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Figure 1: Sketches of the 2016 (left) and 2019 (right) TMR experimental configurations. The origin of the distance to neutron detectors is also indicated. tering effect. A high number of protons is implanted in the target and since the hydrogen solubility in Be and its diffusion coefficient are low, hydrogen atoms start to accu- mulate. Blisters appear and damage the target. With the beryllium target used for the experiment no blistering ef- fect has been observed so far. The main difference regard- ing the 2016 and 2019 TMR experimental configurations are sketched in Figure 1 and summarized in Table 2. The Geant4 simulation also agrees with the data taken in the 2019 configuration inside their uncertainties as presented in Figure 2 (IPHI-2019 legends).

Figure 1 Sketches of the 2016 (left) and 2019 (right) TMR experimental configurations. The origin of the distance to neutron detectors is also indicated. tering effect. A high number of protons is implanted in the target and since the hydrogen solubility in Be and its diffusion coefficient are low, hydrogen atoms start to accu- mulate. Blisters appear and damage the target. With the beryllium target used for the experiment no blistering ef- fect has been observed so far. The main difference regard- ing the 2016 and 2019 TMR experimental configurations are sketched in Figure 1 and summarized in Table 2. The Geant4 simulation also agrees with the data taken in the 2019 configuration inside their uncertainties as presented in Figure 2 (IPHI-2019 legends).

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Table 1: Neutronic and thermal properties for different material foreseen as a target. The neutron yields are from [3]
Table 1: Neutronic and thermal properties for different material foreseen as a target. The neutron yields are from [3]
Figure 2: Thermal neutron flux (E,<400 meV) as a func- tion of the distance (see Figure 1), for the 2016 and 2019 TMR configurations. Geant4 simulation results are com- pared with experimental data.
Figure 2: Thermal neutron flux (E,<400 meV) as a func- tion of the distance (see Figure 1), for the 2016 and 2019 TMR configurations. Geant4 simulation results are com- pared with experimental data.
Table 2: Main differences between the 2016 and 2019 TMR experimental configurations performed at IPHI. Between the 2016 and 2019 TMR configurations (Figure 2), at 2 meters, the simulated ¢,, dropped by a factor 1.7. This is mainly because the target thickness in- creases from 0.5 mm to 20 mm (Table 2). Indeed, in beryl- lium, a 640 keV (average initial neutron energy) neutron has a mean free path around 17 mm and the neutron mean free path averaged over the initial neutron energy spec- trum is around 22 mm. These are similar to the 20 mm target thickness. Therefore, in average neutrons interact once before reaching the moderator. This spatially dilutes the neutrons leading to a less efficient neutron extraction. This target thickness was initially chosen for mechanical and thermal constraint reasons. This underlines the need to find the best compromise between mechanical, thermal, and neutronic constraints to maximize the neutron flux.
Table 2: Main differences between the 2016 and 2019 TMR experimental configurations performed at IPHI. Between the 2016 and 2019 TMR configurations (Figure 2), at 2 meters, the simulated ¢,, dropped by a factor 1.7. This is mainly because the target thickness in- creases from 0.5 mm to 20 mm (Table 2). Indeed, in beryl- lium, a 640 keV (average initial neutron energy) neutron has a mean free path around 17 mm and the neutron mean free path averaged over the initial neutron energy spec- trum is around 22 mm. These are similar to the 20 mm target thickness. Therefore, in average neutrons interact once before reaching the moderator. This spatially dilutes the neutrons leading to a less efficient neutron extraction. This target thickness was initially chosen for mechanical and thermal constraint reasons. This underlines the need to find the best compromise between mechanical, thermal, and neutronic constraints to maximize the neutron flux.
nel is used. The thermal fluxes obtained with polyethylene (CH2) and light water (H2O) are similar (Figure 3). How- ever the thermal beam quality, defined by @in/@yor, is higher by a factor 1.5 with CH) compare to H2O because !°O has a higher mass than '*C. To conclude, polyethylene is the best moderator to get a well collimated beam along with a high thermal neutron flux for one extraction channel and a low beam power. For higher beam power H2O will be considered because of the CH2 low thermal conductivity. For multiple beam extractions, DO could be also a good candidate. Figure 3: Thermal neutron flux as a function of the dis- tance (see Figure 1), for different moderator materials. The TMR configuration is the 2016 one with the 2019 ac- celerator/target part.
nel is used. The thermal fluxes obtained with polyethylene (CH2) and light water (H2O) are similar (Figure 3). How- ever the thermal beam quality, defined by @in/@yor, is higher by a factor 1.5 with CH) compare to H2O because !°O has a higher mass than '*C. To conclude, polyethylene is the best moderator to get a well collimated beam along with a high thermal neutron flux for one extraction channel and a low beam power. For higher beam power H2O will be considered because of the CH2 low thermal conductivity. For multiple beam extractions, DO could be also a good candidate. Figure 3: Thermal neutron flux as a function of the dis- tance (see Figure 1), for different moderator materials. The TMR configuration is the 2016 one with the 2019 ac- celerator/target part.
Figure 4: Thermal neutron flux as a function of the dis- tance (see Figure 1), for different polyethylene moderator dimensions.
Figure 4: Thermal neutron flux as a function of the dis- tance (see Figure 1), for different polyethylene moderator dimensions.
Figure 5: Thermal neutron flux as a function of the dis- tance, for different reflector material. In this configura- tion a ~10 cm thick reflector material is placed around a 15x15x20 cm? polyethylene moderator. The impact of the polyethylene moderator dimensions on ¢, has then been studied. Figure 4 shows that the bigger the moderator size is, the higher ¢,, is (neutrons are more thermalized). It also shows that starting from a 30x30x40 cm? moderator, ¢,, starts to converge to an asymptotic value as the moderator size increases. It is then not necessary to increase the moderator volume above this volume. 5 Conclusion
Figure 5: Thermal neutron flux as a function of the dis- tance, for different reflector material. In this configura- tion a ~10 cm thick reflector material is placed around a 15x15x20 cm? polyethylene moderator. The impact of the polyethylene moderator dimensions on ¢, has then been studied. Figure 4 shows that the bigger the moderator size is, the higher ¢,, is (neutrons are more thermalized). It also shows that starting from a 30x30x40 cm? moderator, ¢,, starts to converge to an asymptotic value as the moderator size increases. It is then not necessary to increase the moderator volume above this volume. 5 Conclusion
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