PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB. Development of a wavelength-shifting fiber-based photon detector for LBNE

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1 PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB LIGHT DETECTION IN NOBLE ELEMENTS (LIDINE 2013) 29 th 31 st MAY 2013, FERMI NATIONAL ACCELERATOR LABORATORY ILLINOIS, U.S.A. RECEIVED: July 22, 2013 ACCEPTED: August 31, 2013 PUBLISHED: October 16, 2013 Development of a wavelength-shifting fiber-based photon detector for LBNE R. Wasserman 1 and N. Buchanan Colorado State University, Fort Collins, Colorado, U.S.A. Ryan.Wasserman@colostate.edu ABSTRACT: The proposed LBNE experiment will employ liquid argon TPCs for the far detector. We are developing a photon detector prototype based on wavelength-shifting fibers and utilizing silicon photomultipliers for potential use in the LBNE far detector. This paper describes progress and plans of the prototype development. An update on the development of a cryogenic detector development test facility, which includes a 500 L cryostat designed for testing full-scale photon detector components for LBNE will also be covered. KEYWORDS: Photon detectors for UV, visible and IR photons (solid-state) (PIN diodes, APDs, Si- PMTs, G-APDs, CCDs, EBCCDs, EMCCDs etc); Noble liquid detectors (scintillation, ionization, double-phase); Cryogenics 1 Corresponding author. c 2013 IOP Publishing Ltd and Sissa Medialab srl doi: / /8/10/c10008

2 Contents 1 Long Baseline Neutrino Experiment 1 2 Research and development Fiber-based prototype Facilities 4 3 Conclusion 5 1 Long Baseline Neutrino Experiment The Long Baseline Neutrino Experiment (LBNE) is a proposed neutrino oscillation experiment that will direct a neutrino beam from FermiLab, near Chicago, to the Homestake Mine in Lead, South Dakota where the far detector will be located. Primary physics goals of the experiment include determining the orientation of the neutrino mass hierarchy and searching for charge-parity violation in the lepton sector. The current design for the LBNE far detector consists of a 10 kt liquid argon (LAr) time projection chamber (TPC) along with a photon detection system located in the space between the drift regions. The current phase 1 design has the detector located on the surface at the Homestake Mine site. The final LBNE design has a 34 kt detector located at the 4850 level of the Homestake mine. The 10 k-ton far detector will be composed of two 5 kt modules each containing both TPC and a photon detector (PD). A schematic of the detector can be seen in figure 1. Each 5 kt module will be instrumented with alternating high voltage cathode and anode planes to create drift fields for electrons within the TPC volume. The entire detector will be filled with purified LAr and equipped with a filtering and condensing system to cycle the gaseous argon back into the detector. Along with charge collection from drift electrons within the TPC the far detector will collect scintillation light produced by charged particles traveling through LAr. In the TPC, LAr will produce around 40,000 photons/mev for minimum ionizing particles at 128 nm, which is the vacuum ultra-violet (VUV) region of the spectrum [1]. One third of the scintillation light is emitted within 5 ns, and the remainder after 1.2 µs [2]. The promptly emitted light can be used to determine t 0 for the event. The goal of the PD system is to detect enough of these VUV photons to provide information that will enhance the detector s ability to reject backgrounds, such as the large flux of cosmic ray background events that would reach a surface detector. A photon detection system also has the ability to provide a trigger for non beam-related events such as those from proton decay and supernovae neutrinos if the detector is located underground. Designs under study for the PD make use of wavelength shifting fluors coated on or doped into acrylic fibers or bars that will shift the VUV scintillation light into the visible region of the spectrum and guide it onto photosensors. Tetraphenyl butadiene (TPB) has been shown to be a compound 1

3 Figure 1. Schematic of design for the LBNE far detector. Figure 2. Measurement of voltage signal from SiPM submerged in liquid nitrogen. capable of providing wavelength-shifting the VUV light and is currently being used for photon detector prototypes [3]. Other alternatives, such as Bis-MSB are also being investigated. The baseline design for the photon detector utilizes silicon photo-multipliers (SiPMs), which have been shown to operate well in the cold with a very low noise level and the capability to distinguish single photons [4]. A measurement taken at CSU, figure 2, has confirmed this result. This data was gathered by illuminating a SiPM submerged in liquid nitrogen with a 395 nm LED. Doing this we were able to distinguish events of up to 8 single photo electrons above the noise pedestal for the device. 2

4 2 Research and development Figure 3. The design for housing SiPM in the LBNE detector. Our group is currently focused on the development and testing of a fiber-based prototype system. This includes both designing a system capable of single photon detection in liquid argon as well as developing a cryogenic facility, CDDF (cryogenic detector development facility) to be used for development, testing, and quality assurance of the photon detector prototypes. 2.1 Fiber-based prototype We are developing a prototype PD based on optical fibers doped with TPB, or potentially other wavelength shifters. Cost reduction and the potential for improved spatial information, which may be helpful for identifying cosmic ray backgrounds for a surface detector, motivated the fiber-based design. The initial design for the prototype was built around Hamamatsu SiPMs used for the PiZero detector for the T2K near detector and 1 mm diameter fibers. This design allowed the reuse of infrastructure used for fiber mounting and SiPM housings (figure 3) the CSU group had developed for the T2K experiment. A second generation design is currently under development and uses larger SiPMs from SensL. The larger (6 mm by 6 mm) devices require a new design for the SiPM and fiber mounting system. Larger, 3 mm by 3 mm unclad fibers with a square cross-sectional profile and doped with TPB have been ordered from Saint-Gobain. It will be possible to read out four fibers with a single SiPM and the prototypes will be used to characterize the fibers and, through the use of simulation how to best arrange them to maximize physics potential while maintaining an appropriate budget. Geometric arrangements will include all four fibers grouped together straight out from the SiPM to the cases where the fibers are spread out to cover a larger area of the detector while still converging onto a single SiPM. The current baseline design is a bar-based prototype, under development at Indiana University. The bar-based design consists of four 1 in 0.25 in bars mounted in a row with each read out by 2-3 SiPMs located on either end of the bar. In order to allow rapid prototyping of both the fiber and bar-based photon detectors, a common mounting structure has been designed and used by both. The mounting system has also been designed to allow the prototypes to be mounted in any of the test cryostats used for the project such 3

5 Figure 4. Mounting system for wavelength shifting bars. as the 35 ton prototype membrane cryostat at Fermilab, or the 500 liter test cryostat in the CSU CDDF. The frames will be built at different lengths to accommodate shorter (< 1 m) prototypes, or the 2.2 m full scale PD prototypes. Figure 4 shows the test PD frame. A modified version of this frame will be used for the PD as part of the APA (anode plane assembly) when the full TPC is constructed. This frame will hold the PD modules securely in place both during the wire wrapping of the APA frame and during operation of the LBNE far detector. In the figure the bars are held within a stainless steel frame at both ends. The photon detection devices are located at the end of the bar with cabling passing though the top of the mounting device to feed-throughs in the cryostat lid for readout electronics. 2.2 Facilities As part of our PD research and development program we are developing a cryogenic test facility. The CDDF will house 3 research dewars (20 l, 40 l, and 500 l), the largest of which can accommodate full scale PD components. The two larger vessels were designed to allow evacuation/gas purge/liquid fill cycles. Each of these systems uses a dedicated dry scroll pump to allow rough vacuum to be achieved prior to the gas purge cycle to ensure all nitrogen is removed from the dewar 1 The 40 l dewar, figure 5, is roughly 10% scaled version of the 500 l unit. The small, 20 l, dewar is an open top dewar to allow mechanical and electrical tests to be quickly performed using LAr or 1 Nitrogen contamination causes a reduction in the emission of scintillation light from the LAr [5 7]. 4

6 LN2. The large 500 l vessel, figure 6, (3 m in height and 0.5 m in diameter) is surrounded by a large platform providing access to the top. A 1 ton electric crane has been installed above the platform to allow insertion and removal of detector components into the 500 l dewar. Due to the large scale of the 500 l dewar safety is a significant concern. A large stainless steel catch-basin was installed under the large dewar with a drain to the outside of the building. A heavy, clear, nylon barrier was installed between the bottom of the access platform and the top of the drain pan to ensure LAr and gaseous argon would be contained in the unlikely event of a structural failure of the dewar. A vent to the outside is connected to a fan to allow purging of any gas accumulating on the tented area under the platform. Oxygen deficiency hazard (ODH) monitors have been installed in the tented area, in the CDDF room, and in the hallway outside the room to monitor oxygen levels. A security camera system has been installed inside the facility, as well as in the hallway outside to ensure 24-hour monitoring. Cameras have been fixed on each of the ODH monitors as well. It is expected that the facility will be fully operational in July, Conclusion The LBNE experiment s far detector will employ both a TPC and photon detection system in order to receive information from neutrino interactions. At Colorado State University we have begun development and testing of a fiber-based photon detection system for potential use in the LBNE far detector. Parts of the design will be used to house whatever PD technology is ultimately chosen. Our group has also commissioned a cryogenic detector development facility for development and testing of detection components. 5 Figure 5. CSU 40 L test cryostat with optical bar mounting device.

7 Acknowledgments We would like to thank David Warner, of Colorado State University, for work on the design and engineering of PD detector components, Forrest Craft and Thomas Cummings, of Colorado State University for their assistance on both the PD prototypes and CDDF construction and commissioning. We would also like to thank the College of Natural Science, Department of Physics, and High-Energy Physics and Astroparticle Program of Research and Scholarly Excellence for their support for the CDDF. References [1] M. Suzuki and S. Kubota, Mechanism of Proportional Scintillation in Argon, Krypton, and Xenon, Nucl. Instrum. Meth. 164 (1977) 177. [2] T. Doke, K. Masuda and E. Shibamura, Estimation of Absolute Photon Yields in Liquid Argon and Xenon for Relativistic (1 MeV) Electrons, Nucl. Instrum. Meth. A 291 (1990) 617. [3] V.M. Gehman et al., Fluorescence Efficiency and Visible Re-emission Spectrum of Tetraphenyl Butadiene Films at Extreme Ultraviolet Wavelengths, Nucl. Instrum. Meth. A 654 (2011) 116. [4] J. Fox, N. Woodard and G. Lafyatis, Characterization of cooled large-area silicon avalanche photodiodes Rev. Sci. Instrum. 70 (1999) [5] B. Jones et al., A Measurement of the Absorption of Liquid Argon Scintillation Light by Dissolved Nitrogen at the Part-Per-Million Level, 2013 JINST 8 P07011 [arxiv: ]. [6] S. Himi, T. Takahashi, J.Z. Ruan and S. Kubota, Liquid and Solid Argon, and Nitrogen-doped Liquid and Solid Argon Scintillators, Nucl. Instrum. Meth 203 (1982) 153. [7] R. Acciarri et al., Effects of Nitrogen Contamination in Liquid Argon, 2010 JINST 5 PO Figure 6. CSU 500 L cryostat with support structure.