Results and Discussion
6.3 The result of neutron source measurement and its comparison to the simulationand its comparison to the simulation
6.3.1 The P u238-C13 source spectra
The results of the NE213 detector
a) The result of measurement The measured spectrum is shown in Fig. 6.7.
However, this is not the real neutron spectrum. As mentioned before, neutrons are only indirectly detected, and the measurement of neutron energies is through the proton recoil. The result in Fig. 6.7 is the proton energy spectrum.
Figure 6.7: Measured spectrum from NE213 detector. The channels range of the time constant spectrum is from the channel 160 to the channel 190.
b) The result of simulation We use GEANT4 toolkit to perform the simu-lation. Fig. 6.8 shows the result of NE213 detector simusimu-lation. This result includes up to the triple recoil of the proton in the NE213 detector.
Figure 6.8: Neutron detection simulation for NE213 detector.
c) Comparison Fig. 6.9 shows the comparison of measurement and simula-tion results. The normalizing range is between 2.5 MeV and 8 MeV. As one can see, for the NE213 detection system, there is a cutoff at about 2.5 MeV. This is the energy threshold for our measurement.
Figure 6.9: The comparison of simulation and measurement for the proton recoil spectrum. The area under the curve between 2.5 MeV and 7 MeV is used to set the normalization.
The result of 3He proportional tube
a) The result of measurement The measured spectrum of Helium-3 pro-portional detector is shown in Fig. 6.10. The peak in the spectrum is the thermal neutron reaction energy which is 0.764 keV. We divide 0.764 keV by the chan-nel number which is at the peak location and we obtain the energy per chanchan-nel.
Therefore we obtain the energy scale for neutron.†
b) The result of simulation Fig. 6.11 shows the result of3He proportional tube simulation.
†The events lower than 0 MeV are the wall effect of the detector. They are the energies of the proton or triton. More information about wall effect is given in Ref. [5]
Figure 6.10: Measured spectrum by Helium-3 detector.
Figure 6.11: Neutron detection simulation for He3 proportional tube.
c) Comparison Fig. 6.12 shows the comparison between measurements and simulations. We use the maximum value of the results to normalize the data. It is easy to see that the simulation is greater than measurement when the energy is more than 0 MeV. However, for events due to the wall effect, the experiment is more sensitive than the simulation.
Figure 6.12: The normalized result for the simulation and experiment of the 3He proportional tube.
6.3.2 The monoenergic neutron source spectra
The results of the NE213 detector
a) The result of measurement Fig. 6.13 shows the recoil proton spec-trum resulted from the monoenergic neutron source generated by Van de Graff accelerator.
Figure 6.13: The measured spectrum by NE213 detector. The channels range of the time constant spectrum are from the channel 160 to the channel 190.
b) The result of simulation We are do not simulate the process,7Li(p, n)7Be.
However, since neutron is known to be monoenergetic, we set the neutron kinetic energy as 2.7 MeV.
Figure 6.14: The simulated spectrum in NE213 detector resulted from the mo-noenergic neutron source.
Obviously, these two results are hard to compare, since the lower limit of the detecting system is around 2.5 MeV; while the maximum value for recoil proton energy is 2.7 MeV. We do not know the trend less than 2.5 MeV in the measured spectrum.
The result of 3He proportional tube
a) The result of measurement Fig. 6.15 shows the neutron spectrum mea-sured by 3He proportional tube.
Figure 6.15: The neutron spectrum measured by 3He proportional tube.
b) The result of simulation Fig. 6.16 shows the simulated neutron spec-trum in3He proportional tube.
c) Comparison Fig. 6.17 shows the comparison of normalized results. Simi-lar to the previous case, the experimental result is more sensitive at the wall effect and the simulation result is more sensitive at energies larger than 0 MeV.
Figure 6.16: The simulated neutron spectrum in 3He proportional tube.
Figure 6.17: The normalized result of the simulation and experiment of the 3He proportional tube.
Chapter 7 Conclusion
The result of neutron energy measurement by NE213 detecting system is consistent with the result by simulation except for the energy lower than 2.5 MeV. The 3He proportional tube detector is however not suitable for measuring fast neutrons.
To produce monoenergic neutron source, we have protons with a kinetic energy 4.4 MeV impact on the LiF target, the neutrons were generated with the maximum of the kinetic energy 2.73 MeV at the zero degree position which can be estimated by Eq. 5.1. It is unreasonable to observe neutron energy greater than 2.73 MeV.
However, there always exists a peak at 4 MeV in the result of NE213 detector, as shown by Fig. 6.13. We also see the same peak at different time constant ranges, as shown by Fig. 7.1 and Fig. 7.2. It is hard to explain where does those events come from. So we suggest another measurement of this effect with different neutron detection method. Furthermore, for producing the monoenergic neutron, we can also use the other processes, such (d, n) reaction. The neutron energy from these processes is larger than (p, n) reaction. It might be more preferred for the NE213 detecting system.
Figure 7.1: The measured spectrum by NE213 detector. The channels range of the time constant spectrum are from the channel 46 to the channel 60.
Figure 7.2: The measured spectrum by NE213 detector. The channels range of the time constant spectrum are from the channel 75 to the channel 86.
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