Proton Polarimetry for the EIC - Stony Brook...

Proton Polarimetry for the EIC

2014.06.27 EIC Users Meeting 1

Andrei Poblaguev Brookhaven National Laboratory

Electron Ion Collider Users Meeting June 24-27, 2014 at Stony Brook University

Outline

• Proton Polarimetry at RHIC • Discussion of systematic errors • Projection to eRHIC


Polarizaed beams at RHIC

AGS LINAC BOOSTER

Polarized Source

200 MeV Polarimeter

Hydrogen Jet Polarimeter

PHENIX STAR

Siberian Snakes

Siberian Snakes

Spin Flipper

Carbon Polarimeters

RF Dipole AGS Internal Polarimeter

AGS pC Polarimeter

Strong Snake

Tune Jump Quads Helical Partial Snake

Spin Rotators



Polarimeters at RHIC Complex

• Linac absolute 200 MeV polarimeter - counting of protons scattered at 12 and 16 degrees in scintillator counters - absolute Polarization measurements - every second bunch is measured • AGS relative pCarbon Polarimeter - detection of 400-900 keV recoil Carbons in 96 Si strips. - monitoring of the beam polarization extracted to RHIC - a tool for beam development studies - statistical accuracy - detection of 400-900 keV recoil Carbons in 96 Si strips. - monitoring of the beam polarization extracted to RHIC 𝛿𝛿𝑃𝑃 ≈ 2 ÷ 3% per a few minute measurement. About 4,000 measurements per RHIC run. • 4 RHIC relative pCarbon Polarimeters (two per RHIC beam) - detection of 400-900 keV recoil Carbons in 72 Si strips (each polarimeter). - monitoring of polarization profiles, polarization decays, bunch by bunch and fill by fill polarization in both RHIC beams. - few 1 minute measurements per RHIC store. • RHIC absolute Polarized Hydrogen Jet Target Polarimeter (measures both beams) - detection of 1-5 MeV recoil protons in 96 Si strips - the jet (target) polarization 92%. - continuous measurement of average beam polarization - statistical errors 𝛿𝛿𝑃𝑃 ≈ 3% per 8-hour store. • Local relative polarimeters at STAR (BBC) and Phenix (ZDC) - monitoring transverse component of the polarization after rotators.


Polarization Measurement Schema in the pCarbon and H-Jet

To suppress systematic errors the asymmetry is calculated as

Beam polarization P can be measured from the production asymmetry a: If average analyzing power is known

• In the pCarbon we use predefine analyzing power AN(E) • In the H-Jet, AN is internally measured:

Spin dependent amplitude:

Rate in the detector:

Average Beam Polarization (measured by polarimeter):

Polarization in Collision Experiments


Intensity and Polarizations profiles, I(x,y) and P(x,y), are needed for the analysis

Gaussian Approximation: (similar for y coordinate).

• In the pCarbon polarimeters, x- and y-

profiles may be measured using moving target

(vertical and horizontal, respectively). In a

fixed target run, the Pmax is measured.

• In the H-Jet polarimeter, average beam

polarization is measured.

Average Polarization in experiment: (single spin)

A model dependence between <P> and R: (W. Fischer and A. Bazilevsky, Phys.Rev.ST Accel.Beams 15 (2012) 041001) If the development of polarization profiles it the primary reason for the reduction of the average polarization : P0 = Psource is “zero-emittance polarization”.


Square Root Formula

There is no systematic errors in measurement of asymmetry 𝒂𝒂.

R L +

-

Number of events in a detector:

Exact solution

a – polarization asymmetry ε – acceptance asymmetry λ – intensity asymmetry

If physics, acceptance, and intensity asymmetries are uncorrelated then


Possible correlation between asymmetries

Actually δε, δ±, δLR are systematic errors in measurements of the a, ε, and λ, respectively

It was evaluated in analysis of the AGS pCarbon data:

WCM

Above the 𝜹𝜹𝜹𝜹~𝟎𝟎.𝟏𝟏𝟏 level, errors in calculation of average analyzing power are the only sources of polarization systematic errors.


AGS p-Carbon Detectors: Rate Corrections

Run 58731 • In a single Si strip, rate per bunch is r ≈ 0.05-0.10 (depends on intensity, emittance, target, …) • The Data Acquisition may take only one events per bunch. • No good event may be detected even if both coincide signals are good. • The measured Polarization is underestimated:

• The parameter k may be evaluated using experimental data (separately for each detector) with accuracy about 20%: • ONLINE the value of k=1 is used (consistent with previous runs)

In Run13, the average rate correction is 6% (for RHIC ref. runs). Uncertainty in the parameter k propagates to a ~ 1% uncertainty in the measured polarization.

Rate corrections are non-linear effect which was not accounted by the asymmetry correlations. Rate corrections are essential only for the AGS polarimeter.

Polarization in RHIC reference runs

2012

Horizontal Polarization Profile

Sources of the systematic errors in pCarbon

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• Analyzing power 𝐴𝐴𝑁𝑁 𝐸𝐸 used in data analysis. • Errors in measurement of signal amplitude (e.g. due to (RF) noise) • Energy Calibration • Background, −1% < 𝛿𝛿𝑃𝑃<0 • Energy losses in the target, −1% < 𝛿𝛿𝑃𝑃<0 • Rate Corrections, 𝛿𝛿𝑃𝑃 ≤ 1%

AGS pCarbon: Analyzing power

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AN(t

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2014 2013 2012

Measured Analyzing Power ( <P> is determined with theor. AN(t) )

• For data analysis we use Analyzing Power theoretically derived from the E950 (21 GeV/c).

• We can measure AN(t) up to a scaling factor. • Results of measurements are well reproducible. • Discrepancy between theoretical and measured

analyzing powers may be caused by wrong energy calibration.

For relative measurements : 𝜹𝜹𝜹𝜹𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔/𝜹𝜹 ≤ 𝟐𝟐 ÷ 𝟑𝟑𝟏

RF noise

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Standalone measurements with FADC250. (Superimposed signal waveforms)

• Prompt • Carbon • Scattering pulse • RF noise

In regular measurements, signal amplitude and time are calculated in the 8-bit, 140x3 MHz WFD firmware. A simple algorithm assume a flat base time.

• In RHIC pCarbons we suppress RF noise by reducing cavity voltage during the measurement

• For AGS pCarbon, the RF noise is a problem which affects time and amplitude measurements and may corrupt the energy calibration.

• Improving of the RF shielding is needed. • At minimum, RF noise should be monitored and properly accounted in

measurements.

AGS p-Carbon: Energy Calibration


Dead Layer

Since AN=AN(E), energy calibration is crucial for the polarization measurement δP/P≈δE/E

From experimental data, can find the dependence between measured time and amplitude:

If t0 is known, we can calibrate detector in a model independent way:

ADC gain is calibrated using α-source 241Am : Edep = αA

A dead-layer approximation: Stopping range: A dead-layer condition: Using MSTAR parameterization for the dE/dx, we can determine t0 and xDL from the data fit

Energy losses may be accounted : 𝑬𝑬𝒌𝒌𝒌𝒌𝒌𝒌 𝑨𝑨 = 𝜶𝜶𝑨𝑨 + 𝑬𝑬𝒍𝒍𝒍𝒍𝒔𝒔𝒔𝒔(𝑬𝑬𝒌𝒌𝒌𝒌𝒌𝒌,𝒙𝒙𝑫𝑫𝑫𝑫)

Comments about Dead-Layer based calibration


This calibration is very sensitive to small variations of the stopping power and dead-layer model.

Comparison of the “standard” and “modified” calibrations

tm is measured time tA is time derived from the measured amplitude Normalized stopping range L0 (E) = LMSTAR(E) / xDL Fit function: L(Et) – L(αA) = 1

The modified stopping range L(E) = p0L0(E) + p1L02(E) fits data much better.

Energy calibrations are significantly different. Better fit does not garantee better calibration.

This example shows why there is a a concern about reliability of the energy calibration. More reliable method is needed. Determination of t0 may solve the problem.

Calibration using fast (punch through) protons


Bethe-Bloch formula: Carbons

Fast Protons

• The method worked well only in few channels.

• Results are affected by “induced pulse”.

• Extension of the WFD range may solve the problem.

RHIC p-Carbon: Target in Beam

Beam heats up the target to glow • Targets graphitize from operation

Target is electrostatically attracted to the beam • Mechanical stress on target, can break → need replacement • Material in beam is hard to control

Induced charge from wake field on target ends • Change to insulated ladder construction

Ultra-thin (𝟓𝟓 𝝁𝝁𝝁𝝁/𝒄𝒄𝒄𝒄𝟐𝟐) Carbon ribbon target thickness 30 𝒌𝒌𝒄𝒄 width 10 𝝁𝝁𝒄𝒄

• Targets are broken often • Target attraction to beam can affect the results of profile measurement.


• 255 GeV/c proton beams. • 6 detectors (98 channels) • Ran with two beam simultaneously separated vertically by 3-4 mm dictated by the machine beam-beam requirements. • Alpha-source runs were taken separately from physics runs. • Full waveform was recorded for every triggered event • Recoil protons were selected within energy range 1 – 5 MeV • Recoil proton asymmetry relative to the beam and jet polarization was mesured simultaneously aBeam = AN(t) PBeam & aJet = AN(t) PJet PBeam = (aBeam /aJet ) × PJet

2014.06.27 17 EIC Users Meeting

Polarized Hydrogen Jet Polarimeter (H-Jet)


Systematic Errors in H-Jet

• The Breit-Rabi polarimeter measures only polarization (96%) of atomic hydrogen. Average polarization of the jet (including molecular hydrogen) is about 92%. The admixture of the molecular hydrogen in the jet is not monitored continuously. We consider this as a biggest contribution to the systematic error of polarization measurement.

• Systematic errors due to background - scattering on beam gas - inelastic pp scattering can be studied using acquired data.

Elastic pp Beam gas background

pp 250 GeV

Elastic pp + mπ

Elastic pp


Plans for RHIC Run15

Upgrade of the H-Jet: New detectors: - larger acceptance - extended energy range of detected protons (0.5 – 9 MeV) - factor 4 effective gain in statistics New DAQ : - based on 12 bit, 250 MHz FADC250 (JLab) - expect improvement in background suppression - study for future upgrade of the p-Carbon DAQ

The upgrade of the H-Jet may help us to resolve some problems discussed above.


eRICH The eRHIC proton beam conditions are likely similar to the current in that the bunch spacing is still 114 nsec, but shorter bunches, but • reduced bunch intensity (factor 5-10) • factor 10 smaller emittance resulting factor 3 smaller transverse beam size

𝜎𝜎𝑥𝑥, 𝑦𝑦~200 𝜇𝜇𝜇𝜇 at polarimeter location.

• shorter bunches, 𝜎𝜎 = 5 𝑐𝑐𝜇𝜇 = 170 𝑝𝑝𝑝𝑝

Desirable accuracy of absolute polarization measurement 𝛿𝛿𝑃𝑃 𝑃𝑃⁄ = 2%


H-Jet at eRICH For reduced beam intensity, statistical accuracy of measurements is expected to be about

10% per 8-hour store. very long time will be needed to achieve required statistical accuracy stability of p-Carbon performance becomes very important possible solutions:

o add unpolarized hydrogen jet target (factor 10 higher jet density) o new Si detectors which will be tested in RHIC Run 15 may effectively increased

statistics by factor 4. Continuous molecular hydrogen component measurements has to be implemented.

p-Carbon at eRICH Rate at p-Carbon detectors will be reduced by factor ~3. We may consider increasing of target thickness by factor 3. Polarization profile (transverse) measurements will still be available. To measure longitudinal polarization profile, the time resolution should be better than 𝜎𝜎 ≤ 50 𝑝𝑝𝑝𝑝.

Such a resolution could be provided by FADC250. Due to noise, time resolution is constrained by a value of about 𝜎𝜎~ 500 𝑝𝑝𝑝𝑝. Energy resolution should be of order of 10−3 since (𝛿𝛿𝛿𝛿 𝛿𝛿 ⁄ = −𝛿𝛿𝐸𝐸 2𝐸𝐸⁄ ) It is unlikely to measure longitudinal polarization profile with existing Si detectors.


3He2+ beam at eRich Yousef Makdisi, EIC14, March 20, 2014


Summary

Based on experience with proton polarimetry at RHIC, a 2% accuracy of absolute polarization measurement of proton beam at eRICH seems to be achievable with existing detectors , but significant improvements are needed including

• continuous monitoring of molecular hydrogen component in H-Jet • improving of the carbon targets for RHIC p-Carbon • reducing and monitoring of the RF noise • reliable energy calibration for p-Carbon detectors • upgrading of the DAQ

It is expected that p-Carbon polarimeters can be used for relative polarization

measurements of the 3He2+ beam. More study is needed to find a solution for absolute polarization measurement of

the 3He2+ beam.

Backup



Absolute Proton Beam Polarimeter at 200 MeV

• The polarimeter is based on the elastic proton-Carbon scattering at 16.2 degree angle, where analyzing power is closed to 100% and is measured with a high accuracy.

• Inelastic protons background is suppressed by 41 mm Cu absorber

• The high rate inclusive 12 degree polarimeter is calibrated using the 16.2 degree measurements.

AGS pCarbon Polarimeter (Run 2013 configuration)

EIC Users Meeting 26 2014.06.27

inner outer

Righ

t

Left

Every detector consists of 12 Si strips.

Carbon target foils: Thickness: 27 nm Width: 50 and 125 µm (Vertical) 75 and 125 µm (Horizontal)

The polarimeter is employed for 1. Monitoring of the polarization extracted to the

RHIC 2. Monitoring the polarization in the beam

development studies. • A regular measurement (40M events) takes few

minutes and allows to determine the polarization with statistical accuracy of about 2-3%.

• About 4000 measurements per RHIC run.

AGS pCarbon: Polarization Measurements


Event selection

Ramp

Polarization flips at integer values of 𝑮𝑮𝜸𝜸

Fixed Target Moving Target (Profile meas.)

AGS pCarbon: Analyzing power

28 EIC Users Meeting 2014.06.27 An

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Pow

er A

N(t

)

Anal

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wer

AN(t

)

Anal

yzin

g Po

wer

AN(t

)

2014 2013 2012

Measured Analyzing Power ( <P> is determined with theor. AN(t) )

• For data analysis we use Analyzing Power theoretically derived from the E950 (21 GeV/c).

• We can measure AN(t) up to a scaling factor. • Results of measurements are well reproducible. • Discrepancy between theoretical and measured

analyzing powers may be caused by wrong energy calibration.

For relative measurements : 𝜹𝜹𝜹𝜹𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔/𝜹𝜹 ≤ 𝟐𝟐 ÷ 𝟑𝟑𝟏

RHIC pCarbon Polarimeters


Target in Beam

Beam heats up the target to glow

• Targets graphitize from operation

Target is electrostatically attracted to the beam

• Mechanical stress on target, can break → need replacement

• Material in beam is hard to control

Induced charge from wake field on target ends

• Change to insulated ladder construction 2014.06.27 EIC Users Meeting 30

Polarization Profile

Significant polarization profiles are observed

𝑅𝑅 =𝜎𝜎𝐼𝐼2

𝜎𝜎𝑃𝑃2≈ 0.1 − 0.2

in units of intensity 𝜎𝜎𝑥𝑥,𝑦𝑦

Intensity

Polarization


Polarization Decay

Pola

rizat

ion

P (%

) Pr

ofile

R

Polarization losses are correlated to

acceleration

emittance

profile

Provide experiments with

injection

𝑃𝑃 = 𝑃𝑃0 +𝑑𝑑𝑃𝑃𝑑𝑑𝛿𝛿

𝛿𝛿

R = 𝑅𝑅0 +𝑑𝑑𝑅𝑅𝑑𝑑𝛿𝛿

𝛿𝛿

𝑑𝑑𝑃𝑃𝑑𝑑𝛿𝛿 = −1%/h

𝑑𝑑𝑅𝑅𝑑𝑑𝛿𝛿 = 5%/h

Typical values:


Fill Pattern

Blue beam

Yellow beam

Example Fill 17370

Raw asymmetry per bunch

Confirm bunch fill pattern reliably

Averaged over all measurements in a fill

𝑃𝑃↑

𝑃𝑃↓

𝑃𝑃↑

𝑃𝑃↓


• The H-jet polarimeter includes three major parts: polarized Atomic Beam source (ABS), scattering chamber, and Breit-Rabi polarimeter.

• The polarimeter axis is vertical and the recoil protons are detected in the horizontal plane.

• The common vacuum system is assembled from nine identical vacuum chambers 50 cm diameter, which provide nine stages of differential pumping each at 1000 l/s

• Flip jet polarization every 300 sec

• The Jet beam is focused to 6 mm FWHM so it sees the full beam polarization profile

• Thickness: 1.2 x1012 atoms / cm2

• Jet polarization ~ 92%


• 255 GeV/c proton beams. • 6 detectors (98 channels) • Ran with two beam simultaneously separated vertically by 3-4 mm dictated by the machine beam-beam requirements. • Alpha-source runs were taken separately from physics runs. • Full waveform was recorded for every triggered event • Recoil protons were selected within energy range 1 – 5 MeV • Recoil proton asymmetry relative to the beam and jet polarization was mesured simultaneously aBeam = AN(t) PBeam & aJet = AN(t) PJet PBeam = (aBeam /aJet ) × PJet


Running conditions (2013)

Analyzing Power


Average Analyzing Power in Run 13: Variations of the measured value of AN are less than 1%

255 GeV

pp-CNI

Used for polarization measurements

24 GeV: PRD 79, 094014(2009) 31 GeV: Preliminary 100 GeV: PLB 638 (2006) 450 250 GeV: Preliminary

Polarization measurement in the H-Jet in Run 13

Fills 17201 – 17324 (Run13 Lattice)


Preliminary Analysis:




Systematic Errors in the H-Jet Measurements

Jet Polarization: there are 2 hydrogen components in the jet: - atomic with (measured) polarization PBR≈96% - molecular (unpolarized) The admixture of molecular hydrogen was measured to be ε≈ 3% but, but systematic errors of this measurement is not well known. The average polarization Pjet = (1- ε) ×PBR should be used in analysis Background: r ~ 5% is background level For Jet asymmetry α=0. For beam asymmetry α is unknown and may be as large as 1 (e.g for beam gas protons). (some previous experimental estimates gave α≈0)

In ONLINE analysis the value of Pjet = 92% was used.

Calibration methods used in HJet


• α-particles from 241Am and 148Gd (α, xDL)

• high energy (low amplitude) prompt particles (t0)

• geometry based calibration (t0 and α* )

𝐸𝐸∝ 𝐺𝐺𝑑𝑑 = 3.183 MeV

𝐸𝐸∝ 𝐴𝐴𝜇𝜇 = 5.486 MeV


Estimation of background effect. For elastic pp-scattering:

z-profile of the jet (smeared by Si strip size) is approximated by :

Background may be expected to be the same in all strips strip s

•Two methods for background subtraction - from the fit - average background • The peak position is associated with well known (from geometry) energy. In such a calibration t0 may be determined with accuracy of about 200 ps, and proton energy may be calibrated with accuracy better than 2% • By product detector geometry may be aligned. • α-particles and prompt indicated more complicated background

The method is not verified yet !

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