The Gas Čerenkov Detector for the Crystal-

Barrel Experiment at ELSAD. Kaiser

Hemholtz-Institut für Strahlen- undKernphysik der Uni Bonn supported by SFB/TR 16

Bosen 2007D. Kaiser2

Nucleon Resonances

• photoproduction reactions with nucleon

resonances at the CB experiment:

N N* ...

• problem: overlapping resonances

partial wave analysis

polarisation observables

)2cos(1[d

d

d

d

0

ΣlinP

))2sin(( FH circlin PPPx

))2cos(( TP lin

y PP )])2sin(( EG circlinz PPP

Breit-Wigner parametrisation

Bosen 2007D. Kaiser3

Double Polarisation

• polarised target

• polarised photonbeam- linear polarised photon beam by use of

unpolarised electron beam and diamond radiator

50% @ 3.2 GeV

- circular polarised photon beam by use of longitudinally polarised electron beam and amorphous radiator

55% @ 2.4 GeV

Bonn Frozen Spin Target - longitudinally polarised protons

in a butanol target

@ 30-50 mK in frozen-spin mode- polarisation 70 – 90%- repolarisation every ~ 60 h- total relaxation time ~ 360 h

time [hours]

polarisation

Bosen 2007D. Kaiser4

Electron Accelerator ELSA

Bosen 2007D. Kaiser5

Crystal-Barrel Experiment

Crystal-Barrel detector

tagger

MiniTAPS

polarised target

GIM

camera

e- beam dump

goniometer

Gas Čerenkov detector

e- beam

Bosen 2007D. Kaiser6

Electromagnetic Background

for butanol target under forward angles: ~100.000 e± 1 hadr.

due to:

• pair production

• Compton scattering

for hydrogen target in full angular acceptance: ~1.000 e± 1 hadr.

need for supression already on trigger level

Bosen 2007D. Kaiser7

Čerenkov Effect• discovered 1934 by P.A. Čerenkov (and S.I. Vavilov)• Nobelprice 1958, together with I.E. Tamm and I.M. Frank• charged particles with high velocity generate

electromagnetic radiation in dielectric media

cv

cvn

oc

Bosen 2007D. Kaiser8

Čerenkov Effect

22 11

11

ns

s 2mcE ss

nc 1cos

observable radiation for a particular angle c:

cv

threshold for Čerenkov radiation:

with 1noc

v

Bosen 2007D. Kaiser9

Čerenkov Effect

• threshold energy “adjustable“ by use of different Č media

• useful for particle discrimination and determination

• simple set-up with Č medium and photon detector

Čerenkov threshold detector

• other set-ups possible (e.g. RICH)

Bosen 2007D. Kaiser10

Čerenkov Effect

dLN cphotons 2

2 1sin2

approx.1700 photons/cm in water approx. 2 photons/cm in CO2

range of normal PM

Used photomultiplier: Hamamatsu R1584-03SEL

Bosen 2007D. Kaiser11

Gas Čerenkov Detector- drawing and picture

Bosen 2007D. Kaiser12

Gas Čerenkov Detector

forward crystals Gas Čerenkov detector MiniTAPS

- positioning and angular space coverage

Bosen 2007D. Kaiser13

Gas Čerenkov Detector• periodic repolarisation of the target

easy in- and output of the detector is required

Bosen 2007D. Kaiser14

Modifications

• gas system • partial pressure sensor

• scale

Bosen 2007D. Kaiser15

Simulation of the Efficiency• verification of the efficiency of the detector

maximum efficiency of 99.97 %

Bosen 2007D. Kaiser16

Determination of the Efficiency

• done at the old CB area

• e± from pair production / Compton scattering on a carbon target

efficiency of 99.72 % ± 0.45 %

Bosen 2007D. Kaiser17

Installation in the CB Set-up

more adaptions needed:

• electronic• trigger logic• DAQ• analysis software ExPlORA

Bosen 2007D. Kaiser18

Use as Veto Detector

Start of readout if two tagged hits in the Crystal-Barrel-, Forward- or

MiniTAPS detector were found

Start of readout if two tagged hits in the Crystal-Barrel-, Forward- or MiniTAPS detector were found,

with applied Čerenkov veto

Bosen 2007D. Kaiser19

Use as Veto Detector

number of suppressed events in the subdetectors

cut on events correlated in time

Bosen 2007D. Kaiser20

Čerenkov Detector as Time Reference

Bosen 2007D. Kaiser21

Čerenkov Detector as Time Reference

Bosen 2007D. Kaiser22

Summary

• Gas Čerenkov detector modified for use in the Crystal-Barrel experiment

• tests and determination of the efficiency

• detector integrated and in use– as veto detector– as time reference

• data taking has already started