## 1 Introduction  
As a fundamental tool in nuclear medicine procedures, radiopharmaceuticals are unique compounds generally composed of ligands and radioisotopes. The ligands selectively transport medical isotopes to designated tissues or organs and eventually combine with them. The decay of the isotopes emits radiation that can be used for medical diagnosis or treatment. One of the most critical medical isotopes, \({}^{99\text{m}}\)Tc, has a half-life of 6.01 h and emits 140 keV photons during decay. Currently, \({}^{99\text{m}}\)Tc is one of the most widely used medical isotopes in the world. It is used in more than 85% of the nuclear medicine diagnostic processes [1], typically in single photon emission computed tomography (SPECT) after pharmaceutical processing [2]. In China, the supply of \({}^{99\text{m}}\)Tc relies on imports due to the lack of domestic production facilities. The activity of \({}^{99}\)Mo/\({}^{99\text{m}}\)Tc imported to China in 2019 attains -63% of the total activity of all imported radioisotopes [3].  
Owing to the relatively short half-life of \({}^{99\text{m}}\)Tc, its general production process involves irradiating enriched uranium targets with high-flux neutrons produced by a reactor to produce \({}^{99\text{Mo}}\) (T\({}_{1/2}\) = 66 h) through fission reactions. After the irradiated target is dissolved and purified, the processed solution is supplied to a \({}^{99}\)Mo/\({}^{99\text{m}}\)Tc generator to be distributed to medical facilities for clinical or research use. In the hospital, technetate (\({}^{99\text{m}}\)TcO\({}^{4-}\))solution for pharmaceuticals is eluted with saline as needed. However, except for the Open Pool Australian Lightwater (OPAL) reactor, other reactors producing \({}^{99}\)Mo have been operating for more than 50 years and are facing decommissioning or frequent maintenance [4; 5]. Consequently, since 2009, some countries have been experiencing serious global shortages of medical isotopes due to the shutdown of research reactors [2]. The development of alternative \({}^{99}\)Tc production methods has received extensive attention. The most promising ones are as follows:  
* Production of \({}^{99}\)Mo based on a high-current neutron generator and fission reaction [6];
* Production of \({}^{99}\)Tc using a proton cyclotron [7];
* Production of \({}^{99}\)Mo based on a high-energy electron linear accelerator that exploits photonuclear reactions [8].  
Among them, the Canadian National Laboratory of Particle and Nuclear Physics (TRIUMF) has verified the feasibility of the method for direct production of \({}^{99}\)Tc based on their TR-24 cyclotron [9; 10].  
The China Initiative Accelerator Driven System (CiADS) is a scientific program proposed by the Chinese Academy of Science (CAS) to solve the problems of nuclear waste and fuel. The 25 MeV superconducting (SC) linear accelerator prototype in the Institute of Modern Physics (IMP) was designed and built as the demonstration facility for the intense proton source for this system [11]. One of the experimental terminals was allocated to investigate the production of \({}^{99}\)Tc via the \({}^{100}\)Mo (p,2n) \({}^{99}\)Tc reaction.  
The high-current proton beam can produce intense secondary radiations when bombarding a highly enriched \({}^{100}\)Mo target. The secondary particles generated, such as neutrons and photons, must be properly shielded for the safety of personnel and avoidance of damage to crucial elements sensitive to radiation.  
In this work, we performed a shielding assessment and optimization of the experimental terminal that considers both prompt and residual radiation together with the effect of nuclear activation from the components of the target system and the building infrastructure.