Measurements of neutron-induced light-charged particle emission reactions

In the past two decades cooperating with Frank Laboratory of Neutron Physics (FLNP), Joint Institute for Nuclear Research (JINR) measurements of (n, α) reaction cross sections for 6Li, 10B, 25Mg, 39K, 40Ca, 54,56,57Fe, 58Ni, 63Cu, 64,67Zn, 95Mo, 143Nd and 147,149Sm nuclei were performed in the MeV neutron energy region based on the 4.5 MV Van de Graaff accelerator at Peking University. In recent years, our measurements were extended in three aspects. Firstly, measurements were expanded from two-body reactions to three-body reactions such as 10B(n, t2α). Secondly, the neutron energy region was extended from below 8 MeV to 8 11 MeV by using the HI-13 tandem accelerator of China Institute of Atomic Energy (CIAE), with which cross sections of 54,56Fe(n, α)53,51Cr reactions were measured. Thirdly, based on the newly-built China Spallation Neutron Source (CSNS) Back-n WNS (White Neutron Source), differential and angle-integrated cross sections for 6Li(n, t) and 10B(n, α) reactions were measured in the neutron energy region from 1 eV to 3 MeV.


MV Van de Graaff accelerator at Peking University
Cooperating with Frank Laboratory of Neutron Physics (FLNP), Joint Institute for Nuclear Research (JINR), and based on the 4.5 MV Van de Graaff accelerator at Peking University, we have measured a series of (n, α) reactions, including light, medium-mass and heavy nuclei. Twin gridded ionization chambers (GICs) were used as detectors for charged particles with a large solid angle (nearly 4π) and high efficiency (˜100%). NIM block electronics and home-made data acquisition system were employed. The neutron energy range was from 1.0 to 7.7 MeV.
Combining measurements with theoretical analysis, systematic results including angular differential cross sections and/or cross sections were obtained. Details of the reaction channels, the energy points and related publications are listed in Table 1.

Recent developments 3.1 Measurement of the three-body reaction
Based on the 4.5 MV Van de Graaff accelerator at Peking University, measurement of cross sections of the three-body reaction 10 B(n, t2α) was performed [7,8]. A thin-backing 10 B sample was prepared, and the 10 B atom number in the sample was measured relative to the 6 Li atom number in the standard 6 LiF sample using thermalneutron-induced reactions of 10 B and 6 Li. Digital data acquisition system based on wave form digitizers and LAB-VIEW software were developed. Forward as well as backward signals of grids and anodes of the twin GIC were recorded. Methods of anode-grid and forward-backward coincidences were used to select the valid events. Cross sections of the three-body reaction 10 B(n, t2α), as well as the two-body reaction 10 B(n, α) 7 Li were measured simultaneously at 4.0, 4.5 and 5.0 MeV [7,8].

Measurements based on the HI-13 tandem accelerator at CIAE
In addition to the measurements of the 56,54 Fe(n, α) 53,51 Cr cross sections at the Van de Graaff accelerator at Peking University, neutron energy range was extended to 8 -11 MeV based on the HI-13 tandem accelerator of China Institute of Atomic Energy (CIAE). Using deuterium gas target and d-d reaction as the neutron source, cross sections of the 56,54 Fe(n, α) 53,51 Cr reactions were measured at 8.5, 9.5 and 10.5 MeV [15] in which region the measurement results are scarce with big discrepancies among different experiments and evaluations. Since the neutron source was not mono-energetic in this region, the neutron energy spectra were measured using an EJ309 detector by unfolding the measured pulse height spectra [26]. The interference of the low energy neutrons was corrected. A shoulder structure was observed near 10 MeV in both excitation functions of the 56,54 Fe(n, α) 53,51 Cr reactions [15] and was analyzed.

Measurements based on CSNS Back-n WNS
China Spallation Neutron Source (CSNS) is the first pulsed spallation neutron source in China and it was put to service in March, 2018. An associated white neutron source (WNS) exploiting the back-streaming neutrons (Back-n) was built for nuclear data measurement [27]. 6 Li(n, t) 4 He and 10 B(n, α) 7 Li reactions were selected as the first and the second (n, lcp) reactions to be measured. Measurements were performed at the Endstation 1 (ES#1) along the Back-n beam line. The proton beam power during measurements was ∼ 20 kW. The repetition frequency of the proton beam was 25 Hz (double-bunch mode).
Enriched double 6 LiF and double 10 B samples 50 mm in diameter were prepared. The detector system used for the measurement was the Light charged Particle Detector Array (LPDA), which mainly consisted of 15 silicon detectors (2.0 × 2.5 cm 2 , with detection angles ranging from 19.2 • to 160.8 • ) in a vacuum chamber. Based on the CSNS Back-n WNS , and the LPDA system, the 6 Li(n, t) 4 He and 10 B(n, α) 7 Li reactions were measured. The beam durations for measurement of the 6 Li(n, t) 4 He and 10 B(n, α) 7 Li reactions were 196 h and 357 h, respectively.
Measurement data are under analysis and preliminary results are obtained. For the 6 Li(n, t) 4 He reaction, differential cross sections at 80 neutron energies were measured from 1 eV to 3 MeV (selected preliminary results are presented in Fig.1). For the 10 B(n, α) 7 Li reaction, differential cross sections at 67 neutron energies were measured between 1 eV and 2.0 MeV (selected preliminary results are presented in Fig.3). For the 10 B(n, α 0 ) 7 Li and 10 B(n, α 1 ) 7 Li reactions, differential cross sections were measured at 59 neutron energies between 1 eV and 1 MeV (selected preliminary results are presented in Figs.5 and 7, respectively). Moreover, angle-integrated cross sections for the reactions above are also obtained through integration. Preliminary results of the present cross sections for the 6 Li(n, t) 4 He, 10

Conclusion and outlook
For more than 20 years, we have been engaged in the measurements of the (n, lcp) reactions based on the 4.5 MV Van de Graaff accelerator at Peking University. In recent years, we extended our measurements based on the HI-13 tandem accelerator of CIAE and on CSNS. A series of measurements have been performed and systematical results obtained. Further measurements are planned, such as the 58,60,61 Ni(n, α) reactions and the n-p and n-d scattering reactions.