study of background noise in the dvcs experiment hall a - jlab florian itard monastir – december...
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Study of Background Noisein the
DVCS Experiment Hall A - JLab
Florian ITARDMonastir – December 15th 2008
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Stakes
Background noise
Continuous signal Inconvenience the measures
Darkening Distort the measures
New calibration
Need to reduce the background noise
Simulation
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GEANT 3
Search of hot points
Difficulties to identify clairly the origin of particles
Idea: record deposited energy in all the calorimeter,Change geometry of experiment and notice the influenceon the measure
Target length, angle influence, shapes, tickness and nature of shieldings,…
No evident identifications of parameters: « premature » conclusion that the most part of Background noise come directly from the target
Difficulties to code, « heavy » commands with manyparameters, and then code less and less supported byinformatic farms
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GEANT 4News bases of research:
Priority to the local aspect of background noise
Restriction at one unique constraint: reduce the background noise without change the calorimeter position
Benefits of GEANT 4:
Facility of geometry visualisationWritten in a modern languageSimplified access to the informations
Inconvenient:
Still too few informations on commands
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Experiment geometry
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Results for the reference geometry
Beam
Beam BeamBeam
In KeVElectromagneticprocesses
Calorimeter with 2.5 cm aluminiumfront shielding
Normal downstreambeam pipe tube
First depth
Second depth Third depth Fourth depth
Mean depositedenergy in eachpiece of block by electron sentinto the target
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Pavel Ambrozewicz Study
http:/www.jlab.org/~pavel/dvcs/Calorimeter
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Hadronics Processes
Reference
With hadronics processes
ReferenceBloc 8-1
Bloc 8-1Dose function on deposited energy
With hadronics processes
KeV
BeamBeam
First depth
First depth
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6 degrees cone with insertion at 7 degrees with cutted iron beam side
shielding
Reference
KeV
Beam
First depth
First depth
6 degrees cone
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Rectangular downtream beam pipe
Reference 6 degrees cone
First depth
First depth First depth
Rectangular
KeV
Beam
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7 cm tungsten block in front of the first column
Reference 7 degrees cone
KeV
First depth
First depth First depth
7 cm tungsten block
Beam
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Tungsten shielding at the intersection betweenscattering chamber and downstream beam pipe
Reference Tungsten block
KeV
First depth First depth
First depth
Tungsten shielding
Beam
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Tungsten shielding with a 1 cm Tungsten plate along the downstream beam pipe tube
Reference Tungsten shielding
Extended tungsten shielding
First depth
First depth First depth
Beam
KeV
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Extended tungsten shielding and 8 cm polyethylen Front shielding
Reference Extended tungsten shielding
Tungsten and polyethylen
First depth
First depthFirst depth
Beam
KeV
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Extended tungsten shielding and 35 cm LiHfront shielding
Reference Tungsten and polyethylen
Beam
KeV
First depth
First depthFirst depth
Tungsten and LiH
LiH
Tungsten
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Deposited Mean Dose
Bloc 8-7
Bloc 8-1
On 100 MeV of deposited energy, 50% come from particles under 24 MeV
Reference
Reference
Reference
Bloc 8-13
Energy of Pocatello Beam: 20 MeV
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Deposited Dose (Gy) into each block for E00-110 and E03-106 integrated luminosity
Problem: study of pocatello on curing show that only 7 KGy could reduce the transmission by 20%Idea: compare the anode current simulation to the experiment values
300 000 Gy
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Integrated Energy during 10 ns function on time
(1500 p.e. / 10 ns) * (4.104) * (1,6.10-19 C/e) = 960 uA
Experiment anode current = 10 uA Factor 100
Approximation GEANT 3: 1000 photon / GeV
Tungsten and LiH
Tungsten and LiH
Block 8-2
Block 8-13
1500 MeV
Gain PM: 4.104
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Energy frequency
Idea: low energy particles don’t give as much cerenkov photons than high energy particles
On 100 particles reaching the block 8-1, 50% are inferior at 4.6 MeV
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Cerenkov photons: one block study
Mean number of cerenkov photons productedin the PbF2 crystal by a 1.280 GeV incident photon = 80 000
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Cerenkov photons: one block study (suite)
850 photo-electrons by GeV
Detected photons
Photons reaching airSum
Zoom
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Low efficiency reasons in the detection of cerenkov photons
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Low efficiency reasons in the detection of cerenkov photons (suite)
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Cluster of nine blocks: influence of shieldings
Without shielding
2.5 cm aluminiumfront shielding
30 cm LiH and 3.39 cm polyethylen front shielding
Deposited Energy in 9blocks
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Cerenkov photons: 208 blocks Calorimeter
0.25 MeV by ARS chanel
Reference pulse
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Cerenkov photons: 208 blocks Calorimeter (suite)
300 channels ~ 200 mV
Block 8-2
Block 8-13
1.6 ARS channels / mV
200 mV / 50 4 mA
Pre-amplification gain = 8
4 mA / 8 = 500 uA
Factor 50
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Curing?KeV
Depth
Block 8-1
Reference
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Conclusion
Background noise reduction:
factor 8 dans la première colonne factor 3 dans la deuxième colonne reduction of one quarter in the other part of calorimeter
Recommended geometry:
addition of tungsten block with extended plate change of front shielding by a mixture of LiH and polyethyelen shielding according to the place and respect to the length radiation
Avantages of the new geometry: no heavy modifications of the scatering chamber and down beam pipe tube
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Conclusion (suite)
Anode current:
factor 50 with the most irradiated block (after reduction of background noise, otherwise factor 150 with the reference geometry)
Curing:
idiot at 300 000 Gy! law of « all or nothing » at 2000 Gy
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Hall-A akbar