Elsevier

Physics Letters B

Volume 422, Issues 1–4, 12 March 1998, Pages 349-358
Physics Letters B

Measurement of the 14C abundance in a low-background liquid scintillator

https://doi.org/10.1016/S0370-2693(97)01565-7Get rights and content

Abstract

The 14C/12C ratio in 4.8 m3 of high-purity liquid scintillator was measured at (1.94±0.09)×10−18, the lowest 14C abundance ever measured. At this level the spectroscopy of low-energy solar neutrinos, in particular a measurement of the 7Be neutrino flux, will not be obstructed by the 14C β decay intrinsic to a liquid scintillator detector. A comprehensive study of the deviation of the shape of the 14C β spectrum from the allowed statistical shape reveals consistent results with recent observations and calculations. Possible origins of the 14C in the liquid scintillator are discussed.

Introduction

Organic liquid scintillators are widely used in physics research as a detection medium for nuclear and particle radiation. Detectors employing large amounts of liquid scintillator (hundreds of tons) will be used in upcoming experiments that will search for rare events with low energy deposition. These experiments include the spectroscopy of solar neutrinos, the search for a neutrino magnetic moment and searches for non-baryonic dark matter. The feasibility of these experiments depends on a low background counting rate at energies typically below a few hundred keV. Since organic liquid scintillators are predominantly composed of carbon, the intrinsic concentration of the radioisotope 14C (β decay, Q = 156 keV, t1/2=5730 years) can constitute the main background at low energies and hence can restrict the sensitivity of these experiments.

Liquid scintillator mixtures contain aromatic solvents and these in turn are usually synthesized from petroleum. While modern carbon of biological origin has an isotopic ratio 14C/12C of about 10−12, it is expected that petroleum derivatives have a 14C abundance that is orders of magnitude lower. Petroleum deposits are mostly found deep underground and are shielded from the cosmic ray flux present at the Earth's surface responsible for the continuous production of 14C from 14N, via the reaction 14N(n,p)14C. Consequently, the cosmogenic production of 14C in underground petroleum is minimized and any 14C originally present in the precursor organic material to the petroleum will have had millions of years to decay away. Thus one would expect the 14C isotopic abundance in an organic liquid scintillator to be extremely low. In fact, the abundance of 14C in a petroleum-derived scintillator has never been determined.

In the Borexino solar neutrino experiment [1], low-energy neutrinos (<1 MeV) are detected via neutrino–electron scattering in a large target mass of liquid scintillator. The expected neutrino–electron interaction rate in Borexino, for electron recoils above 250 keV, is dominated by 7Be solar neutrinos and amounts to ∼0.5 events/day/ton, assuming the neutrino flux calculated in Standard Solar Models 2, 3. Though the maximum β energy for 14C is 156 keV, finite energy resolution and pile-up can create a tail in the β energy spectrum that extends to higher energy. Thus, the intrinsic level of 14C in the liquid scintillator becomes an important consideration in setting the low-energy threshold for neutrino events in Borexino. From these considerations, it is desirable in Borexino that the 14C/12C ratio not substantially exceed 10−18.

Trace concentrations of 14C are usually measured with accelerator mass spectroscopy (AMS). The sensitivity of this technique is limited by sample preparation at the 10−15 level [4], corresponding to a radiocarbon age of about 60,000 years. In one atypical measurement, CO samples were prepared from purified methane from a natural gas well and underwent an enrichment process to potentially increase the 14C/12C isotope ratio. When analyzed with AMS at the IsoTrace Laboratory in Toronto, Canada, by comparing with blank values, an upper limit of <1.6×10−18 was obtained [5]. This result suggested a petroleum-derived scintillator would be satisfactory for the Borexino solar neutrino experiment. However, a direct measurement of the 14C abundance in a liquid scintillator was considered a necessary prerequisite.

This paper describes the measurement of the 14C/12C ratio in an organic liquid scintillator carried out in the Counting Test Facility (CTF) of the Borexino project, located in the Gran Sasso underground laboratory in Italy. In addition to the 14C abundance measurement, the low background rate of the detector together with its large target mass and low energy threshold allowed a comprehensive study of the 14C β shape to be made. The results obtained for the deviation from the allowed statistical shape are compared with the contradictory experimental findings in recent literature.

Section snippets

Measurement of 14C with the counting test facility

In the case of liquid scintillators, sensitivities better than 10−18 can be achieved by direct measurement of the β decay of 14C. This is only possible if the scintillation detector has a mass of several tons and is constructed with stringent requirements on the selection of materials with low radioactivity (high radiopurity). The detector size is crucial in order to provide for an adequate event rate. A large size also leads to a lower surface-to-volume ratio which assists in lowering the

Data analysis and results

The theoretical β spectrum used for fitting the data has the following general form [9]:N(We)dWe=peWe(W0−We)2F(Z,We)C(We)dWe,where pe,We are the momentum and total energy of the emitted electron, W0 is the endpoint, F(Z,We) is the Fermi function which accounts for the influence of the nuclear Coulomb field on the β spectrum, and C(We) is the “shape factor” for the β decay.

A recent theoretical calculation by Garcı́a and Brown [10]of the 14C β spectrum shape predicts a shape factor correction, C(W

Possible origins of 14C content

At some level, depending on the underground depth, cosmogenic production of 14C still occurs, resulting indirectly from neutrons emitted following μ capture, neutrons produced by spallation, and (more rarely but depending on depth) from nuclear fragmentation directly following an inelastic muon-nucleus collision. However, at sufficient depth underground cosmogenic production becomes negligible compared to other production mechanisms involving natural radioactivity.

Production of 14C deep

Conclusions

By direct counting of the 14C β decays in a 4.8 m3, low-background liquid scintillation detector, sensitivities at the 10−19 level were achieved. A 14C/12C ratio of (1.94±0.09)×10−18 was found in the scintillator investigated. An examination of the shape of the β spectrum found results consistent with the theoretical prediction [10]and with the recent experimental result [12]for the deviation from the allowed statistical shape. The shape values reported in Ref. [11]can be excluded.

The 14C

Acknowledgements

This work was supported by the Istituto Nazionale di Fisica Nucleare (INFN), Italy, the Bundesministerium für Bildung und Forschung (BMBF), Germany, the Deutsche Forschungsgemeinschaft (DFG), Germany, and by the National Science Foundation (NSF), USA.

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    The sources of 14C deep underground were later estimated in connection with the construction of the large (300 ton) BOREXINO [46] scintillation counter, for solar 7Be neutrino detection, to be due to the presence of 14C from alpha particle induced reactions, in 17O, from nearby actinides – or from cosmic ray produced 14C from contamination. At estimated levels as low as 5 × 10−21 for the 14C/C ratios [46] for underground hydrocarbons, the challenge for radiocarbon measurement may be beyond the capability of all present methods of measurement but of course the background level must be determined for each radiocarbon application. This topic of the limiting radiocarbon background in large scintillation counters was discussed, in some detail, much later in section IV B of a 2011 paper on Mass Spectrometry with Accelerators [47].

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