Transcript GlueX collaboration meeting Dec. 11-13, 2003, Newport News Polarized Photon Beam Instrumentation for GlueX Richard Jones, University of Connecticut Part 1: active collimation Part 2: polarimetry.

GlueX collaboration meeting
Dec. 11-13, 2003, Newport News
Polarized Photon Beam
Instrumentation for GlueX
Richard Jones, University of Connecticut
Part 1: active collimation
Part 2: polarimetry
GlueX activity at Connecticut: Part 1
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simulation and software
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crystal radiator
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diamond quality control
crystal mount
photon tagger instrumentation
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Geant simulation
experiment geometry description
software tools
tagging microscope
photon beam instrumentation
beam line shielding
 beam position control
 photon beam polarimetry
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Active collimator project overview
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active stabilization required for collimation
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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
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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.
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Active collimator project overview
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best solution: monitor alignment of both beams
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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
1.9 mm
0.5 mm
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1s contour of electron
beam at 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.
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
This work is the senior project of Connecticut undergraduate
Chris Gauthier
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Beam line simulation
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Detailed photon beam line description is present in
HDGeant
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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.
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Beam line simulation
cut view of simulation geometry through horizontal plane at beam height
Hall D
collimator cave
Fcal
tagger building
vacuum pipe
Cerenkov
spectrometer
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Beam line simulation
overhead view of collimator cave cut through horizontal plane at beam height
12 m
collimators
vac
concrete
air
sweep magnets
vac
iron blocks
lead
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Beam line simulation
3d view of primary collimator with segmented photon rate monitor in front
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Design criteria for photon monitor
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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
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Design choice
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Segmented scintillator
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Ion chamber
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used for the Hall B collimator (lower currents)
not very rad-hard
requires gas system and HV
good choice for covering large area
Tungsten pin-cushion detector
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used on SLAC coherent bremsstrahlung beam line in 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
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Simulation geometry
12 cm
5 cm
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Detector response
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the photons are incident on the back side of the pincushions (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 highenergy 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 mA * 10-4 rad.len. * ln(E0/E1) 1 – 2 nA detector current
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Detector response from simulation
beam centered at 0,0
10-4 radiator
Ie = 1mA
inner ring of
pin-cushion plates
outer ring of
pin-cushion plates
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Beam position sensitivity
using inner ring only for fine-centering
±200 mm of motion
of beam centroil on
photon detector
corresponds to
±5% change in the
left/right current
balance in the inner
ring
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Beam position sensitivity
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Sensitivity is greatest near the center.
Outside the central 1 cm2 region the currents are
non-monotonic functions of the coordinates.
CG demonstrated a fitting procedure that could
invert the eight currents to find the beam center to
an accuracy of ±350 mm anywhere within 3 cm of
the collimator aperture.
Using BPMs and survey data, the electron beam
can be steered to hit a strike zone 6 cm in diameter
from a distance of 75 m.
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Electronics and readout
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tungsten plate is cathode for current loop
anode is whatever stops the knock-ons
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walls of collimator housing
 primary collimator
 for good response, these must be in contact
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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
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requires keeping the input capacitance low (preamp must be placed
near the detector)
differential readout, no ground loops
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Present status and future plans
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A prototype detector is under construction.
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aluminum housing already fabricated at Connecticut
mechanical drawings of tungsten pin-cushions exist
two options for fabrication
1.cut pins from tungsten wire and manually mount them into
machined tungsten wedges (first method tried at SLAC)
2.start off with a thick tungsten wedge and remove the excess
material using Electrostatic Discharge Machining leaving only
base plate and pins (superior result, SLAC)
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The goal is to build and instrument a prototype with
two opposing pin-cushions and test it in the Hall B
photon beam line.
Budget estimate for prototype is about 15K$, and
about 20K$ additional to complete the full detector
and electronics.
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
GlueX activity at Connecticut: Part 2
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simulation and software
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crystal radiator
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

diamond quality control
crystal mount
photon tagger instrumentation
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Geant simulation
experiment geometry description
software tools
tagging microscope
photon beam instrumentation
beam line shielding
 beam position control
 photon beam polarimetry
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Photon beam polarimetry
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Polarimetry is the least worked out aspect of the GlueX beam line
Conceptual Design.
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CDR section copied Hall B plan (1998)
 no follow-up in terms of measurements
 no criteria for precision
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Argument for linear polarization for GlueX is qualitative
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provides initial state with definite parity
separates natural/unnatural exchange production
Making a quantitative argument would involve trying to guess
what the critical channels will be and what backgrounds will be
important to eliminate.
Alternative to a physical argument for precise polarimetry would
be common sense: build a state-of-the-art device and be
prepared to push the precision if the analysis requires it.
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
CB polarimetry project at YerPhI
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Facilites with experience in photon polarimetry
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Connecticut / YerPhI collaboration established 2002
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Saskatoon
Mainz
Hall B – organized Photon Polarimetry Workshop (1998)
Yerevan synchrotron (YerPhI)
develop precise photon polarimetry for coherent
bremsstrahlung beams
funded by CRDF grant AP2-2305-YE-02 (64K$ for 2 years)
Progress so far
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review of 4 methods, two articles in draft form
beam time allocated at YerPhI to test 1 method in 2004
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Polarimetry: method 0
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measure azimuthal distribution of p0 photoproduction from a
spin-0 target
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sometimes called “coherent p0 photoproduction”
elegant – has 100% analyzing power
analysis is trivial – just N(f) ~ 1 + P cos(2f)
coherent scattering is not essential
only restriction: target must recoil in a spin-0 state
practical example: 4He scattering
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must detect the recoil alpha in the ground state
 requires gas target, high-resolution spectrometer for Eg ~ GeV
 cross section suppressed at high energy
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not competitive at GlueX energies
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Polarimetry: method 1
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pair production from a crystal
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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
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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
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Polarimetry: method 2
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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
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systematic errors
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atomic form factor
 multiple scattering in target, tracking elements or slits
 simulation of geometric acceptance of detector
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Polarimetry: method 3
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angular distribution from pair production on atomic electrons
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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
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Polarimetry: method 4
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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
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systematic errors
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atomic form factor used to calculate CB process
model of electron beam and crystal properties
photon beam alignment on the collimator
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Planned activities at YerPhI in 2004
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beam time approved for CB measurements
 electron
beam energy 4.5 GeV
 4-week run during summer 2004
 goniometer, crystal, pair spectrometer already
installed to test IPP method
 results anticipated for next fall
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visits of YerPhI physicists to Jlab
 travel
support provided in CRDF grant
 should be timed to coincide with GlueX meeting
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003
Present status, future plans
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visit to Yerevan 11/03
 R.
Jones visited Yerevan for 8 days in November, 03
 accelerator is quiet (and dark) but group is still active
 both theory and experimental expertise in CB
 good response to seminar on GlueX
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possible role for YerPhI in GlueX
 Deputy
Director A. Sirunian has expressed interest
 group seeks invitation and open projects in GlueX
 polarimetry would be an obvious place to start
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Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003