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Measurement of wavelength-dependent refractive indices of liquid scintillation cocktails
Applied Radiation and Isotopes 82 (2013) 382–388
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Measurement of wavelength-dependent refractive indices of liquid scintillation cocktails Karsten Kossert n Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany
H I G H L I G H T S
Refractive indices of several liquid scintillation cocktails were measured. The wavelengths cover a range from 404.7 nm to 706.5 nm. Measurements were carried out at 16 1C, 18 1C, 20 1C and 22 1C. For some cocktails, mixtures with water or nitromethane were studied.
art ic l e i nf o
a b s t r a c t
Article history: Received 7 August 2013 Received in revised form 28 September 2013 Accepted 10 October 2013 Available online 18 October 2013
Refractive indices of several commercial liquid scintillation cocktails were measured by means of an automatic critical-angle dispersion refractometer in the wavelength range from 404.7 nm to 706.5 nm. The results are needed for various applications. In particular, detailed Monte Carlo simulations of liquid scintillation counters that include the computation of optical light require these data. In addition, the refractive index is an important parameter for studies of micelle sizes by means of dynamic light scattering. In this work, the refractive indices were determined for Ultima Gold™, Ultima Gold™ F, Ultima Gold™ LLT, Ultima Gold™ AB, Hionic Fluor™, Permafluors E þ, Mineral Oil Scintillator, Insta-Gel Plus, OptiPhase HiSafe 2, OptiPhase HiSafe 3, Ultima Gold™ XR, Insta-Gel Plus, AquaLight, MaxiLight and Ultima Gold™ MV at 16 1C, 18 1C, 20 1C and 22 1C. The carbon dioxide absorber Carbo-Sorbs E was also analyzed. For some scintillators, various batches were compared and mixtures with water or nitromethane were studied. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Refractive index measurement Liquid scintillation cocktail Dispersion
1. Introduction Liquid scintillation (LS) counting is widely used for activity determination and can be considered as an active research field (Broda et al., 2007). Some researchers are developing Monte Carlo simulations which comprise the modelling of the scintillation photons (Hurtado et al., 2009; Bobin et al., 2012; Thiam et al., 2012). Such simulations require excellent knowledge of material parameters since the transmission, reflection and refraction of light is emulated using software packages, like the GEANT4 tool (Agostinelli et al., 2003). The refractive index of scintillation cocktails belongs to these parameters. For accurate simulations, it must be taken into account that the refractive index depends on the wavelength of light. When the emitted light is not monochromatic, as in the case of Čerenkov light, dispersion plays an important role.
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In many cases, LS counting is used to measure aqueous radioactive solutions. Thus, the cocktails usually contain a surfactant which ensures the miscibility of the organic and the aqueous phases. The radioactivity usually remains within the aqueous phase which is embedded in the organic phases in the form of (reverse) micelles. Electrons which are ejected as a result of a radioactive decay may lose a part of their kinetic energy and, consequently, only a reduced energy remains to create scintillation light (Grau Carles, 2007). The effects are very small in LS samples with low water content (Kossert and Grau Carles, 2010; Bergeron, 2012), but they might increase when using gel samples with high water content. Tarancόn Sanz and Kossert (2011) have also shown that the general idea of subtracting the energy loss in the aqueous phase works well when using plastic scintillation beads in an aqueous radioactive medium. The energy loss within the micelles and the corresponding reduction of the counting efficiency are referred to as the micelle size effect. For accurate studies of this effect, it is crucial to know the dimensions of micelles. Rodríguez et al. (1998) and Bergeron (2012) have shown that the size of
K. Kossert / Applied Radiation and Isotopes 82 (2013) 382–388
micelles can be evaluated by applying the Stokes–Einstein equation. To this end, the viscosity and the diffusion coefficient are required. The latter can be measured by means of dynamic light scattering (DLS). This measurement technique, however, also requires information about the refractive index of the samples under study. Thus, mixtures of LS cocktails and water as well as mixtures of cocktails and the quenching agent nitromethane (CH3NO2) were included in this study. These measurements are of course also important since most activity measurements are carried out with corresponding mixtures.
2. Description of measurements The measurements were carried out with an automated multiwavelength refractometer DSR-λ (Schmidt þHaensch, 2010) between 17 January and 13 June 2013. According to the manufacturer's manual, the instrument can be used to measure the “refractive index of liquid media independent of opacity, viscosity and colour”. The refractive index is determined via the measurement of the critical angle for total reflection. To this end, the position of the dark line is determined by means of a high resolution CCD linear array with 2048 elements. Seven LEDs are part of the system, allowing the sequential measurements at different wavelengths in the range from 404.7 nm to 706.5 nm. The system comprises a heater as well as a Peltier cooler and can be used for measurements in a temperature range from 10 1C to 80 1C. The system used in this work allows measurements of the refractive index in a range from 1.33 to 1.70 at 589 nm. According to the manufacturer's description, the accuracy is 70.0001 at 20 1C. In this work, small volumes of about 1 mL of the liquid to be measured were placed on the surface of the sapphire prism. All measurements were carried out according to the following sequence: – Cleaning of the measurement chamber and the sapphire surface – Measurement with distilled water to ensure sufficient cleaning – Cleaning and drying of the prism surface – Placing the sample to be measured on the surface
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– After waiting period of several minutes to ensure stable and homogeneous temperature distribution, the automated measurement is started with 5 repetitions at each of the 9 wavelengths. A waiting period of two minutes between the measurements was adjusted using the instrument software. In this way, several commercial LS cocktails as well as the carbon dioxide absorber CarboSorb E were measured at 16 1C, 18 1C, 20 1C and 22 1C. Information on these liquids is summarized in Table 1. In addition, some measurements were carried out using mixtures of scintillation cocktail and water. We define the ratio Vr ¼ Vwater/Vscintillator, where Vwater and Vscintillator are the volumes of water and the scintillator, respectively. LS samples with Vr ¼ 0.067, Vr ¼0.033 and Vr ¼0.010 were prepared using the scintillators Ultima Gold and AquaLight, respectively. The ratio Vr ¼ 0.067 is also obtained when mixing 15 mL of a scintillator and 1 mL of water. Such a sample composition is frequently used at PTB. In addition, the influence of the chemical quenching agent nitromethane was studied. To this end, a mixture of nitromethane and pseudocumene (1:1 by volume) was prepared and small amounts of this quenching agent were added to pure Ultima Gold as well as to a mixture of Ultima Gold and water with Vr ¼0.067. All liquids were stored at a room temperature of about 20 1C in a dark place prior to the measurements. The measurements with distilled water were also used as a validation of the experiment in a manner similar to that conducted Table 2 Measured refractive index of distilled water at 20 1C as measured in this work compared to reference values, nref(λ), which were calculated according to Thormählen et al. (1985) for 19.993 1C. λ in nm
n(λ) this work
nref(λ)
n(λ)–nref(λ)
404.7 435.8 486.1 546.1 587.6 589.3 632.8 656.3 706.5
1.3428 1.3403 1.3372 1.3345 1.3331 1.3330 1.3318 1.3312 1.3301
1.3432 1.3408 1.3377 1.3350 1.3335 1.3334 1.3321 1.3314 1.3301
0.0004 0.0005 0.0005 0.0005 0.0004 0.0004 0.0003 0.0002 0.0001
Table 1 LS cocktails and CarboSorb E used in this study. No.
Name of LS cocktail
Solvent
Expiry date
LOT no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Ultima Gold Ultima Gold F Ultima Gold LLT Ultima Gold AB Hionic Fluor CarboSorb E Permafluor E þ Permafluor E þ High Efficiency Mineral Oil Scintillator Insta-Gel Plus OptiPhase 'HiSafe' 2 OptiPhase 'HiSafe' 3 Ultima Gold Ultima Gold AB Ultima Gold F Ultima Gold LLT Ultima Gold XR Insta-Gel Plus Hionic Fluor AquaLight MaxiLight Ultima Gold MV
DIN DIN DIN DIN Pseudocumene – Pseudocumene Pseudocumene White mineral oil Pseudocumene DIN DIN DIN DIN DIN DIN DIN Pseudocumene Pseudocumene DIN DIN DIN
January 2011 July 2010 June 2014 May 2006 April 2007 June 2010 July 2012 March 2014 March 2013 December 2008 Unknown 2003 July 2014 January 2015 January 2014 June 2014 May 2014 June 2015 April 2015 2014 2014 August 2015
77-090601 78-090601 97-1121 91-050501 55-040901 99-071102 90-091201 90-11311 137-100801 95-060501 1222836 0416733 77-12501 91-12241 78-12491 97-12471 79-12441 95-12451 55-12361 Unknown Unknown 80–13051
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K. Kossert / Applied Radiation and Isotopes 82 (2013) 382–388
by Yeo et al. (2010). The measured data were compared with reference values from Daimon and Masumura (2007) and Thormählen et al. (1985), respectively. Table 2 shows some results at 20 1C compared to reference values. The differences to the reference values were 0.0005 or lower. Similar differences were observed in repetition measurements which were carried out prior to the measurements of the scintillators. If the differences were larger, a further cleaning of the surface of the refractometer was carried out and the check measurements with distilled water were repeated. The maximum difference of 0.0005 from Table 2 was also taken as the uncertainty component ascribed to the instrument accuracy, which is five times larger than the accuracy stated by the manufacturer. Further uncertainty components for the display accuracy (7 0.00001) and the limited reproducibility (70.00030) were taken into account. The latter component was evaluated by comparing results of several repetition measurements for a given scintillator. Finally, the standard deviation of not
less than 5 repetition measurements was taken into account. For pure scintillators, this standard deviation was found to be (70.00010) or lower. To simplify the uncertainty evaluation, the maximum standard deviation was adopted for all results, yielding an overall uncertainty of 70.0006. For the mixtures of scintillator with water and/or nitromethane, the maximum standard deviation (7 0.00035) was found to be considerably larger than for pure scintillators and the overall uncertainty is evaluated to be 70.0007. The larger standard deviations are due to increasing refractive indices when repeating the measurements after a couple of minutes. This sample instability might be due to differences between the aqueous and the organic parts of the mixture. According to the refractometer manual, the wavelength accuracy is 7 2 nm and the temperature measuring accuracy is 70.1 1C. All uncertainties stated in this article are standard uncertainties with an expanding factor k ¼1, if not otherwise stated.
Table 3 Refractive indices measured at 16.0(1) 1C. The standard uncertainty for the refractive index is evaluated as 70.0006. No. (acc. to Table 1)
Cocktail
n(404.7 nm)
n(435.8 nm)
n(486.1 nm)
n(546.1 nm)
n(587.6 nm)
n(589.3 nm)
n(632.8 nm)
n(656.3 nm)
n(706.5 nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
UG UG F UG LLT UG AB HF CarboS PFEþ PFEþ MO IGþ HS2 HS3 UG UG AB UG F UG LLT UG XR IGþ HF AL ML UG MV
1.5800 1.6189 1.5706 1.5724 1.5170 1.4341 1.5251 1.5247 1.5052 1.5280 1.5864 1.5699 1.5804 1.5686 1.6190 1.5663 1.5652 1.5277 1.5127 1.5694 1.6187 1.5642
1.5690 1.6052 1.5601 1.5617 1.5100 1.4305 1.5169 1.5166 1.5001 1.5206 1.5749 1.5596 1.5694 1.5582 1.6053 1.5560 1.5553 1.5204 1.5061 1.5589 1.6050 1.5540
1.5571 1.5903 1.5486 1.5502 1.5021 1.4261 1.5078 1.5075 1.4942 1.5123 1.5623 1.5484 1.5574 1.5469 1.5904 1.5448 1.5445 1.5122 1.4985 1.5476 1.5902 1.5429
1.5479 1.5791 1.5399 1.5413 1.4958 1.4226 1.5007 1.5003 1.4894 1.5057 1.5527 1.5399 1.5483 1.5382 1.5791 1.5363 1.5362 1.5056 1.4925 1.5389 1.5789 1.5345
1.5435 1.5737 1.5357 1.5370 1.4927 1.4207 1.4971 1.4968 1.4869 1.5024 1.5481 1.5357 1.5439 1.5340 1.5737 1.5322 1.5321 1.5024 1.4894 1.5347 1.5735 1.5303
1.5434 1.5735 1.5356 1.5369 1.4926 1.4206 1.4970 1.4967 1.4868 1.5023 1.5480 1.5356 1.5437 1.5339 1.5735 1.5320 1.5320 1.5022 1.4893 1.5346 1.5733 1.5302
1.5398 1.5691 1.5321 1.5334 1.4900 1.4191 1.4941 1.4938 1.4848 1.4996 1.5442 1.5322 1.5401 1.5305 1.5692 1.5287 1.5287 1.4996 1.4868 1.5312 1.5689 1.5268
1.5381 1.5671 1.5306 1.5318 1.4888 1.4184 1.4928 1.4925 1.4839 1.4984 1.5425 1.5307 1.5385 1.5289 1.5672 1.5272 1.5272 1.4983 1.4857 1.5296 1.5669 1.5253
1.5353 1.5636 1.5278 1.5291 1.4867 1.4171 1.4904 1.4901 1.4822 1.4963 1.5395 1.5279 1.5356 1.5262 1.5636 1.5244 1.5245 1.4962 1.4837 1.5269 1.5634 1.5226
Table 4 Refractive indices measured at 18.0(1) 1C. The standard uncertainty for the refractive index is evaluated as 7 0.0006. No. (acc. to Table 1)
Cocktail
n(404.7 nm)
n(435.8 nm)
n(486.1 nm)
n(546.1 nm)
n(587.6 nm)
n(589.3 nm)
n(632.8 nm)
n(656.3 nm)
n(706.5 nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
UG UG F UG LLT UG AB HF CarboS PFEþ PFEþ MO IGþ HS2 HS3 UG UG AB UG F UG LLT UG XR IGþ HF AL ML UG MV
1.5791 1.6178 1.5697 1.5727 1.5162 1.4328 1.5240 1.5236 1.5044 1.5271 1.5854 1.5702 1.5782 1.5683 1.6182 1.5663 1.5643 1.5265 1.5118 1.5690 1.6178 1.5640
1.5682 1.6041 1.5592 1.5620 1.5092 1.4291 1.5159 1.5155 1.4993 1.5197 1.5740 1.5599 1.5673 1.5578 1.6045 1.5560 1.5544 1.5192 1.5052 1.5585 1.6041 1.5538
1.5562 1.5893 1.5478 1.5504 1.5013 1.4248 1.5068 1.5064 1.4934 1.5114 1.5614 1.5486 1.5553 1.5465 1.5897 1.5448 1.5436 1.5110 1.4976 1.5471 1.5892 1.5427
1.5471 1.5780 1.5392 1.5415 1.4950 1.4212 1.4996 1.4993 1.4886 1.5048 1.5519 1.5400 1.5463 1.5379 1.5784 1.5363 1.5353 1.5045 1.4916 1.5385 1.5780 1.5342
1.5427 1.5727 1.5349 1.5372 1.4919 1.4194 1.4961 1.4957 1.4861 1.5016 1.5473 1.5359 1.5419 1.5337 1.5730 1.5322 1.5313 1.5012 1.4885 1.5343 1.5726 1.5301
1.5425 1.5725 1.5348 1.5371 1.4918 1.4193 1.4960 1.4956 1.4860 1.5014 1.5471 1.5357 1.5417 1.5335 1.5728 1.5320 1.5311 1.5011 1.4884 1.5342 1.5724 1.5299
1.5390 1.5681 1.5314 1.5336 1.4892 1.4178 1.4931 1.4928 1.4840 1.4988 1.5434 1.5323 1.5382 1.5301 1.5684 1.5286 1.5278 1.4984 1.4860 1.5307 1.5680 1.5265
1.5373 1.5661 1.5298 1.5320 1.4881 1.4171 1.4918 1.4914 1.4831 1.4975 1.5416 1.5308 1.5365 1.5286 1.5664 1.5271 1.5264 1.4972 1.4848 1.5292 1.5660 1.5250
1.5345 1.5626 1.5271 1.5292 1.4860 1.4158 1.4894 1.4891 1.4814 1.4954 1.5386 1.5280 1.5337 1.5258 1.5629 1.5244 1.5237 1.4951 1.4828 1.5264 1.5625 1.5223
K. Kossert / Applied Radiation and Isotopes 82 (2013) 382–388
3. Results The results of the measurements carried out with pure scintillators at the four temperatures are summarized in Tables 3–6. Fig. 1 shows the refractive index n(435.8 nm) at 20 1C. The results were sorted in decreasing order starting with the highest result on the left side. Results are only shown for scintillators, i.e. CarboSorb was excluded in the figure. The scintillators, which are based on the solvent di-isopropylnaphthalene (DIN), have a higher refractive index as those using pseudocumene. The largest values were found for the DIN-based cocktail Ultima Gold F, which does not contain any surfactant since it is designed for measurements of organic liquids and dry filters. Almost the same high refractive index was found for MaxiLight which is also used for non-aqueous samples. For the mineral oil scintillator, the refractive index was found to be slightly lower than for the pseudocumene-based
385
cocktails. For some LS cocktails, two different batches were included in this study, and, in most cases, the corresponding results appear as direct neighbours in Fig. 1. The results of the two batches of Ultima Gold F are in excellent agreement and the same is true for the samples of Ultima Gold, Insta-Gel Plus and Permafluor Eþ . It should be noted that the results are often not exactly the same but the measured refractive indices agree within the uncertainties. The situation is different for Ultima Gold AB, Ultima Gold LLT and Hionic Fluor, for which significant differences between the two batches were found. The origin of these differences is not clear. It might be that the differences are due to changes in the cocktail composition or the production process. However, it is more likely that the differences are simply due to aging effects of the respective liquids which also change other characteristics of a scintillator, such as its colour or the maximum achievable counting efficiency. It should be noted that some of the
Table 5 Refractive indices measured at 20.0(1) 1C. The standard uncertainty for the refractive index is evaluated as 7 0.0006. No. (acc. to Table 1)
Cocktail
n(404.7 nm)
n(435.8 nm)
n(486.1 nm)
n(546.1 nm)
n(587.6 nm)
n(589.3 nm)
n(632.8 nm)
n(656.3 nm)
n(706.5 nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
UG UG F UG LLT UG AB HF CarboS PFEþ PFEþ MO IGþ HS2 HS3 UG UG AB UG F UG LLT UG XR IGþ HF AL ML UG MV
1.5791 1.6169 1.5695 1.5718 1.5156 1.4311 1.5226 1.5221 1.5035 1.5264 1.5845 1.5694 1.5787 1.5684 1.6171 1.5660 1.5634 1.5263 1.5107 1.5680 1.6168 1.5632
1.5681 1.6032 1.5590 1.5611 1.5087 1.4275 1.5145 1.5140 1.4985 1.5190 1.5731 1.5591 1.5677 1.5580 1.6034 1.5557 1.5536 1.5189 1.5041 1.5576 1.6031 1.5530
1.5561 1.5884 1.5476 1.5495 1.5008 1.4231 1.5054 1.5050 1.4926 1.5107 1.5605 1.5478 1.5557 1.5466 1.5886 1.5444 1.5428 1.5107 1.4965 1.5462 1.5883 1.5419
1.5470 1.5771 1.5389 1.5406 1.4945 1.4196 1.4983 1.4979 1.4878 1.5042 1.5510 1.5393 1.5466 1.5379 1.5773 1.5359 1.5345 1.5041 1.4905 1.5376 1.5771 1.5334
1.5425 1.5718 1.5347 1.5363 1.4913 1.4178 1.4948 1.4944 1.4853 1.5009 1.5464 1.5351 1.5422 1.5337 1.5719 1.5318 1.5305 1.5009 1.4875 1.5334 1.5717 1.5293
1.5424 1.5716 1.5345 1.5362 1.4912 1.4177 1.4947 1.4943 1.4852 1.5008 1.5462 1.5349 1.5420 1.5336 1.5717 1.5316 1.5303 1.5007 1.4874 1.5333 1.5715 1.5291
1.5388 1.5672 1.5311 1.5327 1.4887 1.4162 1.4918 1.4914 1.4832 1.4981 1.5425 1.5316 1.5385 1.5301 1.5674 1.5283 1.5270 1.4981 1.4849 1.5299 1.5671 1.5258
1.5372 1.5652 1.5295 1.5311 1.4875 1.4155 1.4905 1.4901 1.4823 1.4969 1.5408 1.5300 1.5368 1.5286 1.5654 1.5267 1.5255 1.4969 1.4838 1.5283 1.5651 1.5242
1.5343 1.5617 1.5268 1.5283 1.4854 1.4142 1.4882 1.4878 1.4806 1.4948 1.5378 1.5273 1.5339 1.5258 1.5619 1.5240 1.5229 1.4947 1.4818 1.5256 1.5616 1.5215
Table 6 Refractive indices measured at 22.0(1) 1C. The standard uncertainty for the refractive index is evaluated as 7 0.0006. No. (acc. to Table 1)
Cocktail
n(404.7 nm)
n(435.8 nm)
n(486.1 nm)
n(546.1 nm)
n(587.6 nm)
n(589.3 nm)
n(632.8 nm)
n(656.3 nm)
n(706.5 nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
UG UG F UG LLT UG AB HF CarboS PFEþ PFEþ MO IGþ HS2 HS3 UG UG AB UG F UG LLT UG XR IGþ HF AL ML UG MV
1.5776 1.6160 1.5686 1.5708 1.5146 1.4306 1.5219 1.5213 1.5026 1.5253 1.5836 1.5685 1.5777 1.5675 1.6162 1.5650 1.5625 1.5251 1.5102 1.5675 1.6158 1.5625
1.5666 1.6023 1.5581 1.5602 1.5077 1.4269 1.5138 1.5133 1.4976 1.5180 1.5722 1.5582 1.5668 1.5571 1.6025 1.5548 1.5527 1.5179 1.5036 1.5571 1.6022 1.5523
1.5547 1.5875 1.5467 1.5486 1.4998 1.4226 1.5048 1.5042 1.4918 1.5097 1.5596 1.5469 1.5548 1.5457 1.5877 1.5436 1.5419 1.5097 1.4960 1.5458 1.5874 1.5412
1.5456 1.5763 1.5380 1.5397 1.4935 1.4191 1.4976 1.4971 1.4869 1.5031 1.5501 1.5384 1.5457 1.5371 1.5764 1.5351 1.5337 1.5031 1.4900 1.5371 1.5762 1.5327
1.5412 1.5709 1.5338 1.5355 1.4904 1.4172 1.4941 1.4936 1.4845 1.4999 1.5455 1.5342 1.5413 1.5329 1.5711 1.5309 1.5296 1.4999 1.4870 1.5329 1.5708 1.5286
1.5410 1.5707 1.5336 1.5353 1.4903 1.4172 1.4940 1.4935 1.4844 1.4998 1.5454 1.5341 1.5412 1.5327 1.5709 1.5308 1.5295 1.4998 1.4869 1.5328 1.5706 1.5284
1.5375 1.5664 1.5302 1.5318 1.4877 1.4156 1.4912 1.4906 1.4824 1.4971 1.5416 1.5307 1.5376 1.5293 1.5665 1.5274 1.5262 1.4971 1.4844 1.5294 1.5662 1.5251
1.5358 1.5644 1.5287 1.5303 1.4865 1.4150 1.4898 1.4893 1.4815 1.4959 1.5399 1.5292 1.5360 1.5278 1.5645 1.5259 1.5247 1.4959 1.4833 1.5278 1.5643 1.5236
1.5329 1.5609 1.5259 1.5274 1.4845 1.4137 1.4875 1.4870 1.4798 1.4938 1.5369 1.5264 1.5331 1.5250 1.5610 1.5232 1.5221 1.4938 1.4813 1.5251 1.5607 1.5208
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analyzed batches were already rather old, and, in some cases, the expiry date had passed some years prior to the measurements described in this study. Fig. 2 shows the refractive indices at the nine different wavelengths for selected cocktails measured at 20 1C. The data were taken from Table 5 and show the anticipated wavelength dependence with decreasing refractive index for increasing wavelength. Uncertainty bars are not marked since they would be smaller than the shown symbols. A comparison of Tables 3–6 also reveals a small temperature dependence. On average, the refractive index decreases by 0.0008 when the temperature is increased by 2 1C. However, the situation depends also on the scintillation cocktail. In rare cases, the value increases with increasing temperatures, but these changes are not significant. The measured refractive indices of mixtures of the scintillators Ultima Gold or AquaLight with distilled water are shown in Tables 7 and 8 for temperatures of 20 1C and 22 1C, respectively. As expected, the refractive index decreases when increasing Vr. This makes clear that the aqueous portion must also be considered for accurate Monte Carlo simulations. For low water content, the refractive index can be estimated using a weighted mean, i.e. using nmix;calc ¼
nwater V water þ nscintillator V scintillator : V water þ V scintillator
without any quenching agent. As expected, these results are almost identical to the corresponding results for pure Ultima Gold in Table 5. Finally, Table 10 lists the measurement results for mixtures of Ultima Gold, water and the above-mentioned quenching agent. Again, the first data row was obtained without the quenching agent and the results are in excellent agreement with the first data row in Table 7.
4. Summary and conclusions The refractive indices of several commercial LS cocktails were measured at nine different wavelengths in the visible region. In addition, the temperature dependence was investigated and several mixtures with water and a quenching agent were included in this study. The data can be used for Monte Carlo simulations as well as for the design of other experiments. The method described here can also be used to measure other LS cocktails and mixtures. The sensitivity of most photomultiplier tubes used in LS and Čerenkov counting covers part of the UV region and, thus, it would
ð1Þ
When using Eq. (1) for Vr ¼0.010, the differences between the measured values from Tables 7 and 8 and the calculated refractive indices nmeas nmix_calc are 0.0008 or lower. For Vr ¼0.033, the highest difference was found to be nmeas nmix_calc ¼0.0028, and a similar maximum value of nmeas nmix_calc ¼0.0031 was obtained for Vr ¼ 0.0067. Thus, Eq. (1) can only be considered as a rough approximation and it is recommended to use the experimentally determined values. The results of Ultima Gold with varying amounts of the quenching agent are summarized in Table 9. The quenching agent was prepared by mixing equal volumes of nitromethane and pseudocumene. Here, too, the refractive index decreases with increasing amount of the quenching agent. It is to be noted that the first data row in Table 9 corresponds to a measurement
Fig. 2. Wavelength-dependent refractive index of selected scintillation cocktails measured at 20.0(1) 1C. The numbers in parentheses in the legend correspond to the numbers in Table 1.
Fig. 1. Refractive index at a wavelength of 435.8 nm for the scintillation cocktails measured at 20.0(1) 1C. The uncertainty bars indicate expanded standard uncertainties with an expanding factor k ¼2. The numbers in parentheses correspond to numbers in Table 1.
K. Kossert / Applied Radiation and Isotopes 82 (2013) 382–388
387
Table 7 Measured refractive indices at 20.0(1) 1C of mixtures of the scintillators Ultima Gold (No. 13 from Table 1) or AquaLight (No. 20) and water using different ratios Vr ¼ Vwater/ Vscintillator. The standard uncertainty for the refractive index is evaluated to be 7 0.0007. Vr
Scintillator
n(404.7 nm)
n(435.8 nm)
n(486.1 nm)
n(546.1 nm)
n(587.6 nm)
n(589.3 nm)
n(632.8 nm)
n(656.3 nm)
n(706.5 nm)
0.067 0.033 0.010 0.067 0.033 0.010
UG UG UG AL AL AL
1.5670 1.5732 1.5767 1.5566 1.5625 1.5666
1.5565 1.5625 1.5658 1.5466 1.5523 1.5562
1.5450 1.5508 1.5539 1.5358 1.5412 1.5449
1.5363 1.5419 1.5449 1.5275 1.5328 1.5364
1.5320 1.5375 1.5405 1.5235 1.5287 1.5322
1.5319 1.5374 1.5404 1.5234 1.5285 1.5321
1.5284 1.5339 1.5368 1.5201 1.5252 1.5287
1.5269 1.5323 1.5352 1.5186 1.5237 1.5271
1.5241 1.5294 1.5323 1.5160 1.5210 1.5244
Table 8 Measured refractive indices at 22.0(1) 1C of mixtures with the scintillators Ultima Gold (No. 13 from Table 1) and AquaLight (No. 20) and water using different ratios Vr ¼Vwater/Vscintillator. The standard uncertainty for the refractive index is evaluated to be 7 0.0007. Vr
Scintillator
n(404.7 nm)
n(435.8 nm)
n(486.1 nm)
n(546.1 nm)
n(587.6 nm)
n(589.3 nm)
n(632.8 nm)
n(656.3 nm)
n(706.5 nm)
0.067 0.033 0.010 0.067 0.033 0.010
UG UG UG AL AL AL
1.5654 1.5729 1.5760 1.5558 1.5613 1.5657
1.5549 1.5622 1.5651 1.5459 1.5512 1.5553
1.5435 1.5504 1.5532 1.5351 1.5402 1.5441
1.5348 1.5415 1.5442 1.5269 1.5318 1.5355
1.5306 1.5372 1.5398 1.5228 1.5276 1.5313
1.5304 1.5370 1.5397 1.5227 1.5275 1.5312
1.5270 1.5335 1.5361 1.5194 1.5242 1.5278
1.5254 1.5319 1.5345 1.5179 1.5227 1.5263
1.5227 1.5291 1.5317 1.5153 1.5200 1.5235
Table 9 Measured refractive indices at 20.0(1) 1C of mixtures with Ultima Gold scintillator (No. 13 from Table 1) and varying amounts of a quenching agent (1:1 nitromethanepseudocumene mixture). The standard uncertainty for the refractive index is evaluated to be 70.0007. Vquench in μL per 3 mL UG
n(404.7 nm)
n(435.8 nm)
n(486.1 nm)
n(546.1 nm)
n(587.6 nm)
n(589.3 nm)
n(632.8 nm)
n(656.3 nm)
n(706.5 nm)
0 20 100 200
1.5786 1.5775 1.5730 1.5676
1.5677 1.5666 1.5622 1.5570
1.5557 1.5546 1.5504 1.5454
1.5466 1.5456 1.5415 1.5366
1.5422 1.5412 1.5371 1.5323
1.5420 1.5410 1.5370 1.5322
1.5385 1.5374 1.5335 1.5287
1.5368 1.5358 1.5319 1.5271
1.5339 1.5329 1.5290 1.5243
Table 10 Measured refractive indices at 20.0(1) 1C of mixtures with Ultima Gold scintillator (No. 13 from Table 1) plus water (Vr ¼ 0.067) and varying amounts of a quenching agent (1:1 nitromethane-pseudocumene mixture). The standard uncertainty for the refractive index is evaluated to be 7 0.0007. Vquench in μL per 3 mL UGþ 0.2 mL H2O n(404.7 nm) n(435.8 nm) n(486.1 nm) n(546.1 nm) n(587.6 nm) n(589.3 nm) n(632.8 nm) n(656.3 nm) n(706.5 nm) 0 20 100 200
1.5670 1.5662 1.5626 1.5590
1.5565 1.5557 1.5523 1.5488
1.5450 1.5442 1.5410 1.5376
be desirable to extend the measurement range towards lower wavelengths. However, the maximum emission of most commercial LS cocktails is above 400 nm due to the usage of secondary fluors like POPOP, dimethyl-POPOP and bis-MSB. Hence, the refractive indices in the visible region as measured in this work are indispensable.
Acknowledgements The author wishes to thank Ms. Katrin Kalitschke for her valuable support during the measurements, and Hidex Oy for providing some LS cocktails which were used for the measurements. References Agostinelli, S., Allison, J., Amako, K., Apostolakis, J., Araujo, H., Arce, P., Asai, M., Axen, D., Banerjee, S., Barrand, G., Behner, F., Bellagamba, L., Boudreau, J., Broglia, L., Brunengo, A., Burkhardt, H., Chauvie, S., Chuma, J., Chytracek, R., Cooperman, G., 2003. GEANT4—a simulation toolkit. Nucl. Instrum. Methods Phys. Res. Sect. A 506, 250–303.
1.5363 1.5356 1.5324 1.5291
1.5320 1.5313 1.5282 1.5250
1.5319 1.5312 1.5280 1.5248
1.5284 1.5277 1.5246 1.5215
1.5269 1.5262 1.5231 1.5199
1.5240 1.5234 1.5203 1.5172
Bergeron, D.E., 2012. Determination of micelle size in some commercial liquid scintillation cocktails. Appl. Radiat. Isot 70, 2164–2169. Bobin, C., Thiam, C., Chauvenet, B., Bouchard, J., 2012. On the stochastic dependence between photomultipliers in the TDCR method. Appl. Radiat. Isot. 70, 770–780. Broda, R., Cassette, P., Kossert, K., 2007. Radionuclide metrology using LS counting. Metrologia 44, S36–S52. Daimon, M, Masumura, A., 2007. Measurement of the refractive index of distilled water from near-infrared to the ultraviolet region. Appl. Opt. 46, 3811–3820. Grau Carles, A., 2007. MICELLE, the micelle size effect on the LS counting efficiency. Comput. Phys. Commun. 176, 305–317. Hurtado, S., Villa, M., Manjon, G., Garcia-Tenorio, R., 2009. Monte Carlo simulation of a liquid scintillation counter using GEANT4 code. LSC2008, Advances in LS Spectrometry. In: Proceedings of the 2008 International Conference on LS Spectrometry, Davos, 2008, The University of Arizona, Tucson, Arizona, USA. Kossert, K., Grau Carles, A., 2010. Improved method for the calculation of the counting efficiency of electron-capture nuclides in liquid scintillation samples. Appl. Radiat. Isot. 68, 1482–1488. Rodríguez, R., Vargas, S., Fernández-Velasco, D.A., 1998. Reverse micelle systems composed of water, Triton X-100, and phospholipids in organic solvents—1. Phase boundary titrations and dynamic light scattering analysis. J. Colloid. Interface. Sci. 197, 21–28. Schmidt þHaensch, 2010. User manual for Automatic critical-angle DispersionsRefractometer DSR-λ. SCHMIDT þ HAENSCH GmbH & Co, Berlin, Germany.
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Tarancόn Sanz, A., Kossert, K., 2011. Application of a free parameter model to plastic scintillation samples. Nucl. Instrum. Methods Phys. Res., Sect. A 648, 124–131. Thiam, C., Bobin, C., Chauvenet, B., Bouchard, J., 2012. Application of TDCR-Geant4 modeling to standardization of 63Ni. Appl. Radiat. Isot. 70, 2195–2199. Thormählen, I., Straub, J., Griguli, U., 1985. Refractive index of water and its dependence on wavelength, temperature and density. J. Phys. Chem. Ref. Data 14, 933–945.
Yeo, I.S., Jang, J.H., Kim, M.S., Ahn, J.K., Baik, S.R., Choi, E.I., Choi, Seonho, Choi, Y., Jang, H.I., Jang, J.S., Jeon, E.J., Jeong, I.S., Joo, K.K., Kim, B.C., Kim, H.S., Kim, J.Y., Kim, S.B., Kim, S.H., Kim, W., Kim, Y.D., Lee, J., Lim, I.T., Ma, K.J., Oh, Y.D., Pac, M. Y., Park, I.G., Park, J.S., Park, K.S., Shin, J.W., Stepanyan, S.S., Yang, B.S., Yu, I., 2010. Measurement of the refractive index of the LAB-based liquid scintillator and acrylic at RENO. Phys. Scr. 82 (065705), 5.
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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Measurement of wavelength-dependent refractive indices of liquid scintillation cocktails Karsten Kossert n Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany
H I G H L I G H T S
Refractive indices of several liquid scintillation cocktails were measured. The wavelengths cover a range from 404.7 nm to 706.5 nm. Measurements were carried out at 16 1C, 18 1C, 20 1C and 22 1C. For some cocktails, mixtures with water or nitromethane were studied.
art ic l e i nf o
a b s t r a c t
Article history: Received 7 August 2013 Received in revised form 28 September 2013 Accepted 10 October 2013 Available online 18 October 2013
Refractive indices of several commercial liquid scintillation cocktails were measured by means of an automatic critical-angle dispersion refractometer in the wavelength range from 404.7 nm to 706.5 nm. The results are needed for various applications. In particular, detailed Monte Carlo simulations of liquid scintillation counters that include the computation of optical light require these data. In addition, the refractive index is an important parameter for studies of micelle sizes by means of dynamic light scattering. In this work, the refractive indices were determined for Ultima Gold™, Ultima Gold™ F, Ultima Gold™ LLT, Ultima Gold™ AB, Hionic Fluor™, Permafluors E þ, Mineral Oil Scintillator, Insta-Gel Plus, OptiPhase HiSafe 2, OptiPhase HiSafe 3, Ultima Gold™ XR, Insta-Gel Plus, AquaLight, MaxiLight and Ultima Gold™ MV at 16 1C, 18 1C, 20 1C and 22 1C. The carbon dioxide absorber Carbo-Sorbs E was also analyzed. For some scintillators, various batches were compared and mixtures with water or nitromethane were studied. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Refractive index measurement Liquid scintillation cocktail Dispersion
1. Introduction Liquid scintillation (LS) counting is widely used for activity determination and can be considered as an active research field (Broda et al., 2007). Some researchers are developing Monte Carlo simulations which comprise the modelling of the scintillation photons (Hurtado et al., 2009; Bobin et al., 2012; Thiam et al., 2012). Such simulations require excellent knowledge of material parameters since the transmission, reflection and refraction of light is emulated using software packages, like the GEANT4 tool (Agostinelli et al., 2003). The refractive index of scintillation cocktails belongs to these parameters. For accurate simulations, it must be taken into account that the refractive index depends on the wavelength of light. When the emitted light is not monochromatic, as in the case of Čerenkov light, dispersion plays an important role.
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In many cases, LS counting is used to measure aqueous radioactive solutions. Thus, the cocktails usually contain a surfactant which ensures the miscibility of the organic and the aqueous phases. The radioactivity usually remains within the aqueous phase which is embedded in the organic phases in the form of (reverse) micelles. Electrons which are ejected as a result of a radioactive decay may lose a part of their kinetic energy and, consequently, only a reduced energy remains to create scintillation light (Grau Carles, 2007). The effects are very small in LS samples with low water content (Kossert and Grau Carles, 2010; Bergeron, 2012), but they might increase when using gel samples with high water content. Tarancόn Sanz and Kossert (2011) have also shown that the general idea of subtracting the energy loss in the aqueous phase works well when using plastic scintillation beads in an aqueous radioactive medium. The energy loss within the micelles and the corresponding reduction of the counting efficiency are referred to as the micelle size effect. For accurate studies of this effect, it is crucial to know the dimensions of micelles. Rodríguez et al. (1998) and Bergeron (2012) have shown that the size of
K. Kossert / Applied Radiation and Isotopes 82 (2013) 382–388
micelles can be evaluated by applying the Stokes–Einstein equation. To this end, the viscosity and the diffusion coefficient are required. The latter can be measured by means of dynamic light scattering (DLS). This measurement technique, however, also requires information about the refractive index of the samples under study. Thus, mixtures of LS cocktails and water as well as mixtures of cocktails and the quenching agent nitromethane (CH3NO2) were included in this study. These measurements are of course also important since most activity measurements are carried out with corresponding mixtures.
2. Description of measurements The measurements were carried out with an automated multiwavelength refractometer DSR-λ (Schmidt þHaensch, 2010) between 17 January and 13 June 2013. According to the manufacturer's manual, the instrument can be used to measure the “refractive index of liquid media independent of opacity, viscosity and colour”. The refractive index is determined via the measurement of the critical angle for total reflection. To this end, the position of the dark line is determined by means of a high resolution CCD linear array with 2048 elements. Seven LEDs are part of the system, allowing the sequential measurements at different wavelengths in the range from 404.7 nm to 706.5 nm. The system comprises a heater as well as a Peltier cooler and can be used for measurements in a temperature range from 10 1C to 80 1C. The system used in this work allows measurements of the refractive index in a range from 1.33 to 1.70 at 589 nm. According to the manufacturer's description, the accuracy is 70.0001 at 20 1C. In this work, small volumes of about 1 mL of the liquid to be measured were placed on the surface of the sapphire prism. All measurements were carried out according to the following sequence: – Cleaning of the measurement chamber and the sapphire surface – Measurement with distilled water to ensure sufficient cleaning – Cleaning and drying of the prism surface – Placing the sample to be measured on the surface
383
– After waiting period of several minutes to ensure stable and homogeneous temperature distribution, the automated measurement is started with 5 repetitions at each of the 9 wavelengths. A waiting period of two minutes between the measurements was adjusted using the instrument software. In this way, several commercial LS cocktails as well as the carbon dioxide absorber CarboSorb E were measured at 16 1C, 18 1C, 20 1C and 22 1C. Information on these liquids is summarized in Table 1. In addition, some measurements were carried out using mixtures of scintillation cocktail and water. We define the ratio Vr ¼ Vwater/Vscintillator, where Vwater and Vscintillator are the volumes of water and the scintillator, respectively. LS samples with Vr ¼ 0.067, Vr ¼0.033 and Vr ¼0.010 were prepared using the scintillators Ultima Gold and AquaLight, respectively. The ratio Vr ¼ 0.067 is also obtained when mixing 15 mL of a scintillator and 1 mL of water. Such a sample composition is frequently used at PTB. In addition, the influence of the chemical quenching agent nitromethane was studied. To this end, a mixture of nitromethane and pseudocumene (1:1 by volume) was prepared and small amounts of this quenching agent were added to pure Ultima Gold as well as to a mixture of Ultima Gold and water with Vr ¼0.067. All liquids were stored at a room temperature of about 20 1C in a dark place prior to the measurements. The measurements with distilled water were also used as a validation of the experiment in a manner similar to that conducted Table 2 Measured refractive index of distilled water at 20 1C as measured in this work compared to reference values, nref(λ), which were calculated according to Thormählen et al. (1985) for 19.993 1C. λ in nm
n(λ) this work
nref(λ)
n(λ)–nref(λ)
404.7 435.8 486.1 546.1 587.6 589.3 632.8 656.3 706.5
1.3428 1.3403 1.3372 1.3345 1.3331 1.3330 1.3318 1.3312 1.3301
1.3432 1.3408 1.3377 1.3350 1.3335 1.3334 1.3321 1.3314 1.3301
0.0004 0.0005 0.0005 0.0005 0.0004 0.0004 0.0003 0.0002 0.0001
Table 1 LS cocktails and CarboSorb E used in this study. No.
Name of LS cocktail
Solvent
Expiry date
LOT no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Ultima Gold Ultima Gold F Ultima Gold LLT Ultima Gold AB Hionic Fluor CarboSorb E Permafluor E þ Permafluor E þ High Efficiency Mineral Oil Scintillator Insta-Gel Plus OptiPhase 'HiSafe' 2 OptiPhase 'HiSafe' 3 Ultima Gold Ultima Gold AB Ultima Gold F Ultima Gold LLT Ultima Gold XR Insta-Gel Plus Hionic Fluor AquaLight MaxiLight Ultima Gold MV
DIN DIN DIN DIN Pseudocumene – Pseudocumene Pseudocumene White mineral oil Pseudocumene DIN DIN DIN DIN DIN DIN DIN Pseudocumene Pseudocumene DIN DIN DIN
January 2011 July 2010 June 2014 May 2006 April 2007 June 2010 July 2012 March 2014 March 2013 December 2008 Unknown 2003 July 2014 January 2015 January 2014 June 2014 May 2014 June 2015 April 2015 2014 2014 August 2015
77-090601 78-090601 97-1121 91-050501 55-040901 99-071102 90-091201 90-11311 137-100801 95-060501 1222836 0416733 77-12501 91-12241 78-12491 97-12471 79-12441 95-12451 55-12361 Unknown Unknown 80–13051
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K. Kossert / Applied Radiation and Isotopes 82 (2013) 382–388
by Yeo et al. (2010). The measured data were compared with reference values from Daimon and Masumura (2007) and Thormählen et al. (1985), respectively. Table 2 shows some results at 20 1C compared to reference values. The differences to the reference values were 0.0005 or lower. Similar differences were observed in repetition measurements which were carried out prior to the measurements of the scintillators. If the differences were larger, a further cleaning of the surface of the refractometer was carried out and the check measurements with distilled water were repeated. The maximum difference of 0.0005 from Table 2 was also taken as the uncertainty component ascribed to the instrument accuracy, which is five times larger than the accuracy stated by the manufacturer. Further uncertainty components for the display accuracy (7 0.00001) and the limited reproducibility (70.00030) were taken into account. The latter component was evaluated by comparing results of several repetition measurements for a given scintillator. Finally, the standard deviation of not
less than 5 repetition measurements was taken into account. For pure scintillators, this standard deviation was found to be (70.00010) or lower. To simplify the uncertainty evaluation, the maximum standard deviation was adopted for all results, yielding an overall uncertainty of 70.0006. For the mixtures of scintillator with water and/or nitromethane, the maximum standard deviation (7 0.00035) was found to be considerably larger than for pure scintillators and the overall uncertainty is evaluated to be 70.0007. The larger standard deviations are due to increasing refractive indices when repeating the measurements after a couple of minutes. This sample instability might be due to differences between the aqueous and the organic parts of the mixture. According to the refractometer manual, the wavelength accuracy is 7 2 nm and the temperature measuring accuracy is 70.1 1C. All uncertainties stated in this article are standard uncertainties with an expanding factor k ¼1, if not otherwise stated.
Table 3 Refractive indices measured at 16.0(1) 1C. The standard uncertainty for the refractive index is evaluated as 70.0006. No. (acc. to Table 1)
Cocktail
n(404.7 nm)
n(435.8 nm)
n(486.1 nm)
n(546.1 nm)
n(587.6 nm)
n(589.3 nm)
n(632.8 nm)
n(656.3 nm)
n(706.5 nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
UG UG F UG LLT UG AB HF CarboS PFEþ PFEþ MO IGþ HS2 HS3 UG UG AB UG F UG LLT UG XR IGþ HF AL ML UG MV
1.5800 1.6189 1.5706 1.5724 1.5170 1.4341 1.5251 1.5247 1.5052 1.5280 1.5864 1.5699 1.5804 1.5686 1.6190 1.5663 1.5652 1.5277 1.5127 1.5694 1.6187 1.5642
1.5690 1.6052 1.5601 1.5617 1.5100 1.4305 1.5169 1.5166 1.5001 1.5206 1.5749 1.5596 1.5694 1.5582 1.6053 1.5560 1.5553 1.5204 1.5061 1.5589 1.6050 1.5540
1.5571 1.5903 1.5486 1.5502 1.5021 1.4261 1.5078 1.5075 1.4942 1.5123 1.5623 1.5484 1.5574 1.5469 1.5904 1.5448 1.5445 1.5122 1.4985 1.5476 1.5902 1.5429
1.5479 1.5791 1.5399 1.5413 1.4958 1.4226 1.5007 1.5003 1.4894 1.5057 1.5527 1.5399 1.5483 1.5382 1.5791 1.5363 1.5362 1.5056 1.4925 1.5389 1.5789 1.5345
1.5435 1.5737 1.5357 1.5370 1.4927 1.4207 1.4971 1.4968 1.4869 1.5024 1.5481 1.5357 1.5439 1.5340 1.5737 1.5322 1.5321 1.5024 1.4894 1.5347 1.5735 1.5303
1.5434 1.5735 1.5356 1.5369 1.4926 1.4206 1.4970 1.4967 1.4868 1.5023 1.5480 1.5356 1.5437 1.5339 1.5735 1.5320 1.5320 1.5022 1.4893 1.5346 1.5733 1.5302
1.5398 1.5691 1.5321 1.5334 1.4900 1.4191 1.4941 1.4938 1.4848 1.4996 1.5442 1.5322 1.5401 1.5305 1.5692 1.5287 1.5287 1.4996 1.4868 1.5312 1.5689 1.5268
1.5381 1.5671 1.5306 1.5318 1.4888 1.4184 1.4928 1.4925 1.4839 1.4984 1.5425 1.5307 1.5385 1.5289 1.5672 1.5272 1.5272 1.4983 1.4857 1.5296 1.5669 1.5253
1.5353 1.5636 1.5278 1.5291 1.4867 1.4171 1.4904 1.4901 1.4822 1.4963 1.5395 1.5279 1.5356 1.5262 1.5636 1.5244 1.5245 1.4962 1.4837 1.5269 1.5634 1.5226
Table 4 Refractive indices measured at 18.0(1) 1C. The standard uncertainty for the refractive index is evaluated as 7 0.0006. No. (acc. to Table 1)
Cocktail
n(404.7 nm)
n(435.8 nm)
n(486.1 nm)
n(546.1 nm)
n(587.6 nm)
n(589.3 nm)
n(632.8 nm)
n(656.3 nm)
n(706.5 nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
UG UG F UG LLT UG AB HF CarboS PFEþ PFEþ MO IGþ HS2 HS3 UG UG AB UG F UG LLT UG XR IGþ HF AL ML UG MV
1.5791 1.6178 1.5697 1.5727 1.5162 1.4328 1.5240 1.5236 1.5044 1.5271 1.5854 1.5702 1.5782 1.5683 1.6182 1.5663 1.5643 1.5265 1.5118 1.5690 1.6178 1.5640
1.5682 1.6041 1.5592 1.5620 1.5092 1.4291 1.5159 1.5155 1.4993 1.5197 1.5740 1.5599 1.5673 1.5578 1.6045 1.5560 1.5544 1.5192 1.5052 1.5585 1.6041 1.5538
1.5562 1.5893 1.5478 1.5504 1.5013 1.4248 1.5068 1.5064 1.4934 1.5114 1.5614 1.5486 1.5553 1.5465 1.5897 1.5448 1.5436 1.5110 1.4976 1.5471 1.5892 1.5427
1.5471 1.5780 1.5392 1.5415 1.4950 1.4212 1.4996 1.4993 1.4886 1.5048 1.5519 1.5400 1.5463 1.5379 1.5784 1.5363 1.5353 1.5045 1.4916 1.5385 1.5780 1.5342
1.5427 1.5727 1.5349 1.5372 1.4919 1.4194 1.4961 1.4957 1.4861 1.5016 1.5473 1.5359 1.5419 1.5337 1.5730 1.5322 1.5313 1.5012 1.4885 1.5343 1.5726 1.5301
1.5425 1.5725 1.5348 1.5371 1.4918 1.4193 1.4960 1.4956 1.4860 1.5014 1.5471 1.5357 1.5417 1.5335 1.5728 1.5320 1.5311 1.5011 1.4884 1.5342 1.5724 1.5299
1.5390 1.5681 1.5314 1.5336 1.4892 1.4178 1.4931 1.4928 1.4840 1.4988 1.5434 1.5323 1.5382 1.5301 1.5684 1.5286 1.5278 1.4984 1.4860 1.5307 1.5680 1.5265
1.5373 1.5661 1.5298 1.5320 1.4881 1.4171 1.4918 1.4914 1.4831 1.4975 1.5416 1.5308 1.5365 1.5286 1.5664 1.5271 1.5264 1.4972 1.4848 1.5292 1.5660 1.5250
1.5345 1.5626 1.5271 1.5292 1.4860 1.4158 1.4894 1.4891 1.4814 1.4954 1.5386 1.5280 1.5337 1.5258 1.5629 1.5244 1.5237 1.4951 1.4828 1.5264 1.5625 1.5223
K. Kossert / Applied Radiation and Isotopes 82 (2013) 382–388
3. Results The results of the measurements carried out with pure scintillators at the four temperatures are summarized in Tables 3–6. Fig. 1 shows the refractive index n(435.8 nm) at 20 1C. The results were sorted in decreasing order starting with the highest result on the left side. Results are only shown for scintillators, i.e. CarboSorb was excluded in the figure. The scintillators, which are based on the solvent di-isopropylnaphthalene (DIN), have a higher refractive index as those using pseudocumene. The largest values were found for the DIN-based cocktail Ultima Gold F, which does not contain any surfactant since it is designed for measurements of organic liquids and dry filters. Almost the same high refractive index was found for MaxiLight which is also used for non-aqueous samples. For the mineral oil scintillator, the refractive index was found to be slightly lower than for the pseudocumene-based
385
cocktails. For some LS cocktails, two different batches were included in this study, and, in most cases, the corresponding results appear as direct neighbours in Fig. 1. The results of the two batches of Ultima Gold F are in excellent agreement and the same is true for the samples of Ultima Gold, Insta-Gel Plus and Permafluor Eþ . It should be noted that the results are often not exactly the same but the measured refractive indices agree within the uncertainties. The situation is different for Ultima Gold AB, Ultima Gold LLT and Hionic Fluor, for which significant differences between the two batches were found. The origin of these differences is not clear. It might be that the differences are due to changes in the cocktail composition or the production process. However, it is more likely that the differences are simply due to aging effects of the respective liquids which also change other characteristics of a scintillator, such as its colour or the maximum achievable counting efficiency. It should be noted that some of the
Table 5 Refractive indices measured at 20.0(1) 1C. The standard uncertainty for the refractive index is evaluated as 7 0.0006. No. (acc. to Table 1)
Cocktail
n(404.7 nm)
n(435.8 nm)
n(486.1 nm)
n(546.1 nm)
n(587.6 nm)
n(589.3 nm)
n(632.8 nm)
n(656.3 nm)
n(706.5 nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
UG UG F UG LLT UG AB HF CarboS PFEþ PFEþ MO IGþ HS2 HS3 UG UG AB UG F UG LLT UG XR IGþ HF AL ML UG MV
1.5791 1.6169 1.5695 1.5718 1.5156 1.4311 1.5226 1.5221 1.5035 1.5264 1.5845 1.5694 1.5787 1.5684 1.6171 1.5660 1.5634 1.5263 1.5107 1.5680 1.6168 1.5632
1.5681 1.6032 1.5590 1.5611 1.5087 1.4275 1.5145 1.5140 1.4985 1.5190 1.5731 1.5591 1.5677 1.5580 1.6034 1.5557 1.5536 1.5189 1.5041 1.5576 1.6031 1.5530
1.5561 1.5884 1.5476 1.5495 1.5008 1.4231 1.5054 1.5050 1.4926 1.5107 1.5605 1.5478 1.5557 1.5466 1.5886 1.5444 1.5428 1.5107 1.4965 1.5462 1.5883 1.5419
1.5470 1.5771 1.5389 1.5406 1.4945 1.4196 1.4983 1.4979 1.4878 1.5042 1.5510 1.5393 1.5466 1.5379 1.5773 1.5359 1.5345 1.5041 1.4905 1.5376 1.5771 1.5334
1.5425 1.5718 1.5347 1.5363 1.4913 1.4178 1.4948 1.4944 1.4853 1.5009 1.5464 1.5351 1.5422 1.5337 1.5719 1.5318 1.5305 1.5009 1.4875 1.5334 1.5717 1.5293
1.5424 1.5716 1.5345 1.5362 1.4912 1.4177 1.4947 1.4943 1.4852 1.5008 1.5462 1.5349 1.5420 1.5336 1.5717 1.5316 1.5303 1.5007 1.4874 1.5333 1.5715 1.5291
1.5388 1.5672 1.5311 1.5327 1.4887 1.4162 1.4918 1.4914 1.4832 1.4981 1.5425 1.5316 1.5385 1.5301 1.5674 1.5283 1.5270 1.4981 1.4849 1.5299 1.5671 1.5258
1.5372 1.5652 1.5295 1.5311 1.4875 1.4155 1.4905 1.4901 1.4823 1.4969 1.5408 1.5300 1.5368 1.5286 1.5654 1.5267 1.5255 1.4969 1.4838 1.5283 1.5651 1.5242
1.5343 1.5617 1.5268 1.5283 1.4854 1.4142 1.4882 1.4878 1.4806 1.4948 1.5378 1.5273 1.5339 1.5258 1.5619 1.5240 1.5229 1.4947 1.4818 1.5256 1.5616 1.5215
Table 6 Refractive indices measured at 22.0(1) 1C. The standard uncertainty for the refractive index is evaluated as 7 0.0006. No. (acc. to Table 1)
Cocktail
n(404.7 nm)
n(435.8 nm)
n(486.1 nm)
n(546.1 nm)
n(587.6 nm)
n(589.3 nm)
n(632.8 nm)
n(656.3 nm)
n(706.5 nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
UG UG F UG LLT UG AB HF CarboS PFEþ PFEþ MO IGþ HS2 HS3 UG UG AB UG F UG LLT UG XR IGþ HF AL ML UG MV
1.5776 1.6160 1.5686 1.5708 1.5146 1.4306 1.5219 1.5213 1.5026 1.5253 1.5836 1.5685 1.5777 1.5675 1.6162 1.5650 1.5625 1.5251 1.5102 1.5675 1.6158 1.5625
1.5666 1.6023 1.5581 1.5602 1.5077 1.4269 1.5138 1.5133 1.4976 1.5180 1.5722 1.5582 1.5668 1.5571 1.6025 1.5548 1.5527 1.5179 1.5036 1.5571 1.6022 1.5523
1.5547 1.5875 1.5467 1.5486 1.4998 1.4226 1.5048 1.5042 1.4918 1.5097 1.5596 1.5469 1.5548 1.5457 1.5877 1.5436 1.5419 1.5097 1.4960 1.5458 1.5874 1.5412
1.5456 1.5763 1.5380 1.5397 1.4935 1.4191 1.4976 1.4971 1.4869 1.5031 1.5501 1.5384 1.5457 1.5371 1.5764 1.5351 1.5337 1.5031 1.4900 1.5371 1.5762 1.5327
1.5412 1.5709 1.5338 1.5355 1.4904 1.4172 1.4941 1.4936 1.4845 1.4999 1.5455 1.5342 1.5413 1.5329 1.5711 1.5309 1.5296 1.4999 1.4870 1.5329 1.5708 1.5286
1.5410 1.5707 1.5336 1.5353 1.4903 1.4172 1.4940 1.4935 1.4844 1.4998 1.5454 1.5341 1.5412 1.5327 1.5709 1.5308 1.5295 1.4998 1.4869 1.5328 1.5706 1.5284
1.5375 1.5664 1.5302 1.5318 1.4877 1.4156 1.4912 1.4906 1.4824 1.4971 1.5416 1.5307 1.5376 1.5293 1.5665 1.5274 1.5262 1.4971 1.4844 1.5294 1.5662 1.5251
1.5358 1.5644 1.5287 1.5303 1.4865 1.4150 1.4898 1.4893 1.4815 1.4959 1.5399 1.5292 1.5360 1.5278 1.5645 1.5259 1.5247 1.4959 1.4833 1.5278 1.5643 1.5236
1.5329 1.5609 1.5259 1.5274 1.4845 1.4137 1.4875 1.4870 1.4798 1.4938 1.5369 1.5264 1.5331 1.5250 1.5610 1.5232 1.5221 1.4938 1.4813 1.5251 1.5607 1.5208
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analyzed batches were already rather old, and, in some cases, the expiry date had passed some years prior to the measurements described in this study. Fig. 2 shows the refractive indices at the nine different wavelengths for selected cocktails measured at 20 1C. The data were taken from Table 5 and show the anticipated wavelength dependence with decreasing refractive index for increasing wavelength. Uncertainty bars are not marked since they would be smaller than the shown symbols. A comparison of Tables 3–6 also reveals a small temperature dependence. On average, the refractive index decreases by 0.0008 when the temperature is increased by 2 1C. However, the situation depends also on the scintillation cocktail. In rare cases, the value increases with increasing temperatures, but these changes are not significant. The measured refractive indices of mixtures of the scintillators Ultima Gold or AquaLight with distilled water are shown in Tables 7 and 8 for temperatures of 20 1C and 22 1C, respectively. As expected, the refractive index decreases when increasing Vr. This makes clear that the aqueous portion must also be considered for accurate Monte Carlo simulations. For low water content, the refractive index can be estimated using a weighted mean, i.e. using nmix;calc ¼
nwater V water þ nscintillator V scintillator : V water þ V scintillator
without any quenching agent. As expected, these results are almost identical to the corresponding results for pure Ultima Gold in Table 5. Finally, Table 10 lists the measurement results for mixtures of Ultima Gold, water and the above-mentioned quenching agent. Again, the first data row was obtained without the quenching agent and the results are in excellent agreement with the first data row in Table 7.
4. Summary and conclusions The refractive indices of several commercial LS cocktails were measured at nine different wavelengths in the visible region. In addition, the temperature dependence was investigated and several mixtures with water and a quenching agent were included in this study. The data can be used for Monte Carlo simulations as well as for the design of other experiments. The method described here can also be used to measure other LS cocktails and mixtures. The sensitivity of most photomultiplier tubes used in LS and Čerenkov counting covers part of the UV region and, thus, it would
ð1Þ
When using Eq. (1) for Vr ¼0.010, the differences between the measured values from Tables 7 and 8 and the calculated refractive indices nmeas nmix_calc are 0.0008 or lower. For Vr ¼0.033, the highest difference was found to be nmeas nmix_calc ¼0.0028, and a similar maximum value of nmeas nmix_calc ¼0.0031 was obtained for Vr ¼ 0.0067. Thus, Eq. (1) can only be considered as a rough approximation and it is recommended to use the experimentally determined values. The results of Ultima Gold with varying amounts of the quenching agent are summarized in Table 9. The quenching agent was prepared by mixing equal volumes of nitromethane and pseudocumene. Here, too, the refractive index decreases with increasing amount of the quenching agent. It is to be noted that the first data row in Table 9 corresponds to a measurement
Fig. 2. Wavelength-dependent refractive index of selected scintillation cocktails measured at 20.0(1) 1C. The numbers in parentheses in the legend correspond to the numbers in Table 1.
Fig. 1. Refractive index at a wavelength of 435.8 nm for the scintillation cocktails measured at 20.0(1) 1C. The uncertainty bars indicate expanded standard uncertainties with an expanding factor k ¼2. The numbers in parentheses correspond to numbers in Table 1.
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387
Table 7 Measured refractive indices at 20.0(1) 1C of mixtures of the scintillators Ultima Gold (No. 13 from Table 1) or AquaLight (No. 20) and water using different ratios Vr ¼ Vwater/ Vscintillator. The standard uncertainty for the refractive index is evaluated to be 7 0.0007. Vr
Scintillator
n(404.7 nm)
n(435.8 nm)
n(486.1 nm)
n(546.1 nm)
n(587.6 nm)
n(589.3 nm)
n(632.8 nm)
n(656.3 nm)
n(706.5 nm)
0.067 0.033 0.010 0.067 0.033 0.010
UG UG UG AL AL AL
1.5670 1.5732 1.5767 1.5566 1.5625 1.5666
1.5565 1.5625 1.5658 1.5466 1.5523 1.5562
1.5450 1.5508 1.5539 1.5358 1.5412 1.5449
1.5363 1.5419 1.5449 1.5275 1.5328 1.5364
1.5320 1.5375 1.5405 1.5235 1.5287 1.5322
1.5319 1.5374 1.5404 1.5234 1.5285 1.5321
1.5284 1.5339 1.5368 1.5201 1.5252 1.5287
1.5269 1.5323 1.5352 1.5186 1.5237 1.5271
1.5241 1.5294 1.5323 1.5160 1.5210 1.5244
Table 8 Measured refractive indices at 22.0(1) 1C of mixtures with the scintillators Ultima Gold (No. 13 from Table 1) and AquaLight (No. 20) and water using different ratios Vr ¼Vwater/Vscintillator. The standard uncertainty for the refractive index is evaluated to be 7 0.0007. Vr
Scintillator
n(404.7 nm)
n(435.8 nm)
n(486.1 nm)
n(546.1 nm)
n(587.6 nm)
n(589.3 nm)
n(632.8 nm)
n(656.3 nm)
n(706.5 nm)
0.067 0.033 0.010 0.067 0.033 0.010
UG UG UG AL AL AL
1.5654 1.5729 1.5760 1.5558 1.5613 1.5657
1.5549 1.5622 1.5651 1.5459 1.5512 1.5553
1.5435 1.5504 1.5532 1.5351 1.5402 1.5441
1.5348 1.5415 1.5442 1.5269 1.5318 1.5355
1.5306 1.5372 1.5398 1.5228 1.5276 1.5313
1.5304 1.5370 1.5397 1.5227 1.5275 1.5312
1.5270 1.5335 1.5361 1.5194 1.5242 1.5278
1.5254 1.5319 1.5345 1.5179 1.5227 1.5263
1.5227 1.5291 1.5317 1.5153 1.5200 1.5235
Table 9 Measured refractive indices at 20.0(1) 1C of mixtures with Ultima Gold scintillator (No. 13 from Table 1) and varying amounts of a quenching agent (1:1 nitromethanepseudocumene mixture). The standard uncertainty for the refractive index is evaluated to be 70.0007. Vquench in μL per 3 mL UG
n(404.7 nm)
n(435.8 nm)
n(486.1 nm)
n(546.1 nm)
n(587.6 nm)
n(589.3 nm)
n(632.8 nm)
n(656.3 nm)
n(706.5 nm)
0 20 100 200
1.5786 1.5775 1.5730 1.5676
1.5677 1.5666 1.5622 1.5570
1.5557 1.5546 1.5504 1.5454
1.5466 1.5456 1.5415 1.5366
1.5422 1.5412 1.5371 1.5323
1.5420 1.5410 1.5370 1.5322
1.5385 1.5374 1.5335 1.5287
1.5368 1.5358 1.5319 1.5271
1.5339 1.5329 1.5290 1.5243
Table 10 Measured refractive indices at 20.0(1) 1C of mixtures with Ultima Gold scintillator (No. 13 from Table 1) plus water (Vr ¼ 0.067) and varying amounts of a quenching agent (1:1 nitromethane-pseudocumene mixture). The standard uncertainty for the refractive index is evaluated to be 7 0.0007. Vquench in μL per 3 mL UGþ 0.2 mL H2O n(404.7 nm) n(435.8 nm) n(486.1 nm) n(546.1 nm) n(587.6 nm) n(589.3 nm) n(632.8 nm) n(656.3 nm) n(706.5 nm) 0 20 100 200
1.5670 1.5662 1.5626 1.5590
1.5565 1.5557 1.5523 1.5488
1.5450 1.5442 1.5410 1.5376
be desirable to extend the measurement range towards lower wavelengths. However, the maximum emission of most commercial LS cocktails is above 400 nm due to the usage of secondary fluors like POPOP, dimethyl-POPOP and bis-MSB. Hence, the refractive indices in the visible region as measured in this work are indispensable.
Acknowledgements The author wishes to thank Ms. Katrin Kalitschke for her valuable support during the measurements, and Hidex Oy for providing some LS cocktails which were used for the measurements. References Agostinelli, S., Allison, J., Amako, K., Apostolakis, J., Araujo, H., Arce, P., Asai, M., Axen, D., Banerjee, S., Barrand, G., Behner, F., Bellagamba, L., Boudreau, J., Broglia, L., Brunengo, A., Burkhardt, H., Chauvie, S., Chuma, J., Chytracek, R., Cooperman, G., 2003. GEANT4—a simulation toolkit. Nucl. Instrum. Methods Phys. Res. Sect. A 506, 250–303.
1.5363 1.5356 1.5324 1.5291
1.5320 1.5313 1.5282 1.5250
1.5319 1.5312 1.5280 1.5248
1.5284 1.5277 1.5246 1.5215
1.5269 1.5262 1.5231 1.5199
1.5240 1.5234 1.5203 1.5172
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