fast gas cherenkov luminosity monitor progress update
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Fast gas Cherenkov Luminosity Monitor Progress Update. O. Atramentov, J.Hauptman. Iowa State University. NLC requirements on performance. The NLC design luminosity places rather tight constraints on the performance of NLC detectors:. - PowerPoint PPT PresentationTRANSCRIPT
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!