[Paper Review] SciBath: A Novel Tracking Detector for Measuring Neutral Particles Underground

R.J. Cooper, L. Garrison+4 more|arXiv (Cornell University)|Oct 20, 2011

Neutrino Physics Research6 references3 citations

TL;DR

SciBath-768 is a prototype 3D tracking detector using 768 wavelength-shifting fibers in liquid scintillator to reconstruct neutrino and neutron events with high precision. It achieves 30% detection efficiency and 30% energy resolution for 1–10 MeV neutrons, demonstrating viability for underground measurements of neutral particles in neutrino and dark matter experiments.

ABSTRACT

The SciBath-768 detector is a prototype neutral particle detector offering high-precision reconstruction of neutrino and neutron events. It consists of a three dimensional grid of 768 wavelength-shifting fibers immersed in 82 liters of liquid scintillator. Initially conceived as a charged particle detector for neutrino studies, it is also sensitive to fast neutrons (1-100 MeV). Simulation results show 30% efficiency and 30% energy resolution for 1-10 MeV tagged neutron events. The apparatus has been commissioned and will be deployed in Fall 2011 to measure neutrinos and neutrons 100 meters underground in the Fermilab MINOS near-detector area.

Motivation & Objective

• To develop a high-resolution, cost-effective tracking detector for reconstructing low-energy proton recoils from neutrino neutral-current elastic scattering.
• To measure cosmic-ray-induced fast neutrons (1–100 MeV) as a background source in low-rate underground experiments.
• To validate the SciBath technology as a prototype for the full-scale FINeSSE detector, enabling precise spin structure measurements of the proton.
• To directly map fast neutron fluxes at different depths using in-situ measurements at Fermilab.
• To demonstrate the feasibility of tagging neutron events via prompt recoil proton tracks and subsequent 2.2 MeV gamma signals from neutron capture.

Proposed method

• The detector uses a 3D orthogonal grid of 768 wavelength-shifting (WLS) fibers (1.5 mm diameter, 2.5 cm spacing) embedded in 82 L of liquid scintillator doped with PPO.
• Scintillation light from charged particles is captured by WLS fibers and guided to multi-anode photomultiplier tubes (MAPMTs) for position and energy reconstruction.
• Particle tracks are reconstructed using a least-squares method for perpendicular offsets combined with the MINUIT minimization routine in ROOT for optimal fit accuracy.
• Neutron events are identified by a two-part signature: a short recoil proton track followed by a 2.2 MeV gamma pulse from n(p,d)γ capture after ~186 μs.
• Calibration is performed using cosmic muons and a custom pulsed LED system, one per fiber, to ensure uniform response across the detector.
• Simulations are performed with GEANT4 to model detector response and predict performance for neutrinos and neutrons.

Experimental results

Research questions

• RQ1 Can a 3D fiber tracker using wavelength-shifting fibers achieve sub-millimeter position resolution and sub-degree angular resolution for charged particle tracks?
• RQ2 What is the detection efficiency and energy resolution of SciBath-768 for 1–100 MeV fast neutrons via proton recoil and gamma capture tagging?
• RQ3 Can the detector distinguish and reconstruct low-energy proton recoils from neutrino neutral-current elastic scattering with sufficient precision for proton spin structure measurements?
• RQ4 What is the expected rate of cosmic-ray-induced fast neutrons at 100 m underground, and can SciBath-768 measure it directly?
• RQ5 How does the performance of the SciBath design scale to a full-scale 19,200-fiber detector for the FINeSSE experiment?

Key findings

• The SciBath-768 prototype achieved a position resolution of 3 mm and angular resolution of 5° for through-going cosmic muon tracks in simulation.
• For 1–10 MeV neutrons, the detector is predicted to achieve 30% detection efficiency and 30% energy resolution using the proton recoil and 2.2 MeV gamma signature.
• At 100 MeV, detection efficiency drops to ~10% and energy resolution degrades to ~60% due to energy loss from escaping particles.
• The detector is expected to record between 100 and 10,000 neutrino events in a 2-month run at Fermilab’s MINOS near-detector hall, depending on beam configuration.
• Cosmic-ray-induced fast neutron rates are estimated at ~20 events per day at 100 m depth, which SciBath-768 is designed to measure directly.
• The detector has been commissioned and is operating with full data acquisition and calibration systems, including cosmic muon and LED-based calibration.