Muon Tracker

Overview

The muon tracker is a new subsystem installed at KamLAND this year by the Berkeley group. The system will be used to track a subset of the cosmic ray muons incident on the KamLAND detector. At present, muons entering KamLAND are identified by the high levels of charge deposited in the inner and outer PMT arrays; each muon trajectory is inferred from the distribution of arrival times of its deposited charge. The data independently provided by the new muon tracker will be used to calibrate the muon reconstruction algorithm used for the main detector, and to measure the efficiency of the muon tagging procedure.

The new muon tracker system assembled inside the clean area of KamLAND is shown below. The edge of the calibration chimney can be seen in the left foreground. The sphere which houses the main detector lies directly beneath the floor.

Motivation

Reactor Anti-neutrino Measurements

Improved knowledge of muon trajectories inside KamLAND is desirable for several reasons. Energetic muons traversing the detector produce neutrons and unstable light nuclei by spallation. These spallation products,particularly delayed neutron beta emitters such as ⁹Li and ⁸He, contribute to backgrounds that mimic the anti-neutrino signal. Such backgrounds can be mitigated by rejecting events correlated in space and time to the muon track. The current reactor analysis conservatively vetoes the detector for 2 seconds in a cylinder of radius 3m around an identified muon track. This introduces ~10% detector dead time. While the 2 second veto interval is driven by the lifetime of the offending spallation products, the 3m cylinder radius is driven primarily by uncertainty in the reconstructed muon track. If muon tracking were better understood, these spallation vetos could be optimised and some detector live time could be recovered. Also, by studying events in KamLAND that follow those muons that trigger the muon tracker but not the main detector, it may be possible to further characterise and thus better reject backgrounds from untagged spallation products and fast neutrons.

Solar Neutrino Sensitivity

KamLAND is currently transitioning to an ultra-low-background phase of operation. In this phase, it is anticipated that background levels in the sub-MeV energy window will be reduced by several orders of magnitude, making it possible to investigate ⁷Be solar neutrinos. However, the potential also exists to study solar pep and CNO neutrinos, and to perhaps reduce experimental limits on the solar pp-neutrino flux. Assuming the anticipated radiopurity levels are achieved, the dominant background in the CNO/pep signal window will be ¹¹C, a long-lived (~21 minutes) spallation isotope continually produced by muons passing through the detector. Hagner et al., Astro. Phys. 14 1 (2000), have measured the production profile of ¹¹C relative to the parent muon track using a high energy muon beam at CERN. These measurements demonstrate that a 50cm cylindrical cut around the muon track would reduce the ¹¹C background by ~95%. Given its long lifetime, very stringent spatial cuts are required in order to veto this background while maintaining a practical signal live time. The muon tracking resolution needs to be improved in order to achieve this.

Description of the Apparatus

The core of the muon tracker consists of 48 proportional tube counters (PTCs) with a pitch size of 1.27cm. The PTCs are arranged on the support frame to sample 2 points on the muon track at a vertical separation of 2m. The multiplication gas is a 90:10 mixture of ArCO₂. The system is triggered by coincidence of two planes of plastic scintillator mounted on top of both the higher and lower planes of PTCs. This arrangement has an active area of 5.4m² and should achieve ~7cm resolution on tracks at the center of KamLAND. The support frame can be reconfigured to change the position of the bottom layer of PTCs relative to the top layer. This will enable muon tracking with a greater range of impact parameters than a static setup, making it possible to study muons passing through the liquid scintillator, the mineral oil buffer region, the outer detector, and particularly at the respective interfaces of these three volumes where muon tracking is currently the least well understood.

Much of the hardware for the muon tracker was salvaged from an older experiment. Following refurbishment and repairs, the full system was assembled and tested at LBNL last year. These tests demonstrated the functionality, stability, and safety of the system. Last summer it was disassembled, cleaned, and packed in preparation for shipment to the clean area of KamLAND. Members of the muon tracker team are shown below cleaning the system in preparation for shipment and depolyment in the KamLAND clean area.

The full system was shipped at the end of the summer and arrived on site in Fall 2007. Installation and commissioning is currently under way. The on-site construction phase was completed by the installation team in late December 2007; Running and testing will continue in Spring 2008. Once commissioning is complete, we expect to operate the system for about one year.

Data and Results

The data is currently being evaluated and we expect to update this section very soon ... !