Polarized Photon Beam Instrumentation for GlueX

Polarized Photon Beam Instrumentation for GlueX

Part 1: active collimationPart 2: polarimetry

Richard Jones, University of Connecticut

GlueX collaboration meetingDec. 11-13, 2003, Newport News

2Richard Jones, GlueX meeting, Newport News, Dec. 11-13, 2003

GlueX activity at Connecticut: Part 1 simulation and software

Geant simulation experiment geometry description software tools

crystal radiator diamond quality control crystal mount

photon tagger instrumentation tagging microscope

photon beam instrumentation beam line shielding beam position control photon beam polarimetry


Active collimator project 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


This work is the senior project of Connecticut undergraduate Chris Gauthier


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.


Beam line simulation

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


Beam line simulation

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 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


Simulation geometry

12 cm 5 cm


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


Beam position sensitivity

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 m 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.


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


Present status and future plans A prototype detector is under construction.

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)

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.


GlueX activity at Connecticut: Part 2

simulation and software Geant simulation experiment geometry description software tools

crystal radiator diamond quality control crystal mount

photon tagger instrumentation tagging microscope

photon beam instrumentation beam line shielding beam position control photon beam polarimetry


Photon beam polarimetry Polarimetry is the least worked out aspect of the GlueX beam line

Conceptual Design. CDR section copied Hall B plan (1998) no follow-up in terms of measurements no criteria for precision

Argument for linear polarization for GlueX is qualitative 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.


Facilites with experience in photon polarimetry Saskatoon Mainz Hall B – organized Photon Polarimetry Workshop (1998) Yerevan synchrotron (YerPhI)

Connecticut / YerPhI collaboration established 2002 develop precise photon polarimetry for coherent

bremsstrahlung beams funded by CRDF grant AP2-2305-YE-02 (64K$ for 2 years)

Progress so far review of 4 methods, two articles in draft form beam time allocated at YerPhI to test 1 method in 2004

CB polarimetry project at YerPhI


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


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


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


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


Planned activities at YerPhI in 2004

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

visits of YerPhI physicists to Jlab travel support provided in CRDF grant should be timed to coincide with GlueX meeting


Present status, future plans

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

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

Polarized Photon Beam Instrumentation for GlueX

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