A Study Of Calculation Of Induced Radioactivity Of Heavy Concrete In Proton Therapy Room

Proton Radiotherapy is one of the most advanced tumor treatment methods in the world,mainly due to the proton-specific "Bragg Peak" dose distribution and superior radiobiological effects.A proton therapy center uses heavy concrete as part of the shielding wall of the machine room,the proportion of iron in heavy concrete is 47.425%,the secondary neutrons produced during proton therapy will activate 56Fe in heavy concrete to 56Mn,which in turn causes the heavy concrete shield to generate induced radioactivity.In order to study the influence of induced radioactivity on the workers in the machine room,the induced radioactivity generated by the heavy concrete shielding wall in the proton therapy room was calculated,and the shielding effect of heavy concrete and ordinary concrete was compared.In this study,the Monte Carlo application software package Geant4 10.05(Geometry And Tracking)was used to simulate the shielding design scheme of a proton therapy machine room in China,and a heavy concrete shielding wall model was established.The induced radioactivity of the heavy concrete shielding wall was calculated,and the shielding effects of heavy concrete and ordinary concrete were compared and calculated.When calculating the induced radioactivity of heavy concrete shielding wall,the model of shielding wall is 2.5 m thick,6.0 m high,and 12.4 m wide.The proton beam energy is 245 MeV,and the beam intensity is 3 nA.The simulation is carried out in two steps,the first step is to simulate the secondary neutron energy spectrum produced by the proton beam irradiating the water phantom,and the second step is to use the simulated secondary neutron energy spectrum to irradiate the heavy concrete shielding wall and record the generated radioactive 56Mn and its distribution;then according to the distribution law of radionuclide 56Mn in the shielding wall,the shielding wall is layered at every 10 cm thickness to calculate the ambient dose equivalent rate in the treatment room by the radionuclide 56Mn in the first three layers of shielding wall.The calculation refers to the calculation formulas related to neutron activation and radiation dosimetry,and calculates the ambient dose equivalent rate generated by the radionuclide 56Mn in the first three layers of shielding wall;finally,the shielding properties of secondary neutrons with energies of 5,20,50,100,150,200 and 245 MeV in heavy concrete and ordinary concrete are studiedThrough simulation and statistics,under the maximum beam irradiation condition(1.872×1010),when simulating secondary neutrons irradiating a 10 cm thick shielding wall,In the range of 0°?5°,5°?10°,10°?15°,15?20°,20?25°,25°?30°,30°?35°,35°?40°,40°?45° from the beam central axis,the number of 56Mn are 5.26 × 106,3.04 × 106,1.74 × 106,6.02 × 105,4.20 × 105,1.45 × 105,4.50 × 104,4.00 × 103 and 0,the number of 56Mn in the range of 0°?5° from the beam center axis is the largest.When doing hierarchical calculations,the number of radionuclide 56Mn in the first three layers of shielding walls are 3.10×108,1.60×108,9.33×107;the ambient dose equivalent rate at a distance of 1 m from the treatment room are 2.13×10-3,8.82×10-4,9.10×10-4?Sv/h,the total ambient dose equivalent rate for the first three layers was 3.92×10-3 ?Sv/hFor the comparative study of the shielding effect of heavy concrete shielding wall and ordinary concrete shielding wall,when the thickness of the shielding wall is the same,for single energy neutrons with energies of 5,50,100,150,200,245 MeV,the transmittance of ordinary concrete shielding walls is greater than that of heavy concrete shielding walls.When the thickness of the two shielding walls is the same with the neutron energy is the same,the neutron transmission in heavy concrete is always smaller than that in ordinary concrete.When the neutron energy is 5,50 MeV,the neutron transmittance of the two shielded walls has the maximum difference when the wall thickness is 50 cm,which is 0.0579 and 0.0744 respectively.When the neutron energy is 100,150,200,245 MeV,the maximum difference between the neutron transmission of the two shielding walls is 0.1007,0.1179,0.1027 and 0.1135 respectively when the wall thickness is 100 cm.In conclusion,when using proton therapy,the closer to the beam central axis,the stronger the induced radioactivity of the shielding wall.As the distance increases,the induced radioactivity gradually decreases and approaches zero,the neutrons at the front of the shielding wall activated iron produces the strongest induced radioactivity.The number of induced radionuclides 56Mn decreases exponentially as the thickness of the shielding wall increases.The higher the energy,the greater the penetration ability of the neutrons,and the shielding design of the proton therapy room should consider the protection of the highest energy secondary neutrons.For neutrons with energy less than 100 MeV,heavy concrete has the greatest advantage in protection performance when the shield wall is 50 cm thick;for neutrons with energy greater than or equal to 100 MeV,heavy concrete is better than ordinary concrete when the shield wall is 100 cm thick has the greatest advantage in protection performance.In general,the shielding effect of heavy concrete shielding walls is better than that of ordinary concrete shielding walls.