Photonuclear Production of $^{195m}$Pt for Medical Applications: Cross Sections and Energy Thresholds

TL;DR

This work tackles the feasibility of producing the medically relevant isomer $^{195m}$Pt via photonuclear reactions. It employs activation measurements with a quasi-monoenergetic γ-ray beam to determine the cross section for $^{197}$Au(γ,pn)$^{195m}$Pt near threshold, using a linear deconvolution of decays from $^{195m}$Pt and $^{195}$Au across multiple cooling times. The results show cross sections that rise with energy, providing the first experimental constraint near the reaction threshold and indicating that practical production requires end-point γ energies around $50$–$60$ MeV. These findings guide future production efforts and motivate higher-energy measurements to enable accelerator-based, high-specific-activity $^{195m}$Pt radiopharmaceuticals for Auger-electron therapy and imaging.

Abstract

Platinum radioisotopes are of growing interest for targeted cancer therapy and diagnostic imaging because their decay delivers highly localized radiation doses in tissue, herewith enabling precise DNA damage through Auger-electron emission. Developing production technologies that provide platinum isotopes with high specific activity is therefore essential, and photonuclear reactions on stable nuclei offer a viable accelerator-based route when supported by reliable cross-section data. We report photonuclear cross-section measurements for the $^{197}$Au($γ$,pn)$^{195m}$Pt reaction at incident $γ$-ray energies of 27, 29, and 31~MeV using the activation method. The measurements were performed by irradiating a stack of concentric-ring gold targets with a quasi-monoenergetic $γ$-ray beam provided by the High Intensity Gamma-ray Source (HI$γ$S). The induced $^{195m}$Pt activity was quantified using off-line $γ$-ray spectroscopy. The present results provide the first experimental information in the energy region where the $^{197}$Au($γ$,pn)$^{195m}$Pt reaction becomes significant and demonstrate the minimum energy requirements and feasibility of producing $^{195m}$Pt via photonuclear reactions using electron accelerator facilities. These measurements indicate that electron energies of $\ge$ 50 MeV are required to achieve practically meaningful production yields.

Figures (3)

• Figure 1: Simplified decay schemes for the $^{195}$Au and $^{195m}$Pt nuclei. The selected $\gamma$-ray lines (blue) were used in the analysis. The decay $\gamma$ energies and relative intensities are adopted from the National Nuclear Data Center (NNDC) 12.
• Figure 2: Full $\gamma$-ray spectrum for target segment 1 measured a few hours after the end of bombardment (EOB) (a), and expanded $\gamma$-ray spectra around the 98.9-keV peak measured a few hours after EOB (b), one week (c) and two weeks later (d). The 98.9 keV $\gamma$-ray line emitted by both $^{195m}$Pt and $^{195}$Au is indicated. The accumulated measurement times for the three measurements were 5, 7, and 24 h, respectively.
• Figure 3: Excitation function of the $^{197}$Au($\gamma$,pn)$^{195m}$Pt reaction as a function of incident $\gamma$-ray energy. Red symbols represent the cross sections measured in this work. Vertical error bars indicate the total experiment uncertainties defined in Eq. \ref{['eq:12']}, while horizontal error bars indicate the energy spread of the incident $\gamma$-ray beam. The experimental results are compared with theoretical predictions calculated using the PHITS-3.34 (solid black line) 10 and TALYS-2.2 codes (dotted black line) 11.