### Introduction

_{3}with cement and gypsum at energies 365.5, 661.6, 1,173.2, and 1,332.5 keV. Simulated results were compared with available experimental data and found comparable in literature. The present investigation using standard simulation geometry for MCNPX (version 2.4.0) code would be very useful for studies of photon interraction with various types of samples for shielding and dosimetry applications.

### Materials and Methods

### 1. MCNPX code

_{3}. MCNPX is a general purpose radiation transport code for modeling the interactions of radiation with materials and also tracks all particles at wide range energies. MCNPX is fully three-dimensional and it utilizes extended nuclear cross section libraries and uses physics models for particle types [2]. Validation of MCNPX code have been studied by considering the different applications in the field of radiation [3–11]. In present simulation, each simulation parameters have been defined in input file by considering the material properties and experimental setup. In this study, an isotropic radiation source has been defined and located at the beginning of the collimator. To avoid the backscatter into detection point, full collimation from radiation source point until the target shielding material was provided. Afterwards, energetic photons have interacted with target shielding material. Transmitted photons and their numerical quantities have been calculated in detection field. The simulation geometry of mass attenuation coefficient calculations can be seen in Figure 1. The geometrical forms and physical parameters for simulation have been defined in cell card and surface card of MCNPX input file. On the other hand, source definition has been defined with different variables such as CEL, ERG, DIR, POS, and PAR. The energy source has been defined as a point source on the central axis as well. To obtain the average cell flux in defined detection area, mesh tally (type F4) has been used. This type of tally in MCNPX code reports the sum of the contributions in considered cell. This type of tally in MCNPX scores energy deposition data in which energy deposited per unit volume from all particles is included. MCNPX calculations were completed by using Intel

^{®}Core

^{TM}i7 CPU 2.80 GHz computer hardware.

### 2. Theoretical estimation

*I*

*and*

_{0}*I*are the incident and transmitted photon intensities, respectively,

*μ*(cm

^{−1}) represents linear attenuation coefficient of the material and

*t*(cm) is the thickness of the target material/sample. Rearrangement of Equation 1 yields the following equation for the linear attenuation coefficient:

*w*

*is the weight fraction, (*

_{i}*μ*/

*ρ*) is the mass attenuation coefficient of the material,

*μ*

*is mass attenuation coefficient of element. The*

_{m}*w*

*can be defined as follows:*

_{i}*A*

*is the atomic weight of the sample,*

_{i}*n*

*is a number of formula units.*

_{i}### 3. Validation

### Results and Discussion

_{3}materials. The mass attenuation coefficients for attenuator samples doped by different percentages of PbCO

_{3}were calculated for the four different energies 356, 662, 1,173, and 1,333 keV and shown in Figure 2. The standard XCOM data has been used for comparison with obtained MCNPX results. Figure 2 shows mass attenuation coefficinents of cement, gypsum and small doping of PbCO

_{3}in both the shielding materials. It is found that the mass attenuation coefficients of pure and doped cement or gypsum are decreasing with increase in photon energy. This variation of mass attenuation coefficients can be explained using fundamental photon interaction process of photoelectric effect, compton effect and pair production for low-, intermediate- and high energy photons, respectively, which varies with atomic number of elements of compostions. From Figure 2A and 2C, it is to be noted that the mass attenuation coefficients of 100% cement and 100% gypsum estimated using MCNPX is lesser than theoretical data and comparable with experimental results. However, from Figure 2B and 2D it is observed that using MCNPX simulation, the mass attenuation coefficients of 100% cement and gypsum are lesser than 30% doping of PbCO

_{3}. Doped cement and gypsum with PbCO

_{3}are found with large difference of mass attenuation coefficients at low energy (photoelectric effect region) compared with high energy (pair production region). It is because of Pb element of PbCO

_{3}, which dominant for interaction in photoelectric effect region (interaction cross sectioin α Z

^{4–5}).

### Conclusion

_{3}has been investigated using monte carlo MCNPX. The simulated values of mass attenuation coefficients were compared available experimental results, theoretical XCOM values and found good comparability of the results. Standard simulation geometry for sample prepration of the present investigation would be very useful for various types of photon interaction investigations using MCNPX without experimental analysis for pure and mixture of elements. In addition, radiation safety inside medical areas, research centers, houses, nuclear shelters especially those close to nuclear power stations can be estimated by the calculation of shielding features of common cement and gypsium mixture materials.