polarized photons for gluex richard jones, university of connecticut gluex photon beamline-tagger...

Polarized Photons for GlueX

Richard Jones, University of Connecticut

GlueX Photon Beamline-Tagger Review Nov. 14-15, 2005, Newport News

presented by

GlueX Tagged Beam Working GroupUniversity of Glasgow

University of ConnecticutCatholic University of America

2Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005

Photon Beam: coherent bremsstrahlung

CriteriaCriteriahigh energy 8-9 GeVlinear polarization 0.5high flux 108 s-1

energy resolution 10 MeV r.m.s.low background

Design optionsDesign options

A.laser backscatter

B.synchrotron backscatter

C.bremsstrahlung

D.coherent bremsstrahlung

energy polarization flux resolution () () () ()

background

A B C

() with tagging

D

“figure of merit” = (rate in 8-9 GeV window) * (polarization)2 @ constant b.g.


Photon Beam: energy spectrum

effects of collimation at 80 m distance from radiatorincoherent (black) and coherent (red) kinematics

effects of collimation: reduce low-energy background and increase polarization


Photon Beam: collimation

< 20 r0 < 1/3 c

d > 70 m

With increased collimator distance: polarization grows low-energy backgrounds shrink tagging efficiency drops off


Photon Beam: rates and background

peak energy 8 GeV 9 GeV 10 GeV 11 GeV

N in peak 185 M/s 100 M/s 45 M/s 15 M/s

peak polarization 0.54 0.41 0.27 0.11 (f.w.h.m.) (1140 MeV) (900 MeV) (600 MeV) (240 MeV)

peak tagging eff. 0.55 0.50 0.45 0.29 (f.w.h.m.) (720 MeV) (600 MeV) (420 MeV) (300 MeV)

power on collimator 5.3 W 4.7 W 4.2 W 3.8 W

power on H2 target 810 mW 690 mW 600 mW 540 mW

total hadronic rate 385 K/s 365 K/s 350 K/s 345 K/s(in tagged peak) (26 K/s) (14 K/s) (6.3 K/s) (2.1 K/s)

1. Total hadronic rate is dominated by the resonance region

2. For a given electron beam and collimator, background is almostindependent of coherent peak energy, comes mostly from incoherent part.


Photon Beam: collimation

line

ar

po

lariz

atio

n

effects of collimation on polarization spectrum

collimator distance = 80 m

5

effects of collimation on figure of merit:

rate (8-9 GeV) * p2 @ fixed hadronic rate


Photon Beam: coherent gain factor

gain increases with electron beam energy gain vanishes at the end-pointeffects of endpoint energy on figure of merit:

rate (8-9 GeV) * p2 @ fixed hadronic rate


Photon Beam: polarizationcircular polarization transfer from electron beam reaches 100% at end-point

linear polarization determined by crystal orientation vanishes at end-point


Electron Beam: expected properties

energy 12 GeV

r.m.s. energy spread 7 MeV

transverse x emittance 10 mm µr

transverse y emittance 2.5 mm µr

electron polarization available

minimum current 100 pA

maximum current 5 µA

x spot size at radiator 1.6 mm r.m.s.

y spot size at radiator 0.6 mm r.m.s.

x spot size at collimator 0.5 mm r.m.s.

y spot size at collimator 0.5 mm r.m.s.

position stability ±200 µm


Diamond crystal: properties

limits on thickness from multiple-scattering rocking curve from X-ray scattering

natural fwhm


Diamond crystal: goniometer mount

temperature profile of crystalat full operating intensity

oC


Tagging spectrometer focal plane

RequirementsRequirementsspectrum coverage

25% -- 95% in 0.5% steps

70% -- 75% in 0.1% steps

energy resolution10 MeV r.m.s.

rate capabilityup to 500 MHz per GeV


Tagging spectrometer: two dipole designradiator

quadrupole

dipole 1dipole 2

photon beamfull-energy electrons

focal plane

final bend angle: 13.4o

dipole magnetic field: 1.5 T focal plane length: 9.0 m energy resolution: 5 MeV r.m.s.

gap width: 3 cm pole length: 3.1 m dipole weight: 38 tons coil power: 30 kW


Tagger focal plane: fixed array

141 counters, spaced every 60 MeV (0.5% E0) plastic scintillator paddles oriented perpendicular to rays not designed for complete recoil electron coverage conventional phototube readout essential for diamond crystal alignment useful as a monitor of photon beam can run in counting mode at full source intensity


Tagger focal plane: microscope

focal planeelectron trajectory

SiPM sensors

scintillating fibers

clear light fibers

Design parametersDesign parameters

square scintillating fibers

size 2 mm x 2 mm x 20 mm

clear light guide readout

aligned along electron direction

exploits the maximum energy

resolution of the spectrometer

readout with silicon photomultipliers

(SiPM devicesSiPM devices)

dispersion is 1.4 mm / 0.1% at 9 GeV

2D readout to improve tagging efficiency


Beam line simulation

Detailed photon beam line description is present in HDGeant beam photons tracked from exit of radiator assumes beam line vacuum down to a few cm from entry to

primary collimator, followed by air beam enters vacuum again following secondary collimator and

continues down to a few cm from the liquid hydrogen target includes all shielding and sweep magnets in collimator cave monitors background levels at several positions in cave and hall simulation has built-in coherent bremsstrahlung generator to

simulate beam line with a realistic intensity spectrum The same simulation also includes the complete GlueX

target and spectrometer, detector systems, dump etc.



Hall Dcollimator cave

tagger buildingvacuum pipe

Fcal

Cerenkov

spectrometer

cut view of simulation geometry through horizontal plane at beam height


Beam line simulationoverhead view of collimator cave cut through horizontal plane at beam height

collimators

sweep magnets iron blocks

concrete

lead

12 m

airvac vac


Active collimator overview

active stabilization required for collimation distance from radiator to collimator 75 m radius of collimator aperture 1.7 mm size of real image on collimator face 4 mm r.m.s. size of virtual image on collimator face 0.5 mm r.m.s. optimum alignment of beam center on collimator aperture

±0.2 mm in x and y

steering the electron beam BPMs on electron beam measure x and y to ±0.1 mm BPM pairs 2 m apart gives ±4 mm at collimator BPM technology might be pushed to reach alignment goal,

under the assumption that the collimator is stationary in this ref. sys.


Active collimator project overview

best solution: monitor alignment of both beams monitor on electron beam position is needed anyway to control

the spot on the radiator BPM precision in x is affected by the large beam size along this

axis at the radiator

independent monitor of photon spot on the face of the collimator guarantees good alignment

photon monitor also provides a check of the focal properties of the electron beam that are not measured with BPMs.

0.5 mm

1.9 mm

1contour of electron beam at radiator



3d view of primary collimator with segmented photon rate monitor in front


Design criteria for photon monitor

radiation hard (up to 5 W of gamma flux) require infrequent access (several months) dynamic range factor 1000 good linearity over full dynamic range gains and offsets stable for run period of days sampling frequency at least 60 Hz at operating

beam current, 600 Hz desireable fast analog readout for use in feedback loop


Design choice Segmented scintillator

used for the Hall B collimator (lower currents) not very rad-hard

Ion chamber requires gas system and HV good choice for covering large area

Tungsten pin-cushion detector used on SLAC coherent bremsstrahlung beam line since 1970’s SLAC team developed the technology through several iterations,

refined construction method reference Miller and Walz, NIM 117 (1974) 33-37 SLAC experiment E-160 (ca. 2002, Bosted et.al.) still uses them,

required building new ones performance is known


Active Collimator Simulation

12 cm 5 cm


12 cmx (mm)

y (m

m)

current asymmetry vs. beam offset

20%

40%

60%

Active Collimator Simulation


Detector response the photons are incident on the back side of the pin-

cushions (opposite the pins) showers start in the base plates (~2 radiation lengths) showers develop along the pins, leaking charges into the

gaps charge flow is asymmetric (more e- than e+) due to high-

energy delta rays called “knock-ons” asymmetry leads to net current flow on the plates

proportional to the photon flux that hits it SLAC experience shows that roughly 1-2 knock-ons are

produced per incident electron

1 A * 10-4 rad.len. * ln(E0/E1) 1 – 2 nA detector current


Detector response from simulation

inner ring ofpin-cushion plates

outer ring ofpin-cushion plates

beam centered at 0,0

10-4 radiatorIe = 1A


Beam position sensitivityusing inner ring only for fine-centering

±200 m of motionof beam centroil onphoton detector

corresponds to

±5% change in theleft/right currentbalance in the innerring


Electronics and readout

tungsten plate is cathode for current loop anode is whatever stops the knock-ons

walls of collimator housing primary collimator for good response, these must be in contact

tungsten plate support must be very good insulator – boron nitride (SLAC design)

uses differential current preamplifier with pA sensitivity experience at Jlab (A. Freyberger) suggests that noise levels as low

as a few pA can be achieved in the halls requires keeping the input capacitance low (preamp must be placed

near the detector) differential readout, no ground loops


Beam position sensitivity

Sensitivity is greatest near the center. Outside the central 1 cm2 region the currents are

non-monotonic functions of the coordinates. A fitting procedure has been demonstrated that

could invert the eight currents to find the beam center to an accuracy of ±350 m anywhere within 3 cm of the collimator aperture.

Using BPMs and survey data, the electron beam must be able to hit a strike zone 6 cm in diameter from a distance of 75 m.


Beam line: shieldingoverhead view of collimator cave cut through horizontal plane at beam height

collimators

sweep magnets iron blocks

concrete

lead

12 m

airvac vac


Beam line: physics simulation

GEANT-based Monte CarloGEANT-based Monte Carlo based on a coherent

bremsstrahlung generator good description of

electromagnetic processes extended to include hadronic

photoproduction processes

complete description of beam line including collimator and shielding

integrated with detector sim.

1. ,N scattering2. ,A single nucleon knockout3. , pion photoproduction


Photon beam polarimetry Several methods have been considered by GlueX

hadronic reactions coherent 0 photoproduction on spin-0 target rho or omega photoproduction (helicity conservation)

pair production from a crystal incoherent pair production triplet production coherent bremsstrahlung spectrum analysis

Detailed comparisons have been carried out A PHOTON BEAM POLARIMETER BASED ON NUCLEAR E+ E- PAIR PRODUCTION IN

AN AMORPHOUS TARGET, F. Adamian, H. Hakobian, Zh. Manukian, A. Sirunian, H. Vartapetian (Yerevan Phys. Inst.), R. Jones (Connecticut U.),. 2005. 9pp. Published in Nucl.Instrum.Meth.A546:376-384,2005

Polarimetry of coherent bremsstrahlung by analysis of the photon energy spectrum, S. Darbinyan, H. Hakobyan, R. Jones, A. Sirunyan and H. Vartapetian,Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, In Press, Corrected Proof, Available online 26 August 2005.


Polarimetry: method 0

measure azimuthal distribution of 0 photoproduction from a spin-0 target sometimes called “coherent 0 photoproduction” elegant – has 100% analyzing power analysis is trivial – just N() ~ 1 + P cos(2) coherent scattering is not essential only restriction: target must recoil in a spin-0 state

practical example: 4He scattering must detect the recoil alpha in the ground state requires gas target, high-resolution spectrometer for E ~ GeV cross section suppressed at high energy

not competitive at GlueX energies


Polarimetry: method 1 pair production from a crystal

makes use of a similar coherent process in pair production as produced the photon in CB

requires counting of pairs, but not precision tracking analyzing power increases with energy (!) second crystal, goniometer needed asymmetry is in rate difference between goniometer settings can also be done in attenuation mode, using a thick crystal

sources of systematic error sensitive to choice of atomic form factor for pair-target crystal shares systematic errors with calculation of CB process in addition to theoretical uncertainty, it involves a model of the

beam and crystal properties



angular distribution from nuclear pair production sometimes called “incoherent pair production” polarization revealed in azimuthal distribution of plane of pair requires precise tracking of low-angle pairs, which becomes

increasingly demanding at high energy analyzing power is roughly independent of photon energy analyzing power depends on energy sharing within the pair best analyzing power for symmetric pairs requires a spectrometer for momentum analysis

systematic errors atomic form factor multiple scattering in target, tracking elements or slits simulation of geometric acceptance of detector



angular distribution from pair production on atomic electrons sometimes called “triplet production” polarization reflected in a number of observables azimuthal distribution of large-angle recoil electrons is a

preferred observable analyzing power is roughly constant with photon energy no need for precision tracking of forward pair pair spectrometer still needed to select symmetric pairs

systematic errors reduced dependence on atomic physics – “exact” calculations analyzing power sensitive to kinematic cuts relies on simulation to know acceptance



analysis of the shape of the CB spectrum not really polarimetry – this you do anyway probably the only way to have a continuous monitor of the beam

polarization during a run polarimetry goal would be to refine and calibrate this method for

ultimate precision takes advantage of tagging spectrometer requires periodic measurements of tagging efficiency using a total

absorption counter and reduced beam current

systematic errors atomic form factor used to calculate CB process model of electron beam and crystal properties photon beam alignment on the collimator


Measurements at YerPhI in 2005

beam time obtained for CB measurements electron beam energy 4.5 GeV 4-week run during Fall 2005 goniometer, crystal, pair spectrometer commissioned

installed to test IPP method results anticipated soon

YerPhI measurements supported by CRDF grant

polarized photons for gluex richard jones, university of connecticut gluex photon beamline-tagger...

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