Study of Background Noise in the DVCS Experiment Hall A - JLab Florian ITARD Monastir – December 15th 2008

Stakes Background noise Continuous signalInconvenience 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 influence on 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 many parameters, and then code less and less supported by informatic farms

GEANT 4 News 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 visualisation Written in a modern language Simplified access to the informations Inconvenient: Still too few informations on commands

Experiment geometry

Results for the reference geometry Beam In KeV Electromagnetic processes Calorimeter with 2.5 cm aluminium front shielding Normal downstream beam pipe tube First depth Second depth Third depthFourth depth Mean deposited energy in each piece of block by electron sent into the target

Pavel Ambrozewicz Study

Hadronics Processes Reference With hadronics processes Reference Bloc 8-1 Dose function on deposited energy With hadronics processes KeV Beam First depth

6 degrees cone with insertion at 7 degrees with cutted iron beam side shielding Reference KeV Beam First depth 6 degrees cone

Rectangular downtream beam pipe Reference6 degrees cone First depth Rectangular KeV Beam

7 cm tungsten block in front of the first column Reference 7 degrees cone KeV First depth 7 cm tungsten block Beam

Tungsten shielding at the intersection between scattering chamber and downstream beam pipe Reference Tungsten block KeV First depth Tungsten shielding Beam

Tungsten shielding with a 1 cm Tungsten plate along the downstream beam pipe tube ReferenceTungsten shielding Extended tungsten shielding First depth Beam KeV

Extended tungsten shielding and 8 cm polyethylen Front shielding ReferenceExtended tungsten shielding Tungsten and polyethylen First depth Beam KeV

Extended tungsten shielding and 35 cm LiH front shielding Reference Tungsten and polyethylen Beam KeV First 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 Bloc 8-13 Energy of Pocatello Beam: 20 MeV

Deposited Dose (Gy) into each block for E and E 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 Gy

Integrated Energy during 10 ns function on time (1500 p.e. / 10 ns) * ( ) * (1, C/e) = 960 uA Experiment anode current = 10 uAFactor 100 Approximation GEANT 3: 1000 photon / GeV Tungsten and LiH Block 8-2 Block MeV Gain PM:

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 producted in the PbF2 crystal by a GeV incident photon =

Cerenkov photons: one block study (suite) 850 photo-electrons by GeV Detected photons Photons reaching air Sum 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 aluminium front shielding 30 cm LiH and 3.39 cm polyethylen front shielding Deposited Energy in 9 blocks

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 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 Gy! law of « all or nothing » at 2000 Gy

Hall-A akbar