Carrier-free radioactive isotopes

The phenomenon of very diluted solutions is well known in radiochemistry. Carrier-free radioactive isotopes could be mentioned as an example. The term denotes a radioisotope of an element in pure form, that is, essentially undiluted, with a stable isotope. The chemical concentration of these radioisotopes is usually very low. For example, 1 kBq radioactivity (applied typically in a tracer experiment) is equivalent to cca. 2 10 12 mol in the case of 137Cs or 90Sr isotopes. In the case of such low concentrations, no chemical system can be considered homogeneous because all surfaces, the wall of the laboratory vessels, or any contaminants in the solution (such as air bubbles, small particles, great molecules, etc.) can initiate interfacial processes and the subsequent formation of heterogeneous phases (adsorption, colloid formation, precipitation, etc.). This is the result of the simple fact that the number of molecules on the surfaces is more than, or at least similar to, the number of particles in the solution. Even in a solution containing... 

The equations of heterogeneous isotope exchange are simpler than ion-exchange equations because the two ions are chemically the same. In the treatment by the law of mass action, it means that the equilibrium constant is equal to 1. The selectivity coefficients at X = 0 and X = 1 can be determined by measuring heterogeneous isotope exchange in which the concentration of the radioactive isotopes is very low and approaches zero (carrier-free radioactive isotope). 

Nagy, N. M., J. Kdnya, and Gy. Wazelischen-Kun. 1999. The sorption and desorption of carrier-free radioactive isotopes on clay minerals and Hungarian soils. Coll. Surf. 152 245-250. 

Sorption and Migration of Carrier-Free Radioactive Isotopes in Rocks... 

A carrier-free radioactive sample is usually one in which the radionuclide is not diluted with isotopic atoms. In reactor production of Na from target Na, each Na is diluted with a large number of Na atoms. Na cannot be made carrier-free in a reactor. If a carrier-free radionuclide has been produced, e.g. through accelerator irradiation, which thm must be purified, its concentration is so low that it may not follow the normal chemical rules. A macroscopic amount of carrier, either isotopic or not, may have to be added to carry the radionuclide through the proper chemical purification steps. We discuss this further in 9.2. 

The only respect in which the hot atom chemistry of organometallic compounds has so far been applied to other fields of study is in the area of isotope enrichment. Much of this has been done for isolation of radioactive nuclides from other radioactive species for the purpose of nuclear chemical study, or for the preparation of high specific activity radioactive tracers. Some examples of these applications have been given in Table II. The most serious difficulty with preparation of carrier-free tracers by this method is that of radiolysis of the target compound, which can be severe under conditions suited to commercial isotope production, so that the radiolysis products dilute the enriched isotopes. A balance can be struck in some cases, however, between high yield and high specific activity (19, 7J),... 

The specific activity of an isotope indicates the activity per unit mass or volume and is quoted as becquerels per gram (Bq g-1) or millicuries per gram (mCi g-1). A sample in which all the atoms of a particular element are radioactive is said to be carrier-free and is very difficult to achieve in practice. 

The same problems of separating radioactive materials occur of course with the fission products of uranium where the task is often to separate a much larger number of different carrier-free radio-elements than occurs in normal targets. The mixture is complex and consists of elements from zinc to terbium and several hundred radioactive isotopes of varying half-life. 

Crystallisation was one of the earliest methods used for separation of radioactive microcomponents from a mass of inert material. Uranium X, a thorium isotope, is readily concentrated in good yield in the mother liquors of crystallisation of uranyl nitrate (11), (33), (108). A similar method has been used to separate sulphur-35 [produced by the (n, p) reaction on chlorine-35] from pile irradiated sodium ot potassium chloride (54), (133). Advantage is taken of the low solubility of the target materials in concentrated ice-cold hydrochloric acid, when the sulphur-35 as sulphate remains in the mother-liquors. Subsequent purification of the sulphur-35 from small amounts of phosphorus-32 produced by the (n, a) reaction on the chlorine is, of course, required. Other examples are the precipitation of barium chloride containing barium-1 from concentrated hydrochloric acid solution, leaving the daughter product, carrier-free caesium-131, in solution (21) and a similar separation of calcium-45 from added barium carrier has been used (60). 

In other words, 1 MBq of tritium contains about 3 ng of tritium. Thus, an important feature of radionuclides becomes apparent—we routinely work with extremely small quantities of material. Pure samples of radioisotopes are called carrier free. Unless a radionuclide is in a carrier-free state, it is mixed homogeneously with the stable nuclides of the same element. It is, therefore, desirable to have a simple expression to show the relative abundances of the radioisotope and the stable isotopes. This specification is readily accomplished by using the concept of specific activity, which refers to the amount of radioactivity per given mass or other similar units of the total sample. The SI unit of specific activity is Bq/kg. Specific activity can also be expressed in terms of the disintegration rate (Bq or dpm), or... 

In the present study, bread in the form of rolls was prepared from whole grain flour (100% extraction rate) with a zinc content of 22 mg/kg and from white wheat flour of about 72% extraction rate, where the zinc content was 7 mg/kg. The water added to the dough, contained an amount of almost carrier free isotope solution, corresponding to 0.25y/ Ci of Zn for each roll. The isotope solution was made by diluting a stock solution of 65 Zn CI2 in 0.1 M HCl (0.1 to 0.5 Ci/g Zn, Radiochemical centre, Amersham, England) with physiological saline to a final radioactivity of Ci/ml. 

Sulfur-35 is the only long-lived radioactive isotope of sulfur and it may be produced by the direct neutron irradiation of elemental sulfur S (n,7)S . However, the specific activity attained is quite low even in high-neutron fluxes. It is preferable to irradiate potassium chloride and take advantage of the reaction CP (n,p)S . Four weeks irradiation of ten grams of potassium chloride in a thermal-neutron flux of 10 neutrons cm. sec. yields about 5 millicuries of which when dissolved in water is isolable as carrier-free sulfate ion. Nevertheless, the isotopes CP , P , K, and Na are concurrently produced in the neutron irradiation of potassium chloride, and a chemical... 

The term specific activity (activity per unit mass) is important for radiochemical methods like isotope dilution. Samples of carrier-free radionuclides have the highest possible specific activity for this radionuclide. By the addition of carrier material the specific activity is lower, the total amount of material (stable and radioactive materials) is increased, and some handling procedures, like precipitation, might be easier. The change of specific activity is the basis of isotopic dilution techniques. 

Intensive quantities related to the activity. In most samples, the radionuclides are not carrier free but inactive isotopes carry them and/or an inactive substance called matrix contains them. (The term no-carrier-added or n.c.a. only means that the production process of a radionuclide sample does not involve adding carrier to it. However, it may not be carrier free.) The specific activity of a radioactive sample is defined as the (absolute) activity of the radioactive sample divided by its mass (unit Bq/kg). Similar quantities are the molar activity (unit Bq/mol) and the activity concentration (unit Bq/dm ), defined as the activity of the sample divided by its molar amount and volume, respectively. 

Radioactive tracers are as a rule accompanied with a certain amount of their stable isotope(s). When the amount of the stable isotope(s) is significant in the sense of ordinary chemistry or for the specific purpose of an experiment, they are called a "carrier. When not, the tracer is carrier-free. Another term no carrier added is also used with almost the same meaning. The amount of carrier is expressed by its weight or in terms of the elemental specific activity of the tracer, that is, the activity divided by the mass of the element in question (Bq/g element) or by that of the chemical species in question (Bq/g species). 

Chemical methods apply to the separation and purification of radioactive substances in the same way they apply to stable substances. A radiochemical separation is judged in terms of both the yield and the purity of the separated material this is particularly important when analyte concentrations are low or contaminant levels are high. Purity and fractional recovery are often evaluated with the tracer technique. Radiochemical and mass spectrometric detection methods are quite sensitive, and it is possible to work with trace amoimts of analyte. However, radiochemical procedures involving the presence of an isotopic carrier (which can also function as a tracer) are often simpler to design than are carrier-free procedures losses from adsorption on vessel walls or suspended particles may negatively affect the recovery of the analyte in a tracer-level sample. 

Bie most common supply of carrier-free lead radioisotopes has been natural radium or thorium samples, Due to the presence of appreciable amounts of stable lead In pitch-blend which Is the principle source of radium or In thorium ores It Is necessary to obtain the carrier-free material from a sample which has previously been purified from lead. A convenient separation Is that of diffusion of the noble gas member of the radium or thorium decay chain from the parent material and its subsequent decay to radioactive lead Isotopes. Very early In the studies of radioactivity It was found that very high specific RaD(Pb ) sources could be obtained by... 

The presence of radioactive decay products In lead sources or tracers obtained from natural sources is undesirable for some applications. A number of lead Isotopes can be produced in carrier-free form and In high yield by helium-Ion bombardment of mercury or by protnn or deuteron bombardment of thallium targets. The isotopes prepared in this way and the relevant reactions are indicated In Table I. 

Probably the most comprehensive published assay of DU used in armor pen-etrators was reported on the basis of analysis of an unfired CHARM-3 penetrator (Trueman et al. 2004). A sample from the penetrator was dissolved in 9 M HCl, spiked with U as a yield monitor, and the uranium was separated from impurities on an ion-exchange resin. The isotopic composition of uranium was determined by mass spectrometric techniques. Actinides ( - Am and Np) were determined in the uranium-free solution by gamma spectrometry and 239+24opy and Pu were measured by alpha spectrometry and their presence was confirmed by ICPMS. Technetium-99 was determined by ICPMS when rhenium was used as a carrier and interferences from iron were eliminated by precipitating with ammonia while ruthenium and molybdenum were removed by separation on a chromatographic resin. The content of these radioactive nuclides is summarized in Table 2.7. 

---