Upload
abie
View
63
Download
0
Tags:
Embed Size (px)
DESCRIPTION
GlueX Photon Beamline-Tagger ReviewNov. 14-15, 2005, Newport News. Polarized Photons for GlueX. presented by. Richard Jones, University of Connecticut. GlueX Tagged Beam Working Group. University of Glasgow University of Connecticut Catholic University of America. - PowerPoint PPT Presentation
Citation preview
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.
3Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
4Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
5Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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.
6Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
7Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
8Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
Photon Beam: polarizationcircular polarization transfer from electron beam reaches 100% at end-point
linear polarization determined by crystal orientation vanishes at end-point
9Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
10Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
Diamond crystal: properties
limits on thickness from multiple-scattering rocking curve from X-ray scattering
natural fwhm
11Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
Diamond crystal: goniometer mount
temperature profile of crystalat full operating intensity
oC
12Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
13Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
14Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
15Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
16Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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.
17Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
Beam line simulation
Hall Dcollimator cave
tagger buildingvacuum pipe
Fcal
Cerenkov
spectrometer
cut view of simulation geometry through horizontal plane at beam height
18Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
19Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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.
20Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
21Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
Beam line simulation
3d view of primary collimator with segmented photon rate monitor in front
22Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
23Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
24Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
Active Collimator Simulation
12 cm 5 cm
25Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
12 cmx (mm)
y (m
m)
current asymmetry vs. beam offset
20%
40%
60%
Active Collimator Simulation
26Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
27Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
Detector response from simulation
inner ring ofpin-cushion plates
outer ring ofpin-cushion plates
beam centered at 0,0
10-4 radiatorIe = 1A
28Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
29Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
30Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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.
31Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
32Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
33Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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.
34Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 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
35Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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
36Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
Polarimetry: method 2
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
37Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
Polarimetry: method 3
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
38Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
Polarimetry: method 4
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
39Richard Jones, GlueX Beamline-Tagger Review, Newport News, Nov. 14-15, 2005
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