Carrier-free isotopes

Cation exchange can be conveniently studied by radioisotopic tracer methods because of the fairly broad concentration range that can be used, from tracer (carrier-free isotopes) to high (applying an inactive carrier) concentrations. 

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. 

SPECIFIC ACnviTY OF CARRIER-FREE ISOTOPES Problem 6-6... 

As a typical example. Figure 6 shows the decay scheme of Co which populates the 14.4 keV Mossbauer level of Fe with a lifetime of T=140ns. The isotope Co can be produced in a cyclotron by the nuclear reaction Fe(d,n) Co. The decay of Co occurs essentially by electron capture (99.8 %) from the K-shell leaving a hole in this shell which is tilled from higher shells under emission of a 6.4 keV X ray. Sources of Co are usually prepared by electrochemically depositing the carrier-free isotope on metallic supports and then diffusing it into the metal at high temperatures. 

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 two Mossbauer levels of Pt, 99 keV and 130 keV, are populated by either EC of Au(fi/2 = 183 days) or isomeric transition of Pt(fi/2 = 4.1 days). Only a few authors, e.g., [323, 324] reported on the use of Pt, which is produced by thermal neutron activation of " Pt via " Pt(n, y) Pt. The source used in the early measurements by Harris et al. [322, 325] was carrier-free Au diffused into platinum metal. Walcher [326] irradiated natural platinum metal with deuterons to obtain the parent nuclide Au by (d, xn) reactions. After the decay of short-lived isotopes, especially Au(fi/2 = 6.18 days), Au was extracted with ethyl acetate, and the Au/Pt source prepared by induction melting. Buym and Grodzins [323] made use of (a, xn) reactions when bombarding natural iridium with... 

Imura and Suzuki36 have prepared labelled organotin compounds from artificial tin isotopes produced in a cyclotron. The carrier-free tin-113 radioisotope was produced by irradiating indium-115 oxide with 40-MeV protons (equation 33). 

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. 

The basis of co-precipitation and adsorption methods for the purification of carrier-free radioisotopes is the use of a non-isotopic carrier for the required product. The carrier must behave chemically similar to the product in enough reactions to enable purification to be effective, its bulk being necessary for manipulation in precipitations. Its chemistry must be sufficiently different, however, to enable a simple separation of the carrier and carrier-free product to be obtained when a satisfactory purity of the radio-active material has been reached. A good example... 

It is common practice for analytical purposes as in the analysis of carrier-free mixtures, such as the fission-products of uranium, to add the isotopic carriers for the main constituents (37), but the exercise is then one of normal analytical practice. This aspect will not be discussed here nor will the classical investigations of radioisotope behaviour with carriers such as is discussed in several books (30), (90), (130). 

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). 

H. R. Haymond, W. M. Garrison and J. G. Hamilton Carrier-free Radio isotopes from Cyclotron Targets. I. Preparation and Isolation of ll3Sn and luIn from Cadmium. J. chem. Physics 17, 1005 (1949). 

Hg-195m/Au-195m Th Daughter High photon yield Acceptable production rate Carrier free parent isotope Ik Parent somewhat short... 

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... 

Astatination by means of nucleophilic halogen exchange, occasionally with the help of catalysts, and electrophilic replacement via demetalation seem to have become the preferred techniques. Short synthesis and separation time together with the possibility of carrier-free preparation of labelled compounds are especially important factors, bearing in mind the short half-life of astatine isotopes and the requirement of high specific activity for chemical and biomedical investigations. 

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 second case is an example of the formation of different chemical species at very low concentration ranges. Let us see the complex formation of a cation in the presence of an excess of anions. The increase of the ratio of anion cation will obviously increase the stability of the formed complex. In the range of macroscopic concentration, if the concentration of anion and cation are similar, no complex formation can be detected. For example, in the case of lead and chloride ions, a slightly soluble compound, PbCl2, is formed. However, when the concentration of lead ions is extremely low, using, for example, the carrier-free Pb-212 isotope, and chloride anions are present in macroscopic concentration, negatively charged [PbClJ(n 2) are formed. 

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... 

The term carrier-free is often used to indicate the absence of stable isotopes or longer-lived radioisotopes of the radionuclide considered. However, due to the omnipresence of most stable elements, carrier-free radioisotopes of stable elements are, in general, not available. The presence of stable isotopes or longer-lived radioisotopes has to be taken into account, and the specific activity is smaller than calculated by eq. (13.2). As long as the presence of such other isotopes cannot be excluded, it is more correct to distinguish no-carrier-added (n.c.a.) and carrier-added radionuclides. On the other hand, radioisotopes of radioelements are carrier-free if longer-lived radioisotopes are absent. 

An important application of isotope dilution in radiochemistry is the determination of a radionuclide by dilution with an inactive nuclide (inactive compound), also called reverse isotope dilution. This apphcation is very valuable if the radionuclide is present in carrier-free form. Again, quantitative separation is avoided a measured amount mi of an inactive isotope of the element to be determined is added and after a non-quantitative separation the amount m2 is measured. The ratio wa/wi is the yield of the separation procedure and the activity of the carrier-free radionuclide (Ax = 0) is obtained from the measured activity A2 ... 

The excitation function for production of As radioisotopes via Ge (p, xn) reactions has been determined . Only As is obtainable in sufficiently pure form after the appropriate cooling time . Employing Ge or Ge enriched targets, the As could be produced in higher yields via (p,n) or (p,3n) nuclear reactions. An isotope generator for carrier-free As, the daughter of neutron-deficient Se, has been proposed . ... 

For instance 1 produced by a (n,y) reaction on ethyl iodide is recoiled and hence, when shaken with water, it is transferred into the water phase in carrier free state (ideally speaking). This is the first example of Szilard-Chalmers reactions which are now extensively studied and used for the production of some isotopes in high specific activity. 

All of them are in carrier-free solutions and sealed in glass ampoules for highest purity with certified isotopic composition and each sample individually documented. 

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