Receiver development for BICEP Array, a next-generation CMB polarimeter at the South Pole

TLDR: The Bicep/Keck Array (BK) program targets the degree angular scales, where the power from primordial B-mode polarization is expected to peak, with ever increasing sensitivity and has published the most stringent constraints on inflation to date as discussed by the authors.

Abstract: A detection of curl-type (B-mode) polarization of the primary CMB would be direct evidence for the inflationary paradigm of the origin of the Universe. The Bicep/Keck Array (BK) program targets the degree angular scales, where the power from primordial B-mode polarization is expected to peak, with ever-increasing sensitivity and has published the most stringent constraints on inflation to date. Bicep Array (BA) is the Stage-3 instrument of the BK program and will comprise four Bicep3-class receivers observing at 30/40, 95, 150 and 220/270 GHz with a combined 32,000+ detectors; such wide frequency coverage is necessary for control of the Galactic foregrounds, which also produce degree-scale B-mode signal. The 30/40 GHz receiver is designed to constrain the synchrotron foreground and has begun observing at the South Pole in early 2020. By the end of a 3-year observing campaign, the full Bicep Array instrument is projected to reach σr between 0.002 and 0.004, depending on foreground complexity and degree of removal of B-modes due to gravitational lensing (delensing). This paper presents an overview of the design, measured on-sky performance and calibration of the first BA receiver. We also give a preview of the added complexity in the time-domain multiplexed readout of the 7,776-detector 150 GHz receiver.

Figures

Figure 3. Left: Comparison of atmospheric transmission at the South Pole with the bandpasses of Bicep/Keck Array and Bicep Array. Median atmospheric transmission during the observing season is shown in black, bracketed by the 10th and 90th percentiles. Transmission drops only slightly across 200–300 GHz, making dust observations in the upper part of this window effective, with similar dust sensitivity to the 220 GHz band. Right: Minimally processed timestream pair-sum and pair-difference noise spectra from Keck Array . The stable Antarctic atmosphere enables observations at all of these frequencies that are low-noise across the indicated science band from 0.1–1 Hz, corresponding to 25 . ` . 250.

Table 4. Simulated magnetic shielding performance in Bicep Array, listed as separate sub-system contributions.

Figure 6. The focal plane distribution board gathers all the readout cabling from warmer stages and distributes them to the corresponding detector module at 280 mK. The SSA board is located at 4 K, and houses the SQUID Series Arrays (SSA). Each MCE connects to a single SSA board via five 100-way cables. The 30/40, 95, and 150 GHz receivers have one, three and six SSA boards, respectively. Shown above is the 150 GHz configuration.

Figure 10. Left: Bicep Array 40 GHz temperature map covering 570 deg2, obtained from 3,187 50-minute scansets from the 2020 observing season. Right: Planck 44 GHz45 reobserved temperature map, for comparison. Color bar units: µK.

Figure 9. Band-averaged far-field beam map (FFBM) composites, made by coadding all per-detector FFBM composites in a frequency band. Per-detector FFBM composites are made by coadding all individual FFBM for a detector, which are taken at a range of boresight angles to get maximum coverage of the source. Left: 30 GHz. Right: 40 GHz.

Figure 4. Bicep Array optical diagram and cross-section of the cryogenic receiver. All optical components, except for the Zotefoam filters, are anti-reflection coated to minimize in-band reflections. The HDPE lenses, nylon filter, and aperture stop are precision-mounted in the receiver and cooled to 4 K to reduce detector loading. The focal plane assembly, which houses 12 detector modules, is cooled to 280 mK by a sorption fridge and surrounded by a superconducting Nb magnetic shield. The focal plane is actively temperature-controlled to achieve detector baseline stability over timescales of hours. The radially-symmetric receiver design allows well-matched beams for orthogonally polarized detectors at the focal plane.

Full text

PROCEEDINGS OF SPIE
SPIEDigitalLibrary.org/conference-proceedings-of-spie
Receiver development for BICEP
Array, a next-generation CMB
polarimeter at the South Pole
Moncelsi, L., Ade, P. A. R., Ahmed, Z,, Amiri, M., Barkats,
D., et al.
L. Moncelsi, P. A. R. Ade, Z, Ahmed, M. Amiri, D. Barkats, R. Basu Thakur, C. A.
Bischoff, J. J. Bock, V. Buza, J. R. Cheshire, J. Connors, J. Cornelison, M. Crumrine,
A. J. Cukierman, E. V. Denison, M. Dierickx, L. Duband, M. Eiben, S. Fatigoni, J. P.
Filippini, N. Goeckner-Wald, D. Goldfinger, J. A. Grayson, P. Grimes, G. Hall, M.
Halpern, S. A. Harrison, S. Henderson, S. R. Hildebrandt, G. C. Hilton, J. Hubmayr,
H. Hui, K. D. Irwin, J. H. Kang, K. S. Karkare, S. Kefeli, J. M. Kovac, C. L. Kuo, K.
Lau, E. M. Leitch, K. G. Megerian, L. Minutolo, Y. Nakato, T. Namikawa, H. T.
Nguyen, R. O'brient, S. Palladino, N. Precup, T. Prouve, C. Pryke, B. Racine, C. D.
Reintsema, A. Schillaci, B. L. Schmitt, A. Soliman, T. St. Germaine, B. Steinbach, R.
V. Sudiwala, K. L. Thompson, C. Tucker, A. D. Turner, C. Umiltà, A. G. Vieregg, A.
Wandui, A. C. Weber, D. V. Wiebe, J. Willmert, W. L. K. Wu, E. Yang, K. W. Yoon, E.
Young, C. Yu, L. Zeng, C. Zhang, S. Zhang, "Receiver development for BICEP Array,
a next-generation CMB polarimeter at the South Pole," Proc. SPIE 11453, Millimeter,
Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy X,
1145314 (13 December 2020); doi: 10.1117/12.2561995
Event: SPIE Astronomical Telescopes + Instrumentation, 2020, Online Only
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Receiver development for BICEP Array, a next-generation
CMB polarimeter at the South Pole
L. Moncelsi
a
, P. A. R. Ade
b
, Z. Ahmed
c,d
, M. Amiri
e
, D. Barkats
f
, R. Basu Thakur
a
,
C. A. Bischoff
g
, J. J. Bock
a,h
, V. Buza
i,j
, J. Cheshire
k
, J. Connors
l,f
, J. Cornelison
f
,
M. Crumrine
m
, A. Cukierman
d,c,n
, E. V. Denison
l
, M. Dierickx
f
, L. Duband
o
, M. Eiben
f
,
S. Fatigoni
e
, J. P. Filippini
p,q
, N. Goeckner-Wald
d,n
, D. C. Goldfinger
f
, J. Grayson
n
,
P. Grimes
f
, G. Hall
k,n
, M. Halpern
e
, S. Harrison
f
, S. Henderson
c,d
, S. R. Hildebrandt
h,a
,
G. C. Hilton
l
, J. Hubmayr
l
, H. Hui
a
, K. D. Irwin
c,d,n,l
, J. Kang
a,n
, K. S. Karkare
i,f
, S. Kefeli
a
,
J. M. Kovac
f,j
, C. L. Kuo
n,c,d
, K. Lau
m
, E. M. Leitch
i
, K. G. Megerian
h
, L. Minutolo
a
,
Y. Nakato
m,n
, T. Namikawa
r,n
, H. T. Nguyen
h
, R. O’Brient
h,a
, S. Palladino
g
, N. Precup
m
,
T. Prouve
o
, C. Pryke
k,m
, B. Racine
f
, C. D. Reintsema
l
, A. Schillaci
a
, B. L. Schmitt
f
,
A. Soliman
a
, T. St. Germaine
f,j
, B. Steinbach
a
, R. V. Sudiwala
b
, K. L. Thompson
d,n
,
C. Tucker
b
, A. D. Turner
h
, C. Umilt`a
p,g
, A. G. Vieregg
s,i
, A. Wandui
a
, A. C. Weber
h
,
D. V. Wiebe
e
, J. Willmert
m
, W. L. K. Wu
c,d,n
, E. Yang
n
, K. W. Yoon
n,c,d
, E. Young
d,c,n
,
C. Yu
n
, L. Zeng
f
, C. Zhang
a
, and S. Zhang
a
a
Department of Physics, California Institute of Technology, Pasadena, California 91125, USA
b
School of Physics and Astronomy, Cardiff University, Cardiff, CF24 3AA, United Kingdom
c
SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025
d
Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita
Mall, Stanford, CA 94305
e
Department of Physics and Astronomy, University of British Columbia, Vancouver, British
Columbia, V6T 1Z1, Canada
f
Center for Astrophysics, Harvard & Smithsonian, Cambridge, MA 02138, U.S.A
g
Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, USA
h
Jet Propulsion Laboratory, Pasadena, California 91109, USA
i
Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
j
Department of Physics, Harvard University, Cambridge, MA 02138, USA
k
Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis, 55455, USA
l
National Institute of Standards and Technology, Boulder, Colorado 80305, USA
m
School of Physics and Astronomy, University of Minnesota, Minneapolis, 55455, USA
n
Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305
o
Service des Basses Temp´eratures, Commissariat `a lEnergie Atomique, 38054 Grenoble, France
p
Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
q
Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, Illinois
61801, USA
r
Department of Applied Mathematics and Theoretical Physics, University of Cambridge,
Cambridge CB3 0WA, UK
s
Department of Physics, Enrico Fermi Institute, University of Chicago, Chicago, IL 60637
ABSTRACT
A detection of curl-type (B-mode) polarization of the primary CMB would be direct evidence for the inflationary
paradigm of the origin of the Universe. The Bicep/Keck Array (BK) program targets the degree angular scales,
email: moncelsi@caltech.edu
Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy X,
edited by Jonas Zmuidzinas, Jian-Rong Gao, Proc. of SPIE Vol. 11453, 1145314
© 2020 SPIE · CCC code: 0277-786X/20/$21 · doi: 10.1117/12.2561995
Proc. of SPIE Vol. 11453 1145314-1
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where the power from primordial B-mode polarization is expected to peak, with ever-increasing sensitivity and
has published the most stringent constraints on inflation to date. Bicep Array (BA) is the Stage-3 instrument of
the BK program and will comprise four Bicep3-class receivers observing at 30/40, 95, 150 and 220/270 GHz with
a combined 32,000+ detectors; such wide frequency coverage is necessary for control of the Galactic foregrounds,
which also produce degree-scale B-mode signal. The 30/40 GHz receiver is designed to constrain the synchrotron
foreground and has begun observing at the South Pole in early 2020. By the end of a 3-year observing campaign,
the full Bicep Array instrument is projected to reach σ
r
between 0.002 and 0.004, depending on foreground
complexity and degree of removal of B-modes due to gravitational lensing (delensing). This paper presents an
overview of the design, measured on-sky performance and calibration of the first BA receiver. We also give a
preview of the added complexity in the time-domain multiplexed readout of the 7,776-detector 150 GHz receiver.
Keywords: Cosmic Microwave Background, Polarization, Instrumentation, Cosmology, B-Modes, Inflation
1. INTRODUCTION
Measurements of the polarization of the Cosmic Microwave Background (CMB) provide crucial information to
further our understanding of the early Universe. The Lambda cold dark matter (ΛCDM) model predicts an
E-mode polarization pattern in the CMB at the level of a few µK as well as a B-mode polarization signal arising
from gravitational lensing of E-modes by the large-scale structure of the Universe. Inflationary gravitational
waves are predicted to produce an excess of B-mode power on degree angular scales that is scaled by the
tensor-to-scalar ratio r. The value of r is directly tied to the energy scale of inflation.
1
While certain classes
of inflation models predict extremely small values of r, a detection of primordial B-mode polarization would
be direct evidence for the theory of inflation. However, polarized Galactic dust and synchrotron foregrounds
hamper our ability to measure primordial gravitational waves and need to be disentangled from the degree-scale
B-mode polarization signal by observing the sky at multiple frequencies with exquisite sensitivity. Additionally,
the removal of B-mode power due to gravitational lensing (delensing) can lead to improved constraints on r, but
it requires higher resolution observations.
2
The most stringent published constraint on the tensor-to-scalar ratio is r
0.05
< 0.06 at 95% confidence from
Bicep/Keck Array data in conjunction with Planck and WMAP measurements
4
(see Figure 1, which includes
the most recent Planck points
3
). Over the past 15 years, our experimental strategy of designing small-aperture,
cryogenic, refracting wide-field telescopes observing from the South Pole has proven successful in probing the
degree-scale polarization of the CMB (multipoles 35 < l < 300) with tight control of systematic errors. Bicep1
was the first polarimeter designed specifically to target the B-mode signal and operated from January 2006
through December 2008 with 49 orthogonal pairs of polarization-sensitive NTD bolometers observing at 100
and 150 GHz. Bicep2 observed the sky with 500 antenna-coupled transition-edge sensor (TES) bolometers at
150 GHz from 2010 to 2012, and reported an excess of B-mode power over the base lensed-ΛCDM model in the
range 30 < l < 150,
5
which was later attributed to polarized dust emission.
6
The Keck Array consisted of five
Bicep2 -like 260 mm aperture receivers and started observations at 150 GHz in 2012. The interchangeable Keck
Array receivers allowed us to diversify the frequency coverage over the years: 95 GHz receivers were deployed in
2014, 220 GHz in 2015, and 270 GHz in 2017. The higher-throughput Bicep3 receiver replaced Bicep2 in its
mount in 2015, with a 520 mm aperture and 2500 detectors all operating at 95 GHz for an on-sky instantaneous
instrument sensitivity of 6.7 µK
cmb
√
s.
7
Figure 2 shows the progression of the Bicep/Keck Array program, to
larger apertures, larger focal planes, and wider frequency coverage.
2. THE BICEP ARRAY INSTRUMENT
The latest instrument in our program is Bicep Array,
8, 9
which adopts the same interchangeable concept used in
Keck Array and is comprised of four Bicep3-class receivers in six frequency bands spanning from 30 to 270 GHz
(Figure 3), which are all 27% wide. The lowest (30/40 GHz) and highest (220/270 GHz) frequency receivers are
dual-band receivers each featuring a checker-board of focal plane modules with single-band detectors centered
at the two adjacent bands (Figure 2). Splitting the atmospheric windows this way provides more spectral
information on the Galactic synchrotron and dust emission, thus improving the constraints on the foreground
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10
1
10
2
10
3
10
−4
10
−3
10
−2
10
−1
10
0
10
1
10
2
lensing
lensing/5
r=0.05
r=0.01
BK15 (2018)
CMB component
DASI (2005)
Boomerang (2006)
CBI (2007)
CAPMAP (2008)
QUAD (2009)
QUIET (2012)
WMAP (2013)
BICEP1 (2014)
ABS (2018)
Polarbear (2019)
Planck (2020)
Polarbear (2017)
SPTpol (2019)
ACTPol (2020)
Multipole
l(l+1)C
l
BB
/2π [µK
2
]
Figure 1. B-mode polarization measurement by different experiments as of Nov. 2020. The Planck points are on arXiv.
3
Receiver Nominal Nominal Single Beam Survey Weight
Observing Band Number of Detector NET FWHM Per Year
(GHz) Detectors (µK
cmb
√
s) (arcmin) (µK
cmb
)
−2
yr
−1
Keck Array
95 288 288 43 24,000
150 512 313 30 30,000
220 512 746 21 2,750
270 512 1,310 17 800
Bicep3
95 2,560 265 24 240,000
Bicep Array

30
40
192
300
260
318
76
57
19, 500
20, 500
95 4, 056 265 24 380, 000
150 7, 776 313 15 455, 600

220
270
8, 112
12, 288
746
1, 310
11
9
58, 600
19, 200
Table 1. Receiver parameters as used in sensitivity projections. Boldface numbers are actual/achieved quantities for
existing receivers. The remaining values in the survey weight column are scaled from the achieved survey weights using
only the ratio of the number of detectors, plus, if necessary to change frequency, the ratio of nominal NET values squared.
model parameters. Scaling from previous on-sky performance, Table 1 shows the expected sensitivities and survey
weights
*
of the 4 receivers, which amount to 32,000+ photon-noise limited polarization-sensitive detectors.
*
Survey Weight is a single number representing the total raw experimental sensitivity achieved, and is defined as
W = 2A/N
2
where A is the map area and N is the noise level in the Q/U maps. Survey weight is useful because it scales
linearly with integration time, number of detectors, and statistical sensitivity to r.
10
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Figure 2. The progression of the Bicep/Keck Array program leading to the Bicep Array. Bottom row: the beam patterns
of the focal planes on the sky shown on a common scale. Each square represents a single receiver, and the colors indicate
different observing frequencies: light red for 30/40 GHz, red for 95 GHz, green for 150 GHz, and blue for 220/270 GHz.
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Frequently asked questions

Q1. What are the contributions in "Receiver development for bicep array, a next-generation cmb polarimeter at the south pole" ?

By the end of a 3-year observing campaign, the full Bicep Array instrument is projected to reach σr between 0. 002 and 0. 004, depending on foreground complexity and degree of removal of B-modes due to gravitational lensing ( delensing ). This paper presents an overview of the design, measured on-sky performance and calibration of the first BA receiver. The authors also give a preview of the added complexity in the time-domain multiplexed readout of the 7,776-detector 150 GHz receiver. 

Q2. How many days of observation can the fridge hold?

With a design total loading of 70 and 15µW at the 300 and 250 mK stages, respectively, the fridge has sufficient cooling capacity to maintain a 3-day uninterrupted observing schedule between thermal cycles. 

Q3. What is the significance of the survey weight?

Survey weight is useful because it scales linearly with integration time, number of detectors, and statistical sensitivity to r.10Proc. of SPIE Vol. 

Q4. How much sensitivity is expected to be achieved at the end of the program?

The authors expected to reach σ(r) ∼ 0.003 at the end of the program, depending on the level of delensing that can be achieved using higherresolution data from the South Pole Telescope. 

Q5. What is the ring on the stage at the bottom of the fridge volume?

The ring on the stage at the bottom of the truss connects to the aluminum walls of the fridge volume and the 4 K base plate, which then connects to the optics tube. 

Q6. What was the first polarimeter designed specifically to target the B-mode signal?

Bicep1 was the first polarimeter designed specifically to target the B-mode signal and operated from January 2006 through December 2008 with 49 orthogonal pairs of polarization-sensitive NTD bolometers observing at 100 and 150 GHz. 

Q7. What is the cooling capacity of the sub-kelvin detectors?

Sub-Kelvin cooling for the detectors is provided by a three-stage helium (4He/3He/3He) sorption fridge from CEA Grenoble,23 with heat intercepts at 2 K (4He stage), 300 mK (intermediate cooler, or IC), and 250 mK (ultra cooler, or UC). 

Q8. When was the BA1 cryostat cooled to base temperatures?

The 30/40 GHz BA1 cryostat was successfully integrated with the new Bicep Array mount and cooled to base temperatures in January 2020. 

Q9. What is the design of the module?

The housing is constructed with superconducting niobium and aluminum, which, along with a high-µ A4K sheet inside the module, are designed to achieve high magnetic shielding performance (Section 2.5). 

Q10. What is the simplest implementation of the TES?

One possible implementation is the microwave SQUID readout (µMux35) system that has been already been operated with their current TES detectors for a year as an on-sky demonstrator. 

Q11. What type of filter connectors are used to protect the cold electronics from RF interference?

C48-00063-01 **) at the 4 K base plate protect the cold electronics from RF interference picked up in wiring outside the cryostat. 

Q12. How many detectors are tiled onto the focal plane?

Twelve detector modules are tiled onto the focal plane, each containing 32 to 2048 detectors, depending on the observing frequency (see Table 1). 

Q13. What is the sensitivity of the Bicep Array?

With the full Bicep Array sensitivity (see Figure 11), the authors expect to achieve and surpass the Planck map depths at all frequencies after only a few months of observations. 

Q14. What is the latest instrument in the program?

The latest instrument in their program is Bicep Array,8,9 which adopts the same interchangeable concept used in Keck Array and is comprised of four Bicep3-class receivers in six frequency bands spanning from 30 to 270 GHz (Figure 3), which are all 27% wide.