The 16N calibration source for the Sudbury Neutrino Observatory

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Abstract

A calibration source using γ-rays from 16N (t1/2=7.13 s) β-decay has been developed for the Sudbury Neutrino Observatory (SNO) for the purpose of energy and other calibrations. The 16N is produced via the (n,p) reaction on 16O in the form of CO2 gas using 14-MeV neutrons from a commercially available Deuterium-Tritium (DT) generator. The 16N is produced in a shielding pit in a utility room near the SNO cavity and transferred to the water volumes (D2O or H2O) in a CO2 gas stream via small diameter capillary tubing. The bulk of the activity decays in a decay/trigger chamber designed to block the energetic β-particles yet permit the primary branch 6.13 MeV γ-rays to exit. Detection of the coincident β-particles with plastic scintillator lining the walls of the decay chamber volume provides a tag for the SNO electronics. This paper gives details of the production, transfer, and triggering systems for this source along with a discussion of the source γ-ray output and performance.

Introduction

The Sudbury Neutrino Observatory (SNO) is a heavy water Cherenkov detector designed primarily for the detection of solar neutrinos. SNO identifies neutrinos via the detection of Cherenkov light from energetic electrons produced by charged current (CC) and elastic scattering (ES) interactions, and neutron capture γ-rays resulting from the neutral current (NC) interactions. It does this with an array of ≈9500 photomultiplier tubes surrounding the D2O target volume (1 kt) which is contained within a 12 m diameter acrylic vessel. The entire assembly is immersed within a≈7 kt water shield and located under ≈2100 m of rock in INCO's Creighton mine near Sudbury, Canada. A complete description of the SNO detector and its many subsystems is presented in Ref. [1]. A short description of the SNO energy response will be presented here to provide the context for the discussion of the 16N calibration source.

A measure of SNO's energy is the number of photomultiplier tubes registering as hit in an event, Nhit. In addition, the hit pattern (in time and space) provides the information that can be used to reconstruct event interaction position, direction, and energy. Nhit and the associated hit patterns are dependent not only on the amount of light (number of photons) produced via the Cherenkov process, but also on the details of the detector optics and the response of the individual photomultiplier tubes. A proper understanding of the light production, propagation, and detection is required in order to understand the response of the detector to neutrinos, muons and radioactive backgrounds. Therefore, a detailed Monte Carlo modeling of the detector was developed along with an extensive program for optical and energy calibrations. A discussion of the Monte Carlo and the overall program for detector calibration can be found in Ref. [1].

The response of the SNO detector is calibrated using a variety of sources providing isotropic light [2], [3], [4], γ-rays [5], [6], [7], [8], β-particles [9], [5], and neutrons. The present work describes one of these sources which provides nearly mono-energetic, primarily 6.13 MeV, γ-rays following the β-decay of 16N. The high-energy γ-rays are used for a number of calibration tasks, including the primary energy scale calibration, verification of the energy resolution and energy scale position dependence, and verification of reconstruction and data reduction algorithms. A feature essential to the calibration analysis is detecting the β-particle coincident with the γ, allowing calibration event identification. The 16N β-decay is also used as an untagged calibration source for the SuperKamiokande imaging H2O Cherenkov detector [10].

The four major systems that comprise the 16N calibration source are illustrated in Fig. 1. The source positioning system is common to all SNO calibration devices and has been discussed in Ref. [1]. The other systems, however, are unique to the 16N source and are therefore described in this paper. Earlier discussions of the systems can be found in Refs. [5], [6]. The systems used to produce and transport the short-lived 16N are discussed in 2 , 3 The transfer system, respectively. The design of the decay/trigger chamber (hereafter referred to as the decay chamber), where the 16N decays and is tagged, is described in Section 4. The γ-ray output and how it is influenced by the decay chamber geometry is discussed in Section 5. Finally, in Section 6, the complete source performance is demonstrated with laboratory measurements and calibration data obtained in SNO.

Section snippets

16N production

SNO uses a commercially available DT generator, the MF Physics model A-3205, to produce fast neutrons. A DT generator is a miniature particle accelerator that generates 14-MeV neutrons by accelerating a mixed beam of deuterium and tritium onto a target containing both deuterium and tritium. This results in the fusion reactiond+t→n+4He(Q=17.6MeV)where the neutrons produced are emitted nearly isotropically.

The A320 was chosen because of its

The transfer system

The purpose of the gas capillary transfer system is to transport the short-lived isotopes quickly from the target chambers to the deck above the detector, then down to the decay chamber inside the D2O or H2O volume. For 16N, CO2 from a compressed gas bottle is sent via polyethylene tubing to a control panel that directs the gas at the chosen flow rate and head pressure to the target chamber at the bottom of the pit. From there, the 16N produced is entrained in the gas stream and sent via Teflon

16N Decay chamber design

The 16N decay chamber design results from a compromise between the need to minimize γ-ray attenuation and maximize β-particle containment. The design is illustrated in Fig. 5. The main casing is a smooth cylindrical tube of stainless steel that is 41.9 cm long, 10.16 cm in diameter and has a wall thickness of 0.476 cm. The stopping power in steel is 2 and 1.6 MeV cm2/g for 10 MeV and 3–4 MeV β-particles, respectively. Therefore, the wall thickness selected for use in the decay chamber, together with

16N Decay chamber function and γ-ray emission

A simplified 16N decay scheme is presented in Fig. 6 [19]. The branch of primary interest for calibration produces a β-particle with end point energy 4.3 MeV and a 6.1-MeV γ-ray (66.2%). There are other branches that produce γ-rays in coincidence with β-particles (6%). There is also a direct branch to the ground state, resulting in a 10.4-MeV endpoint β-particle without an associated γ-ray (28%). Thus, each β-particle will produce a trigger for SNO, but not all triggered events constitute a

Source performance

The following discusses the performance of the 16N source based on both off-line measurements and measurements taken in SNO. The discussion focuses on the yield and the trigger efficiency.

The yield is primarily a measure of how well the production and transfer systems work. The method for estimating the yield was outlined in 2 , 3 The transfer system and the predicted yield was ≈460/s. To test these calculations, measurements were taken with the SNO transfer configuration and the optimal flow

Conclusion

A calibration system using the decay of 16N has been developed and successfully deployed into the SNO detector. The source has achieved all its design goals including high 16N production yield (⩾300 s−1) and high trigger efficiency (⩾95%). The γ-ray emission from the source is also well understood via Monte Carlo simulation. The systematic uncertainty in the energy calibration due to using this source is <0.5%.

Acknowledgements

The SNO project has been financially supported in Canada by the Natural Sciences and Engineering Research Council, Industry Canada, National Research Council of Canada, Northern Ontario Heritage Fund Corporation and the Province of Ontario, in the United States by the Department of Energy, and in the United Kingdom by the Science and Engineering Research Council and the Particle Physics and Astronomy Research Council. The heavy water has been loaned by AECL with the cooperation of Ontario Power

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Cited by (0)

1

Los Alamos National Laboratory Los Alamos, NM 87545, USA.

2

Department of Physics and Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada.

3

Institut de Physique, Université de Neuchâtel CH-2000, Neuchâtel, Switzerland.

4

Nuclear and Astrophysics Laboratory, Oxford University, Keble Road, Oxford, OX1 3RH, UK.

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