Fabrication of a Liquid Scintillator based on 7-Diethylamino-4-Methylcoumarin for Radiation Detection

Organic liquid scintillation detectors are widely used to measure the presence of radiation. With these devices, there are advantages in that they are easy to manufacture, large in size, and have a short fluorescence decay time. However, they are not suitable for gamma spectroscopy because they are composed of a low-atomic-number material. In this regard, alternative materials for the secondary solute used in basic organic liquid scintillators have been investigated, and the applicability of alternative materials, the detection characteristics, and neutron/gamma identification tests were all assessed. 7-Diethylamino-4-methylcoumarin (DMC), selected as an alternative material, is a benzopyrone derivative in the form of colorless crystals with high fluorescence, a high quantum yield in the visible region, and excellent light stability. In addition, it has a large Stokes shift, and solubility in a solvent is good. Through an analysis in this study, it was found that the absorption wavelength range of DMC coincides with the emission wavelength range of PPO, which is the primary solute used with DMC. Finally, it was confirmed that the optimal concentration of DMC was 0.08 wt%. As a result of performing gamma and neutron measurement tests using a DMC-based liquid scintillator, it was found to perform well (FOM = 1.42) compared to a commercial liquid scintillator, BC-501A.


Introduction
Gamma ray and neutron detection technology is necessary for nuclear security and non-proliferation.Gamma ray measurements are widely used as a non-destructive measurement method, for measurements at decommissioned sites of nuclear facilities, and for radioactive waste classification.With regard to gamma rays, shielding is possible with materials composed of a high-Z material.However, unlike gamma rays, neutrons have high penetrability.Neutron detection technology is potentially useful for detecting neutron-emitting materials such as nuclear materials [1].Given that neutron detection technology is used for nuclear security, fast and efficient neutron-sourcedetection technologies are required.In addition, because neutrons always accompany gamma rays, a technology for distinguishing gamma rays from neutrons is essential [2][3][4].In this study, a liquid scintillator that can be used not only non-destructive measurements and decommissioning site measurement fields but also in the nuclear security field was fabricated.
Solutes commonly used in conventional plastic and liquid detection sensors include 2,5-diphenyloxazole (PPO, primary solute) and 1,4-bis[5-phenyl-2-oxazolyl]benzene (POPOP.secondary solute) [5].In general, POPOP in an organic compound as a secondary solute has low solubility and shows a broad emission peak in the wavelength.To solve this secondary solute problem, 7-diethylamino-4-methylcoumarin (DMC) [6][7][8], which has good solubility in a solvent and a wavelength suitable for PMT, was used as an alternative to POPOP.In this study, the feasibility of DMC was evaluated.The fluorescence process in an organic material is caused by transitions between the energy levels of single molecules and is determined by the type of molecule.For example, anthracene is observed to fluoresce in a solid polycrystalline state, as a vapor, or as part of a solution consisting of a large component [9][10][11][12][13].Most organic scintillators are based on some organic molecule with a symmetrical property known as the π-electron structure.The fluorescence of an organic material is caused by an energy transfer of molecules [14].The primary solute used in this study is PPO (2,5-diphenyloxazole) and the secondary solutes are POPOP (1,4-bis [5-phenyl-2-oxazolyl] benzene) and DMC (7-Diethylamino-4-methylcoumarin).The physical-chemical properties of PPO, POPOP and DMC are shown in Table 1 [15,16].The role of PPO is to convert some of the energy transferred to the base polymer material by radiation into light, and it is important to find the content that can generate the maximum amount of light.In addition, the secondary solute absorbs light generated from the primary solute and shifts it to a wavelength that matches the photomultiplier tube (PMT) for measurement.It is necessary to select a material with good solubility in a solvent and that can generate the maximum amount of light.The greatest disadvantage of POPOP, which is used as an additive solute for organic liquid scintillators, is its solubility in solvents.DMC has good solubility, with other properties similar to those of POPOP [17,18].DMC as selected in this study is a benzopyrone derivative in the form of colorless crystals.DMC has high fluorescence and a high quantum yield in the visible light region and has excellent light stability.In addition, given the large Stokes shift, light loss caused by self-absorption is minimized.In this study, DMC was selected as an alternative to POPOP, which is generally used as a secondary solute and its applicability was evaluated.

Characterization
For the characterization of the spectroscopy of the organic materials, an absorption/transmission analysis, a Raman analysis, fluorescence spectroscopy, and a decay time analysis were conducted.The equipment used to perform each characteristic evaluation is as follows.First, the absorption and emission spectra were analyzed using a TECAN Microplate Reader (Infinite 200 PRO, Switzerland).Raman spectroscopy equipment (NanophotonKorea, RAMANtouch) was utilized to analyze the energy changes of the molecules with an excitation wavelength of 532 nm.The detector used a highly sensitive TE-cooled CCD device (1,650 pixels).The weakest point of a Raman microscope used to be the extraordinary long measurement time.However, the equipment (RAMANtouch) used in this study can scan by emitting laser beams along the most appropriate paths without any preliminary information on the samples, obtaining images at speeds five to ten times faster than a conventional scanning Raman microscope.In addition, a time-resolved fluorescence spectrophotometer (Spectrophoto-fluorometer, HORIBA, Fluorolog3) was used for fluorescence spectroscopy and for the fluorescence decay time analysis of the liquid scintillator.The fluorescence and phosphorescence can be measured in the UV-Vis-NIR region with high sensitivity for a material, and the time-correlated single-photon counting (TCSPC) method using a pulsed nanoLED and a pulsed laser diode as the light source is used to measure the decay times of the fluorescence and phosphorescence of a material.The photoluminescence lifetime of a fluorescent material is affected by both the radiative and non-radiative transition processes of excited electrons.The radiative transition is determined by the molecular structure of the phosphor, and the non-radiative transition changes sensitively due to the phosphor-fluorescence interaction, the phosphor-solvent interaction, and the energy transfer.

Fabrication of a Liquid Scintillator
Liquid scintillators were manufactured with various contents.PPO was used as the primary solute, with each scintillator having PPO added at a constant rate of 0.1 wt%.The content of the secondary solute, DMC, was changed.DMC was added at 0.02, 0.04, 0.06, 0.08, and 0.1 wt%.In addition, a comparative sample was prepared by adding 0.04 wt% of POPOP, which is a commercially used secondary solute.The quartz cell used here had a diameter of 50 mm (thickness 3 mm) and an optical path length of 50 mm.The quartz cell has a volume of 86 ml, and the contents of the primary and secondary solutes for its capacity of 86 ml were calculated [19].After each scintillation material was added to the solvent, mixing took place at room temperature for two hours or more.The PPO and DMC had good solubility in toluene, a solvent, but the POPOP was confirmed to have poor solubility in toluene.POPOP, a commonly used secondary solute, emits in the wavelength range of 370 to 450 nm.The disadvantages of POPOP include not only poor solubility in solvents but also multiple emission peaks and a wide emission range.However, in the case of DMC, which was used as an alternative to POPOP, it showed a single emission peak in the wavelength range of 400 to 450 nm, and the emission region was narrower than that of POPOP.This characteristic not only improves the sensitivity of the PMT, which converts the fluorescence signal into an electrical signal, but also improves the photoluminescence quantum efficiency.In order to use the aforementioned substances for the purposes of this study, it is necessary to optimize their contents.In order to optimize the content of DMC, a test was performed after fabricating a liquid scintillator with various concentrations.The fabricated liquid scintillator was additionally taped with Teflon and black tape, as shown in Fig. 1.Teflon acts as a reflector, and the black tape was used to block out miscellaneous light.Measurement data using a liquid scintillator were compared with the results of a commercial liquid scintillator, the BC-501A type, which has a diameter of three inches (Saint-Gobain, St. Great Lakes Parkway, Ohio, USA).

Gamma and Neutron Detection
To evaluate the measurement performance of the secondary solute, a detection system was constructed by connecting certain electronic devices including a PMT (ET-9266 KB, ET Enterprises) and MCA (DT5730, CAEN).The ET-9266 KB is a 51 mm(2 inch) diameter, end window photomultiplier with a blue-green sensitive bialkali photocathode and 10 high gain, high stability, SbCs dynodes of linear focused design for good linearity and timing.The digitizer(DT5730) is an electronic module which is responsible for sampling and digitizing the voltage signal coming from the PMT.When constructing the detection system, in order to remove the afterglow of the PMT generated during the liquid scintillator replacement process, the liquid scintillator was connected and stored in a dark room for about 6 to 12 h to secure the stability of the measurement sensor.In addition, the radiation sources used in the measurement experiment are a gamma radiation source ( 137 Cs) and a neutron/gamma radiation source ( 252 Cf), and the distance from the measurement sensor is 12 cm (Fig. 2).Data were obtained and analyzed for 600 s with the configured system.When considering the half-life of the radioactivity of the source, the radioactivity of 137 Cs was 331.4 kBq and the radioactivity of 252 Cf was 3.0944 MBq.
It has been reported that a neutron/gamma discrimination test can distinguish between neutrons and gamma rays through pulse shape discrimination when a high concentration (> 20 wt%) of a fluorescent material is added to the scintillator [20][21][22].Since radiation generates different types of signals according to energy, mass, and charge, the type of radiation can be classified using the shape of the signal.The classification of radiation by using the difference in the waveform of the signal generated by the detector is referred to as pulse shape discrimination (PSD).Pulse shape discrimination is basically used to distinguish different radiations (γ, n).Among the PSD methods, the charge comparison method is a method of classifying the type of radiation by using the ratio of the total charge amount of the measured pulse and the charge amount of the falling part.When comparing the PMT output signals of neutrons and gamma rays, the decay time of neutrons is longer than that of gamma rays, so the ratio of the amount of charge in the falling part to the total amount of charge in the neutron is higher than that of gamma rays.PSD (pulse shape discrimination) is analyzed based on a simple model of energy transfer and triplet-triplet annihilation.Excitation of the singlet state through an energy transfer causes immediate fluorescence from individual solute molecules [23,24].Fluorescence is light emitted when electrons in an excited state change to a stable 'ground state' within a short period of time.The triplet-triplet annihilation process that enables a PSD analysis requires the proximity of interacting molecules, and the excited triplet behaves as an energy trap because direct fluorescence from the triplet is not possible.In a scintillator containing a low concentration of solute, the probability of identifying a pulse for PSD is low because solute molecules cannot interact due to the relatively large intermolecular distance and low collision probability.However, in a scintillator containing a high concentration of solutes, the pulse identification probability for PSD increases above a certain concentration threshold due to the enhancement of delayed light due to the increased triplet-triplet collision probability and the increased number of interacting solute molecules.At this time, it has been reported in several studies that the optimal concentration of solute for PSD is 20-30 wt% [20][21][22].In this study, a measurement test using a neutron/gamma source ( 252 Cf) was conducted to evaluate the PSD performance, and lead was installed in front of the detector to reduce the effect of gamma.The FOM (figure of merit) value, an evaluation index of PSD, was calculated.The FOM calculation formula is expressed here as Eq. ( 1) below [4].In Eq. ( 1), r means gamma and n means neutron.When the interval between the neutron peak and the gamma peak is defined as Δ, the ratio of Δ to the sum of the half widths of the neutron peak and the gamma peak is the FOM value.The larger the FOM is, the higher the peak discrimination performance becomes. (1)

Properties of Solutes for a Liquid Scintillator
First, the absorption and emission analysis results are presented.Figure 3 shows the absorption and emission spectra of the materials used in this study.As shown in Fig. 3, the absorption and emission spectra of the secondary solute overlap well with the emission spectra of the primary solute, demonstrating that an efficient radiative transfer can be induced.Absorbance measurements are feasible for measuring the intensity at which a sample absorbs light, and the visible and ultraviolet absorption spectra of a material change depending on the chemical structure of the material.Therefore, the material can be identified by measuring the absorption intensity at various wavelengths.Figure 3 shows that the emission peak of PPO and the absorption peak of DMC almost overlap.As a result of calculating the FWHM based on Fig. 3, it was analyzed that the FWHM of POPOP was 60 nm whereas DMC was 45 nm.Because the overlap of the absorption and emission peaks of DMC is minor, it is judged that the effect on self-absorption will be small.
In addition, a liquid scintillator was manufactured using a material having such characteristics, and the luminescence characteristics were confirmed by irradiating UV after manufacturing a toluene-based liquid scintillator.The UV irradiation test is a method of directly confirming the luminescence of a sample through irradiation with 365 nm of UV.Blue light is emitted when irradiated with UV light of 365 nm, and it was confirmed that the added solutes were well dissolved in the solvent.Figure 4a represents a liquid scintillator based POPOP (0.04 wt%) as a secondary solute, and Fig. 4b represents a photograph based DMC (0.08 wt%) as a secondary solute under natural light and UV wavelength at 365 nm.
Below the results of the Raman analysis are presented.Various types of scattered light are generated from the sample irradiated with the laser.Most of the light is Rayleigh-scattered light having the same wavelength as the incident light along with Raman-scattered light having a slightly different wavelength from the incident light also being generated.The Rayleigh-scattered light is filtered, and the Raman-scattered light is detected in a CCD (charged coupled device) as a spectrum by a spectrometer.The Raman spectrum contains information about the molecules and crystals.Molecules excited by the incident photons generate energy losses.Therefore, when an excited molecule returns to a stable state, the molecule does not return to its initial state.At this time, a Raman shift equal to the energy loss is generated.Figure 5a-c represents the Raman spectrum of PPO, POPOP and DMC, respectively.Figure 5d is a spectrum of mixed PPO and POPOP.Figure 5e is a spectrum of mixed PPO and DMC.It was confirmed that PPO and POPOP have similar vibration bands and DMC can confirm peaks such as C-H that did not appear in PPO or POPOP.DMC has stretching vibration bands of C = O carbonyl groups and carboxylate groups (1,705, 1,542, 1,347 and 764 cm −1 ).In addition, an analysis showed that it has bands of lactone bonding (987 cm −1 ) and aromatic ring deformation vibration (683 cm −1 ) bands.With regard to PPO, the characteristic C = C stretching vibration bands of the phenyl group appear at 1,445, 1,478 and 1,604 cm −1 , and the 1,537 cm −1 peak can be assigned to C = N vibration.For POPOP of the oxazole group, the characteristic C = C stretching vibration bands of the phenyl group appear at 1,490 and 1,510 cm −1 , and the 1,610 cm −1 peak can be assigned to C = N vibration [25][26][27].
As shown in Fig. 6, PPO emits light at a wavelength of about 380 nm and DMC emits light at a wavelength of 420 nm.For the sample with a mix of PPO and POPOP, it was found that a shoulder of the peak appeared not only at 440 nm but also around 470 nm.For the sample with a mix of PPO and DMC, a peak shoulder was found at about 440 nm.It was also found that the resolution of the peak is better for DMC than for POPOP, and through this, DMC is judged to have a wavelength suitable for PMT.In addition, the decay time of the fluorescence extinction curve is calculated based on the raw data through the Eq. ( 2).The Eq. ( 2) is a formula for multiexponential fitting of the spectrum over time, where A n is the amplitude of component nth and τ n is the lifetime of component n th .
Figure 7 shows the results of each analysis at a 316 nm excitation wavelength.As shown in Fig. 7, the decay time of DMC is found to be very short at 0.948 ns.This is shorter than that of POPOP, which shows a decay time of about 1.788 ns. (2)

Gamma Detection
To optimize the efficiency of gamma detection, the gamma detection was evaluated using 137 Cs as a gamma radiation source in concentrations manners of DMC.Here, the energy spectrum is expressed as counts per channel or counts per energy.The measured energy spectrum can be expressed in two ways.One is an energy spectrum shown as a general count per channel, and the other is an energyweighted spectrum.As a result, the effect of Compton scattering was observed.Thus, many studies have been conducted to emphasize Compton edge peaks by applying Energy Weighting Algorithm (EWA).When EWA is applied, the low-energy noise signal is reduced, and the peak of the Compton edge would be emphasized.And it is possible to distinguish artificial radionuclides from natural radionuclides through the emphasized Compton Edge.From the Eq. ( 3), EWA is calculated by fitting the energy (keV) corresponding to each channel in the energy spectrum, and then is multiplied by the counts for each channel.C i is the counts value of i-channel, and E i is the energy (keV) corresponding to i-channel [28][29][30].Table 2 represents compared efficiencies to the concentration manners of DMC based on Fig. 8a-c.The relative efficiency was calculated by Eq. ( 4).Weighted energy spectrum was applied by designating the area corresponding to 80% of the maximum peak in the normalized spectrum in Fig. 8c as the Compton edge range in the total counts [29].
As shown in Fig. 8a-c, a photoelectric peak was not observed while the Compton edge was observed.The Compton edge of 137 Cs was at 477 keV.The measurement was conducted after placing the source at a distance of 120 mm from the liquid scintillator.Considering the halflife, the radioactivity of 137 Cs is 331.4 kBq.Because the liquid scintillator was dominated by Compton scattering, energy correction was performed using the Compton edge and the measurement test was conducted for 600 s.The efficiency and the relative light yield were calculated using the channel of the Compton edge compared to the previously used secondary solute, POPOP.Based on the efficiency and relative light yield values, 0.08 wt% was found to be the most optimal DMC content.When analyzing the results of the 137 Cs source, all samples except without DMC showed better efficiency than POPOP.The optimal content of DMC was 0.08 wt%.The efficiency was higher than that of the commercial liquid scintillator within ranges of 116 ~ 160%.
As shown in Fig. 8a-c and Table2, the detection characteristics of 137 Cs for was analyzed.Figure 8a-c show the individual measured energy spectrum and energy weighted spectra for 137 Cs, where this source has specific theoretical Compton edges 477 keV.A liquid scintillator is composed of a low-atomic-number material, meaning that a  .08wt% was found to be the most optimal DMC content.When analyzing the results of the 137 Cs source, all samples except for the sample without DMC showed better efficiency than POPOP.Therefore, considering the test results, the optimal content of DMC is 0.08 wt%.The efficiency was higher than that of the commercial liquid scintillator, and it was 116 ~ 160% of that of the commercial scintillator.Afterwards, a neutron/gamma identification experiment (PSD) was conducted using a sample containing 0.08 wt%, the optimal concentration of DMC. Figure 9 shows relative and detection efficiency of Compton region for concentration manner of DMC.Due to the significantly increased efficiency at optimized 0.08wt% DMC, it had much clearer peak distributions.This results showed that optimized condition could facilitate the clear determination of the Compton edge and positive identification under conditions with insufficient counts due to the low activity of 137 Cs source.

Pulse Shape Discrimination
Not only gamma radionuclide detection experiments but also pulse shape discrimination (PSD) were conducted to discriminate neutrons and gamma radionuclides using a neutron source.PSD is a method for identifying and measuring particles using the ratio of two components.In this experiment, the spectrum of the neutron/gamma source ( 252 Cf) was measured using the prepared liquid scintillator.Two samples were prepared for the PSD test sample, and the performance was compared with that of a commercial liquid scintillator (BC-501A).A 30 wt% of PPO as a primary solute was consistently added to make a liquid scintillator, and 0.08 wt% of DMC and POPOP as secondary solutes were added to make a liquid scintillator.Here, 0.08 wt% is an optimized content of the secondary solute.A PSD performance was confirmed by calculating the FOM value, which is the identification index of the neutron and gamma-ray peaks.Here, FOM refers to the value obtained by dividing the events of gamma rays and neutrons by the sum of the full width at half maximum (FWHM) in the neutron gamma spectrum.The radioactivity of 252 Cf used in this experiment is 3.0944 MBq when considering the half-life.The charge integration technique for PSD applies two time gates to each pulse and sets the ratio between them.First, the charge pulse of the commercial liquid scintillator (BC-501A) is integrated by the gate width (Short gate (22 ns) and long gate (124 ns)).The charge pulses of POPOP and DMC-based liquid scintillators are integrated by gate widths (Short gate (38 ns) and long gate (250 ns)).In addition, the FOM value was calculated by applying a threshold of 800 ± 55 keVee to the three scintillators.
The PSD experimental results are shown in Fig. 10.When compared with the results of commercial liquid scintillators, the results of liquid scintillators with DMC and POPOP added showed good results.The FOM values of the commercial liquid scintillator, the POPOP sample, and the DMC sample were 1.29, 1.49, and 1.43, respectively.Liquid scintillators including POPOP and DMC are 2-inch in size, commercial liquid scintillator is 3 inches in size.Nevertheless, PSD performance was analyzed to be superior.Therefore, the liquid scintillator developed in this study can replace the existing commercial liquid scintillator (BC-501A).

Conclusion
A secondary solute generally functions as a wavelength shifter.The existing secondary solute, POPOP, receives the energy of the primary solute, PPO, and emits it at 400 ~ 450 nm, which is a suitable wavelength range for PMT, which amplifies and processes signals.In order to receive the energy of the primary solute efficiently, the emission wavelength of the primary solute must match the absorption wavelength of the secondary solute.Therefore, in this study, the possibility of using DMC as a secondary solute was evaluated to compensate for the disadvantages of the existing secondary solute, POPOP.The absorption wavelength range of DMC coincides with the emission wavelength with a primary solute such as PPO and exhibits a narrower emission peak than POPOP.In addition, it was confirmed that DMC has excellent solubility in toluene, a solvent and that the emission wavelength of DMC is 400 ~ 450 nm.The greatest advantage of DMC is that the Stokes shift is large, meaning that the effect on self-absorption is less than that of POPOP.Based on these characteristics, it was confirmed that DMC could be expanded as a secondary solute due to solubility in water, aqueous acid solutions, resins, varnishes, vinyls, ether, acetone, ordinary organic solvents, and alcohols.In the concentrations of manners of DMC, gamma radiation sources ( 137 Cs) were used, and it was eventually confirmed that 0.08 wt% was the most optimized amount.In addition, neutron/ gamma discrimination (PSD) showed a difference in FoM of less than 4% compared to POPOP-based liquid scintillator, which means DMC has sufficient potential as a secondary solute.Finally, DMC was demonstrated to be feasible for use instead of POPOP.More importantly, DMC as alternative secondary solute, it will be possible to manufacture and use simple and meaningful liquid scintillator for radiation detection.

Fig. 1
Fig. 1 Fabrication of liquid scintillator based DMC Fig. 2 The configuration of neutron/gamma ray detection system used in this experiment

Fig. 3
Fig. 3 Absorption and emission spectra of a PPO b POPOP and c DMC(Toluene was used as the solvent)

Fig. 5
Fig. 5 Raman spectrum analysis.a PPO b POPOP c DMC d Mixing sample of PPO and POPOP e Mixing sample of PPO and DMC

Fig. 8 137
Fig. 8 137 Cs measurement based on concentration manners of DMC a Energy spectrum for 137 Cs b Spectrum applied with energy weighted algorithm c Normalization of spectrum applied with energy weighted algorithm

Fig. 9
Fig. 9 Relative and Detection efficiency (%) of Compton region for concentration manner of DMC. and stand for relative and detection efficiency of 0.04 wt% POPOP used as a reference, respectively

Table 1
Properties and chemical structures of POPOP and DMC

Table 2
Comparison of results according to DMC content change