, but with the effective decay constant λ^e = λ + λ^f. The composite effective half-life then can be found from the relationship

Figure 17.2. Flow decay.

This formula is seen to be of the same form as the one developed in Section 16.1 for radioactive materials in the body. Here, the flow half-life takes the place of the biological half-life.

Soon after Watson and Crick explained the structure of DNA in 1951, tracers P-13 and S-35 were used to prove that genes were associated with DNA molecules. Tritium-labeled thymidine, involved in the cell cycle, was synthesized. The field of molecular biology expanded greatly since then, leading to the Human Genome Project (see References), an international effort to map the complete genetic structure of human beings, involving chromosomes, DNA, genes, and protein molecules. Its purpose is to find which genes cause various diseases and to enable gene therapy to be applied. Part of the complex process of mapping is hybridization, in which a particular point on the DNA molecule is marked by a radioactive or fluorescent label.

An outgrowth of genetic research is DNA fingerprinting, a method of identifying individual persons, each of which (except for identical twins) has a unique DNA structure. In one of the techniques (RFLP) the procedure involves treating samples of blood, skin, or hair with an enzyme that splits DNA into fragments. The membrane containing them is exposed to a radioactive “probe” and dark bands appear on an X-ray film. The method is accurate but requires a large sample and a long exposure of film. An alternate method (PCR-STR) is more popular. It involves making multiple copies of DNA. The processes are used in crime investigation and court cases to help establish guilt or innocence and to give evidence in paternity disputes.

17.3. Radiopharmaceuticals

Radionuclides prepared for medical diagnosis and therapy are called radiopharmaceuticals. They include a great variety of chemical species and isotopes with half-lives ranging from minutes to weeks, depending on the application. They are generally beta- or gamma-ray emitters. Prominent examples are technetium-99m (6.01 h), iodine-131 (8.04 d), and phosphorus-32 (14.28 d).

A radionuclide generator is a long-lived isotope that decays into a short-lived nuclide used for diagnosis. The advantage over the use of the short-lived isotope directly is that speed or reliability of shipment is not a factor. As needed, the daughter isotope is extracted from the parent isotope. The earliest example of such a generator was radium-226 (1599 y), decaying into radon-222 (3.82 d). The most widely used generator is molybdenum-99 (65.9 h) decaying to technetium-99m (6.01 h). The Tc-99m is said to be “milked” from the Mo-99 “cow.” Tc-99m is the most widely used radioisotope in nuclear medicine because of its favorable radiations and half-life. The parent isotope Mo-99 comes from Canada and other countries. If for any reason the United States borders were closed to imports of radioactive materials, innumerable medical tests would cease.

Several iodine isotopes are used. One produced by a cyclotron is I-123 (13.2 h). The accompanying isotopes I-124 (4.18 d) and I-126 (13.0 d) are undesirable impurities because of their excessively energetic gamma rays. Two fission products are I-125 (59.4 d) and I-131 (8.04 d).

Table 17.1 illustrates the variety of radionuclides used, their chemical forms, and the organs studied.

Table 17.1. Radiopharmaceuticals Used in Medical Diagnosis

Radionuclide	Compound	Use
Technetium-99m	Sodium pertechnate	Brain scanning
Hydrogen-3	Tritiated water	Body water
Iodine-131	Sodium iodide	Thyroid scanning
Gold-198	Colloidal gold	Liver scanning
Chromium-51	Serum albumin	Gastrointestinal
Mercury-203	Chlormerodrin	Kidney scanning
Selenium-75	Selenomethionine	Pancreas scanning
Strontium-85	Strontium nitrate	Bone scanning

Specialists in radiopharmaceuticals are called radiopharmacists, who are concerned with the purity, suitability, toxicity, and radiative characteristics of the radioactive drugs they prepare.

17.4. Medical Imaging

Administering a suitable radiopharmaceutical to a patient results in a selective deposit of the radioactive material in the tissue or organ under study. The use of these radionuclides to diagnose malfunctions or disease is called “medical imaging.” Approximately 20 million diagnostic nuclear medicine studies are performed each year in the United States. In imaging, a photographic screen or a detector examines the adjacent area of the body and receives an image of the organ, revealing the nature of some medical problem. A scanner consists of a sodium iodide crystal detector, movable in two directions, a collimator to define the radiation, and a recorder that registers counts in the sequence of the points it observes. In contrast, an Anger scintillation camera is stationary, with a number of photomultiplier tubes receiving gamma rays through a collimator with many holes, and an electronic data-processing circuit.

The Anger camera provides a view of activity in the form of a plane. The introduction of computer technology has made possible more sophisticated displays, including three-dimensional images. Such a process is called tomography, of which there are several types. The first is single photon emission computer tomography (SPECT), which has a rotating camera that takes a series of planar pictures of the region containing a radionuclide. A sodium iodide crystal detects uncollided photons from the radioactive source and produces electric signals. Data from 180 different angles are processed by a computer to give 2D and 3D views of the organ. SPECT is used especially for diagnosis of the heart, liver, and brain. The second is positron emission tomography (PET), in which a positron-emitting radiopharmaceutical is used. Three important examples are oxygen-15 (2 min), nitrogen-13 (10 min), and carbon-11 (20 min). They are isotopes of elements found in all organic molecules, allowing them to be used for many biological studies and medical applications, especially heart disease. A fourth, fluorine-18 (110 min), is especially important in brain studies, in which there is difficulty getting most chemicals through what is called the blood–brain barrier. In contrast, F-18 forms a compound that acts like glucose, which can penetrate brain tissue and show the location of a disease such as stroke or cancer. The isotopes are produced by a cyclotron on the hospital site, and the targets are quickly processed chemically to achieve the desired labeled compound. The gamma rays released in the annihilation of the positron and an electron are detected, taking advantage of the simultaneous emission (coincidences) of the two gammas and their motion in opposite directions. The data are analyzed by a computer to give high-resolution displays. PET scans are analogous to X-ray computerized axial tomography (CT) scans, but better for some purposes. Figure 17.3 compares the ability of CT and PET to locate a brain tumor.

Figure 17.3. CT and PET scans of a brain tumor.

(Courtesy of Lawrence Berkeley National Laboratory).

An alternate diagnostic method that is very popular and does not involve radioactivity is magnetic resonance imaging (MRI). It takes advantage of the magnetic properties of atoms in cells. Formerly it was called nuclear magnetic resonance (NMR), but physicians adopted the new name to avoid the association with anything “nuclear.” There are approximately 900 MRI units in the United States. References are included for the interested reader.

17.5. Radioimmunoassay

Radioimmunoassay, discovered in 1960 by Yalow and Berson, is a chemical procedure that uses radionuclides to find the concentration of biological materials very accurately in parts per billion and less. It was developed in connection with studies of the human body's immune system. In that system a protective substance (antibody) is produced when a foreign protein (antigen) is introduced. The method makes use of the fact that antigens and antibodies also react. Such reactions are involved in vaccinations, immunizations, and skin tests for allergies.

The object is to measure the amount of an antigen present in a sample containing an antibody. The latter has been produced previously by repeatedly immunizing a rabbit or guinea pig and extracting the antiserum. A small amount of the radioactively labeled antigen is added to the solution. There is competition between the two antigens, known and unknown, to react with the antibody. For that reason the method is also called competitive binding assay. A chemical separation is performed, and the radioactivity in the products is compared with those in a standard reaction. The method has been extended to many other substances including hormones, enzymes, and drugs. It is said that the amounts of almost any chemical can be measured very accurately, because it can be coupled chemically to an antigen.

The method has been extended to allow medical imaging of body tissues and organs. Radiolabeled antibodies that go to specific types of body tissue provide the source of radiation. As noted in Section 18.1, the same idea applies to radiation treatment. The field has expanded to include many other diagnostic techniques not involving radioactivity (see References).

17.6. Dating

There would seem to be no relationship between nuclear energy and the humanities such as history, archaeology, and anthropology. There are, however, several interesting examples in which nuclear methods establish dates of events. The carbon dating technique is being used regularly to determine the age of ancient artifacts. The technique is based on the fact that carbon-14 is and has been produced by cosmic rays in the atmosphere (a neutron reaction with nitrogen). Plants take up CO² and deposit C-14, and animals eat the plants. At the death of either, the supply of radiocarbon obviously stops and the C-14 that is present decays, with half-life 5715 y. By measurement of the radioactivity, the age within approximately 50 y can be found. This method was used to determine the age of the Dead Sea Scrolls, as approximately 2000 y, making measurements on the linen made from flax; to date documents found at Stonehenge in England, by use of pieces of charcoal; and to verify that prehistoric peoples lived in the United States, as long ago as 9000 y, from the C-14 content of rope sandals discovered in an Oregon cave. Carbon dating proved that the famous Shroud of Turin was made from flax in the 14th century, not from the time of Christ.

Even greater accuracy in dating biological artifacts can be obtained by direct detection of carbon-14 atoms. Molecular ions formed from are accelerated in electric and magnetic fields and then slowed by passage through thin layers of material. This sorting process can measure three atoms of out of 10¹⁶ atoms of . Several accelerator mass spectrometers are in operation around the world (see References).

The age of minerals in the Earth, in meteorites, or from the Moon can be obtained by a comparison of their uranium and lead contents. The method is based on the fact that Pb-206 is the final product of the decay chain starting with U-238, half-life 4.47 × 10⁹ y. Thus the number of lead atoms now present is equal to the loss in uranium atoms, i.e.,where

Elimination of the original number of uranium atoms (N^U)⁰ from these two formulas gives a relationship between time and the ratio N^Pb/N^U. The latest value of the age of the Earth obtained by this method is 4.55 billion y.

For intermediate ages, thermoluminescence (heat and light) is used. Radiation shifts electrons in atoms to higher orbits (Section 2.3), whereas heating causes electrons to drop back. Thus the firing of clay in ancient pottery “starts the clock.” Over the years, traces of radioactive U and Th cause a cumulative shifting, which is measured by heating and observing the light emitted. An elementary but entertaining account of the applications of this technique is provided by Jespersen and Fitz-Randolph (see References).

For the determination of ages ranging from 50,000 y to a few million y, an argon method can be used. It is based on the fact that the potassium isotope K-40 (half-life 1.25 × 10⁹ y) crystallizes in materials of volcanic origin and decays into the stable argon isotope Ar-40. An improved technique makes use of neutron bombardment of samples to convert K-39, a stable isotope of potassium, into Ar-39. This provides a substitute for measuring the content of K. These techniques, described by Taylor and Aitken (see References), are of special interest in relation to the possible collision of an asteroid with the Earth 65 million y ago and the establishment of the date of the first appearance of man. Dating methods are used in conjunction with activation analysis described in the next section.

17.7. Neutron Activation Analysis

This is an analytical method that will reveal the presence and amount of minute impurities. A sample of material that may contain traces of a certain element is irradiated with neutrons, as in a reactor. The gamma rays emitted by the product radioisotope have unique energies and relative intensities, in analogy to spectral lines from a luminous gas. Measurements and interpretation of the gamma-ray spectra, with data from standard samples for comparison, provide information on the amount of the original impurity.

Let us consider a practical example. Reactor design engineers may be concerned with the possibility that some stainless steel to be used in moving parts in a reactor contains traces of cobalt, which would yield undesirable long-lived activity if exposed to neutrons. To check on this possibility, a small sample of the stainless steel is irradiated in a test reactor to produce Co-60, and gamma radiation from the Co-60 is compared with that of a piece known to contain the radioactive isotope. The “unknown” is placed on a Pb-shielded large-volume lithium-drifted germanium Ge(Li) detector used in gamma-ray spectroscopy as noted in Section 10.4. Gamma rays from the decay of the 5.27-y Co-60 give rise to electrons by photoelectric absorption, Compton scattering, and pair production. The electrons produced by photoelectric absorption then give rise to electrical signals in the detector that are approximately proportional to the energy of the gamma rays. If all the pulses produced by gamma rays of a single energy were equal in height, the observed counting rate would consist of two perfectly sharp peaks at energy 1.17 MeV and 1.33 MeV. A variety of effects cause the response to be broadened somewhat as shown in Figure 17.4. The location of the peaks clearly shows the presence of the isotope Co-60 and the heights tell how much of the isotope is present in the sample. Modern electronic circuits can process a large amount of data at one time. The multichannel analyzer accepts counts caused by photons of all energy and displays the whole spectrum graphically. When neutron activation analysis is applied to a mixture of materials, it is necessary after irradiation to allow time to elapse for the decay of certain isotopes whose radiation would “compete” with that of the isotope of interest. In some cases, prior chemical separation is required to eliminate interfering isotope effects.

Figure 17.4. Analysis of gamma rays from cobalt-60.

(Courtesy of Jack N. Weaver of North Carolina State University).

The activation analysis method is of particular value for the identification of chemical elements that have an isotope of adequate neutron absorption cross section and for which the products yield a suitable radiation type and energy. Not all elements meet these specifications, of course, which means that activation analysis supplements other techniques. For example, neutron absorption in the naturally occurring isotopes of carbon, hydrogen, oxygen, and nitrogen produces stable isotopes. This is fortunate, however, in that organic materials including biological tissue are composed of those very elements, and the absence of competing radiation makes the measurement of trace contaminants easier. The sensitivity of activation analysis is remarkably high for many elements. It is possible to detect quantities as low as a millionth of a gram in 76 elements, a billionth of a gram in 53, or even as low as a trillionth in 11.

Prompt gamma neutron activation analysis (PGNAA) is a variant on the method just described. PGNAA measures the capture gamma ray from the original (n,γ) reaction resulting from neutron absorption in the element or isotope of interest, instead of measuring gammas from new radioactive species formed in the reaction. The distinction between NAA and PGNAA is shown in Figure 17.5, which shows the series of reactions that can result from a single neutron.

Figure 17.5. Nuclear reactions involved in neutron activation analysis (PGNAA)

(Courtesy of Institute of Physics).

Because the reaction rate depends on the neutron cross section, only a relatively small number of elements can be detected in trace amounts. The detection limits in ppm are smallest for B, Cd, Sm, and Gd (0.01 to 0.1), and somewhat higher for Cl, Mn, In, Nd, and Hg (1 to 10). Components that can readily be measured are those often present in large quantities such as N, Na, Al, Si, Ca, K, and Fe. The method depends on the fact that each element has its unique prompt gamma-ray spectrum. The advantages of PGNAA are that it is nondestructive, it gives low residual radioactivity, and the results are immediate.

A few of the many applications of neutron activation analysis are now described.

• Textile manufacturing. In the production of synthetic fibers, certain chemicals such as fluorine are applied to improve textile characteristics, such as the ability to repel water or stains. Activation analysis is used to check on inferior imitations by comparison of the content of fluorine or other deliberately added trace elements.
• Petroleum processing. The “cracking” process for refining oil involves an expensive catalyst that is easily poisoned by small amounts of vanadium, which is a natural constituent of crude oil. Activation analysis provides a means for verifying the effectiveness of the initial distillation of the oil.
• Crime investigation. The process of connecting a suspect with a crime involves physical evidence that often can be accurately obtained by NAA. Examples of forensic applications are the comparison of paint flakes found at the scene of an automobile accident with paint from a hit-and-run driver's car; the determination of the geographical sources of drugs by comparison of trace element content with that of soils in which plants are grown; verification of theft of copper wire by use of differences in content of wire from various manufacturers; distinguishing between murder and suicide by measurement of barium or antimony on hands; and tests for poison in a victim's body. The classic example of the latter is the verification of the hypothesis that Napoleon was poisoned, by activation analysis of arsenic in hair samples.
• Authentication of art work. The probable age of a painting can be found by testing a small speck of paint. Over the centuries the proportions of elements such as chromium and zinc used in pigment have changed, so that forgeries of the work of old masters can be detected.An alternate method of examination involves irradiation of a painting briefly with neutrons from a reactor. The radioactivity induced produces an autoradiograph in a photographic film, so that hidden underpainting can be revealed.It was desired to determine the authenticity of some metal medical instruments, said to be from Pompeii, the city buried by the eruption of Vesuvius in ad 79. PGNAA was applied, and by use of the fact that the zinc content of true Roman artifacts was low, the instruments were shown to be of modern origin.
• Diagnosis of disease. Medical applications (see References) include accurate measurements of the normal and abnormal amounts of trace elements in the blood and tissue as indicators of specific diseases. Other examples are the determination of sodium content of children's fingernails and the very sensitive measurement of the iodide uptake by the thyroid gland.
• Pesticide investigation. The amounts of residues of pesticides such as DDT or methyl bromide in crops, foods, and animals are found by analysis of the bromine and chlorine content.
• Mercury in the environment. The heavy element mercury is a serious poison for animals and human beings even at low concentrations. It appears in rivers as the result of certain manufacturing waste discharges. By the use of activation analysis, the Hg contamination in water or tissues of fish or land animals can be measured, thus helping to establish the ecological pathways.
• Astronomical studies. Measurement by NAA of the variation in the minute amounts of iridium (parts per billion) in geological deposits led to some startling conclusions about the extinction of the dinosaurs some 65 million years ago. A large meteorite, 6 km in diameter, is believed to have struck the Earth and to have caused atmospheric dust that reduced the sunlight needed by plants eaten by the dinosaurs. The theory is based on the fact that meteorites have a higher iridium content than the Earth. The sensitivity of NAA for Ir was vividly demonstrated by the discovery that contact of a technician's wedding ring with a sample for only 2 seconds was sufficient to invalidate results.Evidence is mounting for the correctness of the idea. Large impact craters and buried structures have been discovered in Yucatan and Iowa. They are surrounded by geological debris whose age can be measured by the K-Ar method (see Section 17.6 and References).
• Geological applications of PGNAA. Oil and mineral exploration in situ of large-tonnage, low-grade deposits far below the surface has been found to yield better results than does extracting small samples. In another example, measurements were made on the ash on the ground and particles in the atmosphere from the 1980 Mount St. Helens volcano eruption. Elemental composition was found to vary with distance along the ground and with altitude. Many other examples of the use of PGNAA are found in the literature (see References).

An alternative and supplement to NAA and PGNAA is X-ray fluorescence spectrometry. It is more accurate for measuring trace amounts of some materials. The method consists of irradiating a sample with an intense X-ray beam to cause target elements to emit characteristic line spectra (i.e., to fluoresce). Identification is accomplished by either (a) measurements of the wavelengths by diffraction with a single crystal, comparison with a standard, and analysis by a computer, or (b) use of a commercial low-energy photon spectrometer, a semiconductor detector. The sensitivity of the method varies with the element irradiated, being lower than 20 ppm for all elements with atomic number above 15. The time required is much shorter than for wet chemical analyses, making the method useful when a large number of measurements are required.

17.8. Radiography

The oldest and most familiar beneficial use of radiation is for medical diagnosis by X-rays. These consist of high-frequency electromagnetic radiation produced by electron bombardment of a heavy-metal target. As is well known, X-rays penetrate body tissue to different degrees depending on material density, and shadows of bones and other dense materials appear on the photographic film. The term “radiography” includes the investigation of internal composition of living organisms or inanimate objects by use of X-rays, gamma rays, or neutrons.

For both medical and industrial use, the isotope cobalt-60, produced from Co-59 by neutron absorption, is an important alternative to the X-ray tube. Co-60 emits gamma rays of energy 1.17 MeV and 1.33 MeV, which are especially useful for examination of flaws in metals. Internal cracks, defects in welds, and nonmetallic inclusions are revealed by scanning with a cobalt radiographic unit. Advantages include small size and portability and freedom from the requirement of an electrical power supply. The half-life of 5.27 y permits use of the device for a long time without need for replenishing the source. On the other hand, the energy of the rays is fixed and the intensity cannot be varied, as is possible with the X-ray machine.

Other isotopes that are useful for gamma-ray radiography are: (a) iridium-192, half-life 73.8 d, photon energy approximately 0.4 MeV, for thin specimens; (b) cesium-137 (30.2 y), because of its long half-life and 0.662 MeV gamma ray; (c) thulium-170, half-life 128.6 d, emitting low-energy gammas (0.052, 0.084, 0.16 MeV), useful for thin steel and light alloys because of the high cross section of the soft radiation.

The purpose of radiography that uses neutrons is the same as that which uses X-rays, namely to examine the interior of an opaque object. There are some important differences in the mechanisms involved, however. X-rays interact principally with the electrons in atoms and molecules, and thus are scattered best by heavy high-Z elements. Neutrons interact with nuclei and are scattered according to what isotope is the target. Hydrogen atoms have a particularly large scattering cross section. Also, some isotopes have very high capture cross section (e.g., cadmium, boron, and gadolinium). Such materials are useful in detectors as well. Figure 17.6 shows the schematic arrangement of a thermal neutron radiography unit, where the source can be a nuclear reactor, a particle accelerator, or a radioisotope. Exposure times are least for the reactor source because of the large supply of neutrons; they are greatest for the isotopic source. A typical accelerator reaction that uses neutrons is the (d,n) reaction on tritium or beryllium.

Figure 17.6. Schematic diagram of a thermal neutron radiography unit.

Source can be an accelerator, a reactor, or a radioisotope.

Several of the radioisotopes sources use the (γ,n) reaction in beryllium-9, with gamma rays from antimony-124 (60.20 d), or the (α,n) reaction with alpha particles from americium-241 (432 y) or curium-242 (163 d). An isotope of the artificial element 98, californium-252, is especially useful as a neutron source. It decays usually (96.9%) by α-particle emission, but the other part (3.1%) undergoes spontaneous fission releasing approximately 3.5 neutrons on average. The half-lives for the two processes are 2.73 y and 85.6 y, respectively. An extremely small mass of Cf-252 serves as an abundant source of neutrons. These fast neutron sources must be surrounded by a light-element moderator to thermalize the neutrons.

Detection of transmitted neutrons is by the small number of elements that have a high thermal neutron cross section and that emit secondary radiation that readily affects a photographic film and record the images. Examples are boron, indium, dysprosium, gadolinium, and lithium. Several neutron energy ranges may be used—thermal, fast and epithermal, and “cold” neutrons, obtained by passing a beam through a guide tube with reflecting walls that select the lowest energy neutrons of a thermal distribution.

Examples of the use of neutron radiography are:

• Inspection of reactor fuel assemblies before operation for defects such as enrichment differences, odd-sized pellets, and cracks.
• Examination of used fuel rods to determine radiation and thermal damage.
• Inspection for flaws in explosive devices used in the United States space program. The devices served to separate booster stages and to trigger release of reentry parachutes. Items are rejected or reworked on the basis of any one of 10 different types of defects.
• Study of seed germination and root growth of plants in soils. The method allows continued study of the root system without disturbance. Root diameters down to ⅓ mm can be discerned, but better resolution is needed to observe root hairs.
• “Real-time” observations of a helicopter gas turbine engine at Rolls-Royce, Ltd. Oil flow patterns that use cold neutrons are observable, and bubbles, oil droplets, and voids are distinguishable from normal density oil.

17.9. Radiation Gauges^[†]

^† Appreciation is extended to the late William Troxler for valuable information in this section.

Some physical properties of materials are difficult to ascertain by ordinary methods but can be measured easily by observing how radiation interacts with the substance. For example, the thickness of a layer of plastic or paper can be found by measuring the transmitted number of beta particles from a radioactive source. The separated fission product isotopes strontium-90 (29.1 y, 0.546 MeV beta particle) and cesium-137 (30.2 y, 0.514 MeV beta particle) are widely used for such gauging.

The density of a liquid flowing in a pipe can be measured externally by detection of the gamma rays that pass through the substance. The liquid in the pipe serves as a shield for the radiation and attenuation of the beam dependent on macroscopic cross section and thus particle number density.

The level of liquid in an opaque container can be measured readily without the need for sight glasses or electric contacts. A detector outside the vessel measures the radiation from a radioactive source mounted on a float in the liquid.

Portable gauges for measurement of both moisture and density are available commercially. A rechargeable battery provides power for the electronics involving a microprocessor. Gamma rays for density measurements in materials such as soil or asphalt paving are supplied by a cesium-137 source. For operation in the direct-transmission mode, a hole is punched into the material being tested and a probe rod with radioactive source in its end is inserted. A Geiger-Mueller gamma ray detector is located at the base of the instrument, as shown in Figure 17.7. A typical calibration curve for the instrument is also shown. Standard blocks of test material with various amounts of magnesium and aluminum are used to determine the constants in an empirical formula that relates density to counting rate. If the source is retracted to the surface, measurements in the back-scattering mode can be made. The precision of density measurements is 0.4% or better. For moisture measurements by the instrument, neutrons of average energy 4.5 MeV are provided by an americium-beryllium source. Particles of approximately 5 MeV from americium-241, half-life 432 y, bombard beryllium-9 to produce the reaction ⁹Be(α,n)¹²C. Neutrons from the source, located in the center of the gauge base, migrate through the material and slow down, primarily with collisions with the hydrogen atoms in the contained moisture. The more water that is present, the larger is the thermal neutron flux in the vicinity of the gauge. The flux is measured by a thermal-neutron detector consisting of a helium-3 proportional counter, in which the ionization is created by the products of the reaction ³He(n,p)³H (σ^a = 5330 barns). Protons and tritons (hydrogen-3 ions) create the ionization measured in the detector. The gauge is calibrated by use of laminated sheets of the hydrocarbon polyethylene and of magnesium. The moisture content can be measured to approximately 5% in normal soil. The device requires correction if there are significant amounts of absorbers such as iron, chlorine, or boron in the ground or if there are hydrogenous materials other than water present.

Figure 17.7. Direct-transmission radiation gauge to measure soil density.

(Courtesy of Troxler Electronic Laboratories, Inc).

A newer portable nuclear gauge^[†] for measuring density and moisture content has several special features: remote control by a hand-held PDA (personal digital assistant), GPS (global positioning system) position recording, and software for transferring data to a PC. Its nuclear sources are an 8 mCi cesium-137 for gammas and a 40 mCi americium-241 plus beryllium-9 for neutrons.

^† Troxler Model 3451 Enhanced RoadReader Plus Nuclear Density Gauge.

Several nuclear techniques are used in the petroleum industry. In the drilling wells, the “logging” process involves the study of geological features. One method consists of the measurement of natural gamma radiation. When the detector is moved from a region of ordinary radioactive rock to one containing oil or other liquid, the signal is reduced. A neutron moisture gauge is adapted to determine the presence of oil, which contains hydrogen. Neutron activation analysis of chemical composition is performed by lowering a neutron source and a gamma ray detector into the well.

17.10. Summary

Radioisotopes provide a great deal of information for human benefit. The characteristic radiations permit the tracing of processes such as fluid flow. Pharmaceuticals are radioactively tagged chemicals used in hospitals for diagnosis. Scanners detect the distribution of radioactivity in the body and form images of diseased tissue. Radioimmunoassay measures minute amounts of biological materials. The dates of archaeological artifacts and of rock formations can be found from carbon-14 decay data and the ratios of uranium to lead and of potassium to argon. The irradiation of materials with neutrons gives rise to unique prompt gamma rays and radioactive decay products, allowing measurement of trace elements for many applications. Radiography uses gamma rays from cobalt-60 or neutrons from a reactor, accelerator, or californium-252. Radiation gauges measure density, thickness, ground moisture, water/cement ratios, and oil deposits.

17.11. Exercises

1. A radioisotope is to be selected to provide the signal for arrival of a new grade of oil in an 800-km-long pipeline, in which the fluid speed is 1.5 m/s. Some of the candidates are:

    

   Isotope	Half-life	Particle, Energy (MeV)
   Na-24	14.96 h	β, 1.389; γ, 1.369, 2.754
   S-35	87.2 d	β, 0.167
   Co-60	5.27 y	β, 0.315; γ, 1.173, 1.332
   Fe-59	44.5 d	β, 0.273, 0.466; γ, 1.099, 1.292

   Which would you pick? On what basis did you eliminate the others?
2. The radioisotope F-18, half-life 1.83 h, is used for tumor diagnosis. It is produced by bombarding lithium carbonate (Li²CO³) with neutrons, with tritium as an intermediate particle. Deduce the two nuclear reactions.
3. The range of beta particles of energy 0.53 MeV in metals is 170 mg/cm². What is the maximum thickness of aluminum sheet, density 2.7 g/cm³, that would be practical to measure with a Sr-90 or Cs-137 gauge?
4. The amount of environmental pollution by mercury is to be measured with neutron activation analysis. Neutron absorption in the mercury isotope Hg-196, present with 0.15% abundance, activation cross section 3 × 10³ barns, produces the radioactive species Hg-197, half-life 2.67 days. The smallest activity for which the resulting photons can be accurately analyzed in a river water sample is 10 dps. If a reactor neutron flux of 10¹² cm⁻²−s⁻¹ is available, how long an irradiation is required to be able to measure mercury contamination of 20 ppm (μg/g) in a 4-milliliter water test sample?
5. The ratio of numbers of atoms of lead and natural uranium in a certain moon rock is found to be 0.05. What is the probable age of the sample?
6. The activity of C-14 in a wooden figure found in a cave is only of today's value. Estimate the date the figure was carved.
7. Examine the possibility of adapting the uranium-lead dating analysis to the potassium-argon method. What would be the ratio of Ar-40 to K-40 if a deposit were 1 million y old? Note that only 10.72% of K-40 decay yields Ar-40, the rest going into Ca-40.
8. The age of minerals containing rubidium can be found from the ratio of radioactive Rb-87 to its daughter Sr-87. Develop a formula relating this ratio to time.
9. It has been proposed that radioactive krypton gas of 10.73 y half-life be used in conjunction with film for detecting small flaws in materials. Discuss the concept, including possible techniques, advantages, and disadvantages.
10. A krypton isotope of half-life 13.1 sec is prepared by charged particle bombardment. It gives off a gamma ray of 0.19 MeV energy. Discuss the application of the isotope to the diagnosis of emphysema and black-lung disease. Consider production, transportation, hazards, and other factors.
11. Tritium has a physical half-life of 12.32 y, but when taken into the human body as water, it has a biological half-life of 12.0 d. Calculate the effective half-life of tritium for purposes of radiation exposure. Comment on the result.
12. With the half-life relationship as given in Section 17.2, calculate the effective half-life of californium-252.
13. The spontaneous fission half-life of Cf-252 is 85.6 y. Assuming that it releases 3.5 neutrons per fission, how much of the isotope in micrograms is needed to provide a source of strength of 10⁷ neutrons per second? What would be the diameter of the source in the form of a sphere if the Cf-252 had a density as pure metal of 20 g/cm³?
14. Three different isotopic sources are to be used in radiography of steel in ships as follows:

     

    Isotope	Half-life	Gamma Energy (MeV)
    Co-60	5.27 y	1.25 (ave.)
    Ir-192	73.8 d	0.4 (ave.)
    Cs-137	30.2 y	0.66

    Which isotope would be best for insertion in pipes of small diameter and wall thickness? For finding flaws in large castings? For more permanent installations? Explain.
15. The number of atoms of a parent isotope in a radionuclide generator such as Mo-Tc given by N^p = N^p0E^p, where E^p = exp(−λ^pt), with N^p0 as the initial number of atoms. The number of daughter atoms for zero initially iswhere k is the fraction of parents that go into daughters and E^d = exp(−λ^dt).
    • Find the ratio of Tc-99m atoms to Mo-99 atoms for very long times, with k = 0.87.
    • What is the percent error in the use of the ratio found in (a) if it takes one half-life of the parent to ship the fresh isotope to a laboratory for use?
16. Pharmaceuticals containing carbon-14 (5715 y) and tritium (12.32 y) are both used in a biological research laboratory. To avoid an error of greater than 10% in counting beta particles, as a result of accidental contamination of C-14 by H-3, what must be the upper limit on the fraction of atoms of tritium in the sample? Assume that all betas are counted, regardless of energy.
17. The atom fraction of C-14 in carbon was approximately 1.2 × 10⁻¹² before bomb tests. How many counts per minute would be expected from a 1-gram sample of carbon? Discuss the implications of that number.

Computer Exercise

1. Recall the computer program RADIOGEN (see Computer Exercise 3.D) giving activities of parent and daughter isotopes.
   • Apply the radionuclide generator of Section 17.3 by use of half-lives 65.9 h for Mo-99 and 6.01 h for Tc-99 m, with k = 0.87. Carry the calculations out to at least 66 h in steps of 1 h.
   • From the formula in Computer Exercise 3.D, show that the ratio of activities of daughter to parent at very long times is
   • Find out how much error there is with the formula of (b), rather than the ratio calculated by RADIOGEN, if it takes exactly one half-life of Mo-99 to ship the generator to a laboratory for use.

17.12 References

Biology Links Biology Links

http://mcb.harvard.edu/BioLinks.html http://mcb.harvard.edu/BioLinks.html

Harvard University Department of Molecular and Cellular Biology Harvard University Department of Molecular and Cellular Biology.

DOE Information Bridge DOE Information Bridge

http://www.osti.gov/bridge http://www.osti.gov/bridge

Search on “A Vital Legacy” for atoms in biology. 4807K pdf file Search on “A Vital Legacy” for atoms in biology. 4807K pdf file.

Adelstein and Manning, 1995 S. James Adelstein, Frederick J. Manning, Isotopes for Medicine and the Life Sciences 1995 National Academy Press Washington Describes sources of stable and radioactive isotopes and recommends a national dedicated accelerator

United States Radioisotope Production United States Radioisotope Production

http://www.nuclear.energy.gov http://www.nuclear.energy.gov

Search on Radiological Facilities Management Search on Radiological Facilities Management

Radiopharmaceuticals Radiopharmaceuticals

http://nucmedicine.com/images/Radiopharmaceuticals-1.pdf http://nucmedicine.com/images/Radiopharmaceuticals-1.pdf

Attractive and informative slide show by Samy Sadak Attractive and informative slide show by Samy Sadak.

United States Radioisotope Supply United States Radioisotope Supply

http://www.ans.org/pi/ps/docs/ps30.pdf http://www.ans.org/pi/ps/docs/ps30.pdf

ANS Position Statement 30, June 2004 ANS Position Statement 30, June 2004

DOE Isotope Production and Distribution DOE Isotope Production and Distribution

http://www.ornl.gov/sci/isotopes/catalog.htm http://www.ornl.gov/sci/isotopes/catalog.htm

Catalog of stable and radioactive isotopes Catalog of stable and radioactive isotopes.

Human Genome Project Information Human Genome Project Information

http://www.ornl.gov/hgmis http://www.ornl.gov/hgmis

Comprehensive information Comprehensive information.

Access Excellence Classical Collection Access Excellence Classical Collection

http://www.accessexcellence.com/AE/AEC/CC http://www.accessexcellence.com/AE/AEC/CC

“A Visit with Dr. Francis Crick,” “A Visit with Dr. Francis Crick,” “DNA Structure,” and “Restriction Nucleases.”

DNA and Molecular Genetics DNA and Molecular Genetics

Search Google with “DNA radioactivity Farabee” Search Google with “DNA radioactivity Farabee”

Tutorial on DNA Tutorial on DNA.

DNA Forensics DNA Forensics

http://www.forensicmag.com/articles.asp?pid=17 http://www.forensicmag.com/articles.asp?pid=17

Comparison of DNA fingerprinting methods RFLP and PCR-STR Comparison of DNA fingerprinting methods RFLP and PCR-STR.

Krawczak and Schmidtke, 1998 M. Krawczak, J. Schmidtke, DNA Fingerprinting 2nd Ed. 1998 Springer Verlag New York Genetics background, applications to forensics, and legal and ethical aspects

Guidebook on Radioisotope Tracers in Industry Guidebook on Radioisotope Tracers in Industry 1990 International Atomic Energy Agency Vienna Methodology, case studies, and trends

Harbert and da Rocha, 1984 John Harbert, Antonio F.G. da Rocha, Textbook of Nuclear Medicine 2nd Ed. 1984 Lea & Febiger Philadelphia Volume I: Basic Science, Volume II: Clinical Applications Volume I contains good descriptions of radionuclide production, imaging, radionuclide generators, radiopharmaceutical chemistry, and other subjects. Volume II gives applications to organs

Wilson, 1998 Michael A. Wilson, Textbook of Nuclear Medicine 1998 Lippincott Williams & Wilkins Philadelphia, PA

Saha, 2005 Gopal B. Saha, Fundamentals of Nuclear Pharmacy 5th Ed. 2005 Springer New York Instruments, isotope production, and diagnostic and therapeutic uses of radiopharmaceuticals. Reflects the continued growth of the field

Introduction to MRI Introduction to MRI

http://mritutor.org/mritutor/index.html http://mritutor.org/mritutor/index.html

Elementary tutorial by Ray Ballinger Elementary tutorial by Ray Ballinger.

The Basics of MRI The Basics of MRI

http://www.cis.rit.edu/htbooks/mri http://www.cis.rit.edu/htbooks/mri

Comprehensive treatment by Dr. Joseph P. Hornak. Click on skull image Comprehensive treatment by Dr. Joseph P. Hornak. Click on skull image.

Hendee and Ritenour, 2002 W.R. Hendee, E.R. Ritenour, Medical Imaging Physics 4th Ed. 2002 Wiley-Liss New York

Medical Physics and Bioengineering Resources Medical Physics and Bioengineering Resources

http://www.medphys.ucl.ac.uk/inset/resource.htm http://www.medphys.ucl.ac.uk/inset/resource.htm

List of textbooks, videos, slides, and CD's List of textbooks, videos, slides, and CD's. By University College London.

Mettler and Guiberteau, 2005 Fred A. Mettler Jr., Milton J. Guiberteau, Essentials of Nuclear Medicine Imaging 5th Ed. 2005 W.B. Saunders Co Philadelphia After presenting background on radioactivity, instruments, and computers, the book describes the methods used to diagnose and treat different tissues, organs, and systems of the body

Chandra, 2004 Ramesh Chandra, Nuclear Medicine Physics: the Basics 2004 Lippincott Williams & Wilkins Philadelphia, PA Intended for resident physicians. Covers scintillation cameras and computer tomography

Brucer, 1990 Marshall Brucer, A Chronology of Nuclear Medicin e 1600–1989 1990 Heritage Public ations St. Louis Interesting and informative discussion with abundant references

Chard, 1995 T. Chard, An Introduction to Radioimmunoassay and Related Techniques 1995 Elsevier Amsterdam Concept, principles, and laboratory techniques. Immunoassays in general, labeling techniques, and commercial services

Price and Newman, 1991 Christopher P. Price, David J. Newman, Principles and Practices of Immunoassay 1991 Stockton Press New York A great variety of immunoassay techniques

Knoche, 1991 Herman W. Knoche, Radioisotopic Methods for Biological and Medical Research 1991 Oxford University Press New York Includes mathematics of radioimmunoassay and isotope dilution

Choppin et al., 2001 G. Choppin, J. Rydberg, J.O. Liljenzin, Radiochemistry and Nuclear Chemistry 3rd Ed. 2001 Butterworth-Heinemann Oxford Includes chapters on isotope uses in chemistry and on nuclear energy

Radiocarbon Web-info Radiocarbon Web-info

http://www.c14dating.com http://www.c14dating.com

Links to many applications of C-14 dating, by T. Higham Links to many applications of C-14 dating, by T. Higham.

Accelerator Mass Spectrometry Accelerator Mass Spectrometry

http://www.phys.uu.nl/ams http://www.phys.uu.nl/ams

Principle, method, and applications Principle, method, and applications

Jespersen and Fitz-Randolph, 1996 James Jespersen, Jane Fitz-Randolph, Mummies, Dinosaurs, Moon Rocks: How We Know How Old Things Are 1996 Atheneum Books for Young Readers New York

Bowman, 1991 Sheridan Bowman, Science and the Past 1991 University of Toronto Press Toronto Qualitative technical treatment of dating with emphasis on the artifacts

Taylor and Aitken, 1997 R.E. Taylor, Martin J. Aitken, Chronometric Dating in Archaeology 1997 Plenum Press New York Principle, history, and current research for a number of dating techniques. Articles are written by experts in the methods

Heydorn, 1984 K. Heydorn, Neutron Activation Analysis for Clinical Trace Element Research Vols. I & II 1984 CRC Press Boca Raton

North Carolina State University's Nuclear Reactor Program North Carolina State University's Nuclear Reactor Program

http://www.ne.ncsu.edu/NRP/reactor_program.html http://www.ne.ncsu.edu/NRP/reactor_program.html

Select Facilities Select Facilities

INL Gamma-Ray Spectrometry Center INL Gamma-Ray Spectrometry Center

http://www.inl.gov/gammaray/spectrometry http://www.inl.gov/gammaray/spectrometry

Display of spectra and decay schemes Display of spectra and decay schemes.

Molnar, 2004 Gabor Molnar, Handbook of Prompt Gamma Activation Analysis with Neutron Beams 2004 Kluwer Dordrecht

Alfassi and Chung, 1995 Zeev Alfassi, Chien Chung, Prompt Gamma Neutron Activation Analysis 1995 CRC Press Boca Raton Includes in vivo measurements of elements in the human body and in situ well-logging in the petroleum industry

The Medical Radiography Home Page The Medical Radiography Home Page

http://home.earthlink.net/~terrass/radiography/medradhome.html http://home.earthlink.net/~terrass/radiography/medradhome.html

Internet resources by Richard Terrass of Massachusetts General Internet resources by Richard Terrass of Massachusetts General.

Halmshaw, 1995 R. Halmshaw, Industrial Radiology, Theory and Practice 2nd Ed. 1995 Chapman & Hall London Principles and equipment using X-rays, gamma-rays, neutrons, and other particles. Computers and automation in quality control and nondestructive testing

Domanus, 1992 J.C. Domanus, Practical Neutron Radiography 1992 Kluwer Dordrecht Sources, collimation, imaging, and applications (e.g., nuclear fuel)

Harms and Wyman, 1986 A.A. Harms, D.R. Wyman, Mathematics and Physics of Neutron Radiography 1986 D. Reidel Dordrecht, Holland Highly technical reference

X-ray WWW Server X-ray WWW Server

http://xray.uu.se http://xray.uu.se

Links to synchrotron radiation facilities Links to synchrotron radiation facilities.

D'Amico et al., 1996 Kevin L. D'Amico, Louis J. Terminello, David K. Shuh, Synchrotron Radiation Techniques in Industrial, Chemical, and Materials, Science 1996 Plenum Press New York Selected papers from two conferences

Nuclear Gauges Nuclear Gauges

http://troxlerlabs.com http://troxlerlabs.com

Details on company's products Details on company's products