CLYC scintillator is based on the nuclear reaction method to measure neutron. The main reaction channels of thermal neutrons (0.025 eV) and fast neutron (>105 eV) detected by the CLYC scintillator are ⁶Li (n, t) a reaction and ³⁵Cl (n, p) 35S reaction, respectively. The specific reaction channels are shown in Equations (1) and (2). The secondary charged particles generated by the nuclear reaction of neutron entering the scintillator lose energy in the scintillator and cause the scintillator luminescence.

n 0 1 + L 3 6 i = T + H 2 4 e + 4.782   MeV (1)

n 0 1 + C 17 35 l = S 16 35 + p + 0.614   MeV (2)

The interaction between γ-ray and CLYC scintillator mainly loses energy through the photoelectric effect, Compton effect and electron pair effect [11]. CLYC scintillator under γ-ray irradiation can induce Core-to-Valence Luminescence-CVL (Core-to-Valence Luminescence-CVL) with attenuation time of about several ns between 220 - 320 nm light waves; this mechanism does not exist between neutron and scintillator. This makes the neutron and gamma rays in the CLYC scintillator will cause different pulse shapes, so the neutron and gamma can be separated by the pulse shape discrimination (PSD) method, which is also the basis for the application of CLYC scintillator detectors.

The luminous efficiency indicates the ability of the scintillator to transform the absorbed ray energy into a light pulse [12]. The higher the photon yield of the scintillator is, the more photons converge to the photoelectric equipment, and the higher the energy resolution is, and the better the performance of the scintillator is. (Absolute) luminous efficiency is defined as the ratio of the number of photons produced in the scintillation process to the energy loss of particles in the scintillator [13]. The expression is:

Y p h = n p h / Δ E (3)

In Formula (3), Y_ph is the luminous efficiency, unit MeV⁻¹. n_ph is the total number of photons generated in the scintillation body. ∆E is the energy lost by incident rays or particles in the scintillation body, in MeV. Here, the F1 card in MCNPX program is used to record the optical integral flow on the left side of the photoconductor, namely (absolute) luminous efficiency.

The scintillator detector detects neutrons and gamma rays using the detection efficiency of ε_s source for calibration. The secondary particles generated by gamma rays or neutrons interact with crystals to produce photoelectrons, which are collected by photocathodes and output electrical signal pulses through photoelectric conversion devices. ε_s source detection efficiency, also known as absolute detection efficiency, is defined as the ratio of the number of pulses generated by the interaction of rays and scintillators to the number of incident particles [14] [15]. The expression is:

ε s = S i / N (4)

In Formula (4), ε_s is the source detection efficiency; S_i is the number of pulses or particles recorded; N is the number of particles emitted by the radioactive source. Here, the F8 card in the MCNPX program is used to record the pulse height spectrum of γ-ray in a scintillation crystal; E8 card is used to record the detection efficiency of the scintillation detector for γ-ray; the F4 card and FM4 multiplier card are used to record the detection efficiency of thermal neutron (n, t) reaction and fast neutron (n, p) reaction.

The detection efficiency and luminous efficiency of the CLYC-6 scintillator with different bottom diameters, different thicknesses and different kinds of particle energy were simulated. Figures 1-3 show the changes in detection efficiency and luminescence efficiency of the CLCY-6 scintillator under 0.661 MeV γ-ray, 0.025 MeV thermal neutron and 1 MeV fast neutron irradiation, respectively. It can be seen from Table 2 and Figure 3 that the detection efficiency gradually increases with the increase of CLYC scintillator size. When the bottom diameter of the CLYC scintillator is constant, the greater the thickness, the greater the detection efficiency. When the thickness of the CLYC scintillator is constant, the larger the bottom diameter, the greater the detection efficiency. But as the size increases, the growth trend of detection efficiency will gradually slow down. It can be seen from Table 2 and Figure 2 that the luminous efficiency decreases with the increase of the size of the CLYC scintillator. When the bottom diameter of the CLYC scintillator is constant, the luminous efficiency decreases with the increase of thickness. When the thickness of the CLYC scintillator is constant, the larger the bottom diameter, the greater the detection efficiency. In general, the influence of the CLYC scintillator geometry on the detection efficiency and luminescence efficiency of γ-rays, thermal neutrons and fast neutrons conforms to the same rule; that is, with the increase of CLYC scintillator size, the number of particles entering the scintillator to react increases and the detection efficiency will gradually increase. The photons generated by this process will deposit in the crystal with the increase of the crystal size and cannot pass to the other side of the photoconductivity, which will reduce the luminous efficiency with the increase of the size of the scintillator. It can be seen that the continuous improvement of crystal size does not bring sustained growth in detection efficiency and luminous efficiency.

Ce³⁺ ion, as the luminescent center of the CLYC scintillation crystal, plays a key role in the performance of the crystal and detector. The appropriate doping concentration can maximize the performance of the crystal. The influence of Ce³⁺ ion doping concentration on the detection efficiency and luminous efficiency of the CLYC-6 scintillator with the size of Φ2.54 cm × 17.78 cm was simulated. The incident particle is 0.661 MeV energy gamma-ray, and the Ce³⁺ ion doping concentration is 0.1% - 1.0%. The simulation results are shown in Table 3. Ce³⁺ ion doping concentration has no effect on the detection efficiency of the CLYC-6 scintillator, and the luminous efficiency reaches the maximum when the doping concentration is 0.50%.

From Tables 1-3, the detection efficiency and luminous efficiency of the CLYC detector are affected by the geometric factors of CLYC crystal, doping concentration and source distance. When the CLYC detector detects three different single source particles, the changing trend of detection efficiency and luminous efficiency is similar. The detection efficiency increases with the increase of CLYC crystal size, and the luminous efficiency decreases with the increase of CLYC crystal size. It can be seen that the CLYC detector is not the bigger, the better. The CLYC-6 detector is rich in ⁶Li, and the detection efficiency of thermal neutrons is nearly three orders of magnitude higher than that of fast neutrons. The CLYC-7 detector (containing ⁷Li up to 99.9%) can be selected for fast neutron measurement. The CLYC-6 scintillator detector has the optimal luminous efficiency and excellent luminous efficiency when the size is Φ2.54 cm × 17.78 cm and the doping concentration is 0.50%.