Measuring the 14C content in liquid scintillators

This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Title: Year: Version: Please cite the original version: All material supplied via JYX is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Measuring the 14C content in liquid scintillators Enqvist, T.; Barabanov, I. R.; Bezrukov, L. B.; Gangapshev, A. M.; Gavrilyuk, Y. M.; Grishina, V. Yu.; Gurentsov, V. I.; Hissa, J.; Joutsenvaara, J.; Kazalov, V. V.; Krokhaleva, S.; Kutuniva, J.; Kuusiniemi, P.; Kuzminov, V. V.; Kurlovich, A. S.; Loo, Kai; Lubsandorzhiev, B. K.; Lubsandorzhiev, S.; Morgalyuk, V. P.; Novikova, G. Y.; Pshukov, A. M.; Sinev, V. V.; Slupecki, Maciej; Trzaska, Wladyslaw; Umerov, Sh. I.; Veresnikova, A. V.; Virkaj arvi, A.; Yanovich, Y. A.; Zavarzina, V. P. Enqvist, T., Barabanov, I. R., Bezrukov, L. B., Gangapshev, A. M., Gavrilyuk, Y. M., Grishina, V. Y., Gurentsov, V. I., Hissa, J., Joutsenvaara, J., Kazalov, V.V., Krokhaleva, S., Kutuniva, J., Kuusiniemi, P., Kuzminov, V.V., Kurlovich, A. S., Loo, K., Lubsandorzhiev, B. K., Lubsandorzhiev, S., Morgalyuk, V. P., . . . Zavarzina, V. P. (2016). Measuring the 14C content in liquid scintillators. In N. Fornengo, M. Regis, & H.-S. Zechlin (Eds.), XIV International Conference on Topics in Astroparticle and Underground Physics (TAUP 2015) (Article 062018). Institute of Physics Publishing Ltd.. Journal of Physics: Conference Series, 718. https://doi.org/10.1088/17426596/718/6/062018 2016 This content has been downloaded from IOPscience. 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Ser. 718 062018 (http://iopscience.iop.org/1742-6596/718/6/062018) Home Search Collections Journals About Contact us My IOPscience Measuring the 14C content in liquid scintillators T Enqvist1, I R Barabanov2, L B Bezrukov2, A M Gangapshev2, Y M Gavrilyuk2, V Yu Grishina2, V I Gurentsov2, J Hissa1, J Joutsenvaara1, V V Kazalov2, S Krokhaleva2, J Kutuniva1, P Kuusiniemi1, V V Kuzminov2, A S Kurlovich2, K Loo3, B K Lubsandorzhiev2, S Lubsandorzhiev2, V P Morgalyuk2, G Y Novikova2, A M Pshukov2, V V Sinev2, M S lupecki3, W H Trzaska3, Sh I Umerov2, A V Veresnikova2, A Virkaj¨arvi1, Y A Yanovich2, V P Zavarzina2 1Oulu Southern Institute and Department of Physics, University of Oulu, Finland 2Institute for Nuclear Research, Russian Academy of Sciences, Moscow, Russia 3Department of Physics, University of Jyv¨askyl¨a, Finland E-mail: [email protected] Abstract. We are going to perform a series of measurements where the 14C/12C ratio will be measured from several liquid scintillator samples with a dedicated setup. The setup is designed with the aim of measuring ratios smaller than 10−18. Measurements take place in two underground laboratories: in the Baksan Neutrino Observatory, Russia and in the Pyh¨asalmi mine, Finland. In Baksan the measurements started in 2015 and in Pyh¨asalmi they start in the beginning of 2015. In order to fully understand the operation of the setup and its background contributions a development of simulation packages has also been started. Low-energy neutrino detection with a liquid scintillator requires that the intrinsic 14 C content in the liquid is extremely low. In the Borexino CTF detector at Gran Sasso, Italy the 14C/12C ratio of 2 ×10−18 has been achieved being the lowest 14 C concentration ever measured. In principle, the older the oil or gas source that the liquid scintillator is derived of and the deeper it situates, the smaller the 14 C/12 C ratio is supposed to be. This, however, is not generally the case, and the ratio is probably determined by the U and Th content of the local environment. 1. Introduction The intrinsic 14C (T1/2≃5700 a) concentration in a liquid causes the main background at very low energies in high-purity liquid scintillation detectors. The measured concentration values are shown in Table 1 for scintillators based on PC (Pseudocumene; C9H12), PXE (Phenylxylylethane; C16H18) and Dodecane (C12H26). The values are essentially between 10−17 and 10−18 in 14C/12C. There are no published data for the 14C concentration values in the LAB (Linear alkylbenzene; C6H5CnH2n+1,n=10–16) being currently the most favourable liquid scintillator in large-volume detectors (e.g. SNO+ and JUNO). The β-decay end-point energy of 14C is quite low, Q=156 keV, and the counting rate from 14C may be often handled by setting the appropriate threshold energy. However, too high concentration of 14C in the liquid may results in pile-ups of pulses. For example, in the Borexino detector the trigger rate is largely dominated by the 14C isotope [5] (with 14C/12C≃2×10−18). XIV International Conference on Topics in Astroparticle and Underground Physics (TAUP 2015) IOP Publishing Journal of Physics: Conference Series 718 (2016) 062018 doi:10.1088/1742-6596/718/6/062018 Content from this work may be used under the terms of theCreative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 Table 1. Results of previous 14C/12C concentration measurements in some liquid scintillators (CTF = Counting Test Facility). 14C/12C (×10−18) Liquid scintillator & fluor Experiment Ref. 1.94 ±0.09 PC + PPO Borexino CTF [1] 9.1 ±0.4 PXE + p-Tp + bis-MSB Borexino CTF [2] 3.98 ±0.94 PC-Dodecane + PPO KamLAND [3] 12.6 ±0.4 PXE + PPO Dedicated setup [4] Based on the analysis of the 14C concentration in liquid scintillators derived from deep oil and gas fields [6], values lower than 10−18 should be possible if the source is carefully chosen. The contamination from reaction 14N(n,p)14C is expected to be the main source of 14C also deep underground but now neutrons are emitted by U and Th isotopes (and their daughters). In order to measure the 14C concentration in liquid scintillators at the level lower than approximately 10−15, a dedicated experimental setup is most probably required (for example, similar to the Borexino CTF [1] or to the one in Ref. [4], or to the present work) since it is currently the lower limit achieved by Accelerator Mass Spectrometry (AMS) method [7]. A campaign has been started to measure the 14C/12C ratio in several different liquid scintillator samples (based on oil, gas and coal derivatives obtained from different locations) with the aim of finding out ratios smaller than 10−18. Measurements are being carried out simultaneously, with essentially similar instruments and rock overburden, in two deep underground laboratories: in the Baksan Neutrino Observatory, Russia [8] and in the new CallioLab laboratory in the Pyh¨asalmi Mine, Finland [9]. In Baksan the measurements have already been started (but no results available yet). In Pyh¨asalmi the preparations have been started and the construction of the setup begins in January, 2016. The present paper describes the procedure and plans for Pyh¨asalmi measurements. For comparison, relevant information is also given from the setup in Baksan. 2. Experimental details The central part of the detector setup consists of two low-activity PMTs (3” ET 9302B), two shaped acryllic light guides and a quartz (or acryllic) vessel of 1.6 litres. The setup is schematically illustrated in Fig. 1. The vessel and light guides are wrapped around by VM2000 reflecting foil. The shieldings against γand neutron background are slightly different in the two laboratories: In Pyh¨asalmi thick layers (10–15 cm) of copper and lead around the central part are used. Paraffin layer (approximately 10 cm, as the outer layer) may also be used to thermalize neutrons from the rock. The central part of the setup is flushed with nitrogen for reducing the background from radon. In Baksan the measurements are carried out in a dedicated low-background chamber where the central part is yet surrounded by a thick copper layer. Measurements are carried out deep underground in both sites: 4900 mwe in Baksan and 4000 mwe in the Pyh¨asalmi. The DAQ will be realized with the DRS4 evaluation board (V5) [10] based on the DRS4 Switched Capacitor Array chip designed at the Paul Scherrer Institute, Villigen, Switzerland. The two PMTs are directly connected to the inputs of the DRS4 board which is connected to the DAQ Laptop via an USB connector. The DRS4 samples the pulse in 1024 bins of the width of 0.2 ns with the maximum sampling speed of 5 GS per second. An iseg NHQ 203M HV (2-chn) module is used to power the PMTs. XIV International Conference on Topics in Astroparticle and Underground Physics (TAUP 2015) IOP Publishing Journal of Physics: Conference Series 718 (2016) 062018 doi:10.1088/1742-6596/718/6/062018 2 Figure 1. Schematic illustration of the experimental setup. It consists of a cylindrical vessel of 1.6 litres, two shaped light guides and two low-background 3-inch PMTs. Figure 2. Calibration spectra of (1) 133Ba, (2) 109Cd and (3) 241Am sources for the measurement performed in Baksan. For 133Ba and 109Cd the position of their Compton edges are used while for 241Am the full absorption peak is possible. 3. Measurements The liquid scintillator samples are purified using Al2O3column and then mixed with ∼2 g/` of PPO and bubbled with nitrogen for removing oxygen. The purification is performed in the room air. A special purification system where the full process could be performed in a nitrogen buffer is in the design phase. The energy calibrations are performed with γ-ray sources using the position of their Compton edges. Three calibration sources are used in Pyh¨asalmi (57Co, 133Ba and 137Cs) and four sources in Baksan (109Cd, 133Ba, 137Cs and 241Am). 241Am provides also the full-energy peak. Essentially linear calibration curves will be expected. In Fig. 2 the calibration spectra measured with 109Cd, 133Ba and 241Am sources in Baksan are shown as an example. In the data processing the digitized waveforms will be analyzed and the signal shape will be used to reduce αand neutron induced backgrounds. XIV International Conference on Topics in Astroparticle and Underground Physics (TAUP 2015) IOP Publishing Journal of Physics: Conference Series 718 (2016) 062018 doi:10.1088/1742-6596/718/6/062018 3 Figure 3. Simulated 14Cβ-decay energy spectrum in a 1.6-`LAB sample with the concentration of 10−17 (14C/12C) and measurement time of 28 days. Realistic assumptions for the light yield and PMTs were made (see the text for details). 4. Results There are no experimental results available yet from Baksan or Pyh¨asalmi measurements and also the Geant4-based simulation package is not yet complete. We present here a result of a simulation that was performed for studying the 14C energy spectrum and energy resolution, i.e., number of photoelectrons released in the PMT cathodes. In the simulations LAB (1.6 `, C18H29,ρ=0.856 g/cm3, average light yield 10000 photons/MeV) was selected as the liquid. A realistic case was considered in the simulations by assuming the 14C concentration of 10−17 and the measurement time of 672 h (=28 days). The quantum efficiency of (28±2)% and gain of (1.0±0.1)×107were assumed for the two PMTs. The vessel and the light guides were wrapped around by the VM2000 reflecting foil in the simulations for increasing the photon collection. The obtained 14Cβ-decay energy spectrum (presented as a function of emitted photoelectrons at the cathodes) is shown Fig. 3 5. Summary A series of measurements have been started where the concentration of 14C will be determined in several liquid scintillator samples. The measurements are being carried out in two deep underground laboratories: in Baksan, Russia and in Pyh¨asalmi, Finland. References [1] Alimonti G et al 1989 Phys. Lett. B422 349 [2] Back H O et al 2008 Nucl. Instrum. Methods A585 48 [3] Keefer G 2011 Preprint arXiv:1102.23876v1 [physics.ins-det] [4] Buck C et al 2012 Instruments and Experimental Techniques 55 34 [5] Bellini G et al 2014 Phys. Rev. D89 112007 [6] Bonvicini G, Harris N and Paolone V Preprint arXiv:hep-ex/0308025v2 [7] Fahmi S M, Wacker L, Synal H-A and Szidat S 2013 Nucl. Instrum. Methods B294 302 [8] Gavriljuk Ju M et al 2013 Nucl. Instrum. Methods A729 576 [9] https://www.calliolab.com [10] http://www.psi.ch/drs/documentation XIV International Conference on Topics in Astroparticle and Underground Physics (TAUP 2015) IOP Publishing Journal of Physics: Conference Series 718 (2016) 062018 doi:10.1088/1742-6596/718/6/062018 4