Transport model predictions of ²²⁵Ac production cross sections via energetic p, d and α irradiation of ²³²Th targets

Highlights

• Cross section measurements for proton, deuteron, and alphainduced reactions on Natural Thorium.

• Comparison between Monte Carlo transport codes (PHITS and MCNP6) and experimental measurement have been reported.

• Cross section measurements for ^225,227Ac, ^227,229Th, ^223,225Ra, and ^229,230,231Pa.

• Reasonable agreement with experimental data on the systems considered.

Abstract

Monte Carlo transport codes PHITS and MCNP6 were used to calculate the production cross sections of ^225,227Ac, ^227,229Th, ^223,225Ra, and ^229,230,231Pa via the bombardment of a²³²Th target with energetic protons, deuterons, and α-particles. The incident projectile energies ranged between 10 and 800 MeV/nucleon. When possible, the predicted production cross sections were compared with the available experimental data and other predictions. The degree of the codes' abilities to match the measured data provides a qualitative assessment of the codes’ abilities to predict data from similar, but unmeasured, projectile/target systems. In addition, a comparison between calculated cross sections and data may provide insight into possible improvements in the physics models employed by those transport codes.

Introduction

There is growing interest in the medical community regarding the use of α-emitting radioisotopes for the treatment of cancer and other diseases. Targeted Alpha Therapy (TAT), one of the techniques used for cancer therapy, takes advantage of the short-range and high linear energy transfer (LET) of particles emitted from radioisotopes. With a LET on the order 100 keV/μm, the emitted α particles can produce a lethal dose to the tumor cell in human tissue and effectively destroy it while minimizing the damage to the healthy cells surrounding the tumor. Actinium-225 (t_1/2 = 9.92 d) and its decay chain product ²¹³Bi (t_1/2 = 45.59 m) are strong therapeutic candidates for TAT. With the increasing demand for ²²⁵Ac for clinical trials and the limited supply of this isotope, new production methods for ²²⁵Ac need to be explored.

Currently, the most common method to produce ²²⁵Ac is by extracting it from ²²⁹Th (t_1/2 = 7,880 y), where the ²²⁹Th is produced by chemical separation from a supply of ²³³U (t_1/2 = 1.6 × 10⁵ y). Uranium-233 was produced in 1960's and 1970's as part of the molten salt breeder reactor program in the United States. Currently, the ²³³U is in long-term storage with no immediate or anticipated need. The three main world suppliers of ²²⁵Ac are Oak Ridge National Laboratory (ORNL), United States, Institute of Physics and Power Engineering (IPPE), Russia, and the Institute of Transuranium Elements (ITE), Germany, with a combined annual supply of 1–2 Ci of ²²⁵Ac (IAEA, 2013). In addition, the Canadian Nuclear Laboratory in Chalk River, Canada, has recently constructed a 10 mCi ²²⁹Th/²²⁵Ac generator (IAEA, 2018). The ²²⁵Ac produced from the decay of ²²⁹Th is considered as a “carrier-free” nuclide and supports a limited number of clinical studies. With the increase in clinical studies and potential widespread use of this isotope, the need for additional production channels has become apparent. An alternate approach to produce ²²⁵Ac is by high-energy proton spallation of ²³²Th targets (Duchemin et al., 2015; Ermolaev et al., 2012; Griswold et al., 2018; Robertson et al., 2019; Weidner et al., 2012a, 2012b). After irradiation, ²²⁵Ac is chemically separated from the thorium target and other product nuclei like radium, protactinium, as well as other spallation, and fission products. In this production route, the challenge is the presence of ²²⁶Ac (t_1/2 = 29.37 h) and ²²⁷Ac (t_1/2 = 21.8 y) contaminants because they cannot be chemically separated from ²²⁵Ac. Given its short 29-h half-life, ²²⁶Ac has little consequence on the use of ²²⁵Ac in TAT.

Because of its much longer half-life ²²⁷Ac (and its decay products) is a critical contaminate in ²²⁵Ac preparation. Consequently, any methodology, such as those evaluated in our study, which results in co-production of ²²⁷Ac with ²²⁵Ac must be evaluated for the amount of ²²⁷Ac that is produced in order to determine the viability of the method as a potential production mode.

In this paper, we describe the prediction of reaction cross sections for the interaction of energetic protons, deuterons, and α-particles with a²³²Th target resulting in formation of ^225,227Ac, ^227,229Th, ^223,225Ra, and ^229,230,231Pa. We used Monte Carlo particle transport codes PHITS and MCNP6 to calculate the cross sections at incident projectile energies between 10 and 800 MeV/nucleon. The predicted production cross sections were compared with earlier measured data and predictions when available.