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Purity Monitors - Overview

Purity Monitor Hardware

MicroBooNE uses three purity monitors to check electronegative contamination levels of the argon beyond the ranges of the gas analyzers. Of particular concern are the electronegative molecules of oxygen and water. The purity monitors themselves are double-gridded ion chambers immersed in the liquid. The monitors themselves are composed of four circular electrodes, all parallel to each other. The first is a photocathode, supported by a stainless steel disk. The second and third are open wire grids, acting as cathodes and anodes respectively. The fourth is a stainless steel disk acting as an anode.

Our purity monitors use a 2.54 cm × 3.18 cm × 0.8 mm aluminum plate coated with 50 ̊A of titanium and 1000 ̊A of gold as the photocathode fixed to the cathode disk. To supply 1ight to the photocathode, we use a xenon flash-lamp external to the cryostat to produce UV light that is then steered to the photocathode using three quartz fibers, each 0.6 mm in diameter.

When the flash-lamp is fired, UV light frees electrons from the photocathode that then drift towards the cathode grid. The freed electrons induce a current on the cathode. After passing through the cathode grid, the electrons experience the field from the anode grid. The anode and cathode wire grids act to prevent induced currents from appearing on the cathode or anode while the electrons are flowing through the drift volume. Once past the anode grid, the electrons then induce a current on the anode. The induced currents on the anode and cathode are then amplified and integrated.

A diagram of a purity monitor appears here:

The MicroBooNE Purity Monitor Configuration

MicroBooNE has two purity monitors inside the cryostat and one monitor inside of an inline vessel downstream of the liquid argon filters. The two inside the cryostat monitors are of differing lengths and therefore offer different electron drift distances. A short purity monitor sits near the bottom of the cryostat and has a total drift distance of 19 cm. The other monitor inside of the cryostat has a total drift distance of 50 cm and will henceforth be referred to as the long purity monitor.

During normal operation, the short and long purity monitors take data every hour. The long purity monitor is thus far the most reliable and has the greatest dynamic range of the two inside of the cryostat.

The inline monitor is currently inactive as its vessel is difficult to keep full of liquid. It remains on standby in the instance that it's needed for debugging the cryogenics system.

The locations of the long and short monitors in relation to the TPC and beam direction appears here:

Purity Monitor Analysis

The data analysis seeks to compare pulses that arrive at the anode to that originating at the cathode. The purity monitor DAQ code currently carries out the calculation as data is taken. An overview of the analysis with all of the details can be found in MicroBooNE DocDB entry 4852.

An example of the traces used for the analysis appears here: