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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
2014.06.27 EIC Users Meeting 2
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
2014.06.27 EIC Users Meeting 3
2014.06.27 EIC Users Meeting 4
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.
2014.06.27 EIC Users Meeting 5
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
2014.06.27 EIC Users Meeting 6
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”.
2014.06.27 EIC Users Meeting 7
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
2014.06.27 EIC Users Meeting 8
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.
2014.06.27 EIC Users Meeting 9
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
10 EIC Users Meeting 2014.06.27
• 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
11 EIC Users Meeting 2014.06.27 An
alyz
ing
Pow
er A
N(t
)
Anal
yzin
g Po
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
RF noise
12 EIC Users Meeting 2014.06.27
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
2014.06.27 EIC Users Meeting 13
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
2014.06.27 EIC Users Meeting 14
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
2014.06.27 EIC Users Meeting 15
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.
2014.06.27 EIC Users Meeting 16
• 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)
2014.06.27 18 EIC Users Meeting
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
2014.06.27 19 EIC Users Meeting
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.
2014.06.27 20 EIC Users Meeting
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%
2014.06.27 21 EIC Users Meeting
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.
2014.06.27 22 EIC Users Meeting
3He2+ beam at eRich Yousef Makdisi, EIC14, March 20, 2014
2014.06.27 23 EIC Users Meeting
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
2014.06.27 EIC Users Meeting 24
2014.06.27 EIC Users Meeting 25
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
2014.06.27 EIC Users Meeting 27
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
alyz
ing
Pow
er A
N(t
)
Anal
yzin
g Po
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
EIC Users Meeting 29 2014.06.27
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
2014.06.27 EIC Users Meeting 31
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:
2014.06.27 EIC Users Meeting 32
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↓
2014.06.27 EIC Users Meeting 33
• 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%
2014.06.27 EIC Users Meeting 34
• 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 35 EIC Users Meeting
Running conditions (2013)
Analyzing Power
2014.06.27 36 EIC Users Meeting
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)
Fills 17328 – 17601 (Run12 Lattice)
Preliminary Analysis:
2014.06.27 37 EIC Users Meeting
Fills 17328 – 17601 (Run12 Lattice)
2014.06.27 EIC Users Meeting 38
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
EIC Users Meeting 39 2014.06.27
• 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
2014.06.27 EIC Users Meeting 40
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 !