Speaker
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It has been a constant research endeavor to find new ways of producing radionuclides for diagnostic and therapeutic purposes in nuclear medicine. The research area has ever grown due to technological developments and facilities like heavy-ion accelerators. The iodine isotopes e.g., $^{131}$I has been widely used as a radiotracer for thyroid-related diseases. This neutron-rich isotope ($^{131}$I) is produced using a reactor. Although often difficult to procure locally, but it can be transported easily because of its long lifetime ($\tau_{1/2}$= 8.04 d). Another isotope $^{123}$I produced using cyclotron, is also utilized in recent times. The suitable $\gamma$-ray energy (E$_\gamma$) region is 100-600 keV which is high enough to be sufficiently penetrating through the medium of the human body, and can be fully absorbed in the detector. These two isotopes of iodine – $^{131}$I emitting the photon of energy 364 keV and $^{123}$I of energy 159 keV – qualify the requirement.
We investigated the production of four isotopes of iodine $^{123,124,126,128}$I via incomplete fusion reactions (ICF) by bombarding the beams of $^{10,11}$B (60-78 MeV) on $^{122,124}$Sn foils using the 14-UD Pelletron accelerator at the Tata Institute of Fundamental Research, Mumbai, India. The experimental results for our initial experiment $^{11}$B+$^{122}$Sn were presented [1] in the conference NN2012. Later, we utilized the same experimental procedure for the other reactions as well. Our purpose was two-fold. Firstly, to measure the cross sections of all the long-lived (minutes to days) reaction products using the off-line γ-ray spectrometry. The main products (Cesium nuclei) of the complete fusion reactions (CF) were produced with high cross sections as expected, and were in the good agreement with the statistical model code PACE4 [2] predictions. Secondly, our main objective was to identify all the ICF products – with enhanced cross sections over the PACE4 predictions – and understand the reaction mechanism. Special attention was paid to iodine isotopes because of their relevance in nuclear medicine. Our results turned out to be quite promising. All the iodine isotopes produced through ICF (α-channels) – $^{123}$I ($\tau_{1/2}$= 13.2 h, E$_\gamma$=159 keV), $^{124}$I ($\tau_{1/2}$= 4.2 d, E$_\gamma$=603 keV), $^{126}$I ($\tau_{1/2}$= 12.9 d, E$_\gamma$=389 keV), and $^{128}$I ($\tau_{1/2}$= 25 m, E$_\gamma$=443 keV) – were suitable radiotracers because of long lifetimes and $\gamma$-ray energies. Moreover, through the process of ICF reactions their cross sections were an order of magnitude higher than the expected CF process.
Earlier works on ICF reactions in literature were carried out at high beam energies, and the sum rule model (original-SRM) [3] was quite successful in explaining the observed results. However, the original-SRM underestimated the ICF cross sections at our utilized low beam energies. We therefore made modification in the model mainly to incorporate the energy dependence in the definition of critical angular momentum. Using our modified-SRM, we found a significant improvement in predicting the enhancement in the cross sections. The present work has much potential in nuclear medicine, as it can improve the accessibility of many iodine isotopes for the better scope in their utilization and circulation.
1. B. Bhujang et al., Journal of Physics: Conference Series 420, 012128 (2013).
2. A. Gavron, Phys. Rev. C 21, 230 (1980).
3. J. Wilczyński et al., Phys. Rev. Lett. 45, 606 (1980).
