Electron Spectrometer

Progress Report

Simon Jolly19th October 2012

Spectrometer Specifications

• Wakefield accelerated electrons ejected collinear with proton beam: need to separate the 2 and measure energy of electron beam only.

• Must be able to resolve energy spread as well as energy: spectrometer must accept a range of energies, probably 0-5 GeV.

• Current conceptual layout:– Dipole mounted ~2 m of plasma exit induces

dispersion in electron beam.– Scintillator screen 1 m downstream of dipole

intercepts electron beam ONLY.– Dispersion gives energy-dependent position spread

on screen.– Scintillator imaged by intensified CCD camera

viewing upstream face of scintillator screen.

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Spectrometer Layout

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Camera

Scintillator ScreenCERN 1 m dipole

Plasma cell

Protons

Electrons + Protons

• Edge of scintillator screen is aligned with dipole coils (position will depend on resolution).

• Screen mounted at 45 degrees to beam axis.• Camera is 4 m from centre of screen, mounted at

90 degrees to beam axis.• Camera shown in horizontal bending plane.• Camera can also be mounted vertically, directly

above screen, with screen tilted at 45 degrees to vertical as well as 45 degrees to beam axis. Dipole to screen distance remains unchanged (independent of screen-camera orientation).

Electron Trajectories (Alexey)

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With Quadrupole Doublet (Alexey)

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CERN 1 m Dipole (Edda, Alexey)

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Scintillator Alternatives

• Concerns expressed at last phone meeting over quantity of light emitted for low numbers of electrons.

• Also problems with unwanted particles:– Lots of crap ejected from plasma along

with accelerated electron.– Bremsstrahlung production in dipole.

• Do we need to look for an alternative technology to optimise spectrometer?

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Stray Particle Hits• The screen sits downstream of >1 m of iron dipole magnet yoke.

– Low energy side of the screen should be well shielding.– High energy side – closer to proton beam – more exposed.

• Add collimators upstream of dipole to reduce the particle flux reaching the screen.

• Add some additional shielding upstream of the dipole if space (no quadrupole doublet).– Need some input on particle species at plasma exit.– How much shielding do we need?

• Bremsstrahlung production within dipole unavoidable, but unlikely to be significant background given low numbers of electrons.– Also check with simulation.

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Collimators

Dipole

Protons + collinear crap

Electrons + bremsstrahlung photons

Sensitive Electron Detection• SEM Grid/Harp:

– Secondary emission of electrons within wires gives beam intensity at transverse beam location.

– Reconstruct 1-D intensity profile.– VERY small spatial coverage (eg.

100 micron wires at 5mm spacing): we need ~100% coverage…

• Drift chamber:– Many wires suspended inside

gaseous medium at high voltage.– Could give 100% coverage– Sample the beam at regular

intervals by integrating the current along each wire.  

– Timing not an issue since we have single very short beam pulse.

– Too many particles to do proper tracking so only single detector layer.

– Can we measure >105 particles per pulse?

– Very high R&D cost compared to camera + screen.

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MICE Sci-Fi Tracker

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• Measure muons produced in MICE cooling channel.

• 5 scintillating fibre (SciFi) stations track individual particles.

• Each layer contains 3 sets of fibres at 120° giving redundant position.

• 350 m fibres on 427 m pitch with doublet layer gives 450 m resolution.

• Might provide a suitable alternative to drift chamber.

• Not clear it will handle multiple hits.

• Expensive…?

Future Plans

• Model beamline using GPT + GEANT4 to simulate unwanted particle flux reaching screen (Scott Mandry).

• Exhaustive search of available scintillators (Dan Hall):– Accelerators (survive higher current):

• Al2O3 (“ruby” powder, used in LHC BTV).• YAG:Ce (high light output).• Quartz (very rad hard, low light output).

– Detectors (high light output):• CdWO4/PbWO4/ZnWO4 (CMS ECal).• CsI(Tl) (very high light output, not rad hard).• BGO/LYSO (more rad hard for lower light).• Plastics (less rad hard, cheap).

– Plasma Wakefield (large area):• Lanex (large screens, vacuum safe?)• P43/P46/P47 phosphors (x-ray phosphors).

• Investigate feasibility of SciFi tracker for low particle flux.

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Spectrometer Camera• Camera already purchased.• Andor iStar 340T iCCD

camera:– 2048 x 512 total pixels

(1850 x 512 active with 25 mm intensifier).

– 13.5 um pixels.– Gen-2 W-AGT P43 intensifier,

gated at 7 ns.– Nikon F-mount lens mount.– 16-bit readout, 150 ke- pixel

full well.– http://www.andor.com/scientifi

c_cameras/istar_iccd_camera/340-intensified-sensor/

• Camera setup:– DAQ and control software.– Data analysis software to give

online measurement of witness beam energy spectrum.

– Mechanical design of camera support system for tests and installation in SPS tunnel (comes later).19/10/12 12Simon Jolly, UCL, AWAKE Collaboration

Meeting