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Ken Livingston, University of Glasgow, Scotland

g8 summary

coh. peak (GeV)

2.072.25

unpolarised

triggers (M)

1400350100

Increased world data set on by x 100 for polarised photons in the 1.75 - 2.25 GeV range.

crystal thickness 50umangle to beam ~1mradbeam current 7nA

current statuscalibration almost completepass 0 cooking to begin in ~2 weeks

g8 school report card for coherent bremsstrahlung

component desired qualities (personal) score / 10

photon beam

diamond

goniometer

collimation

tagger

stable, low emittance

thin, uniform monocrystal

high precision (<10 urad), reliable

tight, easily positioned, (active ? )

highly segmented, < 2ns timing res.fast feedback via scalers

6

5

5

8

6

goniometer and diamond radiator

goniometer problemsoversizeno feedback ?got lost, stuck

for g8bgoni to get "full service"

20 um diamond radiator problemscoherent peak jumped around in anunpredictable way

for g8bfind out why

goniometer installed by GWU group

tagger and e-counter scalers

a few noisy channels

... 2 weeks later

Tagger had major upgrade + ecounter scalers. Commissioned at start of g8 run period

tagger problemssome teething troubles with upgrade, but generally performed well.rate in coherent peak limited by big T counters

for g8bnew tagger electronics, scalers can be gated by downstream detector

coherent peak instability

problem with 20um crystal

coherent peak "jumps" aroundby very large amounts.

possible reasons

crystal damaged by grinding to 20umfrom original 100um

beam not stable enough(ruled out by 50 um results)

goniometer (or other) vibration

crystal "bowed" on mounting wires

- to be tested with laser (next week)

electron beam at 5.7 GeV in Hall B

beam problemsStandard quadropole for beam tuning in hall B is downstream of goniometer.Coherent peak is worse for "para" than "perp" due to non-symmetric beam.Angular divergence of beam dominates (lose advantage of thin crystal)

for g8bA new quadrupole to be fitted upstream of the goniometer

conclusion

Overview of g8

g8 was the first run group to use linearly polarised at JlabVery successful first run period.

problemsSeveral minor problems due to first use of system, easily fixed for g8bProblems due to "retrofitting": tagger and beam focussing to be improved for g8bOutstanding problem - 20um crystal

Hall Dbig advantageDesigned specifically for coherent bremsstrahlung.Should be able to avoid most hall B problems.

problemsAnticipate using a 10um crystal. Still need to understand the "jumping peak" problem.

** Important to decouple coherent brem source from main experimental DAQ