of 35
D
RAFT VERSION
O
CTOBER
4, 2021
Typeset using L
A
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style in AASTeX62
B
ICEP
/
Keck
XV: THE B
ICEP
3 CMB POLARIMETER AND THE FIRST THREE YEAR DATA SET
P. A. R. A
DE
,
1
Z. A
HMED
,
2
M. A
MIRI
,
3
D. B
ARKATS
,
4
R. B
ASU
T
HAKUR
,
5
C. A. B
ISCHOFF
,
6
D. B
ECK
,
2, 7
J. J. B
OCK
,
5, 8
H. B
OENISH
,
4
E. B
ULLOCK
,
9
V. B
UZA
,
10
J. R. C
HESHIRE
IV,
9
J. C
ONNORS
,
4
J. C
ORNELISON
,
4
M. C
RUMRINE
,
11
A. C
UKIERMAN
,
7, 2
E. V. D
ENISON
,
12
M. D
IERICKX
,
4
L. D
UBAND
,
13
M. E
IBEN
,
4
S. F
ATIGONI
,
3
J. P. F
ILIPPINI
,
14, 15
S. F
LIESCHER
,
11
N. G
OECKNER
-W
ALD
,
7
D. C. G
OLDFINGER
,
4
J. G
RAYSON
,
7
P. G
RIMES
,
4
G. H
ALL
,
11
G. H
ALAL
,
7
M. H
ALPERN
,
3
E. H
AND
,
6
S. H
ARRISON
,
4
S. H
ENDERSON
,
2
S. R. H
ILDEBRANDT
,
5, 8
G. C. H
ILTON
,
12
J. H
UBMAYR
,
12
H. H
UI
,
5
K. D. I
RWIN
,
7, 2, 12
J. K
ANG
,
7, 5
K. S. K
ARKARE
,
4, 10
E. K
ARPEL
,
7
S. K
EFELI
,
5
S. A. K
ERNASOVSKIY
,
7
J. M. K
OVAC
,
4, 16
C. L. K
UO
,
7, 2
K. L
AU
,
11
E. M. L
EITCH
,
10
A. L
ENNOX
,
14
K. G. M
EGERIAN
,
8
L. M
INUTOLO
,
5
L. M
ONCELSI
,
5
Y. N
AKATO
,
7
T. N
AMIKAWA
,
17
H. T. N
GUYEN
,
8
R. O’B
RIENT
,
5, 8
R. W. O
GBURN
IV,
7, 2
S. P
ALLADINO
,
6
T. P
ROUVE
,
13
C. P
RYKE
,
11, 9
B. R
ACINE
,
4, 18
C. D. R
EINTSEMA
,
12
S. R
ICHTER
,
4
A. S
CHILLACI
,
5
R. S
CHWARZ
,
11
B. L. S
CHMITT
,
4
C. D. S
HEEHY
,
19
A. S
OLIMAN
,
5
T. S
T
. G
ERMAINE
,
4, 16
B. S
TEINBACH
,
5
R. V. S
UDIWALA
,
1
G. P. T
EPLY
,
5
K. L. T
HOMPSON
,
7, 2
J. E. T
OLAN
,
7
C. T
UCKER
,
1
A. D. T
URNER
,
8
C. U
MILT
`
A
,
6, 14
C. V
ERG
`
ES
,
4
A. G. V
IEREGG
,
20, 10
A. W
ANDUI
,
5
A. C. W
EBER
,
8
D. V. W
IEBE
,
3
J. W
ILLMERT
,
11
C. L. W
ONG
,
4, 16
W. L. K. W
U
,
2
H. Y
ANG
,
7
K. W. Y
OON
,
7, 2
E. Y
OUNG
,
7, 2
C. Y
U
,
7
L. Z
ENG
,
4
C. Z
HANG
,
5
AND
S. Z
HANG
5
(B
ICEP
/
Keck
C
OLLABORATION
)
1
School of Physics and Astronomy, Cardiff University, Cardiff, CF24 3AA, United Kingdom
2
Kavli Institute for Particle Astrophysics and Cosmology, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California 94025, USA
3
Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, V6T 1Z1, Canada
4
Center for Astrophysics, Harvard & Smithsonian, Cambridge, MA 02138, U.S.A
5
Department of Physics, California Institute of Technology, Pasadena, California 91125, USA
6
Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, USA
7
Department of Physics, Stanford University, Stanford, California 94305, USA
8
Jet Propulsion Laboratory, Pasadena, California 91109, USA
9
Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA
10
Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
11
School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA
12
National Institute of Standards and Technology, Boulder, Colorado 80305, USA
13
Service des Basses Temp
́
eratures, Commissariat
`
a l’Energie Atomique, 38054 Grenoble, France
14
Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
15
Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
16
Department of Physics, Harvard University, Cambridge, MA 02138, USA
17
Kavli Institute for Physics and Mathematics of the Universe (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
18
Aix-Marseille Universit
́
e, CNRS/IN2P3, CPPM, Marseille, France
19
Physics Department, Brookhaven National Laboratory, Upton, NY 11973
20
Department of Physics, Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
ABSTRACT
We report on the design and performance of the B
ICEP
3 instrument and its first three-year data set collected
from 2016 to 2018. B
ICEP
3 is a 52 cm aperture, refracting telescope designed to observe the polarization of
the cosmic microwave background (CMB) on degree angular scales at 95 GHz. It started science observation
at the South Pole in 2016 with 2400 antenna-coupled transition-edge sensor (TES) bolometers. The receiver
first demonstrated new technologies such as large-diameter alumina optics, Zotefoam infrared filters, and flux-
activated SQUIDs, allowing
10
×
higher optical throughput compared to the
Keck
design. B
ICEP
3 achieved
instrument noise-equivalent temperatures of 9.2, 6.8 and 7.1
μ
K
CMB
s
and reached Stokes
Q
and
U
map depths
of 5.9, 4.4 and 4.4
μ
K-arcmin in 2016, 2017 and 2018, respectively. The combined three-year data set achieved
a polarization map depth of 2.8
μ
K-arcmin over an effective area of 585 square degrees, which is the deepest
CMB polarization map made to date at 95 GHz.
Corresponding author: Howard Hui
hhui@caltech.edu
arXiv:2110.00482v1 [astro-ph.IM] 1 Oct 2021
2
Keywords:
cosmic background radiation — cosmology: observations — gravitational waves — inflation —
instrumentation: polarimeters — telescopes
1.
INTRODUCTION
Inflation, a brief period of exponential expansion in the
early Universe, was postulated to solve the horizon, flatness
and monopole problems which arise from the
Λ
CDM “stan-
dard model” of the Universe (Brout et al. 1978; Starobinsky
1980; Kazanas 1980; Guth 1981; Linde 1982; Albrecht &
Steinhardt 1982). The perturbations under this paradigm
are adiabatic, nearly Gaussian and close to scale-invariant,
which are consistent with precise cosmic microwave back-
ground (CMB) observations (Planck Collaboration et al.
2020a). Moreover, many models of inflation predict the
existence of primordial gravitational waves (PGWs) which
would leave a unique degree-scale
B
-mode polarization pat-
tern in the CMB (Kamionkowski et al. 1997; Seljak & Zal-
darriaga 1997). If detected, PGWs can serve as a probe of
the very early Universe and high energy physics inaccessible
with existing particle accelerators.
The B
ICEP
/
Keck
experiments are a series of telescopes de-
signed to search for this degree-scale
B
-mode polarization
of CMB originating from PGWs. These instruments are lo-
cated at the Amundsen-Scott South Pole Station in Antarc-
tica. The
10,000 ft altitude and extreme cold make the
Antarctic plateau one of the driest places on earth. During
the winter season, the 6 months of continuous darkness pro-
vides exceptionally low and stable atmospheric
1
/f
noise,
which allows our telescopes to observe the sky without the
need of an active instrument modulation at these large angu-
lar scales (Kuo 2017).
We first reported an excess of
B
-mode signal at 150 GHz
in BICEP2 collaboration et al. (2014a). In a subsequent joint
analysis with the
Planck
collaboration, it was found that
polarized emission from dust in our galaxy could account
for most of the signal (BICEP2/Keck and Planck collabo-
rations et al. 2015). Dust is currently the dominant fore-
ground contaminant to CMB polarization measurements, and
is most powerful at high frequencies. Subsequent modeling
shows synchrotron may potentially be another source of fore-
ground emission at lower frequencies (Krachmalnicoff, N.
et al. 2018). In order to probe the physics of the early Uni-
verse, we need a dedicated strategy to separate these fore-
grounds from the potential faint primordial signal.
The B
ICEP
/
Keck
instruments are small-aperture, com-
pact, on-axis refracting telescopes, emphasizing high opti-
cal throughput and low optical loading with dedicated cal-
ibration campaigns to control instrument systematics. Five
separate instruments spanning the past two decades have
been deployed to date. B
ICEP
1 operated from 2006 through
2008 with 98 neutron transmutation doped (NTD) germa-
nium thermistors at 95, 150 and 220 GHz (Chiang et al.
2010; Takahashi et al. 2010). B
ICEP
2 replaced B
ICEP
1 and
observed from 2010 through 2012 with 512 planar antenna
transition edge sensors at 150 GHz (BICEP2 collaboration
et al. 2014b).
Keck
utilized the same optical and detector
technologies as employed in B
ICEP
2, comprising five inde-
pendent receivers. It observed at 150 GHz, and later at 95
and 220 GHz, installed in a separate telescope mount pre-
viously used for DASI (Leitch et al. 2002) and QUaD (Ade
et al. 2008). It began science observations in 2012, observ-
ing until 2019 (Kernasovskiy et al. 2012; Staniszewski et al.
2012).
After B
ICEP
2 was decommissioned at the end of 2012, B
I
-
CEP
3 was installed in the same telescope mount in November
2014 and started scientific observation in 2016 with 2400 de-
tectors at 95 GHz. It employed a conceptually similar design
to its predecessor, but with multiple technological improve-
ments allowing an order of magnitude increase in mapping
speed compared to a single
Keck
95 GHz receiver. Benefiting
from a modular receiver design,
Keck
was gradually adapted
from an all-150 GHz receiver configuration into a high fre-
quency ‘dust telescope’, observing at 220 and 270 GHz,
with B
ICEP
3 continuing observations at 95 GHz, where fore-
grounds are minimal. In late 2019,
Keck
was decommis-
sioned and replaced with a new telescope mount (Crum-
rine et al. 2018) to accommodate four B
ICEP
3-like receivers
that will form the next phase of the experiment, B
ICEP
Ar-
ray. The first receiver in B
ICEP
Array started observation
at 30/40 GHz in 2020 to probe the low frequency polarized
synchrotron signal. B
ICEP
Array will cover 6 distinct bands
from 30 to 270 GHz when fully deployed. In the meantime,
the B
ICEP
Array telescope mount carries a mixture of
Keck
and B
ICEP
Array receivers, while B
ICEP
3 continues to ob-
serve. Table 1 shows the B
ICEP
/
Keck
experiments from 2010
to 2020 and their frequency coverage.
This paper provides an overview of the B
ICEP
3 instrument
design and performance with the three-year dataset from
2016 to 2018. Fig. 1 shows the overall layout of B
ICEP
3
as it is installed at the South Pole. The following sections
describe the details of each of the subcomponents: telescope
mount (§2); optics (§3); cryostat (§4); focal plane unit (§5);
transition-edge sensor bolometers (§6); and data acquisition
and control system (§7).
In particular, B
ICEP
3’s 520 mm diameter aperture is
2
times the size of the
Keck
design. This is realized by the
large diameter alumina optics shown in §3.1. The increase
in aperture size allowed us to accommodate 2400 detectors
in the focal plane, compared to 288 detectors in the previous
3
Table 1.
Frequency coverage in the B
ICEP
/
Keck
experiment from 2010 to 2020. Brackets in the table indicate an engineering receiver (270 GHz
Keck
in 2017 was a prototype of high-frequency focal plane, B
ICEP
3 in 2015 only had a partially filled focal plane, and the 150 GHz
Keck
in 2019 was a demonstration of the
μ
Mux readout (Cukierman et al. 2020)), and are not included in science analyses.
Keck
was replaced by
B
ICEP
Array in 2020. In its first season, one slot was fitted with the 30/40 GHz B
ICEP
Array receiver, and three
Keck
receivers were put back
into the new telescope mount. This paper uses the data collected by B
ICEP
3 from 2016 through 2018.
Receiver
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
B
ICEP
2
150GHz
150GHz
150GHz
Keck Rx0
150GHz
150GHz
95GHz
95GHz
220GHz
220GHz
220GHz
220GHz
220GHz
Keck Rx1
150GHz
150GHz
150GHz
220GHz
220GHz
220GHz
220GHz
[150GHz]
Keck Rx2
150GHz
150GHz
95GHz
95GHz
220GHz
220GHz
220GHz
220GHz
220GHz
Keck Rx3
150GHz
150GHz
150GHz
220GHz
220GHz
220GHz
220GHz
220GHz
Keck Rx4
150GHz
150GHz
150GHz
150GHz
150GHz
[270GHz]
270GHz
270GHz
270GHz
B
ICEP
3
[95GHz]
95GHz
95GHz
95GHz
95GHz
95GHz
BA Rx0
30/40GHz
Reflective Groundshield
Absorptive Forebaffle
Building Roof
BICEP3 Cryostat
Telescope Mount
3 meters
3 meters
Figure 1.
The B
ICEP
3 telescope in the mount, looking out through the roof of the Dark Sector Laboratory (DSL) located
1100
m from
the geographic South Pole. The insulating environmental shield shown in the bottom right photo is hidden in the CAD layout. The three-
axis mount previously used in B
ICEP
1 and B
ICEP
2 allows for motion in azimuth, elevation, and boresight rotation. A co-moving absorptive
forebaffle extends skyward beyond the cryostat receiver to intercept stray light outside the designed field-of-view. Additionally, the telescope
is surrounded by a stationary reflective ground shield which redirects off-axis rays to the cold sky.
Keck
95 GHz receivers. The new modular focal plane de-
sign in §5 allows rapid rework and dramatically reduces risk.
The high number of detectors also requires a mature mul-
tiplexing readout. B
ICEP
3 is the first experiment to adapt
the new generation flux-activated time domain multiplexing
system described in §7. Most CMB experiments utilize low
temperature, superconducting detectors that operate below
1 K. Rapid development in mechanical compressor cryocool-
ers allowed ground-based telescopes to phase out the need
of liquid Helium, but the high-pressure Helium lines in the
system between the telescope and compressor induce signif-
icant wear in a continuous rotating mount. We address this
by integrating a helium rotary joint into the telescope mount
system, allowing for continuous rotation while maintaining a
high-pressure seal and electrical connectivity (§2).
The achieved performance characteristics of the receiver
and detector properties of B
ICEP
3 are presented in §8, the
observing strategy is presented in §9, and in §10 we show
4
the first three-year data set taken from 2016 to 2018, report-
ing its internal consistency validation, sensitivity, and map
depth. The cosmological analysis using
Planck
, WMAP, and
B
ICEP
/
Keck
observations through the 2018 observing season
are presented in BICEP/Keck et al. (2021a).
2.
TELESCOPE MOUNT, FOREBAFFLE AND GROUND
SHIELD
2.1.
Telescope mount
B
ICEP
3 is installed in the Dark Sector Laboratory build-
ing, approximately a kilometer away from the South Pole
Station. The base of the telescope mount is supported by
a platform on the second floor of the building, with a 2.4 m
diameter opening in the roof for telescope access to the sky
(Fig. 1). The warm indoor environment of the building is
extended beyond the roof level by a flexible insulating envi-
ronmental shield, so that only the receiver window is exposed
to the Antarctic ambient temperature.
B
ICEP
3 uses a steel three-axis mount built by Vertex-RSI
1
.
It was originally built for B
ICEP
1 and also housed B
ICEP
2
until 2013. The mount structure was modified in 2014 to
accommodate the larger B
ICEP
3 receiver.
The mount moves in azimuth and elevation, with the third
axis rotating about the boresight of the telescope (“deck” ro-
tation). The range of motion of the mount is
48
to
110
in
elevation and
400
in azimuth, capable of scanning at speeds
of
5
/s in azimuth. The B
ICEP
3 cryostat houses a pulse tube
cryocooler which limits the accessible deck angle to less than
a full
360
rotation in the B
ICEP
mount. However, the design
still allows the telescope to scan with two sets of
180
oppos-
ing deck angles, offset from each other by
45
, retaining an
effective set of observation schedules in order to probe sys-
tematic errors, as shown in §9.
2.2.
Helium rotary joint
The pulse tube cryogenic cooler comprises two sub-
systems: a coldhead installed inside the receiver, and a
helium compressor located in the building, away from the
telescope mount. This pulse tube provides cooling by ex-
panding a high pressure helium gas volume, and requires
high and low pressure helium flexible lines to be routed from
the compressor, through the three mount axes (azimuth, ele-
vation, and boresight), to the coldhead in the receiver.
During an observing schedule, movements in elevation and
boresight are intermittent and span a limited range of angles,
unlike the azimuth axis which scans back and forth continu-
ously in azimuth with a
130
range. To avoid wear on the
compressor lines in the helium line wrap, B
ICEP
3 uses a
1
Now General Dynamics Satcom Technologies, Newton, NC 28658,
http://www.gdsatcom.com/vertexrsi.php
commercial high-pressure-gas rotary joint from DSTI
2
that
enables the two pressurized helium gas lines to pass through
the azimuth motion. In this joint, shown in Fig. 2, one set
of lines remains static at the base of the mount and connects
to the helium rotary joint, from which a second set of lines
rotate with the azimuth axis of the mount. Therefore the az-
imuth cable carrier only needs to handle the much more flex-
ible electrical cables.
During the 2015 engineering season, the original design
used a basic 2-channel rotary joint (DSTI model: GP-421)
to connect the compressor’s high and low pressure helium
channels at 290 and 90 PSI, respectively. However, helium
gas can permeate materials and gaps much more easily than
other larger gas molecules, and this commercial rotary joint
was not designed specifically for helium gas. We found the
overall system lost 3-5 PSI of pressure per day, originating
in the dynamic seals of the rotary joint. Such a large leak
resulted in the need to refill the compressor system multiple
times a week to maintain optimal pulse tube performance. In
addition to being extremely labor-intensive for the telescope
operator, these repeated helium refills introduced contamina-
tion into the pulse tube, and eventually degraded the cooling
performance.
To remedy the high leak rate, the rotary joint was replaced
with a 4-channel model (DSTI model: GP-441) before the
2016 season. The 4 channels were configured such that the
two working high pressure helium lines would be guarded
by two outer channels, serving as pressurized buffers. Thus,
the dynamic seals between the two inner high pressure lines
would only ‘sense’ the small differential pressure to the pres-
surized buffers (
10
PSI) instead of the much larger differ-
ential to atmospheric pressure (
>
100
PSI). This configura-
tion reduced the leak rate of the active channels to between
0.1 and 0.5 PSI per day over an entire season
3
. The reduced
helium leak rate requires less frequent refills, and enables op-
timal pulse tube performance throughout a full season. The
HRJ dynamic seals receive a complete replacement once per
year.
2.3.
Ground shield and absorptive baffle
A warm, absorptive forebaffle as shown in Fig. 1 extends
skyward beyond the cryostat receiver to intercept stray light
outside the designed field-of-view. The forebaffle is mounted
directly to the receiver and therefore co-moving with the axes
of motion of the telescope. The forebaffle is constructed from
a large aluminum cylinder, 1.3 m in diameter and height, with
2
Dynamic Sealing Technologies,
Inc.,
Andover,
MN 55304,
www.dsti.com
3
The guard channels still have similar leak rate as the 2-channel de-
sign, but this is acceptable since refills for them do not affect the pulse tube
performance.