Hall C Compton PolarimeterPreliminary Designby the

Qweak Polarimetry Working Group

S. Kowalski, M.I.T. (chair)D. Gaskell, Jefferson LabR.T. Jones, K. Joo, U. Connecticut

with modifications forQweak collaboration meetingBoston, October 10-11, 2003

Hall C Polarimetry WorkshopNewport News, June 9-10, 2003

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Outline

Overview of Qweak Qweak plan for polarimetry Criteria for the Compton design The Compton chicane Pulsed vs. coincidence operation Monte Carlo simulation Laser options Detector options Outlook

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Overview of Qweak

Precision measurement of proton weak form factor at low Q2

At Q2 0 interpretation is clean: running of sin2w

Interesting proposals for New Physics show deviations from

SM at the level 0.5% in sin2w

Qweak of proton (1 - 4sin2w) is a sensitive measure:

Qw/Qw = 5% sin2w/sin2w = 0.5%

Measuring Qweak to 5% requires measuring ALR in polarized

electron scattering at the level 3%.

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Beam requirements for Qweak

E = 1.165 GeV (1-pass) I = 180 A P = 80% (known to ±1%) ALR(proton) ~ 3·10-7 at Q2 ~ .03 GeV2

– beam position stability 100 m (40 nm)– beam size stability --- (2 m)– beam angle stability 100 r (60 nr)– beam energy stability 10-3 (10-8)– P expected to vary > 1% during run

continuous monitoring of polarization

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Qweak plan for polarimetry

Design goal: 1% overall uncertainty on P Moller runs: measure P at fixed intervals

– requires reduction in current to few A

– sufficient precision reached in short time (30 min.)

– reliable for absolute measurement at 1%

– can be used to calibrate the Compton

Build a Compton polarimeter for Hall C– runs continuously

– should be capable of 1% systematic error over periods between Moller runs

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Qweak plan for polarimetry, cont.

Relevant parameter is average P over run– want luminosity-weighted average

– corrections are second order in ALR

Information from Hall A useful for monitoring stability and performing consistency checks.

Qweak should be able to measure polarization and verify accuracy independent of what is going on in other halls.

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Criteria for the Compton design

Measure luminosity-weighted average polarization over period of ~1 hour with combined statistical and systematic errors of 1.5% under Qweak running conditions

Control systematic errors at 1% level

Coexist with Moller on Hall C beamline

Configurable for running at higher energies, up to 11 GeV.

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The Compton chicane

10 m

2 m

D1

D2 D3

D4

Comptondetector

Comptonrecoildetector

D

4-dipole design accommodates both gamma and recoil electron detection small beam-laser crossing angle (~1 degree)

– protects mirrors from direct synchrotron radiation– implies significant cost in luminosity– simplifies alignment

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The Compton Chicane, cont.

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The Compton Chicane, cont.

Alex Bogacz (CASA) has found a way to fit the chicane into the existing Hall C beamline.

– independent focusing at Compton and target

– last quad triplet moved 7.4 m downstream

– two new quads added, one upstream of Moller and one between Moller arms

– fast raster moves closer to target, distance 12 m.

– beamline diagnostic elements also have to move

Focus with = 8 m near center of chicane

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The Compton Chicane, cont.

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The Compton Chicane, cont.

3 configurations support energies up to 11 GeV

Beam energy bend B D xe (=514nm)

(GeV) (deg) (T) (cm) (cm)

1.165 10 0.67 57 2.4 2.0 1.16 4.1 2.5 1.45 5.0 2.5 4.3 0.625 25 2.2 3.0 0.75 2.6 6.0 1.50 4.9 4.0 2.3 0.54 13 1.811.0 1.47 4.5

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Pulsed vs. coincidence operation

Detect both gamma and recoil electron– two independent detectors

– different systematics – consistency checks

Two methods to reject background counts1. gamma-electron coincidence

– rates should not be a limitation

– gets rid of some backgrounds

2. pulsed laser operation– backgrounds suppressed by duty factor of laser

– gets rid of additional bg, eg. bremsstrahlung

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Illustration of pulsed-mode operation

detectorsignal

signal gate

background gate

laseroutput

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Advantages of pulsed-mode operation

Two independent asymmetry measurements

More flexible choice of high-power lasers

Can provide high luminosity without the cost of a

mode-locked cavity.– A resonant cavity design requires high-reflectivity mirrors

which are sensitive to synchrotron light.

– To shield the mirrors generally requires a crossing angle of a degree or so.

– In general L ~ 1 / crossat such angles.

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Luminosity vs. crossing angle

Assume a green laser

= 514 nm Fix electron and laser foci

= 100 m Emittance of laser beam

scaled by diffraction limit

= M (/ 4

Scales like 1/cross down

to scale of beam divergence.

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How to “count” in pulsed-mode

Cannot count individual gammas because pulses overlap within a single shot.

Q. How is the polarization extracted?

A. By measuring the energy-weighted asymmetry.

Consider the general weighted yield:

Then for a given polarization, the asymmetry in Y depends in general on the weights wi used.

i

iw Y

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How to “count” in pulsed-mode

What is the optimal weight to use when forming the asymmetry?

The answer must depend on the Compton analyzing power

where ±(k) is shorthand for the polarized differential cross section, which depends on c.m. scattering angle or equivalently on lab scattered photon energy k.

)()(

)()()A(

kk

kkk

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Problem can be solved analytically

wi = A(k)

Solution is statistically optimal, maybe not for systematics.

Standard counting is far from optimal

wi = 1

Energy weight is better!

wi = k

How to “count” in pulsed-mode

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Define a figure-of-merit for a weighting scheme

How to “count” in pulsed-mode

f (ideal) f (wi=1)> f (wi=k)

514nm 2260 9070 3160

248 nm 550 2210 770

193 nm 340 1370 480

N

fp )ˆ(V

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Systematics of energy-weighted counting– measurement independent of gain– no need for absolute calibration of detector– no threshold

Can electron counter use a similar technique?– would need to be segmented– rate per segment should be < 1/shot– one scalar on each segment– weighting used when combining results from different

segments

How to “count” in pulsed-mode

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Monte Carlo simulations

Needed to study systematics from– beam-laser misalignment– detector misalignment– beam-related backgrounds– crossing angle effects– detector nonlinearities

Processes generated– Compton scattering from laser– synchrotron radiation in dipoles (with secondaries)– bremsstrahlung from beam gas (with secondaries)– standard Geant3 list of physical interactions

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Monte Carlo simulations

Compton-geant: based on original Geant3 program by Pat Welch

dipole chicane

backscatter exit portgamma detector

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Monte Carlo simulations

Several events superimposed

Compton recoil electron not yet simulated, coming soon

electron beam

Compton backscatter (and bremsstrahlung)

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Monte Carlo simulations

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Laser options

1. External locked cavity (cw)– Hall A used as reference

2. High-power UV laser (pulsed)– large analyzing power (10% at 180°)

– technology driven by industry (lithography)

– 65W unit now in tabletop size

3. High-power doubled solid-state laser (pulsed)– 100W commercial unit available

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Laser options: comparison

laser l P Emax rate <A> t (1%)option (nm) (W) (MeV) (KHz) (%) (min)

Hall A 1064 1500 23.7 480 1.03 5

UV ArF 193 32 119.8 0.8 5.42 100

UV KrF 248 65 95.4 2.2 4.27 58

Ar-Ion (IC) 514 100 48.1 10.4 2.10 51

DPSS 532 100 46.5 10.8 2.03 54

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Detector options

Photon detector– Lead tungstate – Lead glass

Electron detector– Silicon microstrip– Quartz fibers

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Summary

Qweak collaboration would like two independent methods to measure beam polarization.

A Hall C Compton polarimeter would complement the Moller and measure the average polarization during the experiment.

Concept for a chicane that imposes minimal disturbance to the present Hall C beamline has been worked out.

Using a pulsed laser system is feasible, and offers advantages in terms of background rejection.

Options now exist that come close to Qweak requirements with a green or UV laser, that use a simple one-pass setup.

Monte Carlo studies are underway to determine tolerances on detector performance and alignment required for 1% accuracy.

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Addendum: recent progress

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Addendum: recent progress

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Addendum: laser choices

High-power green laser (100 W @ 532 nm)– sold by Talis Laser– industrial applications– frequency-doubled solid state laser– pulsed design

D. Gaskell: visit from Talis Laser reps June 2003– not confident that they could deliver– product no longer being advertised (?)

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Addendum: laser choices

High-power UV laser (50 W @ 248 nm)– sold by several firms– industrial applications: micromachining and lithography– excimer laser (KrF)– pulsed design

R. Jones: visit from Lambda Physik reps Fall 2003– sales team has good technical support – plenty of experience with excimer lasers– strong interest in our application

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Addendum: laser choices

Properties of LPX 220i– maximum power: 40 W (unstable resonator)– maximum repetition rate: 200 Hz– focal spot size: 100 x 300 m (unstable resonator)– polarization: should be able to achieve ~90%

with a second stage “inverted unstable resonator”– maximum power: 50 W– repetition rate unchanged– focal spot size: 100 x 150 m– polarization above 90%

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Addendum: laser choices

two passes make up for losses in elements– small crossing angle: 1°– effective power from 2 passes: 60 W– mirror reflectivity: 97%– length of figure-8: 100 cm

UV laser

electron beam

monitor

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Addendum: laser choices

purchase cost for UV laser system– LPX-220i (list): 175 k$– LPX-220 amplifier (list): 142 k$– control electronics: 15 k$– mirrors, ¼ wave plates, lenses: 10 k$

cost of operation (includes gas, maintenance)– per hour @ full power: $35 (single)

$50 (with amplifier)

– continuous operation @ full power: 2000 hours