On the scintillation efficiency of carborane-loaded liquid scintillators for thermal neutron detection

Nuclear Instruments and Methods in Physics Research A 769 (2015) 112–122

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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

On the scintillation efficiency of carborane-loaded liquid scintillators for thermal neutron detection Zheng Chang a,n, Nkemakonam C. Okoye a, Matthew J. Urffer b, Alexander D. Green b, Kyle E. Childs a, Laurence F. Miller b a b

The Applied Radiation Sciences Laboratory, South Carolina State University, Orangeburg, SC 29117, USA Department of Nuclear Engineering, University of Tennessee, Knoxville, TN 37996, USA

art ic l e i nf o

a b s t r a c t

Article history: Received 23 July 2013 Received in revised form 15 September 2014 Accepted 25 September 2014 Available online 6 October 2014

The scintillation efficiency in response to thermal neutrons was studied by loading different concentrations of carborane (0–8.5 wt%) and naphthalene (0 and 100 g/L) in five liquid organic scintillators. The sample was characterized in Pb and Cd shields under the irradiation of the thermal neutrons from a 252Cf source. A method was developed to extract the net neutron response from the pulse-height spectra. It was found that the order of scintillation efficiencies for both γ-rays and thermal neutrons is as follows: diisopropylnaphthalene4toluene (concentrated solutes) 4toluene
pseudocumene
m-xylene. The quench constants, obtained by fitting the Stern–Volmer model to the plots of light output versus carborane concentration, are in the range of 0.35–1.4 M  1 for all the scintillators. The Birks factors, estimated using the specific energy loss profiles of the incident particles, are in the range of 9.3–14 mg cm  2 MeV  1 for all the samples. The light outputs are in the range of 63–86 keV electron equivalents (keVee) in response to thermal neutrons. Loading naphthalene generally promotes the scintillation efficiency of the scintillator with a benzene derivative solvent. Among all the scintillators tested, the diisopropylnaphthalene-based scintillator shows the highest scintillation efficiency, lowest Birks factor, and smallest quench constants. These properties are primarily attributed to the double fused benzene-ring structure of the solvent, which is more efficient to populate to the excited singlet state under ionizing radiation and to transfer the excitation energy to the fluorescent solutes. Published by Elsevier B.V.

Keywords: Neutron detection Liquid organic scintillator Carborane Scintillation efficiency Birks factor Quench constant

1. Introduction Neutron detection technology is critical for a number of growing fields including radiological imaging, neutron therapy, neutron radiography, nuclear forensics, and nuclear security applications. Recently, the development of new thermal neutron detectors has intensified due in large part to the worldwide shortage of 3He, which is the essential material for portable neutron monitors widely used to date [1]. The capability of a nuclear detector is principally defined by its target material, which converts the kinetic energy of incident particles into electronic or photonic signals for detection. Among various target materials, organic scintillators are advantageous in low price, fast pulse-decay time, good plasticity, high detection efficiency, and inherent radiation resistance [2-4]. For detection of thermal neutrons, a nuclide with high neutron cross-section is often loaded in the organic scintillator to take advantage of the energetic particles produced by the neutron capture. Table 1 lists common nuclides with large cross-sections

n

Corresponding author. E-mail address: [email protected] (Z. Chang).

http://dx.doi.org/10.1016/j.nima.2014.09.066 0168-9002/Published by Elsevier B.V.

and corresponding nuclear reactions. In the table, 155Gd and 157Gd have the largest cross-section and Q-values. They were tested in organic scintillators for thermal neutron detection [5,6]. Due to the fact that Gd neutron captures produce a group of low energy γ-rays, x-rays, and electrons, a large part of energy from the nuclear reaction may escape the scintillator and the scintillation response resembles the feature of background radiation. These can negatively affect the detection sensitivity of Gd-loaded organic scintillators. Both 6Li and 10B are better neutron-capture nuclides for organic scintillators because their neutron captures produce energetic 3H and α-particles. The Q-value of 10B neutron capture (2.31 MeV, 94%) is lower than that of 6Li (4.78 MeV), but its cross section (3840 Barn) is four times larger than that of 6Li (940 barn). Since the natural abundance of 10B (19.8%) is about three times higher than that of 6Li (7.40%), it is unnecessary to use isotope enriched agents in the study of B-loaded organic scintillators for thermal neutron detection. Li compounds are commonly ionic and insoluble in organic solvents, suggesting that molecules with higher polarity (e.g. carboxylates) have to be used to keep the Li-loaded organic solution transparent and chemically stable [7,8]. Polar molecules often lead to quenching and decrease in scintillation efficiency.

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Table 1 Common nuclides loaded in the organic scintillators for thermal neutron detection [28,56]. Nuclide

Natural abundance (%)

Neutron capture

Q-value Kinetic energy (particle) (MeV) (MeV)

3

He Li 10 B

– 7.40 19.8

3

0.764 4.78 2.79

Gd

14.8 15.7

6

Heþ 1n -3Hþ 1H Liþ 1n -3H þ α 10 Bþ 1n -7Li þ α 10 1 B n-7Li þα þ γ (480 keV) 155 Gdþ 1n-156Gdþ γ'sþe  'sþX's 157 Gdþ 1n -158Gd þγ'sþ e  'sþ X's 6

(6%) (94%)

8.54 7.94

0.573 (1H) 2.744 (3H) 1.775 (α) 1.470 (α) γ-rays, x-rays, and electrons at tens to hundreds keV

σ for thermal neutrons (barn) 5330 940 3840 60,800 253,929

efficiency were studied based on light output, quench constant, and Birks factor. Liquid scintillators were selected in this study because their chemical compositions can be readily adjusted. A thorough understanding on the liquid scintillators is important for the study of carborane-loaded polymeric solid scintillators, which are more suited for various types of neutron detectors. Fig. 1. Structures and thermal rearrangements of the icosahedral carborane isomers [57].

It is noted that a group of B compounds, icosahedral carboranes (B10H10C2H2), are especially valuable for organic scintillators to detect thermal neutrons. These molecules are notable members of closo polyhedral carborane family [9], with high B content (74.9 wt%) and no apparent quench groups (e.g. –COOH, –Cl, etc.). Icosahedral carboranes have three isomers with two C atoms being placed at ortho-, meta- and para- positions of the twelve vertices in the cagelike structure (Fig. 1). These molecules have an effective van der Waals radius of 4 Å [10]. The ortho-structure is the most chemically active isomer. Because of the cage-like structure, these molecules are “pseudo-aromatic” in nature and outstanding building blocks for the synthesis of geometric structures and molecular machines [11,12]. The delocalized 3-dimensional bonding in the icosahedral geometry provides strong thermal, kinetic, and photochemical stability. A family of polymers with remarkable heat-resistance was synthesized with carboranes, taking advantage of their thermal stability [13]. Biomedical investigations made use of the hydrophobicities to link carborane motifs to biomolecules for 10B neutron-capture therapy [14]. Moreover, the carboranes are colorless, hydrophobic, and highly soluble in organic solvents. All these suggest that they are effective 10 B carrier for the organic scintillators to detect thermal neutrons. The investigation on carborane-loaded organic scintillators for neutron detection was scarcely reported, though some commercial B-loaded organic scintillators are believed made with carborane. Bell et al., [15] investigated the synthesis of liquid and solid rubber copolymers with phenyl derivatives by dissolving carborane in siloxane oligomer solutions. The B-loading in their samples ranges from 5 to 22 wt%. Carturan et al. [16] reported the cross-linked polysiloxane polymers loaded with 3 wt% carborane. Both studies obtained well-defined peaks in response to thermal neutrons, demonstrating the potential of carborane for organic scintillators. It is noticed that siloxane unit, –Si–O–Si–, is not the best structure for high scintillation efficiency [17]. More investigation is necessary to fully understand the role of carborane in various organic scintillators for thermal neutron detection. In this report, ten sample groups were prepared using five different liquid scintillators. Each group includes a number of samples with the carborane concentration ranging from 0 to 8.3 wt% (0–0.56 M). The pulse-height spectra of the samples in response to γ-rays and thermal neutrons were measured. The effects of scintillator and carborane concentration on scintillation

1.1. Quenching of scintillation The scintillation and fluorescence processes have been widely studied [18–22]. For an organic liquid system composed of one aromatic solvent and two fluorescent solutes, the scintillation process can be outlined in four steps: i) The incident particle dissipates kinetic energy (E) in ionization and excitation of molecules along the track. Most of the ionized and excited molecules are solvent molecules, which make up the majority of the solution. About 90% of the excited molecules, ions, and free radicals will recombine, de-excite, and dissipate their energy thermally. They do not contribute to the scintillation emission [19,20]. ii) About 10% of the excited and ionized solvent molecules are populated to excited π-electronic singlet states. Depending on the nature of the solvent, a fraction of these undergo rapid internal conversion into the lowest excited singlet state (S1) [19,20]. iii) The excitation energy from the S1 state is transferred to the primary solute, and then, to the secondary solute. It is believed that Föster Resonance Energy Transfer (FRET) is the dominant mechanism to transfer the excitation energy between the fluorescent molecules in an organic scintillator [21]. As it is driven by the dipole-dipole coupling, FRET is radiationless and occurs over greater than interatomic distances (10–100 Å) between the donor and acceptor molecules [21-23]; iv) The de-excitation of the secondary solute leads to emission of photons in an appropriate wavelength range, which causes a scintillation pulse (L) at the anode of the photomultiplier (PMT) for detection [20]. The scintillation efficiency, defined as the fraction of the incident particle energy converted to the scintillation response (L/E), is the basic property of a scintillator and is affected by various factors in the steps discussed above. Birks [19] developed a semi-empirical equation to describe the quenching of scintillation related to step i). This quenching is due to the interaction of ionized and excited molecules along the track of incident particle and can be expressed as: dL SðdE=dxÞ ¼ dx 1 þ kBðdE=dxÞ

ð1Þ

where, L is light output. dL/dx refers to specific fluorescence along the track of incident particle. S is the absolute scintillation efficiency, which is normally considered independent of the type

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and energy of the incident particle [19,24-28]. dE/dx is the specific energy loss of the incident particle. B  dE/dx refers to the density of ionization and excitation. k is the quenching parameter due to the interaction of high density ionization and excitation. kB is often regarded as a single parameter and referred as Birks factor [29]. Based on Bethe formula [30], the dE/dx value of a heavy charged particle is often hundreds and thousands of times larger than that of a fast electron at the same energy. Thus, Eq. (1) leads to two approximations under the conditions: a) when kB  dE/dx ⪡1 (for fast electrons) and b) when kB  dE/dx ⪢1 (for heavy charged particles): dL dL S ¼ S; ¼ dE dE kBðdE=dxÞ

ð2Þ

Many studies show that for fast electrons the scintillation efficiency is independent of the kinetic energy (E) when E is in the range of 125 keV–MeV [28]. If the electron energy is below this range, dE/dx increases quickly and the scintillation efficiency is affected by kB (Eq. (1)). For heavy particles, kB is always significant to the scintillation efficiency, which is remarkably lower than that for fast electrons, as predicted by Eq. (2). Quenching can also occur in step iii). The solvent–solute and solute–solute Förster resonance energy transfer (FRET) can be affected by various species and functional groups, known as chemical quenchers [31]. According to the FRET theory [21], the success of FRET is primarily decided by the overlapping of donor's fluorescence spectrum with the acceptor's absorption spectrum. The efficiency of FRET, decreasing with the sixth power of donor–acceptor distance, is affected by factors such as donor's quantum yield, acceptor concentration, medium refractive index, and dipole angular orientation of donor and acceptor molecules [22,23]. Furthermore, the emission of fluorescence light in step iv) may be absorbed by colorful species, which are so-called color quenchers [31,32]. Thus, the absolute scintillation efficiency (S) in Eq. (1) is mainly decided by the property of solvent and the quenching processes in steps iii) and iv) that occurs after the solvent molecules populate to the lowest excited singlet states (S1). The quenching in steps iii) and iv) is generally dependent on the concentration of quencher. Regardless of the mechanism, the classic Stern–Volmer model is often used as the first-order approximation to describe the quench effect when the concentration of quencher is small [33–35]. The Stern–Volmer model is expressed as: L0 ¼ 1 þ kq c L

ð3Þ

where, L0 and L are the light outputs without quencher and with the quencher at concentration c, respectively. kq is referred as quench constant. The larger the kq value, the larger the quench effect. Thus, correct selection of the solvent, solutes, and compound to carry the neutron-capture nuclide, is critical for achieving high scintillation efficiency in the detection of thermal neutrons.

2. Materials and experiments 2.1. Materials Ortho-carborane was obtained from Strem Chemicals, Inc. Toluene (99.85%, 0.8667 g cm  3), m-xylene (99þ%, 0.8661 g cm  3), and pseudocumene (1,2,4-trimethylbezene, 98%, 0.8761 g cm  3) were purchased from Thermo Fisher Scientific, Inc. Naphthalene, PPO (2,5-Diphenyloxazole), and POPOP (1,4-Bis(5-phenyl-2-oxazolyl)benzene) were obtained from Sigma-Aldrich Co. Ultima Gold F scintillation cocktail (UGF, 0.9547 g cm  3) was purchased from Perkin-Elmer, Inc. From the elemental composition [36] and relevant products [37], it is deduced that the UGF solution is made of diisopropylnaphthalene witho2.5 wt% of PPO and bis-MSB (1,4-Bis (2-methystyryl)benzene). BC-501A is a reference liquid scintillator obtained from Saint-Gobain Crystals. It has a light output 78% of anthracene and a pulse-decay time of 3.2 ns according to the manufacture [3]. GS20 (Li glass, 18 wt% Li2O), used as a reference for the neutron density in the experiment, was purchased from Applied Scintillation Technologies [38].

2.2. Sample preparation Ten sample groups were prepared in the experiment. The detailed chemical compositions of all the samples are listed in Table 2. The first five groups, UF, TC, TL, PC, and XL groups, were prepared in diisopropylnaphthalene, toluene (concentrated solutes), toluene, pseudocumene, and m-xylene based scinitllators, respectively. The next five groups, NUF, NTC, NTL, NPC, and NXL groups, have the same primary solvents as UF, TC, TL, PC, and XL groups, respectively, but contain 100 g/L naphthalene as the secondary solvent. Each sample in TL, PC, XL, NTL, NPC, and NXL groups contains 4 g/L PPO and 50 mg/L POPOP as the solutes. The samples in TC and NTC groups contain higher solute concentrations (100 g/L PPO and 1.25 g/L POPOP). The samples in UF and NUF groups containo2.5 wt% of PPO and bis-MSB. In each of these groups, carborane was added from 0 to 8.5 wt% (0 to 6.3 wt% B). The secondary solute, POPOP, is often slow to dissolve, though the concentration required is as low as 50 mg/L. As a preliminary step, a POPOP-toluene solution with a concentration 100 times higher than that required for the samples was prepared by stirring the mixture for a long time. Two drops of this clear concentrated solution (0.01 mL) were added in the sample of 10-mL for a quick addition of POPOP. The other chemicals were added using a balance with the precision of 70.01 g. Thus, the uncertainty related to sample composition is o5%. Right after preparation, a 6-mL glass vial of 1.5 cm O.D.  4.0 cm H. was filled with the sample to the brim and sealed with a PTFE-faced screw cap so that almost no air bubbles were left in the vial. Big air bubbles in the sample affect the scintillation measurement because they scatter light and decrease the apparent

Table 2 Chemical compositions of the liquid samples in each group. Group

Primary solvent

Secondary solvent

Primary solute

Secondary solute

Content of carborane

Sample number

UF TC TL PC XL NUF NTC NTL NPC NXL

diisopropylnaphthalene Toluene Toluene 1,2,4-trimethylbenzene m-xylene diisopropylnaphthalene Toluene Toluene 1,2,4-trimethylbenzene m-xylene

– – – – – Naphthalene Naphthalene Naphthalene Naphthalene Naphthalene

PPO PPO PPO PPO PPO PPO PPO PPO PPO PPO

BisMSB (50 mg/L) POPOP (1.25 g/L) POPOP (50 mg/L) POPOP (50 mg/L) POPOP (50 mg/L) BisMSB (50 mg/L) POPOP (1.25 g/L) POPOP (50 mg/L) POPOP (50 mg/L) POPOP (50 mg/L)

0–8.5 wt% 0–8.5 wt% 0–8.5 wt% 0–8.5 wt% 0–8 wt% 2.7, 6.0 wt% 1.2, 6.0 wt% 2.7, 6.0 wt% 2.7, 6.0 wt% 2.7, 6.0 wt%

5 6 5 5 5 2 2 2 2 2

(100 g/L) (100 g/L) (100 g/L) (100 g/L) (100 g/L)

(4 g/L) (100 g/L) (4 g/L) (4 g/L) (4 g/L) (4 g/L) (100 g/L) (4 g/L) (4 g/L) (4 g/L)

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scintillation efficiency. The sealed sample vials were stored in a dark place for the subsequent measurement. It is noted that all the chemicals in this report were used as obtained from the vendors. Each sample was measured 2–3 times over a period of two years. The scintillation response of each sample was the same within the experimental error (5–10%). Except for the standard BC-501A, the liquid samples were not purged with inert gas before the measurement. This approach avoids uncertainties such as introducing bubbles or varying concentrations when measuring a large number of 6-mL samples 2–3 times over a long period of time. According to the authors' experience and literature [39], the impurities and dissolved O2 in liquid organic scintillators generally cause a 5–10% decrease in the light output. Although it leads to a decrease in the light output, the present approach was chosen to eliminate uncontrollable factors that have larger influence on the relative comparisons of the data measured under the same condition.

2.3. Equipment The setup for pulse-height spectrometric analysis consists of a PMT (Philips XP2202B) mounted on a Canberra 2007P base, an ORTEC 556 HV power supply, an ORTEC 572A amplifier, an ORTEC 926 analog to digital converter and memory multichannel buffer, and a PC computer installed with Maestro-32 emulation software. The photocathode of the PMT is made of bialkali A that has the maximum sensitivity at 410 nm. This wavelength matches the maximum fluorescence peaks of POPOP (415 nm) and bis-MSB (420 nm), which were used as the secondary solutes of the samples (Table 2). The PMT has 10 dynode stages, which can achieve a gain of 1 million at 1200 V. The preamplifier base (2007P) has a bias-voltage input ranging from 0 to þ2000 V DC. It was coupled to the amplifier in the bipolar mode. For all measurements, the electronic settings were kept constant: high voltage is 1200 V and gain is 100. The shaping time was set at 2 μs for better energy resolution because the count rate was normally low. The lower and upper level discriminators were set at 112 and 8000 channels, respectively. The count time was set at 300 s for most of the measurements. To couple the sample with the PMT for scintillation measurement, a sample holder was fabricated and used to hold the 6-mL sample vial firmly against the PMT window, as shown in Fig. 2 (a). The sample holder was made of a transparent acrylic cylinder with the same diameter as that of the PMT window. At the end affixed to the PMT, a well chamber vertical to the cylindrical axis was bored into the holder. The sample holder and PMT were tightly wrapped together with Teflon tape and black insolation tape. The sample vial, coated with a thin layer of optical silicone grease, was snugly inserted in the well chamber so that its side was firmly held against the PMT window. A cylindrical black cover was used to block the ambient light during the measurement. For the γ-response measurement, a sealed disk source (137Cs, 109 Cd, 57Co, or 60Co) was attached to the sample holder. For the neutron-response measurement, a neutron irradiation setup was used. The 252Cf source was purchased from Frontier Technology Corporation located in Xenia, Ohio. This source, with the strength of 1.36  106 neutron/s in July 2009, is enclosed in a 1/8″-thick stainless-steel cup and a 1½″-thick Pb pig. Documentation associated with the source specifies 0.59 μg 252Cf and does not list any other Cf isotopes. A measurement on the decay of source strength over time with Li glass (G20) showed that its half-life is very close to that of 252Cf. Thus, the source is reasonably assumed as a pure 252Cf source for the sake of the present work. According to an MCNPX simulation for a pure 252Cf source with the Watt fission spectra parameters in the code manual, about 18% of the source γ-rays escape the Pb pig. But most of the source γ-rays will be blocked by the shielding materials in the neutron irradiation setup. The γ-rays

Fig. 2. Schematic diagrams of the sample holder (a) and neutron irradiation setup (b) 1) Acrylic sample holder. 2) 6-mL sample vial. 3) PMT. 4) cylindrical cover. 5) 252 Cf source. 6) Cd well. 7) Pb well.

that can reach the scintillation sample are predominately produced by the (n,γ) reactions in the shielding materials, as discussed below. As illustrated in Fig. 2 (b), the neutron irradiation setup consists of two boxes with 2″-thick high density polyethylene (HDPE) walls. The 252Cf source is placed in the small HDPE box, which is in the big HDPE box along one side of the wall. Along the opposite side from the small box, two hollow acrylic wells were positioned at equidistance from the neutron source. One well was wrapped with a 1/8″-thick Cd sheet and another with a 1/8″-thick Pb sheet. Because the thermal-neutron cross-section of Cd is 20,000 times larger than that of Pb [40], the radiation in the Cd well is mainly composed of the secondary radiation from neutron absorptions and epi-thermal neutrons while the radiation in the Pb well is predominantly composed of secondary radiation from neutrons of all energies from the fission source. The center of the detector is about 4″ from the source with 2″–3″ of HDPE in between, depending on the path of the neutron. The neutron spectra at the surface of the wells are not well thermalized. As a result, the neutron count rate measured by G20 (under the peak due to 6Li reaction products) in the Pb well is only about 10 times as high as in the Cd well. It is concluded that about 10% of the neutron count rate is due to neutrons with energies above the Cd cutoff.

3. Results and discussion 3.1. Light transmission The UV–vis spectrum of each sample between 250–800 nm was collected with a 1-cm cuvette against the air. It is found that

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Table 3 Scintillation properties of the samples. Group

Sample

CB (M)

AL410 (m)

chγ  10  3

chn  10  2

kB1(mg cm  2 MeV  1)

kB2(mg cm  2 MeV  1)

keVee

UF

UF0 UF1 UF2 UF3 UF4

0 0.13 0.26 0.40 0.56

2.1 70.1 2.1 70.1 1.9 70.1 2.1 70.1 1.8 70.1

5.17 0.3 5.0 7 0.3 4.8 7 0.2 4.7 7 0.2 4.2 7 0.2

0 7.6 7 0.4 7.0 7 0.4 7.17 0.4 6.17 0.3

– 157 1 167 1 167 1 157 1

– 9.3 7 0.7 107 1 9.5 7 0.7 9.5 7 0.7

– 85 76 80 76 86 76 86 76

NUF

NUF1 NUF3

0.13 0.40

1.8 70.1 1.8 70.1

5.2 7 0.3 4.5 7 0.2

7.5 7 0.4 6.6 7 0.3

167 1 167 1

107 1 9.8 7 0.7

80 76 86 76

TC

TC0 TC1 TC2 TC3 TC4 TC5

0 0.036 0.072 0.21 0.36 0.51

5.4 70.3 4.7 70.2 5.2 70.3 4.8 70.2 5.0 70.3 4.8 70.2

5.0 7 0.3 4.9 7 0.3 4.7 7 0.2 4.17 0.2 3.9 7 0.2 3.5 7 0.3

0 6.17 0.3 6.4 7 0.3 5.0 7 0.3 4.9 7 0.3 4.5 7 0.2

– 197 1 177 1 197 1 187 1 197 1

– 12 71 117 1 12 71 12 71 12 71

– 71 75 76 76 71 75 72 75 72 75

NTC

NTC2 NTC4

0.072 0.36

4.7 70.2 4.8 70.2

5.0 7 0.3 4.2 7 0.2

6.6 7 0.3 5.2 7 0.3

187 1 197 1

117 1 12 71

75 75 73 75

TL

TL0 TL1 TL2 TL3 TL4

0 0.12 0.24 0.36 0.51

3.6 70.2 3.6 70.2 3.6 70.2 3.6 70.2 3.6 70.2

4.8 7 0.2 4.17 0.2 3.4 7 0.2 3.5 7 0.2 3.0 7 0.2

0 4.8 7 0.2 4.4 7 0.2 4.2 7 0.2 3.4 7 0.2

– 207 1 197 1 197 1 217 1

– 137 1 117 1 12 71 137 1

– 69 75 74 75 70 75 68 75

NTL

NTL1 NTL3

0.12 0.36

3.6 70.2 3.6 70.2

4.7 7 0.2 4.17 0.2

5.7 7 0.3 4.5 7 0.2

197 1 227 2

12 71 147 1

70 75 66 75

PC

PC0 PC1 PC2 PC3 PC4

0 0.12 0.28 0.36 0.51

3.7 70.2 3.7 70.2 3.6 70.2 3.6 70.2 3.5 70.2

4.8 7 0.2 4.17 0.2 3.4 7 0.2 3.17 0.2 3.0 7 0.2

0 4.5 7 0.2 3.9 7 0.2 3.5 7 0.2 3.3 7 0.2

– 217 1 217 1 217 1 227 2

– 137 1 137 1 137 1 147 1

– 68 75 68 75 68 75 64 75

NPC

NPC1 NPC3

0.12 0.36

3.4 70.2 3.5 70.2

5.17 0.3 4.6 7 0.2

5.7 7 0.3 5.0 7 0.3

217 1 217 2

137 1 137 1

65 75 65 75

XL

XL0 XL1 XL2 XL3 XL4

0 0.12 0.24 0.36 0.50

3.6 70.2 3.6 70.2 3.7 70.2 3.6 70.2 3.8 70.2

4.3 7 0.2 3.6 7 0.2 3.5 7 0.2 3.2 7 0.2 2.5 7 0.1

0 4.6 7 0.2 4.0 7 0.2 3.7 7 0.2 3.17 0.2

– 187 1 207 1 207 1 197 1

– 12 71 137 1 137 1 12 71

– 72 75 67 75 67 75 72 75

NXL

NXL1 NXL3

0.12 0.36

3.5 70.2 3.4 70.2

4.8 7 0.2 4.6 7 0.2

5.6 7 0.3 4.9 7 0.3

207 1 227 2

137 1 147 1

68 75 63 74

Radiation sources: γ-Source: 5 μCi 137Cs; Neutron source: 252Cf (1.36  106 n/s in July, 2009). Counting condition: HV: 1200 V; Gain: 100; Requisition time: 300 s. Scintillation reference: BC501A; Compton edge (chγ): 4.2  103; Light output relative to anthracene: 78%.

As the PMT (Philips XP2202B) has the maximum sensitivity at 410 nm, the absorbance at 410 nm is obtained from the spectrum and the attenuation length (AL410) is calculated to evaluate the light transmission. AL410 is defined as the distance of 410-nm light traveling through the sample before it decreases to 1/e of the initial intensity. The AL410 values of all the samples are listed in Table 3. It can be seen that the AL410 values range from 1.8 to 5.4 m. The samples in TC and NTC groups show the largest AL410, while the samples in UF and NUF groups have the smallest AL410. For any sample group, the attenuation length does not change with the concentration of carborane within experimental error, indicating that carborane does not cause color quench in any of the scintillators tested. 3.2. Energy calibration Fig. 3. Energy calibration of the scintillation response to different γ-ray sources.

within the range of 300–700 nm all samples show similar spectra and the absorbance is close to zero. Although a 10-cm cuvette will give more precise measurement when the absorbance is low, 1-cm cuvette was used to save carborane, which is the most expensive agent in the experiment.

The pulse-height spectra of two samples, UF0 and BC-501A were obtained in the γ-response measurement with 109Cd, 57Co, 137 Cs, and 60Co. Each spectrum shows a Compton continuum extending from channel 0 to the Compton edge where the count rate drops off sharply. The position of Compton edge (chγ, in channel) is determined by the half maximum of the Compton peak. The uncertainty of chγ is estimated to be 5%. As chγ is equivalent to the scintillation response to the maximum recoil

Z. Chang et al. / Nuclear Instruments and Methods in Physics Research A 769 (2015) 112–122

energy of Compton electrons, it can be used for relative comparison of the light outputs and scintillation efficiency of the samples measured under the same condition. The maximum recoil energy (Ee) of the Compton electrons for 109 Cd, 57Co, 137Cs, and 60Co are 0.023, 0.039, 0.477, and 1.12 MeV, respectively. Fig. 3 shows the plot of chγ versus Ee for UF0 and BC-501A. A phenomenological formula modified from literature [26,41], chg¼sEe  a(1  e  bEe) is used to fit the data. As shown in Fig. 3, it can be seen that the scintillation response for both liquid organic scintillators can be described by the formula. If only considering 5% uncertainty for the 2nd term of the formula, when the electron energy (Ee) is Z3/b, the scintillation response (chγ) has a linear relationship with Ee (chr ¼sEe – a).

PC0

PC1

PC2

PC3

Count Rate (cps)

0.06

0.04

0.02

0.00 2000

3000

4000

5000

6000

Channel Fig. 4. Pulse-height spectra of the samples in PC group in response to

137

Cs γ-rays (chγ)

PC4

0.08

1000

When 3/b 4Ee 40.1/b, the scintillation response is a curve with respect to Ee. When Ee r0.1/b, chγ is proportional to Ee (chr ¼(s  ab)Ee). The curved part should be related to the influence of Birks factor (kB) when dE/dx increases at low Ee, as shown by Eq. (2). Parameter s should be corresponding to the absolute scintillation efficiency (S). It is noticed from Fig. 3 that the exponential term in the formula has limited influence on chγ because parameter a is 10 times less than s for both scintillators. This may be the reason that the absolute scintillation efficiency (S) was found independent of Ee in a wider range (125 keV–several MeV), as discussed in Section 1.1. Such an empirical relationship is considered true for all the liquid scintillators not loaded or loaded with carborane or naphthalene in the following discussion. 3.3. Scintillation response to

0.10

117

137

Cs γ-rays.

Fig. 5. Compton edge (chγ) versus carborane concentration (CB) for different sample groups. Fit curve markers: UF ( ). TC ( ). TL ( ). PC ( ). XL ( ).

The pulse-height spectrum of each sample was obtained in the γ-response measurement with 137Cs. As an example, Fig. 4 shows the pulse-height spectra of the samples in PC group. It can be seen that each spectrum contains a Compton continuum from channel 0 to the Compton edge. Similar γ-response spectra are observed for the other sample groups. The position of Compton edge (chγ) is determined by the half maximum of the Compton peak. The chγ values of all the samples are listed in Table 3. The uncertainty related to determining chγ in the spectrum may be estimated with the method proposed by Garnir et al. [42], which was used to calculate the standard deviation of the peak centroid. If the Compton edge is regarded as the right half of a Gaussian distribution, the uncertainty of chγ can be estimated with 0.415√(2nhf/a), where hf is the right half-width at half maximum and a is the amplitude of the Compton edge peak. According to this method, the samples in each group have 2–3% uncertainty. Furthermore, from the replicate measurements of the samples, the standard deviation is found to be 5%, suggesting the factors such as small bubbles in the sample and coupling of the vial and PMT also contribute to the uncertainty. Thus, it is estimated that chγ generally contains 5% error, which is listed in Table 3. Fig. 5 shows the plot of chγ versus carborane concentration (CB) for different sample groups. It can be seen that chγ decreases with CB in each group, indicating that carborane is a quencher to the scintillator. As CB is low (o0.56 M), the Stern–Volmer model (Eq. (3)) is used to fit the experimental data. For the γ-response measurement, L0 and L in Eq. (3) are given as chγ,0 and chγ, which are the Compton edges of the samples not loaded and loaded with carborane at CB, respectively. The quench constant (kq) is given as kq, γ. The best fitting curve for each sample group is shown in Fig. 5. The goodness of fit is checked with a chi-square test [43]. The degrees of freedom (ν) for UF, TC, TL, PC, and XL groups are 4, 5, 4, 4, and 4, respectively. It is found that the probability (p-Value) of χ2/ν for all the sample groups falls in the range of 41–91%, larger than 5% significance level. Thus, the hypothesis that the experimental data obeys the Stern–Volmer model is not rejected. The chγ,0, kq,γ, and χ2/ ν values obtained for each scintillator are listed in Table 4 for the following discussion. The uncertainties of chγ,0 and kq,γ are

Table 4 Scintillation properties of the scintillators. Scintillator

Diisopropylnaphthalene Toluene (conc. solutes) Toluene Pseudocumene m-xylene

For γ-rays

For thermal-neutrons

chγ,0  10  3

kq,γ(M  1)

χ2/ν

chn,0  10  2

kq,n(M  1)

χ2/ν

5.2 70.1 5.0 70.1 4.6 70.3 4.6 70.2 4.4 70.3

0.357 0.08 0.80 7 0.06 1.17 0.2 1.17 0.1 1.3 7 0.2

0.26 0.94 0.99 0.50 0.99

8.0 7 0.4 6.3 7 0.3 5.6 7 0.5 5.17 0.2 5.3 7 0.2

0.447 0.13 0.86 7 0.17 1.2 7 0.3 1.2 7 0.1 1.4 7 0.1

0.31 1.03 0.79 0.10 0.16

kB1(mg cm  2 MeV  1)

kB2(mg cm  2 MeV  1)

keVee

14 71 18 71 19 72 21 72 19 72

9.3 7 0.5 127 1 127 1 137 1 127 1

85 75 73 74 71 77 67 76 70 75

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Z. Chang et al. / Nuclear Instruments and Methods in Physics Research A 769 (2015) 112–122

1.9

Count Rate (cps)

1.5

1.1

0.7

0.3

-0.1 100

200

300

400

500

600

700

800

channel Fig. 6. Difference spectra (DS) of TL4 against TL0 in the Pb and Cd wells and TL4's net neutron spectrum (NNS).

1.4 TL1

TL2

TL3

TL4

Count Rate (cps)

1.2 1 0.8 0.6 0.4 0.2 0 100

300

500

700

900

Channel Fig. 7. Net neutron spectra (NNS) and normal distribution fit curves of the carborane-loaded samples in TL group.

calculated by regression analysis [43] and are in the ranges of 2–7% and 8–23%, respectively. 3.4. Scintillation response to thermal neutrons (chn) The pulse-height spectra of each sample were measured in the Pb and Cd wells of the neutron irradiation setup. As the sample responds not only to thermal neutrons but also to γ-rays, x-rays, and fast electrons, the following method is used to extract the net neutron response from the pulse-height spectra. For a sample loaded with carborane at CB, its difference spectrum (DS) is calculated by: DS ¼ BLS 

NLS ð1 þ kq;γ C B Þ

ð4Þ

where, BLS is the spectrum of the caroborane-loaded sample. NLS is the spectrum of the same scintillation liquid not loaded with carborane. kq,γ is the quench constant of the scintillator listed in Table 4. Both BLS and NLS are measured in the same well in the neutron irradiation setup. It is found by trial and error that Eq. (4) is more effective in removing the scintillation response to the γ- and x-ray radiations in the final net neutron spectrum, as shown in the following (Eq. (5)). As an example, Fig. 6 shows the DS of sample TL4 against TL0 in the Pb and Cd wells. A large peak is present in the DS of TL4 measured in the Pb well. This peak is attributed to the response of TL4 to the ion particles (α-particle and Li ion) produced from 10B neutron capture. As

it is the response to 10B neutron capture, 10B(n, α)7Li, the peak is referred as (n, α) peak in the following discussion. On the other hand, a continuum with a gradient toward the origin is observed in the DS of TL4 measured in the Cd well, as shown in Fig. 4. The characteristic (n, α) peak is hardly seen on this continuum because the thermal neutron density in the Cd well is much lower than that in the Pb well. The slanted continuum should be related to the quench effect of carborane because the continuum gradient increases with CB. As the quenching increases with CB, the sample spectrum is more compressed toward lower channels. And Eq. (4) becomes less effective in removing the response to the γ- and x-rays at lower channels. Thus, it was found that the continuum gradient increases with CB in each sample group. Assuming that the continuum related to the carborane-loading in the DS has the same amplitude and slope in both wells, the net neutron spectrum (NNS) can be calculated by: NNS ¼ DS in Pb well  DS in Cd well

ð5Þ

It can be seen from Fig. 6 that the (n, α) peak in the NNS of TL4 is more symmetric and has less background compared with the DS in the Pb well. Notice in Fig. 6 that the left side of each peak is truncated because the low-energy cutoff was set at channel 112 in the measurement (Section 2.3). According to the relationship between the scintillation response and energy of Compton recoil electrons (Fig. 3), the chγ values in response to different γ-sources can be approximately estimated from the response to 137Cs. The estimated chγ values for 109Cd and 57 Cd γ-rays are marked on Fig. 6 for a quick comparison of the response to different types and energies of the incident particles. Regression analysis shows that the (n, α) peak in NNS can be described by Gaussian distribution function. As an example, Fig. 7 illustrates the NNS and corresponding Gaussian distribution fit curves for all the carborane-loaded samples in TL group. According to a statistical method for peak symmetry [44], the skewnesses (G1) of the (n, α) peaks are found to be 1.2, 1.1, 0.9, and  0.019 for TL1, TL2, TL3, and TL4, respectively, showing that the peak transforms from moderately right-skewed to symmetric distribution with the increase of CB. The high-energy tail of (n, α) peak appearing at low CB could be originated from the 6% branch of 10B neutron capture, which produces the ion particles at higher kinetic energy instead of emitting a 480-keV photon (Table 1). As the quenching increases with CB (Fig. 5) and the (n, α) peak is compressed toward the lower channels, the right skewness of the peak gradually becomes undetectable. Similar results are also found for the other sample groups. Thus, Eqs. (4) and (5) are used to extract the net neutron spectra for all the carborane-loaded samples. The peak position (chn) is obtained from the peak centroid in the NNS of each sample and listed in Table 3. Similar to chγ in the γ-response spectrum, the uncertainty in determining chn in NNS is estimated with the equation used by Garnir et al. [42]. It is found that this uncertainty is about 2%. From the replicate measurements of the samples, the standard deviation is found to be 5%, which is listed with the chn values in Table 3. Fig. 8 shows the plot of chn versus CB for all the sample groups. Carborane is obviously a quencher because chn decreases with CB in each group. The Stern–Volmer model (Eq. (3)) is thus fit to the data in Fig. 8. For thermal neutrons, L0 and L in Eq. (3) are shown as chn,0 and chn, which are the (n, α) peak positions of the samples not loaded and loaded with carborane at CB in the same group, respectively. The quench constant kq is given as kq,n. The best-fit curve for each sample group is shown in Fig. 8. The goodness of fit is checked with a chi-square test [43]. It is found that the p-Value of χ2/ν for all the sample groups falls in the range of 39–96%, larger than 5% significance level. Thus, the Stern– Volmer model is valid for describing the relationship between chn and CB. The chn,0, kq,n, and χ2/ν values of each scintillator are listed

Z. Chang et al. / Nuclear Instruments and Methods in Physics Research A 769 (2015) 112–122 8.5E+02

7.5E+02

6.5E+02

5.5E+02

4.5E+02

3.5E+02

2.5E+02 0

0.1

0.2

0.3

0.4

0.5

0.6

Fig. 8. (n, α) peak position (chγ) versus carborane concentration (CB) for different sample groups. Fit curve markers:: UF ( ). TC ( ). TL ( ). PC ( ). ). XL (

in Table 4. The uncertainties of chn,0 and kq,n are calculated by regression analysis [43] and are in the ranges of 4–9% and 7–30%, respectively. 3.5. Birks factor (kB) According the ionizing particles involved in the experiment, Eq. (1) can be written as: dLγ ¼

S dE 1 þ kBe ðdE=dxÞe

ð6Þ

dLn ¼

S S dE þ dE 1 þ kBα ðdE=dxÞα 1 þkBLi ðdE=dxÞLi

ð7Þ

where, Lγ and Ln are proportional to the scintillation response, chγ and chn, to 137Cs γ-rays and thermal neutrons, respectively. S refers to absolute scintillation efficiency, which is conventionally considered independent of the type and energy of the incident particle. kBe, kBα, and kBLi are the Birks factors for the fast electron, α-particle, and Li ion, respectively. Some investigations reported that kB can be also regarded as a parameter independent of the type and energy of the incident particle [19,24,29,45]. Especially, Tretyak [29], using the experimental data from dozens of reports to calculate the kB values for different organic and inorganic scintillators in response to various particles (protons, α-particles, and heavy ions), concluded that kB can be empirically considered independent of the type and energy of the particle if the measurements are conducted under the same condition. Thus, assuming that kB is a constant for each sample in Table 3 in response to both fast electrons and heavy ions, one can combine Eqs. (6) and (7) into: RE R Eα ðdE=1 þ kBðdE=dxÞα Þ þ 0 Li ðdE=1 þkBðdE=dxÞLi Þ chn Ln ¼ 0 ð8Þ ¼ R Ee Lγ chγ ðdE=1 þ kBðdE=dxÞe Þ 0

Eq. (8) can be used to calculate kB from chγ, chn, and the specific energy loss profiles (dE/dx vs. E curves) of the incident particles. Several simulation codes are available on the Internet for calculation of the dE/dx profiles in different substances. In this study, the dE/dx profiles for α-particle and Li ion are calculated by the SRIM codes [46]. The dE/dx profile for fast electrons is calculated by the ESTAR codes [47]. Following Tretyak’s method [29], the total stopping power (dE/dx) given by the codes is used for the numerical integration in Eq. (8). The SRIM codes [48] calculate the stopping power (dE/dx) in two steps: 1) estimate the ion stopping by the elemental composition of

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the absorber based on Bragg's rule, and 2) improve the estimation by the “core-and-bond” approximation, which considers the difference in stopping contributions from inner shell electrons (core) and binding electrons (bond). It was claimed that this approach can calculate the dE/dx values for many compounds within an average uncertainty of 1% [48]. The “core-and-bond” approximation results in a so-called “compound correction” (ε) for the SRIM codes. The SRIM codes only provide the ε values for a limited number of compounds [46]. The ε values for compounds such as pseudocumene and diisopropylenaphthalene are not given. In this study, unknown ε values are predicted from the best fit curves to the known ε values of three aromatic compounds (benzene, toluene, and xylene) given by the SRIM codes. As the polynomial curve fitting fits the known points very well, this method is used to estimate the ε values of α-particle and Li ion in pseudocumene, diisopropylenaphthalene, and the samples loaded with carborane and naphthalene. On the other hand, the ESTAR codes only need the elemental composition of the absorber to calculate the dE/dx profile for fast electrons. In addition, the sample density in each group is considered to be constant because the mole fraction of carborane in all samples is r0.13. Accordingly, the Birks factor can be numerically determined by Eq. (8) with Eα ¼1.47 MeV, ELi ¼0.84 MeV, and Ee ¼0.477 MeV. The results are listed as kB1 for each carborane-loaded sample in Table 3 and each scintillator in Table 4. It can be seen that kB1 is in the range of 14–22 mg cm  2 MeV  1 for all the carboraneloaded samples and scintillators. The uncertainty of kB1 is about 5–9% based on the error propagation of chγ and chn. It is noticed that the SRIM codes assume that Li ion carries þ1 charge in the dE/dx calculation. But the knowledge about the Li ion charge right after being released from 10B neutron capture is very limited. If the Li ion carries more than one charge, the dE/dx value will be much larger because it is proportional to the square of the particle's charge [30]. In such a situation, the contribution of the Li ion to chn will be significantly less than that of the α-particle. Assuming that the scintillation of the Li ion is negligible, a new Birks factor is calculated by removing the integration for Li ion in Eq. (8). The calculation results are listed as kB2 in Tables 3 and 4. It can be seen that kB2 falls in the range of 9.3–14 mg cm  2 MeV  1, about 60–70% less than kB1. The Birks factors for various organic scintillators with different calculation methods were reported in literature. Hirshberg and his co-workers [24] observed a value of 10 71 mg cm  2 MeV  1 for polyvinyl toluene-based plastic scintillator NE102 in response to protons. von Krosigk et al. [27] found kB ranging between 8.1 and 8.4 mg cm  2 MeV  1 for different linear alkylbenzene scintillators in response to protons. Broda et al. [49], using triple-to-double coincidence ratio technique [50], measured kB for 13 Ultima Gold and xylene-based liquid scintillators in response to the low-energy β-particles from 3H and 63Ni and found kB in the range of 9.0– 12 mg cm  2 MeV  1. Tretyak [29], calculating the dE/dx profiles, determined the kB values of 2–9 MeV α-particles in liquid pseudocumene and solid polyethylene based scintillators to be 9.0 and 9.4 mg cm  2 MeV  1, respectively. All these reports show that the kB values of different particles in organic scintillators are in the range of 8–12 mg cm  2 MeV  1, which is about half of kB1 (14– 22 mg cm  2MeV  1) but rather close to kB2 (9.3– 14 mg cm  2 MeV  1). This suggests that the Li ion from 10B neutron capture has negligible contribution to chn, possibly due to its carrying more than one charge. Thus, it is believed that the true Birks factor of the sample is more close to kB2. Furthermore, the ratio of kB1 to kB2 for any sample is found basically constant (1.58). Thus, kB2 will be mainly used in the following discussion. A chi-square test is carried out to check the variation of kB2 in each sample group. It is found that the p-Value of χ2/ν is440% assuming kB2 to be constant for all the samples in each group and the corresponding group with 100 g/L naphthalene. This suggests that

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for any given scintillator the Birks factor does not change with the loading of carborane or naphthalene in the experimental range. The average kB2 of the samples in UF and NUF groups are 9.7 mg cm  2 MeV  1, which is significantly smaller than those of the other groups. Except UF and NUF groups, the samples of the other groups can be considered having the same kB2 (averagely 12.4 mg cm  2 MeV  1) according to the chi-square test. 3.6. Effect of scintillator From Table 4, for diisopropylnaphthalene, toluene (concentrated solutes), toluene, pseudocumene, and m-xylene based scintillators, the chγ,0 values are 5.2  103, 5.0  103, 4.6  103, 4.6  103, and 4.4  103, respectively; the chn,0 values are 8.0  102, 6.3  102, 5.6  102, 5.1  102, and 5.3  102, respectively. It can be seen that the light output of each scintillator for 137Cs γ-rays is 6–9 times higher than that for thermal neutrons. For all the scintillators, the order of scintillation efficiencies for 137Cs γ-rays and thermal neutrons changes is similar: diisopropylnaphthalene4toluene (concentrated solutes)4toluene
pseudocumene
m-xylene. This is because chγ,0 is proportional to the absolute scintillation efficiency (S) and chn,0 is equivalent to S/(kB  dE/dx), as shown in Eq. (2). The diisopropylnaphthalene-based scintillator has the smallest kB2, the other scintillators have the same kB2 within the uncertainty, and the dE/dx profile of the 1.47-MeV α-particle is rather similar for all the samples. Thus, the scintillation efficiency for thermal neutrons has the similar order as that for 137Cs γ-rays. Table 4 shows that the diisopropylnaphthalene-based scintillator has the highest light outputs (chγ,0 and chn,0) and smallest Birks factor (kB) among all the scintillators tested. This is primarily attributed to the fused double benzene-ring structure of diisopropylnaphthalene, which is different from the single benzene-ring structure of the solvents for the other scintillators. As the π-bonds of a fused double benzene-ring are more delocalized than those of a single benzenering, diisopropylnaphthalene is more efficient to populate to the excited singlet state under ionizing radiation (lower kB) and to transfer the excitation energy to the solutes (higher S). Thomson [51] reported that diisopropylnaphathalene has apparent larger light output than many benzene derivative solvents in the detection of 3H. His observation agrees with the results in this study. Furthermore, the scintillation efficiency of the toluene (concentrated solutes) based scintillator is 9 and 13% higher than that of the toluene-based scintillator in response to 137Cs γ-rays and thermal neutrons, respectively. This is due to the difference in solute concentration. The PPO and POPOP concentrations in the toluene (concentrated solutes) based scintillator are 25 times higher than those in the toluene-based scintillator. Such a significant increase changes the absolute scintillation efficiency. However, it is noted that too much solutes can lead to quenching because the fluorescent solutes often contains functional groups such as C ¼N, C–O, etc. The low solubility of the solutes is another technical issue, which causes the scintillator solution unstable at high solute concentrations. 3.7. Effect of carborane According to the plots of chγ
CB and chn
CB for each sample group (Figs. 5 and 8), it is clear that carborane quenches scintillation light in all the groups. Based on the UV–vis absorbance results shown in Section 3.1, it is apparent that carborane is not a color quencher. Because of its high chemical stability, carborane is not likely to have any chemical reactions with the solvent or solute molecules in the scintillation process. Thus, the quenching of carborane should not have a static mechanism, which often involves molecular associations between the quencher and solvent or solute molecules [52]. The quenching is more likely to have

a dynamic mechanism, which may occur in either of the two ways: i) carborane molecules collide with the excited solvent or solute molecules and absorb the excitation energy, and ii) carborane competes in the Förster resonance energy transfer (FRET) to absorb the excitation energy. In either of these ways, carborane can turn the excitation energy into thermal energy, and the scintillation efficiency is reduced. Table 4 lists the quench constants kq,γ and kq,n for each scintillator in response to 137Cs γ-rays and thermal neutrons, respectively. For diisopropylnaphthalene, toluene (concentrated solutes), toluene, pseudocumene, and m-xylene based scintillators, the kq,γ values are 0.35, 0.8, 1.1, 1.1, and 1.3 M  1, respectively; the kq,n values are 0.44, 0.86, 1.2, 1.2, and 1.4 M  1, respectively. It can be seen that the order of quench effect for both γ-rays and thermal neutrons changes is: diisopropylnaphthaleneotoluene (concentrated solutes)otoluene
pseudocumeneom-xylene. The diisopropylnaphthalene-based scintillator has the smallest kq,γ and kq,n among all the scintillators tested. The kq,γ and kq,n values in the toluene (concentrated solutes) based scintillator are less than those in the toluene-based scintillator. The m-xylene based scintillator has the largest kq,γ and kq,n values among all the scintillators tested. According to the chi-square analysis, the Birks factor does not change with the concentration of carborane (CB) for any sample group. This is because CB is rather low and carborane does not affect the interaction of ionized and excited solvent molecules along the track of incident particle. Thus, the quench effect mainly affects the absolute scintillation efficiency (S), as discussed in Section 1.1. Based on this analysis, the quench effect of carborane should be independent of the type and energy of incident particle. This is proved by the kq,γ and kq,n values, which are the same within the uncertainty for each scintillator listed in Table 4. 3.8. Effect of naphthalene It can be seen from Tables 3 and 4 that the diisopropylnaphthalenebased scintillator and the carborane-loaded samples in UF group show the highest light outputs, lowest Birks factor, and least quench constants among all the scintillators and samples. As discussed in Section 3.7, these properties are primarily attributed to the fused double benzenering structure of diisopropylnaphthalene. Five more sample groups are used to further test the role of fused double benzene-ring structure in the scintillation process. These groups, NUF, NTC, NTL, NPC, and NXL, were prepared by adding 100 g/L naphthalene as the secondary solvent to carborane-loaded samples in diisopropylnaphthalene, toluene (concentrated solutes), toluene, pseudocumene, and m-xylene based scintillators, respectively, as shown in Table 2. The chγ and chn values of the samples containing 100 g/L naphthalene are plotted with solid markers in Figs. 5 and 8 for comparison. It can be seen that the light outputs of these samples are generally larger than those of the corresponding samples not loaded with naphthalene. The average chγ values of the samples in NUF, NTC, NTL, NPC, and NXL groups are 0.55%, 6.7%, 15%, 29%, and 32% larger than those in UF, TC, TL, PC, and XL groups, respectively. The average chn values of the samples in NUF, NTC, NTL, NPC and NXL groups are  3.7%, 4.7%, 12%, 29%, and 25% larger than those in UF, TC, TL, PC, and XL groups, respectively. For both γ-rays and thermal neutrons, the increase in light output by adding 100 g/L naphthalene follows the order: UFoTCoTLoXL
PC. For the samples in diisopropylnaphthalene, naphthalene-loading does not lead to any change in light output within the uncertainty (5%) in response to both γ-rays and thermal neutrons. This is because both naphthalene and diisopropylnaphthalene have the same fused double benzene-ring structure. The largest increases in light output are found with the samples in XL and PC groups, which have the lowest light outputs among all the groups. According to the chi-square test, loading naphthalene does not change the Birks factor for any given scintillator. This suggests that

Z. Chang et al. / Nuclear Instruments and Methods in Physics Research A 769 (2015) 112–122

naphthalene, similar as carborane, primarily affects the absolute scintillation efficiency (S) or the fluorescence process after the solvent molecules populate to the excited singlet state (Section 1.1). Dobbs [53] studied the effect of naphthalene on the light output of the toluenebased samples containing chemical quenchers. He attributed the light output increase to that naphthalene acts as an intermediate solvent transferring the excitation energy from primary solvent to solutes, thereby bypassing the quench process. According to the fluorescence spectra [20], the absorption peak of naphthalene (half maximum:
250–310 nm) is lower than that of PPO (280–330 nm). Its fluorescence peak (320–350 nm) overlap part of the absorption peaks of PPO and POPOP (330–400 nm). This may explain why naphthalene can provide a competitive pathway to bypass the quenching of carborane and promote the energy transfer to the solutes. 3.9. Electron equivalent energy (keVee) The light outputs in response to heavy charged particles are often expressed in equivalent electron energy (keVee) in order to compare the scintillation efficiency of different scintillators. In this study, the keVee value is obtained with Eq. (8) by determining the electron energy (Ee) that satisfies chn/chγ ¼1. It is found that both calculation methods (considering the integration of 0.84-MeV Li ion with kB1 or neglecting the Li ion with kB2 ) result in the same keVee value. The light outputs in keVee for all the carborane-loaded samples and scintillators in response to thermal neutrons are thus calculated and listed in Tables 3 and 4. The uncertainty of the keVee value is estimated as 5–9% from the error of kB2. From Table 4, the light output are 85, 73, 71, 67, and 70 keVee for diisopropylnaphthalene, toluene (concentrated solutes), toluene, pseudocumene, and m-xylene based scintillators, respectively. From Tables 3 and 4, all the samples in the same scintillator show a constant keVee value within the uncertainty. Loading carborane or naphthalene does not change the keVee value. This is because that the light output in keVee is only related to the Birks factor and specific energy loss (dE/dx). And the Birks factor is independent of the concentrations of carborane and naphthalene, as shown in Table 3. Table 3 shows that the light outputs of all the caborane-loaded samples fall in a range of 63–86 keVee, which are close to the values observed in literature. Swiderski et al. [54] studied the scintillation response of two 10B-loaded liquid scintillators (BC-523A2 and EJ-339A2) and found that both scintillators show a light output of 60 keVee for thermal neutrons. Bentoumi et al. [55] studied carborane-loaded linear alkyl benzene (LAB) liquid scintillator and also observed that the neutron-response peak is positioned at 60 keVee. The samples in this study have larger keVee values. Especially, the samples in UF group have the largest light outputs (80–86 keVee), indicating that diisopropylnaphthalene has better scintillation efficiency for the detection of thermal neutrons.

4. Conclusions It is found that carborane is an effective B-carrier for the organic liquid scintillators to detect thermal neutrons. Carborane is readily soluble in all the five liquid scintillators up to 8.5 wt% to yield colorless and transparent solutions, with the attenuation length at 410 nm ranging between 1.8 and 5.4 m. The quench effect of carborane is obtained. By fitting the Stern– Volmer model to the scintillation response, the quench factors of carborane in all the scintillators are found in the range of 0.35–1.4 M  1. For any scintillator, the quench constants in response to 137Cs γ-rays and thermal neutrons are the same within the uncertainty. As the Birks factor of any scintillator does not change with carborane-loading, it is suggested that the quenching of carborane occurs after the solvent molecules populate to the excited singlet state in the scintillation

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process. The diisopropylnaphthalene-based scintillator has the smallest quench constant among all the scintillators tested. A method was developed to extract net neutron spectrum from the pulse-height spectra in response to thermal neutrons. This method removes the scintillation response to the secondary radiation of neutrons by comparing the spectra of the carboraneloaded and non-loaded scintillators measured in the Pb and Cd shielded wells. It is believed that this method, if combined with other techniques (such as pulse-shape discrimination and γ–n coincidence counting), can increase the detection sensitivity, which is critical for the detection of weak neutron sources against large background noise. The Birks factor is estimated from the scintillation response to 137Cs γ-rays and thermal neutrons and the specific energy loss calculated by the simulation codes (SRIM and ESTAR). It is observed that if the scintillation of the 0.84-MeV Li ion is neglected, the Birks factors of the scintillators are in the range of 9.3–14 mg cm  2 MeV  1, which is close to the data range in literature. Thus, it is concluded that the contribution the Li ion to the light output is negligible. This might be due to that the Li ion carries more than one charge when just after it is released from 10 B neutron capture. It is found that the diisopropylnaphthalene-based scintillator has the smallest Birks factor (kB2 ¼9.370.5 mg cm  2 MeV  1) among all the scintillators investigated. It is interesting to compare the light outputs of the samples in this study with those commercially available. The well-known B-loaded scintillators, EJ-339 (liquid) (ELJEN, 2014), EJ-254 (plastic) (ELJEN, 2014), BC-523 (liquid) [3], and BC-454 (plastic) [3] are 65%, 60%, 65%, and 48% of the light output relative to anthracene, respectively. With the same B-loading (5 wt% B), the diisopropylnaphthalene, toluene (concentrated solutes), toluene, pseudocumene, and m-xylene based samples in this study show 87%, 70%, 63%, 58%, and 58% of the light output of anthracene, respectively. Considering the uncertainty at the level of 5–10%, one can see that toluene, pseudocumene, and m-xylene based samples have similar light outputs as the commercial products. But the diisopropylnapthalene-based scintillator has apparently higher scintillation efficiency than all the commercial scintillators. The diisopropylnapthalene-based samples also have significantly larger keVee than the B-loaded liquid scintillators reported recently. Among all the scintillators tested, the diisopropylnaphthalenebased scintillator shows the highest scintillation efficiency, lowest Birks constant, and smallest quench constants in response to both 137Cs γrays and thermal neutrons. These properties are primarily attributed to the fused double benzene-ring structure of diisopropylnaphthalene, which is more efficient to populate to the excited singlet state under ionizing radiation and to transfer the excitation energy to the fluorescent solutes. This conclusion is supported by the light outputs of the samples with 100 g/L naphthalene as the secondary solvent. References [1] GAO, Neutron detectors, alternatives to using heliuim-3, U.S. Goverment Accountability Office, Washington, 2011. [2] F.D. Brooks, Nucl. Instrum. Methods A 162 (1979) 477. [3] Saint-Gobain, Hiram, USA: Saint-Gobain Crystals (2011) (last time accessed October 12, 2014: www.crystals.saint-gobain.com/uploadedFiles/SG-Crystals/ Documents/SGC%20Organics%20Brochure.pdf). [4] ELJEN, Product documents on ELJEN Technology website (2014) (last accessed on April 25, 2014). [5] K. Banerjee, S. Kundu, S. Mukhopadhyay, T.K. Rana, S. Bhattacharya, C. Bhattacharya, S.R. Banerjee, T.K. Ghosh, G. Mukherjee, T. Bandyopadhyay, A. Dey, J.K. Meena, P. Mukhopadjyay, D. Gupta, S. Pal, D. Pandit, S. Battachary, Nucl. Instrum. Meth. Phys. Res. A 580 (2007) 1383. [6] L. Ovechkina, K. Riley, S. Miller, Z. Bell, V. Nagarka, Phys. Proc. 2 (2009) 161. [7] N. Zaitseva, A. Glenn, H.P. Martinez, L. Carman, I. Pawelczak, M. Faust, S. Payne, Nucl. Instrum. Methods A 729 (2013) 747. [8] S. Ait-Boubker, M. Avenier, G. Bagieu, J.F. Cavaignac, J. Collot, J. Favier, E. Kajfasz, D.H. Koang, A. Stutz, B. Vignon, Nucl. Instrum. Method A 277 (1989) 461. [9] M.F. Hawthorne, Carborane chemistry at work and at play (Special Publication No. 201), in: W. Sibert (Ed.), Proceedings of the Ninth International Meeting on

122

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]

[33] [34] [35]

Z. Chang et al. / Nuclear Instruments and Methods in Physics Research A 769 (2015) 112–122

Boron Chemistry, Advances in Boron Chemistry, The Royal Society of Chemistry, Cambridge, 1996, pp. 261–272. J. Green, N. Mayes, A.P. Kotloby, M.M. Fein, E.L. O’Brien, M.S Cohen, J. Polym. Sci. Part B: Polym. Lett. 2 (1964) 109. W. Jiang, I.T. Chizhevsky, M.D. Mortimer, W. Chen, C.B. Knobler, S.E. Johnson, F.A. Gomez, M.F. Hawthorne, Inorg. Chem. 35 (19) (1996) 5417. J. Morin, T. Sasaki, Y. Shirai, J.M. Guerrero, J.M. Tour, J. Org. Chem. 72 (25) (2007) 9481. M. Patel, A.C. Swain, A.R. Skinner, L.G. Mallinson, G.F. Hayes, Macromol. Symp. 202 (2003) 47. R.H. Pak, F.J. Primus, K.J. Richard-Dickson, L.L. Ng, R.R. Kane, M.F Hawthorne, Proc. Natl. Acad. Sci. USA 92c (1995) 6986. Z.W. Bell, M.A. Miller, L. Maya, G.M. Brown, F.V. Sloop Jr, IEEE Trans. Nucl. Sci. 51 (4) (2004) 1773. S. Carturan, A. Quaranta, T Marchi, F. Gramegna, M. Degerlier, M. Cinausero, V.L. Kravchuk, M. Poggi, Radiat. Prot. Dosim. 143 (2-4) (2011) 471. A. Quaranta, S.M. Carturan, T. Marchi, V.L. Kravchuk, F. Gramegna, G. Maggioni, M. Degerlier, IEEE Trans. Nucl. Sci. 57 (2010) 891. J.B. Birks, Proc. Phys. Soc. (1951) 874. J.B. Birks, The Theory and Practice of Scintillation Counting, Pergamon Press, New York (1964) 185–205. J.B. Birks, Solutes and Solvents for Liquid Scintillation Counting, Koch-Light Laboratories Ltd, New York (1969) 14–39. T. Förster, Modern Quantum Chemistry, Vol. III, in: O. Sin-anoglu (Ed.), Academic Press, New York, 1965, pp. 93–137. G.D. Scholes, Annu. Rev. Phys. Chem. 54 (2003) 57. P. Wu, L. Brand, Anal. Biochem. 218 (1994) 1. M. Hirshberg, R. Beckmann, U. Brandenburg, H. Bruckmann, K. Wick, IEEE Trans. Nucl. Sci. 39 (1992) 511. S. Mouatassim, G.J. Costa, G. Guillaume, B. Heusch, A. Huck, M. Moszynski, Nucl. Instrum. Methods Phys. Res. A 359 (1995) 530. R.E. Pywell, B.D. Sawatzky, J. Ives, N.R. Kolb, R. Igarashi, W.A. Wurtz, Nucl. Instrum. Methods Phy. Res. A 565 (2006) 735. B. von Krosigk, L. Neumann, R. Nolte, S. Rottger, K. Zuber, Eur. Phys. J. C 73 (2013) 2390. G.F. Knoll, Radiation, Detection and Measurement, 4th Ed., John Wiley & Sons, Inc., New York (2010) 223–235. V. Tretyak, Astropart. Phys. 33 (2010) 40. H. Bethe, J. Ashkin, in: E. Segré (Ed.), Experimental Nuclear Physics, J. Wiley, New York, 1953, p. 253. M.W.M Al-Badrani, Bulg. J. Phys. 35 (2008) 142. K.O. Rundt, Quench correction of colored samples in LSC, in: H. Ross, J.E. Noakes, J.D. Spaulding (Eds.), Liquid Scintillatin Counting and Organic Scintillators, Lewis Publishers, Inc, Gatlinburg, Tennessee, 1989, pp. 677–689. O. Stern, M. Volmer, über die Abklingungszeit der Fluorszenz Phys. Z 20 (1919) 183. N.J. Green, S.M. Pimblott, M. Tachiya, J. Phys. Chem. 97 (1993) 196. H. Morii, K. Mizouchi, T. Nomura, N. Sasao, T. Sumida, M. Kobayashi, Y. Murayama, R. Takashima, Nucl. Instrum. Methods A 526 (2004) 399.

[36] PerkinElmer, LSC cocktails – Elemental composition, USA: PerkinElmer Inc., Waltham, 2010 (Retrieved Dec. 28, 2013, from www.nucleide.org/ ICRM_LSC_WG/2010_LSC_cocktails_elementary_composition.pdf). [37] PerkinElmer, Material safety data sheet of Ultima Gold TM, PerkinElmer Inc, Waltham, USA, 2009. [38] AST, Lithium glass scintillators, Applied Scintillation Technology, Harlow, UK, 2012. [39] H.T. Ishikawa, Quantitative measuring of oxygen quenching effect, in: D.L. Horrocks (Ed.), Organic Scintillators and Liquid Scintillation Counting, Academic Press Inc., New York, USA, 1971, pp. 387–394. [40] J. Lamarsh, A. Baratta, Introduction to Nuclear Engineering, 3rd Ed., Prentice Hall, New Jersey (2001) 739–742. [41] R.A. Cecil, B.D. Anderson, R. Madey, Nucl. Instrum. Methods 161 (1979) 439. [42] H.P. Garnir, Y. Bandine-Robinet, P.D. Dumont, Nucl. Instrum. Methods Phys. Res. B28 (1987) 146. [43] W.H. Press, S.A. Teukolsky, W.T. Vetterling, B.P. Flannery, Numerical Recipes in C: The Art of Scientific Computing, 2nd ed., Cambridge University Press, USA (2002) 659–666. [44] D.N. Jones, C.A. Gill, The Statistician 47 (1) (1998) 183. [45] L. Rechhart, Y.D. Akimov, H.M. Araujo, E.J. Barnes, V.A. Belov, A.A. Burenkov, V. Chepel, A. Currie, L. DeViveiros, B. Edwards, V. Francis, C. Ghag, A. Hollingswarth, M. Horn, G.E. Kalmus, A.S. Kobyakin, A.G. Kovalenko, V.N. Lebedenko, A. Lindote, M.I. Lopes, R. Luscher, P. Majewski, A.S. J. Murphy, F. Neves, S.M. Paling, J.P. da Cunha, R. Preece, J.J. Quenby, P.R. Scovell, C. Silva, V.N. Solovov, N.J.T. Simith, P.F. Smith, V.N. Stekhanov, T.J. Sumner, C. Thorne, R.J. Walker, Phys. Rev. C 85 (065801) (2012) 1. [46] Ziegler, J. F., Biersack, J. P., Ziegler, M. D., (2013). SRIM, The stopping and ranges of ions in matter, SRIM Co., SRIM Version 2013. 〈http://www.srim.org〉. [47] Berger, M.J., Coursey, J.S., Zucker, M.A., Chang, J. (2009). Stopping-power and range tables for electrons, protons, and helium ions. Version 1.2, 〈http:// physics.nist.gov/PhysRefData/Star/Text/contents.html〉. [48] J.F. Ziegler, J.M. Manyoyan, Nucl. Instrum. Methods B35 (1988) 215. [49] R. Broda, K. Maletka, T. Terlikowska, P Cassette, Appl. Radiat. Isot. 56 (2002) 285. [50] R. Broda, Appl. Radiat. Isot. 58 (2003) 585. [51] J. Thomson, Di-isopropylnaphthalene – a new solvent for liquid scintillation counting, in: H. Ross, J. Noakdes, J. Spaulding (Eds.), Liquid Scintillation Counting and Organic Scintillators, Lewis Publishers, Inc, Chelsea, Michigan, 1991, pp. 19–34. [52] J. Keizer, J. Am. Chem. Soc. 105 (1983) 1494. [53] H. Dobbs, Nature (1963) 788. [54] L. Swiderski, M. Moszynski, D. Wolski, T. Batsch, J. Iwanowska, A. Nassalski, A. Syntfeld-Kazuch, T. Szczesniak, F. Kniest, M.R. Kusner, G. Pausch, S.W. Klamra, P. Schotanus, C. Hurlbut, IEEE Trans. Nucl. Sci. 57 (1) (2010) 375. [55] G. Bentoumi, X. Dai, H. Fritzsche, G. Jonkmans, L. Li, G. Marleau, B. Sur, Nucl. Instrum. Meth. Phys. Res. A 701 (2013) 221. [56] P. Kandlakunta, L. Cao, P. Mulligan, Nucl. Instrum. Methods Phys. Res. A 705 (2013) 36. [57] G.M. Edvenson, D.F. Gaines, Inorg. Chem. 29 (6) (1990) 1210.