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.
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 9
Li and 8
He, 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
Be 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 11
C, 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
C 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 11
C 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
Much of the hardware for the muon tracker was salvaged from an older
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
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 ... !