Designed free-lead composite shields for diagnostic medicine radiation fields by using MCNPX code

 Introduction:
Nowadays, applications of free-lead radiation shields are increased specially for weighting factor and toxicity of lead elements. Although touching the lead may be results in the nervousand blood system of body, it is not exactly proven yet. Protective measures are the fundamental parts of radiation protection. On the other hand, high weighting factor of lead shields are caused to fallow off using apron just in necessary situations. The primary principle of radiation protection is based on ALARA (As Low as Reasonable Achievable) law. Therefore, free-lead shields are attended of investigator. Three factors were proposed for designing radiation shields consist of: percent of attenuating against radiation in field, weight and cost. Choose of radiation shields are usually based on these three factors.

Materials and Methods:
In this study, attenuating factor, weight and cost are proposed for designing composite radiation shields. A source of radiation is collimated like X-ray machines in diagnostic medicine fields. The source has 90, 100 and 120keV spectrums that is output of IPEM78 software. The composite consists of the following eight elements. Tungsten(W), Barium(Br), Bismuth(Bi), Antimony(Sb), Cadmium(Cd), Cerium(Ce), Gadolinium(Gd) and Tin(Sn) are simulated by MCNPX code. The composites include one, two or three element with variant percentage. The dimension of each designed shield (composites and lead) in MCNPX code is 15*15*0.05cm3. Tallies F4 and F6 are applied for estimating local flux behind the shield. All the simulation codes with a standard deviation less than 0.5% are accepted. The relative flux of each composites is calculated by: Frel= Festimated/ Freal.
Results:
For composites which has two elements consist of W-Ce (55.16%-44.84%), W-Sb (52.38%- 47.62%) and W-Cd (31.25%-68.75%) are 63.4%, 50.4% and 47.7%, respectively. The relative flux of the composites with three elements consist of W-Ce-Cd (50%-30%-20%), W-Bi- Ce (45%-28%-27), W-Sn-Ce (46%-50%-4%), W-Sn-Cd (44%-50%-6%) and W-Sn-Bi (44%-50%-
6%) are Frel 58.1%, 55.9%, 51.6%, 49.5% and 49.2%, respectively. weight of These composites are equal to the weight of a common lead shield, while they have a higher attenuating factor.
Conclusion:
The results show that the pure elements have a better attenuating than oxide and sulfite of them in markets. If the cost wasn’t important, pure Gd could be good choice with the best attenuating factor. If an equipped laboratory was available, the composite with W could be the best choice with a reasonable cost. Optimization of these three factors depends on the importance of the factors.