Study of Background Noisein the

DVCS Experiment Hall A - JLab

Florian ITARDMonastir – December 15th 2008

Stakes

Background noise

Continuous signal Inconvenience the measures

Darkening Distort the measures

New calibration

Need to reduce the background noise

Simulation

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

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

Experiment geometry

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

Pavel Ambrozewicz Study

http:/www.jlab.org/~pavel/dvcs/Calorimeter

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

6 degrees cone with insertion at 7 degrees with cutted iron beam side

shielding

Reference

KeV

Beam

First depth

First depth

6 degrees cone

Rectangular downtream beam pipe

Reference 6 degrees cone

First depth

First depth First depth

Rectangular

KeV

Beam

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

Tungsten shielding at the intersection betweenscattering chamber and downstream beam pipe

Reference Tungsten block

KeV

First depth First depth

First depth

Tungsten shielding

Beam

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

Extended tungsten shielding and 8 cm polyethylen Front shielding

Reference Extended tungsten shielding

Tungsten and polyethylen

First depth

First depthFirst depth

Beam

KeV

Extended tungsten shielding and 35 cm LiHfront shielding

Reference Tungsten and polyethylen

Beam

KeV

First depth

First depthFirst depth

Tungsten and LiH

LiH

Tungsten

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

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

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

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

Cerenkov photons: one block study

Mean number of cerenkov photons productedin the PbF2 crystal by a 1.280 GeV incident photon = 80 000

Cerenkov photons: one block study (suite)

850 photo-electrons by GeV

Detected photons

Photons reaching airSum

Zoom

Low efficiency reasons in the detection of cerenkov photons

Low efficiency reasons in the detection of cerenkov photons (suite)

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

Cerenkov photons: 208 blocks Calorimeter

0.25 MeV by ARS chanel

Reference pulse

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

Curing?KeV

Depth

Block 8-1

Reference

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

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

Hall-A akbar