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Calibration Systems

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.

The MiniCal System

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.

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.

The 4 pi System

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.

The Z-axis System

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

The glovebox

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.