fast gas cherenkov luminosity monitor progress update

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!