Submitted to the Proceedings of the US Community Study

on the Future of Particle Physics (Snowmass 2021)

# The Selena Neutrino Experiment

A.E. Chavarria  Center for Experimental Nuclear Physics and Astrophysics,
University of Washington, Seattle, WA 98195, United States  C. Galbiati
Physics Department, Princeton University, Princeton, NJ 08544, United States
INFN Laboratori Nazionali del Gran Sasso, 67010 Assergi (AQ), Italy  Gran
Sasso Science Institute, 67100 L’Aquila, Italy  B. Hernandez-Molinero
Laboratorio Subterráneo de Canfranc, 22880 Canfranc-Estación, Spain  Al. Ianni
INFN Laboratori Nazionali del Gran Sasso, 67010 Assergi (AQ), Italy  X. Li
Physics Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720,
United States  Y. Mei  Nuclear Science Division, Lawrence Berkeley National
Laboratory, Berkeley, CA 94720, United States  D. Montanino  Dipartimento di
Matematica e Fisica “Ennio De Giorgi”, Università del Salento, 73100 Lecce,
Italy  INFN Sezione di Lecce, 73100 Lecce, Italy  X. Ni  Center for
Experimental Nuclear Physics and Astrophysics, University of Washington,
Seattle, WA 98195, United States  C. Peña Garay  Laboratorio Subterráneo de
Canfranc, 22880 Canfranc-Estación, Spain  Institute for Integrative Systems
Biology (I2SysBio), Valencia, Spain  A. Piers  Center for Experimental Nuclear
Physics and Astrophysics, University of Washington, Seattle, WA 98195, United
States  H. Wang  Physics and Astronomy Department, University of California,
Los Angeles, CA 90095, United States

###### Abstract

Imaging devices made from an ionization target layer of amorphous selenium
(aSe) coupled to a silicon complementary metal-oxide-semiconductor (CMOS)
active pixel array for charge readout are a promising technology for neutrino
physics. The high spatial resolution in a solid-state target provides
unparalleled rejection of backgrounds from natural radioactivity in the search
for neutrinoless $\beta\beta$ decay and for electron neutrino ($\nu_{e}$)
spectroscopy with 82Se. In this white paper, we summarize the broad scientific
program of a large imaging detector with a 10-ton target of 82Se. We review
the detector technology, and outline the ongoing research program to realize
this experiment.

## 1\. Executive Summary

The Selena experiment was proposed in Ref. Chavarria  _et al._ (2017) to
search for the neutrinoless $\beta\beta$ decay of 82Se with unprecedented
sensitivity. Recently, we expanded its scientific case to include solar
neutrino spectroscopy (Sec. B) and sterile neutrino searches (Sec. C) based on
$\nu_{e}$ capture on 82Se. Fig. 1a summarizes the nuclear processes that will
be studied by Selena. Thus, Selena will be a neutrino observatory with a
scientific program that spans fundamental neutrino physics and solar
astrophysics.

The Selena detector will consist of towers of imaging modules made from
$\sim$5 mm-thick amorphous selenium (aSe)—isotopically enriched in
82Se—deposited on a CMOS active pixel charge sensor (APS). Free charge
liberated by ionizing particles in the aSe is drifted by an applied electric
field and collected by the electrodes of CMOS pixels to form high resolution
images. From these images, Selena can i) measure the deposited energy, ii)
identify the type and number of ionizing particles (electrons, $\alpha$’s,
etc.) from the track topologies, and iii) identify radioactive decay sequences
by spatio-temporal correlations. This strategy has been realized to some
extent—in the context of dark matter searches—by the DAMIC experiment to
constrain the activities in the silicon target of 32Si, and every isotope in
the 238U and 232Th decay chains Aguilar-Arevalo  _et al._ (2021). For example,
Fig. 1b shows the identification of a 32Si decay ($Q$-value $=225$ keV,
$\tau_{1/2}=150$ y) from its spatial correlation with the $\beta$ track of its
daughter 32P ($Q$-value $=1.71$ MeV, $\tau_{1/2}=14$ d) many days later. Note
that the start and end points of the $\beta$ track from 32P decay can be
distinguished by the presence of a high density of charge, _i.e._ , the Bragg
peak, at the end of the track. In the context of Selena, this experimental
strategy will allow for background-free spectroscopy of $\beta\beta$ decay and
solar neutrinos in exposures $>$100 ton-year.

Selena is particularly efficient in its use of the isotopically-enriched
active target since background suppression relies on event-by-event
identification of the signal and not self shielding. Furthermore, our
background estimates assume background event rates comparable to those already
achieved by kg-scale solid-state detectors for rare event searches, _e.g._ ,
the MAJORANA DEMONSTRATOR Alvis  _et al._ (2019). Thus, the requirements in
the radiopurity of the construction materials and processes, although
challenging, are already possible.

Finally, we note that Selena will operate at room temperature, the CMOS APS
for its imaging modules will be fabricated with standard commercial CMOS
foundry processes, and the aSe deposited following industrial-scale processes
developed for the fabrication of medical devices, making the project cost-
effective at scale.

We are in the final R&D stage to validate the aSe/CMOS imaging technology for
Selena. In the next years, we plan to fabricate and test the first detector
modules that will serve as the building block for a larger experiment. The
first detector will be a 100-kg Selena “demonstrator,” which will directly
demonstrate the background suppression capabilities of the experimental
technology in an underground environment. This compact detector (of overall
volume $\sim$0.1 m3) will place the strongest lower limit on the half-life of
the neutrinoless $\beta\beta$ decay of 82Se and detect $\sim$five _pp_ solar
neutrinos with zero background in a one-year run. The Selena program will then
proceed with an even larger detector.

The white paper is structured as follows. In Sec. 2, we present the scientific
case for a 10 ton Selena detector with a 100 ton-year exposure. In Sec. 3, we
describe the detector technology behind Selena, including recent R&D
milestones and the expected performance of a large detector. Finally, in Sec.
4, we outline the research program for the upcoming years, with a clear path
that addresses the biggest challenges for a large detector.

Figure 1: a) Diagram showing two natural channels for 82Se to transmute into
82Kr. Both $\beta\beta$ decay and $\nu_{e}$ capture will be detected with high
efficiency by Selena for nuclear physics studies. b) The identification of a
single 32Si atom by DAMIC at SNOLAB: a sequence of two $\beta$ decays
separated in time by many days and with the start points of the electron
tracks detected at the same location. Pixels toward the red end of the
spectrum collect more charge.

## 2\. Scientific Motivation

In the next sections, we present the scientific potential of a 10 ton Selena
detector in three specific areas of neutrino physics and astrophysics.

### 2 A. Neutrinoless $\beta\beta$ decay of 82Se

The search for neutrinoless $\beta\beta$ decay probes the Dirac-Majorana
nature of the neutrino. This process is forbidden in the lepton-number-
conserving Standard Model (SM) and is only allowed if neutrinos are Majorana
in nature. Observation of this process would demonstrate that lepton number is
not conserved, with deep implications for grand unification and the matter-
antimatter asymmetry of the universe.

When $\beta\beta$ decay occurs in a calorimetric detector, the summed energy
of the two emitted $\beta$s is observed. In the SM process involving the
emission of two neutrinos, the summed $\beta$ energy follows a continuous
spectrum extending up to the $Q$-value of the decay ($Q_{\beta\beta}$). If no
neutrinos are emitted, the two $\beta$s carry away the full decay energy, and
a monoenergetic peak would be observed at $Q_{\beta\beta}$. The shape of the
peak is determined by the detector energy resolution. Thus, the strategy for
any neutrinoless $\beta\beta$ search involves maximizing exposure to make the
amplitude of the peak as large as possible, and minimizing the background rate
within a full-width half-maximum of $Q_{\beta\beta}$.

In a minimal SM extension in which neutrinoless $\beta\beta$ decay is mediated
by SM neutrinos. The half-life is inversely proportional to the square of the
effective neutrino mass $m_{\beta\beta}=|\sum_{i}U_{ei}^{2}m_{i}|$, where
$U_{ei}$ are PMNS neutrino-mixing matrix elements and $m_{i}$ is the mass of
the $i^{th}$ neutrino mass eigenstate. Results from neutrino oscillation
experiments predict that for the inverted mass ordering (IO: $m_{3}<m_{1}$)
there is a lower limit on $m_{\beta\beta}$ at
$(m_{\beta\beta}^{min})_{\textrm{IO}}\sim 20$ meV (Fig. 2a). Multiple proposed
efforts aim to achieve this sensitivity by operating ton-scale experiments for
5–10 years in several isotopes (namely ^76Ge, ^100Mo and ^136Xe) Agostini  _et
al._ (2021); Hallman (2021). For the case of the normal mass ordering (NO:
$m_{1}<m_{3}$), most of the parameter space would remain in the range 1 meV
$<m_{\beta\beta}<(m_{\beta\beta}^{min})_{\textrm{IO}}$

Figure 2:  a) Allowed values for $m_{\beta\beta}$ as a function of the mass of
the lightest neutrino for different orderings of the neutrino masses Dell’Oro
_et al._ (2016). The width of the bands represent the uncertainties in the
parameters of the PMNS matrix. The dashed line shows the value of
$m_{\beta\beta}$ that corresponds to $\tau_{1/2}=10^{28}$ y in ^82Se. b)
Predicted $\beta\beta$-decay spectrum of 82Se about $Q_{\beta\beta}$ in a 100
ton-year exposure if $\tau_{1/2}=10^{28}$ y.

Our initial study on the search for neutrinoless $\beta\beta$ decay with a
conceptual Selena detector predicts background rates at $Q_{\beta\beta}=3$ MeV
below $6\times 10^{-5}$ per keV per ton-year Chavarria  _et al._ (2017). This
extremely low background level is possible because of a combination of
factors. First, the high $Q_{\beta\beta}$ of 82Se is at an energy greater than
most backgrounds from primordial 238U and 232Th radiocontaminants, which leads
to a relatively low raw event rate. Second, spatio-temporal correlations
effectively reject any radioactive decays in the bulk or the surfaces of the
imaging modules. Finally, external $\gamma$-ray backgrounds, which mostly
produce single-electron events from Compton scattering or photoelectric
absorption, are suppressed by the requirement that the $\beta\beta$ signal
events have two clearly identified Bragg peaks (this selection retains 50% of
signal events while rejecting 99.9% of the single-electron background).

The superb background suppression does not require much more than a realistic
pixel pitch $<15$ $\mu$m and an imager uptime close to 100%, specifications
that are within reach of current technology (Sec. 3). The spectroscopic
identification of the neutrinoless $\beta\beta$ signal over the two-neutrino
$\beta\beta$ decay background is a bigger challenge. Figure 2b shows the
$\beta\beta$ decay spectrum assuming our experimentally-informed best estimate
for the energy resolution (red marker in Fig. 8b), and $\tau_{1/2}=10^{28}$ y
for the neutrinoless channel. Although there is interference from the two-
neutrino channel, a limit of $\tau_{1/2}>5\times 10^{28}$ y (90% C.L.) would
be possible in a 100 ton-year exposure.

In the fortunate event that one of the upcoming ton-scale experiment discovers
neutrinoless $\beta\beta$ decay, Selena will play a followup role by being the
first experiment to confirm the process in the isotope ^82Se, and the first to
confirm that it indeed occurs by the emissions of two electrons. Other
possible details of the decay mechanism could be uniquely probed by Selena
from the angular correlations between the outgoing electrons Ali  _et al._
(2007); Deppisch  _et al._ (2020).

### 2 B. Solar neutrino spectroscopy

Solar neutrinos have proved to be crucial probes to understand the fundamental
properties of neutrinos Aharmim  _et al._ (2013) and stellar astrophysics
Agostini  _et al._ (2018). Solar neutrinos are produced by fusion reactions in
the sun, which transmute four hydrogen nuclei to one ^4He nucleus. Neutrinos
are emitted both by primary and secondary nuclear processes, with the
neutrinos emitted by each reaction labeled a “species.” The different neutrino
species can be distinguished from their characteristic energies, _i.e._ , by
their spectrum. They provide critical information about the fusion processes
in the sun and the fundamental properties of neutrinos as they travel in the
dense solar matter and transition into empty space Gann  _et al._ (2021).

Solar $\nu_{e}$ captures in Selena can be tagged on an event-per-event basis
with high efficiency to perform solar $\nu$ spectroscopy concurrently with the
search for neutrinoless $\beta\beta$ decay. The threshold for $\nu_{e}$
capture of 172 keV provides sensitivity to all solar $\nu$ species, and leads
to the decay sequence shown in Figure 1a. The first step with $\tau_{1/2}=7.2$
ns is too fast to be distinguished from the electron with energy $E_{\nu}-172$
keV emitted following $\nu_{e}$ capture. Together they constitute the “prompt”
event, whose spectrum is shown in Figure 3 with the energy resolution given by
the red line in Fig. 8b. The prompt event is followed by a sequence of two
decays, which are expected to occur at exactly the same spatial location with
time delays of $\sim$5 min and $\sim$1 day, respectively. Although these time
separations may seem large, the probability of any two accidental events
occurring at exactly the same pixel (_i.e._ , in $\sim$10 $\mu$g of aSe) is
negligible. Furthermore, any charged particle reaction to produce 82mBr
($2^{-}$ state) from 82Se would have remarkably different prompt event
topology. Additionally, no background isotope has been identified that could
mimic the $\nu_{e}$ capture sequence in time separation, event energies and
topologies. So far, it appears that zero-background solar $\nu$ spectroscopy
with a large Selena detector may be possible.

Figure 3: Expected spectrum of the prompt event from solar $\nu_{e}$ captures.
Neutrino species from different solar-fusion reactions are labeled. Capture
cross section from Ref. Hernandez-Molinero  _et al._ (2022). Solar neutrino
fluxes from Ref. Vinyoles  _et al._ (2017). Neutrino oscillation parameters
from Ref. Capozzi  _et al._ (2017).

(a)

(b)

Figure 4: a) Number of $\nu_{e}$ captures as a function of distance from an intense ^51Cr source at the center of a 10 ton Selena detector. Best-fit parameters from Ref. Barinov  _et al._ (2021). b) Expected spectrum of prompt events from ^51Cr $\nu_{e}$ captures over the solar neutrino background. Species |  $E$ range [keV] |  $N$ in 100 ton-year | $1/\sqrt{N}$
---|---|---|---
pp | 29–278 | 5359 | 1.4%
7Be | 665–775 | 1814 | 2.3%
pep | 1230–1360 | 130 | 8.8%
CNO | 278–655, 785–1220 | 136 | 8.6%
8B | (1.5-15)$\times 10^{3}$ | 209 | 6.9%
Table 1: Expected results for the capture rate of solar neutrinos from a
counting experiment. $N$ is the expected number of $\nu_{e}$ captures for the
different species in the specified energy ($E$) range. The last column
provides an estimate of the statistical uncertainty in the measured rate.

Table 1 presents the capture rates for different solar neutrino species
obtained by integrating Fig. 3 in the specified energy regions. Neutrino
charged current cross sections on 82Se have been taken from Ref. Hernandez-
Molinero  _et al._ (2022). With a 100 ton-year exposure, Selena is expected to
measure the $pp$ rate to $\sim$1% for a strong constraint on the neutrino
luminosity111The uncertainty in the $pp$ flux will be dominated by the
uncertainty in the $\nu_{e}$ capture cross section. For example, calibration
to the $\nu$-$e$ elastic scattering cross section from a comparison between
the 7Be neutrino rates measured by Selena and Borexino Agostini  _et al._
(2018) would contribute a 3.5% uncertainty., the $pep$ rate to $\sim$8% to
probe the onset of the Mikheyev-Smirnov-Wolfenstein (MSW) effect for solar
neutrinos Mikheev and Smirnov (1987), and the CNO-cycle rate to $\sim$10% to
demonstrate the presence of the CNO-cycle in the Sun with 5$\sigma$
significance and to resolve the solar metallicity problem Serenelli  _et al._
(2009).

Figure 5: Sensitivity to oscillation parameters from an intense ^51Cr source
at the center of a 10 ton Selena detector. The best-fit from Ref. Barinov  _et
al._ (2021) is shown for comparison. Blue line: only the total rate of the
neutrino capture line within the detector volume is considered. Red line: the
oscillation pattern is exploited to enhance sensitivity to neutrino
parameters.

The shape of the energy spectrum of neutrino lines is modified by thermal
effects. On the low energy side of the line, the dominant effect is first-
order Doppler broadening caused by nuclear motion. On the high energy side of
the line, the dominant effect is exponential broadening caused by center of
energy electron-nucleus. The asymmetric broadening causes a shift in the mean
energy of the solar neutrino lines. The 7Be electron-capture decay line is
expected to be shifted by 1.3 keV, relative to the neutrino line in the
laboratory Bahcall (1994), directly correlated with the solar core
temperature. There is a corresponding shift of the _pep_ line by 7.6 keV, much
larger due to the lighter nuclei involved in the electron capture Hernandez-
Molinero  _et al._ (2022). We expect a Selena experiment with a 100 ton-year
exposure to measure, with a dedicated energy calibration better than 1%, these
line shifts with high statistical significance for a $\sim$30% estimate of the
solar core temperature, in what would be the first ever direct measurement of
the Solar core temperature.

### 2 C. Sterile neutrinos

Recent results from the BEST experiment Barinov  _et al._ (2021) confirm the
longstanding “gallium anomaly” Kostensalo  _et al._ (2019), where
radiochemical detectors based on the target isotope ^71Ga observe a $\nu_{e}$
capture rate that is $80\pm 5$% of the expected from electron-capture sources.
These radioactive sources (^51Cr, ^37Ar) emit $\sim$0.8 MeV neutrinos and were
deployed at short distances (typically $<$1 m) from the detector. This
discrepancy has been interpreted as $\nu_{e}$ oscillations into a sterile
neutrinos ($\nu_{s}$) Laveder (2007). Since the BEST detector saw no
difference between the capture rate in its separate inner and outer volumes,
if the oscillation hypothesis is correct, the oscillation length must be $<$2
m.

If a 3.5 MCi ^51Cr source were to be placed in the center of a 10 ton Selena
detector, in a similar configuration to the BEST experiment, $\sim 2.3\times
10^{4}$ $\nu_{e}$ capture events would be tagged with high efficiency222For
reference, consider the proposed LENS-Sterile experiment Grieb  _et al._
(2007), which features a segmented real-time calorimetric $\nu_{e}$ detector
with a response that is qualitatively similar to Selena’s.. This estimation
takes into account a source transportation time after irradiation of 4 days
and an exposure of 60 days. Calculations method is similar to the one
discussed in Ref. Ianni  _et al._ (1999). Furthermore, Selena would be able to
resolve the spatial location of every $\nu_{e}$ capture to search for
oscillations with lengths as small as $\sim$0.1 m, limited only by the
physical size of the radioactive source. In terms of neutrino oscillation
parameters, this corresponds to probing the short-baseline $\nu_{s}$
oscillation hypothesis in the range 1 eV${}^{2}<\Delta m^{2}<$10 eV2. Fig. 4
shows the simulated result where the gallium anomaly is explained by
$\nu_{e}\rightarrow\nu_{s}$ oscillations. Selena would be able to clearly
resolve the oscillation pattern to conclusively test the hypothesis. The same
figure shows the expected spectrum for a 10 ton Selena detector with the
source at the center. In order to probe the sensitivity of this detector to
neutrino oscillation parameters, we compared the predicted $\nu_{e}$ capture
rate with and without the source. The result is shown in Fig. 5, where the
best-fit from BEST from Ref. Barinov  _et al._ (2021) is shown for comparison.
Inclusion of the oscillation pattern enhances significantly the potential of
Selena in this framework.

## 3\. Detector technology

Amorphous selenium is a well-established photoconductor commonly used as an
X-ray converter in flat panel imagers employed in the medical industry for
radiographic and fluoroscopic digital imaging Zhao and Rowlands (1995);
Yorkston and Rowlands (2000). Incident radiation creates electron-hole pairs
in the target material and these carriers are separated by a strong electric
field (typically tens of V/$\mu$m) applied across the detector. The charge
carriers drift towards the electrode where they induce a measurable signal
proportional to the amount of charge generated. Current medical devices use
thin-film transistor (TFT) technology to measure the charge in each pixel,
resulting in $\sim$1000 $e^{-}$ of noise per pixel, making this style detector
insufficient for single electron detection and spectroscopy333For example, a
50 keV X ray generates a signal of 1000 $e^{-}$ at an electric field of 10
V/$\mu$m.. Thus, in Ref. Chavarria  _et al._ (2017), we proposed to read out
the ionization charge from aSe with a lower noise CMOS APS to resolve
individual electron tracks. Details of the proposed detector for the Selena
experiment are discussed in the following sections.

### 3 A. Imaging Capabilities

We recently demonstrated the imaging of single electron tracks with a hybrid
aSe/CMOS device for the first time. The device was based on the _Topmetal-II
-_ CMOS APS An  _et al._ (2016), which features a rectangular array of
72$\times$72 pixels with 83 $\mu$m pitch. We achieved a pixel noise of 23
$e^{-}$ with the sensor coupled to a 500 $\mu$m thick aSe target layer. Fig.
6a shows photographs of the device before and after aSe deposition. Fig. 6b
shows electron tracks from a ^90Sr $\beta$ source obtained with the device
operated at high voltage.

Figure 6: a) _Topmetal-II -_ CMOS APS before and after aSe deposition. b)
Electron tracks observed with our first hybrid aSe/CMOS device operated at a
drift electric field of 4 V/$\mu$m at room temperature.

The Selena experiment will consist of much larger imagers made from $\sim 5$
mm-thick amorphous ^82Se layers coupled to an array of $\sim 5$ cm2 CMOS
active pixel arrays tiled on a 300-mm diameter silicon wafers. The aSe layers
are sufficiently thick to fully-contain the mm-long $\beta\beta$ decay tracks
with total energy of $Q_{\beta\beta}$. Each module is made from two back-to-
back imagers that share a common electrode that provides the high voltage
(HV). A detector is built from towers of these modules inside a radio-pure
lead shield in an underground laboratory to minimize background.

The CMOS APS will feature standard parameters for an imaging device: pixels
$10\times 10$ $\mu$m2 in size, a pixel readout noise of 10 $e^{-}$ and a pixel
dynamic range of 70 dB. Each pixel will be instrumented to measure the total
integrated charge collected by a pixel and the time of arrival of the charge.
The integrated charge provides an estimate of the deposited energy, while the
time of arrival will be used to reconstruct the relative $z$ coordinate of the
energy depositions for high-resolution 3D imaging. Considering the drift speed
of the charge carriers, a 5 ns time resolution would correspond to a
resolution in $z$ of 10 $\mu$m. We will also infer the absolute $z$ position
of the events from the lateral spread of charge on the pixel array. Fig. 7
shows the differences caused by lateral charge diffusion of the same simulated
$\beta\beta$ decay event at different ($1$ mm vs. $5$ mm) depths in the
detector.

Figure 7: Simulated $\beta\beta$ decay event with total energy of
$Q_{\beta\beta}$ in an imager with 10$\times$10 $\mu$m2 pixel size. The color
axis corresponds to the relative $z$ position. In the left plot, we simulated
lateral diffusion of charge with an overall offset on the $z$ position of 1
mm, while the right plot has an overall $z$ offset of 5mm. The Bragg peaks are
identified and labeled.

### 3 B. Energy Response

The energy resolution at $Q_{\beta\beta}$ determines the capability of Selena
to spectrally resolve the neutrinoless $\beta\beta$ decay signal over the two-
neutrino $\beta\beta$ decay background spectrum. In Ref. Li  _et al._ (2021),
we reported our measurement of the response of aSe to 122 keV $\gamma$ rays
from a 57Co radioactive source as a function of the electric field in the aSe.
Fig 8a shows the measured energy resolution compared to our full detector
simulation, which includes particle tracking, all details of the charge
generation and transport, and an electronics simulation. The data is best
explained by a recombination probability that has a strong dependence on the
ionization density along the electron tracks, _i.e._ , by a charge yield along
that has an inverse relation with $dE/dx$. This leads to an energy resolution
that is dominated by fluctuations in $dE/dx$ and not charge-carrier
statistics. Fortunately, the imaging capabilities of Selena will allow for a
local measurement of $dE/dx$ along the electron tracks from which we can
estimate the energy by correcting for the varying charge yield. From our
latest simulations based on our recombination model, we expect an energy
resolution at $Q_{\beta\beta}$ of 1.1% RMS after correcting for the varying
charge yield. Although further improvements may be possible, we use the
extrapolation shown in Fig. 8b to build the science case in Sec. 2.

The calibration of the energy response of Selena is straightforward since the
detector identifies individual electron tracks from single-scattering events.
Photoelectric absorption of monoenergetic $\gamma$ rays leads to a
monoenergetic sample of electron tracks that are easily distinguishable from
the continuous spectrum of electrons from Compton scattering since their
energies are significantly above the Compton edge. By exposing the Selena
modules with $\gamma$ ray sources of various energies up to $Q_{\beta\beta}$,
we would be able to calibrate with high accuracy the entire energy region of
interest for solar neutrino spectroscopy and $\beta\beta$ decay. Furthermore,
the data will contain not only the total ionization signal of the events but
high resolution 3D images of the ionization tracks. With sufficient
statistics, we should be able to experimentally determine the charge yield as
a function of $dE/dx$ to validate our recombination model and optimize the
energy resolution.

Figure 8: a) Fractional energy resolution of the 122 keV $\gamma$-ray line
from ^57Co as a function of drift electric field in aSe. Our full detector
simulation that includes our recombination model (red line) is in good
agreement with the data. b) Extrapolation with our detector simulation of the
fractional energy resolution up to $Q_{\beta\beta}$ at a drift electric field
of 50 V/$\mu$m.

### 3 C. Background Sources and Rejection

In Ref. Chavarria  _et al._ (2017), we studied by simulation how the imaging
capabilities of a large Selena detector could be employed to suppress
backgrounds from natural radioactivity in the search for neutrinoless
$\beta\beta$ decay. We performed an extensive survey of radiogenic and
cosmogenic background sources considering radiocontaminants in the bulk and on
the surfaces of the modules, and an external $\gamma$-ray flux. We note that
since only a very small fraction of $\gamma$ rays from ^238U and ^232Th decay
chains can produce single electrons with energies of $Q_{\beta\beta}$, a
significant contribution to the relevant $\gamma$-ray flux comes from
cosmogenic isotopes activated _in-situ_ and deexcitation of nuclei following
neutron capture. This leads to unusual constraints in the construction
materials since it introduces considerations (_e.g._ , activation and neutron
capture cross sections) beyond material radiopurity. We argued that
classification of events by spatio-temporal correlations could suppress
backgrounds originating in the bulk or on the surfaces of the modules to a
negligible level. A $10^{3}$ reduction in background from the external
$\gamma$-ray flux was achieved by discarding events without two Bragg peaks
identified by an automated algorithm. This could perhaps be improved by
leveraging on more advanced machine learning techniques Kekic  _et al._
(2021). The final value of $6\times 10^{-5}$ per keV per ton-year would allow
us to perform background-free $\beta\beta$ decay spectroscopy in 100 ton-year.

The number of accidentals that mimic the “triple sequence” used to identify
$\nu_{e}$ captures (Fig. 1a) is $<10^{-4}$ in 100 ton-year in an underground
detector, whose background rate is dominated by two-neutrino $\beta\beta$
decay. Charged particle reactions to produce ^82mBr from ^82Se—the traditional
background for radiochemical experiments—have a prompt event topology that is
easily distinguishable from a fast electron. A more likely background for the
production of ^82mBr would be from neutron capture on trace ^81Br in the aSe,
where the deexcitation $\gamma$ ray only interacts once very close to the
decay site to mimic the prompt event. However, the contamination of Br in aSe
is not known at this time. The dominant background will likely come from
neutron captures on ^82,80,78Se, which lead to sequences of three or four
decays that emit fast electrons and $\gamma$ rays. However, they can all be
suppressed by appropriate selections on event topology, time separation and
decay energies. Finally, we performed a survey of isotopes that can be
cosmogenically activated in ^82Se and could not identify any that could be
produced at any significant rates and could mimic the triple sequence closely
enough in topology and time separation. Thus, so far, it appears that zero-
background solar neutrino spectroscopy is possible in a 100 ton-year exposure
with Selena.

## 4\. The Selena Research program

In the next sections, we describe technical details and ongoing developments
to realize the Selena Neutrino Experiment.

### 4 A. Pixel structure

The pixel structure of Selena will be optimized for _i)_ charge collection
efficiency $>$99.9%, _ii)_ noise on charge sensing per pixel of $\sim$10
$e^{-}$, _iii)_ time-of-arrival estimate of the charge to the pixel with an
accuracy of 5 ns, _iv)_ pixel pitch of 10 $\mu$m, and _v)_ minimal power
consumption for the readout of large pixel arrays.

Each pixel in the CMOS charge sensor will focus the charge coming from the aSe
layer to the collection electrode and capture the charge signal with circuitry
embedded in the pixel. The outputs of the pixel circuitry, which correspond to
the charge amplitude and time of arrival, will be driven out of the pixel
array and recorded by external readout. The _Topmetal-II -_ An  _et al._
(2016) realized most of these goals but was not specifically optimized for aSe
operations, which resulted in sub-optimal performance.

To achieve maximum charge collection efficiency and minimize inter-pixel cross
talk, we plan to implement a charge collection and focusing electrode
structure in the topmost metal layer of CMOS as shown in Fig. 9a. The center
electrode collects charge and transfers it to the input of the CSA. The guard
ring, which surrounds the center electrode and forms a cross-pattern between
pixels, is biased to focus the charge to the electrode and to isolate pixels
from each other. The results from electrostatic simulations shown in Fig. 9b
suggest that 100% collection efficiency is possible with a guard ring
potential of only 1.5% of the drift HV.

We also plan to improve the in-pixel circuitry by implementing, per pixel, a
DAC tunable CSA feedback, a sample-hold structure of the CSA output, and a
time-of-arrival circuit (Fig. 9c). The per-pixel tunable DAC allows fine-
tuning the time constant of the CSA in each pixel, to counter the process
variation from pixel to pixel and ensure sufficient signal gain and retention.
Further, a sample-hold structure shall be added to the output of the CSA to
act upon a trigger to capture the signal and wait for readout sometime later.
The sample switch is opened by a discriminator on the pixel (not shown but
present in the current _Topmetal-II -_ design) when its value crosses a
programmable threshold set by a DAC. These two additions are expected to
significantly improve the uniformity and overall resolution of the sensor. The
same discriminator circuit can be used to sample a clock to provide the time-
of-arrival of the charge to the pixel444An existing version of _Topmetal_ ,
_Topmetal-M_ , already features time-of-arrival measurement capabilities Ren
_et al._ (2020)..

Figure 9: a) Proposed pixel structure for the next CMOS APS iteration. b)
Electrostatic simulation results (electric field lines and equipotential
lines) for the proposed pixel structure. Collection efficiency of $\sim$100%
is achieved with a guard ring potential of 1.5% of the HV. c) Schematic of the
pixel circuitry of the proposed design. Parts in red (a sample/hold for the
pixel value, and a DAC to set the feedback resistance of the pixels
individually) are new additions to the baseline _Topmetal-II -_ design.

### 4 B. Large-area detector module

The Selena detector modules will consist of a large array of stand-alone
$2\times 2.5$ cm2 CMOS APSs on a 300 mm-diameter wafer. To efficiently contain
electron tracks in the energy region of interest in a single module, the aSe
layer must be $\sim 5$ mm thick, which correspond to a 82Se target mass of 2
kg per module. A significant challenge will be the application of the HV to
the modules. Unlike test devices, where the HV electrode is fully exposed on
top, Selena modules will feature face-to-face devices where the HV comes in
from the side. This becomes challenging for a design that maximizes the
thickness of the aSe layer.

We will approach this challenge with the conceptual design shown in Fig. 10.
The pixel pattern will be hexagonal (Fig. 10a) instead of square to reduce the
maximum field near electrode corners while maintaining the same hole
collection efficiency. The profile of the pixel array will remain rectangular.
To maximize the active pixel area and simplify the packaging, we will bring
the CMOS APS connections to the backside of the wafer with through-silicon
vias (TSVs). The front side of the wafer (Fig. 10b) will be held at ground
potential, by the active pixel arrays, and by a conductive coating above the
gaps between CMOS APSs and the edges of the wafer deposited post-fabrication.
A thin flexible PCB will be attached to the backside of the wafer and wire-
bonded to the TSVs (Fig. 10e). The PCB will deliver power and control signals
to CMOS APSs and bring digitized signals outside the Selena module. The CMOS
APS design will feature power regulation and complete digital control, thus no
external electric components will be mounted on the PCB for power
distribution, chip control, or signal multiplexing. The PCB will be protected
from HV by a conductive frame around the wafer (Fig. 10c). The frame can be
metal, or insulator with conductive coating; the later is preferred to reduce
radioactive contamination and neutron capture near the modules. The wafer-PCB-
frame assembly will be sent out for aSe deposition. Making connections prior
to aSe deposition avoids mechanically or thermally stressing the aSe film,
which is critical for handling high electric field. A thin layer of gold will
be deposited on top of aSe as the HV electrode with a 2 cm margin to the edge
of the wafer. A conductive Kapton film aperture expended by a conductive ring
will deliver HV to the gold electrode (Fig. 10d). The aperture in the center
will prevent electrons crossing the top of the aSe film from losing energy in
the Kapton film. The ring provides mechanical support and a rounded edge far
away from ground potential. The film will be gently sandwiched by two Selena
modules face-to-face, constituting one basic detector unit. Finally, multiple
pairs of modules will be stacked together as a tower (Fig. 10e, the overall
support structure is not shown). The entire assembly will be hosted in vacuum
at room temperature. Additional heat dissipation structures attached to the
backside of the wafer may be necessary.

Figure 10: Conceptual design of the large-area detector module. a) Hexagonal
pixel array. b) Top surface of the 300-mm diameter wafer. Blue rectangles
represent CMOS APSs and gray areas are grounded with post-fabrication coating.
c) Single module with gold electrode (gold) deposited on aSe (dark red) on the
top wafer surface shown in (b), supporting frame (light gray), and flexible
PCB (khaki). d) HV ring placed on top of (c). The brown aperture is conductive
Kapton. The gray ring provides mechanical strength and a rounded edge. e)
Cross section of a stack of three face-to-face module pairs. Slots on the
flexible PCBs are for connecting to TSVs.

### 4 C. The Selena 100 kg demonstrator

A “demonstrator” detector made from 50 large-area modules and a target mass of
100 kg of enriched 82Se will be a stepping stone in the Selena program. The
demonstrator will be designed to minimize backgrounds from natural
radioactivity and deployed underground (_e.g.,_ at the Laboratorio Subterráneo
de Canfranc (LSC) in Spain). Its main goal will be to demonstrate the
technology (including scalability), performance and background rejection
capabilities necessary for a large-scale Selena detector.

The fabrication of the demonstrator will require the enrichment and deposition
of aSe at scale. Our industrial partner, Hologic, Inc., deposits $\sim$1 ton
of aSe per year for their commercial imagers. We will setup a dedicated aSe
deposition facility for Selena, which will a recovery process to minimize the
waste of aSe enriched in 82Se.

Enrichment of 82Se is commonly achieved by centrifugation of gaseous selenium
hexafluoride (SeF_6), an inefficient process that would dominate the cost of
the demonstrator. A more attractive approach is isotopic enrichment by
cryogenic distillation of hydrogen selenide (H_2Se), for which a hint of
isotopic separation of ^82Se was observed with a meter-long column Mills  _et
al._ (1988). We will explore this possibility as part of the Selena program.
We note the synergies with other efforts in our community, _e.g.,_ the Aria
project, which aims to remove 39Ar from natural argon by distillation Agnes
_et al._ (2021).

The demonstrator will provide the opportunity to develop a calibration
strategy with $\gamma$-ray sources for Selena, and will also acquire a large
sample of two-neutrino $\beta\beta$ decay events. After one year of operation,
the Selena demonstrator will place an lower limit on the neutrinoless decay
half-life of ^82Se $\tau_{1/2}>2\times 10^{26}$ y (90% C.L.), and detect $\sim
5$ _pp_ solar neutrinos with zero background.

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