# Fluorescence Emission and Decay Time Studies on Doped 1, 3, 5- Triphenylbenzene Scintillator Crystal Grown by Solution Growth Technique

N Durairaj, S Kalainathan, R Kumar

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N Durairaj, S Kalainathan, R Kumar. Fluorescence Emission and Decay Time Studies on Doped 1, 3, 5- Triphenylbenzene Scintillator Crystal Grown by Solution Growth Technique. Mechanics, Materials Science & Engineering MMSE Journal. Open Access, 2017, 9, 10.2412/mmse.4.45.791. hal-01503818

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# Fluorescence Emission and Decay Time Studies on Doped 1, 3, 5-Triphenylbenzene Scintillator Crystal Grown by Solution Growth Technique

N. Durairaj¹, S. Kalainathan¹,ᵃ, R. Kumar²

1. Centre for Crystal Growth, VIT University Vellore-632014, Tamilnadu, India

2. Nuclear and Radioanalytical Chemistry, IGCAR, Kalpakkam-603102, Tamilnadu, India

a. kalainathan@yahoo.com

<figure><img src="image_2.png" /></figure>

DOI 10.2412/mmse.4.45.791 provided by [Seo4U.link](https://link.springer.com/10.2412/mmse.4.45.791)

**Keywords:** organic scintillator, fluorescence, decay time, scintillator detector.

**ABSTRACT.** Detection of fast neutrons in the presence of gamma radiation background is possible in the existence of two decay fluorescence of organic molecules. Doped organic molecules are enhancing the fluorescence emission and decay time which enables the large events of neutron-gamma detection. The present work is to investigate the organic scintillator crystal 1,3,5-triphenylbenzene (3PB) doped with 1,4-diphenyl-1,3-butadiene (DPB) and 2,5-diphenyloxzole (PPO) with different concentration of mole percentage ratios (0.01, 0.03, 0.05, & 0.07). The fluorescence emission and decay time studies were carried out which reveals that importance of growing 3PB scintillator crystal. Among the four different concentrations, 0.01mol% of DPB and 0.05mol% of PPO obtained the maximum emission intensity. Fluorescence decay time was calculated by time-correlated single photon counting (TCSPC) method which reveals the reduction of decay time compare to pure 3PB and other organic scintillators.

**Introduction.** Scintillation detection is a high sensitive technique to identify the invisible radiations. To search the materials for efficient scintillation detection for high-energy particles in radiation background which begins in the mid of the 20th century has directed to a number of single crystal scintillators [1]. The particle discrimination in complex radiation field is possible by two decay (prompt and delayed) fluorescence molecules. Some of the organic single crystals, which is existing two decay fluorescence implemented for the neutron-gamma discrimination in high energy radiation field [2]. Nowadays it is considered to make industrialize scintillation detectors to be a base for security and environment control systems, as well as for radiation medicine [3, 4]. However, the conventional crystal scintillators have a number of unavoidable drawbacks, such as high production cost, specialised equipment required for its production as well as difficulties in obtaining large area scintillators. Hence overcome this issues many researchers has found new organic scintillators and grown the larger single crystals with low-cost techniques [5-7]. 3PB is the one of organic scintillator crystal; which gives the better pulse shape discrimination [7]. The present authors also have grown the larger unidirectional 3PB single crystal by Sankaranarayanan-Ramasamy (SR) method and reported decay time and other preliminary studies [8]. For the scintillator application, a larger light output and shorter decay time would be most welcomed to cover the large events of particle discrimination [9]. But 3PB has lower light output value [7, 8]. Hence, we are motivated to improve the light output of 3PB by an addition of small bright fluorophores such as DPB and PPO with the pure material (3PB).

**Experimental Technique.** Pure and doped 3PB single crystal is grown by slow evaporation technique. According to the solubility data [8], at room temperature 22.54g of the 3PB (solute) is dissolved in 100ml tetrahydrofuran solvent. The distribution of dopants DPB and PPO is calculated as a ratio of the solute concentration in solution. The 3PB is doped by DPB and PPO of different
concentration ratios 0.01, 0.03, 0.05, and 0.07. The saturated solution is filtered and transferred into a crystal growth vessel (petri dish) by using the Whatman filter paper. The growth vessel with the saturated solution was tightly covered with aluminium foil with few fine holes, which make the controlled evaporation of the solvent. Due to continue evaporation of the solvent, the solution will attain the supersaturation. When the solution attains the supersaturation, then it initiated the nucleation. After 16 days, spontaneously nucleated good optical quality crystals harvested. Similar size of the harvested crystal (Fig.1a&b) was selected for fluorescence emission and decay time analyses. The fluorescence spectrum was recorded using Edinburgh FLS 980 spectrometer with excitation wavelength 300 nm at room temperature. The emission spectrum of pure, DPB and PPO doped 3PB crystals are shown in Fig. 2. The lifetime measurement was carried out by Time-Correlated Single Photon Counting (TCSPC) method by using the Jobin Yuvon (FL3-11) spectrofluorometer with the emission wavelength 360 nm. Table.1 reveals some scintillation properties of 3PB and DPB and PPO molecules.

<figure><img src="image_3.png" /><figcaption>Fig. 1. (a) DPB Doped 3PB crystals, (b) PPO Doped 3PB crystals grown by slow evaporation technique.</figcaption></figure>

<table><caption>Table 1. Some scintillation properties of host and guest molecules.</caption><thead><tr><td>S.No.</td><td>Properties</td><td>3PB (C<sub>24</sub>H<sub>18</sub>)<br>(Host) [8]</td><td>DPB (C<sub>16</sub>H<sub>14</sub>)<br>(Guest) [9]</td><td>PPO (C<sub>15</sub>H<sub>11</sub>NO)<br>(Guest) [9]</td></tr></thead><tbody><tr><td>1</td><td>Excitation</td><td>300 nm</td><td>320 nm</td><td>300 nm</td></tr><tr><td>2</td><td>Emission</td><td>360 nm</td><td>350 nm</td><td>416 nm</td></tr><tr><td>3</td><td>Decay time</td><td>12 ns, 30ns</td><td>-</td><td>6 ns</td></tr></tbody></table>

## Result and Discussion

**Fluorescence studies.** The result of doping DPB and PPO in 3PB single crystal is increasing the light output values or increasing the number of photons produced. Increasing light output values in a doped single crystal is compared with undoped one, which can be explained by the active transfer of electronic excitation energy from the host molecules (3PB) to the guest molecules (DPB and PPO). At the same time, the distribution of guest molecules also is important in 3PB. When increasing the concentration of guest molecules, there is a chance to suppress the light output values which is due to concentration quenching effect [10]. The emission spectrum was measured for the pure and all concentration of doped crystals (Fig.2 a&b). It is noticed that the concentration of 0.01 mol% of DPB and 0.05 mol% of PPO doped crystals shows the maximum emission intensity which is due to the exchange coupling and triplet mobility increased in doped crystals. The decrease in fluorescence intensity above these concentrations (0.01 mol% of DPB and 0.05 mol% of PPO) of doped crystals
is due to concentration quenching effect [11]. The fluorescence intensity of pure 3PB is lower than compare to doped crystals which are due to the less populated excited states in it. The fluorescence spectra are the evidence of dopant incorporation into the 3PB crystals.

<figure><img src="image_4.png" /><figcaption>Fig. 2. Fluorescence spectra of (a) DPB doped crystals (b) PPO doped crystal excited at 300nm.</figcaption></figure>

**Fluorescence decay analysis.** A distinctive feature of the organic scintillator as a detector application is to exhibit different lifetime at the same emission wavelength. The lifetime profile was calculated by time-correlated single photon counting method (TCSPC) [12]. Figure 3 (a, b) reveals the decay time spectrum of the 0.01% DPB 0.05% PPO doped crystals. The analysis of decay time measurement was fitted with the two exponential decay components:

$$F(x) = A_1 e^{-\frac{t}{\tau_1}} + A_2 e^{-\frac{t}{\tau_2}} \quad (1)$$

where $A_1$ and $A_2$ are amplitudes of prompt and delayed emissions respectively, $\tau_1$ and $\tau_2$ are lifetimes.

The calculated fluorescence lifetime of the pure, 0.01mol% of DPB and 0.05mol% of PPO doped crystal is listed in Table.1. The fast scintillation decay time in organic molecules is typically 2-30ns. From the literature, the existing organic scintillator lifetime is listed with the present work. Compare to earlier reports the present 3PB crystal doped with 0.01% of DPB and 0.05% of PPO exhibit the very short decay time. The prompt fluorescence corresponds to direct de-excitation of singlet energy level transition and delayed emission was due to collisional interaction pairs of molecules in the lowest excited triplet state. The significant of the collisional interaction of triplet energy level was used for neutron detection in an organic scintillator [2, 12].
<figure><img src="image_5.png" /><figcaption>Fig. 3. Fluorescence decay curve (a) 0.01% DPB (b) 0.05% PPO doped 3PB crystal.</figcaption></figure>

<table><caption>Table 2. Comparison of scintillation decay time with existing materials.</caption><thead><tr><td>S.No.</td><td>Compound</td><td>Prompt</td><td>Delayed</td><td>References</td></tr></thead><tbody><tr><td>1</td><td>Anthracene</td><td>30 ns</td><td>-</td><td>[6]</td></tr><tr><td>2</td><td>Stilbene</td><td>4.5 ns</td><td>-</td><td>[6]</td></tr><tr><td>3</td><td>Pure Naphthalene</td><td>14.4 ns</td><td>78.6 ns</td><td>[2]</td></tr><tr><td>4</td><td>PPO doped Naphthalene</td><td>7.5 ns</td><td>50.2 ns</td><td>[8]</td></tr><tr><td>5</td><td>P-terphenyl</td><td>3.3 ns</td><td>-</td><td>[9]</td></tr><tr><td>6</td><td>Pure 3PB</td><td>12 ns</td><td>30 ns</td><td>[5]</td></tr><tr><td>7</td><td>0.05% PPO doped 3PB</td><td>3.8 ns</td><td>17.8 ns</td><td>Present work</td></tr><tr><td>8</td><td>0.01 DPB doped 3PB</td><td>2.08 ns</td><td>11.37 ns</td><td>Present work</td></tr></tbody></table>

**Summary.** DPB and PPO doped 3PB crystal was grown by slow evaporation technique. The distribution of dopant was in 3PB analysed by different concentration. Among the four different concentrations, 0.01mol% of DPB and 0.05mol% of PPO obtained the maximum emission intensity. The fluorescence emission and decay time studies were carried out which reveals that importance of growing 3PB scintillator crystal. The fluorescence spectra are the evidence of the dopant incorporation into the 3PB crystals. Fluorescence decay time was measured by time-correlated single photon counting (TCSPC) method which reveals the reduction of decay time compare to pure 3PB and other organic scintillators. The present study exhibits the maximum emission intensity and short decay time (2.08ns) for the 0.01mol% DPB doped crystal which is better result compare to the pure and scintillator crystals. It is due to the exchange coupling and mobility of triplet in the guest and host molecules.

**Acknowledgement**

The authors 1 and 2 thank the DAE-BRNS (Project sanction no. 34/14/22/2014-BRNS0294 dated 16 May 2014) for providing financial support and the VIT University management for their constant encouragement. The authors would like to thank Prof. C.K.Jayasankar and MoU-DAE- BRNS Project (No.2009/34/36/BRNS/3174), Department of Physics, S.V. University, Tirupati, India for extending the experimental fluorescence facility.

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Cite the paper

N. Durairaj, S. Kalainathan, R. Kumar, (2017). Fluorescence Emission and Decay Time Studies on Doped 1,3,5-Triphenylbenzene Scintillator Crystal Grown by Solution Growth Technique. Mechanics, Materials Science & Engineering, Vol 9. DOI 10.2412/mmse.4.45.791