CMS EMU ageing tests

• Complete information about ageing tests at PNPI is given in our publications.
• Information about ageing tests on GIF at CERN can be found in Ageing page

The ageing studies of the CMS EMU prototypes in PNPI were performed with the ⁹⁰Sr source at the Ageing Test Station. The ageing tests were carried out with three types of working gas mixtures: Ar(30%)+CO₂(50%)+CF₄(20%), Ar(40%)+CO₂(50%)+CF₄(10%) and Ar(30%)+CO₂(70%). The last mixture was investigated to estimate a contribution of CF₄ to the ageing resistance of the CMS chambers.
The accumulated charges were:

Gas mixture	Dose,C/cm
Ar(30%)+CO2(50%)+CF4(20%)	13.56
Ar(30%)+CO2(70%)	0.76
Ar(40%)+CO2(50%)+CF4(10%)	13.35+2.2

The set-up

The tests were performed using four identical prototypes of the CMS chambers with the active area of 175 x 240 mm² . One of them was used as a reference, and three others as test chambers. The applied high voltage for each gas mixture was chosen so as to provide the gas gain 4x10⁴ . The gas flow was kept at the rate 75 volumes per day till the accumulated charge 13.56 C/cm was obtained. The rest 2.2 C/cm charge was accumulated with small gas flows 0.1 V/day and 1 V/day.

The given scheme presents the test point distribution and cross section of the prototype chamber.

Ageing results

For Ar(30%)+CO₂(50%)+CF₄(20%) mixture gas gain reduction was not observed up to the accumulated charge Q = 13.56 C/cm. However, the dark current in the chamber was noticeable starting already from Q = 2 C/cm.

The absence of the CF₄ in the Ar/CO₂ gas mixture caused fast aging deterioration, and Q = 0.2 C/cm was enough to obtain 50% of the gas gain reduction.

In the test with Ar(40%)+CO₂(50%)+CF₄(10%) mixture the gas gain reduction and the dark current increase were not observed up to Q = 13.35 C/cm. The reduction of the gas flow rate to 0.1 volume per day caused 50% of the gain decrease due to the gas pollution by fragments of dissociated gas molecules. The gain restored after turning back to the 1 volume per day flow rate.

Studies of the cathode and wire samples with SEM/XEM methods find the traces of silicon. It may be assumed that the most probable source of the chamber contamination is the Si-reach RTV sealing and cathodes honeycomb FR4.

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The ageing chambers were irradiated with the ¹³⁷Cs (2 GBq) source at the GIF (Gamma Irradiation Facility).
For two tested chambers the doses were:

Chamber ID	001	002
Layer 1 Dose (C/cmwire)	0.281	0.250
Layer 2 Dose (C/cmwire)	0.144	0.107

The set-up

The chambers had a modified design of a bottom segment of an ME2/1 chamber, with addition of an extra guard strip in the middle of the sensitive volume. The working gas mixture was Ar(40%)+CO₂(50%)+CF₄(10%) and the gas flow rate was 1 volume per day. The working point was chosen so as to provide the average gas gain at the level of 4x10⁴ . The location of the test points is shown below.

Results

The CMS EMU prototype 002 after ageing run has been dismounted. The cathodes and wires planes have been visually inspected.

Cathode surface: Visual inspection of the cathode planes determined the white coating covering practically the whole cathode surface. The detailed analysis using SEM/XEM methods has shown the presence of the silicon, calcium and bromine on the cathode surface.

The white crystals at the cathode surface are seen at the SEM-micrograph. The corresponding XEM-spectrum is shown below. The presence of the bromine and calcium peaks allows to conclude that the main source of the silicon deposits was the mashined fiberglass of the cathode honeycomb.

Anode wire surfaces: The wire analysis demonstrated clear presence of the silicon and carbon deposits on the wires. It must be noted that the most noticeable amount of deposits was observed on the wires located in the vicinity of the guard strips. The most typical view of the coated wire is presented.

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