Mauro L. Bonardi

Excitation functions and yields for cyclotron production of radiorhenium via nat W(p,ߙxn)181-186gRe nuclear reactions and tests on the production of 186gRe using enriched 186W

Abstract Excitation functions and thin-target yields for the 181-186gRe radionuclides have been measured by the stacked-foil activation technique on tungsten foils of natural isotopic composition for different proton energies up to 22.0 MeV. A further check on the cross sections was done by irradiation of thick-targets and comparing the irradiated thick-target yields with those calculated by analytical integration from the thin-target yields. The production of 186gRe was also studied by the irradiation of thick-target of enriched 186W with a 13.6±0.2 MeV proton beam. The results for 186W(p,ߙn)186gRe were compared also to those calculated by the EMPIRE II code (version 2.19), due to 186gRe extensive applications in nuclear medicine for metabolic radiotherapy of tumours. It was found that the maximum percentage of 186gRe by irradiation of natural tungsten is about 20% only, which confirms the conclusion that high radionuclidic purity and specific activity of 186gRe necessitate the use of highly enriched 186W target.

True radiotracers: are we approaching theoretical specific activity with Tc-99m and I-123?

Nuclear medicine is based on the RadioTracer principle of György von Hevesy, who was awarded the 1943 Nobel Prize in Chemistry [1] and on the Magic Bullet concept of Ehrlich [2]. These two are interrelated, especially as molecular imaging focuses on targeted imaging. Both principles require that the minimal mass be injected. In radiopharmaceutical terms, high specific activity is required in order to not alter the biochemistry but also necessary for those targets of relatively low target protein density such as receptors, which could be saturated even though the total mass injected is still rather small [3]. The concept of specific activity (AS according to International Union for Pure and Applied Chemistry (IUPAC) terminology) started as a definition of the radioactivity found in naturally occurring radioactive materials. This led to the definition of the potency of radium-226 in terms of (radio) activity (i.e., radioactivity is the physical phenomenon, but activity is the SI-derived quantity, measured in becquerel) permass of isotopic carrier (i.e., Bq.kg in SI units). Another important measurement is the activity per mass resulting from the (n,γ) reaction in a thermal reactor. Since the target and the product are most often the same element, this definition of specific activity is defined by the activity produced divided by the targetmaterial in the irradiated sample, unless mass spectrometry (MS) is used to separate the isotopes or the hot atom Szilard–Chalmers method is applied. Several definitions used in nuclear medicine evolved because of the need to point out that nuclear transformations on the cyclotron did not produce the theoretical specific activity, which is defined as carrier-free, AS(CF) [4]. The concept of specific activity as it applies to radiopharmaceuticals has evolved from the classical definition of the ratio of the number of radioactive atoms to the total number of atoms of a given element (in the same chemical or physical form, whenever fast isotopic exchange is minimal). Since the specific activity of the final product is the important factor in radiopharmaceuticals, other definitions have been described for different situations that occur when analyzing the final radioactive product. Kilbourn [4] defined apparent specific activity where the denominator is the total mass eluted at the same time as the radioactive product in the process of high-performance liquid chromatography (HPLC) or gas chromatography purification. The isolated nonra-

Excitation functions and yields for cyclotron production of radiorhenium via deuteron irradiation: natW(d,xn)181,182(A+B),183,184(m+g),186gRe nuclear reactions and tests on the production of 186gRe using enriched 186W

Abstract Excitation functions, thin- and thick-target yields for the 181−186gRe and 187W radionuclides were measured by the activation stacked-foil thecnique on natural tungsten foils for deuteron energies up to 18.0 MeV. These cross sections were validated by comparing the experimental results for thick-target yields with those calculated by integration of the thin-target yields. It was found that the maximum 186gRe content by irradiation of natural tungsten is about 55%, a higher value compared with the one found for proton beam, but not sufficient to use natural tungsten for medical purposes yet. Thus, in order to have a higher specific activity AS of 186gRe, the use of enriched 186W target is necessary. Therefore the irradiation of a thick target of enriched 186W by accelerated deuterons was studied and the results for the production of 186gRe were compared with those obtained from the irradiation of the same target by accelerated protons. It was found that the deuteron irradiation is preferable for three reasons: larger yield, less contamination by tantalum radioisotopes and smaller required amount of the target, which simplify the separation of the 186gRe from the target itself.