BESIII Muon Counter

Jiawen ZHANG

BESIII Workshop June 5-6 2002

Introduction (1)

Outmost subsystem Main function: Measur

e the muons of the end particles produced during reaction. Identify muons from hadrons (especially pions).

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Introduction(2) Muon counter is very important The channel of decay to muons clean J/Ψ discovery cross section measurement is one of the important ev

idences mass precise measurement on BES m

easured mainly through

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Introduction(3)

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i Z/ff In reaction, many muons are produced during D decays and decays.

The momentum distribution muons produced during D decays and decays near 2×2.0 GeV.

Introduction(4)

Lower muon end momentum Greater cover solid angle Higher detection efficiency Safer working gas Suitable location precision

Detector Choice (1)

Brief introduction to (RPC)• The RPC is developed by R. Santonico in the

early 80’s. • Several large experiments have used it. For in

stance, Belle, BaBar, CMS, ATALAS, L3 and ARGO have used RPC

• Much successful experience was accumulated

Detector Choice(2)RPC structure• Two parallel high resistive plate electrodes • gas room • pickup strip

Detector Choice(3)

Detector Choice(4)

Detector Choice(5)

RPC’s characteristics • Simple and solid

structure • Superior time and

spatial property • High detection

efficiency and little dead space

• Flexible signal readout

• Occupation of small space

• Mature technology • Good radiation

hardness • Easy management and

maintenance • Big signal • Long lifetime

Detector Choice(6)

RPC comparison with PST

RPC and PST resemble very much in many places,

• working in streamer mode• the signal read out

through the pickup strip,• big signal• graphite to be sprayed

Simple comparison of RPC and PST

• Time performance • Spatial resolution • Detection efficiency • The crucial material

Monte Carlo Simulation (1)

Careful simulation studies were made for initial designing and optimizing

Geant 3.21 Condition 13.5 radiation lengths CsI, All of the other inner detectors equal to 5cm

Fe plate

Monte Carlo Simulation (2) detection efficiency a

nd contamination

If we choice suitable thickness of the absorber, the pion contamination to muon can be reduced to a lower level when the muons detection efficiency is ensured.

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Thickness of Fe(cm)

Effi

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m: P=0. 35GeV p: P=0. 35GeVm: P=0. 4GeV p: P=0. 4GeVm: P=0. 45GeV p: P=0. 45GeVm: P=0. 5GeV p: P=0. 5GeVm: P=0. 6GeV p: P=0. 6GeVm: P=0. 7GeV p: P=0. 7GeVm: P=0. 8GeV p: P=0. 8GeVm: P=0. 9GeV p: P=0. 9GeVm: P=1. 0GeV p: P=1. 0GeVm: P=1. 1GeV p: P=1. 1GeVm: P=1. 2GeV p: P=1. 2GeV

Monte Carlo Simulation (3) hits position distribution The sigma of the hit posi

tion distribution of moun will be about 4 to 8cm

If the position resolution is to be increased, only increase the electronics readout channels, but the effect to identify muon and pion is not obvious.

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P=0. 35Gev P=0. 4GeV

P=0. 45GeV P=0. 5GeV

P=0. 6GeV P=0. 7GeV

P=0. 8GeV P=0. 9GeV

P=1. 0GeV P=1. 1GeV

P=1. 2GeV

Monte Carlo Simulation (4)

The muon detection efficiency in one dimension readout will be higher than that of 2-dimension readout

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Monte Carlo Simulation (5)

The contamination of muon by pion will be increased, but to a very limited level.

In the high momentum range, the contamination of muon by pion has no difference 0

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Monte Carlo Simulation (6)The influence of noise background 2-dimension read-out can reduce the influence o

f noise background properly, and it is superior to one-dimension readout .

But we can use the time gate to reduce the noise background during the electronics readout.

If the noise of RPC reaches 1 KHz/ ㎡ , we use 2000 ㎡ RPC, the time gate width is 100ns, for every event, will record 2000 ㎡ ×1000 Hz/ ㎡ ×10-7s=0.2 signal, so the physics analysis will not be affected .

Overall Structure (1)High detection efficiency for muon Large solid angle coverage Large momentum range (the mini

mum momentum ～ 400MeV) Ability of rejects other charged par

ticles Appropriate positioning precision

Overall Structure(2)The barrel muon counter is subdivided into 8 pieces. Its inner radius is 1700mm and its outer radius 2600mm.

Overall Structure(3) The barrel part has 10

layers of RPC and 9 layers of absorbing iron

9 layers of absorbing iron, thickness of each layer being respectively 3, 3, 3, 4, 4, 8, 8, 8, and 8cm. The total Fe thickness is about 49cm.

4cm gap for arrangement the RPC

Overall Structure(4) The barrel part adopts the rectangle RPC of

different size to overlap the arrangement in order to reduce its dead space

Overall Structure(5) The end cap muon counter has

nine layers of counters and nine layers of absorbing iron.

9 layers of absorbing iron, thickness of each layer being respectively 3, 3, 3, 3, 3, 4, 8, 8, and 8cm. The total Fe thickness is about 43cm

4cm gap for arrangement the RPC

Overall Structure(6) 4 pieces at each en

d 2cm iron between

every two pieces each piece consists

of 4 trapezoids of right angle RPC

pushed into the gap of the yoke from the left side or the right side

The RPC Structure (1)

The RPC consists of two parallel sheets of 2.0mm Bakelite.

The bulk resistivity of the Bakelite is 1011-1012Ω·cm at room temperature.

The plates are separated by 2.0mm thick circular spacers made of insulating material.

The span between every spacer is 100mm. The mixture gas of certain proportion passes th

e gap of the plates as working gas.

The RPC Structure (2) The places round the ga

p are sealed with T-shape spacer made of insulating material

This spacer can reduce the dark current, and guarantee the span of the resistive plate, guarantee the glue thickness and the binding strength

The RPC Structure (3)

The outer surface of the Bakelite is coated with graphite. The surface resistivity adopts 105—106Ω/□.

When graphite is sprayed to the resistive plate, it will not be sprayed to the place which corresponds to the place of spacers for the reduction of the dark current

The RPC Structure (4)

The two layers of RPC and one layer of aluminum pickup strip between the two layers of absorbing iron constitute a superlayer

High Voltage System apply positive voltage to the anodes and negative voltage

to the cathodes The modules typically operate with a total gap voltage of

8—9 KV Each layer of the barrel RPC uses one group of high

voltage, including one positive and one negative high voltage

Each end is divided into 3 groups, with 3 layers forming one group

One CEAN high voltage crate, controlled by computer

Gas System (1) argon ＋ F134A ＋ isobutane control system adopts the mass flow control

system The flow rates from the mass flow controlle

r are monitored via a network connection and the high voltage is automatically lowered if a deviation from the desired flow rate is detected

The flow rate is controlled to approximately one volume change per day

Gas System (2)

The Expected Performance (1)

the total solid angle coverage of the barrel and end cap parts will reach 0.89

monolayer location resolution in Φ direction is 1.2 cm,

The minimum momentum of muon that can be detected reaches 0.35 GeV , momentum larger than 0.4 GeV, the detection efficiency can reach 95% in different degrees of angle

The Expected Performance (2)

Muon separation efficiency and contamination from pion versus momentum

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The R&D of Muon counter

The selection of RPC material and research on the technology of surface treatment

The research on readout strip The choice of gas The influence of environmental temperature and

humidity The choice of spacer and sealed material The shape and the structure of the spacer and seal frame To make a test model according to the design of RPC

and carry out an overall test of its performance

Schedule

Estimated Budget

Thanks