The Resistive Plate Chamber detectors - PowerPoint Presentation

Slide1

The Resistive Plate Chamber detectors at the Large Hadron Collider experiments

Roberto

Guida Paolo Vitulo

PH-DT-DI Univ. Pavia

CERN

EDIT school 2011

Slide2

1949:

Keuffel

 first

Parallel Plate Chamber 1955: Conversi used the “PPC idea” in the construction of the flash chambers 1980: Pestov  Planar Spark chambers – one electrode is resistive – the discharge is localised 1982: Santonico  development of the Resistive Plate Chamber – both electrode are resistive RPC applications: ‘85: Nadir (n-n\bar oscillation) – 120 m2 (Triga Mark II – Pavia) ‘90: Fenice (J/  n-n\bar) – 300 m2 (Adone – Frascati)‘90: WA92 – 72 m2 (CERN SPS)‘90: E771– 60 m2; E831 – 60 m2 (Fermilab)1992: development of RPC detector suitable to work with high particle rate  towards application at LHC 1994-1996: L3 – 300 m2 (CERN-LEP)1996-2002: BaBar – 2000 m2 (SLAC)

Some history

Slide3

Identikit of RPC detectors for LHC

Basic parameter for a detector design:

Gap width

Single gap/double gap/multi gap design Gas mixture Gas flow distribution Bakelite bulk resistivity Linseed oil electrode coatinges

Slide4

The RPC detector

Gap

width 2 mm:

Good compromise between good efficiency, time resolution and rate capability More gaps:Increase time resolution and efficiency Double gap design:Best ratio induced/drift charge, therefore best signal/charge ratio Freon based mixture:Higher efficiency (at the same gas gain) and lower streamer probability Bakelite bulk resistivity = 1-6 1010 W cm:Good compromise between high rate capability and low current and noise Linseed oil treatment:Lower current and noise rate. No ageing effect observed

Slide5

Why the RPC?

Drift chambers (cylindrical geometry) have an important limitation:

Primary electrons have to drift close to the wire before the charge multiplication starts

limit in the time resolution  0.1s Not suitable for trigger at LHC+ In a parallel plate geometry the charge multiplication starts immediately (all the gas volume is active).+ much better time resolution ( 1 ns)+ less expensive ( 100 €/m2)However:-Smaller active volumeElectrical discharge may start more easilyRelatively expensive gas mixtureQuite sensitive to environmental conditions (T and RH)

Slide6

Lab. activity:

Switch on the RPCs

HV scan

Pulse height Pulse charge

Slide7

Avalanche signal

Streamer signal

Towards a new operation regime

Originally RPC were operated in Streamer mode:Ar-based mixtureHigher signal (100 pC) but also high current in the detectorVoltage drop at high particle rate  loss of efficiency  poor rate capability (< 100 Hz/cm2) Operation with high particle rate possible in Avalanche mode:Freon-based mixturelower signal ( pC) but also lower current in the detectorLess important high voltage drop at high particle rate  good rate capability ( 1 kHz/cm2)R. Santonico et al.ATLAS Muon TDR

Slide8

Ionizing particles passing through the gas are producing primary ionization

Primary electrons accelerated in the electric field will start to produce further ionization

n

total: total number e-/IonE: total energy lossWi: <energy loss>/(total number e-/Ion)Working principle

Slide9

Gain

Primary e

-

Charge multiplication = first Townsend coefficient= attachment coefficient = - = effective Townsend coefficientx  20  M  108 Reather breakdown limit

Slide10

Time constant for charge development is related to drift velocity and multiplication

Time constant for recharge the elementary cell is related to the RC

discharge << recharge Are the most important improvement with respect to previous generationsSince recharge >> discharge the arrival of the electrons on the anode is reducing the electric field and therefore the discharge will be locally extinguished.the electrode are like insulator after the first charge development Self-extinguish mechanismHV



discharge

=

1/

v

d

~ 10 ns



recharge

=

~ 10 ms

Why resistive electrodes?



Slide11

The RPC detector: gap width

The gap width is affecting both

time performance charge distribution

Narrow gap  better time resolution, but “more critical” charge spectrum (R seems to be the driving parameter)  reduce number of primary electrons R = l/R > 1R < 1 cluster density (primary clusters)g =const = 18M. Abbrescia et al. NIMA 471 55-59

Slide12

The RPC detector: single vs double gap

The double gap layout:

allow to operate at lower

 (i.e. lower high voltage)best ration qind/qtot (single 0.7; double 1.4; 3 gaps 0.8)Better charge distributionTwice expensive per unit of active area

Slide13

Formation of the induced signal

Equivalent circuit of double RPC near the discharge region:

Ratio between total charge and induced charge on the strips:

Slide14

The RPC detector: gas mixture

Key parameters:

 cluster density (primary clusters) – only gas dependent

 first Townsend coefficient (charge multiplication) – gas, HV dependent  attachment coefficient – gas (electronegative and/or quencher), pressure dependent  effective Townsend coefficient =  -  High  gas: high efficiency and low streamer probability at the same !i.e. for avalanche regime: better Freon ( = 5.5 mm-1) than Ar ( = 2.5 mm-1)Voltage increasing

Slide15

The RPC detector: gas mixture

Key parameters:

Moreover quencher (iC

4H10) and electronegative (SF6) gases will help in containing the charge development and reducing the streamer probability Charge distribution:avalanche streamerFraction of streamer vs High voltage:M. Abbrescia et al. NIMA 508 142-146R. Santonico Scientifica Acta Pavia

Slide16

The RPC detector: gas mixture

Key parameters:

Large  are giving also a better charge distribution (constant )

 ≤ 4 mm-1 spectrum monotonically decreasingCharge spectrum vs Streamer probabilityM. Abbrescia et al. NIMA 431 413-427

Slide17

The RPC detector: gas flow distribution

Experience says that the gas flow has to be at least

 0.3 vol / h even without radiation

Is the gas distribution inside the gap the origin of this limit?Is there a space for future improvements? Some preliminary results coming from a finite element simulation: Waldemar MaciochaAntonio RomanazziThe velocity field shows areas in which the gas is hardly movingThanks toGas velocity (m/s)inlets

Slide18

RPC detector: bakelite resistivity

The detector rate capability is strongly dependent on the bakelite resistivity.

At high particle rate (r) the current through the detector can become high enough to produce an important voltage drop (V

d) across the electrode:s: electrode thickness<Qe>: average pulse charge: bakelite resistivity In order not to lose efficiency  Vd < 10 V Therefore  r ~ 1010 W cm The time constant of an elementary cell is lower at lower resistivity:the cell is recovering faster (it is quicker ready again) after a discharge took place inside it.

Slide19

RPC detector:

linseed oil surface treatment

RPC electrodes are usually treated with linseed oil:

better quality of the internal electrode surfaceit acts as a quencher for UV photonsbetter detector performance …but…More time needed during constructionAgeing problems? (Not observed)What is the linseed oil:Drying oil (consists basically of triglycerides)Drying is related to C=C group in fatty acidCross-linking (polymerization) in presence of air (O2 play important role) due to C=C

Slide20

RPC detector:

linseed oil surface treatment

Few SEM photos (S.Ilie, C.Petitjean EST/SM-CP EDMS 344297):

Defect on Bakelite surface possibly covered with linseed oilThickness of the layer: ~ 5 mThe linseed oil layer is damaged by a surface outgassing

Slide21

RPC detector:

linseed oil surface treatment

Effect on UV photons hitting the electrode internal surface:

Linseed oil absorbance8 eV5.6 eVUV sensitivity for coated and non coated BakeliteC.Lu NIMA 602 761-765P.Vitulo NIMA 394 13-20

Slide22

RPC detector:

linseed oil surface treatment

Chamber Performance:

With linseed oil coated electrodesLower current (1/10) Lower noise rate (1/10) M. Abbrescia et al. NIMA 394 13-20

Slide23

RPCs for LHC experiments

Why RPCs for application in LHC experiments need a particular “care”?

Huge (

5000 m2 of sensitive area) and very expensive (6 106 CHF) systems (for comparison BaBar was about 2000 m2)Very long period of operation expected (at least 10 years)Very high level of background radiation expected Integrated charge never reached before: 50 mC/cm2 for ALICE and CMS 500 mC/cm2 in ATLASLarge detector volume  basically impossible to operate the gas system in open mode  closed loop operation  gas mixture quality

Slide24

RPCs for LHC experiments

Where are the RPCs systems at LHC?

ATLAS experiment:

- Active surface 4000 m2 - 94.7% C2H2F4; 5% iC4H10; 0.3 % SF6- Gas Volume 16 m3 - 40% Rel.Humidity- Expected rate ~ 10 Hz/cm2 - Closed loop operationCMS experiment:

Slide25

Closed loop gas circulation

Large detector volume (

~

16 m3 in ATLAS and CMS) use of a relatively expensive gas mixture closed-loop circulation system unavoidable. Nowadays with 5-10 % of fresh gas replenishing rate  cost is 700 €/dayBut….Several extra-components appear in the return gas of irradiated RPCsDetector performances can be affected if impurities are not properly removed Purifiers:

Slide26

Gas analysis results: chromatography

Many extra components identified in the return mixture from detector

Operated with open mode gas system

Under high gamma radiation Concentration of the order of 10 ppmMainly hydrocarbonsother Freon

Slide27

With respect to reference bakelite surface:

High fluorine concentrationNa signal appeared

N signal disappeared

Linseed oil and melamine layer etched:Na is used as a catalyser for phenolic resin (bulk)Normal surface layer (made on melamine resin) contain N Bakelite SEM resultsWe analyzed few bakelite samples from an RPC with relatively high current. The visual inspection of the surface shows at least two different kinds of surface defects:Reference bakelite

Slide28

Operation of RPC detectors

The operation of a large area detector is never simple.

“Second order” problems may come from anywhere and anytime.

Few example:Gas quality is a crucial issue for all gaseous detectors (therefore also for RPC)Environmental conditions (like temperature and relative humidity) are affecting the detector performances (complex network of sensors is needed in order to understand behaviours)Gas leaks in the detector (unfortunately is a weak point) In the following, for time reason, I will discuss only an example concerning the gas quality

Slide29

RPC detectors at LHC: some data

A

CMS event with

muon track reconstructedLab. activity: Switch on the RPCs

Slide30

Lab. activity:

Pulse charge spectra

Avalanche vs. Streamer

SignalsTime behaviorChargeNoise charge spectra

Slide31

0.05% SF6

0% SF6

0.2% SF6

Slide32

0% SF6

0,05 % SF6

0,2 % SF6