Speaker
Mr
Tobias Wels
(Forschungszentrum Jülich)
Description
Selenium-73 (half-life: 7.1 h; positron energy = 1.3 MeV) is a very promising radionuclide for application in positron-emission-tomography (PET). With its optimal radionuclidic properties it is the only suitable option for PET-tracer analogues of sulfur-compounds and for authentic labelling of organic selenium molecules [1]. It is preferably produced by the nuclear reaction As-75(p,3n)Se-73 in the proton energy range of 40 → 30 MeV [2]. To produce adequate amounts of Se‑73 for radiosyntheses, a suitable target material withstanding high beam‑currents is necessary. For this purpose an alloy of copper and arsenic has proven useful [3].
The currently used dry distillation method [4] for isolation of the n.c.a. radionuclide did not prove reliable enough for routine use. Therefore a new wet-chemical method was developed for its separation from the irradiated target.
Selenium‑75 (half-life: 119.8 d), produced by the (p,n)-reaction on the same target material with 17 MeV protons, was used for the determination of mass distribution coefficients and separation factors, due to its longer half-life and easier production possibility at a smaller cyclotron. Therewith a two-step wet-chemical separation was developed and optimized. First, the bulk material of copper and co-produced radiozinc were separated from radioselenium and the bulk of arsenic by use of the cation-exchange resin DOWEX 50Wx8 and diluted hydrochloric acid as eluent. In a second step the separation of radioselenium from arsenic was attained using the anion-exchange resin DOWEX 1x8 and diluted nitric acid as eluent. With this new separation method up to 94 ± 2% of radiochemically pure radioselenium was obtained in 2 h without the use of any isotopic carrier.
For a precise evaluation of the produced radioactivity the cross section data of the main nuclear reactions and relevant side reactions are necessary. Up to now, for arsenic-74 which is the major by-product in the production of selenium-73 no cross section data in the proton energy range under consideration were reported. To close the existing gap and strengthen the available data for Se-73,75 production cross sections of the proton induced reactions As‑75(p,pn)As-74, As‑75(p,n)Se-75 and As-75(p,3n)Se-73 were measured in the energy range from 40 to 25 MeV. Based on the experimental results an excitation function for the formation of As-74 could be developed for the first time.
References:
[1] T. Blum, J. Ermert, H. H. Coenen, Nucl. Med. Biol. 2003, 30, 361.
[2] A. Mushtaq, S. M. Qaim, G. Stöcklin, Appl. Radiat. Isot. 1988, 39, 1085.
[3] G. Blessing, R. Weinreich, S. M. Qaim, G. Stöcklin, Appl. Radiat. Isot. 1982, 33, 333.
[4] G. Blessing, N. Lavi, K. Hashimoto, S. M. Qaim, Radiochim. Acta, 1994, 65, 93.
Primary author
Mr
Tobias Wels
(Forschungszentrum Jülich)
Co-authors
Prof.
Heinz H. Coenen
(Forschungszentrum Jülich)
Dr
Ingo Spahn
(Forschungszentrum Jülich)
