A radioactive isotope (as distinguished from a stable isotope) of an element. Atomic nuclei are of two types, unstable and stable. Those in the former category are said to be radioactive and eventually are transformed, by radioactive decay, into the latter. One of the three types of particles or radiation (alpha particles, beta particles, and gamma rays) is emitted during each stage of the decay. See also: Isotope; Radioactivity
The term radioisotope is also loosely used to refer to any radioactive atomic species. Whereas approximately a dozen radioisotopes are found in nature in appreciable amounts, hundreds of different radioisotopes have been artificially produced by bombarding stable nuclei with various atomic projectiles.
The production, separation, purification, and shipment of radioisotopes are usually considered primary production; secondary processing of the primary materials into compounds, special shapes, radiation sources, and medical radiopharmaceuticals is usually required before use in hundreds of applications that have been developed. Radioisotopes are produced by irradiation of the elements with high-energy particles (protons, deuterons) in accelerators and in the high-intensity neutron fluxes of relatively small research-type reactors.
A few radioactive elements occur in nature, for example, uranium, radium, and thorium, which were produced when the Earth was formed. Some of these naturally occurring radioisotopes (primary parents of series) have half-lives (time in which one-half of the atoms decay) greater than 108 years, and therefore have not had time to disappear. During the early years of work with radioactive materials, only natural radioactivity was available; The first radioactive tracer experiment was in 1934 with radiolead (212Pb) obtained from thorium decay products.
The first artificially induced radioactivity was also produced in 1934 by irradiating aluminum foil with alpha particles, thus initiating the use of radioactivity on a wide scale in scientific work. A very wide variety of radioisotopes are produced in particle accelerators, such as the cyclotron. Charged particles, such as deuterons (D+) and protons (H+), are accelerated to great speeds by high-voltage electrical fields and allowed to strike targets in which nuclear reactions take place; for example, proton in, neutron out (p,n), increasing the target-atom atomic number by one without changing the atomic mass; and deuteron in, proton out (d,p), increasing the atomic mass by one without changing the atomic number. The target elements become radioactive because the nuclei of the atoms are unbalanced, having an excess or deficit of neutrons or protons. Although the particle-accelerating machines are most versatile in producing radioisotopes, the amount of radioactive material that can be produced is relatively smaller than that made in a nuclear reactor [less than curie amounts; a curie (abbreviated Ci) is that quantity of a radioisotope required to supply 3.7 × 1010 disintegrations per second or 3.7 × 1010 becquerels (Bq)]. For large-scale production, nuclear reactors with neutron fluxes of 1 × 1010 to 5 × 1015 neutrons per square centimeter per second are required. See also: Nuclear reaction; Nuclear reactor; Particle accelerator; Reactor physics; Units of measurement
Radioisotope production proceeds in a series of steps. First, target materials containing the desired element are selected in the most stable heat- and radiation-resistant form (for example, metal oxides) and the highest chemical purity, determined by chemical, spectrographic, or neutron activation analysis. The target material is encapsulated in a low-neutron-absorbing material (such as aluminum or magnesium) and sealed by heli-arc welding or an equivalent process, and the capsule is tested for leakage. Next, the capsule is irradiated for the optimum time period, determined by the half-life of the desired radioisotope, the absorption cross section of the target atom, and the growth of possible undesired impurities. The irradiated capsule is then removed to a remote-control hot cell for chemical processing, and the preparation is analyzed and assayed by the most advanced methods that are amenable to remote-control operations. Aliquots are dispensed as needed for users or secondary processors, and the radioisotope is packaged, inspected for radioactive contamination, and shipped by common carriers. Shipment may be subject to local, state, federal, or international shipping regulations, depending upon the nature, size, and destination of the radioactive material.
Fig. 1 Aluminum capsule target for high-neutron-flux reactor.
Most radioisotopes produced in quantity are made in the nuclear reactor by one of four reactions (Table 1): neutron-gamma, neutron-proton, neutron-alpha, and neutron-fission. The neutron-gamma reaction is the most common, because many elements have a good neutron-capture cross section (relative ability for capturing neutrons). By simple neutron capture, important radioisotopes, such as 24Na, 59Fe, 60Co, and 198Au, are made. The procedure is quite simple in this case. Highly purified materials (to prevent contamination by neutron-capturing impurities) in amounts ranging from a few milligrams to several hundred grams are sealed in pure aluminum cans and placed in the reactor. In the higher-flux reactors (flux greater than 1014 neutrons per square centimeter per second) the highly purified target material is sealed in quartz ampules which are in turn weld-sealed in small aluminum tubes, well designed for heat removal (Fig. 1). Pneumatic or hydraulic tubes are used for passing samples in and out of the reactor. Also, reactors may have special magnesium, beryllium, or aluminum holders for insertion into the reactor lattice. Care must be taken to avoid putting maerials that decompose easily, such as organic compounds, into the reactor because gas pressure may be produced in the container. In general, thermally stable materials are also fairly stable under neutron irradiation. Metals or stable oxides of elements are usually used as target materials.
The production rate for a radioisotope in a reactor depends upon the neutron flux, amount of target atom, the half-life of the radioisotope, and the neutron-activation cross section of the target atom. The formula written below
can be used for calculating the amount of radioisotope produced, where A is the activity in disintegrations per second, N is the number of target atoms, φ is the neutron flux in neutrons per square centimeter per second, σ is the activation cross section for the reaction in square centimeters per atom, and the expression (1 − e−λ t) is the saturation factor. The irradiation time t and the decay constant λ are in compatible units (λ = 0.693/half-life). The activity A, in disintegrations per second, can be converted to millicuries by dividing by 3.70 × 107 disintegrations per second per millicurie.
Target materials of some elements can be obtained quite pure; when such elements have only one isotope that has a high activation cross section, radioisotopes are easy to produce and little chemical purification is required after irradiation. In other cases there is multiple production in the target material, sometimes four or five different radioactive species in one target, which must be chemically separated from the main product. Virtually every known chemical-separation procedure is used in this kind of work, which usually must be done by remote methods because of the high radiation levels involved. See also: Nuclear chemistry
When radioisotopes are produced by neutron-proton or neutron-alpha reactions or by a neutron-gamma reaction followed by beta decay, the radioelement is of a different chemical species from the target element and can be separated from it chemically to produce carrier-free radioisotopes.
Very often concentrations of radioelements are too low for them to be precipitated directly, so they are carried on the surface of a flocculent precipitate, such as Fe(OH)3; similarly, others are coprecipitated, where isomorphous compounds are brought down together, for example, 140BaSO4 with PbSO4. Many methods are more adaptable to work with low concentrations of material or amenable to remote operation, such as solvent extraction and ion exchange. A preferred purification method is gasification, because if the radioisotope goes through various stages in the gaseous phase, very high purity usually results. Distillation is sometimes used, as in purification of 131I (distilled as the element) and 103-106Ru (distilled as the tetroxide). Ion exchange is practically the only method for fractionating the rare earths in the low concentrations usually found in radioisotope work.
The fission products, fragments of the fissioned uranium or plutonium atom, are an extremely important group of radioisotopes, ranging from zinc (atomic number 30) to samarium (atomic number 62). The fission-product fragments occur in two groups: light atoms with atomic masses between 72 and 110 units and heavy atoms with masses between 110 and 162 units. Those fission products with high yields occur at one of the peaks, and the companion fission fragment occurs with the same high yield on the other peak; for example, mass 140 (Xe through Ba) has companion fragments of about mass 95 (Y through Mo). Fission-product yields are given as the percent of fissions that result in fragments of a certain mass; there are two fragments for each fission, so the overall yield arithmetically totals up to 200%. See also: Nuclear fission
The fission products can be separated and purified for use as radioisotopes and fall into two groups (Table 2).
With the exception of 131I, the short-lived fission products were first separated almost entirely for research purposes. The extraordinary growth in the use of 99Mo to produce the daughter 99mTc for medical use has made 99Mo the most important commercial radioisotope. The parent radioisotope, 67-h 99Mo, is also made by neutron-gamma reaction on natural molybdenum, or, in some cases, enriched 98Mo target material is irradiated. The very high specific activity fission product 99Mo does have some advantages for making 99Tc generators. However, because of the high-radiation-level, highly technical separation process, very few commercial radioisotope producers have been interested in taking it over.
The short-lived fission products are made by irradiating a small piece of 235U-Al alloy in the reactor for a few weeks. The target is promptly dissolved in nitric acid upon discharge from the reactor, the radioiodine and xenon separated from the gases are discharged during dissolution, and the uranium-aluminum nitric acid solution is processed by solvent extraction and ion exchange, principally to obtain 99Mo, 95Zr-95Nb, 89Sr, and the rare earths.
The long-lived fission products, such as 90Sr and 137Cs, are separated from long-cooled waste from the processing of spent fuel from power reactors. Krypton-85 is also separated from the waste gases at the fuel reprocessing plants. The long-lived fission products have potential as sources of radiation and heat.
The production of radioisotopes using high-energy particles is more versatile than reactor production, especially for radioisotopes that are neutron-deficient (most reactor radioisotopes have an excess of neutrons in the nucleus). Many of the neutron-deficient radioisotopes decay by K-shell electron capture with the emission of soft x-rays, or by positron emission with accompanying annihilation radiation (the collision of positive and negative electrons, e− and e+, with each electron mass converted to 0.51-MeV gamma radiation). Particularly in nuclear medicine, short-lived radioisotopes are needed that have little beta radiation and thus cause minimum tissue damage, or radioisotopes with directional annihilation radiation to aid location in scanning.
Small cyclotrons have been developed that will accelerate protons, deuterons, and alpha particles. A few of these are now devoted to radioisotope production.
Attachments have been put onto a few large research cyclotrons to get protons of extremely high energy to produce radioisotopes by spallation. This occurs when the particle hitting the target atom is so energetic that “pieces fly off.” Many of the particles making up the nucleus are lost and a variety of radioisotopes of lower atomic number and weight are produced. See also: Spallation reaction
Fig. 2 Radioisotope shipping containers. (a) Lead shield, stainless-steel encased for gamma sources, such as 60Co. (b) Lead shield, stainless-steel encased, in wooden block for pile units and small sources. (c) Gas shipping container with internal sealed cylinder for 85Kr and tritium gas. (d) Nonreturnable lead shield for processed radioisotope bottles. (e) Solid stainless-steel container for bottles of processed 90Sr. (f) Lead shield, stainless-steel encased, on a shipping pallet for distributing weight in aircraft, for pile units and small gamma sources. (Oak Ridge National Laboratory)
The packaging and shipment of radioisotopes present some unusual problems—the penetrating radiation must be shielded and leakage must be prevented because of the extreme toxicity of many radioisotopes. Packaging must be done by remote control. Most radioisotopes are dispensed as water solutions by pipetting from glass storage bottles to glass shipping bottles. Tempered-glass bottles are commonly used, with closures lined with polyethylene inserts. For some special preparations, such as carrier-free 45Ca or 32P, polyethylene storage and shipping bottles are used to cut down losses by adsorption on the walls of the container. High standards of cleanliness must be maintained, although no attempt is made to ensure sterility of solutions except for the special medical radioisotope preparations shipped by pharmaceutical suppliers.
A few preparations have sufficient mass per unit of radioactivity and weak enough radioactivity to permit handling and packaging as dry solids. An example is 14C, which is dispensed as barium carbonate. Materials that are not chemically processed, such as reactor irradiation units or metallic cobalt, are sent out as solids in metal containers.
Two general kinds of shipping container are used; returnable, on which a deposit is paid; and nonreturnable, which the receiver may keep. Returnable containers are usually more massive and expensive, especially those used for shipping large amounts of 60Co or fission products. Pure beta emitters, such as 32P, require little shielding other than the absorbent packing material. Gases are shipped in special glass or metal containers (Figs. 2 and 3).
Fig. 3 Nonreturnable shipping container for radioisotopes. (Oak Ridge National Laboratory)
Shipment of radioactive materials by rail or truck is covered by federal government regulations (Department of Transportation) and international regulations, coordinated through the International Atomic Energy Agency. See also: Isotope separation; Radioactive tracer
Arthur F. Rupp
Accelerator-Produced Nuclides, Brookhaven National Laboratory, BNL-50448, 3 vols., 1975, 1978, 1983
F. Helus (ed.), Radionuclides Production, 2 vols., 1983
International Atomic Energy Agency, Radioisotope Production and Quality Control, Tech. Rep. Ser. 128, 1971
International Atomic Energy Agency, Radiopharmaceuticals and Labelled Compounds, vol. 1, IAEA-SM-171/35, 1973
F. F. Knapp and T. A. Butler (eds.), Radionuclide Generators: New Systems for Nuclear Medicine Applications, 1984
U.S. Department of Energy, Isotope Production and Distribution, 1994