## 1 Introduction  
Currently, reproducing tissue seems impossible due to different compositions among humans [1]. Nevertheless, new opportunities to develop materials that look and feel as close to human tissue as possible are presented with every innovation in manufacturing and material science. These are known as tissue equivalent materials (TEMs). They can be used in diagnostic and therapeutic physics for quality assurance and control. Therefore, is there a unified selection criteria for a material to be a tissue equivalent in terms of radiation response? Recently, researchers have been influenced by the application of therapy or imaging in their studies. For example, to develop TEMs in ultrasound, a set of international standards [2] is typically used. However, it is not possible to create TEMs under standard operating procedures for MRI, surgery, or thermal therapies. Surgery is the only field that has published guidelines on developing TEMs [3]. For gamma-ray irradiation, the key parameters for material specification are the linear attenuation coefficients (LACs) for photoelectric absorption, Compton scattering, and Rayleigh scattering (using NIST databases [4], ICRU 44 [5], and ICRU 46 [6]) [7]. Radiological properties are required to mimic the electron density and the mass energy-absorption coefficient as well as the density and atomic number. Based on the application, TEMs must match the absorption and scattering characteristics of body tissue to a high degree of accuracy [7]. Using the Hounsfield Unit (HU), the CT number measures how far tissue LAC differs from that of water. Typically, a particular tube voltage is used to characterize the tissue. One limitation of this method of determining tissue properties is that it only applies to a particular photon energy [8], and the density of lung and bone tissues is affected by beam hardening due to the patient's size [9]. In addition, measured CT numbers are somewhat dependent on scanner-related factors such as calibration and imaging protocol. During this work, we will introduce for the first time another tuning parameter for the TEM selection process called the buildup factor (BUF). As a quantification of the scattered to the unscattered beam, the BUF can inform us with the radiation exposure or the energy absorption leading to the exposure buildup factor (EBF) or to the energy absorption buildup factor (EABF). As it is well known that introducing buildup factors for comparison purposes is effective at energies where Compton scattering dominant, here we will adress the study of such phenomena in second order for choosing tissue equivalency after the effective atomic number wich remains in first order. Thus, we can talk about tuning of the TEM searching criteria. According to the particle type (photon, neutron,...), energy, and the material elemental composition, BUFs can be calculated using tabulated data from ANSI/ANS-6.4.3-1991 [10]. Those data were generated according to the five geometric progression (GP) fitting method, depending on the particle energy and the penetration depth in terms of mean free path (mfp). Moreover, the CT number, as a selection criteria, was generally calculated at 120 kVp standard beam. Here, we extended the calculation to 30 and 80 kVp in order to cover the mammography and the thorax imaging kVp intervals. Our goal can be summarized as follows: (i) verification of the in-house C++ developed program to reproduce some standard data, (ii) introduction of the concerned tissues and their, costly effective compared to commercially available, equivalent material candidates, and (iii) search and proposition of TEMs. Thus, after checking the ability of our computation to reproduce the energy absorption and exposure buildup factors of water, we carried out the calculation of a set of physical properties.