Design And Performance Testing Of An Epithermal Neutron Flux Detector For BNCT

Boron neutron capture therapy(BNCT)is a promising and rapidly developing targeted cancer radiotherapy.BNCT selectively kills tumor cells by using?particles and ⁷Li recoil nuclei produced from the ¹⁰B(n,?)⁷Li reaction occurred in tumor cells while with little damage to the healthy tissues.BNCT has the characteristics of precise targeting,high relative biological effect and short course of treatment.Therefore,BNCT has been a new hotspot of energetic particle therapy and one of the most important choices for future cancer radiotherapy.A suitable neutron source is one of the necessary conditions for BNCT.Epithermal neutron(0.5 eV<E<10 keV)flux,as one of the basic characteristic parameters of the BNCT radiation field,is directly related to the analysis of BNCT parameters such as BNCT neutron source quality,treatment time,prescription dose and neutron irradiation dose.It is very important to measure the epithermal neutron flux in the BNCT radiation field accurately and quickly for the quality assessment of the BNCT neutron source and development of the treatment planning system.BNCT radiation field is a mixed radiation field with quite high neutron flux(?1×10⁹ n/cm²/s),which has a wide neutron energy range and contains thermal neutrons(<0.5 eV),epithermal neutrons and fast neutrons(>10 keV).Therefore,compared with other types of detectors,the activation detector is more suitable for measuring the characteristic parameters of the BNCT radiation field.In this work,a BNCT epithermal neutron flux detector based on ⁵⁵Mn(n,?)⁵⁶Mn activation reaction is designed by Monte Carlo simulations.The designed detector is spherical,and its structure from inside to outside is:manganese(Mn)foil(10 mm in diameter and 0.1 mm in thickness)as activation material,boron carbide(B₄C)layer(0.18 mm in thickness)as thermal/epithermal neutron absorber,high-density polyethylene(HDPE)sphere(62.2 mm in diameter)as neutron moderator and cadmium(Cd)layer(0.025 mm in thickness)as thermal neutron absorber.The Mn foil is located at the geometric center of the whole detector.The simulation results of the detector sensitivities in the neutron energy range from 0.01 eV to 10 MeV show that the designed detector is sensitive to epithermal neutrons and has a flat sensitivity curve in epithermal neutron energy range,but it is not sensitive to fast and thermal neutrons.After the design,two methods are used to evaluate the performance of the designed detector in this work.The first method is to design the experimental scheme and test the detector performance by using the D-D neutron source;the second method is to build simulated BNCT radiation fields based on the research results of the BNCT neutron source at home and abroad,and test the detector performance in these simulated BNCT radiation fields by using Monte Carlo simulations.For the first method mentioned above,the radiation field of D-D neutron source cannot be directly used to test the performance of the detector because the average energy of neutrons in the radiation field is 2.5 MeV.Therefore,in this work,a moderator based on the D-D neutron source is designed by using Monte Carlo simulations,and a suitable configuration of the moderator is obtained,that is,the combination of 5 cm thick polyethylene(PE)layer+24 cm thick titanium trifluoride(TiF₃)layer+22 cm thick magnesium fluoride(MgF₂)layer is used as the moderator material,20 cm thick nickel(Ni)layer is employed as the reflector and 0.11 cm thick lithium(Li)sheet is selected as the thermal neutron filter.The simulation results show that a suitable radiation field,which can be used to test the detector performance,can be formed when the D-D neutron beam passing through the designed moderator.The detector performance testing results show that the measurement accuracy of the BNCT epithermal neutron flux detector based on ⁵⁵Mn(n,?)⁵⁶Mn reaction designed in this work is better than that of the detector based on ⁷¹Ga(n,?)⁷²Ga reaction.Meanwhile,under the same conditions,compared with the detector based on⁷¹Ga(n,?)⁷²Ga reaction,the measurement time of the detector based on ⁵⁵Mn(n,?)⁵⁶Mn reaction is shorter.Therefore,the BNCT epithermal neutron flux detector designed in this work is more efficiently applicable to measure the characteristic parameters of the BNCT radiation field,and it will be helpful for the quality assessment of the BNCT neutron source and development of the treatment planning system.