Over the course of the KamLAND experiment, a variety of energetic
sources &mdash both radioactive and LED &mdash have been routinely lowered
into the detector in order to calibrate its response to different types of
physics events. Designing the systems that deploy such sources presents
many unique engineering challenges, because the deployment systems must
operate in corrosive liquid scintillator, possess low intrinsic
radioactivity, and exhibit precise and reproducible positioning.
To date, KamLAND has employed two successive source deployment systems:
the Z-axis system, followed by the
4&pi system. As its name suggests, the
Z-axis system was capable of raising and lowering sources along the central,
vertical z axis of the detector. In 2005 it was replaced by the more
versatile 4π system, which can position a source at nearly any point
inside the KamLAND balloon. We intend to soon replace the 4π system with
a new z-axis system named MiniCal which
satisfies even more stringent radiopurity requirements and is therefore more
suitable for use in the low-background phase of the experiment.
The performance of these systems and their impact on KamLAND is
described in more detail below.
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The MiniCal System
(2009–present)
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MiniCal is a compact, low-mass, ultra-clean, z-axis source deployment system
used for performing detector calibrations in the low-background phase of
KamLAND. MiniCal was installed and commissioned on site at KamLAND between
February 25 – March 11, 2009, and the system has already completed
several calibration scans.
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MiniCal works in basically the same way as the original Z-axis system, but
MiniCal uses a much thinner cable that introduces fewer contaminants into
the liquid scintillator. Moreover, MiniCal's components are sealed inside
a stainless steel box to provide an additional barrier between the liquid
scintillator and the outside world. These precautions are crucial for the
low-background phase of the experiment, when the liquid scintillator will
be highly purified and contamination must be minimized as much as possible.
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The 4 pi System
(2005–present)
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The 4π system can deploy a radioactive source to any position inside
the KamLAND detector. This enables detailed study of how the detector's
response varies with event position.
In total, nearly 500 hours of detector live time were spent collecting
off-axis calibration data, at over 60 unique positions, using three
different types of radioactive sources. This data reduced the uncertainty
in the fiducial volume by more than a factor of two and thus significantly
improved the target mass uncertainty — heretofore the largest source
of systematic uncertainty in the anti-neutrino rate measured by KamLAND.
The off-axis calibration data also enabled the study of small, systematic
biases in vertex reconstruction.
The 4π off-axis calibration data played a crucial role in obtaining
the latest KamLAND results (published in the June 6, 2008 issue of PRL)
which provide the most precise value to date for
Δm221.
Its off-axis mission having been satisfied in early 2007, 4π has
operated in z-axis mode ever since, as preparations were made for the
low-background phase of KamLAND.
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The Z-axis System
(2002–2005)
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The Z-axis system was KamLAND's first calibration system. It was
capable of deploying radioactive and LED sources along the detector's
z-axis. Sources were attached to the end of a cable which was lowered
into the detector using a remotely controlled motor. The source's
position inside the detector was determined from the length of cable
payout, as measured by an electronic shaft encoder connected the
system's upper pulley
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All of the deployment systems are installed inside a sealed glovebox
that sits atop the detector. The interior of the box can be accessed
through gloveports, allowing operators to change sources, exchange
objects through the airlock transfer box, etc. The glovebox is purged
with nitrogen gas before the 6-inch and 16-inch gate valves are opened
to permit the deployment of sources into the detector.
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