Transcript PPT - SLAC

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
NLC 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 speedof-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:
• The Cherenkov photon signal exits the calorimeter volume at
the velocity of light
• Gas has index of refraction n = 1+, ( 10-3), therefore
Cherenkov angle is small
sin θ C  2  .05
and energy threshold for electrons is high
me
Eth 
 11.2 MeV
2
• 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.
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
Generic geometry – transverse
segmentation of the absorber
Example geometries:
• “Spagetti”
• “Honey-Comb”
• “Cylindrical Lasagna”
O. Atramentov, American Linear Collider Workshop, Cornell U.
13-16 July 2003
Geometry: “Cylindrical Lasagna”
Y
Z
X
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
•
•
•
•
index of refraction
reflectivity
absorber material
geometric parameters:
Seemingly trivial: the higher the better,
but alas: close to 100% reflectivity is notoriously
hard to achieve (especially in UV)
Energy resolution critically
depends on these parameters.
– characteristic size of the light guides,
– gas/absorber ratio
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.
Watch, however, for resident light
from scintillation!
gas
n-1
CH4
0.00081
C2H6
0.00140
C3H8
0.00200
C4H8
0.00258
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
Rohit Nambyar
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
… high reflectivity is not trivial – such detector
requires a large area of high quality surface.
Such reflectivity (~95%) can be achieved with a very
smooth surfaces coated with Al.
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
…tests DAQ’s response to the bunch-to-bunch
interactions and optical system.
Requirements:
• train of several 20-50ps wide pulses;
• every 1.4 ns;
• Cherenkov spectrum;
• table-top.
O. Atramentov, American Linear Collider Workshop, Cornell U.
13-16 July 2003
Oesa Walker
Cherenkov Light Generation
~5 mm
~1-2 cm
• Aluminized tube with lead “plug” at bottom
Hexane
(ρ~1, UV)
• 1mm diameter hole in plug allows only
electrons with correct direction to pass
Pb
• Upper portion of tube filled with
hexane to generate and transmit
Cherenkov light
O. Atramentov, American Linear Collider Workshop, Cornell U.
13-16 July 2003
90Sr
Oesa Walker
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)
O. Atramentov, American Linear Collider Workshop, Cornell U.
13-16 July 2003
Oesa Walker
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
O. Atramentov, American Linear Collider Workshop, Cornell U.
13-16 July 2003
Oesa Walker
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
 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.
 reflectivity measurement of the reference mirror.
 Coat polished samples with Al, Al+MgF2 and perform reflectivity
measurements
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 “honeycomb” 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!
O. Atramentov, American Linear Collider Workshop, Cornell U.
13-16 July 2003