CMB Polarimetry with BICEP:
Probing Inflationary Gravitational Waves
IAS - Polarization 2005
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Denis Barkats
Background Imager of Cosmic Extragalactic Polarization
Caltech / JPL
Andrew Lange
Denis Barkats
John Battle
James Bock
Cynthia Chiang
Darren Dowell
Greg Griffin
Viktor Hristov
UC Berkeley
Bill Holzapfel
Yuki Takahashi
CEA Grenoble
Lionel Duband
Eric Hivon
John Kovac
Chao-Lin Kuo
Pete Mason
Hien Nguyen
Ian Sullivan
Ki Won Yoon
UC San Diego
Brian Keating
Evan Biermann
Tom Renbarger
Cardiff
Peter Ade
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State of the CMB polarization field in August 2005
EE and TE polarization
WMAP
BOOMERANG 03
CBI
DASI
CAPMAP
BICEP
What do we need?
• Significant
advance system sensitivity
BB
polarization
• Control
systematic
Upper limits
~ 2 mK2 errors at < 0.1 mK levels
• Distinguish
from Galactic Foregrounds
(from DASI,CMB
CBI, B03)
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T/S < 0.36 from
CMB+LSS Signal from Lensing
• Distinguish
Gravity-wave
Overview of the BICEP instrument
Minimize polarization systematics
• Simple refractor, no mirrors
• Azimuthal symmetry
• Alt-az telescope, with continuous boresight rotation
• Wide-band polarization modulation with FRMs
Optimize to 10 < < 400
• Beam sizes ~ 1 deg, 0.6 deg
• Field of view ~ 17 deg
• Observed sky fraction ~ 5%
10 cm thick
Zotefoam window
IR-blocking filters
Cold polyethylene
lenses
Feed horn array
Faraday rotators
PSBs
Separate CMB / foregrounds
• Two frequency bands at
100 and 150 GHz
Helium sorption
refrigerator
Signal-to-noise considerations
• 49 light PSB pairs at 250 mK (98 bolos)
• Multiple levels of modulation
• South Pole location: long integration
over contiguous patch of sky, reduced
atmospheric loading
Cryostat
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New Technology: Polarization Sensitive Bolometers
Parallel metal lines,
Polarization in x-axis
Polarization
in y-axis
Photon
Courtesy of Jamie Bock, NASA JPL
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Upward-looking
cryostat with toroidal
LN2 / LHe tanks
Cryostat and mount
Long hold time (4-5 days)
Helium sorption fridge to
cool focal plane to 250 mK
Az-El mount +
boresite rotation
Rotation essential for
map cross-linking
4He, 1.5 K
3He, 350 mK
3He, 250 mK
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Hold time ~ 2 days, cycle ~ 5 hours
25 cm
Front-end optics
180 cm
• Telecentric design for flat focal plane
• AR-coated filters and lenses
• Cross-polarization < 0.01%,
instrumental polarization < 1%
from physical optics simulations
Teflon
Zotefoam
Polyethylene
IR-blocking
window
lenses
filters
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The BICEP insert
Lens tube mounted on top of insert
Drop-in focal plane
Cold JFETs for signal buffering,
refrigerator, thermal connections
to focal plane
Trussed structure
to isolate 250 mK
plate from 4 K
All RF-sensitive components
enclosed in large faraday cage
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Full focal plane map
100 GHz
150 GHz
• 49 spatial pixels =
98 polarizationsensitive bolometers
• Divided into 6
similar hextants
• Each hextant has 4
pixels at each band
• Adjacent hextants
measure Q and U.
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Image: J. Kovac
Image: K. Yoon
A closer look at the focal plane
Primary beam-defining
feed horn (4 K)
Faraday rotator module
A photon's view of the focal plane
Re-expanding horn (4 K)
Polarizationsensitive
bolometers
Band-defining metal
mesh filters, baffles
Refocusing horn (250 mK)
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Home, sweet (frozen) home:
South Pole station, Antarctica
“150 GHz band”
“100 GHz band”
Bandwidth: 24 GHz Bandwidth: 41 GHz
Optical efficiency:27.5%Optical efficiency:25%
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What can BICEP do for you?
- Winter 2005 / 2006: deployment to south pole
- Return information on polarized foreground emission
- Measure EE polarization at intermediate
, complementary to current detections
BB polarization: if anyone's gonna do it (in the next N years), we will!
N ~ number of years before SPIDER, EBEX, CMBpol, Planck etc.
1 year of Planck ~ 6 x sensitivity of 8 years of WMAP
BICEP has similar instantaneous sensitivity to polarization as Planck, and is
more sensitive to peak BB signal due to concentration on a small patch of sky
Detect BB polarization if T/S > 0.05 (assuming “reasonable” integration time),
otherwise set a bomber upper limit
http://www.astro.caltech.edu/~lgg/bicep_front.htm
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Expected BICEP performance
System NET ~ 35 uK√s
(350 uK√s per PSB, 98 detectors)
EE
BB
Total integration time: somewhere
between 100 days and 200 days
Observed sky fraction ~ 5%
Beam width ~ 1 degree
T/S = 0.05 and t = 0.17
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