Transcript BLAST Experiment South Hall Ring

The MIT-Bates Compton Polarimeter
for the South Hall Ring
W.A. Franklin for the BLAST Collaboration
SPIN2004 Conference
Trieste, Italy
October 14, 2004
1. Physics Motivation
2. Experimental Apparatus
3. Results
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BLAST
• Research program in South Hall Ring using Bates Large
Acceptance Spectrometer Toroid
• Study comprehensively nucleons and light nuclei at low Q2
Collaborating Institutions
MIT, UNH, Arizona St., Duke,
Dartmouth, Vrije University, U.S.
Naval Acad., Ohio, Boston U.
12 Ph.D. students
• Neutron Electric Form Factor
V. Ziskin, 10/15 Session 5
• e-p Asymmetries on Deuterium
A. Maschinot, 10/15 Session 5
• Electric-Magnetic Form Factor
Ratio of Hydrogen
C. Crawford, 10/15 Session 5
Experiments underway 2003, running throughout 2004
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BLAST Experiment
Measure asymmetries using polarized beams and targets
• South Hall Ring: Intense
(175 mA) stored CW
polarized electron beams in
at 850 MeV
• BLAST Atomic Beam
Source: (E. Tsentalovich,
10/15 Session 8)
• BLAST: Symmetric
detector with wide
momentum transfer bite
• Beam-target polarization product from BLAST asymmetry.
• Need rapid nondestructive measurement of beam polarization.
• Laser backscattering can provide.
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Compton Polarimetry Overview
• Compton scattering in highly relativistic frame
 Angular distribution compressed into narrow kinematic cone
 Photon frequencies shifted into gamma regime
 Detect backscattered photons with compact detector
• Compton scattering cross section
• Well known theoretically
• Term dependent on electron spin and laser helicity
 Can extract e- polarization by measuring asymmetries in
scattering rates for circularly polarized laser light
Transverse pol. yields asymmetry
in azimuthal distribution of
scattered photons.
Longitudinal pol. yields
asymmetry in scattered photon
energy spectrum
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Compton Polarimetry Below 1 GeV
• Compton polarimetry is well
532 nm laser light
Apol
established at high energy
accelerators (Apol~0.5)
• Different challenges exist in
applying at energies below 1 GeV.
> Analyzing power falling with
energy (Apol < 0.05)
> Interaction mechanism varies
with gamma ray energy
> Broader angular distribution for
photons
> Background from low energy
photons
> Beam lifetime less than 1 hour
Compton Analyzing Power
HERA
JLab
Bates
Electron Energy (MeV)
• Bates seeks precise polarization measurement for each ring fill
(15 minutes) for experiments with BLAST.
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SHR Compton Polarimeter
• Design Considerations
• Based on NIKHEF Compton
Polarimeter
• Located upstream of BLAST
target to reduce background
• Measures longitudinal
projection of electron pol.
• Scattered gamma trajectory
defined by electron momentum
• Polarimeter Layout
• Laser in shielded hut with 18 m
flight path
• Interaction with electron
beam in 4 m straight section
• Laser mirrors moved remotely
• CsI calorimeter 10 m from IR
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SHR Injection Line
Laser exit
Ring
Dipole
Interaction
Region
Electron beam
Remotely controlled
mirrors
Scattered
photons
Laser line
CsI detector
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Laser hut
BLAST
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Laser System
Y angle
• Laser
X angle
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• Solid-state continuous-wave, very stable
• 5W output at 532 nm
• Optical Transport
•Simple, robust lens arrangement for
transport to IR and focusing
• Mechanical chopper wheel allowing
background measurements
• Circular polarization by Pockels Cell for
rapid helicity reversal
• Phase-compensated mirror arrangement
• Interaction Region
• 4 degrees of freedom for laser scans.
• Laser intercepts stored beam at < 2 mrad
• EPICS Control System for slow controls
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Scattered Gamma Ray Line
• Align electron beam first to align using bremsstrahlung
background.
• Movable collimators used to eliminate background from beam halo
• Sweep magnet, veto scintillator reject charged particles
• Scintillator hodoscope provides position information for beam alignment
• Pure CsI calorimeter offers resolution and speed for single photon mode
• High rate PMT bases for linear response, stable gain
• Variable thickness stainless steel absorbers for high intensity operation
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High Intensity Operation
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•
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Measurements made up to 190 mA.
Stainless steel absorbers act as
neutral density filter to control rate.
Signal-to-background tractable at
high currents (beam size increases)
Energy calibration stable on short
time scale for high rates in CsI
• Small systematic correction to
asymmetry for absorber thickness
• Polarization reduction sometimes
observed in raising beam current (tune
spreading).
• Rapid feedback for retuning Ring.
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Data Acquisition
Fast and reliable data acquisition system is very important
•
•
VME-based system
Rapid digitization
– 100 MHz, 12-bit buffered ADC
– External triggering
– Single event mode
•
High readout and sorting speed
– DMA for high CW event rates
(> 100 kHz)
– Pulse shape discrimination and
pile-up rejection
– Rapid spin sorting capability
•
Integration
– Linked with BLAST analysis
– Synch with BLAST event
stream
Developed locally (T. Akdogan)
– Include EPICS information
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Data Analysis
• Analysis begins with raw ADC spectra
–
L1 - Laser on, right-handed Pcirc
– L2 - Laser on, left-handed Pcirc
– B1 - Laser off, right-handed Pcirc
– B2 - Laser off, left-handed Pcirc
• Establish energy calibration based on
Compton edge and ADC pedestal
• Normalization for background
subtraction from bremsstrahlung tail
• Asymmetry between laser helicities
formed as function of gamma energy
• Fit asymmetry data with function
representing polarimeter analyzing power
(GEANT simulation)
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Polarization
Fill-by-Fill Polarization Results
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Time (hours)
Polarization reversed in electron source on fill-by-fill basis
Polarization monitored continuously
Typical precision of 4-5% for ~15 minute fill
Gaussian profile to results
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Cumulative Results
• Database of results for BLAST experiment in blocks of ~4 hrs
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•
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Polarization stable within few percent as a function of time.
Changes usually correlated with beam properties.
Mean polarization (2004): 0.663, (July-Sep, 2004): 0.654
Long term errors determined by systematics
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Stored Polarization
Consistency Checks
Wien Filter Voltage
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•
•
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Expected sinusoidal dependence on injection angle
Avg. Injection polarization 0.71 +/- 0.04 (20 MeV transmission pol.).
Measure zero asymmetries with unpolarized beam
Consistency of results for two helicity states
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Spin Flipping in South Hall Ring
• Spin flipper
• Reverse direction of beam
pol. while stored
• Separate instrumental
asymmetries from polarization
differences from source
• Adiabatic spin flip
• Rf magnetic field
Resonance Scan
• Performance in South Hall Ring
• Achieved efficiency > 98%
(Michigan Spin Physics Group)
Pfinal
• Ramp through resonant
frequency.
• < 1 sec to induce spin flip.
• Spin flip period of ~5 min.
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frf (kHz)
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Automated Spin Flip
• Spin flipper controlled
Polarization
through Compton DAQ for
time synchronization.
• Execute flip based on time
or beam current
• Polarization data sorted
from beginning of fill to flip
(dark) and post-flip (open)
• Eliminates false asymmetry
due to geometric effects
• Verifies equality of beam
polarization for two
helicities at injection to < .01
Time (hrs)
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Systematic Error Estimation
Small analyzing power makes systematic error reduction crucial
Error Contribution
DP
Modeling of Apol and
energy calibration
0.03
Pile-up
0.01
Beam misalignment
and PITA
~0.05 single pol. state,
< 0.01 average pol
Laser circular polarization
0.005
Spin precession
uncertainty
0.003
•Total systematic error in avg. beam pol. estimated at 0.04.
•Working to reduce, sufficient for BLAST experiments.
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Summary
• MIT-Bates operates a laser backscattering Compton
polarimeter during internal target experiments with BLAST
at 850 MeV.
• High intensity operation has been successful with stored
currents exceeding 175 mA.
• Polarization results are generally stable with time. Certain
changes in beam conditions can produce depolarization.
• Systematic error control and calibration uncertainty meet
level needed for BLAST experiments. Use of consistency
checks provides reduction of systematic errors.
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