Calculating S values of the 225Ac decay chain using MCNP simulating code

Rezazadeh-Sefideh, Fatemeh; Hosseini, Seyed Abolfazl; Shirvani, Simindokht; Zolghadri, Samaneh; Yousefnia, Hassan; Bahrami-Samani, Ali

doi:10.24200/nst.2025.1769.2091

Calculating S values of the 225Ac decay chain using MCNP simulating code

Articles in Press

Document Type : Research Paper

Authors

Fatemeh Rezazadeh-Sefideh ¹

Seyed Abolfazl Hosseini ²

simindokht shirvani ³

Samaneh Zolghadri ⁴

Hassan Yousefnia ⁴

Ali Bahrami-Samani ⁴

¹ Radiation Medicine Department, Faculty of Energy, Sharif University of Technology, P.O.Box: 1458889694, Tehran-

² . Radiation Medicine Department, Faculty of Energy, Sharif University of Technology, P.O.Box: 1458889694, Tehran-Iran

³ دکتر

⁴ Nuclear Science and Technology Research Institute, AEOI, P.O.Box: 14395-836, Tehran – Iran

https://doi.org/10.24200/nst.2025.1769.2091

Abstract

Among alpha-emitting radioactive nuclei for clinical applications, Actinium-225 (²²⁵Ac), with a half-life of 9.9 days, is widely used. This radionuclide undergoes a decay chain, emitting five alpha particles in the energy range of 5.8 to 8.4 MeV. The decay cascade also includes beta emissions from 0.6 to 2 MeV and gamma radiation from Francium-221 (²²¹Fr) and Bismuth-213 (²¹³Bi). This study aimed to calculate S-values, the absorbed dose per decay from the source organ to the target organ in mSv/MBq.s, for ²²⁵Ac and its daughters in the adult male ORNL phantom (225 source-target organ pairs) using the MCNPX2.7 code. The accuracy of the code was verified by computing the specific absorbed fraction (SAF) and comparing it with ORNL data. The results of MCNP and ORNL data showed strong agreement when the source-target organ distance and size were appropriate. However, in cases with greater distances or complex geometries, discrepancies were observed, attributed to the different computational methods used by ORNL in these situations. This study confirmed that ²²⁵Ac and its daughters, especially Polonium-213 (²¹³Po), deliver a high dose to target organs. The MCNPX simulation showed good accuracy compared to ORNL data and can aid in optimizing targeted alpha therapy.

Keywords

225Ac decay chain

S-value

ORNL

MCNP

Main Subjects