Photonuclear production of the medical isotope 67Cu
Introduction
One of the many applications of radioisotopes is in medicine. Physicians use medical isotopes for diagnostic and therapeutic purposes. Physical and nuclear characteristics of therapeutic and diagnostic isotopes are not the same. It is a usual practice to use short-living gamma or positron emitters for diagnostic purposes and relatively long-living particle emitters for therapeutic purposes, with the emitted particles having a range compatible with the cell size. However, there is a need for specific “theranostic” single radionuclides or radionuclide pairs that have emissions that allow pre-therapeutic low-dose imaging as well as high-dose therapy in the same patient. The reason is that the use of one isotope for radioimmunotherapy and another one for performing biodistribution and imaging studies to predict the dosimetry and toxicity before initiating the therapy is hazardous [1]. Two different elements, even with similar biochemistry, can have some important dissimilarities, resulting in an inconsistency between diagnostics and therapy. Therefore, it is best, if possible, to use the same theranostic radionuclide with the same electronic structure and, therefore, with the same chemical and biochemical properties for both diagnostics and therapy. In this case, imaging would predict biodistribution and dosimetry in a reliable manner and would also predict which patients would respond to the radionuclide therapy procedure being considered [1]. Several theranostic radionuclides or radionuclide pairs exist, with Copper-67 being one of them [2].
Therapeutic amounts of 67Cu can be produced on accelerators through several reactions with zinc: 68Zn(p,2p)67Cu, 70Zn(p,α)67Cu, 68Zn(α,p)67Cu [3], and 70Zn(d,n) [4]. In this paper, we discuss photonuclear production of 67Cu at the bremsstrahlung endpoint energies of 30 and 40 MeV. The attractiveness of the photonuclear production route of 67Cu is the lack of co-produced isotopes [3]. Reported yields of the 67Cu radioisotope depend on irradiation parameters such as beam flux, length of irradiation, and shape and size of the target [5], [6], [7], [8], [9]. The reaction cross-section is one of the most important parameters for estimating the efficiency of the production route. It depends only on the initial particle energy E0. The cross-sections of photonuclear reactions on zinc were measured using quasi-mono energetic photons in the energy range of 12 to 30 MeV [10], [11], [12], and bremsstrahlung photons [13], [14]. Flux-weighted average cross-sections of natZn(γ,xn)62Zn, natZn(γ,xn)63Zn, natZn(γ,xn)65Zn, natZn(γ,xn)69mZn reactions at the endpoint of bremsstrahlung of 50, 55, 60, and 65 MeV were reported in [15]. The cross-section per equivalent quantum of 68Zn(γ,p)67Cu reaction at Eγmax = 116 MeV was reported in [16]. Photonuclear reaction data are important not only for choosing the isotope production route and estimating its effectivity [1], [2], [3], [4], [19], [8] but also for a number of other applications such as theoretical model approval [17], [18] and astrophysical applications [20].
Section snippets
Experimental procedure
The experiment was carried out using the electron linear accelerator LUE-75 of the A.I. Alikhanian National Science Laboratory (Yerevan, Armenia). Zinc targets of natural composition were irradiated for 1 h with 30 and 40 MeV electrons. The isotopic composition of natural zinc is given in Table 1.
The bremsstrahlung photons are produced when the electron beam hits a 2-mm-thick titanium converter target. The material and thickness of the convertor were chosen based on calculations using PENELOPE
Reaction cross-sections per equivalent quantum
In the experiment, the bremsstrahlung photons are produced when the electron beam hits a titanium converter. Photon flux in this case can be calculated using the following equation [15], [24]:where N is the observed number of γ-rays under the photopeak; λ is the decay constant; ε is the detector efficiency; is monitor reaction cross-section per equivalent quantum, is the partial intensity of the product gamma line; t1 is the irradiation time; t2 is
Conclusions
We have considered the photonuclear production of 67Cu radioisotope. Zinc targets of natural composition were irradiated with bremsstrahlung photons with endpoint energies of 30 and 40 MeV. The cross-sections per equivalent quantum of natZn(γ,xn)62Zn, natZn(γ,xn)63Zn, natZn(γ,xn)65Zn, natZn(γ,xn)69mZn, natZn(γ,pxn)64Cu, and natZn(γ,pxn)67Cu reactions were measured. The specific activities of 67Cu were estimated to be 2.04 and 4.75 µCi/(µA h g) at the end of the bombardment for bremsstrahlung
CRediT authorship contribution statement
G.H. Hovhannisyan: Conceptualization, Writing - review & editing, Funding acquisition. T.M. Bakhshiyan: Software, Data curation, Investigation. R.K. Dallakyan: Resources, Investigation, Methodology.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors are thankful to the staff of the electron linear accelerator LUE-75 of the A.I. Alikhanian National Science Laboratory, Armenia, for the excellent operation of the linac and good beam parameters.
This work was made possible in part by a research grant from the Yervant Terzian Armenian National Science and Education Fund (ANSEF) based in New York, USA.
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