o. atramentov, american linear collider workshop, cornell u. 13-16 july 2003 fast gas cherenkov...
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O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Fast gas Cherenkov Luminosity Monitor
Progress Update
O. Atramentov, J.Hauptman
Iowa State University
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
NLCNLC requirements on performance
The NLC design luminosity places rather tight constraints on the performance of NLC detectors:
…bunch-to-bunch time interval of 1.4ns suggests almost speed-of-light response…
…large background of low energy e±, suggests a detector with a 10-20 MeV energy threshold
…large IR radiation dose will radioactivate the detector mass, suggesting an energy threshold above 8 MeV
…large radiation dose will damage detector components, requiring radiation-hard detector
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Gas Cherenkov calorimeter satisfies these four requirements:
• Gas has index of refraction n = 1+, ( 10-3), therefore Cherenkov angle is small
and energy threshold for electrons is high
MeV2.112
e
th
mE
• The Cherenkov photon signal exits the calorimeter volume at the velocity of light
• Decay products from radioactivation of the calorimeter mass are below Eth and therefore invisible
• A calorimeter made wholly of gas and metal cannot be damaged by any dose of radiation.
05.2θsinC
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Calorimeter design
• The Cherenkov light is generated by shower particles that cross gas gaps between absorber elements.
e-
• Shower particles co-move with the Cherenkov light as two overlapped pancakes. The width of these pancakes is about 10 ps.
• Inside surfaces must be highly reflective at grazing incidence.
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
production of Cherenkov photons by 10 GeV electron transversing 2mm gas conduits in Pb.
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Geometry
•“Spagetti”
•“Honey-Comb”
•“Cylindrical Lasagna”
Generic geometry – transverse segmentation of the absorber
Example geometries:
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
X
Y
Z
Geometry: “Cylindrical Lasagna”
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Geometry: Hexagonal Array
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Simulation
Energy resolution critically depends on these parameters.
• index of refraction
• reflectivity
• absorber material
• geometric parameters: – characteristic size of the light guides,
– gas/absorber ratio
Seemingly trivial: the higher the better,
but alas: close to 100% reflectivity is notoriously hard to achieve (especially in UV)
We need a detailed detector simulation
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Simulation
Change of reflectivity from 100% to 90% reduces # of photons by a factor of two.
Rohit Nambyar
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Simulation
We would like to have gas with the highest possible n.
Rohit Nambyar
gas n-1
CH4 0.00081
C2H6 0.00140
C3H8 0.00200
C4H8 0.00258
Watch, however, for resident light from scintillation!
β-butylene( n=1.00131 NTP ) might be a better candidate (than alkanes): scintillation/Cherenkov~10-5
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Simulation
Conversion from number of photons to energy seems to be independent on the energy of incoming electron.
Time spread ~ 11ps!
Rohit Nambyar
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Optical Surfaces
Such reflectivity (~95%) can be achieved with a very smooth surfaces coated with Al.
… high reflectivity is not trivial – such detector requires a large area of high quality surface.
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Optical Surfaces
Technique for obtaining optical quality of the metallic surfaces is well underway: polishing machine is built; surface roughness ≤30nm; reflectivity at grazing angles down to 200nm is coming (being fine tuned).
Reference mirror, glass substrate.
Polished stainless steel shim (Ukraine).
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
1.4ns Pulser
•train of several 20-50ps wide pulses;
•every 1.4 ns;
•Cherenkov spectrum;
•table-top.
…tests DAQ’s response to the bunch-to-bunch interactions and optical system.
Requirements:
Oesa Walker
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Cherenkov Light Generation
• Aluminized tube with lead “plug” at bottom
Oesa Walker
• 1mm diameter hole in plug allows only electrons with correct direction to pass
• Upper portion of tube filled with hexane to generate and transmit Cherenkov light
Hexane (ρ~1, UV)
Pb
90Sr
~5 mm
~1
-2
cm
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Delay Design 1
• Use lens to focus light from generator into beam
• Two beam-splitters break beam into three parts
• Path lengths differ by 1.4ns (approx. 42 cm)
Oesa Walker
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Delay Design 2
• Connect 3 liquid light guides of differing lengths to end of generator tube
• Lengths differ by 1.4ns (approx. 30cm in light guide liquid)
• Coil light guides to ensure light emerges from same distance from PMT
Oesa Walker
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Summary & To-Do List: DAQ
Existing components: sub-nanosecond FPGA-base ADCs are commercially available 16-anod fast PMTs (e.g. H6568 – 200nm, 150ps).
We will work with other LC collaborators on DAQ chain.
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
Summary & To-Do List: Optical Surfaces
reflectivity measurement of the reference mirror.
Coat polished samples with Al, Al+MgF2 and perform reflectivity measurements
surface quality control: roughness less then 30nm
reflectivity measurement at grazing angles in UV
polishing techniques give surface finish comparable to high quality commercial mirrors.
O. Atramentov, American Linear Collider Workshop, Cornell U.13-16 July 2003
To-Do List: Simulation
G4OpBoundary class is being now fixed by Geant4 team to work with complex geometries.
thus it should be possible to properly implement “honey-comb” geometry (its construction now has become feasible due to improved shim polishing techniques)
find optimal absorber, shape and size of conduits, gas/absorber ratio.
Stay tuned!