# The DAMIC-M Low Background Chamber

I. Arnquist  N. Avalos  P. Bailly  D. Baxter  X. Bertou  M. Bogdan  C.
Bourgeois  J. Brandt  A. Cadiou  N. Castelló-Mor  A.E. Chavarria  M. Conde  J.
Cuevas-Zepeda  A. Dastgheibi-Fard  C. De Dominicis  O. Deligny  R. Desani  M.
Dhellot  J. Duarte-Campderros  E. Estrada  D. Florin  N. Gadola  R. Gaïor
E.-L. Gkougkousis  J. González Sánchez  S. Hope  T. Hossbach  M. Huehn  M.
Kallander  B. Kilminster  L. Iddir  A. Lantero-Barreda  I. Lawson  H. Lebbolo
S. Lee  P. Leray  A. Letessier Selvon  H. Lin  P. Loaiza  A. Lopez-Virto  I.
Lounes  D. Martin  K.J. McGuire  T. Milleto  P. Mitra  D. Moya Martin  S.
Munagavalasa  D. Norcini  C. Overman  S. Paul  D. Peterson  A. Piers  O.
Pochon  P. Privitera  D. Reynet  P. Robmann  R. Roehnelt  M. Settimo  S. Smee
R. Smida  B. Stillwell  T. Van Wechel  M. Traina  R. Vilar  P. Vallerand  A.
Vollhardt  G. Warot  D. Wolf  R. Yajur  and J-P. Zopounidis

###### Abstract

The DArk Matter In CCDs at Modane (DAMIC-M) experiment is designed to search
for light dark matter (mχ<10 GeV/c2) at the Laboratoire Souterrain de Modane
(LSM) in France. DAMIC-M will use skipper charge-coupled devices (CCDs) as a
kg-scale active detector target. Its single-electron resolution will enable
eV-scale energy thresholds and thus world-leading sensitivity to a range of
hidden sector dark matter candidates. A DAMIC-M prototype, the Low Background
Chamber (LBC), has been taking data at LSM since 2022. The LBC provides a low-
background environment, which has been used to characterize skipper CCDs,
study dark current, and measure radiopurity of materials planned for DAMIC-M.
It also allows testing of various subsystems like readout electronics, data
acquisition software, and slow control. This paper describes the technical
design and performance of the LBC.

##  1 Introduction

A multitude of astrophysical and cosmological observations have revealed that
our universe is mostly made of non-luminous, non-baryonic dark matter (DM) [1,
2, 3]. Measurements of type Ia supernovae [4, 5], cosmic microwave background
fluctuations [6], the large-scale distribution of galaxies [7, 8, 9, 10, 11],
weak gravitational lensing [12, 13, 14, 15, 16], and the abundances of the
light elements (see a review in Ref. [17]) suggest that the Standard Model of
Cosmology, $\Lambda$CDM, remarkably describes the total matter density when
parameterized with a cold DM particle. There are many particle candidates that
fit the constrained properties, yet no experiment has seen evidence for direct
interactions between DM and the Standard Model particles.

Over the last few decades, Weakly Interacting Massive Particles (WIMPs) [18]
have been the most searched-for DM candidate. With a mass larger than the
proton ($\approx 1$ GeV$/c^{2}$), these particles are highly motivated since
if their annihilation cross section is similar to the electroweak-scale, it
results in a thermal relic from the early universe consistent with the
observed DM abundance. As multi-ton xenon experiments have only produced null
results for DM masses larger than 10 GeV [19, 20, 21], the field has recently
expanded its mass range of interest to include light (sub-GeV) DM [22].
Candidates in this regime include light WIMPs and dark or hidden sector
particles [23]. Due to the smaller mass of these particles, the kinematics
favor lower energy recoils that require detectors to be sensitive to sub-keV
energy deposits.

The DAMIC-M (DArk Matter In CCDs at Modane) experiment [24] has been
specifically designed to detect both nuclear and electronic recoils from sub-
GeV DM scattering. The detector will operate deep underground at the
Laboratoire Souterrain de Modane (LSM) and contain up to 208 charge-coupled
devices (CCDs) for a total sensitive mass of silicon of $\sim$700 g. The CCDs
feature skipper readout amplifiers allowing for the detection of DM-induced
ionization events within the silicon with sub-electron resolution through non-
destructive, repeated pixel measurements [25, 26, 27, 28]. The ability to
detect single electrons with low dark current sensors [29, 30] in a very low
background environment (<1 dru 1111 dru = 1 event/keV/kg/day.), will allow
DAMIC-M to lower the energy threshold down to only of a few eV.

Here we report the technical details of the DAMIC-M prototype detector, the
Low Background Chamber (LBC), which is currently in operation at LSM. The
objectives of the detector are discussed in Sec. 2, followed by the
description of the detector design and instrumentation in Sec. 3 and 4. The
efforts to reduce the radioactive background and first results on detector
performance are shown in Sec. 5 and 6, respectively.

##  2 Prototype Objectives

The LBC was built to establish DAMIC-M infrastructure at LSM, test components
and subsystems, and benchmark skipper CCD performance in a low background,
underground environment. The detector has also served as a scientific
instrument, producing preliminary background studies (see Sec. 6 for more
details) and first DAMIC-M dark matter-electron scattering exclusion limits
[24, 31]

LSM is the deepest underground laboratory in Europe, sitting 1700 m below the
Fréjus Peak (4800 m.w.e.) between the French and Italian Alps. In this
location, the muon flux is reduced to 5.4 $\mu$/m2/day [32, 33], i.e., by more
than a factor of 106 compared to sea level. Although several experiments have
previously built facilities at LSM, such as the EDELWEISS dark matter [34] and
the NEMO/SuperNEMO neutrinoless double beta decay [35, 36] experiments, the
DAMIC-M LBC is the first CCD detector to operate at this site. Thus, one of
the major objectives of the prototype was for the collaboration to gain
experience working with the LSM staff and begin building the cleanroom and
supporting infrastructure for DAMIC-M. An ISO 5 class cleanroom was built to
house the LBC and will later be used for the full DAMIC-M detector. More
details about the infrastructure can be found in Sec. 3.6.

An important goal of the LBC was to demonstrate that a background rate below
DAMIC at SNOLAB levels [37] i.e., <10 dru could be achieved at the LSM depth
with cleaner detector components. Extensive R&D work was required to select
the most pure materials, determine procedures to properly clean surfaces, and
to perform radioassays. Both experience from other low background experiments
[38, 39, 40, 37] and a custom Geant4 simulation [41, 42, 43] were used to
inform these decisions, as further described in Sec. 5. In addition, the LBC
also acts as a test bed to validate the operation of detector subsystems for
DAMIC-M (see Sec. 4), such as the cryogenics, CCD electronics, slow control,
and data quality monitoring.

As the first opportunity for DAMIC-M to bring skipper CCDs underground, a
crucial objective of the LBC was to evaluate the performance of these novel
devices. Data were taken in different shielding configurations to examine the
decrease of radiation background, measure the dark current induced by external
radiation and select operating parameters and readout strategies depending on
the background radiation level. Furthermore, observables – such as gain,
noise, energy resolution, dark counts – were optimized and their variation
over time was studied. We checked the production and packaging yield of large-
format CCDs, observed the CCD performance after multiple cooling and warming
cycles, and measured the dark current and clock-induced charge in low
background environment. Sec. 6 gives a summary of the various datasets and
performance outcomes.

Dependent on the successful achievement of goals, the LBC was also designed to
produce science results, because the single-electron resolution and low dark
current of skipper CCDs makes them an ideal target and detector for light dark
matter. The sensitivity of the LBC probes both models of thermal freeze-out
and freeze-in [23] for numerous dark matter masses within a few months of
exposure. The low energy threshold of skipper CCDs also makes them uniquely
suited to search for mediators of the hidden sector.

First results on dark matter-electron scattering [24], including a search for
a daily modulation in the event rate [31], were published, and studies of
alternative low-mass dark matter candidates are forthcoming. In addition,
three doctoral theses describe work on the LBC [44, 45, 46].

##  3 Detector Description

The LBC is designed to provide $O(\mathrm{1})$ dru background environment for
the operation of CCDs with modular electronics. The innermost region of the
detector consists of a cold copper housing, which simultaneously serves to
cool the CCDs and provide an infrared shield from the surrounding room
temperature materials. This cold copper housing is enclosed by 2 cm of ancient
Roman lead from a sunken galley [47, 38], then an additional 4-10 cm of very
low background ("très faible activité" or TFA) lead. Kinked channels provide a
pathway for the two-layer polyimide flex cables, used for CCD operation and
readout, to exit the inner region eliminating a line-of-sight to the inner
copper housing. Cold oxygen-free high-conductivity, or OFHC222In reality, it
is from OFE-OK copper grade by Luvata in Finland. Because it is comparable to
oxygen-free high thermal conductivity copper used in other experiments, we
will use acronym OFHC., copper "fingers" travel through four narrow gaps in
the interior (warm) lead shielding with $\sim$1 mm clearance to provide
thermal contact and mechanical support for the inner copper housing.

Outside of the interior lead shielding are four axially-spaced intermediate
boards, which provide the first stage amplification to the CCD video signal.
The boards connect to flex cables through commercial 50-pin connectors on each
side. The output side of each board connects to a 45 cm long, five-layer CCD
flex cable that exits below the lead shielding to commercial 50-pin D-Sub
vacuum feedthroughs below.

The cryostat is designed in two pieces, each of them machined from OFHC
copper. A top can with an inner diameter of 40 cm and height of 25 cm can be
lifted up and away from the detector with a crane for easy access to the CCDs.
A bottom cross provides structural support as well as all vacuum and
electronics feedthroughs. A set of lead discs are affixed to the top can to
shield the CCDs from above, and are lifted during opening.

The cryostat is housed inside of a custom shielding support structure built
out of aluminum extrusions and steel plates. This structure includes a
mechanism to horizontally open the shielding by sliding half of the shielding
and the cryostat away from a fixed shielding wall. Outside of the cryostat,
the cylindrical shielding consists of lead and high-density polyethylene
(HDPE). Fig. 1 shows a cross sectional cut through the CAD model of the LBC in
the open configuration.

The design allows for easy exchange of CCDs and internal electronics, as well
as for the deployment of calibration sources. This provides an efficient way
to characterize various CCDs and front-end electronics in different background
environments.

Figure 1: A cross-sectional view of the LBC detector in its fully opened
position with major parts labelled. The cryostat and right side of the
external shield can be moved on rails, while the left side of the external
shield is static. Other parts of the detector are labelled.

###  3.1 Charge-Coupled Devices

In the originally commissioned configuration, two CCDs – each of the pixel
format 6144 columns and 4128 rows or 6k$\times$4k – were installed in the LBC
in late 2021333A couple of CCDs of the same format were also installed in the
upgrade of the DAMIC detector at SNOLAB [48].. These CCDs feature a three-
phase polysilicon gate structure with a buried p-channel, with a pixel size of
15$\times$15 $\mu$m2. The CCD thickness after all fabrication steps is 669
$\mu$m [37], totaling $\sim$8.9 g in the active mass. The bulk of these device
consist of high-resistivity ($>$10 k$\Omega$ cm) n-type silicon which allows
for fully depleted operation at substrate bias voltage
V${}_{\mathrm{sub}}\geq$40 V.

The devices were developed by Lawrence Berkeley National Laboratory (LBNL)
[49, 50, 51] and fabricated by Teledyne DALSA Semiconductor with front-side
polishing at Omnisil. The 150 mm diameter silicon wafers were sliced from an
ingot pulled by Topsil GlobalWafers A/S. Wirebonding and packaging were done
at the University of Washington.

These 6k$\times$4k CCD have two skipper amplifiers on one side and two
standard amplifiers on the opposite side. Unlike conventional CCD amplifiers,
skipper amplifiers can be configured to make multiple, non-destructive charge
measurements (NDCMs) [25, 26, 27, 28]. Skipper readout essentially moves the
charge contained in each pixel back-and-forth into the readout node allowing
for many measurements of the same pixel so that they can be averaged. Since
the measurements are uncorrelated, the readout noise is reduced to
$\sigma_{1}/\sqrt{N_{\rm{skip}}}$, where $\sigma_{1}$ is the single-sample
readout noise (the standard deviation of a single charge measurement) and
$N_{\rm{skip}}$ is the number of NDCMs. By taking a large enough number of
NDCMs, the readout noise can reach the sub-electron level and single
charges444For the sake of clarity, we use the term electrons to indicate
charge carriers detected in the CCD. However, holes are held in the pixels of
p-channel LBNL CCDs. are resolved. The single-electron resolution also
provides a straightforward way of calibrating the energy response of the
detectors.

A two-layer CCD flex cable carries the clock voltages to and video outputs
from each CCD. The cable is glued to an extended non-functional area of the
CCD. To improve adhesion, rectangular silicon shims were glued between the CCD
die and the flex. The glue is low-background Epotek 301-2 previously selected
for DAMIC at SNOLAB. The cable lines are wedge bonded with 25-$\mu$m thick Al
wire to corresponding pads on the CCD. Special ESD safety precautions have
been implemented during handling, packaging and testing CCDs to reach high
packaging yield. During this time, care is taken to minimize the exposure of
the CCDs to the air and hence, the accumulation of radon progeny on the CCD
surface.

CCDs were characterized in a surface laboratory and only the devices passing
our science-grade criteria were selected for the LBC. The selection criteria
are as follows: all amplifiers work and have low readout noise; none or only
few defect pixels are in the pixel area; minimal clock-transfer inefficiency
during vertical and horizontal clocking of charge; single electron resolution
by skipper amplifiers; and the CCD has low leakage current. In addition,
dedicated measurements with atmospheric muon tracks were taken to model the
diffusion of charge as it drifts from the back or the bulk to the buried
channel on the front side of the CCD [52].

In the LBC, the two CCDs were placed in a 180$\times$180 mm2 interlocking
copper box. This CCD box consists of one frame for each CCD, top and bottom
lids and t-bars (to clamp the flex cable to the frame). The parts are tightly
connected with brass screws to prevent infrared radiation and to press CCD
cables coming out of the CCD box with t-bars. Fig. 2 (left) shows the CCD
configuration before installing the top lid.

In February 2023, two DAMIC-M CCD prototype modules were installed for
characterization in a low background environment. Each module includes four
CCDs with an active pixel area of 6144 columns and 1536 rows (6k$\times$1.5k)
glued to a silicon pitch adapter with traces for clocks, biases and video
signals. The pitch adapters were fabricated at the Washington Nanofabrication
Facility (WNF) from high-resistivity wafers with 0.5 $\mu$m of thermal oxide.
The traces are made from 1-$\mu$m thick aluminum and their width/spacing were
optimized to minimize electrical resistance and parasitic capacitances. In the
CCD module, only one amplifier is read out from each CCD. The module installed
in the copper frame can be seen in Fig. 2 (right). By installing these two CCD
modules in the LBC, we were able to verify the design and characterize
6k$\times$1.5k CCDs and their electronics in a low background environment.

Figure 2: (Left) Two 6k$\times$4k CCDs installed in the OFHC copper CCD box.
Note, that there is no material between the bottom and top CCD. Two flex
cables from each CCD are fixed with copper t-bars on the way out of the box.
(Right) Two CCD modules installed in the same CCD box as in (left), but only
the the top module can be seen. 6k$\times$1.5k CCDs are glued on a silicon
pitch adapter with traces bringing voltages to all four CCDs from the cable.
Note that the flex from the second module is visible in the right-hand side
photo.

###  3.2 Boards and Cabling

Here, we will describe the electronics chain used for the 6k$\times$4k CCDs
originally installed in the LBC and provide a few details about the
configuration used for the CCD modules. All components were tested in
laboratories at various institutions before their installation at LSM.

The electronics chain starts at the two-layer CCD flex cable, which is wire
bonded directly to two sides of the 6k$\times$4k CCD. This type of a flex
cable has been developed to achieve low background in future experiments, like
DAMIC-M [53]. The flex cables used for the LBC have the same layers but they
are not as radiopure as those described in [53], because they were fabricated
without costly cleaning steps.

Each 17 cm long CCD cable is routed through a thermally insulating
polychlorotrifluoroethylene (PCTFE) sleeve to the outside of the inner lead
shielding castle. A 50-pin D-Sub connector at the end of the cable is plugged
into a intermediate PCB. As these boards and D-Sub-50 connectors are not
radiopure, they are shielded from CCDs by the internal lead.

The intermediate board has an amplifier, bias line filters, clock shaping
circuit and two D-Sub connectors. Right angle connectors were used at the
beginning, but boards with straight connectors were installed later after
comparing radioassay results for both type of connectors.

A second-stage 45 cm long flexible cable, still thermally isolated from warm
lead, goes from the intermediate board down to the cross part of the cryostat,
where it is plugged into a two-to-one board. This board merges two cables
coming from opposite sides of the same CCD and connects to the vacuum
feedthrough. The two-to-one boards were needed only for the 6k$\times$4k CCDs
and were taken out when the CCD modules were installed in early 2023. The CCD
modules have only one cable to carry all signals so new intermediate boards
were also installed.

###  3.3 Cold Copper Parts and IR Shielding

The CCDs are operated at low temperature to achieve low dark current and in
vacuum to avoid humidity condensation, which may cause shorts between exposed
conductive parts. Typically, the LBC operates the CCDs at $\sim$130 K. Images
have also been acquired at higher temperatures (up to 180 K) in runs dedicated
to the identification of hot pixels and columns and studies of temperature
dependence of the dark current. Low, stable temperatures are necessary to
optimally operate the CCDs, thus imposing strict requirements on cold copper
parts.

The CCDs are housed in a copper box which cools the devices and acts as an IR
shield. The dominant thermal path to the CCDs is through the CCD flex cables
which are clamped to the CCD box via t-bars. In this configuration, the
temperature remains stable when the on-chip CCD amplifiers are powered, see
Sec. 6.1.

The CCD box is supported by, and thermally coupled to, four cold fingers which
rise from a copper hand. This hand is atop of an arm which is fixed to a cold
mass table. The cold mass table sits on four polytetrafluoroethylene (PTFE)
standoffs, which are connected to the bottom flange with vented studs. The
drawing of the setup can be seen in Fig. 3. All these copper parts are kept at
cold during the operation and they have sufficient clearance to other parts
inside the cryostat that are kept at room temperature.

The connection to the cold tip of a cryocooler is made by a flexible thermal
strap by Thermal Management Technologies. Individual copper pieces were
tightly connected with brass screws and a thin layer of thermally conductive
grease (i.e. Apiezon N cryogenic high vacuum grease) was applied between them.
For an electrical isolation, two 25 $\mu$m-thick Kapton tapes are placed
between the cold tip and flexible strap.

The copper parts inside the cryostat are made from OFHC copper from Luvata and
were machined at the University of Zürich. The cryostat was made by CINEL in
Italy from the same type of copper. The cold copper parts and electronics
chain inside the cryostat, support structure, and a connection to the
cryocooler are shown in Fig. 3. For the clarity, we do not show the internal
lead shielding.

Because the copper lids are directly above and below CCDs, they are a major
source of radioactive background. OFHC copper lids were used while taking
science data in 2022 [24, 31], but were taken out later the same year. The new
installed lids are from less radioactive, electro-formed copper (EFC) grown
and machined by Laboratorio Subterraneo de Canfranc (LSC) [54]. The EFC lids
not only have much lower radioactive contamination than the commercial OFHC
copper, but also their cosmogenic activation time has been reduced from many
months to less than a week, see Sec. 5.1.

Figure 3: All parts inside the cryostat except the inner lead shielding, RTDs
and heaters; together with the cryocooler (without its air shroud). The CCD
box is lifted above other components to have enough lead below it. Cold copper
pieces connect to the cold tip of the cryocooler (right, bottom). Four
extension flex cables encapsulated in thermal shielding can be seen running
from the CCD box to intermediate boards. The second stage cables run down from
the boards to a couple of two-to-one boards which are plugged to the vacuum
feedthrough. Lead, temperature sensors, and heaters are not shown in the
figure for clarity.

###  3.4 Internal Passive Shielding

A lead castle inside the cryostat provides shielding against high energy gamma
radiation originating from electronics or any other part inside or outside the
cryostat. The castle is built in such a way that there is at least 6 cm of
internal lead shielding the CCD box from the bottom and sides. The innermost 2
cm is made from ancient Roman lead provided by LSM [47, 38] and the rest is
from TFA lead casted by Fonderie de Gentilly in France.

Three lead disks, each 2 cm thick, are attached with brass studs and a copper
ring to the inside of the cryostat top cylinder above the CCD box. The bottom
one directly above the CCD box is from ancient Roman lead. Six, 2 cm-thick
square plates are below the CCD box with the top one from ancient lead. Two of
these plates can be removed without affecting the background level if an
upgrade to a second CCD box will be needed. Vertical pieces on all four sides
are designed to accommodate the thermal guides for CCD cables while providing
sufficient shielding.

The lead castle is kept at room temperature because it is thermally isolated
from cold copper parts. This was achieved either by using materials with low
thermal conductivity or keeping a few millimeters of clearance between lead
and cold copper parts. The internal lead shield is shown in Fig. 4.

Figure 4: The CCD box inside the lead castle. The CCD cable runs in a PCTFE
cable guide to keep it thermally isolated from lead and is plugged to the
intermediate board. In this photo, the intermediate boards have right angle
connectors, but new boards with straight connectors were installed later.

###  3.5 External Passive Shielding and Support Structure

External gamma radiation and neutrons are attenuated by a layer of low
background lead (faible activitè or FA) from LEMER and HDPE by Abaqueplast,
both in France. The shielding around and above the cryostat cylinder, where
the shielding of CCDs is the most important, consists of 15 and 20 cm of lead
and HDPE, respectively, see Fig. 1 and 5. The bottom cross of the cryostat is
surrounded by 5 cm of lead from all sides but below the cryostat and 10 cm of
HDPE. The 41 cm diameter ring of lead shielding above the cryostat together
with HDPE half-disks is supported by 2 cm thick stainless steel plate.

The support structure standing on the floor of the cleanroom is static, but
motorized opening system can move half of the shield on rails to allow easy
access to the cryostat. Neither the motor nor rails use lubrication and thus
do not compromise the cleanroom cleanliness. A photo of fully assembled and
partly opened detector is in Fig. 5.

A movable crane is kept inside the cleanroom. It was employed during the
assembly of the support structure, external shielding and cryostat. It is
utilized to lift the cylindrical cover any time access inside the cryostat is
required.

Figure 5: The LBC is shown in its open configuration with partly disassembled
external shielding. The cryostat in the middle is surrounded by lead and HDPE.
The static part of the support structure is on the right and the left side
moves on rails. One feedthrough flange and parts of the vacuum system are also
visible.

###  3.6 LSM Infrastructure

A new cleanroom was built for DAMIC-M on the ground floor of LSM between the
neutrinoless double-beta decay experiment SuperNEMO [36] and a room with high
purity Germanium (HPGe) detectors [55]. In the future, the LBC will be removed
so that DAMIC-M can be assembled in this cleanroom. The total floor area is
6.2$\times$4.3 m2. The main room is class ISO 5 for air cleanliness and the
gowning room is ISO 6. The room has a roof opening to load big and heavy
pieces with the LSM overhead crane. This option was used before the assembly
of the LBC and the room was cleaned afterwards.

The furniture and equipment inside the cleanroom are electrostatic discharge
(ESD) safe to comply with requirements on handling ESD-sensitive equipment,
like CCDs and electronics. While working with CCDs, we require the humidity
above 35% and we deploy a local humidifier using deionized water if needed.

The radon concentration has been continuously measured every 10 minutes with
Durridge RAD7 in the cleanroom since January 2023. The median radon
concentration is (18.4$\pm$8.5) Bq/m3 with 99% of the data below 42 Bq/m3.
Other backgrounds obtained by different groups are: gamma flux of
3.8$\times$10-6 cm-2s-1 between 4 and 6 MeV [56], thermal neutron flux of
$4.6\times 10^{-6}$ n/cm2/s at the LBC location [57], and fast neutron flux
above 1 MeV of (1.06$\pm$0.60)$\times$10-6 n/cm2/s [58].

##  4 Detector Readout and Instrumentation

The LBC is remotely controlled and monitored by custom data acquisition
systems placed outside the external shield. We describe two different
electronics systems installed in the LBC to run CCDs, of which the first one
collected the data published in two DAMIC-M publications [24, 31]. The slow
control, DAQ software and data quality monitor system are also detailed.

###  4.1 Readout Electronics

A custom data acquisition system based on a commercial Gen-III CCD controller
from Astronomical Research Cameras, Inc. (ARC) was used for initial tests with
the LBC. The controller provides the voltage biases and clocks required for
operation of the CCD and reads out the video signal from two CCD amplifiers.
External low-noise power supplies provide DC voltages for controllers,
amplifiers and Vsub. Two CCD controllers were used to read two 6k$\times$4k
CCDs or two CCDs in two CCD modules.

The signals for the CCDs are generated by a clock (ARC-32) and bias (ARC-33)
board and the CCD signal is read by a video board (ARC-45). Each board has a
separate cable, which are merged by a custom-made breakout board to a 50-pin
connector. The ARC controller communicates to the computer via an ARC-22 board
in the controller crate and an ARC-66x in the PC. A custom-made, 3 m-long
cable harness of 50 $\Omega$ micro-coaxial cables (model 40322-001 by Hitachi)
brings signals to the vacuum feedthrough in the cryostat.

All components of the electronics chain are electrically grounded and kept
isolated from other devices to minimize the noise level on the readout signal
and cross talk.

The new DAMIC-M readout electronics were successfully installed in the LBC in
January 2024 and are used to control CCDs since then. These electronics were
developed for DAMIC-M, have lower readout noise than the ARC CCD controller
and can read all four CCDs in a module. The electronics chain consists of a
custom-made Acquisition and Control Module (ACM) plugged to a VME crate
(Wiener VME 6023x610), a 3 m long cable by Samtec and a custom-made front-end
board. In addition, an adapter board connects the Samtec and D-Sub-50
connector in the vacuum feedthrough.

###  4.2 Instrumentation

As described in Section 3.3, the CCDs are housed inside a light-tight copper
cryostat. The nominal operating temperature is approximately 130 K, providing
a low dark current and high charge carrier mobility. This is achieved by using
an air-cooled Sunpower CryoTel GT cryocooler. It is equipped with an active
balancer to minimize mechanical vibrations and prevent microphonics noise.
Airflow for the cooling of the cryocooler is provided by a 3D-printed tube and
a PC fan. The cryocooler assembly is mounted to a dedicated flange on the
cryostat. The cryocooler’s cold tip is connected to the cold copper (see Sec.
3.3), maintaining electrical isolation via Kapton tape.

The temperature of the cold copper is monitored by resistance temperature
detectors (RTDs) PT-103 in two different positions. The cooling rate and
temperature are set by a Lakeshore 336 controller that monitors the RTDs and
controls the power to a 100 W heater installed next to the RTD closest to the
cryocooler. To minimize temperature-induced stress on CCDs, we use the cooling
rate of 0.1 K/min during temperature cycles. The system maintains the
temperature within $\pm$0.2 K for many months, see Sec. 6.2.

The cryostat is kept at a pressure of $\sim$5$\times$10-6 mbar with a Pfeiffer
HiCube 80 turbo pump. Two Pfeiffer PTR 91 pressure gauges, which are
controlled by a Pfeiffer TPG362 controller, are used to monitor the system
pressure in different locations. One gauge is connected to the vacuum tube
next to the pump and the other next to the vacuum connection to the cryostat.
Both pressure gauges are turned off during data taking because we have
observed an increase of the dark current in the CCDs when they were turned on.

The components of the electronics chain (see Sec. 4.1) are powered by
independent, low-noise power supplies. Specifically, 3-channel Keysight
E3612A’s are used to supply power to the CCD controller bias boards and the
second-stage amplifiers. A high voltage SRS DC205 provides Vsub to the CCDs.

The detector and its components are protected by two-levels of backup power:
(1) the DAMIC-M APC 2200 UPS and (2) the LSM system-wide UPS. In the case of a
power outage, both UPS systems would be able to operate from battery power for
at least one hour. This is adequate time for remote shifters to respond to
alarms (see Sec 4.3) and safely shut down the experiment. In the unlikely
scenario of an outage longer than one hour with no intervention from shift
scientists, instruments would undergo a hard shut down.

###  4.3 Slow Control System

All of the instrumentation discussed in Section 4.2 required to run the
cryostat and electronics are controlled and monitored by a custom slow control
system. The system was adapted from astro-slow-control [59] developed for the
LUX experiment, and centers around a MySQL database populated by independent
processes for telemetry. Each instrument has a process that stores
input/output data to/from the device. Additional processes are used as
watchdog daemons, alarm systems, and other safety features. The backend C
programs encode the different communication protocols required to transmit and
receive messages from the various instruments. A PHP web frontend interface is
used for real-time monitoring, low-level analysis, and setting new control
parameters to the backend programs.

Figure 6: Schematic of the slow control system. This digital system is
controlled by a Slow Control PC through a series of TCP networks (blue) to the
auxiliary devices (purple). The backend C programs control I/O from a MySQL
database. The system was made to be user-friendly through a PHP web interface.
This is for the original setup with ARC CCD Controller (LEACH). LS 336 stands
for Lakeshore temperature controller.

.

Communication between the slow control system and the instrumentation is based
on a private internal network, as illustrated in Figure 6. A host PC running
CentOS8 connects to both the external world for web access and the private
TCP/IP slow control network. The network then plugs into a large Ethernet
switch that interfaces with all of the instruments. Some of the devices can
directly communicate through this line via TCP/IP protocols if they have
physical Ethernet ports, while others must be converted via serial device
servers to translate other protocols, e.g. RS-232. The DAQ PC (Sec. 4.4) has
the ability to interface with the slow control system via a MySQL client.

Conditions that can pose a hazard for the operation of the CCDs are triggered
on by alarm daemons. For example, a failure of the vacuum pump would cause
moisture to condense on the cold CCDs and introduce shorts in the electrical
components. For this reason, the pump is powered by the local UPS which is
monitored by the system. These alarms signal to on-shift users that the CCDs
must be promptly shut down. In addition, a safety watchdog daemon is used to
execute a shutdown if any pump parameters are beyond user-defined thresholds
for more than two consecutive measurements (occurring every 5 s).
Additionally, sound and light alarms would be triggered and notifications sent
to system managers.

###  4.4 DAQ software

Each ARC controller communicates via an optical fiber to the DAQ computer
through the custom-built CCDDrone [60] software, which sends configuration
files, executes the readout sequence and receives the output images. Both are
controlled centrally by the ccd-cdaq [61] Python library, designed to control
CCD experiments. It loads another Python library, libabcd [62], which
implements the MQTT protocol to communicate between processes, and logs, for
debugging purposes, all messages that pass through the MQTT broker. A set of
four applications (run control, DAQ status monitor, CCD manager, and spy) that
run continuously in the background send and receive messages in an orderly
fashion. Two CCD client applications, one for each CCD controller, communicate
with each CCDDrone program using a custom Python library.

The operator sends commands directly to the central DAQ, which automatically
adds the commands to a queue. Each command is executed sequentially, with the
next command waiting for the previous one to return. The operator can pause or
clear the queue at any time, as well as send a series of commands via a
script. They can also decide whether each command applies to a specific CCD
client or to all, and the central DAQ sends the messages to the appropriate
client. This makes it possible to configure each of the CCDs in a different
way and then start the readout of both of them at the same time.

As shown in Fig. 6, the DAQ machine runs the CCD control software. It features
two network cards for internet connection and TCP communication with the power
supply that provides Vsub. The latter allows to perform all operations related
with Vsub in an automated manner.

New firmware and software have been written for the ACM and the ccd-cdaq is
used to operate the ACM and control the CCDs. This new system runs in the LBC
since the installation of the ACM, see Sec. 4.1.

###  4.5 Data quality monitor

An automated data quality monitor (DQM) system has been developed for the LBC
to ensure detector functionality and operational efficiency. The DQM performs
periodic data quality checks and spots acquisition problems and/or
malfunction, whether in the detector hardware or reconstruction software, and
issues corresponding alarms. This proactive approach enables rapid
intervention to minimize downtime and maintain uninterrupted data collection.

The DQM framework provides tools to generate, store and visualize histograms
and scalar elements (monitoring elements), using MongoDB as the database
backend. The first step is low-level data processing of raw images to obtain
the charge value for pixels, calculate median baselines, resolution, dark
current per row, etc. The time evolution of these parameters is monitored, for
example, the stability of dark current which can be used to identify new hot
columns. Standardized algorithms perform statistical tests on the monitoring
elements and automate the data certification processes. Additionally, the DQM
framework enables visualization of monitoring results, retrieval of DQM
quantities from the conditions database, and dataset selections for physics
analyses.

##  5 Radioactive backgrounds

This section describes our effort to control and reduce the radioactive
background in the LBC. Radio-assay analyses were performed for materials used
in the detector – particularly those installed inside the cryostat. Results of
our own or public radio-assay measurements have been utilized in dedicated
Monte Carlo simulations of the whole detector and served us to guide the
development and construction phases of the LBC. We present event rates
measured with two setups in the LBC and compare them with simulations. The
original configuration of the LBC is called Setup 1 and it includes two
6k$\times$4k CCDs in the CCD box with OFHC copper lids. Upgraded detector is
called Setup 2 and consists of two CCD modules, each with four 6k$\times$1.5k
CCDs, and EFC lids.

###  5.1 Material Screening and Activation

All materials used in the detector were selected either based on our radio-
assay measurements or results of other experiments (DAMIC at SNOLAB [37],
MAJORANA [39], SuperCDMS [63], EDELWEISS [38], etc.). Our material samples
have been assayed by various techniques, e.g. HPGe detectors, alpha particle
counting with a XIA instrument and inductively coupled plasma mass
spectrometry (ICP-MS). Results are summarized in Table 1. A few measurements,
e.g., 210Pb in ancient Roman lead, have not been finalized yet.

|  238U |  232Th |  226Ra |  210Pb |  40K
---|---|---|---|---|---
CCD flex | 32.1$\pm$0.5 | 1.06$\pm$0.05 |  |  | 43.9$\pm$12.9
OFHC copper | 0.017$\pm$0.002 | <0.014 |  | 28.4$\pm$8.5 | <0.2
EFC | 0.0013$\pm$0.0002 | 0.0014$\pm$0.0002 |  |  | 0.013$\pm$0.005
Brass |  | <7.9 | <4.8 |  | <14.6
Ancient Pb | <4.18 | <0.058 | <0.026 |  | <0.265
TFA Pb | <60.36 | <2.72 | 0.95$\pm$1.21 | <43780 | 5.72$\pm$11.69
FA Pb | <135 | <9.70 | <5.66 | 54300$\pm$2000 | <20.0
PCTFE | <0.062 | <0.041 |  |  | <15.5
Table 1: Assay results for different isotopes in materials used in the LBC.
The activity is in units of [mBq/kg]. The results for the CCD flex correspond
to DAMIC-M CCD [Com.] in Tab. 4 in Ref. [53].

We have taken steps to control the cosmogenic activation of copper and silicon
parts. Storage and processing times on the surface were tracked, while also
avoiding storage on the surface and transport by air when possible. However,
no expedited fabrication was implemented, and no special shielded storage or
transport boxes were used for the CCDs nor OFHC copper in Setup 1 originally
installed in the LBC. The surface equivalent exposure of the 6k$\times$4k CCDs
was similar to the CCDs installed in DAMIC at SNOLAB [37].

The 6k$\times$1.5k CCDs used in the CCD modules in Setup 2 were fabricated
from the same batch of silicon wafers which is being used for CCDs in the
final DAMIC-M project. Expedited fabrication, special shielding and transport
precautions were implemented before the delivery of wafers to SNOLAB, where
they were stored before CCD production by Teledyne DALSA in Canada. However,
no special shielding – which has been used for CCDs in the final DAMIC-M
detector – was used during CCD production and the devices were allowed to be
transported by air. Therefore, the 6k$\times$1.5k CCDs installed in the LBC
have five times lower cosmogenic activation than original 6k$\times$4k CCDs.
Based on work in Ref. [64], we expect ${}^{3}\rm{H}$ decays to contribute
$\sim$2.6 and $\sim$0.5 dru between 1 and 5 keV in the 6k$\times$4k and
6k$\times$1.5k CCDs, respectively.

Commercial OFHC copper pieces were restricted to only ground transport and
were stored underground after machining. The total exposure time of OFHC
copper was approximately five months, including manufacturing and machining,
before the copper storage at LSM starting in May 2021. The cosmogenic
activation was greatly reduced for EFC lids grown underground at LSC in Spain
and installed in the LBC in September 2022. The machining of these pieces,
which had to be performed on the surface, was limited to only few days. The
total exposure time, including the transport from LSC to LSM with a half-day
stop for chemical cleaning, was 3.1 and 6.5 days for the top and bottom lid,
respectively.

###  5.2 Material Cleaning Procedures

Cleaning procedures help remove radioactive isotopes that accumulate on
material surfaces during production, storage and handling. All copper and lead
inside the cryostat, and the cryostat itself, went through cleaning followed
by passivation aiming to remove contaminants on surfaces and prevent oxide
formation. We used a procedure developed at Pacific Northwest National
Laboratory (PNNL) for copper [65, 66] and lead [39], which had been
successfully used in other experiments. The highest purity chemicals (Optima
grade by Fisher Chemical) and deionized water (18 M$\Omega$-cm) were used. The
cleaning took place in a cleanroom at Laboratoire de Physique Subatomique &
Cosmologie in Grenoble, France.

All instruments, tools and other parts were cleaned, dried and packed in at
least two plastic bags or layers of a plastic foil before their transport to
LSM. This includes copper and lead parts after their chemical cleaning.

###  5.3 Simulation

A Geant4 [41, 42, 43] and Python-based framework was developed by the DAMIC-M
collaboration to reproduce the response of the detector to various particle
interactions. The first code includes a geometry model of the whole LBC,
including a pixelated sensitive detector composed of silicon and polysilicon
layers to model the CCD, and inactive silicon parts [37, 28]. Other parts in
the detector model are the CCD box with CCD flex cables and screws, internal
lead shield, thermal guides, cold copper, cryostat, outer lead, HDPE shield
and support structure.

The Livermore physics list [43] is used for interactions of various simulated
particles ($\gamma$, $\alpha$, $e^{\pm}$, neutrons, etc.). The particles
originate either internally in the LBC (radioactive decays) or from the
outside world, i.e. the laboratory555The information about the gamma and
neutron flux together with the radon level measured at LSM are given in Sec.
3.6..

The Python code is then used to simulate the detector response, i.e. pre-
processing of the Geant4 energy deposits, electron-hole pair creation,
diffusion, pixelation, dark current, readout noise, pixel saturation, and
cluster finder (or clustering). This code is used to reconstruct not only
simulated but also measured data.

The simulations have been invaluable in the design validation and
optimization, and they have provided guidance in the selection of material
type and shape. Results of the simulations for the two installed setups are
summarized in Table 2. The two setups differ in the CCDs and copper lids of
the CCD box, while other detector parts are the same. The rates are provided
as the average for the top and bottom CCDs between 2 and 6 keV – to avoid
silicon and copper fluorescence X-ray lines – in units of dru. We verified
that the contribution from ${}^{210}\rm{Pb}$ deposited on the outside of the
CCD box and the cryostat is negligible. The simulations do not include
external gammas and neutrons – originating either from rocks at LSM or induced
by muons in the external lead.

As can be seen in Table 2, major contributors to the background rate at low
energies are CCDs and the CCD box. Activities used for CCDs are based on the
background model of DAMIC at SNOLAB [37] with two modifications. First, we
rescaled the activity of cosmogenically activated isotopes in the silicon bulk
(3H, 22Na) with our knowledge of the activation history, see Sec. 5.1. Second,
we estimate the activity of 210Pb on surfaces by comparing measured alpha
rates in the LBC and DAMIC at SNOLAB. This contribution is 1.3 and 1.0 dru in
Setup 1 and 2, respectively. In Setup 2, 210Pb deposited on the surface of a
pitch adapter gives an additional 0.7 dru.

From Setup 1 to Setup 2, we have reduced the total background by 5.5 dru
according to simulations. The most significant reduction is from upgrading the
lids in the CCD box to EFC. Another major decrease from exchanging
6k$\times$4k CCDs with two CCD modules. The predicted decrease can be compared
with measurements shown in the last row in Table 2 and discussed in Sec. 5.4.

| Setup 1 | Setup 2 | Difference
---|---|---|---
CCDs | 4.65 | 2.74 | 1.91
CCD box | 5.48 | 1.93 | 3.55
Copper | 0.06 | 0.06 | 0.0
Internal lead | <0.29 | <0.29 | 0.0
External parts |  $\sim$0.1 |  $\sim$0.1 | 0.0
Total | 10.58 | 5.12 | 5.46
Measured | 12.5$\pm$2.8 | 6.7$\pm$1.1 | 5.8
Table 2: The background rates in dru for two setups used in the LBC. Rates are
the average calculated for or measured by the top and bottom CCDs. In
simulations, cold copper includes all copper parts but the CCD box. External
gammas and neutrons were not included in simulations. The background
measurements are described in Sec. 5.4.

###  5.4 Background measurements

Several runs dedicated to the study of the radioactive background were
performed in 2022 and 2023. These runs had various configurations, typically
no pixel binning666Pixel binning is an operating mode of the CCD where the
charge of several pixels is summed before being read out. The charge can be
summed only in the vertical or horizontal direction, or in both directions.
was used for the acquired images and the whole CCD was read out after long
exposures (up to half a day).

We first verified that the external shield significantly reduces the
radioactive background level. The data shows 50 times lower background rate
after closing the external shield. It is important to note, that in any
configuration of the external shielding, the external radiation is still
attenuated by the internal lead shield inside the cryostat.

The next step was to compare the background rate measured in the original
configuration – Setup 1 – and after the installation of Setup 2. The data
presented in this paper have a total exposure of 0.32 and 1.2 kg-days for
Setup 1 and 2, respectively. The event rate above 1 keV decreased from
(1.12$\pm$0.06) ev/g/d in Setup 1 to (0.91$\pm$0.03) ev/g/d in Setup 2. The
background rate between 1 and 6 keV, after excluding the energy range of
silicon K-shell lines, is (12.5$\pm$2.8) and (6.7$\pm$1.1) dru in Setup 1 and
2, respectively. The measured decrease of 5.8 dru agrees with the expectation
calculated with our Geant4 simulations, see Table 2. We can see that measured
and simulated rates agree within one and half standard deviations.

The LBC results are similar to the ones obtained in DAMIC at SNOLAB, where one
CCD was in a copper box made from EFC grown at PNNL enclosed between two
ancient lead bricks, and the remaining CCDs were in a box from OFHC copper
[37]. Further, we have identified contributors to the total radioactive
background in the LBC, which provides important information for the background
mitigation in DAMIC-M.

An analysis of $\alpha$ rates and coincidences, e.g. $\beta-\beta$ for 32Si
and 210Pb, on the surface and in the bulk of the 6k$\times$1.5k CCDs is under
way. Our studies follow previous work done for DAMIC at SNOLAB [67]. The
measurement of contaminants inside the silicon bulk is of primary importance
for the final DAMIC-M detector, because the CCDs in DAMIC-M will be made from
the same silicon ingot as 6k$\times$1.5k CCDs installed in the LBC.

##  6 Detector Stability

Long-term detector stability is a critical requirement on dark matter
searches. In this section, we illustrate the stability of the LBC with the
data collected by the Slow Control system (see Sec. 4.3) and CCDs.

###  6.1 Pressure and Temperature

Our CCDs operate typically in high vacuum and low temperatures, which are
achieved by implementing the following procedure. After closing the cryostat,
the vacuum pump is powered on and monitored to check for any air leaks. The
cryocooler is turned on when pressure inside the cryostat gets below
5$\times$10-4 mbar and cold copper is cooled down from room temperature to 170
K. At this temperature, we start up the CCDs by supplying bias, clock, and
substrate voltages. Necessary part of the CCD startup is the erase procedure
described in Sec. 6.2. The CCDs are then cooled down further to their
operating temperature chosen between 110 and 130 K. To minimize temperature-
induced stress on CCDs and hence, reduce leakage current, we use a cooling
rate of 0.1 K/min.

Pressure is continuously monitored only when the CCDs are turned off to avoid
light emitted by two pressure gauges. The pressure at 130 K stays at
$\sim$5$\times 10^{-6}$ mbar in the cryostat and ten times lower in a vacuum
tube close to the pump.

Unlike pressure gauges, temperature sensors are monitored continuously. The
RTD in the cold mass table, see Fig. 3, is kept at a constant temperature by
using a 100 W heater located 2.5 cm next to it. The reading of an RTD epoxied
in a cold finger, where it is well shielded by lead plates from CCDs, is about
2 K higher. An RTD was kept temporarily on the CCD box during the detector
commissioning and measured 4 K higher than the cold finger. This RTD was then
removed to suppress the radioactive background. The highest temperature
gradient – about 30 K – is between the cold head and the cold mass table and
it is due to a polyimide tape, which provides an electrical isolation but
reduces thermal contact. The temperature of the CCDs has been measured through
independent thermal tests and is typically $\sim$5 K higher than the
temperature measured by an RTD epoxied on the outside of the CCD box.

Over 63 consecutive days of data taking (described in Refs. [24, 31]), the
cold mass table temperature increased from 116.9 K to 117.2 K, see Fig. 7.
This increase may be explained either by heating the cryostat by a flow of air
warmed by the cryocooler body when the external shielding is closed, or
relaxing the connection between cold copper parts. As is shown in Refs. [31],
the dark current did not increase during the same period.

###  6.2 CCD Performance

The LBC serves as a test setup for the DAMIC-M project and allows us to
perform various studies of operating parameters for different CCD formats and
electronics. An extensive optimization program for clock and bias voltages,
and timings were carried out. These tests were performed with originally
installed 6k$\times$4k CCDs and then the CCD modules with 6k$\times$1.5k
devices. The latter is of particular importance, because very similar CCD
modules will be installed in the full-scale DAMIC-M detector.

Details on the image processing and selection criteria are described in Ref.
[28, 24]. The readout noise $\sigma_{1}$ can be obtained from single-skip
images, but multi-skip images ($N_{\rm{skip}}\gg 1$) are necessary to measure
single electron peaks. By fitting the pedestal (0 e-) and 1 e- peaks in the
pixel charge distribution with a multi-Gaussian function777The multi-Gaussian
function is composed of equally spaced Gaussian functions. convolved with a
Poisson distribution, we obtain the readout noise, gain (or calibration
constant), and dark current. These parameters have been extensively studied
with all setups installed in the LBC.

The first studied noise levels were $\sigma_{1}\sim 10$ e- which are higher
than in DAMIC at SNOLAB and test setups located in different institutions.
After implementing noise mitigation steps, the noise level decreased to an
acceptable level of $\sigma_{1}\sim 7$ e- in images taken with ARC CCD
controllers in Setup 1. The averaging in the skipper amplifier reduces any
remaining high frequency noise contributions.

When reading CCD modules with two unsynchronized ARC CCD controllers in Setup
2, a significant correlated noise was observed. Because this will not be the
case with the new electronics developed for DAMIC-M, we have not invested
resources to synchronize the controllers. Reading CCD modules separately and
applying a noise decorrelation algorithm, provided the resolution at the same
level as when we read a single CCD.

The major noise reduction was achieved with the new electronics developed for
DAMIC-M. Noise measured by two CCD modules run synchronously is
$\sigma_{1}\leq 3$ e- rms. When we read the CCDs with $N_{\rm{skip}}=500$, the
skipper averaging reduces noise to $\sim 0.1$ e-.

Various clock and bias voltages were investigated to study the clock induced
charge, amplifier glowing, and charge transfer inefficiency, all three of
which have an important effect on the e- rate. During these studies, we have
employed also a 60Co radioactive source. In low dark current runs, we have
been using rather low substrate bias voltage (V${}_{\mathrm{sub}}=45$ V) and
swing of vertical and horizontal clocks (between 3 and 3.5 V). These
parameters have been modified in other studies, e.g. in background runs.

To achieve a low dark current, the CCD is started up at high temperature (170
K) and an erase procedure is performed as suggested by Ref. [50]. Our erase
procedure consists of the following steps: reduce Vsub to 0 V in a linear way
($\sim$15 V/s) and put clocks in inversion phase (i.e. +9 V). After a few
seconds, we increase Vsub back to its normal operating value and restore the
clocks to their normal values for charge transfer at Vsub=10 V. Temperature is
then reduced to its target value.

The stability of the resolution and gain measured by an amplifier in one CCD
during a 63-day long continuous run in the summer 2022 is shown in Fig. 7. The
corresponding dark current can be found in Fig. 1 in Ref. [31]. Even though
the temperature of cold copper increased by 0.3 K over this period, we did not
observe any change of either the amplifier gain or the resolution. The gain
and resolution of the amplifier U in CCD 6414 had standard deviation of 4.4%
and 2.1%, respectively. Due to the excellent stability of the LBC, we could
constrain DM-electron scattering by exploiting the expected daily modulation
due to DM interactions within the Earth [31].

We performed additional tests to characterize installed skipper CCDs. These
include study the temperature dependence of dark current and the increase in
dark current for different levels of ionizing radiation from the external 60Co
source.

Figure 7: The relative deviation of the gain (blue) and resolution (yellow) of
the U amplifier in CCD 6414 during 63-day run. Temperature of cold copper
(dark violet) in Kelvins is shown on the right y-axis. See text for more
details.

##  7 Conclusions

The DAMIC-M prototype detector, the LBC, was built at LSM in late 2021 and
continues to serve as a low background setup for skipper CCDs. The LBC has
operated without any major issues over its lifetime and its stable operation
has lead to the best published limits on the interaction of DM with electrons
[24, 31]. Of critical importance, we have studied the CCD performance in a low
background environment and after numerous thermal cycles. Thanks to successful
background mitigation steps, the background level was reduced to (6.7$\pm$1.1)
dru between 1 and 6 keV. This result, together with detailed simulations,
provides a clear pathway for further background reduction in the final DAMIC-M
detector, which will be built at LSM in 2025.

##  8 Acknowledgements

The DAMIC-M project has received funding from the European Research Council
(ERC) under the European Union’s Horizon 2020 research and innovation
programme Grant Agreement No. 788137, and from NSF through Grant No. NSF
PHY-1812654. The work at University of Chicago and University of Washington
was supported through Grant No. NSF PHY-2110585. This work was supported by
the Kavli Institute for Cosmological Physics at the University of Chicago
through an endowment from the Kavli Foundation. We thank the College of Arts
and Sciences at UW for contributing the first CCDs to the DAMIC-M project.
Part of this work was conducted at the Washington Nanofabrication Facility /
Molecular Analysis Facility, a National Nanotechnology Coordinated
Infrastructure (NNCI) site at the University of Washington with partial
support from the National Science Foundation via awards NNCI-1542101 and
NNCI-2025489. We also thank the Krieger School of Arts & Science at Johns
Hopkins University for its contributions to the DAMIC-M experiment. IFCA was
supported by project PID2019-109829GB-I00 funded by MCIN/ AEI
/10.13039/501100011033. The Centro Atómico Bariloche group is supported by
ANPCyT grant PICT-2018-03069. The University of Zürich was supported by the
Swiss National Science Foundation. The CCD development work at Lawrence
Berkeley National Laboratory MicroSystems Lab was supported in part by the
Director, Office of Science, of the U.S. Department of Energy under Contract
No. DE-AC02-05CH11231.

We are grateful to LSM and its staff for support through underground space,
logistical and technical services.

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