1. R. Guida PH-DT-DI Boston Students Program CERN, 16/1/2013
Monitoring of the gas mixture quality/composition for particle detectors
R. Guida PH-DT-DI Boston Students Program CERN, 16/1/2013

2. Gaseous particle detectors
Basic design of gaseous detector: a capacitor filled with a well defined gas mixture and with a well defined electric field
Ionizing particles passing through the gas volume produce e--Ion+ couples
 Primary ionization
The primary e--Ion+ couples travel under the effect of the electric field and they are accelerated
At a certain point the primary e- can gain enough energy to produce other e--Ion+ couples  Secondary ionization
The total charge produced is measured by the read-out electronic and therefore the passage of a ionizing particle is detected
16/1/2013
R. Guida PH-DT-DI

3. Introduction
Ionizing particles passing through the gas are producing primary ionization
Primary electrons accelerated in the electric field will start to produce further ionization
Primary and secondary ionization depend on the gas mixture used in the detector: The gas mixture is the sensitive media therefore a correct and stable composition is a basic requirement for a good and long term operation of all detectors
ntotal: total number e-/Ion
E: total energy loss
Wi: <energy loss>/(total number e-/Ion)

4. Gas systems at the LHC experiments
Gas systems for detectors at the LHC experiments are large apparatus:
they extend over about 400 m
Typical gas system for a gas detector at LHC
Gas detectors at the LHC experiments
Monitoring of the mixture composition and relatively simple cases of impurities as O2 and H2O contamination.
Monitoring of more complex impurities as the radicals produced by the breakdown of the main mixture components due to the high radiation background.
16/1/2013
R. Guida PH-DT-DI

5. Gas quality vs detector performances
O2 measured on the return of one RPC gas system.
Dramatic increase of the O2 concentration detected by the gas analysis module.
Several tests allowed to localize the problem in one of the distribution rack and to solve it.
Detector performances were affected
Detector currents (A)
16/1/2013
R. Guida PH-DT-DI

6. Mixture composition: Gas chromatography
A gas chromatograph is a gas analyzer. It allows to separate and identify the components of a gas mixture.
16/1/2013
R. Guida PH-DT-DI

7. Gas chromatography: monitoring
The systematic monitoring of the gas mixture using the gas chromatography stations allows to identify the origin of potential problem like:
Low quality of the gas supply
Out-gassing material
Drift in the calibration of the mass flow controllers used to prepare the gas mixture for the detector
Fault in some gas system components (pressure regulator, valves, etc)
The increased SF6 concentration produced a shift in the detector operating parameters
16/1/2013
R. Guida PH-DT-DI

8. Gas chromatography: monitoring
In the CMS Underground gas room a complex GC station is installed. It allows to analyze the gas mixture coming from different regions of all muon detectors (CSC, DT and RPC) and therefore to study the effect of different radiation levels (mixture coming from different areas of the detectors) as well as to localize possible problems.
The GC automatically controls three multi-position valves allowing to select up to 46 different analysis stream subdivided between the three muon detectors.
CSC DT
Inner
detectors
RPC
16/1/2013
R. Guida PH-DT-DI

9. Conclusions Objectives for the stage
Get familiar with gas detectors, gas systems and gas analyzers
Contribute in the weekly monitoring of the gas mixture for the LHC experiments
Contribute in the start-up of the CMS-GC underground installation.
16/1/2013
R. Guida PH-DT-DI