The enrichment of the thermal and epithermal components of the beam and the corresponding formation of the flow must be performed using a Beam Shaping Assembly (BSA). The main goal of this work is the design of the BSA using the software package GEANT4, which will be useful for medical purposes, particularly for BNCT.
First, the BSA should be a composition of carefully selected moderators to reduce the neutron energy to the thermal/epithermal range. Second, for an effective treatment, it is necessary to use reflectors, collimators and shielding materials to have a proper focusing of the quasi-monochromatic neutron flux, which will also satisfy the IAEA’s criteria.
The therapeutic dose of 10B isotope is greater or equal than 20 µg per gram of tumor tissue or 109 of 10B per cell. At the same time, thermal/epithermal neutron flux must be high enough to be effectively captured by each 10B located inside of the cell, as the main goal of BNCT is to kill the tumor cell by cell.
Thermalization of neutrons could be done by using layers of different types of materials when each layer could be applied for neutrons in the exact energetic range. At the same time, the number of neutrons should not be low or non-applicable for BNCT.
Natural molybdenum with optimal thickness 20 cm, installed as the first layer of BSA, works as a multiplier. With (n, 2n) nuclear reactions, it is possible to decrease the energy of neutrons and at the same time get a higher amount of thermal/epithermal flux. Due to earlier studies , it may be admitted that molybdenum is an optimal material for multiplication of neutron yield and at the same time for thermalization on the base (n, xn) nuclear reactions.
Right after the nat Mo, there are installed 45-cm-thick iron and 45-cm-thick paraffin boric acid to decrease the energy of neutrons as much as possible. Iron is a suitable material to decrease the energy of neutrons down to 1 MeV, and the other important moderator is a mix of paraffin and boric acid to make the flux thermal/epithermal.
Moderators should be covered by the reflector to focus the beam. In this case, 20-cm-thick Pb is used as a reflector, which is known as one of the useful materials and widely used as an optimal one [16,17,18,19].
In this GEANT4 model, the whole BSA system was covered by thick concrete from sides. It was considered that the BSA should be installed in the wall, which will separate the room for patients from the rest of the experimental hall. The created BSA gives the possibility to achieve neutron flux, which is suitable with the International Atomic Energy Agency (IAEA) recommendations. Fig. 2 presents the energy distribution of neutrons when BSA consisted of 20-cm-thick Mo, 45-cm-thick Fe and 45-cm-thick paraffin boric acid. As a result of GEANT4 simulations, the registered neutron flux is around 0.968 × 109n/s/cm2 from 6.24 × 10 14 proton/s (when the current is 100 µA) and consisted of 69% epithermal neutrons, around 17% fast neutrons, and thermal neutrons amount is just 14%. Experimentally the results should be proven at A. Alikhanian National Science Laboratory, where the Cyclone 18/18 is installed. Compared with previous result , where the BSA consisted from 5-cm-thick Bi, 55-cm-thick Fe, 10-cm-thick Al, 5-cm-thick graphite and 10-cm-thick 7LiF as moderators and 10-cm-thick Pb as reflector surrounding the moderation system, the total flux was 0.96 × 106 n/s/cm2 from 6.24 × 1014proton/s and only 12% of neutrons were in the energetic range of up to 10 keV, while the rest were fast neutrons.
To apply neutron flux for medical purposes or for BNCT, there is a necessity to study epithermal neutrons with energies up to 10 keV, which loses energy because of tissue penetration and becomes thermalized . Fig. 3a, b presents the aforementioned energetic range neutrons, which was possible to achieve by using BSA consisted from 20-cm-thick Mo neutron multiplier, 45 cm Fe and 45 cm paraffin boric acid as moderators, and 20-cm-thick Pb as reflector/collimator (Fig. 4).
Because of the application for medical purposes, the total dose includes not only the influence of neutrons, but also gamma rays. Fig. 5a presents energy distribution of gamma rays, and almost 99.4% of gammas have energies up to 6 MeV. It is known that even 6 MeV monochromatic gamma ray flux is useful for radiotherapy [21,22,23].
The statistical error for the registered gamma rays from 7 to 9 MeV increased from 22 to 100%. In this case, 0.6% of gamma rays, which are in the aforementioned energetic region, can be even less. Nevertheless, the estimation of gamma rays influence and its contribution on the total dose should be done in greater detail in the future, but as a result of simulations the registered flux is 1.083 × 1010 gamma/s/cm2 from the highest possible proton current, which is 100 µA or around 6.24 × 1014 proton/s. It is important to take into account the focusing of gamma rays as well. Figure 5b is the angular distribution, which shows that gamma flux is mainly focused on the central part of the surface of the register/detector. The aforementioned fact and the appropriate figure also show that the reflector of a BSA focuses not only neutrons but gamma rays too. In the case of BNCT, the impact of gamma rays and fast neutrons should be suppressed. It is worth to be mentioned that the capturing process of neutrons by boron nucleus usually followed by 0.478 MeV gamma rays, which should be detected, for example by SPECT. Due to the existence of gamma rays emitted from the target, as well as, from other components of the BSA, it will make technical difficulties and impede the detection of gamma rays from the capturing process. It is known that Pb is one of the widely used gamma ray filters [24,25,26], but its influence on epithermal neutrons and on the final quality of the neutron beam should be estimated only after deeply detailed investigations.