1. Test and Control Chambers
The proposed replacement for the forward muon detector is called the
new Small Wheel (nSW). A critical component of the nSW is small thin
gap chambers (sTGC). These are wire chambers in which the gap
between the upper and lower plates (1.4mm) is smaller than the
spacing of the wires (1.8mm).
During operation, the
wire chambers are filled
with ionizing gas (a
mixture of 45% pentane
55% carbon dioxide).
They detect muons as
w e l l a s a n y o t h e r
charged particle that
enters the chamber and
ionizes the gas mixture.
The chosen mixture also acts as a quencher to absorb photons
released in ionization, preventing the generation of secondary charge
carriers. The charge carriers produced in ionization are separated in
the electric field of the anode and are collected at the anode wires
(ganged in 5) and the cathode strips and pads on the top and bottom
surfaces of the chamber. The cathode strips record position
information, and act as a trigger better than 25ns timing resolution.
The chambers are capable of spatially accurate measurement since
the wire spacing is grater than the distance between electrodes.
Discussion and Analysis
Test Stand
Ageing
It is important that the gas filling the chambers be circulated because
when it is ionized, photons are generated. These photons can damage
quenching gas molecules causing polymerization on the anode and/or
cathode. With increased exposure to radiation, the rate of degradation
is expected to increase. This degradation is referred to as ageing.
Both the current drawn by the chambers and the collected charge in
the test chamber are relevant parameters in the study of ageing. The
control chamber is exposed to much less radiation resulting in fewer
events and consequently poor statistics.
Small Thin Gap Chambers
The above results, collected over 600 hours of run time, show the
mean charge per event collected by the anode wire, normalized over a
period of one hour for the test (1) and control chambers (2) in blue,
and the current for the test (3) chamber in red. The mean charge is the
parameter of interest in ATLAS, and will indicate the energy of the
detected muon. The current allows to monitor losses etc, in the sTGC.
The atmospheric pressure (black) is common to both chambers.
The average charge accumulation at the beginning and at the end of
the study (indicated by a horizontal line) compare data before and
after ageing. These observations indicate that the accumulated charge
increase from 12.7pC to 12.8pC (test chamber) over the duration of
the study. The results also show that the current seems to be strongly
inversely related to the atmospheric pressure. The temperature was
monitored and appeared uncorrelated.
The Malter Effect may also occur. This is a process in which a
resistive coating collects on the cathode preventing charge dissipation.
Charge buildup eventually leads to the removal of some electrons from
the cathode through the resistive layer. The electrons will then
propagate to the anode and result in a fake signal.
This study determines if the
increase in irradiation would
cause observable changes in
output signal. If ageing were
to occur, a decrease in total
collected charge would be
expected, as polymer would
collect on the electrodes,
weakening the field in the
chamber.
Conclusion