Journal of Radioanalytical and Nuclear Chemistry, Vol. 263, No. 1 (2005) 8792 02365731/2005/USD 20.00 AkadØmiai Kiad, Budapest ' 2005 AkadØmiai Kiad, Budapest Springer, Dordrecht How do we ascertain specific activities in no-carrier-added radionuclide preparations? M. L. Bonardi, 1 * J. J. M. de Goeij 2 1 UniversitÆ degli Studi di Milano and National Institute of Nuclear Physics, INFN-Milano, LASA, Radiochemistry Laboratory, Via F.lli Cervi 201, I-20090 Segrate, Milano, Italy 2 Department of Radiochemistry, Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, NL-2629 JB Delft, The Netherlands (Received April 6, 2004) In nuclear and radiochemistry the concepts of specific activity, radioactive concentration, isotopic and non-isotopic carrier, carrier-free and no- carrier-added are of paramount relevance. In fact, traces of carrier are easily introduced in the radioactive preparations by their presence in the target material, by equipment and chemicals used in target processing, and may also be produced by side nuclear reactions induced in the target by nuclear activation, sometimes followed by a decay chain. Very sensitive analytical and radioanalytical techniques allow the determination of the real value of specific activity of no-carrier-added radionuclides and labeled compounds. Thus, in this paper we show with several examples that in most practical cases the term carrier-free (CF) is used improperly and has to be substituted by the more suitable term no-carrier-added (NCA). Carrier-free radionuclides and labeled compounds do exist in a few selected cases only. Definitions of specific activity, activity concentration, and isotopic dilution factor In many relevant applications of radionuclides a specific activity (A S ) as high as possible is envisaged. In the last decades many efforts have been made to develop new nuclear production methods of radionuclides without intentional addition of isotopic carrier. The radionuclides obtained in this way are named no-carrier-added (NCA). 13 The isotopic carrier is the total number of atoms (or mass) of the same element (same Z) present in a radioactive preparation. 48 Unfortunately, NCA is a qualitative definition and in most practical cases the radionuclide concerned is diluted in a mixture of both radioactive and stable atoms of the same element. Thus, the true value of specific activity in NCA preparations must be measured experimentally for the applications envisaged. 922 We define quantitatively the specific activity of a radionuclide (Bq . kg 1 , no radionuclidic impurities, radionuclidic purity 100%), under the assumption that a free isotopic exchange is provided in the radioactive preparation and that only stable isotopic carrier is present, as: A S (t) N(t) . . m(t) 1 where N(t) is the number of atoms of the radionuclide concerned at time t (s), m(t) is the total mass (kg) of all atoms (both stable and radioactive) of same Z present in radioactive preparation at time t and ln2 (t 1/2 ) 1 is the decay constant (s 1 ) of radionuclide with half-life t 1/2 (s). 23 In the case of accelerator-produced radionuclides, the order of magnitude of A S (NCA) is very often in the MBq . g 1 to GBq . g 1 range. In a few selected cases a radionuclide can be considered absolutely free of * E-mail: Mauro.Bonardi@mi.infn.it isotopic carrier. 23 In such cases the radionuclide of molar (atomic) mass M (kg . mol 1 ) is named carrier-free and its A S is calculated as: A S (CF) N Av . . M 1 where N Av is the Avogadros constant (mol 1 ). The A S (CF) is a specific physical constant and it is an intensive quantity that cannot depend on the substrate or solvent in which the radionuclide is diluted. 6,7,23 The value of A S (CF) depends primarily on the half-life of radionuclide, which can vary of some 25 orders of magnitude through the Table of Nuclides presently known. 5 Besides, some authors use the non-quantitative term nearly carrier-free (NCF) to describe the chemical- physical state of a NCA radionuclide, whose A S (NCA) is unknown, 1 but it is assumed to be close to the theoretical carrier-free value (e.g., 232 Th of natural isotopic composition and artificially produced 211 At, only if it is free from other At radioisotopes like 210 At). Conversely, we define 23 activity concentration C A (t) of a radionuclide at time t, the ratio between the activity A (Bq) of radionuclide and the mass (kg) or volume (m 3 ) of substrate or solvent in which the radionuclide is diluted, e.g., C A (t) (Bq . kg 1 or Bq . m 3 ) activity of radionuclide [mass or volume of substrate] 1 . It is necessary to stress that in many papers and scientific communities and even in present IUPAC documents and recommendations, 3,24,25 the two quantities A S (t) and C A (t) are considered as synonyms and seem interchangeable, even though the definitions of these two quantities are completely different. In a very few cases these two quantities have the same value, but it is just the exception and not the rule (e.g., 100% enriched 235 U metal, natural 232 Th metal, transactinoids with Z104 and speculated superactinoids 5 of transition series 6f and 5g with Z121). 3,23