Organic single crystal scintillators enable efficient fast neutron detection

On November 2, 2022, Professor Hu Wenping’s team at Tianjin University published a research paper entitled “Dual discrimination of fast neutrons from strong γ noise using organic single-crystal scintillator” in the journal Matter.

This achievement reports a double fast neutron/gamma ray discrimination (FNGD) method based on γ-ray insensitive tetrastyrene (TPE) organic single crystal scintillator (OSCS), which greatly improves the discrimination ability of fast neutrons and provides new inspiration for the design and selection of neutron detection materials.

The corresponding author of the paper is Professor Hu Wenping; The first author is Sun Qisheng.

Fast neutron detection has important applications in radioactive material detection and many applications related to fast neutrons (such as non-destructive testing, tumor treatment, etc.). The two main challenges of current fast neutron detection are: fast neutrons are only sensitive to certain elements (such as H, Gd), and the reaction cross-section with most substances is extremely small; Strong γ rays are often produced along with fast neutrons, which affect the detection results as a background. Therefore, it is necessary to find suitable materials for effective FNGD.

Some hydrogen-rich organic scintillators can be used for fast neutron detection thanks to their high reaction cross-section with fast neutrons and their ability to achieve FNGD. OSCSs have a clear crystal structure, outstanding optical properties, and good light transmission, and are superior candidates for FNGD (such as trans-stilbene (TSB), 9,10-diphenylanthracene (9,10-DPA), etc.). The basic principle of FNGD is that organic scintillators can only produce transient fluorescence by singlet deexcitation under γ irradiation (PF, Figure 1A), while delayed fluorescence can be generated by tritary-trilet annihilation (TTA) under fast neutron irradiation (DF, Figure 1B). FNGD can be achieved by distinguishing PF and DF by pulse shape discrimination (PSD). However, its disadvantages are: it is impossible to further distinguish PF caused by fast neutrons from PF caused by γ rays; Attributing all PF signals to γ events leads to errors in the detection results.

Figure 1: Photophysical process of fluorescence by interaction of γ rays (A) and fast neutrons (B) with OSSCs and a schematic diagram of the TPE-based dual FNGD strategy (C).

Tetrastyrene (TPE) is a polyphenyl ring molecule similar to TSB, 9,10-DPA, and its single crystal may also undergo FNGD through PSD. In addition, TPEs have a good fast neutron response and a weak γ-ray response, which may cause differences in the intensity of light pulses under neutron and γ ray irradiation. It is expected that PF caused by neutrons and γ rays can be further distinguished by pulse height discrimination (PHD) (Figure 1C). The authors selected TPEs and 9,10-DPA for comparative studies.

The authors first predict the FNGD potential of TPEs from the theoretical and optical properties. Theoretically, the realization of FNGD by PSD requires molecules to have the energy level conditions for generating TTA, that is, △ST>0.8 eV and 2T1>S1. Calculations show that both TPE and 9,19-DPA meet the above conditions (Figure 2A). Since FNGD is performed by the fluorescence signal generated by the interaction of scintillators with fast neutron/γ rays, the optical properties of scintillators are also important considerations (Figures 2B-2D). In particular, the fluorescence lifetime of TPEs is only 1.6 ns, which means that in order to ensure that the response to the light pulse signal is completely acquired, the equipment used for signal acquisition needs to have considerable accuracy. In addition, X-ray excitation spectra showed that TPEs have a weak sensitivity to high-energy photons, which coincided with subsequent γ-ray irradiation experiments.

Figure 2: Energy level structure and optical properties of TPE and DPA.

The authors prepared centimeter-scale TPEs and 9,10-DPA single crystals (Figures 3A, 3B) by a level-assisted strategy and analyzed their pulsed signals under irradiation from the γ source Cs-137 and the fast neutron/γ hybrid source Cf-252. The results show that they can produce PF (short pulse) under γ ray excitation and DF (long pulse) under fast neutron excitation, which means that they have the ability to perform FNGD via PSD (Figure 3C). In addition, the pulse height spectrum (Figure 3D) shows that TPEs can only produce pulses of low intensity under γ source irradiation, meaning that FNGD can be performed by PHD.

Figure 3: Preparation of OSSCs and their pulsed signals under irradiation.

The authors presented the FNGD results using a 3D coordinate system, and the fast neutrons were clearly distinguished compared to the results irradiated by the control group Cs-137 gamma source. Further analysis showed that the FoM value of TPE could reach 2.4, thanks to the dual FNGD of PSD and PHD, in which the proportion of sub-count to total count could reach 35.5%.

Figure 4: FNGD results for OSCSs.

This work provides new ideas for the preparation of large-scale organic single crystals and the development of new neutron detection materials. This work was supported by the Ministry of Science and Technology Foundation (2017YFA0204503, 2018YFA0703200), the Natural Science Foundation of China (52121002, 51733004, 51725304, 21875158, U21A6002) and the Tianjin Natural Science Foundation (20JCJQJC00300). (Source: Web of Science)

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