Crystal scintillators for the DM
directionality approach
TWENTIETH LOMONOSOV CONFERENCE ON
ELEMENTARY PARTICLE PHYSICS
Moscow State University, Moscow, August 19-25, 2021
Vincenzo Caracciolo on the behalf of ADAMO coll.
University of Roma “Tor Vergata” and INFN

Signatures for direct detection experiments
In direct detection experiments to provide a Dark Mattersignal identification with respect to the background a model
independent signature is needed•

Model independent annual modulation: annualvariation of the interaction rate due to Earthmotion around the Sun which is moving in the~23km 260°Galaxy/s
30
at present the only feasible one, sensitive tokmJune/s
many DM candidates and scenarios(successfully exploited by DAMA)• Model independent diurnal modulation: due to30

km December/sthe Earth revolution around its axis
2nd order effect•

Diurnal variation: daily variation of the interaction rate due to the different
Earth depth crossed by the Dark Matter particles
only for high cross sections• Directionality: correlation of Dark Matter impinging
direction with Earth's galactic motion
only for DM candidate particle inducing recoils2What the directionality approach is?
Based on the study of the correlation between the arrival direction of those
Dark Matter (DM) candidates able to induce a nuclear recoil and the Earthmotion in the galactic frame
Impinging direction of DM particle is(preferentially) opposite to thevelocity of the Sun in the Galaxy…

DM wind…and because of the Earth's rotation around its
axis, the DM particles average direction withrespect to an observer on the Earth changes witha period of a sidereal day
In case of DM candidate particles giving rise to nuclear recoils, the direction of thelatter ones is expected to be strongly correlated with the direction of the impinging DM
particle. Therefore, the observation of an anisotropy in the distribution of nuclear recoil
direction could give further evidence and information for such candidates

A direction-sensitive detector is neededDirectionality sensitive detectors: TPC• Detection of the tracks’ directions
 Low Pressure Time Projection Chamber might be suitable; in fact the range of recoiling nuclei is of
the order of mm (while it is μm in solid detectors)
In order to reach a significant sensitivity, a realistic TPC experiment needs e.g.:
1. extreme operational stability
2. high radiopurity
3. extremely large detector size
4. great spatial resolution
5. low energy threshold
NEWAGE
Not yet competitive sensitivity
DRIFT-IId

Background dominatedby Radon Progeny Recoils(decay of 222Rn daughter
nuclei, present in thechamber)

μ-PIC(Micro PixelChamber) is a twodimensional position
sensitive gaseous detector

DM-TPC• The “4-‐Shooter” 18L (6.6 gm) TPC
4xCCD, Sea-level@MIT• moving to WIPP• Cubic meter funded, design underway
9Directionality sensitive detectors:anisotropic scintillators
o The use of anisotropic scintillators to study the directionality signature proposed forthe first time in refs. [P. Belli et al., Il Nuovo Cim. C 15 (1992) 475], where the case ofanthracene was analysed; some preliminary activities have been carried out [N.J.C.
Spooner et al, IDM1997 Workshop; Y. Shimizu et al., NIMA496(2003)347]: the idea was revisitedin [R. Bernabei et al., EPJC28(2003)203]
First indication of anisotropy propertieso Anisotropic Scintillator:

for a ZnWO4 crystal scintillator:
a/b ratio for ZnWO4• for heavy particles the light output and the pulse
shape depends on the particle impinging direction
with respect to the crystal axes
• for g/e the light output and the pulse shape are
isotropic

NIM A 544(2005)553

o ZnWO4 anisotropic scintillator: a very promising detector (NIMA544(2005)553, Eur.
Phys. J. C 73 (2013) 2276): i) very good anisotropic features; ii) high level ofradiopurity; iii) high light output, that is low energy threshold feasible; iv) high stability
in the running conditions; v) sensitivity to small and large mass DM candidate
10particles; vi) detectors with  kg masses feasible

Strategy and advantages to develop and study
the ZnWO4 anisotropic response to nuclear
recoils for the ADAMO project
Eur. Phys. J. C 73 (2013) 2276

✓Advantages of the ZnWO4 crystal
Very good anisotropic features✓

High level of radio-purity✓

High light output, that is low energy threshold feasible✓High stability in the running conditions✓

Sensitivity to small and large mass DM candidate particles✓

Detectors with  kg masses

The main ongoing R&Ds and studies:•➢ Further increase the radio-purity level➢ Improve the optical properties➢ Increase the light yield to further decrease theenergy threshold➢ Study the anisotropies property at energy ofinterest for DM particle nuclear recoils
See next slides••

Optimization of purification procedureof the starting materials for crystal
growthOptimization of crystallization protocols
Study the light yield response vs theoperation temperature

JINST15(2020)07,C07037; JINST15(2020)05,C05055;
NIMA935(2019)89; NIMA833(2016)77;JPCS718(2016)4,042011;
EPJC73(2013)2276; NIMA626-627(2011)3; JP38(2011)115107
NPA826(2009)256; PLB658(2008)193

Measurements of ZnWO4 anisotropic response tonuclear recoils for the ADAMO project• In summer 2018 a campaign of measurements using a dedicated ZnWO4
crystal to study the anisotropic features of the detector for low energy
nuclear recoils started• Preliminary measurements with a collimated a source have been
performed• After a calibrations a campaign of measurements at ENEA-Casaccia with a14 MeV neutron beam has been carried outZnWO4 crystal = 10 x 10 x 10 mm3 (detector ofreduced dimensions to investigate neutron
single-scattering)

Very transparent and radio-pure EPJA 56 (2020) 3

Eur.Phys.J.A 56 (2020) 83

12Studying the response of the ZnWO4 with241Am a source
Calibration set-up:• PMT Hamamatsu H11934-200 (transit time  5 ns) + ZnWO4• LeCroy Oscilloscope 24Xs-A, 2.5 Gs/s, 200MHz bandwidth• Pulse profiles acquired in a time window of 100 ms

Energy spectra of 4.63 MeV α particles impinging along thethree axes of the crystal. The crystal was irradiated in thedirections along the crystal axes I (blue on-line), II (green online), and III (red on-line), respectively

Eur.Phys.J.A 56 (2020) 83

Ref. [11]: NIM A 544(2005)553

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•Studying the response of the ZnWO4with a neutron gun
Set-up:✓ ZnWO4 Crystal (10 x 10 x 10 mm3 )✓ Two Hamamatsu PMTs: HAMA-H11934-200✓ 2 Neutron detectors (Scionix EJ-309)✓ Neutron Gun, Thermo Scientific
MP320: 14 MeV neutrons

Eur.Phys.J.A 56 (2020) 83•Strategy: search for coincidence betweena scattered neutron at a fixed angle andscintillation event in ZnWO4 occurred in awell defined time window (ToF)•Once fixed the q angle, the recoil
direction and energy are fixed•

Measurements performed at different qangles

The response of ZnWO4 to neutrons: results
Energy distributions in ZnWO4 for coincidence events when neutrons are identified in EJ-309
and two ToF windows are considered (q=80°) to consider the neutron induced recoils and tocharacterize the random coincidences
First evidence at low energyThe anisotropy is significantly evident also foroxygen nuclear recoils in the energy region
down to hundreds keV at 5.4 σ confidence
level.
Eur.Phys.J.A 56 (2020) 83

How can we profit of the anisotropic scintillators features?
As a consequence of the anisotropy light response for heavy particles, recoil nuclei induced by
the considered DM candidates could be discriminated from the background thanks to the
expected variation of their low energy distribution along the day
The expected signal counting rate in the energy window (E1,E2) is a function of the time t(vd(t) the detector velocity in the galactic rest frame)
DM particle
velocity in thelaboratory frame

local DM halodensitynumber of targetnuclei (n) per
mass unit

differential crosssection in thec.m. frame

quenching factor, it dependson out the output
direction of the nuclear
recoil in the lab frame
recoiling nucleus kinetic
energy in thelaboratory frame

𝝃nuclear recoil
direction in thecenter of mass frame

DM particlemass

DM velocity distribution in
the galactic rest frame

detector energy
resolution

NB: Many quantities are model dependent and a model framework has to be fixed: in this example, forsimplicity, a set of assumptions and of values have been fixed, without considering the effect of the17existing uncertainties on each one of them and without considering other possible alternatives

… the model framework considered here
• a simple spherical isothermal DM halo model with Maxwellian velocity distribution, 220 km/s
local velocity, 0.3 GeV/cm3 local density (0) and 650 km/s escape velocity;• DM with dominant spin-independent coupling and the following scaling law (DM-nucleus
elastic cross section, n, in terms of the DM elastic cross section on a nucleon, p):

• a simple exponential form factor:

Quenching factor adopted in the following example:

where qn,i is the quenching factor value for a given nucleus, n, with respect to the i-th axis of theanisotropic crystal and out = (g,) is the output direction of the nuclear recoil in the laboratory frame
qn,i have been calculated following ref. [V.I. Tretyak, Astropart. Phys. 33 (2010) 40] considering the dataof the anisotropy to a particles of the ZnWO4 crystal

Energy resolution: 𝐹𝑊𝐻𝑀 = 2.4 𝐸(𝑘𝑒𝑉)18

Example of expected signal
Expected rate as a function of sidereal time and days of the year[2-3] keV p=5×10-5 pb
mDM= 10 GeV[6-7] keV p=5×10-5 pb
mDM= 100 GeV→ Identical sets of crystals placed in the same set-up with different axis
orientation will observe consistently different time evolution of the rate
→ The diurnal effect will refer to the sidereal day and not to the solar day
Eur. Phys. J. C 73 (2013) 2276

19

ADAMO project:
OZn, W→ light masses→ high masses

example of reachable sensitivity in agiven scenario

Assumptions:• simplified model framework

EPJC73(2013)2276• 200 kg of ZnWO4• 5 years of data taking• 2 keVee threshold• four possible time independent
background levels in the low energy
region:➢ 10-4 cpd/kg/keV➢ 10-3 cpd/kg/keV➢ 10-2 cpd/kg/keV➢ 0.1 cpd/kg/keV

The directionality approach can reach in the given scenario a sensitivity to the cross section atlevel of 10-5 – 10-7 pb, depending on the particle mass
Allowed regions obtained with a corollary analysis of the 9.3 C.L. DAMA model independent
result in terms of scenarios for the DM candidates considered here (green, red and blue) 20

ZnWO4 – work in progress…
❖ A cryostat for low temperature
measurement with scintillation
detectors has been realized❖ Test of the cryostat is in progress❖ Lowering the energy threshold (newPMT with higher QE optimized to thefluorescence light emission andtemperature operation)

An example of theradio-purity
improvements afterthe recrystallization

NIMA 833 (2016)
JINST 15 (2020) 05,C05055❖ New measurements of anisotropy atlow energy with MP320 Neutron
Generator (En = 14 MeV) at ENEACasaccia is ongoing❖ Further improvement of the radiopurity
22

Conclusions• Directionality Dark Matter experiments could obtain, with a completelydifferent new approach, further evidence for the presence of DM
candidates inducing nuclear recoils in the galactic halo and/or provide
complementary information on the nature and interaction type of DM
particle candidates.
• The anisotropic ZnWO4 detectors are promising to investigate thedirectionality for DM candidates inducing nuclear recoils• First evidence of anisotropy in the response of ZnWO4 crystal scintillator
to low energy nuclear recoils reported•The data presented here confirm the anisotropic response of the ZnWO4
crystal scintillator to α particles in the MeV energy region. The anisotropy
is significantly evident also for oxygen nuclear recoils in the energy region
down to some hundreds keV at 5.4 σ confidence level.

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