36. Particle Detectors for Non-Accelerator Physics

1

36. Particle Detectors for Non-Accelerator Physics
Revised 2024. See the various sections for authors.

36.1
36.2

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
High-energy cosmic-ray hadron and gamma-ray detectors . . . . . . . . . . . . . .
36.2.1 Atmospheric fluorescence detectors . . . . . . . . . . . . . . . . . . . . . . . .
36.2.2 Atmospheric Cherenkov telescopes for high-energy gamma ray astronomy . .
36.3 Large neutrino detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36.3.1 Deep liquid detectors for rare processes . . . . . . . . . . . . . . . . . . . . .
36.3.2 Neutrino telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36.3.3 Radio emission from (ultra-)high energy particle showers . . . . . . . . . . . .
36.4 Large time-projection chambers for rare event detection . . . . . . . . . . . . . . .
36.4.1 Dark matter and other low energy signals . . . . . . . . . . . . . . . . . . . .
36.4.2 0νββ Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36.5 Sub-kelvin detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36.5.1 Motivation for Sub-kelvin Detectors . . . . . . . . . . . . . . . . . . . . . . .
36.5.2 Detector Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36.5.3 Experimental Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36.6 Low-radioactivity background techniques . . . . . . . . . . . . . . . . . . . . . . .
36.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36.6.2 Radio-purity assay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36.6.3 Radon and its progeny . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36.6.4 Surface backgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36.6.5 Mitigation of backgrounds and active background discrimination . . . . . . .

1
2
2
6
8
8
12
19
26
30
33
34
34
36
40
44
44
45
46
47
48

36.1 Introduction
Non-accelerator experiments have become increasingly important in particle physics and astrophysics. From them comes the evidence of physics beyond the SM, with the discovery of neutrino
oscillations and adiabatic flavor conversion. Explored energies range from the meV scale to above
the EeV, some 24 orders of magnitude. The physics and the design of the detectors vary as a consequence. Some experiments look at astrophysical high-energy phenomena using the atmosphereas a detector in the fluorescence and Cherenkov observatories or the polar ice and the ocean waterin neutrino telescopes. Experiments on extremely rare events, such as neutrino-less double beta
decay, solar neutrinos and dark matter induced scattering, need dedicated fully equipped deep
underground laboratories, existing in different countries. Critical is the research to push back theultra-low radioactive background frontier, with dedicated facilities in these laboratories. Detectors range from hyper-pure liquid scintillators, both organic and not, to thermalized and ballistic
phonon detectors at the sub-Kelvin temperature, to dual phase noble fluid TPCs, etc. Space-based
detectors also use some unique instrumentation, but these are beyond the present scope of thisreview. Gravitational wave detectors are not included as well.
S. Navas et al. (Particle Data Group), Phys. Rev. D 110, 030001 (2024)
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36. Particle Detectors for Non-Accelerator Physics

36.2 High-energy cosmic-ray hadron and gamma-ray detectors
36.2.1 Atmospheric fluorescence detectors
Revised March 2024 by L.R. Wiencke (Colorado School of Mines).
Cosmic-ray fluorescence detectors (FDs) use the atmosphere as a giant calorimeter to measureisotropic scintillation light that traces the development profiles of extensive air showers. An extensive air shower (EAS) is produced by the interactions of ultra high-energy (E > 1017 eV) subatomic
particles in the stratosphere and upper troposphere. The amount of scintillation light generated by
an EAS is proportional to the energy deposited in the atmosphere and nearly independent of the
primary species. With energies extending beyond 1020 eV, ultra high energy cosmic rays (UHECRs) are the highest energy subatomic particles known to exist. In addition to particle arrival
directions, energy spectra and primary composition, the astroparticle science investigated with
FDs also includes multi-messenger studies, searches for high energy photons, neutrinos, monopoles
and deeply penetrating forms of dark matter. The Pierre Auger Observatory FD also measures
UV scintillation that traces the development of ring-shaped atmospheric transient luminous events,
called Elves, in the ionosphere that are initiated by strong lightning [1].
Previous experiments with FDs included the pioneering Fly’s Eye [2,3], and the High Resolution
Fly’s Eye (HiRes and HiRes prototype) [4]. The history of the fluorescence technique includes earlier
studies in the 1950’s and 1960’s [5, 6]. The current generation of experiments include the Telescope
Array (TA) [7] in the northern hemisphere, and the larger Pierre Auger Observatory (Auger) [8]in the southern hemisphere. (Discussion, with extensive references, of current and possible future
UHECR experiments can be foundx[ in [9].) Auger and TA are hybrid observatories. Their FD
telescopes overlook sparse arrays of particle detectors on the ground. Select parameters are listed
in Table 36.1. TA and Auger have each one FD site populated with additional telescopes thatview up to 60◦ in elevation to measure lower EASs using a combination of scintillation and direct
Cherenkov light. As part of a fourfold coverage upgrade of TA (TAx4), 12 HiRes refurbishedtelescopes have been installed at the north and south-east sites of TA. A set of prototype FD
telescopes, dubbed FAST [10], have observed EASs at the TA site using design that features
wide field of view PMTs, fast timing and economical optics for a next-generation ground-based
observatory. A second prototype has been installed at the Pierre Auger Observatory.
The fluorescence light is emitted primarily between 290 and 430 nm (Figure 36.1) with majorlines at 337, 357, and 391 nm, when relativistic charged particles, primarily electrons and positrons,
excite nitrogen molecules in air, resulting in transitions of the 1P and 2P systems. Reviews andreferences for the pioneering and recent laboratory measurements of fluorescence yield, Y (λ, P, T, u),
including dependence on wavelength (λ), temperature (T ), pressure (p), and humidity (u) may befound in Refs. [11–13]. The results of various laboratory experiments have been combined (Figure36.2) to obtain an absolute average and uncertainty for Y(337 nm, 800 hPa, 293 K, dry air) of
7.04 ± 0.24 ph/ MeV after corrections for different electron beam energies and other factors. The
units of ph/ MeV correspond to the number of fluorescence photons produced per MeV of energy
deposited in the atmosphere by the electromagnetic component of an EAS.
An FD element (telescope) consists of a non-tracking spherical mirror of less than astronomical
quality, a“camera” of photomultiplier tubes (PMTs) near the focal plane, and a flash ADC readout
system with a pulse and track-finding trigger scheme [8, 16]. The major experiments listed inTable 36.1 all use conventional PMTs (for example, Hamamatsu R9508 or Photonis XP3062) withgrounded cathodes and AC coupled readout. Segmented mirrors have been fabricated from slumped
or slumped/polished glass with an anodized aluminum coating or fabricated using shaped aluminumthat was then chemically anodized with AlMgSiO5 . A broadband UV filter (custom fabricated, BG3, or Schott MUG-6) reduces background light such as starlight, airglow, man-made light pollution,
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36. Particle Detectors for Non-Accelerator Physics

3

Table 36.1: Parameters of major fluorescence detectors. Note 1: Year when all FD sites wereoperational. Note 2: At TA 1 of the 3 FD sites features 24 telescopes from the HiRes experiment.
Note 3: A-C for one telescope where A is the full area and C the area obscured by the cameraand support structures. Thus A-C is the effective light collecting area. For the modified Schmidt
design at Auger, the area of the entrance pupil, A, is listed because the pupil is smaller than the
mirror and thus defines the entrance aperture. For the other experiments, the area of the mirror,A, is listed
Observatory
LocationStart-End
Sites (note 1)
Separation
Telescopes/site
Pixel FOV
Telescope FOV
Azi×Elv
Light collection
area (note 3)
Energy Scale
Uncertainty

Fly’s Eye
Dugway UT US
1981-1992
2 (1986)
3.3 km67,18
5.5◦≈18◦ ×≈18◦

HiRes
Dugway UT US
1996-2006
2 (1999)12.6 km21,42
1◦
16◦ ×13.5◦

Telescope Array
Delta UT US
2008-present
3 (2008)
31-40 km
12+8,12,14+10+4
1◦
18◦ ×15◦ (note 2)

Pierre Auger
Malargüe AR
2005-present
4 (2008)
39-62 km6, 6, 6, 6+3
1.5◦
30◦ ×28.1◦

1.95 m2 - 0.25 m2

3.72 m2 - 0.5 m2

≤40%

≈20%

6.8 m2 - 0.85 m2(for 2 sites)≈20%

3.80 m2 - 0.80 m2(modified schmidt)
14%

Relative intensity

15

10

5

0
290 300 310 320 330 340 350 360 370 380 390 400 410 420
Wavelength (nm)Figure 36.1: Measured fluorescence spectrum excited by 3 MeV electrons in dry air at 800 hPa
and 293 K. Airfly experiment. Figure from Ref [14].

and airplane strobe-lights.
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36. Particle Detectors for Non-Accelerator Physics

4<Y 337> = 7.04 ± 0.24 ph/MeV
AirflyDandl
AirLight
FLASH

Theoretical

MACFLY
Lefeuvre
NaganoKakimoto
3

4

5

6

7

8

9

10

Y 337 (ph/MeV)
Figure 36.2: Fluorescence yield values and associated uncertainties at 337 nm (Y337 ) in dry air
at 800 hPa and 293 K The methodology and corrections that were applied to obtain the averageand the uncertainty are discussed extensively in this reference. The vertical axis denotes different
laboratory experiments that measured FY. The gray bars show three of the original measurements
to illustrate the scale of the corrections applied. Figure from Ref [15].
At 1020 eV, where the flux drops below 1 EAS/km2 century, the aperture for an eye of adjacent
FD telescopes that span the horizon can reach 104 km2 sr. FD operation requires (nearly) moonless nights and clear atmospheric conditions, which typically imposes a duty cycle of about 10%.
Arrangements of LEDs, calibrated diffuse sources [17], pulsed UV lasers [18], LIDARs 1 and IR
detectors that are sensitive to clouds are used for photometric calibration, atmospheric calibration [19], and determination of exposure [20]. For purposes of optical transmission, the atmosphereis treated as having a dominant molecular component and a secondary aerosol component. The
latter is well described [21] by molecular scattering theory and models derived from radiosonde measurements. The aerosol component can include dust, haze and pollution and the aerosol opticaldepth profile must be measured on site in the UV during FD data taking.
The EAS generates a track consistent with a light source moving at v = c across the FOV. The
number of photons (Nγ ) as a function of atmospheric depth (X) can be expressed as [12]
tot
dEdep
dNγ
=
Y (λ, P, T, u) · τatm (λ, X) · εFD (λ)dλ,(36.1)
dX
dX
where τatm (λ, X) is the atmospheric transmission, including wavelength (λ) dependence, and εFD (λ)
is the FD efficiency. εFD (λ) includes geometric factors and collection efficiency of the optics, quantum efficiency of the PMTs, and other throughput factors. The typical systematic uncertainties,τatm (10%) and εFD (photometric calibration 10%), currently dominate the systematic uncertainty

Z

1
LIDAR stands for "Light Detection and Ranging" and refers here to systems that measure atmospheric properties
from the light scattered backwards from laser pulses directed into the sky.

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36. Particle Detectors for Non-Accelerator Physics

5

data
total light
Mie Cherenkov
direct Cherenkov
Rayleigh Cherenkov
multiple scattered

350
300
250
200
150

dE/dX [PeV/(g/cm2)]

detected light [photons/m2 /100ns]

the absolute EAS energy scale. FD energy resolution, defined as event-to-event statistical uncertainty, is typically less than 10% for final data samples used for science analysis.

100

50

χ 2/Ndf= 42.45/44

40
30
20
10

50
0
0

250

300

350

400

200

450

400

600

800

1000

1200

1400

1600

slant depth [g/cm2 ]time slots [100 ns]

Figure 36.3: Example light profile (left) of one EAS recorded by the Pierre Auger FD and thecorresponding profile (right) of energy deposited in the atmosphere vs atmospheric slant depth.
The light profiles include the estimated components of Cherenkov light that have been scatteredout of the forward beam by the molecular and aerosol (Mie) components of the atmosphere. The
reconstructed energy of this EAS was 3.0 ± 0.2 × 1019 eV. Figure from Ref [22].
Analysis methods to reconstruct the EAS profile and deconvolve the contributions of re-scattered
scintillation light, and direct and scattered Cherenkov light are described in [2] and more recently
in [23]. The EAS energy is typically obtained by integrating over the Gaisser-Hillas function [24](Xmax −X0 )/λX − X0e(Xmax−X)/λ ]dX,
(36.2)
X
−
X
0
max
0
where Ecal is the energy of electromagnetic energy component of the EAS and Xmax is the atmospheric slant depth at which the shower reaches its maximum energy deposit rate. This maximum
dE/dX is denoted as wmax . X0 and λ are two shape parameters. The energy of the primary cosmic
ray is obtained by correcting Ecal upward by about 10% to account for the invisible energy carriedby particles that do not interact in the atmosphere. Auger reported a data-driven method to estimate the invisible energy from the muon number at ground level and Xmax to reduce systematic
uncertainties [25]. Energy resolution, ∆E/E, of 15-20% is achievable, provided the geometric fitof the EAS axis is constrained, typically by multi-eye stereo projection or hybrid observations,and the profile fit of EAS development along the track is constrained by the observed rise and fall
about Xmax . An example of a recorded EAS light profile and its corresponding dE/dX development
profile are shown in Fig. 36.3. The EAS generates a track consistent with a light source moving at
v = c across the FOV. The number of photons (Nγ ) as a function of atmospheric depth (X) can be
expressed as [12] In 2023 the Pierre Auger Collaboration published a public data release [26] of the
100 highest energy cosmic ray events collected over a 17-year span through 2020 with an additional
9 energetic hybrid events used in the calibration proceedure. In 2024 the collabaration reported
the first comparision of Xmax measurements [27] between fluorescence and radio detectors at thesame location. This comparision includes a subset of 53 airshowers that were observed by the two
detectors simultaneously.
An FD that would look down on the earth’s atmosphere from space orbit to view a muchlarger area than ground based instruments is an active area of R&D. Prototypes that have been
built and flown in orbit include the pioneering TUS instrument [28], [29] operated 2016-2018 onboard the Lomonosov satellite. The JEM-EUSO collabration has flown prototype FD telescopes

Z ∞Ecal =





[wmax

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36. Particle Detectors for Non-Accelerator Physics

on stratospheric balloon flights in 2014 (EUSO-Balloon) [30], 2017 (EUSO-SPB1) [31] and 2023
(EUSO-SPB2) [32]. The Mini-EUSO [33] FD continues recording terrestial UV emission by looking
down through a 25 cm diameter UV-transmitting window inside the International Space Station
since 2019. The Probe of Extreme Multimessenger Astrophysics (POEMMA) project has completed
a detailed conceptual design study for a twin-satellite mission [34] that would observe UHECRs
and PeV scale cosmogenic tau neutrinos.
36.2.2 Atmospheric Cherenkov telescopes for high-energy gamma ray astronomy
Revised October 2023 by J. Holder (Delaware U.; Delaware U., Bartol Inst.).
A wide variety of astrophysical objects are now known to produce high-energy γ-ray photons.
Leptonic or hadronic particles, accelerated to relativistic energies in the source, produce γ-rays typically through inverse Compton boosting of ambient photons or through the decay of neutral pions
produced in hadronic interactions. At energies below ∼30 GeV, γ-ray emission can be efficiently
detected using satellite or balloon-borne instrumentation, with an effective area approximately
equal to the size of the detector (typically < 1 m2 ). At higher energies, a technique with muchlarger effective collection area is desirable to measure astrophysical γ-ray fluxes, which decrease
rapidly with increasing energy. Atmospheric Cherenkov detectors achieve effective collection areasof >105 m2 by employing the Earth’s atmosphere as an intrinsic part of the detection technique.
As described in Chapter 30, a hadronic cosmic ray or high energy γ-ray incident on the Earth’s
atmosphere triggers a particle cascade, or air shower. Relativistic charged particles in the cascade
generate Cherenkov radiation, which is emitted along the shower direction, resulting in a lightpool on the ground with a radius of ∼130 m. Cherenkov light is produced throughout the cascade
development, with the maximum emission occurring when the number of particles in the cascade islargest, at an altitude of ∼10 km for primary energies of 100 GeV–1 TeV. Following absorption andscattering in the atmosphere, the Cherenkov light at ground level peaks at a wavelength, λ ≈ 300–
350 nm. The photon density is typically ∼100 photons/m2 for a 1 TeV primary, arriving in a brief
flash of a few nanoseconds duration. This Cherenkov pulse can be detected from any point within
the light pool radius by using large reflecting surfaces to focus the Cherenkov light on to fast photon
detectors (Fig. 36.4).
Modern atmospheric Cherenkov telescopes, such as those built and operated by the VERITAS[35], H.E.S.S. [36] and MAGIC [37] collaborations, consist of large (> 100 m2 ) segmented mirrors
on steerable altitude-azimuth mounts. A camera made from an array of photosensors is placed at
the focus of each mirror and used to record a Cherenkov image of each air shower. In these imaging
atmospheric Cherenkov telescopes, single-anode photomultipliers tubes (PMTs) have traditionallybeen used (2048, in the case of H.E.S.S. II), but silicon devices now feature in more modern designs.
The telescope cameras typically cover a field-of-view of 3 − 10◦ in diameter. Images are recorded
at kHz rates, the vast majority of which are due to showers with hadronic cosmic-ray primaries.
The shape and orientation of the Cherenkov images are used to discriminate γ-ray photon eventsfrom this cosmic-ray background, and to reconstruct the photon energy and arrival direction. γ-ray
images result from purely electromagnetic cascades and appear as narrow, elongated ellipses in thecamera plane. The long axis of the ellipse corresponds to the vertical extension of the air shower,
and points back towards the source position in the field-of-view. If multiple telescopes are used
to view the same shower (“stereoscopy”), the source position is simply the intersection point of
the various image axes. Cosmic-ray primaries produce secondaries with large transverse momenta,
which initiate sub-showers. Their images are consequently wider and less regular than those with
γ-ray primaries and, since the original charged particle has been deflected by Galactic magnetic
fields before reaching the Earth, the images have no preferred orientation.
The measurable differences in Cherenkov image orientation and morphology provide the back31st May, 2024

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36. Particle Detectors for Non-Accelerator Physics

10 km

Camera plane

130 m
Figure 36.4: A schematic illustration of an imaging atmospheric Cherenkov telescope array. The
primary particle initiates an air shower, resulting in a cone of Cherenkov radiation. Telescopes
within the Cherenkov light pool record elliptical images; the intersection of the long axes of theseimages indicates the arrival direction of the primary, and hence the location of a γ-ray source inthe skyground discrimination which makes ground-based γ-ray astronomy possible. For point-like sources,
such as distant active galactic nuclei, modern instruments can reject over 99.999% of the triggered
cosmic-ray events, while retaining up to 50% of the γ-ray population. In the case of spatiallyextended sources, such as Galactic supernova remnants, the background rejection is less efficient,but the technique can be used to produce γ-ray maps of the emission from the source. The angularresolution depends upon the number of telescopes which view the image and the energy of the
primary γ-ray, but is typically less than 0.1◦ per event (68% containment radius) at energies above
a few hundred GeV.
The total Cherenkov yield from the air shower is proportional to the energy of the primaryparticle. The image intensity, combined with the reconstructed distance of the shower core fromeach telescope, can therefore be used to estimate the primary energy. The energy resolution of
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836. Particle Detectors for Non-Accelerator Physics

this technique, also energy-dependent, is typically 15–20% at energies above a few hundred GeV.
Energy spectra of γ-ray sources can be measured over a wide range, depending upon the instrument
characteristics, source properties (flux, spectral slope, elevation angle, etc.), and exposure time. The
effective energy range is typically from 30 GeV to 100 TeV and peak sensitivity lies in the range
from 100 GeV to a few TeV.
The first astrophysical source to be convincingly detected using the imaging atmospheric Cherenkov
technique was the Crab Nebula [38], with an integral flux of 2.1 × 10−11 photons cm−2 s−1 above
1 TeV [39]. Modern imaging atmospheric Cherenkov telescopes have sensitivity sufficient to detectsources with less than 1% of the Crab Nebula flux in a few tens of hours. The TeV source catalog
now consists of over 200 sources (see e.g. Ref. [40]). A large fraction of these were detected byscanning the Galactic plane from the southern hemisphere with the H.E.S.S. telescope array [41].
Recent reviews of the field include [42] and [43], and a historical overview can be found in [44].
Major upgrades of the existing telescope arrays include the 2012 addition of a 28 m diameter
central telescope to H.E.S.S. (H.E.S.S. II). Cherenkov telescopes also play an important role in hybrid air-shower detector systems such as TAIGA [45] and LHAASO [46]. Development for the next
generation instrument, the Cherenkov Telescope Array Observatory (CTAO), is now at an advanced
stage. CTAO will consist of a northern and a southern hemisphere observatory, with a combined
total of more than 60 telescopes [47]. Telescopes of three different sizes are planned, spread overan area of > 1 km2 , providing wider energy coverage, improved angular and energy resolutions,and an order of magnitude improvement in sensitivity relative to existing imaging atmospheric
Cherenkov telescopes. A number of prototype telescopes are already operating, exploiting technological developments in telescope structural design and high-speed data acquistion. Novel featuresof some designs include dual mirror optics and silicon photo-detectors. In particular, the first 23 m
diameter “Large Size Telescope” is now operating regularly on La Palma, and is producing scientificresults [48]. The wide range of physics questions that CTAO will adress is summarized in [49].

36.3 Large neutrino detectors
36.3.1 Deep liquid detectors for rare processes
Revised October 2023 by K. Scholberg (Duke U.) and C.W. Walter (Duke U.).
Deep, large detectors for rare processes tend to be multi-purpose with physics reach that includes
not only solar, reactor, supernova and atmospheric neutrinos, but also searches for baryon number
violation and lepton number violation, searches for exotic particles and beyond-the-standard-model
physics, and neutrino and cosmic-ray astrophysics in different energy regimes. The detectors may
also serve as targets for long-baseline neutrino beams for neutrino oscillation physics studies. In
general, detector design considerations can be divided into high- and low-energy regimes, for whichbackground and event reconstruction issues differ. The high-energy regime, from about 100 MeV toa few hundred GeV, is relevant for proton decay searches, atmospheric neutrinos and high-energy
astrophysical neutrinos. The low-energy regime (a few tens of MeV or less) is relevant for supernova,
solar, reactor and geological neutrinos.
Large water Cherenkov and scintillator detectors (see Table 36.2) usually consist of a volumeof transparent liquid viewed by photomultiplier tubes (PMTs) (see Sec 35.2); the liquid serves asactive target. PMT hit charges and times are recorded and digitized, and triggering is usually basedon coincidence of PMT hits within a time window comparable to the detector’s light-crossing time.
Because photosensors lining an inner surface represent a driving cost that scales as surface area,
very large volumes can be used for comparatively reasonable cost. Some detectors are segmented
into subvolumes individually viewed by PMTs, and may include other detector elements (e.g.,
tracking detectors). Devices to increase light collection, e.g., reflectors or waveshifter plates, may
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Table 36.2: Properties of large detectors for rare processes. If total target mass is divided intolarge submodules, the number of subdetectors is indicated in parentheses. Projects with first data
expected in 2024 or later are indicated in italics.
Detector

Mass, kton
(modules)
BUST
0.33, scint (3150)
MACRO
0.56, scint (476)LVD
1, scint. (840)KamLAND
0.41∗ , scint
Borexino
0.1∗ , scint
SNO+
0.78, scint‡
CHOOZ
0.005, scint (Gd)Double Chooz 0.017, scint (Gd)(2)Daya Bay
0.160, scint (Gd)(8)RENO
0.032, scint (Gd)(2)JUNO
20.0∗ , scint
IMB-1
3.3∗ , H2 O
IMB-2
3.3∗ , H2 O
Kam I
0.88/0.78∗ , H2 O
Kam II
1.04∗ , H2 O
Kam III
1.04∗ , H2 O
SK I
22.5∗ , H2 O
SK II
22.5∗ , H2 O
SK III-V
22.5∗ , H2 O
SK-Gd
22.5∗ , H2 O (Gd)Hyper-K187∗ , H2 Ok
SNO
1, D2 O/1.7, H2 O
DUNE40∗ , Ar (4)PMTs(diameter, cm)
1/module (15)2-4/module (20)
3/module (15)1325(43)+554(51)†2212 (20)9394 (20)192 (20)534/module (20)
192/module (20)342/module (25)
17612 (51)/25600 (8)2048 (12.5)
2048 (20)1000/948 (51)948 (51)948 (51)11146 (51)5182 (51)11129 (51)11129 (51)>20000 (51)∗∗9438 (20)TBD‡‡ξp.e./MeV

Dates

segmented
segmented
segmented
34%
30%
47%
15%
13%
5.6%§
12.6%
77.9%
1%
4.5%
20%
20%
20%¶
40%
19%
40%
40%
>20%
31%††
TBD‡‡

40
18
15
460
500
400–600
130
180
100
100
1200
0.25
1.1
3.4
3.4
4.3
6
3
6
6
>6
9
TBD‡‡

1980–
1989–2000
1992–
2002–
2007–2021
2021–
1997–1998
2011–2017
2011–20202011–2024 (exp.)
1982–1985
1987–1990
1983–1985
1986–1990
1990–1995
1996–2001
2002–2005
2006–20202020–2027 (exp.)
1999–2006
2028 (exp.)§§∗

Indicates typical fiducial mass used for data analysis; this may vary by physics topic.
Measurements made before 2003 only considered data from the 43 cm PMTs.
‡
SNO+ ran with water fill from May 2017 to July 2019.
§
The effective Daya Bay coverage is 12% with top and bottom reflectors.
¶
The effective Kamiokande III coverage was 25% with light collectors.
kA second staged module is being investigated.
∗∗
Additional photosensor modules and PMTs are planned.
††
The effective SNO coverage was 54% with light collectors.
‡‡
Photodetector technology and coverage varies according to TPC type and is not yet fully determined.
§§
Modules will be constructed in a phased approach.
†be employed. A common configuration is to have at least one concentric outer layer of liquid material
separated from the inner part of the detector to serve as shielding against ambient background. If
optically separated and instrumented with PMTs, an outer layer may also serve as an active vetoagainst entering cosmic rays and other background events. The PMTs for large detectors typicallyrange in size from 20 cm to 51 cm diameter, and typical quantum efficiencies are in the 20–25%
range for scintillation and water-Cherenkov photons. PMTs with higher quantum efficiencies, 35%
or higher, are now available. The active liquid volume requires purification and there may be
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36. Particle Detectors for Non-Accelerator Physics

continuous recirculation of liquid. For large homogeneous detectors, the event interaction vertex isdetermined using relative timing of PMT hits, and energy deposition is determined from the numberof recorded photoelectrons. A “fiducial volume” is usually defined within the full detector volume,
some distance away from the PMT array. Inside the fiducial volume, enough PMTs are illuminatedper event that reconstruction is considered reliable, and furthermore, entering background from theenclosing walls is suppressed by a buffer of self-shielding. PMT and detector optical parameters
are calibrated using laser, LED, or other light sources. Quality of event reconstruction typicallydepends on photoelectron yield, pixelization and timing.
Because in most cases one is searching for rare events, large detectors are usually sited underground to reduce cosmic-ray-related background (see Chapter 30). The minimum depth required
varies according to the physics goals [50].
36.3.1.1 Liquid scintillator detectors
Past and current large underground detectors based on hydrocarbon scintillators include LVD,
MACRO, BUST (Baksan), Borexino, KamLAND and SNO+; JUNO is a future detector. Experiments at nuclear reactors include CHOOZ, Double CHOOZ, Daya Bay, and RENO. Organic liquid
scintillators (see Section 35.3) for large detectors are chosen for high light yield and attenuation
length, good stability, compatibility with other detector materials, high flash point, low toxicity,
appropriate density for mechanical stability, and low cost. They may be doped with waveshifters
and stabilizing agents. Popular choices are pseudocumene (1,2,4-trimethylbenzene) with a few g/Lof the PPO (2,5-diphenyloxazole) fluor, and linear alkylbenzene (LAB), with light yield ∼ 104
photons/MeV. In a typical detector configuration there will be active or passive regions of undoped
scintillator, non-scintillating mineral oil or water surrounding the inner neutrino target volume. A
thin vessel or balloon made of nylon, acrylic or other material transparent to scintillation light may
contain the inner target; if the scintillator is buoyant with respect to its buffer, ropes may hold theballoon in place. For phototube surface coverages in the 20–40% range, yields in the few hundredsof photoelectrons
per MeV of energy deposition can be obtained. Typical energy resolution is aboutp
5 − 7%/ E(MeV) [51, 52], and typical position reconstruction resolution is a few tens of cm at ∼1 MeV, scaling as ∼ N −1/2 , where N is the number of photoelectrons detected.
Shallow detectors for reactor neutrino oscillation experiments require excellent muon veto capabilities. For ν̄e detection via inverse beta decay on free protons, ν̄e + p → n + e+ , the neutron is
captured by a proton on a ∼180 µs timescale, resulting in a 2.2 MeV γ ray, observable by Compton
scattering and which can be used as a tag in coincidence with the positron signal. The positronannihilation γ rays may also contribute. Inverse beta decay tagging may be improved by additionof Gd at ∼0.1% by mass, which for natural isotope abundance has a ∼49,000 barn cross-section forneutron capture (in contrast to the 0.3 barn cross-section for capture on free protons). Gd capture
takes ∼30 µs, and is followed by a cascade of γ rays adding up to about 8 MeV. Gadolinium dopingof scintillator requires specialized formulation to ensure adequate attenuation length and stability.
Scintillation detectors have an advantage over water Cherenkov detectors in the lack of Cherenkov
threshold and the high light yield. However, scintillation light emission is nearly isotropic, andtherefore directional capabilities are relatively weak. Liquid scintillator is especially suitable fordetection of low-energy events. Radioactive backgrounds are a serious issue, and include long-lived
cosmogenics such as 14 C. To go below a few MeV, very careful selection of materials and purificationof the scintillator is required (see Section 36.6). Fiducialization and tagging can reduce background.
One can also dissolve neutrinoless double beta decay (0νββ) isotopes in scintillator. This has beenrealized by KamLAND-Zen, which deployed a 1.5 m-radius balloon containing enriched Xe dissolvedin scintillator inside KamLAND, and 130 Te is planned for SNO+.
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36.3.1.2 Water Cherenkov detectors
Very large imaging water detectors reconstruct ten-meter-scale Cherenkov rings produced bycharged particles (see Section 35.5). The first such large detectors were IMB and Kamiokande.
The only currently existing instance of this class of detector, with fiducial mass of 22.5 kton andtotal mass of 50 kton, is Super-Kamiokande (Super-K, SK). Hyper-Kamiokande (Hyper-K) plansat least one, and possibly two, detectors with 187-kton fiducial mass. For volumes of this scale,absorption and scattering of Cherenkov light are non-negligible, and a wavelength-dependent factor
exp(−d/L(λ)) (where d is the distance from emission to the sensor and L(λ) is the attenuation
length of the medium) must be included in the integral of Eq. (35.6) for the photoelectron yield.
Attenuation lengths on the order of 100 meters have been achieved.
Cherenkov detectors are excellent electromagnetic calorimeters, and the number of Cherenkov
photons produced by an e/γ is nearly proportional to its kinetic energy. For massive particles,
the number of photons produced is also related to the energy, but not linearly. For any type ofparticle, the visible energy Evis is defined as the energy of an electron which would produce thesame number of Cherenkov photons. The number of collected photoelectrons depends on the scattering and attenuation in the water along with the photo-cathode coverage, quantum efficiency andthe optical parameters of any external light collection systems or protective material surrounding
them. Event-by-event corrections are made for geometry and attenuation. For a typical case, inwater Np.e. ∼ 15 ξ Evis (MeV), where ξ is the effective fractional photosensor coverage. Cherenkov
photoelectron yield per MeV of energy is relatively small compared to that for scintillator, e.g.,∼ 6 pe/MeV for Super-K with a PMT surface coverage of ∼ 40%. In spite of light yield andCherenkov threshold issues, the intrinsic directionality of Cherenkov light allows individual particle
tracks to be reconstructed. Vertex and direction fits are performed using PMT hit charges andtimes, requiring that the hit pattern be consistent with a Cherenkov ring.
High-energy (∼100 MeV or more) neutrinos from the atmosphere or beams interact with nucleons; for the nucleons bound inside the 16 O nucleus, nuclear effects must be considered both at theinteraction and as the particles leave the nucleus. Various event topologies, with final-state particles
contained, exiting, or entering the detector, can be distinguished by their timing and fit patterns,
and by presence or absence of light in a veto. At high energies, multi-photoelectron hits are likely
and the charge collected by each PMT (rather thanpthe number of PMTs firing) must be used; theenergy resolution in this case is approximately 2%/ ξ Evis (GeV). The absolute energy scale in thisregime can be known to ∼2–3% using cosmic-ray muon energy deposition, Michel electrons and π 0from atmospheric neutrino interactions. Typical vertex resolutions for GeV energies are a few tensof cm [53]. Angular resolution for determination of the direction of a charged particle track is a few
degrees. For a neutrino interaction, because some final-state particles are usually below Cherenkov
threshold, knowledge of direction of the incoming neutrino direction itself is generally worse thanthat of the lepton direction, and dependent on neutrino energy.
Multiple particles in an interaction (so long as they are above Cherenkov threshold) may bereconstructed, allowing for the exclusive reconstruction of final states. In searches for proton decay,
multiple particles can be kinematically reconstructed to form a decaying nucleon. High-quality
particle identification is also possible: γ rays and electrons shower, and electrons scatter, which
results in fuzzy rings, whereas muons, pions and protons make sharp rings. These patterns can bequantitatively separated with high reliability using maximum likelihood methods [54]. Sources ofbackground for high energy interactions include misidentified cosmic muons and anomalous lightpatterns when the PMTs sometimes “flash” and emit photons themselves. The latter class of
events can be removed using its distinctive PMT signal patterns, which may be repeated. Moreinformation about high energy event selection and reconstruction may be found in reference [55].
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1236. Particle Detectors for Non-Accelerator Physics

reconstructing low-energy events, down to e.g. ∼ 4-5 MeV for Super-K [56]. Low-energy neutrino
interactions of solar neutrinos in water are predominantly elastic scattering off atomic electrons;
single electron events
are then reconstructed. At solar neutrino energies, the visible energy resp
olution (∼ 30%/ ξ Evis (MeV)) is about 20% worse than photoelectron counting statistics would
imply. Using an electron LINAC and/or nuclear sources, approximately 0.5% determination of theabsolute energy scale has been achieved at solar neutrino energies. Angular resolution is limited
by multiple scattering in this energy regime (25–30◦ ). At these energies, radioactive backgrounds
become a dominant issue. These backgrounds include radon in the water itself or emanated fromdetector materials, and γ rays from the rock and detector materials. In the few to few tens of MeV
range, radioactive products of cosmic-ray-muon-induced spallation are troublesome, and are removed by proximity in time and space to preceding muons, at some cost in dead time. Gadolinium
doping using 0.2% Gd2 (SO4 )3 has now been initiated for Super-K to improve selection of low-energyν̄e and other events with accompanying neutrons [57].
The Sudbury Neutrino Observatory (SNO) detector [58] was the only instance of a large heavy
water detector. In addition to an outer 1.7 kton of light water, SNO contained 1 kton of D2 O,
giving it unique sensitivity to neutrino neutral current (νx + d → νx + p + n), and charged current(νe + d → p + p + e− ) deuteron breakup reactions. The neutrons were detected in three ways: via
capture on deuterons, via capture on dissolved 35 Cl, and via specialized 3 He counters.
36.3.1.3 Noble liquid detectors
Noble liquids scintillate and can be used as the active medium for particle detection. Detectors
employing argon and xenon are also used as time-projection chambers (TPCs), either as dual-phase
low-energy recoil detectors, or as track-imaging detectors. Noble-liquid detectors with low energy(few to few tens of keV) capability for detecting electronic and nuclear recoils are employed for dark
matter and other rare event searches and are described in Sec. 36.4. These detectors can also be
employed for some of the same physics (baryon number violation, astrophysical neutrino transient
searches, etc.) as for the other large detectors described here, especially as they approach tens of ton
scale and higher (e.g., DARWIN, DarkSide-20, ARGO). Track-imaging time-projection chambers,
which are described in detail Section 36.4, have a dynamic range reaching down to the few to few tensof MeV scale, enabling sensitivity to e.g., solar and supernova burst neutrinos. Surface LArTPCs
have significant cosmic backgrounds, but may still have sensitivity to astrophysical transients suchas supernova burst neutrinos. DUNE will be sufficiently deep to have sensitivity to steady-state
low-energy neutrino sources such as solar neutrinos.
36.3.2 Neutrino telescopes
Revised February 2024 by U.F. Katz (Erlangen U.) and C. Spiering (DESY, Zeuthen).
The primary goal of neutrino telescopes (NTs) is the detection of astrophysical neutrinos, inparticular those which are expected to accompany the production of high-energy cosmic rays inastrophysical accelerators. NTs in addition address a variety of other fundamental physics issues
like the indirect search for dark matter, studies of neutrino oscillations, searches for exotic particles
like magnetic monopoles or study of cosmic rays and their interactions [59–61]. Electromagnetic
radio frequency detectors for high energy neutrinos are discussed in "Radio emission from (ultra-)high energy particle showers" section 36.3.3.
NTs are large-volume arrays of “optical modules” (OMs) installed in open transparent media
like water or ice, at depths that completely block the daylight. The OMs are sensitive to individual
photons of the Cherenkov light induced by charged secondary particles produced in reactions ofhigh-energy neutrinos in or around the instrumented volume. The time of photon-induced signals(“hits”) is registered with a precision of a few nanoseconds. The neutrino energy, Eν , and direction
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36. Particle Detectors for Non-Accelerator Physics

13

log effective area (m²)

490° <  < 120°
(just below horizon)

32

150° <  < 180°
(upward)10-1

0° <  < 30°(downward)

-2
-3

2

3

4

5
6
log E (GeV)

7

8

9

Figure 36.5: Average over the effective νµ and ν µ areas for IceCube as an example of a cubickilometre NT, as a function of neutrino energy for three intervals of the zenith angle θ. The values
shown here correspond to a specific event selection for point source searches.

can be reconstructed from the hit pattern recorded. NTs typically target an energy range Eν &100 GeV; sensitivity to lower energies is achieved in dedicated setups with denser instrumentation.
In detecting cosmic neutrinos, three sources of backgrounds have to be considered: (i) atmospheric neutrinos from cosmic-ray interactions in the atmosphere, which can be separated fromcosmic neutrinos on a statistical basis, or, for down-going neutrinos, by vetoing accompanying
muons; (ii) down-going punch-through atmospheric muons from cosmic-ray interactions, which aresuppressed by several orders of magnitude with respect to the ground level due to the large detector depths and can be further reduced by selecting upward-going or high-energy neutrinos orby self-veto methods; (iii) random backgrounds due to photomultiplier (PMT) dark counts, 40 K
decays (mainly in sea water) or bioluminescence (only water). Atmospheric neutrinos and muons
allow for investigating neutrino oscillations and cosmic ray anisotropies, respectively. Since 2012,it has become obvious that a precise measurement of the energy-zenith-distribution of atmospheric
neutrinos in the energy range from a few to about 100 GeV may allow for determining the neutrinomass hierarchy by exploiting matter-induced oscillation effects in the Earth [62, 63].
Neutrinos can interact with target nucleons N through charged current (,ν-` N → `∓ X, CC)or neutral current (,ν-` N → ,ν-` X, NC) processes. A CC reaction of a ,ν-µ produces a muon track
and a hadronic particle cascade, whereas all NC reactions and CC reactions of ,ν-e produce particle
cascades only. CC interactions of ,ν-τ can have either signature, depending on the τ decay mode.
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36. Particle Detectors for Non-Accelerator Physics

Of particular interest is the so-called double-bang signature, where the τ decays sufficiently far
away from the primary interaction to create a second, distinguishable cascade (typically at PeV
energies and above). In most astrophysical models, neutrinos are expected to be produced throughthe π/K → µ → e decay chain, i.e., with a flavour ratio νe : νµ : ντ ≈ 1 : 2 : 0. For sources outside
the solar system, neutrino oscillations turn this ratio to νe : νµ : ντ ≈ 1 : 1 : 1 upon arrival on
Earth.
The total neutrino-nucleon cross section is about 10−34 cm2 at Eν = 20 TeV and rises roughly
linearly with Eν below this energy and as Eν0.3–0.5 above, flattening out towards high energies.
The CC:NC cross-section ratio is about 2:1. At energies above several TeV, neutrino absorption in
the Earth becomes noticeable; for vertically upward-moving neutrinos (zenith angle θ = 180◦ ), thesurvival probability is 74 (27, < 2)% for 10 (100, 1000) TeV. The energy transferred to the final-state
lepton varies between 0 and 100% of Eν , with a mean of 50% (65%) for neutrinos (antineutrinos)
at 100 GeV and 75% for both neutrinos and antineutrinos at 10 PeV.
The final-state lepton follows the initial (anti)neutrino direction with an average mismatchangle of about hφν` i ≈ 1◦ /(Eν /TeV)0.55 , with a steeper decrease beyond 10 TeV, reaching 0.005◦ at
1 PeV [64]. These values indicate the intrinsic kinematic limit to the angular resolution of NTs. For
CC ,ν-µ reactions at energies above about 10 TeV, the angular resolution is dominated by the muonreconstruction accuracy of a few times 0.1◦ at most. For muon energies Eµ & 1 TeV, the increasing
light emission due to radiative processes allows for reconstructing Eµ from the measured Cherenkov
light intensity with an accuracy of σ(log Eµ ) ≈ 0.3; at lower energies, Eµ can be estimated from thelength of the muon track if it is contained in the detector. These properties make CC ,ν-µ reactions
the prime channel for the identification of individual astrophysical neutrino sources.
Hadronic and electromagnetic particle cascades at the relevant energies are 5–20 m long, i.e.,
short compared to typical OM spacings. The total amount of Cherenkov light provides a direct
measurement of the cascade energy with an accuracy of about 20% at energies above 10 TeV and
10% beyond 100 TeV for events contained in the instrumented volume. Except for double-bang
events, the neutrino flavour and reaction mechanism can, however, be determined on a statisticalbasis at best, and neutrinos from NC reactions or τ decays may carry away significant “invisible”
energy. Above 100 TeV, the average directional reconstruction accuracy of cascades is better than
10 (2) degrees in polar ice (sea water), the difference being due to the inhomogeneity of the ice andstronger light scattering in ice. These features, together with the small background of atmospheric,ν-e and ,ν-τ events, makes the cascade channel particularly interesting for searches for a diffuse,high-energy excess of extraterrestrial over atmospheric neutrinos. Cascade events can also be used
to complement the muon channel in searches for point sources or transient signals, albeit withinferior angular accuracy compared to muon tracks.
The detection efficiency of a NT is quantified by its effective area, e.g., the fictitious area forwhich the full incoming neutrino flux would be recorded (see Figure 36.5). The increase with Eν is
due to the rise of neutrino cross section and muon range, while neutrino absorption in the Earthcauses the decrease at large θ for large Eν . Identification of downward-going neutrinos requires
stringent event selection to suppress atmospheric muons, hence the cut-off towards low Eν at low
θ. Due to the small cross section, the effective area is many orders of magnitude smaller than thegeometrical dimension of the detector; a ,ν-µ with 1 TeV can, e.g., be detected with a probability ofthe order 10−6 if the NT is on its path.
Due to the long muon range, CC interactions of up-going ,ν-µ can be detected from far outside theinstrumented volume. This method also works for horizontal neutrinos up to about 10◦ above thehorizon (depth dependent), where the background from atmospheric muons become prohibitive.
Alternatively, one can select events that start inside the instrumented volume and thus remove
incoming muons that generate early hits in the outer layers of the detector. Such a veto-based event
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15

selection is sensitive to neutrinos of all flavours from all directions, albeit with a reduced efficiency
since a part of the instrumented volume is sacrificed for the veto. Such a muon veto, or vetoing
events with a coincident signal in the surface array, also rejects down-going atmospheric neutrinos
that are accompanied by muons from the same air shower and thus reduces the atmosphericneutrino background. Actually, the breakthrough in detecting high-energy cosmic neutrinos was
first achieved with this technique.
Note that the fields of view of NTs at the South Pole and in the Northern hemisphere arecomplementary for each reaction channel and neutrino energy.
36.3.2.1 The Projects
Table 36.3 lists past, present and future neutrino telescope projects and their main parameters.
Table 36.3: Past, present and future NT projects and their main parameters. The milestone years give the times of project start, of first datataking with partial configurations, of detector completion, and of projecttermination. Projects with first data expected past 2024 are indicated initalics. The size refers to the largest instrumented volume reached during
the project development. The status of projects under construction is reported as of February 2024. See [61] for references to the different projects
where unspecified.
Experiment

Milestones

Location

Size Remarks(km3 )DUMAND
1978/–/–/1995
Pacific Ocean
Terminated due totechnical/funding problems
NT-2001980/1993/1998/2015 Lake Baikal
10−4 First proof of principle
GVD [65]
2012/2015/–/–Lake Baikal
ca. 1 High-energy ν astronomy
first 12 clusters installed
AMANDA
1990/1996/2000/2009 South Pole
0.015 First deep-ice NT
IceCube [66]2001/2005/2010/–
South Pole
1.0 First km3 -sized detector
IceCube-Gen2 [67, 68] 2014/–/–/–South Pole
5–10 Planned future extension of
IceCube covering low and highenergies, a surface array andradio detection
NESTOR
1991/–/–/–
Med. Sea
2004 data taking with prototype
NEMO
1998/–/–/–
Med. Sea
R&D project, prototype tests
ANTARES
1997/2006/2008/2022 Med. Sea
0.010 First deep-sea NT
KM3NeT/ARCA [63] 2013/2021/–/–
Med. Seaca. 1 High-energy configuration forneutrino astronomy.
Under construction,
data taking with 28 strings
KM3NeT/ORCA [63] 2014/2020/–/–Med. Sea
0.007 Low-energy configuration forneutrino mass hierarchy.
Under construction,
data taking with 18 strings
KM3NeT Phase 3
2013/–/–/–
Med. Seaca. 3 Possible future extension,
6 ARCA blocks + ORCA
P-ONE [69]2018/–/–/–
Pacific Ocean
O(1) Planned future NT, R&D phase
TRIDENT [70]2021/–/–/–South China Sea O(10) Planned future NT, R&D phase

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36. Particle Detectors for Non-Accelerator Physics

36.3.2.2 Properties of media
The efficiency and quality of event reconstruction depend strongly on the optical properties(absorption and scattering length, intrinsic optical activity) of the medium in the spectral rangeof bialkali photocathodes (300–550 nm). Large absorption lengths result in a better light collection, large scattering lengths in superior angular resolution. Deep-sea sites typically have effective
scattering lengths of > 100 m and, at their peak transparency around 450 nm, absorption lengthsof 50–65 m. The absorption length for Lake Baikal is 22–24 m. The properties of South Polar icevary strongly with depth; at the peak transparency wave length (400 nm), the scattering length is
between 5 and 75 m and the absorption length between 15 and 250 m, with the best values in thedepth region 2200–2450 m and the worst ones in the layer 1950–2100 m.
Noise rates measured by PMTs with a diameter of 25 cm in deep polar ice are about 0.5 kHz
per PMT and almost entirely due to radioactivity in the OM components. The corresponding rates
in sea water are typically 60 kHz, mostly due to 40 K decays. Bioluminescence activity can locallycause rates on the MHz scale for seconds; the frequency and intensity of such “bursts” depends
strongly on the sea current, the season, the geographic location, and the geometry of the detector
elements. Experience from ANTARES shows that these backgrounds are manageable without amajor loss of efficiency or experimental resolution.
36.3.2.3 Technical realisation
Optical modules (OMs) and PMTs: An OM is a pressure-tight glass sphere housing one or several
PMTs with a time resolution in the nanosecond range, and in most cases also electronics for control,
high-voltage generation, operation of calibration LEDs, time synchronisation and signal digitisation.
Hybrid PMTs with 37 cm diameter have been used for NT-200, conventional hemispheric PMTs
with 20 cm diameter for AMANDA and with 25 cm diameter for ANTARES, IceCube and BaikalGVD. A novel concept has been chosen for KM3NeT. Each OM (43 cm) is equipped with 31 PMTs
(7.5 cm), plus control, calibration and digitisation electronics. Advantages are that (i) the overall
photocathode area exceeds that of a 25 cm PMT by more than a factor of 3; (ii) the individual
readout of the PMTs results in a very good separation between one- and two-photoelectron signals
which is essential for online data filtering and random background suppression; (iii) the hit pattern
on an OM provides directional information; (iv) no mu-metal shielding against the Earth magnetic
field is required. Figure 36.6 shows the OM designs of IceCube and KM3NeT.
Readout and data filtering: In current NTs the PMT data are digitised in situ: for ANTARES andBaikal-GVD in special electronics containers close to the OMs, for IceCube and KM3NeT inside
the OMs. For IceCube, data are transmitted via electrical cables of up to 3.3 km length, dependingon the location of the strings and the depth of the OMs; for ANTARES, KM3NeT and Baikal-GVD
optical fibre connections have been chosen (several 10 km for the first two and 4 km for GVD).
The full digitised waveforms of the IceCube OMs are transmitted to the surface for pulses
appearing in local coincidences on a string; for other pulses, only time and charge information isprovided. For ANTARES (time and charge) and KM3NeT (time and time over threshold), all PMT
signals above an adjustable threshold are sent to shore.
The data are subsequently processed on online computer farms, where multiplicity- and topologydriven filter algorithms are applied to select event candidates. The filter output data rate is about
10 GByte/day for ANTARES and of the order 1 TByte/day for IceCube (100 GByte/day transferred
via satellite) and KM3NeT.
Calibration: For efficient event recognition and reconstruction, the OM timing must be synchronised
at the few-nanosecond level and the OM positions and orientations must be known to a few 10 cm
and a few degrees, respectively. Time calibration is achieved by sending synchronisation signals to
the OM electronics and also by light calibration signals emitted in situ at known times by LED
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36. Particle Detectors for Non-Accelerator Physics

Figure 36.6: Schematic views of the digital OMs of IceCube (left) and KM3NeT (right).
or laser flashers (ANTARES, KM3NeT). Precise position calibration is achieved by measuring thetravel time of light calibration signals sent from OM to OM (IceCube) or acoustic signals sent fromtransducers at the sea floor to receivers on the detector strings (ANTARES, KM3NeT, BaikalGVD). Absolute pointing and angular resolution can be determined by measuring the “shadow of
the moon” (i.e., the directional depletion of muons generated in cosmic-ray interactions). IceCube
and ANTARES have both shown that they have angular resolution below 1◦ , confirming MC calculations which indicate a precision of ≈ 0.5◦ for energies above 10 TeV. For KM3NeT, simulations
indicate that sub-degree precision in the absolute pointing can be reached within a few weeks ofoperation.
Detector configurations: IceCube [66] (see Figure 36.7) consists of 5160 Digital OMs (DOMs) installed on 86 strings at depths of 1450 to 2450 m in the Antarctic ice; except for the DeepCore
region, string distances are 125 m and vertical distances between OMs 17 m. 324 further DOMs are
installed in IceTop, an array of detector stations on the ice surface above the strings. DeepCore isa high-density sub-array at large depths (i.e., in the best ice layer) at the centre of IceCube.
The NT200 detector in Lake Baikal at a depth of 1100 m consisted of 8 strings attached to anumbrella-like frame, with 12 pairs of OMs per string. The diameter of the instrumented volume
was 42 m, its height 70 m. Meanwhile (February 2024), the Baikal collaboration has installed thefirst 12 clusters of a future cubic-kilometre array, GVD [65]. It will consist of 18 clusters, each with
288 OMs at 8 strings, plus several single strimgs between the clusters; its completion is scheduled
for 2027.
The operation of ANTARES (see [61] and references therein) was terminated in 2022. It comprised 12 strings with lateral distances of 60–70 m, each carrying 25 triplets of OMs at verticaldistances of 14.5 m, located at depths of 2.1–2.4 km, starting 100 m above the sea floor. An additional string held devices for calibration and environmental monitoring. A system to investigate
the feasibility of acoustic neutrino detection was also implemented.
KM3NeT will consist of building blocks of 115 strings each, with 18 OMs per string. Operationof the first strings deployed has successfully verified the KM3NeT technology and sensitivity. In theupcoming phase of its staged implementation, KM3NeT aims at two building blocks for neutrino
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18

50 m

IceCubeLab

1450 m2450 m324 m
Eiffel Tower

Figure 36.7: Schematic view of the IceCube neutrino observatory comprising the deep-ice detector
including its nested dense part DeepCore, and the surface air shower array IceTop. The IceCube
Lab houses data acquisition electronics and the computer farm for online processing. Operation of
AMANDA was terminated in 2009.
astronomy, with vertical distances between OMs of 36 m and a lateral distance between adjacentstrings of 90 m (ARCA, for Astroparticle Research with Cosmics in the Abyss) and at one block forthe measurement of the neutrino mass hierarchy, with vertical distances between OMs of 9 m anda lateral distance between adjacent strings of about 20 m (ORCA, for Oscillation Research with
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36. Particle Detectors for Non-Accelerator Physics

Cosmics in the Abyss) [63]. The installation of ARCA near Capo Passero, East of Sicily (depth
3440 m) and of ORCA near Toulon (depth 2450 m) is ongoing and as of now (February 2024) 28/18
strings have been deployed for ARCA/ORCA and are continuously operated. Completion of thefull ARCA and ORCA arrays is planned for 2028.
P-ONE (Pacific Ocean Neutrino Experiment) [69] is a new initiative in its R&D phase, envisaging a large NT in the Pacific Ocean off the Canadian coast. It is intended to use an existing
deep-sea cable infrastructure and to optimise the sensitivity for horizontal neutrinos with energies
of about 100 TeV and beyond.
TRIDENT (Tropical Deep-Sea Neutrino Telescope) is a proposed future neutrino telescope in
the South China Sea with an instrumented volume of 7.5 km3 . R&D and site exploration efforts are
ongoing and a pathfinder (TRIDENT Phase-1) is planned to be commissioned around 2026 [71].
36.3.2.4 Results
See Sect. 30.4 for a summary of the results from neutrino telescopes.
36.3.2.5 Plans beyond 2023
Within the future IceCube-Gen2 project, it is planned to extend the sensitivity of IceCube
towards both higher and lower energies. To increase the detector sensitivity at high energies, alarge-volume extension is envisaged, combined with a radio array for highest-energy neutrinos anda surface array providing also a powerful veto against atmospheric events [67, 68]. A substantiallydenser instrumentation of a sub-volume of DeepCore will be achieved with 7 closely spaced strings
to be deployed in 2025/26 (the IceCube Upgrade), aiming to cover a low-energy program, to better
calibrate the existing IceCube detector and the archival data, and to test new technologies. Moreinformation on the future extensions of GVD and KM3NeT and on P-ONE and TRIDENT aregiven above and in Table 36.3.
36.3.3 Radio emission from (ultra-)high energy particle showers
Revised August 2023 by S.R. Klein (NSD LBNL; UC Berkeley) and A. Nelles (DESY, Zeuthen;Erlangen U.).
Coherent radio-frequency (RF) electromagnetic radiation is an attractive signature to searchfor particle cascades produced by interactions of high-energy particles. RF signatures have beenused to study both cosmic-ray air showers and to search for neutrino-induced showers. This article
will discuss the radio signal generation, the relevant energy regime and the application in detectors.
Air showers are discussed in more detail in two recent reviews [72,73]. At lower energies, incoherent
optical Cherenkov radiation is frequently used, as discussed in "Neutrino telescopes" section 36.3.2.
This article uses the general definitions and properties of neutrino telescopes as described in 36.3.2.
36.3.3.1 Signal generation and its characteristics
As discussed in the "Passage of Particles Through Matter" review Sec. 34 the electromagnetic
component of a high energy shower gives rise to radio emission. For the signal generation itself thetype of primary particle is irrelevant. However, the signal medium is important. It must be nonconducting (and non-absorptive at RF frequencies). The different shower length scales (radiation
length, X0 ) between air and the solid materials used for neutrino searches leads to surprisingly
large differences in shower development. Due to the interaction with the medium the shower
contains more electrons than positrons, which results in a net charge excess and leads to coherent
Cherenkov emission, also known as Askaryan effect [74–76]. In air, during propagation throughthe geomagnetic field, the relative motion of electrons and positrons is affected, which leads to avarying transverse current, usually referred to as the geomagnetic effect [72]. Both effects may
be described more generally as being due to radiation from a time-varying net charge [77]. Theirrelative importance is governed by the density of the medium, as the electric field scales as the
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36. Particle Detectors for Non-Accelerator Physics

net charge excess, and a lower density allows the geomagnetic effect to gain importance. Thus, insolid materials the emission from the Askaryan effect dominates, but it is a < 20% correction in
air showers.
Coherent radiation is possible at wavelengths longer than the instantaneous thickness of theshower along an observer’s line of sight. Since air showers have a larger extent than showers insolid media, their coherent radiation appears at lower frequencies [78].
High-frequency radiation is concentrated around the Cherenkov angle θC . Viewed directly on
the Cherenkov cone, the electric field strength, Ch at a frequency f from an electromagnetic showerfrom a νe with energy Eν in ice may be roughly parameterized as [79, 80]
Ch (V /mMHz) = 2.53 × 10−7

Eν f
1
.
1TeV fc 1 + (f /fc )1.44


(36.3)The electric field strength increases linearly with frequency, up to a cut-off frequency fc , which
is set by the transverse size of the shower [81, 82]. The maximum wavelength c/fc is roughly the
Moliere radius divided by cos(θC ) where θC is the Cherenkov angle. The cutoff frequencies dependon the density (which affects the Moliere radius). They are about 1 GHz in ice, about 3 GHz in
the lunar regolith, and below 100 MHz in air.
Near fc , radiation is narrowly concentrated around θC [81,82]. At lower frequencies, the limited
length of the emitting region leads to a broadening in emission angle around the Cherenkov cone.
Away from θC , the electric field from Eq. (36.3) is reduced by [79],!

1(θ − θC )2= exp −.
Ch
2 (2.2◦ × [1GHz/f ])2


(36.4)

The angular distribution of the signal around θC can be parameterized by a Gaussian peak modulated by a sin θ. In both ice and the lunar regolith, θC is about 56◦ , in air only 1◦ . Close to θC ,the 1 GHz maximum frequency in ice/regolith leads to a generated pulse width of ≈ 1 nsec.
These equations are appropriate for ice. More general parameterizations can be found in [80,83].
More accurate calculations of the predicted radio signal, in particular air showers are not easily
parameterized, but require detailed Monte Carlo simulations. For air showers, these are built on
microscopic air shower simulations, calculate the emission from all individual particles in the shower
development and add them for different observer positions [72]. For neutrinos, most approaches
calculate (directly or from a parameterization) the Askaryan signal from a shower profile. The
signal is then propagated through the medium and into an antenna model [80].
At energies above 1016 eV in ice, the Landau-Pomeranchuk-Migdal effect lengthens electromagnetic showers, by reducing the cross-sections for bremsstrahlung and pair production [84]. The
lengthening of the shower leads to a narrowing of the radio emission around the Cherenkov cone,
and a reduction in high-frequency emission away from the cone [80]. At higher energies, this leads
to two separate components of the Askaryan radiation from a neutrino interaction: an un-alteredcomponent from the hadronic portion of the shower and an angularly narrowed component from
1/3
the LPM-lengthened electromagnetic shower. The width of the narrowed component scales as Eν .
If these two components can be observed separately, they could, in principle, be combined to determine the inelasticity of the neutrino interaction [85], allowing for improved measurements of low−x
parton distributions and searches for beyond-standard-model interactions.
Similarly, energetic outgoing µ± and τ ± from neutrino interactions will dominantly lose energy
via stochastic pair production and photonuclear interactions. These secondary particles will produce electromagnetic showers that can be detected by radio detectors, if they are above threshold
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36. Particle Detectors for Non-Accelerator Physics

energy. This will enable multiple detections of the same particle track and thus present interesting
reconstruction opportunities [86].
At still higher energies, above 1020 eV, the LPM effect strengthens, and the electromagnetic
shower splits into multiple subshowers with significant separation. When these separations becomelarge enough, the subshowers will effectively become independent radiators, with the total emission
showing substantial event-by-event variation, depending on the division into subshowers [84]. Because of this, many experiments that study higher energy (well above 1020 eV) neutrinos focus on
the hadronic shower from the struck nucleus. This contains an average of only about 20% of theenergy, but with fewer large fluctuations.
36.3.3.2 Energy regime of radio detectors
The electric field amplitude is linearly proportional to the shower energy. Since the signal isa radio wave, the field amplitude decreases as 1/R, plus potential absorption in the intervening
medium, while the energy fluence decreases as 1/R2 , again, plus potential absorption. The detection
threshold depends on the distance to the antenna and the bandwidth and noise characteristics ofthe antenna and detector. For an antenna located in the detection medium, at a distance of 1 km
the typical threshold is around 1017 eV. For stand-off (remote sensing) detectors, the threshold rises
roughly linearly with the distance. These thresholds can be reduced by using directional antennas
and/or combining the signals from multiple antennas using beam-forming techniques.
RF detectors are used to search for energetic neutrinos from three types of sources: astrophysicalobjects (i.e. extending measurements of the neutrino energy spectrum observed at TeV to PeV
energies upward in energy), cosmogenic neutrinos associated with cosmic-ray-cosmic microwave
background radiation (CMBR) interactions, and neutrinos from beyond-standard-model physics.
These types are very roughly associated with energies below 1018 eV, the energy range 1018 to 1020
eV, and above 1020 eV.
Cosmogenic neutrinos are produced when ultra-high energy (UHE) protons with energy E > 5×
1019 eV interact with photons from the CMBR, infrared light from old stars, and other extragalactic
background light. These protons are excited to a ∆+ resonance which may decay via ∆+ → nπ + ,leading to neutrinos with energies above 1018 eV [87, 88]. The cosmogenic neutrino signal depends
heavily on the fraction of UHE cosmic-rays that are protons. For a 100% proton composition(disfavored by most data [89]), observing a cosmogenic neutrino signal of at least a few events per
year requires a solid or liquid detector with an active volume of about 100 km3 .
To reach the effective volumes necessary to observe the expected low fluxes of UHE neutrinos,
common, naturally occurring, non-conducting solid (or potentially liquid) media, with a long absorption length for radio waves are needed. Optical Cherenkov and acoustical detectors are limitedby short (< 100 m) attenuation lengths [90] so would require a prohibitively expensive number of
sensors. The radio detection technique has been used to detect air showers, targeting neutrinos aswell as cosmic rays, and to search for neutrino showers in ice, salt domes and the lunar regolith.
36.3.3.3 Reconstruction of particle energy and direction and background suppression
Since radio detectors view the interaction from afar, the reconstruction techniques differ fromoptical neutrino telescopes.
Radio detection is a calorimetric measurement, thus provides good energy estimates of theshower energy. The energy fluence (integrated pulse power) of the signal scales quadratically withshower energy. It also depends on the distance to the shower, through potential attenuation losses
and the usual 1/R loss in electric field amplitude. The arrival times in antennas and a sphericalwave approximation can be used to determine the interaction vertex, although some uncertainty
due to the viewing angle with respect to the Cherenkov angle may remain, if not corrected forby using the frequency information. If the radio signal travels through media where the index of
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36. Particle Detectors for Non-Accelerator Physics

refraction varies (like the firn of glacial ice), then ray-tracing techniques may be required to follow
the signal back to the interaction point. For buried antennas, the bending of the signal trajectories
due to the index of refraction creates an opportunity. For some geometries, there may be twopaths to the detector: a ‘direct’ path, with minor bending, and a second where the signal is bent
beyond horizontal, bouncing off the surface before reaching the antenna. By measuring the timedifference between the two paths, the distance to the interaction vertex may be determined; thisgreatly simplifies the energy determination [91, 92]. For most neutrino interactions (except for νecharged-current interactions), the shower energy is less than the neutrino energy. The uncertainty
on the interaction inelasticity is a major contributor to the uncertainty in the neutrino energy,
along with uncertainties on the distance between the antenna(s) and the interaction vertex [93].
Reconstruction of the neutrino arrival direction depends on several aspects of the signal. First,
the direction from the antenna to the interaction site must be determined. This can be done byusing the relative timing from separated antennas, or using beam-forming techniques with multielement arrays. For air showers, the signal arrival direction is (almost) equal to the particle arrival
direction, with corrections being obtainable by fitting a hyperbolic wavefront [72, 73].
For showers in solid/liquid media, the arrival direction with respect to the interaction point antenna vector is determined from two additional angles. The frequency spectrum can be used todetermine the angle between signal arrival direction and Cherenkov cone according to Eq. (36.4) [94].
The second angle can be determined from the polarization of the signal. The radio signal is producedwith a linear polarization in the plane containing both the particle direction and the radio wave
direction. These two angles can be combined to determine the direction, subject to a (usually) fourfold ambiguity, due to uncertainty as to whether the antenna is inside or outside the Cherenkov
cone, and because the particle direction can be flipped 180◦ without affecting the observed signal.
Often, some of these solutions can be rejected because they correspond to long path lengths throughthe Moon or the Earth, where the neutrino would be absorbed.
Spectral information is crucial for the reconstruction and background rejection. However, large
bandwidth antennas typically disperse (i.e. broaden) the pulses. As long as the dispersion can becompensated for and backgrounds controlled, a large bandwidth detector is the most sensitive.
All radio experiments must contend with background. Common sources are anthropogenic
noise, antenna/preamp noise, charge generated by blowing snow, lightning, and, at low frequencies,
radiation from the Milky Way. While narrowband noise impacts triggering and contaminates signal
quality, impulsive backgrounds could mimic a signal. One of the major issues for radio-detection
experiments is anthropogenic noise. Most anthropogenic noise has distinctive characteristics (suchas being narrow-band, and coming from near the horizon) which makes it relatively easy to reject
during data analysis, via narrow-band filters and other techniques [72]. However, these factors
complicate triggering and reduce data purity. This is even an issue in Antarctica, where communication radios and passing satellites can mimic showers, at least at the trigger level. The needto limit anthropogenic noise has led most experimental groups to select remote locations for theirdetectors. Still, experiments have used approaches to reduce trigger-level noise, and/or to rejectbackground at the analysis level. For example, for multi-element arrays, the threshold drops as thesquare root of number of antennas, since the signal adds in-phase while the backgrounds add withrandom phases [95]. It seems, however, that triboelectric signals recorded in periods with winds
exceeding 10 m/s are a problematic trigger-level background, for experiments in polar regions [96]and will require removal during analysis.
Most dedicated air shower experiments have used radio antennas in combination with at least
one other detector technology, such as scintillation counters, if the site quality is not sufficient
and/or computing power on autonomous stations is limited. One exception is ARIANNA, which
is located in an uninhabited part of Antarctica, enabling them to efficiently self-trigger on air
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36. Particle Detectors for Non-Accelerator Physics

showers [97].
Lunar experiments (discussed below) use different techniques to reduce the anthropogenic background. Some experiments use multiple antennas, separated by at least hundreds of meters; byrequiring a coincidence within a small time window, anthropogenic noise can be rejected. With
good enough timing, beam-forming techniques can be used to further reduce the background. An
alternative approach is to use beam forming with multiple feed antennas viewing a single reflector,
to ensure that the signal points back to the moon.
Due to the similarity in the radio emission of air showers and neutrino showers, the more abundant cosmic rays can act as background to neutrino searches. In-ice detectors need to suppress
in-air emission that is refracted into the ice, emission that is created from developing air showers continuing in the ice, as well as from stochastic energy-losses of atmospheric muons. Lunar
experiments may faces challenges in separating neutrino interactions from cosmic ray interactions.
36.3.3.4 Recent experiments
Figure 36.8 shows some current limits from neutrino searches, including from prototype arrays.
Except for LOFAR, which is fully operational, projected limits from future experiments are notshown in the figure.
i. IceThe most common dense medium transparent to radio waves is ice. Natural ice is an attractive
medium for neutrino detection with radio attenuation lengths from over 300 m to 1 km [108]. The
attenuation length varies with frequency and ice temperature, with higher attenuation in warmer
ice. Although glacial ice is mostly uniform, the top ≈ 100 m of ice, the ’firn,’ exhibits a gradualtransition from packed snow at the surface (typically ρ = 0.35 g/cm3 ) to solid ice (ρ = 0.92 g/cm3 )below [109]. The thickness of the firn varies with location; it is thicker in central Antarctica thanin the coastal ice sheets or in Greenland. The varying density has several implications.
The index of refraction depends linearly on the density, so radio waves curve downward in thefirn. This bending reduces the effective volume of surface or aerial antennas. A surface antenna
cannot see near-surface interactions at large horizontal distances. There are also indications thatthe increase in firn density is non-monotonic [110, 111]. This leads to non-monotonic changes inindex of refraction which may create waveguides that trap a small fraction of the radio energy andpropagate it horizontally.
In one type of experiment, antennas mounted on high-altitude balloons observe the ice fromabove. Radio signals from in-ice neutrino interactions propagate to the surface, traverse the ice-air
interface, and then travel to the balloon. The surface roughness of the ice can affect signals as
they transition from the ice to the atmosphere. The best known example, ANITA, has made four
flights around Antarctica, floating at an altitude around 35 km [112]. Its 32/40/48 (depending on
the flight) dual-polarization horn antennas scanned the surrounding ice, out to the horizon (650km away). Because of the small angle of incidence, ANITA could use polarization information to
separate signals from background; ν signals should be vertically polarized, while most backgroundfrom cosmic-ray air showers should be horizontally polarized.
As with all radio-detection experiments, ANITA had to contend with anthropogenic backgrounds. The ANITA collaboration uses their multiple antennas as a phased array to achieve good
pointing accuracy, with a resolution of 0.2-0.4◦ in elevation, and 0.5-1.1◦ in azimuth. They rejected
all events that pointed toward known or suspected areas of human activity. ANITA has set the moststringent flux limits yet on neutrinos with energies above 1020 eV [102]. The ANITA experiment has
also reported several anomalous events, matching cosmic ray signals, but with unexpected polarization signature, which the collaboration has indicated might be from Earth-skimming ντ [113, 114].
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36. Particle Detectors for Non-Accelerator Physics

E 2 (all-flavor)[GeV cm 2 s 1 sr 1]

24

10 5

ARIANNA

10 6

ARA ANITA

10 7

Auger
IceCubeNuMoonLUNASKA/
ParkesLOFAR 200h

10 8
10 9
10 10

107

Allowed region of cosmogenic neutrinos

108

109 1010 1011 1012 1013 1014 1015

Neutrino energy [GeV]

Figure 36.8: Representative 3-flavor (summed, assuming equal fluxes of each flavor) differential(over one decade in energy) limits from different experiments and prototype experiments. Shown arelimits from the IceCube ultra-high energy ν search [98], the Auger search for earth-skimming ντ [99],the LUNASKA/Parkes [100] and NuMoon lunar searches [101], the ANITA balloon experiment[102], ARA [103] and ARIANNA prototypes [104], along with projections for the LOFAR array[105]. The dashed blue line is the extrapolation of the IceCube through-going νµ flux measured atlower energies (few 10s of TeV to 10 PeV), with spectral index α = −2.37 [106]. Because of the longextrapolation, this should only be treated as a rough reference. The ARA and ARIANNA limits
are from prototype arrays, and indicate the energy range that might be covered, with far higher
sensitivity by larger arrays. The shaded area is the allowed region for cosmogenic neutrinos, froma recent global analysis that included the measured cosmic-ray spectrum and composition [107].

However, this interpretation is controversial.
Because of the significant source-detector separation, ANITA is most sensitive at energies above
1019 eV. A lower energy threshold requires a smaller antenna-target separation.
Other ice based experiments use antennas located within the active volume, allowing themto reach thresholds around 1017 eV, or lower with phased array antennas. This approach waspioneered by the RICE experiment [115] which buried 18 half-wave dipole antennas in holes drilled
for AMANDA at the South Pole, at depths from 100 to 300 m. The hardware was sensitive from
200 MHz to 1 GHz. Each antenna fed an in-situ preamplifier which transmitted the signals tosurface digitizing electronics.
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36. Particle Detectors for Non-Accelerator Physics

More recently, two groups have deployed prototype arrays which have explored different detector
concepts. The Askaryan Radio Array (ARA) deployed surface and buried antennas at the South
Pole [116], while the Antarctic Ross Iceshelf Antenna Neutrino Array (ARIANNA) installed surface
antennas on the Ross Ice Shelf [104], about 110 km north of McMurdo station. ARIANNA offered
the possibility of detecting downward-going ν, from the radio waves reflected off the ice-sea waterinterface on the bottom of the Ross Ice Shelf, while ARA took advantage of the colder, deeper ice at
the South Pole, with its longer radio attenuation length. ARA buried antennas up to 200 m deep tobe able to observe a larger portion of ice, due to the refraction of the signal in the firn. In contrast,
ARIANNA deployed antennas just below the surface, allowing them to use high-gain, but large log
periodic dipole antennas. Recently, phased-array trigger techniques have been demonstrated thatcan reduce the energy threshold by a factor of several [95, 117].
Both experiments use stations which operate independently, spaced far enough to maximize
sensitivity, but where only a small fraction of neutrino events will be visible in multiple stations.
Each station includes multiple antennas, which will be sensitive to both horizontal and verticalpolarization. The expected angular resolution is a few degrees [94].
In 2021, the Radio Neutrino Observatory Greenland (RNO-G) started deploying stations at
Summit Station. RNO-G is planned to consist of 35 stations, which employ ARIANNA-stylesurface antennas, an ARA-style phased array and deep antennas, and draw heavily on ANITA’s
electronics heritage [118].
ii. The Moon
Because of its large size and non-conducting regolith, and the availability of large radio-telescopes,
the Moon is an attractive target [119]. Conventional radio-telescopes are quite well suited to lunar
neutrino searches, with natural beam widths not too dissimilar from the size of the Moon. Still,there are experimental challenges. The attenuation length is typically estimated to be 9m/f(GHz)[120], so only near-surface interactions can be studied. The composition of the lunar regolith is
not well known, and there are significant uncertainties due to this uncertainty. One big limitationof lunar experiments is that the 385,000 km target-antenna separation leads to energy thresholds
above 1020 eV.
The effective volume probed by experiments depends on the geometry, which itself depends onthe frequency range used. At high frequencies f , the electric field strength is high, leading to alower energy threshold, but the sensitive volume is limited because the Cherenkov cone only points
toward the Earth for a narrow range of geometries. Lower frequency radiation is more isotropic,
so the effective volume is larger, but, because the electric field is weaker, the energy thresholdis higher. The 1/f dependence of the attenuation length in the lunar regolith further increases
the effective volume at low frequencies. The frequency range affects the energy dependence of thesensitivity. As can be seen in Fig. 36.8, a low-frequency experiment like NuMoon (which covered
115-180 MHz) has good sensitivity, but only above about 1014 GeV, while Lunaska/Parkes, whichobserved in the range 1200-1500 MHz, has a higher flux limit, but is sensitive above about 1012.5
GeV.
Current limits and projected sensitivities are sensitive to many details. A recent review [121]compared different radio-detection experiments using a common framework, and found some significant shifts in sensitivities due to, e.g. different assumptions about lunar composition and inelasticities.
With modern technology, it is increasingly viable to search over very broad frequency ranges[122]. One technical challenge is due to dispersion (frequency dependent time delays) in the ionosphere. Dispersion can be largely removed with a de-dispersion filter, using either analog circuitry
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36. Particle Detectors for Non-Accelerator Physics

or post-collection digital processing.
iii. AirRadio detection in air is sensitive to all particles inducing air showers. Radio-detection can be used
to determine the energy of cosmic rays, as done by e.g. the Auger and Tunka-Rex experiments[123, 124]. Radio signals can also be used to infer the altitude for shower-maximum, where theshower contains the most particles, as done by e.g. the LOFAR and Tunka-Rex collaborations[124, 125]. This altitude is sensitive to the cosmic-ray composition. Reconstructing the particle
arrival direction is much easier in air since the Cherenkov angle, and thus the radio wavefront,aligns with the axis to 1◦ . Radio-detection is also useful for energy cross-calibrations between
different experiments and may be able to provide an independent energy scale calibration for air
shower arrays [126].
One variation on the radio-detection approach is to look for radio emission from Earth-skimmingντ [127, 128]. Although ντ are much less commonly produced than νµ and νe , over astrophysicaldistances, oscillations lead to a νe : νµ : ντ ratio near 1 : 1 : 1, for almost all non-exotic accelerationand propagation mechanisms.
If the ντ traverse the Earth and interact while traveling upward, the resulting τ ± may exit
the Earth before decaying. 83% of the time, the decay produces a hadronic or electromagnetic
shower in the atmosphere [129]. Experiments have searched for these upgoing showers, and for the
resulting optical Cherenkov and coherent RF radiation. The threshold energy dependence for thesesearches depends on several factors, notably including the average τ ± decay length, which increases
linearly with energy. For somewhat lower energy ντ , with shorter decay lengths, a mountain may
serve as a target, with τ ± expected to emerge from the other side. The Pierre Auger observatory
sets stringent limits on this neutrino flux at energies above 1017 eV [99].
36.3.3.5 Future experiments
Looking ahead, RNO-G [118] will continue deployment until 2026 and reach the largest yearlysensitivity of any radio array thus far. Further out, the proposed IceCube-Gen2 expansion includes
a substantial radio array component [68], which will be sensitive to both neutrinos from the ice andair showers from cosmic rays. PUEO, a successor of ANITA has been funded and is scheduled for
its first flight in 2025 [130].
In the near future, several large radio detector arrays should reach significantly lower energies
for lunar neutrino detection. The LOFAR array is taking data with 36 detector clusters spreadover Northern Europe [105]. In the longer term, the Square Kilometer Array (SKA) with its 1 km2
effective area will push thresholds down to near 1020 eV [122]. The SKA will also study air showers.
A number of dedicated prototype ντ radio-detection experiments exist. The GRAND Collaboration, e.g., is proposing to deploy a large array of simple autonomous radio stations optimized fornear-horizontal signals [131]. The Pierre Auger Collaboration is currently upgrading their surface
array to include radio antennas at all water Cherenkov detectors. This array will improve thesensitivity of the instrument to horizontal showers, both for composition sensitivity for cosmic rays
and for the detection of Earth skimming neutrinos [132].

36.4 Large time-projection chambers for rare event detection
Written August 2023 by L. Baudis (Zurich U.).
Initially developed for particle physics experiments at accelerators (see Section 35.6.5), theconcept of a Time Projection Chamber (TPC) [133] evolved and is now tailored to a large range
of applications, most notably to experiments in astroparticle physics searching for rare interactions
deep underground. Present and proposed experiments designed to observe interactions of dark
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36. Particle Detectors for Non-Accelerator Physics

matter and other exotic particles, second order weak nuclear decays, as well as neutrinos from avariety of sources are based on TPCs operated with pure noble elements, either in gaseous or liquidform. The detectors aim for large target masses, low energy thresholds (keV-scale) and ultra-low
levels of backgrounds from radioactivity and cosmic rays. Historical introductions, reviews of theoperation principles, instrumentation and technological aspects, as well as applications, are foundin Refs. [134–138].
The two most common noble fluids employed as detector media in present TPCs for rare event
searches are argon and xenon, either as a high-pressure gas, or in liquid phase, or in liquid andgas phase (dual-phase). An interaction within the active volume of a TPC will create ionisation electrons and scintillation photons. The prompt scintillation signal is detected with one or
two arrays of photosensors, while the ionisation electrons, which drift in the pure medium under
the influence of an external, uniform electric field, are observed either directly with charge sensors, or via proportional scintillation, or electroluminescence. Since transverse electron diffusion
is small, albeit non-negligible, the ionisation signal provides the x − y information of the interaction site, with the z−information coming from the drift time measurement. The TPC thus yields athree-dimensional event localisation, enabling fiducial volume selections and differentiation between
single- and multiple-scatters in the active volume. These features are crucial for filtering out rare
events from the much higher backgrounds due to residual radioactivity of detector components.
The specific energy loss (dE/dx) depends on the primary interacting particle, and leads to a distinct ratio of scintillation to ionisation for gammas/electrons, alphas and fast neutrons, allowing
for further background discrimination. Two examples for the operation principle of TPCs for rare
event detection are shown in Fig. 36.9.
The properties of liquid argon as detection medium in a TPC are reviewed in Section 35.7.
Here some properties for xenon are given in Tab. 36.4. In the liquid phase, the high xenon density
allows for compact, large and homogeneous detector geometries with efficient self-shielding against
external radiation, given that the cross sections for the photoelectric effect, Compton scattering
7/2
and pair production scale as Z 5 /Eγ , Z/Eγ and Z 2 ln (2Eγ ), respectively, for incoming X-rays andgammas with energy Eγ . The radioactive isotopes 124 Xe, 126 Xe, 134 Xe and 136 Xe have very long
half-lives, and their second-order weak decay modes are subject to investigation (see Section 36.4.2).
In an argon or xenon detector, the energy loss of an incident particle is shared between ionisation,
excitation and sub-excitation electrons liberated in the ionisation process. The average energy
loss in ionisation is slightly larger than the ionisation potential, as it includes multiple ionisation
processes. In their condensed phases, argon and xenon exhibit a band structure of electronic states,with the band gap energy being around 14.3 eV and 9.3 eV for liquid argon and xenon, respectively.
In gaseous xenon, the ionisation potential is about 12.1 eV. Scintillation arises from excited atoms(excitons) and from ions, via recombination, with the emitted vacuum-ultraviolet (VUV) photons
in single bands centred at λ = 128 nm for argon and λ = 175 nm for xenon. The excited dimers (orexcimers), at their lowest excited level, de-excite to the dissociative ground state by the emissionof a single VUV photon. This comes from the large energy gap between the lowest excitationand the ground levels, forbidding other decay channels such as non-radiative transitions. The
scintillation light has two decay components due to de-excitation of spin singlet and spin triplet
states of the excited dimers to the ground state. The singlet and triplet states refer to the total
spin quantum number (s=0 or s=1) of the excited Rydberg electron and the angular momentum
due to the molecular orbit, with the shorter and longer decay shapes being produced by the deexcitation of s=0 states and s=1 states, respectively. The different pulse decay time for different
type of particle interactions, and thus ionisation densities, is employed to discriminate electronic
from nuclear recoils in dark matter detectors. While this is very effective in liquid argon, due to
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36. Particle Detectors for Non-Accelerator Physics

28
HV

HV

E⃗

time

HV

S2

gas
e-

eee-

E⃗

e–
e– e–

HV

high-pressure gas

liquiddrift time
(depth)S1drift time

time
S1

S2

Figure 36.9: Operation principle of TPCs for rare event detection: a high-pressure gas (left) and adual (liquid/gas) phase (right) TPC. A particle interaction yields a prompt scintillation signal (S1)and a delayed, amplified proportional scintillation signal (S2). The latter is caused by ionisationelectrons drifted in a homogeneous electric field (and, for dual phase TPCs, extracted into the gas
phase above the liquid). In these examples, both S1 and S2 signals are observed with photosensor
arrays placed behind the anode and cathode (left), or at the top and bottom (right) of the TPC.
The position information along the drift field is inferred from the time difference between the twolight signals, while the position in the transverse plane to the field is derived from the S2 lightpattern in the photosensor plane close to the electroluminiscent light production site.
the large time separation of (6 ± 1) ns versus (1500 ± 100) ns, it is difficult to put in practice forxenon [139], considering the much smaller separation, (2.2 ± 0.3) ns versus (27 ± 1) ns, of the twocomponents.
The recombination-independent, mean energy required to produce a single detectable quantum(photon or electrons), called the W-value, assumes that each recombining electron-ion pair produces
an exciton, which leads to a photon. In liquid argon, the W-value is (19.5 ± 1.0) eV [148]. In liquid
xenon, the widely-adopted W-value is (13.7±0.2) eV, measured with 122 keV γ-rays from an external
57 Co source [149]. Recently, a lower value of 11.5+0.2 (syst.) eV was obtained in a small TPC with
−0.3
internal sources at energies between 2.8 keV and 42 keV [150], consistent with the value measured
at the MeV-scale, 11.5 ± 0.5(syst.) ± 0.1(stat.) eV with theEXO-200 detector [151]. The ionisationand scintillation W-values in gaseous xenon are 24.8 eV and (76 ± 6) eV, respectively [152].
The partition between excitation and ionisation depends on the density of the electron-ion
pairs produced along the track of a particle, and the recombination fraction depends on the applied
electric field, as well as on the ionisation density in the track. While several phenomenologicalmodels exist to describe electron-ion recombination as a function of stopping power and electric
drift field (see [134] and discussions therein), a solid theory of energy loss of low-energy ions innoble fluids is still missing. For nuclear recoils as generated by interactions of neutrons, neutrinos or
hypothetical WIMPs, an energy-dependent quenching is introduced via the Lindhard factor L [153],
with L being around 0.2 − 0.25 and 0.15 − 0.2 in liquid argon [154] and xenon [155], respectively, at
nuclear recoil energies in the range 3-100 keV. A comprehensive framework to simulate scintillationand ionisation yields in argon and xenon as a function of interaction type, energy and electric field
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drift
field

36. Particle Detectors for Non-Accelerator Physics

29

Table 36.4: Physical properties, volume fraction in the atmosphere, radioactive isotopes of the noble element xenon.
Property [unit]
Atomic number, mean atomic mass [g/mol]
Boiling Tb and melting Tm point at 1 atm [K]Gas density at 1 atm and 298 K (Tb ) [g/l]Liquid density at Tb [g/cm3 ]Volume ratio (ρliduid /ρgas )Dielectric constant of liquid
Scintillation light yield (in liquid, at 122 keV) [140]
Wavelength (peak centred at)
Decay time constants (s=0, s=1)
Refractive index
Electron mobility∗ [142]Volume fraction in Earth’s atmosphere [ppm]Isotopes with spin, abundance [%]Radioactive isotopes,
abundance [%] and T1/2 [y]Xe54, 131.29
165.0, 161.4
5.40 (9.99)
2.94
526
1.95
63 photons/keV
175 nm
2.2 ns, 27 ns
1.69
2
0.29 mm /(µs V) (.100 V/cm)
0.01 mm2 /(µs V) (&100 V/cm)
0.087
129 Xe, 26.44; 131 Xe, 21.18
136 Xe, 8.87; 2.2×1021 [143, 144]
124 Xe, 0.095; 1.1×1022 [145]
134 Xe, 10.4; >8.7×1020 (90% C.L.) [146]
126 Xe, 0.089; >1.9×1022 (90% C.L.) [147]∗Measured values in two benchmark regimes, so-called "cold" and "hot" electrons [141]. In the first case, theelectron energies are due to thermal bath of the xenon fluid and they rapidly gain energy with increasing field; in thesecond case, the electrons have gained most of their energy through acceleration in the field and experience increased
energy loss on their path due to collisions with Xe atoms.

in a TPC is the Noble Element Simulation Technique (NEST) [156].
To calibrate the energy scale and determine the energy resolution of low-background TPCs,
mono-energetic lines from external (e.g., 57 Co, 60 Co, 137 Cs, 228 Th, etc) and internal (83m Kr, 37 Ar)
calibration sources, as well as from neutron activation lines (e.g., 129m Xe, 131m Xe in xenon) areused. Relative energy resolutions (σ/µ) at the level of 4-6% at energies of a few tens of keV
and 2-3% at energies of a few 100 keV were reached in argon and xenon TPCs, respectively. At
higher energies, relevant for the neutrinoless double beta decay of 136 Xe, relative resolutions of
0.67%, 0.8% and 1.2% around 2.5 MeV were obtained by LZ [157], XENON1T [158] and EXO200 [159], respectively. Superior energy resolutions are attained in high-pressure gas TPCs withelectroluminiscent amplification [160], with a relative resolution of 0.4% obtained in the NEXTWhite detector [161]. This is due to the fact that the Fano factor in pure gaseous xenon, FGXe =0.15±0.02, is about two orders of magnitude smaller than in the liquid phase.
To achieve a high collection efficiency for both ionisation and scintillation signals, the concentration of impurities is reduced and maintained at a level below one part per 109 (ppb) oxygen
equivalent. While the light can be strongly reduced by the presence of water vapour, the ionisation signal also requires high purity in terms of molecules with high electronegative affinity. The
purity is commonly expressed via the electron drift lifetime, τe , which is the time over which thenumber of drifting electrons, ne , is reduced by a factor e: ne (t) = ne (t0 ) exp (−t/τe ). Ideally, τeshould be a few times larger than the total drift time in a TPC, to prevent charge losses. Lifetimes
31st May, 2024

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36. Particle Detectors for Non-Accelerator Physics> 10 ms were achieved both in argon and xenon (corresponding to impurity concentrations 1 ppb
O2 equivalent) via continuous recirculation and purification in the liquid or gas phase.
The drift velocity of electrons, which depends on the drift field Ed and the electron mobility µ
as vd = µ · Ed , has been measured for drift fields between ∼ (10 − 104 ) V/cm. It also depends on thetemperature in the medium and the impurity concentration, often making the comparison between
different measurements difficult. In general, the drift velocity raises from about 0.25 mm/µs at low
fields to 2 mm/µs at high fields in both argon and xenon. As electrons drift in the electric field, theinitial electron cloud spreads both in longitudinal and transversal direction to the field, expressed interms of longitudinal DL and transversal DT diffusion coefficients, with DL < DT and DL /DT ≈ 1
for fields approaching zero. In argon, DL was measured as ∼4.2 cm2 /s at 200 V/cm [162], ∼3.7 cm2 /s
at 274 V/cm [163], ∼7.2 cm2 /s at 500 V/cm and DT as ∼12 cm2 /s [164]. In xenon, DL is around100 cm2 /s at low fields, reaching ∼25 cm2 /s at higher drift fields [142], while DT has been measured
to be ∼55 cm2 /s at fields between (20-600) V/cm [165]. As an example,√ the spatial distribution of
an initially point-like electron cloud, after a drift time t, is σD = 2Dt, resulting in a value of∼20 mm for 1 ms drift and a 25 cm2 /s diffusion coefficient.
The scintillation light can be attenuated by absorption on impurities or due to Rayleigh scattering, the latter depending strongly on the wavelengths of the photons and the optical propertiesof the materials. Since only the combined effect is observed, it is non-trivial to disentangle thetwo contributions. A combination of measurements and theoretical calculations yield total attenuation and Rayleigh scattering lengths between 55-110 cm and 60-99 cm for argon and 30-50 cm and30-50 cm for xenon detectors, with refractive indexes between 1.34 and 1.57-1.72, respectively, andabsorption lengths >100 cm [134, 166].
Background sources for rare-event searches, their mitigation techniques and radio-purity assays
are discussed in Section 36.6 of this Review. The main backgrounds in large TPCs are due todecays of other noble isotopes mixed with the argon and xenon (39 Ar, 85 Kr, 222 Rn), from natural,
anthropogenic or cosmogenic radioactivity of detector materials in close proximity to the target(238 U, 232 Th, 40 K, 60 Co), and from muons and their secondary particles, including in situ production of isotopes which are sufficiently long-lived to render their vetoing difficult (e.g., 137 Xe in
xenon TPCs). Astrophysical neutrinos cause largely irreducible backgrounds in dark matter detectors, but deliver exciting physics signatures in their own right. To mitigate the 39 Ar background,argon is extracted from underground sources (CO2 wells), 85 Kr and 222 Rn are reduced by cryogenic
distillation, exploiting the different vapour pressures of the different noble gases, or with gas chromatography using synthetic charcoal. In xenon detectors, nat Kr concentrations of < 50 ppq [167]and 222 Rn concentrations of 0.8 µBq/kg were achieved by cryogenic distillation In argon TPCs,
39 Ar concentrations of (0.73 ± 0.11) mBq/kg were demonstrated [168].
An overview of large TPCs for rare event searches is given in Table 36.5.
36.4.1 Dark matter and other low energy signals
TPCs built to search for particle dark matter are based on the two-phase principle. Startingwith total masses of a few kilogram the detectors evolved and reached active target masses at thetonne- and more recently multi-tonne scale. Concomitantly, the background levels in the mostinner regions constantly decreased, with current electronic recoil rates of 15 events/(t y keV) in theenergy region below 100 keV for xenon detectors [169]. Typically, the TPCs contain two arrays ofphotosensors, one immersed in the liquid, and one in the gas phase above the liquid at similar low
temperature. The xenon scintillation light can be observed directly by PMTs with synthetic silica(quartz) windows or with VUV-sensitive SiPMs, while for argon detectors, tetraphenyl butadiene(TPB) is used to shift the 128 nm light to 420 nm, given that it is absorbed by quartz windows.
The DarkSide-50 experiment [168] operated a 50 kg TPC with liquid argon depleted in 39 Ar until
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31

Table 36.5: Selected experiments using large TPCs for rare event
searches. Shown are experiments that operated until recently, as well as
current, soon-to-be-operating and proposed experiments. UAr stands forunderground argon; HP-GXe stands for high-pressure gaseous xenon. For
some of the proposed experiments, the location or the readout scheme is
not yet decided (marked as "TBD").
Experiment
Dark matter
DarkSide-50
DarkSide-20k
Argo
LUX-ZEPLIN
PandaX-4T
PandaX-xT
XENONnTDARWIN/XLZD
0νββ decay
EXO-200
nEXO
NEXT-White
NEXT-100
NEXT-HD
PandaX-III†∗†

Location

TPC type

Total (active) mass

Readout

LNGS
LNGS
SNOLAB
SURF
CJPL
CJPL
LNGS
TBD

Dual-phase, UAr
Dual-phase, UAr
Dual-phase∗ , UAr
Dual-phase, Xe
Dual-phase, Xe
Dual-phase, Xe
Dual-phase, Xe
Dual-phase, Xe

51.1 kg (46.4 kg)
50 t (50 t)∼400 t (∼400 t)
10 t (7 t)
5.6 t (3.7 t)
47 t (43 t)
8.6 (5.9 t)
50-100 t (40-80 t)

300 PMTs, TPB
SiPMs, TPB
SiPMs, TPB
300 PMTs
300 PMTs
200 PMTs
300 PMTs
300 PMTs

WIPP
SNOLAB
LSC
LSC
TBD
CJPL

LXe, 136 Xe 81%
LXe, 136 Xe 90%
HP-GXe, 136 Xe 91%
HP-GXe, 136 Xe 90%
HP-GXe, 136 Xe 90%
HP-GXe, 136 Xe 90%

175 kg (110 kg)
4.8 t (3.65 t)
5 kg (4.3 kg)
100 kg (80 kg)
1.2 t (1 t)
140 kg (100 kg)

LAAPDs, wires
SiPMs, charge pads
300 PMTs, SiPMs, TPB
300 PMTs, SiPMs, TPB
Fibres + SiPMs, TPB
Micromegas

The option to build Argo as a single-phase detector is also under consideration.
This is an R&D program for a future larger, enriched PandaX-xT detector.

2019. DarkSide-20k [170], to operate 51.1 t of underground LAr in an octogonal TPC, the walls
made of gadolinium-loaded acrylic panels, with two arrays of SiPMs, is under construction in Hall Cof the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. A larger, 400 t LAr detector, Argo, hasbeen proposed for SNOLAB in Canada. After the successful operation of TPCs for LUX, PandaX-II
and XENON1T, the current generation of xenon detectors, LUX-ZEPLIN (LZ) [171] at the Sanford
Underground Facility (SURF) in USA, PandaX-4T [172] at the China Jinping Laboratory (CJPL)and XENONnT [173] at LNGS, employ several tons of LXe as active target mass. Their overall TPC
design is rather similar, with cylindrical, polytetrafluoroethylene (PTFE) enclosed target regions
viewed by two arrays of low-radioactivity, 3-inch diameter Hamamatsu R11410 PMTs. The PTFE
has a high reflectivity for the Xe scintillation light [174]. Next-generation detectors, at the 50 t LXescale or beyond, have been proposed with DARWIN/XLZD [175,176] and PandaX-xT [177]. Recent
constraints on various dark matter candidates are presented in Section 27.6. The search for WIMP
dark matter is ongoing, with the sensitivity goal of current experiments to the spin-independent
WIMP-nucleon cross section around 1.5×10−48 cm2 at 40-50 GeV/c2 mass [171, 173].
Owing to low electronic recoil backgrounds and low energy thresholds in noble liquid TPCs,
high-sensitivity searches for dark matter electron scattering with particle candidates from a hidden
sector, for keV-scale axion-like-particles (ALPs) and dark photons via absorption in LXe, as well
as searches for solar axions, became feasible (see Section 27.6). Looking at ionisation-only signals
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36. Particle Detectors for Non-Accelerator Physics

32

allows for a further reduction of the energy threshold, given the much higher efficiency to detect anionisation electron compared to a primary scintillation photon (typically 90% versus 10%). While
this gain comes at the expense of higher backgrounds, it allows nonetheless to set stringent limits onlight dark matter electron interactions at masses from a few tens of MeV to a few GeV, as shown by
DarkSide-50 [178], PandaX-4T [179], XENONnT [180] and LZ [181]. Further, current-generation
dual-phase TPCs have reached target masses which will allow for the first time the detection of solar
pp neutrinos via elastic neutrino-electron scattering at energies down to 1 keV and of 8 B neutrinos
via coherent elastic neutrino-nucleus scattering (CEνNS).
Dual-phase TPCs were successful in gradually scaling up their target mass and in reducing thebackground levels for each iteration, while maintaining a low-energy threshold of ∼1 keV for electronic recoils. Notwithstanding, the construction of next-generation detectors at the multi-ten-tonsscale poses multiple technological challenges. The background goals are such that electronic andnuclear recoils rates are below the ones from irreducible astrophysical neutrino interactions. Thisrequirement sets the goals for the intrinsic 222 Rn and 85 Kr concentrations: the background from thedecay of these isotopes should be significantly lower than the solar pp-neutrino elastic scattering
rate. This translates into 0.1µBq/kg for 222 Rn2 and 0.1 ppt for nat Kr, assuming a 85 Kr/nat Kr ratioof 2×10−11 mol/mol. While nat Kr concentrations of < 50 ppq were already achieved, for 222 Rn afactor of about 10 reduction compared to the current values is needed. This calls for a high liquidthroughput (close to 1 t/hour) with efficient cooling power based on cryogenic heat pumps andradon-free heat exchangers. Cryogenic distillation or charcoal chromatography must go hand inhand with selection of low radon emanation materials and new coating techniques to prevent radon
emanation from surfaces.
Other challenges related to the liquid target are the continuous purification for electronegative
impurities and water, to maintain high charge and light yields, while not introducing radon into the
TPCs, as well as new solutions for reliable storage and recuperation at large scales. A dedicatedsingle-closed-loop cryogenic system capable of handling 100 t of underground argon was designed
and constructed for DarkSide-20k [182]. Liquid phase purification powered by a liquid xenon pump,as demonstrated in [183], was employed to achieve an electron drift lifetime above 15 ms in about
8.6 t of xenon in XENONnT. A system capable of handling 30 t of xenon in liquid phase, including
long-term storage and and transfer of the cryogenic liquid between storage module and detectors
was constructed for PandaX-xT [177, 184].
Regarding the detector design, electrodes with large (>2.5 m) diameters, with high transparency,
minimal sagging and low spurious electron emission, as well as high-voltage feed-throughs that cansafely deliver 50 kV or more to the cathode must be employed. DarkSide-20k will use for the firsttime anode and cathode plates made out of acrylic coated with a commercial conductive polymer
and with TPB. LZ successfully built custom-woven wire-mesh grids with 1.5 m diameter, producedwith an in-house built loom to weave the wire meshes [185]. Finally the cryostat design must ensurestability, while reducing as much as possible the amount of material, and thus gamma and neutron
emitters in proximity to the TPCs.
Besides Ar and Xe TPCs, detectors using superfluid 4 He, targeted at low-mass dark matter,are under development within the HeRALD [186] and DELight [187] programs, based on the basic
principles demonstrated by the HERON solar neutrino project [188]. The detectors will be instrumented with transition edge sensors or magnetic micro-calorimeters to observe both atomic signals,
such as scintillation light, and quasiparticle (phonon and roton) excitations. As in other noble
elements, the He∗2 dimers appear in a short-lived (<10 ns) singlet and long-lived (13 s) triplet state.
The former decay promptly, emitting photons with energies peaked around 16 eV, while the triplet
2

A 222 Rn concentration of 0.1µBq/kg corresponds to less than one radon atom per 100 mol of xenon. The main
background is due to 214 Bi β-decays which are not accompanied by an α-decay and thus cannot be tagged in the
TPC.
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36. Particle Detectors for Non-Accelerator Physics

states can propagate through the fluid and de-excite at surfaces. Liquid helium is transparent to its
VUV photons, since the first excited state of atomic helium is at 20 eV. Quasiparticles will propagate ballistically through the liquid without decaying, and can also be detected via the evaporationof 4 He atoms from the surface of the liquid, with the helium binding energy being ∼0.7 meV.
Large volume, low-background TPCs are also developed for directional dark matter searches.
An example is CYGNO [189] which aims to built a TPC with 50 cm drift, filled with a He:CF4
gas mixture at room temperature and atmospheric pressure at LNGS. The liberated charge in aparticle interaction is drifted towards an amplification stage consisting of a triple GEM structure,
where the charge is multiplied and also light is produced. The readout will consist of light detectors,
PMTs and scientific CMOS cameras, yielding the energy and z-position of events and the 2D track
projections, respectively. The combined information thus allows for a 3D track reconstruction, withthe goal being to achieve energy thresholds for electronic recoils around 1 keV.
36.4.2 0νββ Decay
Experiments built to search for the 0νββ-decay of 136 Xe, at a Q-values of 2457.8 keV, are basedon single-phase TPCs, filled either with liquid or gas xenon at high pressure. The drift region
is horizontal (EXO-200, NEXT-100) or vertical (nEXO, NEXT-HD, PandaX-xT-enriched). For
EXO-200 and nEXO, the TPCs are enclosed by radio-pure, thin-walled Cu vessels in cryofluid,hydro-fluoro ether, acting thermal bath and radiation shield. In EXO-200, which took data at the
Waste Isolation Pilot Plant (WIPP) until 2018, the charge drifted to crossed-wire planes at eachanode, while the light was collected by arrays of large area avalanche photodiodes placed behind
the wire planes at both ends. In nEXO, the TPC vessel will be a vertical Cu cylinder with 130 cm
height and diameter (for 118.3 cm drift), with the charge collected at the anode by 0.6 cm pitch
and 9.6 m long electrode strips, and the scintillation light observed by SiPMs arranged in a barrel
configuration. Unlike for dark matter TPCs, there are no reflector panels and the field rings andcathode are coated with reflective aluminium deposition. The design drift field is 400 V/cm andthe goal for the relative energy resolution is 0.8%. EXO-200 set a lower limit on the half-life of0νββ-decay of 3.5 × 1025 y with a total exposure of 231.4 kg y [190]. The future nEXO aims for asensitivity of 1.35 × 1028 y with 5 t of enriched xenon and ten years of data taking [191].
TPCs using high-pressure xenon gas aim to reconstruct the tracks from the two electrons emitted in double beta decays, and the decay vertex, thus providing also a topological signature. In theNEXT project at the Laboratorio Subterráneo de Canfranc (LSC), the horizontal TPC, to be operated at 10-15 bar, features a single drift region. The primary (S1) and secondary (S2) scintillation
light, the latter due to electroluminescence when the ionisation electrons cross a high-field region
close to the anode, are collected by arrays of PMTs and SiPMs placed behind the cathode andanode, respectively, for energy measurements and tracking. The surfaces facing the active volume
are coated with TPB to shift the VUV light to the visible spectrum. An inner Cu shield absorbs
radiation from the high-pressure vessel of the cryostat.
The NEXT-White experiment, a prototype for NEXT-100 and for the proposed tonne-scale
NEXT-HD, has been operating a TPC with 20.8 cm radius and 53 cm drift, with 4.3 kg of xenon
at 10 bar. It has shown excellent energy resolution (< 1% FWHM [161]) and demonstrated background rejection using the capability to reconstruct the trajectories of ionising charges in the gas,thus separating the two-electron signature of ββ-decay from single electrons due to backgroundsources [192, 193]. Using data collected with xenon enriched and depleted in 136 Xe, it measured
the half-life of the 2νββ-decay and demonstrated clear Bragg peaks for the 2 electrons emitted
in the decay [194]. NEXT-HD will operate a vertical-drift TPC, 2.6 m in diameter and height, toaccommodate ∼1.1 t of enriched xenon at 15 bar [195]. An intense R&D programe for the in situ
identification of the barium ions (136 Ba2+ , 136 Ba+ ) produced in the 0νββ-decay (called Ba-tagging)
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36. Particle Detectors for Non-Accelerator Physics

is ongoing [196–198]. PandaX-III, an R&D program for a future, enriched PandaX-xT detector,
operates a high-pressure Xe gas TPC, where the xenon is mixed with 1% trimethylamine (TMA) toensure stable operation of the Micromegas employed in the readout planes, and to minimise electron
diffusion [199]. Experimental searches for 0νββ-decay are discussed in more detail in Section 14.9.
Apart from enriched xenon detectors, large TPCs using xenon in its natural isotopic abundance
are also sensitive to second-order weak decays, such as the double beta decay of 134 Xe and 136 Xe, andthe double electron capture process in 124 Xe and 126 Xe. In its electronic recoil channel, XENON1T
observed for the first time the 2νECEC process in 124 Xe by detecting the simultaneously emitted
K-shell X-rays/Auger electrons of the daughter atom 124 Te with a combined energy of 64.33 keV,which is twice the K-shell binding energy. With a half-life of (1.1 ± 0.2stat ± 0.1sys )×1022 y, this is
the slowest process ever measured directly [145,200]. The double electron capture in 124 Xe can also
be studied with high-pressure xenon gas TPCs, as demonstrated by NEXT [201]. LZ, PandaX-4T
and XENONnT will improve upon existing results and will also search for the 2νβ + EC decay.
This channel has a distinct signature due to the two 511 keV gammas emitted after the positronannihilates with an electron in the medium. With a predicted half-life around 1.6 × 1023 y [202] its
first observation is within reach of these running experiments, which will also measure the half-lifeand in particular also the shape of the 2νββ-decay of 136 Xe with high statistics and at low energies
not previously accessed. The current generation of dark matter detectors will also set constraints
on the 0νββ-decays of 136 Xe and 134 Xe, as predicted or shown in [145, 203–205], delivering proofof-principle methods towards higher sensitivity searches in DARWIN/XLZD and PandaX-xT. As
an example, with 40 t of natural xenon in a TPC, a 136 Xe half-life sensitivity of 3 × 1027 y (90%C.L.) after ten years of operation can be reached [206], a number which increases by about a factorof 1.7 for an enlarged xenon mass of 60 t.

36.5 Sub-kelvin detectors
Revised August 2023 by O. Cremonesi (INFN, Milano-Bicocca), L. Hsu (FNAL) and G. Signorelli(Pisa U.; INFN, Pisa).
36.5.1 Motivation for Sub-kelvin Detectors
Detectors operating below 1 K are referred to as low-temperature detectors (LTDs). The advantage of using LTDs over conventional detectors resides in their better energy resolutions, lower
noise, and improved energy thresholds, which can all be achieved with a versatile choice in materials. In certain applications, these advantages outweigh the potential drawbacks of cooling andreading out a detector payload at sub-kelvin temperatures, and thus enable exploration of new
frontiers in fundamental physics, astrophysics and cosmology. Among the endeavors enabled byLTDs are direct searches for dark matter over a wide mass range, precision experiments to measure the electron neutrino mass, searches for neutrinoless double-beta decay, and X-ray observationof the Universe. Large arrays of LTDs are also employed to measure the properties of the cosmic microwave background (CMB) spectrum whose parameters are determined by fundamental
physics. These include dark matter and dark energy densities, the sum of neutrino masses and thenumber of light relativistic species, as well as probing the physics of inflation at energy scales of∼ 1016 GeV. This article presents a brief overview of LTDs, their features, and several applications.
More detailed treatment of this subject is available in the literature [207–210].
The advantages of LTDs are enabled by the detection of very low energy excitations (e.g.
phonons and quasiparticles). In a typical interaction, energy from an incident particle is dissipatedthrough excitation of secondary particles such as electrons, ions, holes, photons, phonons etc.
These particles will in turn produce their own secondaries. Thus there is a cascade down inenergy until the original energy deposit is converted entirely into heat and the detector reaches
thermal equilibrium. Prior to the equilibrium phase, the energy at any given moment is partitioned
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36. Particle Detectors for Non-Accelerator Physics

among multiple excitation modes. Conventional particle detectors work by sensing the higher
energy excitation modes, such as ionization and scintillation, which require an average minimum
energy of few eV to 10’s of eV to produce. For such detectors, a large fraction of the depositedenergy remains undetected in the form of heat. Furthermore, the measurements are subject to thefluctuations inherent in the partition of energy across different excitation modes. Secondaries that
don’t eventually escape the detector, will de-excite or recombine to dissipate their energy in theform of phonons and quasiparticles, which are characterized by energies in the range of meV down
to µeV. These can be detected by LTDs at various stages of their final degradation towards thermal
equilibrium. Thus LTDs allow for energy resolutions and operational thresholds much lower thandetectors that only sense scintillation and ionization. Furthermore, LTDs provide a precise energy
measurement owing to the relatively large number of excitation quanta that can be detected. In
fact, LTDs designed to measure thermal phonons achieve the highest possible energy resolutions
with optimal noise performance.
At thermal equilibrium, energy E deposited in an LTD causes a temperature rise ∆T = E/C
where C is the heat capacity. Thermal equilibrium is characterized by the condition where theaverage heat flowing to an LTD equals the average heat flowing from the LTD (into a proper heatsink or bath). In this state, the ideal intrinsic energy resolution is determined by the statisticalfluctuations in the phonon system. Fluctuations in the total number of phonons in the LTD
absorber have variance C/kB , which yields a minimum resolution of ∆E 2 = kB T 2 C to the device
energy resolution, where kB is Boltzmann’s constant. Thus, the smaller the heat capacity, themore sensitive the response of the calorimeter and the better the energy resolution. The most
relevant feature of this result is that the latter is independent of the energy deposition. The heat
capacity itself is the product of the detector volume (V) and the specific heat (cp ). Optimizationof LTD response can be achieved by using small detector volumes and materials with low specificheat. Noise contributions from additional sources will increase the variance and are generally
parameterized as a multiplicative factor (ξ & 1) to the variance expression above.
Similarly, the power P from incident particles or radiation can be measured through a temperature rise given by ∆T = P/G, where G is the thermal conductance to a weakly linked bathheld at constant temperature. Power fluctuations are limited by G and have a spectral density of
SP = N EP 2 = 4kB T 2 G, where N EP is the noise equivalent power, defined as the power in a 1 Hz
bandwidth that gives a response signal with an equal amplitude to the noise. Hence lower conductance yields better sensitivities. To minimize thermal conductance in precise power measurements,
weak thermal links can be realized by using thin membranes or by decoupling the electron andphonon systems. However, a compromise must be made. While lower G yields better noise metrics,
conversely, larger G is needed to dissipate all incident power (which can be large in the case of CMB
detectors) or to have a faster detector response (the characteristic response time being τ = C/G).
LTDs that measure power are sometimes referred to as bolometers in literature, as opposed tocalorimeters that measure energy, generating some confusion. In principle, there is no clear distinction between a calorimeter and a bolometer. The operation mode is generally determined by theratio of the characteristic time constant and the average time between the arrival of incident particles or quanta [211].Yet another convention is to refer to non-equilibrium LTDs as those detectorsthat measure incident energy or power by counting excitations that have energy  kB T . In suchdetectors, energy resolution is determined by the statistical fluctuations of the energy partition,
similar to conventional ionization detectors but with a much lower average excitation energy andhence a larger number of excitation quanta.
A variety of possible detector materials and sensor technologies makes LTDs very versatile andhighly customizable. Dielectric, superconducting and paramagnetic materials are often used owing
to the fact that at very low temperatures, the specific heat decreases strongly as a function of T (see31st May, 2024

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36. Particle Detectors for Non-Accelerator Physics
Table 36.6: Low temperature dependence on temperature (T) for specific heat, based on different material classes. In the table below ΘD is the
Debye temperature and TC is the transition temperature of the superconductor.
Heat Capacity
∝



T
ΘD

Material

3

Dielectric and diamagnetic∝T

Conductor
∝ exp − 2TTC∝

SuperconductorµB B 22 µB B
sech
kB T
kB T

Paramagnet(in magnetic field B)

table 36.6). Superconductors offer additional advantages: the abrupt change in resistance whena material transitions from its normal to superconducting state enables highly sensitive measuresof temperature changes. Additionally, the existence of a small (less than a meV) but distinct
energy gap that is required to break a Cooper pair, provides a means to measure energy deposits
by counting the resulting quasiparticles (QP). QP relaxation processes are typically faster thanthermal processes making these detectors suitable for high-rate photo-counting. In summary, boththe sensitivity and energy resolution of an LTD benefit greatly from low temperature operation.
36.5.2 Detector Types
A generalized LTD calorimeter consists of an absorber in thermal contact with a phonon or
quasiparticle sensor, and a thermal link to a heat bath at a constant temperature (see figure 36.10).
The absorber provides the mass necessary for the interaction of the particle and a fast and complete thermalization of the deposited energy. The sensor accomplishes the task of translating theparticle interaction into measurable parameters. It is generally sensitive to equilibrium phonons(e.g. thermistors and MMCs) and thus provides a precise measurement of the temperature. However, non-equilibrium (e.g. STJ’s) and mixed (e.g. TES) phonon sensors have been devised andimplemented in many applications as well. The goal of the thermal link is to cool the absorber
down to its equilibrium temperature after the absorption of a particle. In monolithic detectors thethermometer and absorber are identical, while in composite detectors the thermometer is attached
to a separate absorber. This basic design applies to single-event particle detection as well as forcontinuous radiation measurement devices. In the case of the latter, a suitable absorber whose mass
is irrelevant, e.g. an antenna at the end of a waveguide, collects the incident power and dissipates it
onto a resistive (e.g. Au) film, which is put in contact with a sensor capable of detecting tiny temperature changes (e.g. semiconductor thermistors, transition-edge sensors, and kinetic inductancedetectors, to name a few).
Superconducting Tunnel Junctions (STJs) and Kinetic Inductance Detectors (KIDs) are examples of non-equilibrium detectors. STJs exploit Josephson tunnelling of particles and QPs between
two superconductors. The superconductors act as radiation absorbers, and are separated by a thininsulating layer. When such a junction is DC biased at a voltage just below the gap voltage, theexcess quasiparticles generated by the incoming radiation are detected as a tunneling-current proportional to the incoming energy. Statistical fluctuations in the tunnelling process limit the energy
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36. Particle Detectors for Non-Accelerator Physics

Operation Mode:• Discrete - single interactions• Continuous - current mode
Sensor-Absorber Interface:• Glue• Intermediate layer• Eutectic• Embedded (monolithic)

Low Temperature
Detector

Thermalcontact

Radiation
absorber or
CMB antenna

Sensor Types:• Thermistors• SC junctions• TES• KIDs• MMC
Can be multiplexed

Heat sinkStable Bath:• Cooling down to ~4K
with a cryocooler• Cooling down to ~10mK
e.g. with a dilution unit

Arrays for detecting:• X-rays• rare events• CMB

Figure 36.10: A generalized LTD consists of an absorber, a sensor and a thermal link to a stabletemperature bath. Biasing schemes and readout is described in detail in the text for individual
sensor types.
resolution which is given by:σE =qε(F + G)E0 ,(36.5)

where ε ∼ ∆, the band-gap, while F ∼ 0.2 and G > 1 are the Fano factor and tunnelling fluctuation respectively. STJs have been proven to be excellent single photon and UV-VIS spectroscopic
detectors with near theoretical energy resolutions, high detection efficiencies and excellent time
resolution. In astrophysics they are used as mixers to detect radiation in the 100 GHz to 1 THz
range by exploiting the non-linear behaviour of their current versus voltage characteristic curve.
STJs share similar design elements to charge qubits, which are being used for the development of
quantum computers. Such qubits have also recently been used in dark matter searches as single
photon detectors to evade the standard quantum limit in measurement noise [212]. Despite their extremely good energy resolutions, STJs cannot be scaled-up to produce sensors with large observing
volumes due to readout complexity and the difficulty in uniformly suppressing the superconducting
Josephson current, which is superimposed on the QP current.
KIDs exploit the variation of the kinetic energy T stored in a superconductor by Cooper pairs
whose inertia acts as an effective inductance, given by:
1
1
T = nm∗ v 2 = LI 22
2

(36.6)

where n is the number density of Cooper pairs, m∗ = 2me is the Cooper pair mass and I = 2nev isthe Cooper pair current. L = me /2ne is defined as Kinetic Inductance and is inversely proportional
to the number of Cooper pairs [213]. When a KID is placed in series with a superconducting
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36. Particle Detectors for Non-Accelerator Physics

capacitor, the resonance frequency of the circuit will be temporarily shifted by incident radiation,
which converts Cooper pairs to QPs, thus changing the effective inductance. The presence andamount of radiation is observed as a change in amplitude or phase of a tuned sinusoidal signalthat is sent through the circuit. In practice the change in L is small, and very high Q microwave
resonance circuits are needed to sense this variation (hence the name MKIDs, for Microwave-KIDs).
Furthermore, only areas of the film where large currents are flowing will be sensitive to pair breaking,thus making the response of a distributed KID position dependent. To overcome this issue, QP
trapping (by coupling two superconductors with different band-gaps) is used for absorbing opticalphotons and X-rays. To detect lower frequencies (e.g. in the 100-1000 GHz range for CMB) a
lumped element resonator (LEKID), with little current variation across the device, is used. The
device itself, based on a series LC circuit inductively coupled to a microstrip feed line, can act as
the absorber as well as the sensing element in a detector system. Macroscopic devices (few mm2 )can be fabricated by shaping the inductor in the form of a meander coupled to an inter-digitatedcapacitor [214]. The theoretical noise limit of these devices is governed by generation-recombinationnoise and takes the form:snQP
N EPQP = 2∆,
(36.7)
τQP
where nQP and τQP are the QP number and lifetime respectively. KIDs are easy to fabricate, very
sensitive, broad band and easily multiplexable: they can be coupled with a single microstrip thatsimultaneously reads 1000s of detectors resonating at different frequencies. They provide therefore
a promising solution for deploying large arrays of detectors with applications to high-energy physics,
astronomy or CMB measurements, although there are still some challenges, especially at frequencies
below 100 GHz, related to their worse noise performance when compared to other LTDs, and tothe choice of materials with a sufficiently small energy-gap.
Semiconductor thermistors are resistive elements characterised by a strong dependence of theresistance on the temperature. Usually, they consist of small crystals of germanium or silicon witha dopant concentration slightly below the metal-to-insulator transition. However, they can also
be realized in the form of amorphous films such as NbSi. At low temperatures, their resistivity(ρ) is governed by variable range hopping (VRH) conduction and is described by the expression
ρ(T ) = ρ0 exp ((T0 /T )γ ), where T0 and ρ0 are parameters controlled by the doping level, while γ
depends on the compensation level K (ratio of acceptor to donor concentrations). For low valuesof K, γ is well approximated by 1/4 while it converges to 1/2 as K increases.
Semiconductor thermistors are high impedance devices (1–100 MΩ) whose performance is usually parameterized in terms of the logarithmic sensitivity α = d log R/d log T , typically in the range
of 1–10. Silicon thermistors are fabricated using a multiple ion implantation process in high purity silicon wafers to produce a thin and uniformly doped box-like volume. The best germanium
thermistors are fabricated starting from bulk, high-purity germanium crystals doped by means ofneutron irradiation in the core of a nuclear reactor, referred to as nuclear transmutation doping(NTD). Individual sensors are then produced by dicing the irradiated samples and finishing themby hand. The great advantage of NTDs is the highly uniform doping level over large volumes which
results in a better signal to noise ratio with respect to other doping techniques. The doping level
depends on the isotopic composition of the starting material and the irradiation time.
The weak coupling to the heat sink can be provided by the electrical leads used for the read-out.
However, nowadays microelectronic planar technologies and silicon micromachining are more commonly preferred, and sensors are suspended on thin silicon nitride membranes or thin silicon beams.
Thermistors are read-out in an approximately constant current biasing configuration obtained byinserting large load resistors in the bias circuit, which allows for direct conversion of the thermal
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36. Particle Detectors for Non-Accelerator Physics

signal (∆T) into a voltage signal (∆V).
Semiconductor thermistors are very practical to use with some drawbacks. One of these isrelated to their high impedance which requires a JFET front-end placed as close as possible tothe device in order to minimize the signal integration on parasitic electrical capacitances. Thiscan represent a technical challenge, because JFETs must be maintained at significantly higher
temperatures ('100 K). Furthermore, deviations from the exponential behaviour of the conductivity
have been observed at low temperatures. They are usually described in terms of a finite thermal
coupling between electrons and phonons which results in an intrinsic limit to the signal rise times,
which is of the order of hundreds of milliseconds at temperatures below 100 mK. Nevertheless,semiconductor thermistors are an established and robust technology, and arrays of detectors basedon these devices have been widely used for neutrinoless double beta decay searches, neutrino massmeasurements and X-ray spectroscopy. Energy resolutions lower than 5 eV have been achieved withSn or HgTe absorbers.
Superconducting Transition Edge Sensors (TESs) exploit the sharp transition between superconducting and normal conducting phases, yielding a high sensitivity to temperature variations.
Temperature perturbations, and hence resistance changes, are sensed as modulations in the current through the TES. TESs are operated in a constant voltage biasing configuration where Joule
heating, arising from the current flowing in the TES, decreases with a rise in resistance, which thenbrings the TES back to its nominal operating temperature. This electrothermal feedback (ETF) is
2 /R(T ), heats the TES toachieved by providing a TES with a voltage bias whose power, PJ = Vbias
its nominal operation point, the superconducting transition temperature. Operation in ETF mode
improves linearity, speeds up response (to faster than τ = C/G), and in some cases it provides
tolerance for TC (critical temperature) variation between multiple TESs in a large array. The low
impedance in this configuration makes them well-suited for readout by SQUID (Superconducting
QUantum Interference Device) based amplifiers. Logarithmic sensitivities α = d log R/d log T of theorder of several hundreds can be achieved. By using different superconductors, or superconductormetal pairs patterned in suitable shapes, a wide range of resistances, transition temperatures andtime constants are obtainable to meet the requirements of the desired application [215].
Nano-TESs are fabricated by lithographic techniques in the form of long (few µm) and narrow(nm) wires that exhibit extremely small NEPs due to their reduced C and G, enabling single-photon
sensing. Superconducting Nanonwire Single-Photon Detectors (SNSPDs) are similarly patterned
superconductors or superconducting bilayers maintained at a temperature well below their TC .
When a DC current just below the critical current is driven through the nanowire, the absorptionof a single photon causes the formation of a hot spot that drives the superconductor to a normal
conducting state, resulting in a very fast (tens of picoseconds) current pulse through a shunt resistor.
For this reason, nano-TESs and SNSPDs are used as single photon detectors for light dark matterand axion searches in applications where large detecting mass is not critical [216].
Magnetic Metallic Calorimeters (MMC) exploit paramagnetic sensors exposed to a weak magnetic field with a weak thermal link to a heat bath. A temperature rise causes a change in thesensor magnetization, which is sensed by a SQUID magnetometer. A common material is an Au:Er
mixture, with the addition of a few hundred ppm of enriched 166 Er, which is needed to reduce theunwanted contribution of nuclear magnetic moments of other Er isotopes. The read-out is nondissipative and avoids the noise sources common to the dissipative devices. The use of a metallic
host ensures relatively fast time response, since the typical spin-electron relaxation times are of
the order of 0.1 microseconds at ∼50 mK [217]. For an optimized MMC, the energy resolution is√
given by ∆E = 2kB T 2 C (τ0 /τ1 )1/4 , where τ0 (order of micro second) and τ1 (order of milli second)
denote the signal rising and decay times respectively [207]. In order to obtain a fast and efficient
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36. Particle Detectors for Non-Accelerator Physics

energy thermalization, MMC are typically fabricated from gold. The intrinsically large heat capacity of the paramagnet does not spoil the temperature sensitivity and allows the use of relatively
large gold absorbers without degrading the device performance. Furthermore, thermal isolationfrom the heat sink is not generally an issue. Owing to the simple concept of these devices, MMC
can be precisely customized and fabricated by employing standard microtechnology. Planar MMC
arrays characterized by an excellent energy resolution, large dynamic range and good linearity havebeen successfully used for the detection of soft X-rays.
Performance and technological constraints limit the maximum size of LTDs. To improve overall
experimental sensitivities, a large number of sensors must be deployed, typically at temperatures ofa few tens of mK, where cooling power is limited to a few µW. To reduce the readout complexity andthe heat load at the colder stages, multiplexing (MUX) techniques are often employed. Multiplexing
consists of reading multiple detectors out through a limited number of lines that traverse room
temperature to cryogenic temperatures [218]. In frequency-domain multiplexing (FDM), a tuned
superconducting LC circuit is placed in series with each sensor, and a frequency comb of AC biases(usually in the MHz range) is sent to a group of sensors in parallel. A synchronous demodulationof the amplified current signal allows the recovery of the resistance variation of each detector.
Microwave-multiplexing (µMUX) represents a variation of this concept, based on the usage of rfSQUIDs, where the frequency comb is in the GHz range. Alternatively, the signal from a single
detector can be recovered by readout of one detector at a time (time-domain multiplexing, TDM).
KIDs have been gaining popularity owing to the fact that they naturally form resonant circuits,
hence multiplexing in the frequency domain is readily achieved by adjusting their capacitance.
36.5.3 Experimental Applications
LTDs are sensitive over a wide range of energy, from centimetre wavelength (∼ meV) throughthe visible spectrum (∼ eV) up to the X-ray domain (∼ keV) and beyond. Several broad categories
of LTD applications towards measurements of fundamental physics are described in more detail in
the text below and summarized in tables 36.7 and 36.8.
Bolometric detectors are favored as microwave detectors owing to their nearly constant responseover frequency, playing an important role in far-infrared astronomy and in the survey of the cosmic
background radiation. The high-frequency instrument of the Planck satellite (HFI) used spiderweb
bolometers read by NTD-Ge thermistors, but current experiments use mainly TESs or, more recently, KIDs. Depending on the objective of the experiment, antenna coupling or absorber couplingis used. In the former case, the bolometer detects one polarization and one (or a few) modes of theradiation, while absorber-coupled bolometers do not distinguish between polarizations. In order toavoid the absorption or emission from Earth’s atmosphere, many (CMB) instruments are operated
by observatories located at high altitude, dry places (such as the Atacama desert or Antarctica),
balloon-borne platforms, or from space. In Table 36.7, we list a subset of the ongoing or planned
CMB experiments (see Section 29 of this Review for the details on present challenges). In the
Atacama desert in Chile, CLASS is taking data, POLARBEAR is being upgraded to Simons Array
and ACTPol is being upgraded to AdvancedACT-Pol, the next big step being the deployment of
the Simons Observatory. At the South Pole, SPTPol has been upgraded to SPT-3G, while BICEP3
and BICEP array will constitute the South Pole Observatory. The most powerful upgrade in termsof both size and sensitivity will be CMB-S4, a consortium of telescopes both at the Pole and in
Chile. In the Canary islands, QUIJOTE and GroundBIRD are operating and LSPE/STRIP is inpreparation. QUBIC has recently started operations in Argentina and AliCPT is in preparation in
Tibet. Regarding balloon projects, SPIDER and EBEX were already launched while LSPE/SWIPE
and PIPER will be in the near future. Finally from space, after the success of Planck, LiteBIRD
has been selected by the Japan Aerospace Exploration Agency (JAXA) as the next strategic large
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36. Particle Detectors for Non-Accelerator Physics

mission to be launched in the 2020s.
The incoming radiation couples directly to the antenna probes (as in BICEP/Keck) or throughmicro-machined horn waveguides (AdvancedACT-Pol, CLASS) or lenslets (Simons Array, SPT-3G),
depending on the frequency range. All of these experiments have focal planes with hundreds tohundreds of thousands of sensors. To optimize focal plane occupancy, multi-mode or multi-chroic,dual polarization sensitive detectors are used. In the former case, the sensitivity is enhanced bycollecting power from a larger number of modes at the expense of angular resolution. In the latter,
one single mode of the radiation is focused on a broad band antenna, and on-chip polarization
separation and band-pass filters split the signal in different frequency bands directing the power todifferent absorbers and sensors.
Experimental design is driven by a trade-off between the sensitivity and the complexity of√theproduction processes and readout. On the one front, single-sensor NEP at the level of 10−21 W/ Hz
has been achieved in laboratories and research centers. Meanwhile, efforts are in place for theindustrialization of the fabrication processes, which is essential for scaling up production for thelarge number of detectors needed for future experiments.
Massive cryogenic calorimeters have been proposed since the 1980’s as particle detectors for the
search of rare processes (e.g. dark matter, neutrinoless double beta decay) [219]. Almost simultaneously, the use of arrays of small mass calorimeters was suggested for X-ray astrophysics [220]and precision measurements of the neutrino mass [221, 222]. Although essential to understanding
the nature of neutrinos, neutrinoless double beta decay (NDBD) has eluded discovery (or definitive
exclusion) for over 50 years. Calorimeteric techniques provide the best sensitivities to NDBD. However, before the advent of low temperature calorimeters (LTC), only a few isotopes (48 Ca, 136 Xeand 76 Ge) could be utilized for NDBD studies with the calorimetric approach. This limitation wasremoved by the advancement of LTC’s, and in particular by CUORE, which takes advantage ofthe naturally high abundance of 130 Te in TeO2 crystals. Additionally, the operation of CUORE at
LNGS [223] has demonstrated that the technical challenges of operating ton-sized LTD detectors
in a deep, underground location are surmountable. Currently, the most promising future approachis based on the hybrid approach of scintillating LTCs, which unfortunately cannot be used for
130 Te. Now, new projects are being proposed based on different scintillating compounds. In particular 100 Mo is the choice of CUPID [224] (NTD thermistors glued to Li2 100 MoO4 crystals), which
will use the same infrastracture of CUORE and follows from the successful operation of several
demonstrators (CUPID-0 [225] and CUPID-Mo [226]), and AMORE (MMC sensors on Ca100 MoO4
or Li2 100 MoO4 crystals) [227]. With an energy resolution comparable to germanium diodes anda mass of the order of a ton, these experiments aim to probe the inverted hierarchy of neutrino
masses. The slow response of these detectors is still a dominant limitation because pile-up may
prove to be a serious background. Extremely pure materials, careful assembly procedures, and deep
underground laboratories are therefore necessary.
In the 1980’s, the calorimetric technique was recognized as a feasible approach to make a direct
measurement of the neutrino mass from the end-point of a beta spectrum. Thus, LTCs were
proposed as a possible alternative to the standard spectrometric measurements [228]. Calorimetricmeasurements offer a number of advantages: i) a weak dependence on the final excited states, ii)
no source effects (e.g. self-absorption), and iii) lack of back-scattering from the detector. Therefore
LTCs provide a faithful reconstruction of the beta spectral shape over a broad energy range below
the end-point. However, the difficulty in resolving a small fraction of the spectrum near the endpoint is a serious limitation that strongly constrains the source strength and the statistics that need
to be accumulated. Such an inconvenience can be mitigated by selecting beta emitters with a small
Q value, owing to the fact that the fraction of counts in an interval, δ close to Q, scales as (δ/Q)3 .
However, this is generally at the cost of choosing decays with more complex nuclear transitions. In
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36. Particle Detectors for Non-Accelerator Physics

Table 36.7: Some selected experiments using LTDs to measure the CMB. These experiments
constrain the physics of inflation and the absolute mass, hierarchy, and number of neutrino species.
The experiment location determines the part of the sky that is observed. The size of the aperture
determines the angular resolution. The table also indicates the type of sensor used, the number of
sensors, the frequency range, and the number of frequency bands. Data for planned upgrades or
future experiments are provided in parentheses.
Sub-K CMB
Experiment
Ground-based
Atacama Cosmology
Telescope (2007–)BICEP/Keck (2006–)
CLASS (2015–)
GroundBIRD(2021-)POLARBEAR /Simons Array (2012–)
South Pole
Telescope (2007–)
Simons Observatory(2022–)CMB-S4(2024–)
BalloonEBEX (2013–)
PIPER (2016–)
SPIDER (2014–)
LSPE (2022–)
Satellite
Planck HFI(2003-2013)
LiteBIRD (2028–)

Location

Aperture

Sensortype# Sensors(planned)

Frequency(planned)Bands(planned)

Chile

6m

TES

5, 61430–230 GHz

5South Pole
Chile
CanaryIslandChile

26/68 cm
60 cm
30 cm

TES
TES
MKID

2,500
3,488
322

95–270 GHz
40–220 GHz
145–220 GHz

6
4
2

3.5 m

TES

South
Pole
Chile

10 m

TES

1,274
(22,764)
16,260150 GHz(90–280 GHz)
90–220 GHz

1
(4)
3

6 m/0.5 m

TES(60,000)(27–280 GHz)

(6)

Chile +South Pole

21
telescopes

TES(500,000)(20–280 GHz)

(11)

McMurdo
New Mexico
McMurdoLongyearbyen

1.5 m
2m
30 cm
60 cm

TES
TES
TES
TES

∼1,000
5,120
1,959
(326)

150–410 GHz
200–600 GHz
90–280 GHz(140–270 GHz)

3
4
3
(3)

L2

1.5 m

NTD

52

100-857 GHz

9L2

20–40 cm

TES

(4,508)(34–448 GHz)

(15)

addition, LTCs may be affected by specific systematics (e.g. solid state effects). Ultimately it isrecognized that spectrometers and calorimeters have complicated but different systematic effects.
It is therefore critical to develop complementary experiments exploiting both techniques.
LTCs were initially proposed as perfect calorimeters to measure the energy spectrum of a low Q
beta emitter embedded in an absorber. However, the requirements of excellent energy resolution anda low rate (to avoid pileup) requires a very large number, O(104 −106 ), of small mass devices (microcalorimeters). Early experiments used 187 Re, which is a long-lived beta emitter that is naturally
abundant in rhenium samples and is characterised by a very low Q value (2.4709 keV [229]).
A large number of 187 Re based experiments have been developed over the years (MANU [230],
MIBETA [231], MARE [232]). Nowadays a different approach is preferred and is based on themeasurement of the atomic radiation following electron capture, typically in 163 Ho which is alsocharacterized by a very low Q (2.837 keV [233]). Different experiments have been proposed to
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36. Particle Detectors for Non-Accelerator Physics

43

face the challenge: ECHo in Germany [234] (using MMC sensors), HOLMES in Italy [235] andNUMECS [236] in the US (using TESs). The very large number of microcalorimeters needed toobtain sensitivities comparable to spectrometric measurements is a serious challenge, both for thereadout and the thermal heat load. An alternative readout based on the use of KIDs, for theirmultiplexing capability, has been proposed and is presently under development.
Traditional searches for WIMP-like dark matter aim to measure the scatter of a massive dark
matter particle off of a target nucleus. Similar to detectors employed for neutrinoless double beta
decay, these searches benefit from large-mass absorbers for the target because the dark matterinteraction rate scales directly with the number of nuclei in the target and hence its mass. Amongthe most successful experiments to date, are those that combine the detection of phonons withanother channel such as ionization energy (EDELWEISS and SuperCDMS) or scintilation light(CRESST). This simultaneous, dual measurement takes advantage of the fact that the energy
deposited in the absorber is partitioned into these channels differently depending on whether the
initial particle interaction produces electron or nuclear recoils (or both). This particle identification
allows for the rejection of background from natural sources of radiation, which most commonly
manifest themselves as electron recoils in the detector.
Table 36.8: Selected experiments using low temperature calorimeters. The table shows currently
or soon-to-be operating experiments that will search for dark matter or neutrino properties. The
dates refer to the start of the program.
Sub-K
Experiment
WIMPs
CRESST III(2016)EDELWEISS †(_SubGeV)SuperCDMS(2023)
Neutrino mass
ECHo [234](2012)HOLMES [235](2015)NUMECS† [236](2015)0νββ decay
CUORE [223, 237]
(2015)CUPID [224](2015)
AMoRe-I [227](2018)†

Location

Detection
mode

Absorber(Total mass)

Sensortype# Sensor# Crystal

LNGS
Italy
LSM Modane
FranceSNOLAB
CanadaAthermal phononand scint.
Thermal phonon
and ion.
Athermal phonon
and ion.

CaWO4 /Al2 O3

TES

10

Ge

NTD Ge

-

Ge/Si

TES

24

Heidelberg
Germany
Milan
Italy
LANL
USA

ThermalphononThermalphononThermalphononAu:163 Ho(0.2µg)implanted 163 Ho(18 µg)implanted 163 Ho

MMC

16

TES

1000

TES

4096

LNGS
Italy
LNGS
Italy
Y2L
South Korea

ThermalphononPhononand scint.
Phononand scint.

nat TeO2

NTD Ge988

NTD Ge

1596

MMC

13/5

(741 kg)
Li100
2 MoO4(450 kg)
Ca100 MoO4 /Li2 100 MoO4
(6 kg)

No payload size quoted for experiments that are primarily in R&D phase.

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36. Particle Detectors for Non-Accelerator Physics

In recent years, multi-ton liquid noble detectors have outclassed LTD-based technologies insearches for heavy (>10 GeV/c2 ) dark matter owing to their ability to more easily and cheaply
scale to large target masses. However, the lower thresholds achieved by LTDs continue to makethem the technology of choice for low-mass dark matter searches. New advances have enabledthese detectors to reach much lower energy thresholds than previously obtained, albeit sometimes
at the cost of being able to detect energy in more than one channel as described above. For
example, the use of an electric field to generate Neganov-Trofimov-Luke [238, 239] phonons inproportion to the applied voltage, has enabled the detection of single electron hole pairs in Si
detectors with thresholds as low as a few eV (SuperCDMS HVeV). This and similar advances in Ge
LTDs (EDELWEISS_SubGeV) have enabled sensitive searches for dark photons and dark matterthat scatters off electrons [240, 241]. Next generation experiments such as SPICE/HERALD aim
to further optimize the intrinsic energy resolution of TES detectors, coupled with a strategic choiceof target materials (superfluid He and polar crystals) to enable sensitivites to dark matter with
masses below an MeV/c2 [242]. Current state-of-the-art axion searches use SQUID based quantum
amplifies such as Josephson Parametric Amplifiers along with resonant cavities operating below
100 mK to look for a signal above fluctuations in the thermal noise [243]. Future axion experiments
are also working to close the sensitivity gap between particle and wave-like dark matter with thehelp of LTDs. Broad-band axion searches in the THz range are being proposed, which will makeuse of TES, SNSPDs or KIDs for single photon detection [244]. Finally, LTD-based dark matter
detectors are also actively employed to study coherent neutrino scattering, owing to the fact that the
hypothesized signal from dark matter-nucleus scattering is nearly identical to that from neutrinonucleus scattering, with both inducing nuclear recoils in a similar energy range [245, 246].

36.6 Low-radioactivity background techniques
Revised October 2023 by Al. Ianni (INFN, LNGS) and S. Schönert (Munich Tech. U.).
36.6.1 Introduction
The study of rare phenomena in fundamental physics, such as proton decay, neutrinoless double
beta decay, dark matter, and MeV-scale neutrino interactions, requires extremely low levels of
background radiation. Experiments searching for these rare events record electron recoils or nuclear
recoils in the energy scale from a few eV to several MeV. The detector technologies used are multiple
from organic and cryogenic liquid scintillators, to bolometers, solide state calorimeters, gaseous
detectors, and crystal scintillators. As far as the background contamination is concerned at someextent the application defines the requirements, although the common denominator is that anextreme reduction of all background sources is essential. Leading experiments in rare events search
have obtained in the region-of-interest (ROI) a background as low as ∼ 10−4 cts/kg/y/keV. As afirst and crucial step, a dedicated radio-purity assay of the detector set-up components has to be
carried out. Over the last fifty years, special screening and cleaning techniques have been developed
to measure and mitigate ultra-low levels of background. In order to characterize the backgroundsources we refer to Heusser [247] and identify the following five main categories:• environmental radioactivity in the location where the detector is installed;• radioimpurities in the detector and shielding;• radon and its progenies;• cosmic rays and induced radioactivity;• neutrons from natural fission, (α,n) reactions, and from cosmic-ray muons interactions.
The energy range affected by these background sources is mainly <10 MeV. All materials containtraces of long-lived primordial radioimpurities, such as 238 U, 232 Th, 235 U (238 U/235 U ∼ 138), and
40 K (40 K/Knat ∼ 1.17 × 10−4 ). We recall that 1 ppt of 238 U and 232 Th corresponds to 12.36 and31st May, 2024

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36. Particle Detectors for Non-Accelerator Physics

4.06 µBq/kg, respectively; 1 ppb of K nat corresponds to 30.25 µBq/kg. In the Earth’s crust theabundance of uranium and thorium is of the order of 1 − 10 ppm which corresponds to about 10 −100 Bq/kg. Taking into account these contamination levels and the low background requirements, afundamental background reduction and mitigation is essential to carry out rare phenomena research.
Besides primordial radionuclides other radioactive elements are produced through interactions
with matter of secondary cosmic ray particles. Among these so-called cosmogenic radionuclides werecall, in particular, 3 H, 14 C, 7 Be, 39 Ar, 42 Ar , and, in copper, steel and iron often used as shielding
materials, 57,60 Co.
A third category of background source in our environment consists of anthropogenic radionuclides. These are artificially produced mainly through nuclear reactions in nuclear power plants,
nuclear fuel reprocessing plants, and nuclear weapons testing. Anthropogenic background elementsof concern for rare phenomena are 85 Kr, 137 Cs, 241 Am, 60 Co, and 90 Sr. The concentration of 85 Kr(T1/2 =10.76 y, Qβ =687 keV) in air has been slowly increasing since World War Two with a present
activity of order 1 Bq/m3 . As a consequence 85 Kr is a crucial background in experiments making
use of nitrogen, xenon, and argon from air.
Once detector components radioactive backgrounds have been assayed and reduced by a carefulselection campaign, which can last several years, a further step to push the background level beyond
the screening possibilities is needed to reach the required sensitivity. For this purpose special active
vetos, purification methods, and offline analyses have been developed. A meticulous backgroundunderstanding and mitigation is crucial to explain any possible signal excess which may be detected.
Background mitigation techniques are based on:• use of radio-pure materials that absorb ionizing radiation;• identify radio-pure material for detector construction;• perform advanced surface and sub-surface cleaning treatment;• reduce muon flux with underground detector deployment and using active vetos;• exploit advanced detection and tagging techniques to discriminate signal from background.
In the following we describe radio-assay and background mitigation techniques developed and exploited in the framework of rare phenomena searches in deep underground laboratories.
36.6.2 Radio-purity assay
The radio-purity assay of detector components is a basic prerequisite to be carried out in low
counting experiments. Several techniques are exploited for radio-purity assay. They are complementary to characterise the radio-purity of materials for shielding or core detector components.
Next generation experiments radioassay campaign requires a considerable effort and organization
for several years [248–251].
Gamma spectroscopy (GS) via high purity germanium (HPGe) detectors is a powerful andcrucial technique [247]. It is nondestructive and thanks to the energy resolution allows to distinguishvarious radionuclides elements. Radiation from 238 U and 232 Th comes with all decay products in
the radioactive chains. However, if secular equilibrium is broken, this crucial information can beaddressed by separating different gamma-ray lines characterizing elements in the decay chains. In
the 238 U chain one can have three sub-chains out of equilibrium. The first sub-chain can be assessed
through 234 Th direct progeny of 238 U. The second sub-chain, which originates from 226 Ra, can beprobed by 214 Pb and 214 Bi. A third sub-chain, which starts from 210 Pb, cannot be efficiently probed
by GS, yet is of crucial importance and alternative methods must be used. In the 232 Th chain again
one can probe two sub-chains which can be out of secular equilibrium: the first one through 228 Ac
from 228 Ra; the second from 228 Th can be measured through 212 Pb, 212 Bi and 208 Tl. The technology
for HPGe operated in deep underground counting facilities [252,253] or in shallow laboratories with
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36. Particle Detectors for Non-Accelerator Physics

an efficient active veto shielding [254] has been boosted to sensitivities of the order of 10-100 µBq/kg
by carefully selecting detector components, electronics and sample handling systems. The HPGescreening method requires order of ∼ 0.1-a few kg of material, and weeks of acquisition to produce
a reliable measurement.
A second crucial technique is based on inductively coupled plasma mass spectrometry (ICPMS) [255]. This technique can probe primordial parent activities at the level of 1 µBq/kg. It is a
destructive method and often needs special sample preparation on a small quantity of material. The
ICP-MS does not measure the radioactive decay of isotopes but determines their concentration. At
present, it is the most sensitive and rapid screening technique which allows to select materials at
sub-ppt level of impurities. The drawback is that ICP-MS cannot assess whether the uranium andthorium chains are out of equilibrium and to reach ultra-high sensitivities one needs to carefully
prepare and handle the samples in a cleanroom environment. ICP-MS screening must be coupledwith other methods to properly assess the radio-purity of materials in the context of rare events
searches. The Glow Discharge Mass Spectroscopy (GDMS) is a trace element analysis techniquesomehow alternative to ICP-MS. An advantage of GDMS is the possibility to determine the bulk
composition of the sample, assuming homogeneity. Sensitivities of the order of 10 ppt can beachieved.
A third screening technique is based on neutron activation analysis (NAA) [256]. A sample
exposed to a neutrons flux can be activated to form radioactive isotopes which can be detectedusing HPGe detectors or ICP-MS. Considering the difficulties to irradiate samples, this method isless often used. NAA can probe sensitivities at the level of 0.01 µBq/kg.
In case out of secular equilibrium conditions are measured for uranium and thorium from GS
screening, a rigorous assessment cannot avoid radon emanation measurements. This matter isdiscussed in Section 36.6.3.
Complementarity between radio-purity assay techniques is a crucial parameter to design detectors for rare phenomena searches. As it has been pointed out above, requirements can be more
stringent than the best sensitivity which can be obtained with the current radioassay techniques. In
these cases, a number of prototype detectors have been built and operated to prove the feasibility
to reach ultra-low backgrounds.
36.6.3 Radon and its progeny
Radon is considered to be rare in nature because most of its isotopes are short-lived. However,
222 Rn (T222 Rn is produced by 226 Ra and is a
1/2 =3.82 d) is of particular concern in our context.
radioactive noble gas which can move within active detector components. 222 Rn daughters are heavy
metals which can deposit on surfaces. Diffusive 222 Rn, supported by 226 Ra, can deposit on surfaces
the long-lived 210 Pb, which is a major concern for low counting experiments. In addition, due to∼100 keV nuclear recoil energies from alpha decays in the 226 Ra sub-chain, eventually 210 Pb can be
implanted into a sub-surface layer of a material exposed to radon. This sub-surface contamination
can remain even after surface cleaning. Surface contamination of 210 Pb is a serious background for
direct dark matter experiments through alpha decay of 210 Po, which can generate neutrons by (α,n)
reactions. Low energy beta/gamma emissions from 210 Pb are also a concern. Therefore, radon-freecleanrooms are essential for the assembly of the detector components. Effective radon abatement
systems are available for this purpose.
In assembling and commissioning rare events experiments, special care must be dedicated to theestimate of radon emanation of the materials and continuous radon monitoring. For this purpose
different methods for radon assay have been developed and exploited since the beginning of solar
neutrino observations. 222 Rn atoms are collected inside an exhalation chamber for several half-lives
before adsorption and counting. Detection limits of the order of 100 µBq (about 50 222 Rn atoms) can
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36. Particle Detectors for Non-Accelerator Physics

be obtained with ∼50 l stainless steel electro-polished chambers [257]. This limit can be pushed
down to 30 µBq for 1-liter scale chamber. Emanation of large vessels (cryostats, storage tanks,
purification columns) can be determined by collecting exhaled radon into transportable charcoal
traps [258]. The same method can be used for liquid samples. In this case instead of evacuating theexhalation chamber into a charcoal trap, He is flushed through a sparger tube for about 10 times
the volume of the liquid used. Sensitivities of the order of 10 µBq/kg have been reached.
Nitrogen or synthetic air is often used in rare events experiments for purging, stripping, andassembling the experimental apparatus. These gases might contain radon. In gases 222 Rn can also
be detected using electrostatic collection of 218 Po and 214 Po [259, 260]. Sensitivities of the order ofmBq/m3 can be obtained. In Borexino three grades of nitrogen purity were used: regular purity,high purity, and low argon and krypton purity. The regular purity is obtained from boil-off gas andhas radon, measured with the method reported above, at the level of < 100µBq/m3 . For stripping
the purified liquid scintillator a higher purity is needed. To remove radon from regular purity
nitrogen a dedicated absorber plant has been built. This system can reduce radon by a factor of100. Finally, we mention that not only radon is found in nitrogen. For specific applications thelong-lived 39 Ar and 85 Kr in nitrogen are an important source of background.
36.6.4 Surface backgrounds
Surface contamination of long-lived 222 Rn daughters can be challenging in low-counting experiments. Considering required sensitivities of the next generation experiments, this source of
background has to be properly quantified and mitigated. Therefore, exposure to 222 Rn should bemonitored and limited to reduce build-up of 210 Pb on surfaces. In addition, 222 Rn exposure could
also produce sub-surface contamination as discussed in Section 36.6.3. For one cannot avoid radon
contamination in many circumstances during production of detectors components, it is crucial toquantify the effectiveness of cleaning aiming at removing surface contamination of 210 Pb, 210 Bi,
and 210 Po. A simple cleaning procedure, which implies degreasing, wiping, and rinsing the material
surfaces is not effective in removing these surface contaminants. Studies of cleaning procedures
have been carried out exposing stainless steel, copper and other materials to a strong radon source.
Etching and electropolishing with subsequential passivation and rinsing have been investigated ingreat details. Several recipes for etching and electropolishing have been proposed [261]. Electropolishing has been shown to be very effective in reducing 210 Pb, 210 Bi, and 210 Po from both copperand stainless steel by a factor greater than 100. Etching, which is easier to perform than electropolishing, followed by passivation and rinsing with deionized water is effective in reducing 210 Pb and210 Bi by a factor between 50 and 100. However, it is less effective for 210 Po, which in copper is very
poorly reduced. Removing 210 Po from surfaces is crucial, therefore, naturally 210 Po contaminatedcopper and stainless steel surfaces have been deeply investigated with a high sensitivity (1 mBq/m2 )alpha spectrometer [262]. After multi-etching steps (≥ 3), followed by a passivation, a reductionof order 100 has been obtained. On the contrary, static etching (single step) is poorly effective.
Electropolishing, or multi-etching are recommended in case copper is in direct contact with theactive core of the detector and ultra-high radio-purity is essential or when copper electroforming
cannot be used. Electroplating of a thin layer of high radio-pure copper onto the surface of lessradio-pure copper has also been investigate to mitigate surface background [262]. This technique isshown to be efficient in reducing surface activity of 210 Po when electroformed copper is used [262].
Besides copper and steel other materials often used, such as polyethylene and teflon, have been
investigated to understand how to reduce radon plate-out contaminations [263].
As far as surface contamination is concerned particulate fallout in cleanroom environments
could be of concern. In general, chemical composition of dust reflects local composition of soil anddust. This is not necessarily true in cleanroom spaces, where the composition of dust depends on
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36. Particle Detectors for Non-Accelerator Physics

ongoing activities and handled materials [264]. The rate of fallout is an important informationin the framework of rare events experiments, where assembling is performed in cleanrooms. The
210 Pb contamination, inferred from stable lead measured by ICP-MS, due to dust fallout has been
investigated [264]. This contamination has a different origin with respect to 210 Pb from radon
progeny implantation. Ultimately, one can conclude that radon exposure is more crucial thandust fallout. Therefore, the best practice for rare events experiments to face surface backgroundcontamination is to perform cleaning and assembling in a radon-free cleanroom environment.
36.6.5 Mitigation of backgrounds and active background discrimination
In this Section we discuss a selection of different background mitigation techniques used inexperiments to search for rare events in deep underground laboratories. This includes both the
avoidance or reduction of specific radioactive contamination, as well as active background suppression techniques based on specific event features and topologies.
• Mitigation of 222 Rn daughters deposition. In dark matter direct search radiogenic (α,n) reactions due to radioactive decays are of great concern. The emitted neutron can mimic a
nuclear recoil induced by a dark matter particle interaction. In the DarkSide-50 and LUXZEPLIN (LZ) experiments to mitigate this background, cleaning of parts and assembling of
the dual-phase Time Projection Chamber have been carried out in a radon-free cleanroom
with 222 Rn activity of the order of . 50 mBq/m3 .
• Underground argon. Liquid argon is an excellent scintillator to search for dark matter interactions due to the high electron-recoils rejection power through pulse shape discrimination.
Events from β − γ background can be rejected at the level of 107 or better with respect tonuclear-recoils. Liquid argon is very effective as active shield and has been used in experiments for neutrinoless double beta decay. However, a major drawback for dark matter searchusing argon as a target is due to the fact that atmospheric argon contains about 1 Bq/kg of
cosmogenic 39 Ar (T1/2 =269 y, Qβ =565 keV) [265]. For neutrinoless double beta decay searchusing argon as an active veto, 42 Ar (T1/2 =32.9 y, Qβ =599 keV) with its short lived progeny
42 K (T39 Ar limits significantly
1/2 =12 h, Qβ =3525 keV) is a major source of background.
the dark matter and 42 Ar the neutrinoless double beta decay sensitivity search. Therefore, asource of argon with reduced 39 Ar and 42 Ar is crucial. Centrifugation or differential thermal
diffusion are established methods to separate 39/42 Ar and 40 Ar. However, this is an expensive
method for a large fiducial mass. Argon from underground natural gas reservoirs is shownto contain low 39 Ar [266], and it is expected that 42 Ar is similar or even strongly reduced.
Therefore, the use of underground argon mitigates the 39/42 Ar backgrounds. DarkSide-50,
with a mass of 150 kg of underground argon, has shown that this source of argon contains
39 Ar at a level reduced by a factor of (1.4 ± 0.3) × 103 with respect to atmospheric argon [267].
DarkSide-20k and LEGEND-1000 are planning to exploit underground argon from the Urania
facility with a production capacity of 330 kg/day [268]. The former to reduce 39 Ar and thelatter 42 Ar.
• Electro-formed copper. High radio-purity copper is often used as shielding and as core detectors components. The radioimpurities in the copper can be a dominant source of external
background. Copper electro-forming is a technique used to reduce this background component in rare events experiments. Copper electro-forming is a well known process to obtainultra-high radio-purity copper. This technique has been used in the framework of the Majorana experiment to search for neutrinoless double beta decay and in ANAIS to search fordark matter annual modulation. Sub-ppt levels in uranium and thorium have been achieved
with electro-formed copper [269].
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36. Particle Detectors for Non-Accelerator Physics• Background suppression using topological event information. In addition to rigorous selection of high-purity target and shielding materials from external radiation, additional active
suppression techniques must usually be employed in low background experiments to achieve
the appropriate experimental sensitivities. While signal and background events may be indistinguishable if only their energy deposition is measured, their event features may differ
significantly in time and space. Liquid scintillators use the characteristic photon emission
time distributions to distinguish between electron- and alpha-like signals [270] and nuclear
recoils [271], respectively. High-purity germanium detectors use the time evolution of theinduced charges to separate point-like signal candidates for neutrinoless double-beta decay
events from background signals induced, for example, by gamma interactions with multiple
interactions within a crystal, or from β or α events on the n+ or p+ electrodes [272]. The operation of bare high-purity germanium detectors in an instrumented liquid argon shield enabled
the GERDA and LEGEND experiments to identify backgrounds with signal-like event topology within the HPGe detectors, but with characteristic energy deposition in the surrounding
liquid argon. These synergistic background suppression techniques enabled for a first timeever a quasi background-free search for neutrinoless double beta decays with GERDA [273].
EXO-200 [190] exploited topological event information distinghuishing single-site against
multi-site events with a liquid xenon TPC for neutrinoless double beta decay in 136 Xe toreject background and threby enhancing signal discovery sensitivity. With a gas xenon TPC
the NEXT experiment will fully exploit the differences in the spatial ionization patterns ofdouble beta decay and single electron events [274]. The former is characterized by two Bragg
peaks at opposite ends of the tracks, the latter on the contrary displays only one peak. The
combination of topology information and good energy resolution offer a powerful tool forbackground rejection. In general, the 3D reconstruction of detected events in a multivariate
fit, which accounts for spacial surface and bulk distributions of signal and background together with other properties, such as pulse shape and topological features, is a powerful tool
for background mitigation.
• Signal selection in direct Dark Matter experiments. In direct dark matter experiments electron
recoil events have to be mitigated with respect to nuclear recoil events. In semiconductor
bolometers, operating at a few tens of mK under a bias electric field and used as calorimeters,
drifting charges produce a large phonon signal proportional to the ionization, which allows
to discriminate electron recoils by the combination of charge and phonon signals [275]. In
scintillating bolometers the phonon and light signals are used for the same purpose [276].
Cryogenic scintillators, such as xenon and argon, in time projection chambers offer a strong
electron recoils background mitigation through the detection of a primary scintillation signal
in liquid and a secondary signal in gas from the drift and extraction of ionization electrons.
This background mitigation technique is also being used for neutrinoless double beta decay
with 136 Xe. The accurate fiducialization and good rejection of multiple-scattering events allow
dark matter optimized experiments to attempt a search for this very rare phenomenon.
• Neutron tagging. Present and next generation experiments need large neutron tagging detectors. In DarkSide-50 a dedicated active veto has been developed to both suppress andmeasure in situ the rate of neutron-induced background events [277]. The detector consistedof a boron-loaded liquid scintillator, which served both as shielding against γ-rays and as a tag
for neutrons. Neutrons are thermalised and captured on 10 B. Experimental data has showna neutron rejection power greater than 99.1% with 5% concentration of TMB in 30 tonnes
pseudocumene-based liquid scintillator. The LZ direct dark matter detector with a central
TPC of 7 tonnes of liquid xenon makes use of an outer Gd-loaded liquid scintillator neutron

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36. Particle Detectors for Non-Accelerator Physics
tagging veto, which works similarly to the DarkSide-50 veto, replacing boron with gadolinium. The 22 tonnes liquid scintillator is based on linear alkyl benzene (LAB) as solvent.
This detector has been designed to operate with a neutron tagging efficiency greater than
95%. The nuclear-recoil background is reduced by a factor of 10. The XENONnT detector ismaking use of a cylindrical stainless steel tank filled with Gd-loaded water, which surrounds
the cryostat with a TPC of 5.9 tonnes of active liquid xenon. The Gd concentration in water
is 0.2% in mass. Neutrons leaving the TPC volume will be moderated and captured by theGd with a probability of 91%. The gamma-rays emitted after the capture are detected fromCherenkov photons, providing a neutron tagging [278].
Neutron tagging techniques are also crucial for coherent neutrino-nucleus elastic scattering
detection from pion-decay-at-rest neutrino source at spallation sources [279, 280]. Neutrons
can be produced in the Pb shielding from CC and NC interactions and propagate into the
nuclear recoil sensitive volume, generating a beam-related background. Careful shielding
design and implementation of neutron tagging is crucial.
• Mitigation of cosmogenic background. In recent years the required sensitivity to search fordark matter and neutrinoless double beta decay asks for a strong reduction of cosmogenic
background. Muons can produce neutrons that can enter the active volume of the detector
from the surrounding rock or from external detector components. The yield of these so-called
cosmogenic neutrons depend on the muon energies and the material properties of the medium
the muon passes through. In addition, muons can produce by spallation radioisotopes insidethe detector active volume. Cosmogenic backgrounds are a function of depth and experimental design, and can limit the sensitivity to search for rare events. Most of these radioisotopes
are short-lived and their effect can be easily removed by an active veto based on the time correlation with a crossing muon. However, a number of cosmogenic radioisotopes are long-lived
and they require an important consideration. Mitigation of these cosmogenic backgrounds
produced in-situ deep underground are a major challenge for upcoming and future experiments. Optimizing the detector design and analysis strategies at a given depth equals aneffective muon flux reduction. In particular, we mention two discrimination techniques formuon-induced isotopes: 1) 11 C tagging by a three-fold coincidence between the crossing muon,
the capture of the ejected neutron from 12 C, and the 11 C decay [281]; 2) similar delayed coincidence tagging can be exploited to mitigate the background due to 77 Ge and its metastablestate 77m Ge, which have been identified as dominant cosmogenic background in the search
for neutrinoless double beta decay of 76 Ge [282].
• Purification. A number of high efficiency specific purification methods have been developed
for different detectors in order to remove long-lived radio-isotopes, 39 K, 85 Kr, and 210 Pb
progeny. For organic liquid scintillators, distillation and water extraction have been shown tobe very effective to reach radiopurity levels of the order of 10−5 µBq/kg or better in uranium
and thorium, and 10−3 µBq/kg in 210 Pb [283]. Distillation has been used to reduce 85 Kr in
xenon by a factor of 103 [284]. Cryogenic distillation can be used to reduce the isotopic
abundance of 39 Ar in argon extracted from underground with a 350 m column in the ARIA
project [285]. For semiconductors [286] and scintillating crystals [287] zone-refining followedby single-crystal growth has been exploited to remove impurities at the cost of a small fractionof material kept after the purification.
• Direct isotope tagging. In neutrinoless double beta decay in order to explore half-lives greaterthan 1028 y one needs an almost background-free detector. The most robust signature is
the identification of the daughter atom in the decay: for example, for 136 Xe, the 136 Ba2+ .
Important step forwards to establish a valid and promising method for this tagging have been
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36. Particle Detectors for Non-Accelerator Physics
recently made [288, 289].

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