Dark Ma er: New Technologies
Daniel McKinsey LUX Co-Spokesperson UC Berkeley and LBNL
TAUP 2017 Sudbury, Canada July 26, 2017 S
7/26/17 D. McKinsey New DM Technologies 1 Current State of the Field
7/25/17 D. McKinsey New DM Technologies 2 Current State of the Field
More mass (LXe, LAr)
7/25/17 D. McKinsey New DM Technologies 3 Current State of the Field
Lower threshold Lighter targets
More mass (LXe, LAr)
7/25/17 D. McKinsey New DM Technologies 4 There’s plenty of room at the bo om
See Tracy Slatyer’s plenary talk for physics mo va on for light dark ma er (thermal, asymmetric, freeze-in, SIMP, ELDER…)
7/25/17 D. McKinsey New DM Technologies 5 There’s plenty of room at the bo om
Non-trivial requirements: • low thresholds • control of radioactive backgrounds • control of dark counts & instrumental backgrounds 7/25/17 D. McKinsey New DM Technologies 6 US Cosmic Visions: New Ideas in Dark Ma er
• Many of the technologies discussed here were compiled in the Cosmic Visions Dark Ma er effort in the US, driven by the DOE Office of High Energy Physics. • Inves ga ng low-cost & high-impact opportuni es in Dark Ma er (DM) science – The G2 experiments (ADMX, LZ, and SuperCDMS) are flagships of the US Dark Ma er program and obvious priority – “New Ideas in Dark Ma er” workshop focused on complementary science that can be done by small projects <$10M (some much less) – 100+ talks in 4 working groups, presen ng new ideas, proposals, and science and R&D results • See h ps://indico.fnal.gov/conferenceDisplay.py?confId=13702 • See arXiv:1707.04591 • A few of the ideas for direct detec on described in this talk.
7/26/17 D. McKinsey New DM Technologies 7 sub-GeV DM
Dis nguish two types of interac ons, e.g.
σe vs mDM σN vs mDM
• dark photon mediator • dark photon mediator • vector, coupling • vector, coupling predominantly to leptons predominantly to quarks • scalar
Important to test interac ons separately Benchmarks: dark-photon mediators “Heavy” Ultralight
exci ng complementarity ultralight mediator scenario with collider & beam-dump is uniquely probed by probes (for elas c sca ering) Direct Detec on Absorp on
Si or Ge detector
superconductor long-term R&D Direct Detec on Summary Slide from Cosmic Visions
Explore hidden-sector targets: Electron recoil (ER) • ER Projects ready now explore elas c scalar thermal relic, asymmetric, SIMP/ Elder, freeze-in • R&D needed to push to weaker couplings & MeV-mass NR • Long-term R&D to reach keV masses
Opportuni es to explore WIMP-p SD sca ering also ready now
Nuclear recoil (NR) Spin independent 11 Nigel Smith (WIMPs) Gianpaolo Carosi (Axions) This talk XENON ADMX LXe bubble chamber LUX/LZ HAYSTAC Water bubble chamber DEAP CAPP Haloscope (CULTASK?) Direc onal gaseous TPC (CYGNUS) DarkSide-50/-20k ORGAN Graphene (PTOLEMY G3) CDEX Orpheus Internally amplified Ge Edelweiss MADMAX Xenon charge-only CRESST QUAX Scin lla ng crystals (GaAs, NaI) SuperCDMS LC-Circuit Color centers PICO ABRACADABRA Superfluid helium with TES readout DAMIC/SENSEI CASPEr Superfluid helium with ioniza on NEWS-G DRIFT
This is not a fully exhaus ve list; apologies if your favorite technology is not covered!
7/26/17 D. McKinsey New DM Technologies 12 Low Mass Dark Ma er: The Wild West of Direct Detec on
7/25/17 D. McKinsey New DM Technologies 13 Low Mass Dark Ma er: The Wild West of Direct Detec on
Small homesteads, wide open (parameter) spaces 7/25/17 D. McKinsey New DM Technologies 14 Backgrounds for sub-GeV
• Solar neutrino background is small • Radiogenic backgrounds to few-eV electron recoil events likely <1 event/ kg/year/eV (based on projec ons for measured values at O(50 eV)) • For sub-GeV searches, cri cal backgrounds are: • dark counts • EM interference • vibra ons
7/25/17 D. McKinsey New DM Technologies 15 Scin lla ng Xenon Bubble Chamber Prototype (Northwestern University) Muon paddle Bubble Chamber Camera Vacuum Cryostat • Concept: Coincident scin lla on IR illumina@on PMT and bubble nuclea on by nuclear recoils –65 to –50 °C 43 – Extreme electron recoil discrimina on as in freon superheated bubble chambers
Mirrors – Event-by-event energy Scin@lla@on & from scin lla on signal Piezo Bubble – Now demonstrated in –105 °C liquid xenon LXe
arXiv:1702.08861 normal [PRL 118, 231301] To hydraulic controller
TAUP - July 26, 2017 16 Scin lla ng 2 1
Bubble Chamber 0 • Poten al for sub-keV −1 LED Gate Image shown at left −150 −100 −50 0 50 Time from DAQ trigger (ms) thresholds and light nuclei 2 1 1 [PMT pulse area (au)] (Ar, Ne, …) 10 Acoustic amplitude (au) – Be er low-threshold 0 0 log −1 LED Gate Image shown at left −1 electron discrimina on than −150 −100 −50 0 50 Time from DAQ trigger (ms) −0.5 0 0.5 1 1.5 freon chambers Time from acoustic t0 (ms) 1 200 [PMT pulse area (au)] 10
– Acoustic amplitude (au) 0 Low-energy NR calibra ons 0 150 log underway 100 Acous c Signal −1 −10 −0.5 50 0 0.5 1 1.5 • CEnNS physics in reach at Time from acoustic t0 (ms) PMT amp (mV) −20 200 −250 −200 −150 −100 −50 0 −50 0 50 100 150 200 Xenon pressure (psia) Time from DAQ trigger (s) Time from PMT trigger (ns) O(10) kg scale 150 0 • Unique 1100 –10 GeV/c2 WIMP −10 50 Scin lla on Signal PMT amp (mV) −20 sensi vity at ton-scale −250 −200 −150 −100 −50 0 −50 0 50 100 150 200 Xenon pressure (psia) Time from DAQ trigger (s) Time from PMT trigger (ns)
TAUP - July 26, 2017 17 Water as a Bubble Chamber (M. Szydagis, U Albany) • Water does not naturally scin llate. BUT: – Can look at Cerenkov light from high energy interac ons as ER veto – Be er yet, can dope water with quantum dots, some of which can have triple the light yield of LXe
• Natural advantages of using H2O – Cheap, non-cryogenic, and easy to purify; has lots of protons (sub-GeV DM target). – Can be passed through hydrophilic nano-pore membrane, new tech for any bubble chamber (or a TPC)! CYGNUS HD10, a 10 m3 gas me projec on chamber
Gas mixture: SF6:4He, p ~ 1 atm. • Possibility of switching from atmospheric pressure (search mode) opera on to low pressure opera on for (improved) direc onal confirma on of WIMP signal. • Reduced diffusion via nega ve Ion dri (SF6)
Charge amplifica on via Micromegas
HD – high resolu on charge readout via x/y strips (200 μm pitch) for improved • 3D direc onality with head-tail sensi vity • Electron event rejec on • Fiducializa on
Redundant 3D fiducializa on • SF6 minority carriers • Charge cloud profile
Helium target • Improved sensi vity to low mass WIMP • Longer recoil tracks, extending electron event discrimina on to lower energies through Helium target
7/25/17 D. McKinsey New DM Technologies 19 PTOLEMY-G3 Detector: Graphene field-effect transistors (G-FETs) arranged into a fiducialized volume of stacked planar arrays – Graphene cube (G3), with mass 1kg ~ 1010 cm2 ~ 109 cm3
Will look for MeV dark ma er sca ering events that liberate an electron from a graphene target, in the absence of any other ac vity in the G3
See Y. Hochberg, Y. Kahn, M. Lisan , C. Tully, K. Zurek, arXiv:1606.08849
G-FET sensor element (Graphene Nanoribbon Array) FET-to-FET hopping trajectory
z
y x
7/26/17 D. McKinsey New DM Technologies 20 Fiducialized Volume (G3)
G-FET sensor element FET-to-FET hopping trajectory
z
y x
20 Graphene Nanoribbon Array (produced Stacked planar arrays of G-FETs at Princeton University) 1kg ~ 1010cm2 ~ 109cm3 Resistance-Temperature (RT) and Current- Cryogenically cooled (4.2K) Voltage (IV) curves in progress Cryopumping of gas contaminants on G3 Scalability to interdigitated capacitor with surface from line-of-sight trajectories pixel areas of 1mm2 or larger Low mass substrates with ALD dielectric
produce a total cold mass lighter than one 21 LHC magnet
Germanium Detectors with Internal Amplifica on (Dongming Mei, U South Dakota) This experiment would use an ioniza on amplifica on technology for Ge in which very large localized E-fields are used to accelerate ionized excita ons produced by par cle interac on to kine c energies larger than the Ge bandgap, at which point they can create addi onal electron-hole pairs, producing intrinsic amplifica on.
This amplified charge signal could then be read out with standard high-impedance JFET- or HEMT-based charge amplifiers.
Such a system would poten ally be sensi ve to single ionized excita ons produced by DM interac ons with both nuclei and electrons. In addi on, purposeful doping of the Ge could lower the ioniza on threshold by a factor of 10, to ~ 100 meV, making the detector sensi ve to 100 keV DM via electron recoils. 7/26/17 D. McKinsey New DM Technologies 22 Few-electron detec on with two-phase Xe detectors (P. Sorensen, A. Bernstein)
Idea: Deploy a small O(10) kg liquid xenon TPC with a focus on electron coun ng and Prototype mi ga on of e- backgrounds. at LLNL
Ways to reduce electron backgrounds:
1) Larger electron emission field – XENON achieved ~5.5 kV/cm – Suspect >7 kV/cm needed for substan al reduc on of e-train bkgd
2)Infrared photons to liberate trapped e- – Liquid surface trapping poten al is 0.34 eV – 940 nm LEDs readily available (1.3 eV photon), trigger on S2
3) Last resort: HV switching – Divert trapped electrons back to gate electrode – Possible in principle, may actually work quite well
7/26/17 D. McKinsey New DM Technologies 23 Scin llators with transi on edge sensor readout Sensi vity to dark ma er – electron interac ons. No E field = No dark current Philosophy: Running in equilibrium, use TES detec on efficiency and photon energy resolu on to minimize dark counts.
GaAs: Derenzo et al: 1607.01009
Pure NaI, CsI: J. Liu 83 photons/keV seen at 77K
No a erglow seen in GaAs
7/26/17 D. McKinsey New DM Technologies 24 Scin llators with transi on edge sensor readout
Or, choose your favorite scin llator No E field = No dark current (LXe, CaWO4, …)
Maximize light yield and target mass, minimize band gap, a erglow, and background.
No a erglow seen in GaAs
7/26/17 D. McKinsey New DM Technologies 25 Color centers (R. Budnik and collaborators)
7/26/17 D. McKinsey New DM Technologies 26 Why Superfluid Helium?
Light baryonic target with mul ple signal channels, including light, charge, triplet excimers, phonons, and rotons. (W. Guo and D. N. McKinsey, PRD 87, 115001 (2013).
7/25/17 D. McKinsey New DM Technologies 27 Superfluid helium-4 as a detector material
• Search for the neutron electric dipole Measurement of neutron lifetime: moment: R. Golub and S.K. Lamoreaux, P.R. Huffman et al, Nature 403, 62-64 (2000). Phys. Rep. 237, 1-62 (1994).
7/25/17 D. McKinsey New DM Technologies 28 Superfluid helium-4 as a detector material
Proposed for measurement of pp solar neutrino flux using roton detection (HERON): R.E. Lanou, H.J. Maris, and G.M. Seidel, Phys. Rev. Lett. 58, 2498 (1987).
Two signal channels, heat and light. Both measured with a bolometer array.
Also, “HERON as a dark matter detector?” in “Dark Matter, Quantum Measurement” ed Tran Thanh Van, Editions Frontieres, Gif-sur-Yvette (1996)
7/25/17 D. McKinsey New DM Technologies 29 Why Superfluid Helium-4? • Liquid down to 0 K, allowing 10-100 mK-scale TES readout. – Take advantage of the great advances in TES technology – Take advantage of possible ~ 100% detec on efficiency for photons, triplet excimers – Take advantage of the extremely low vapor pressure of superfluid helium at low temperatures, enabling quantum evapora on-based heat signal amplifica on. • Helium is expected to have robust electronic excita on produc on efficiency, with a forgiving Lindhard factor, so nuclear recoil scin lla on signals should be rela vely large. • Negligible target cost • Low nuclear mass and charge -> low backgrounds from neutrino-nucleus sca ering and gamma-nucleus sca ering. • Low vibra on sensi vity: As a superfluid, small veloci es don’t generate excita ons. • Large ioniza on gap -> less signal quanta per keV than in super-, semiconductors. But no electron recoil background below 16 eV. • Impuri es easily removed from helium using cold traps and ge ers, and will literally fall out of the superfluid. 7/25/17 D. McKinsey New DM Technologies 30 Reading Out Singlet Excita ons (16 eV Photons)
Detec ng photons is a simple calorimetry applica on. Opera ng calorimetry in LHe: less Simple detector: box standard. Possible thanks to: with calorimetry inside - Huge LHe-solid Kapitza resistance - Fast conversion of photon energy to non-phonon excita ons (e.g. Al quasipar cles)
Triplet excimers may also be read out using the same calorimetry!
F. Carter et al, J Low Temp Phys (2016) arXiv:1605.00694
7/25/17 D. McKinsey New DM Technologies 31 Phonons and Rotons
Superfluid supports vibra onal modes (some non-intui ve).
Ballis c, ~150 m/s.
Enormous Kapitza resistance, i.e. ny probability of crossing into solid.
Few downconversion pathways.
Most signal expected in R- and R+ rotons, with absorp on probability measured to be 2.8 x 10-3. See Brown and Wya , J. Phys.: Condens. Ma er 15, 4717 (2003).
7/25/17 D. McKinsey New DM Technologies 32 Phonons and rotons can change type when reflec ng from surfaces
Calcula ons based on Tanatarov et al., arXiv:1004.3497
refection mode-change probabilities upper three: solid interface (blue=0, red=1) lower three: vacuum interface R [i −> 1] R [i −> 2] R [i −> 3]
1.5 1.5 1.5
1 1 1 i i i Θ Θ Θ
0.5 0.5 0.5
0 0 0 0 1000 2000 R [i −3000> 1] 4000 5000 0 1000 2000 R [i −3000> 2] 4000 5000 0 1000 2000R [i −3000> 3] 4000 5000 p [eV/c] p [eV/c] p [eV/c] 1.5 1.5 1.5
1 1 1 i i i Θ Θ Θ
0.5 0.5 0.5
0 0 0 7/25/17 0 1000 2000 3000 4000 5000 D. McKinsey New DM Technologies 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 33 5000 p [eV/c] p [eV/c] p [eV/c] Quantum evapora on from superfluid helium – vacuum interface
Heat amplifica on from desorp on – adsorp on process Adsorp on gives 10-40 meV depending on surface
7/25/17 D. McKinsey New DM Technologies 34 Reading out 4He Quasipar cles (Quantum Evapora on) Substan al amplifica on of heat signal through quantum evapora on, followed by helium atom adsorp on
7/25/17 D. McKinsey New DM Technologies 35 Athermal Evapora on – Demonstrated by HERON R&D
7/25/17 D. McKinsey New DM Technologies 36 Superfluid Helium Detector Concept
Signal channels: 1) Scin lla on 2) Ballis c Triplet Excimers 3) Phonons/Rotons
No dri field, and no S2 signal • No worry of few-electron background • Posi on reconstruc on via signal hit pa erns • (Though could apply dri field to detect single electrons via roton/phonon produc on.)
Best for energies down to 300 eV.
Discrimina on using signal ra os
Posi on reconstruc on using signal hit pa erns
7/25/17 D. McKinsey New DM Technologies 37 Electron recoil / nuclear recoil discrimina on
Toy Monte Carlo detec on efficiencies: - singlet UV photons: 0.95 (4pi coverage by calorimetry) - Triplet excimers: 5/6 (only solid surfaces) - IR photons: 0.95 (similar to UV photons)
Excellent predicted discrimina on at sub-keV energies
7/25/17 D. McKinsey New DM Technologies 38 Heat-only Readout?
Signal channels: Phonons Rotons
Energies down to ~ few meV !!
Discrimina on using roton/phonon signal ra os likely. Electron recoils, detector effects, nuclear recoils likely create different roton/ phonon distribu ons.
Posi on reconstruc on using signal hit pa erns
7/25/17 D. McKinsey New DM Technologies 39 Projected sensi vity to keV-scale dark ma er par cles
K. Schutz and K. Zurek, Phys. Rev. Le . 117, 121302 (2016) and S. Knapen, T. Lin, and K. Zurek, arXiv:1611.06228.
Instead of coupling to a single nucleus, couple to virtual mode in the superfluid helium, which in turn decays to mul -excita ons
Produc on of mul -excita ons allows high energy transfer from the low-mass, low-momentum dark ma er par cle
7/25/17 D. McKinsey New DM Technologies 40 Expected Backgrounds
Backgrounds included: - Neutrino nuclear coherent sca ering - Gamma-ray electron recoil backgrounds (similar to SuperCDMS) - Note: Helium itself is naturally radiopure, and easily purified of contaminants - Gamma-ray nuclear recoil backgrounds (see Robinson, PRD 95, 021301 (2017)
Arguments for low “detector” backgrounds: - Low-mass calorimeter, easy to hold - Target mass highly isolated from environment (superfluid: fric on-free interfaces)
7/25/17 D. McKinsey New DM Technologies 41 Projected Sensi vity Ini al sensi vity studies, taking neutrino and gamma ray backgrounds into account: (with S. Hertel, UCB/LBL -> U. Massachuse s, Amherst Junsong Lin, Andreas Biekert, Vetri Velan, UC Berkeley)
7/25/17 D. McKinsey New DM Technologies 42 Projected Sensi vity
Even a small mass (~ kg) of superfluid helium can probe substan al dark ma er parameter space
7/25/17 D. McKinsey New DM Technologies 43 Projected Sensi vity Larger target mass (~ 100 kg) of superfluid helium, with lower energy threshold (~ 10 meV) can push even lower in dark ma er Superfluid He w/ ultra-low threshold mass and cross-sec on
7/25/17 D. McKinsey New DM Technologies 44 Superfluid helium readout via field ioniza on (See arXiv:1706.00117) Goal: an even more sensi ve way to detect He atoms produced through quantum evapora on, reaching energy sensi vity of 1 meV
1 U = ↵ E~ 2 int 2 | |
A) Direct impact B) Orbital capture The polariza on force draws helium to the C) Surface diffusion p, where the field gradient is highest. D) Bouncing
7/25/17 D. McKinsey New DM Technologies 45 Superfluid helium readout via field ioniza on (See arXiv:1706.00117) Field ioniza on detector array
DM par cle
Rotons and phonons
Recoil helium atom
Liquid helium
7/25/17 D. McKinsey New DM Technologies 46 Summary and Outlook
Rising theore cal interest in low-mass dark ma er
Lots of open parameter space, can be probed by small (inexpensive) experiments
Many proposed approaches, which is appropriate as the field discovers which approaches work and which do not.
Much lower energy thresholds will bring new technical challenges, primarily instrumental backgrounds.
Expect rapid development in this area!
7/25/17 D. McKinsey New DM Technologies 47 Backup
7/25/17 D. McKinsey New DM Technologies 48 Helium-4 Nuclei: A Natural Match for Light Dark Ma er Detec on
Another view: maximum recoil energy for various targets, as a func on of WIMP mass.
7/25/17 D. McKinsey New DM Technologies 49 Why Superfluid Helium for Low-mass Dark Ma er Detec on? (W. Guo and D. N. McKinsey, PRD 87, 115001 (2013).
• Kinema c matching with light dark ma er candidates. – Pull the energy deposi ons up in energy, to above threshold. – Gain access to more of the WIMP velocity distribu on, for a given energy threshold.
• Superfluid helium offers mul ple signals to choose from, and to separate dark ma er signal from backgrounds (both electron recoils and detector backgrounds). – Prompt light – Delayed triplet excimers – Charge – Heat (roton and photon quasipar cles)
7/25/17 D. McKinsey New DM Technologies 50 Liquid helium-4 predicted response (Guo and McKinsey, arXiv:1302.0534, Phys. Rev. D 87, 115001 (2013).)
Liquid helium has lower electron scin lla on yield for electron recoils (19 photons/keVee)
But, extremely high Leff, good charge/light discrimina on and low nuclear mass for excellent predicted light WIMP sensi vity
Calculated nuclear recoil signals Measurements in literature!
7/25/17 D. McKinsey New DM Technologies 51 Predicted nuclear recoil discrimina on and signal strengths in liquid helium
for He decreases to about 0.4 at low energies, due to Lindhard effect, but is not equal to the Lindhard factor ! 7/25/17 D. McKinsey New DM Technologies 52 Underway: Development of a NR light yield measurement
Sca er fast neutrons in LHe to measure light yield (as we have done previously in LXe, LAr, and LNe). This is yet to be measured in LHe!
Neutron sources available:
➢ DD neutron generator ○ 2.8 MeV and 106 n/s ➢ 88Y-Be photoneutron ○ 153 keV and ~103 n/s ➢ 124Sb-Be photoneutron ○ 24 keV and ~103 n/s
53 How to detect triplet helium molecules? New demonstra on: Transi on Edge Sensor operated immersed in superfluid helium See F. Carter et al, arXiv:1605.00694
The collec on area here is just Coincidence: prompt singlet photon the transi on edge sensor itself Microscopic. One (max) Non-coincidence: delayed triplet molecule excita on per recoil. (+untagged photons)
7/25/17 D. McKinsey New DM Technologies 54 Phonon/roton reflec vi es have complex energy and angular dependence
Tanatarov et al, arXiv:1004.3497v1
7/25/17 D. McKinsey New DM Technologies 55 A mirrored box for heat collec on
Analogy with light detectors, which collect scin lla on light from a crystal or scin lla ng liquid, using a reflector around the scin llator to efficiently steer the light into a photomul plier.
Here is a standard NaI detector, this one from Ortec:
7/25/17 D. McKinsey New DM Technologies 56 Discrimina on based on electronic excita on/heat ra o:
G. Seidel:
S. Hertel:
7/25/17 D. McKinsey New DM Technologies 57 Discrimina on without electronic excita ons?
For very low energies, electronic excita ons are heavily suppressed. Need to move to a scheme that doesn’t rely on electronic excita ons, only heat.
How to get par cle iden fica on without electronic excita ons?
Possibly could look at roton/phonon ra o, or more generally the momentum distribu on of the quasipar cles. Given that ER and NR have different dE/dx, it’s quite plausible that they give different quasipar cle distribu ons. Higher dE/dx should result in a more thermalized (colder) quasipar cle distribu on.
7/25/17 D. McKinsey New DM Technologies 58 Projected sensi vity to keV-scale dark ma er par cles
7/25/17 D. McKinsey New DM Technologies 59 Liquid helium cryostat at UC Berkeley
➢ Cools down to ~1.5 K ○ 4He is superfluid below ~ 2.1 K ○ 4He scin lla on normal versus superfluid
60 Light detec on with PMTs, opera ng in superfluid helium
➢ Pt underlay to avoid posi ve charge accumulated in cathode
➢ Demonstrated to work at milli-Kelvin temperature
➢ Pt underlay decreases QE ○ 16% with Pt versus 30% in XENON100
61 Helium cell design
➢ Mechanical moun ng for the panel with TPB deposited
➢ A cube has 8 ver ces, 6 panels × 2 = 12. ○ 8 < 12. Cannot use diagonal. Off-diagonal design.
➢ Thickness of panels ○ 0.5 mm in PMTs in helium scenario ○ 2 mm in PMTs in vacuum scenarios
62 How low of an energy can be probed?
Electron recoil Nuclear recoil (excited by electron) (excited by neutron)
Light yield (photons/keV) 22 9 @ ~1 keVnr (predicted prompt sec on, Ito 2013)
TPB conversion efficiency 1.35
Light collec on efficiency 0.5
PMT quantum efficiency 0.14
Photoelectrons/keV 2.1 0.85
Aim for a few keVnr with this setup. The next experiment will use transi on edge sensor readout at < 100 mK temperatures, with ~ 100% quantum efficiency.
63 Simula ons PMTs in liquid helium (submerged) or in vacuum ○ Submerged scenario: More dead helium, thinner acrylic panel ○ Vacuum scenario: Less dead helium, thicker acrylic panel
• No big difference in recoil spectrum • Technically easier to implement design with PMTs submerged in liquid helium.
64