Simultaneous measurement of energy spectrum and fluence of neutrons using a diamond detector


Experiments were performed on d–d and d–t neutron sources separately.

The experiments on the d–d neutron sources were based on the Van de Graaff accelerator at Peking University (PKU) and the HI-13 tandem accelerator at China Institute of Atomic Energy (CIAE). Two neutron energies of 5.0 and 5.5 MeV were measured at PKU and three neutron energies of 8.5, 9.5 and 10.5 MeV were measured at CIAE.

The setup of the experiments on d–d neutron sources is shown in Fig. 3a including the deuterium gas target28a 238U fission chamber, the diamond detector and an EJ-309 liquid scintillator detector. The diamond detector was produced by CIVIDEC corporation. A CIVIDEC fast charge amplifier C6 was used as the pre-amplification29. A compound alpha source28 was used for energy calibration. The output charge signals were recorded using a commercial CAEN DT5730 digitizer (10 bit, 500 MHz). In addition to the diamond detector, the EJ-309 liquid scintillator and the 238U fission chamber were also used to measure the neutron energy spectra and the neutron fluences, respectively, for comparison. Liquid scintillators are commonly used for neutron energy spectrum measurement using the unfolding method26and the 238U(n, f) reaction as the international standard is generally used for fast neutron fluence determination30. The details of the EJ-309 liquid scintillator and the 238U fission chamber are presented in Refs.26 and28, respectively. Combining the relative neutron energy spectra measured from the EJ-309 liquid scintillator and the fluence of the main-energy neutron determined from the 238U fission chamber, the neutron energy spectra with absolute fluence can be obtained.

Figure 3

The setup of the experiments based on the d–d neutron source (a) and d–t neutron source (b).

The experiment on the d–t neutron source was based on the Cockcroft-Walton generator at CIAE, the setup of this experiment is shown in Fig. 3b. The deuteron beam with the energy of 300 keV was incident on a solid tritium-titanium (T-Ti) target of 1.0 mg/cm2 in thickness. The diamond detector was placed 193 cm from the T-Ti target and 80 degrees with respect to the deuterium beam direction. The energy of main neutrons was ~14.2 MeV and the fluence was routinely measured with the associated alpha particle method.

The measured pulse height spectra of the diamond detector are shown in Fig. 4a as two examples for neutron energies of 5.0 and 9.5 MeV. From the measured pulse height spectra and using the GRAVEL iterative unfolding method31, the neutron energy spectra and fluences were obtained. And then the folded back pulse height spectra can be calculated through the convolutions of the obtained neutron spectra with the response matrix. Two examples of the folded back spectra are also shown in Fig. 4a. The consistency between the folded back and the measured pulse height spectra indicates the reliability of the response matrix and the unfolding method.

Figure 4
figure 4

(aThe measured pulse height spectra and the folded back pulse height spectra in unfolding with the neutron energies of 5.0 MeV and 9.5 MeV for the d–d neutron sources. (bThe neutron energy spectra with absolute fluence measured using the diamond detector compared with those measured using the EJ-309 liquid scintillator together with the 238U fission chamber for 5.0 MeV and 9.5 MeV neutrons from the d–d neutron sources. (c) The neutron energy spectrum with absolute fluence measured with the diamond detector for the d–t neutron source.

It should be stressed that the energy spectra and fluence of neutrons are obtained simultaneously after unfolding, and two examples of which are shown in Fig. 4b for 5.0 and 9.5 MeV mono-energetic neutrons. The neutron energy spectra and fluence of 5.0 and 9.5 MeV neutrons measured with the EJ-309 liquid scintillator and the 238U fission chamber are also plotted in Fig. 4b for comparison. One can see from Fig. 4b that the main-energy neutron peaks determined with the diamond detector and the EJ-309 liquid scintillator together with the 238U fission chamber are in good agreement. In addition to the main-energy neutrons, low-energy neutrons measured with the two kinds of detectors are also in agreement. For the energy spectrum of 5.0 MeV neutrons, the low-energy neutron components are mainly from the scattering of the neutron hall. For the energy spectrum of 9.5 MeV neutrons, the low-energy components below 6.0 MeV mainly come from the 2H(d, np)2H reaction32. This is the first simultaneous measurement of neutron energy spectra and fluences based on the diamond detector.

Results of neutron fluences measured with the diamond detector, the 238U fission chamber, the EJ-309 scintillator and the associated alpha particle method are compared in Table 2. As shown in Table 2, the main-energy neutron fluences measured with the diamond detector and the 238U fission chamber for all the neutron energies are consistent. The proportions of the low-energy neutrons measured by the diamond detector agree well with those measured with the EJ-309 scintillator for neutron energies of 5.0, 5.5 and 8.5 MeV. However, for neutron energies of 9.5 and 10.5 MeV, the proportions of the low-energy neutrons measured with the diamond detector are higher than those measured with the EJ-309 liquid scintillator. This discrepancy is mainly due to the insufficient precision of the nuclear reaction data used in the simulation at higher neutron energies. Therefore, it is necessary to carry out accurate measurements for neutron-induced nuclear reactions on carbon, especially for higher neutron energies.

Table 2 The comparation of the main-energy neutron fluences at the diamond position measured using the diamond detector and fission chamber/associated alpha particle method, and the comparation of the proportions of the low-energy neutrons measured with the diamond detector and the EJ- 309 liquid scintillator.

For the measurement of the d–t neutrons, the energy spectrum with absolute fluence measured using the diamond detector is shown in Fig. 4c. One can see from Fig. 4c that the ~14.2 MeV main-energy neutrons as well as low-energy neutrons are measured simultaneously in the neutron energy spectrum. The fluence of the ~14.2 MeV neutrons measured with the diamond detector is consistent with that measured with the associated alpha particle method as shown in Table 2. The accuracy of the spectrum for low-energy neutrons is insufficient, which can be improved by using more reliable nuclear data in simulating the response matrix in the future.

The uncertainty of neutron fluences and neutron energy spectra measured with the diamond detector is not given, because the discrepancy of different evaluated nuclear data for the reactions in Table 1 is too large to analyze the uncertainty.

The above results show that the simultaneous measurement of the neutron energy spectrum and neutron fluence has been realized and the simultaneous measurement of the d–d and d–t neutrons can be realized using a diamond detector.

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