1. D. Svirida for the DANSS Collaboration (ITEP-JINR)
DANSS Reactor Antineutrino Project: Status and Proof of Principle
D. Svirida for the DANSS Collaboration (ITEP-JINR)

2. DANSS Project
Detector of reactor AntiNeutrino based on Solid Scintillator
WWER1000
reactor
KNPP
Kalinin Nuclear Power Plant, Russia
DANSS on a lifting platform
Detection of the reactor antineutrino spectrum through the reaction of inverse β-decay:
Designed to contain no flammable or other dangerous liquids or materials
Located right below the core of 3.1 GW commercial reactor
Uses reactor body and shielding for cosmic background suppression (~50 m w.e.)
Lifting system allows to change the distance between the centers of the detector and of the reactor core from 10.7 to 12.7 m
Sensitive volume 1 m3
Physics goal: sterile neutrino search in the short range region
Engineering goals: power monitoring, fuel composition, actual reaction center position

3. DANSS detector design
Polystyrene based scintillator
WLS fibers to PMT
SiPM
Grooves with fibers
Gd containing coating 1.6 mg/cm2
10 layers = 20 cm
X-Module
1 layer = 5 strips = 20 cm
Y-Module
PMT
100 fibers
Two-coordinate detector with fine segmentation – spatial information
Multilayer closed passive shielding: electrolytic copper frame ~5 cm, borated polyethylene 8 cm, lead 5 cm, borated polyethylene 8 cm
2-layer active μ-veto on 5 sides
2500 scintillator strips with Gd containing coating for neutron capture
Light collection with 3 WLS fibers
Central fiber read out with individual SiPM
Side fibers from 50 strips make a bunch of 100 on a PMT cathode = Module

4. Data Acquisition System
PAs
HVDAC
WFD
Input amplifiers
ADCs
FPGAs
Power and VME buffers
Clean room
Preamplifiers PA in groups of 15 and SiPM power supplies HVDAC for each group inside shielding, current and temperature sensing
STP cables to get through the shielding
Total 46 Waveform Digitisers WFD in 4 VME crates on the platform
WFD: 64 channels, 125 MHz, 12 bit dynamic range, signal sum and trigger generation and distribution (no additional hardware)
2 dedicated WFDs for PMTs and μ-veto
System trigger on certain energy deposit in the whole detector (PMT based) or μ-veto signal
Each channel selftrigger on SiPM noise (with decimation)

5. Digitized signals and shape fitting
ADCu
ADCu
Single pixel SiPM signal
Selftrigger
PMT signal
~27 MeV
System trigger
Exceptionally low electronic noise
High dynamic range
t, ns
t, ns
ADCu
SiPM signal
~4.5 MeV
System trigger
ADCu A n τ t0
Signal fitting
f(t) = A∙exp(n(1+lnξ–ξ)), where ξ = (t–t0)/nτ
t, ns
t, ns

6. Energy calibration
1 pixel
2 pixels
4 pixels
3 pix.
MC energy spectrum in a strip
Landau+Gauss
Mpv=1.598 MeV
SiPM noise
spectrum
Noise spectrum: SiPM calibration in terms of ADCu/p.e., including cross-talk accounting
Temperature dependent – several times per day
Strip light yield, p.e./MeV – stable, once per month
Using vertical muons: PMT tower 100x20x20 cm
Compare MC-simulated energy deposit to the experimental by Mpv
Direct muon calibration for PMTs: ADCu/MeV, similar Mpv technique
Both SiPMs and PMTs have ~20 p.e./MeV
Attenuation ~20%/m, can be corrected
Energy resolution is dominated by p.e. statistics – add SiPM and PMT signals to improve
Experimental energy spectrum Mpv=30.7 p.e.
Strip light yield distribution
(attenuation dependent)

7. IBD basics and backgrounds
Instantaneous continuous ionization
Instantaneous annihilation
Thermalization and wandering
~50 μs max
Capture with Gd and
γ-emission
Ee = Eν – 1806 MeV
Neutron
>3 MeV
Multiplicity >=4
Better signature
Positron
>1 MeV Continuous ionization cluster
events
50 μs max.
backgrounds
Positron: instantaneous continuous ionization track, annihilation -> pair of 512 keV photons -> few soft Compton scatterings. Range of interest Ee =1-9 MeV.
Neutron: thermalization (~5 μs), wandering (till ~50 μs), Gd capture with excitation, cascaded γ-emission (total 8 MeV) followed by numerous Compton scatterings
Event – time correlated ordered pair of candidates. Search for positron 50 μs backwards from neutron
Backgrounds of various nature:
SiPM thermal noise (distort spectrum)
Correlated pairs (fake IBD)
Accidental coincidence (fake single IBD products)
Trigger rate ~1000 Hz, expected IBD rate ~0.1 Hz, need suppression factor of ~105

8. SiPM thermal noise
Number of PMT hits per trigger before cleaning
Average=40
SiPM noise dominating
Number of PMT hits per trigger after cleaning
Average=3.2
Physics hits dominating
Relative time for large SiPM hits
σ=3 ns
slightly worse for small signals
Small amplitude (~70 keV equivalent), but high frequency (200 MHz for the whole detector) – energy distortion up to 3 MeV in the 500 ns trigger window
Suppression based on tight timing requirement: signals should be simultaneous within ±15 ns
Additionally, coincidence requirement with according PMT hit
Average number of hits per trigger decreases 12 times

9. Cosmic muons and correlated background
мкс
мкс
Time between positron candidate and any preceding trigger, excluding μ–veto trigger
Time between μ–veto trigger and neutron candidate
Cosmic muons themselves are well identifiable by μ–veto or large (>20 MeV) energy deposit in the detector (35 Hz)
Can mimic IBD: muon strikes out fast neutron, which produces proton (=positron) and gets thermalized and captured by Gd similarly to IBD neutron
‘Muon’ cut : exclude 100 μs after any muon trigger
Other correlated processes (presumably cascaded decays with short-living intermediate states) – are not detected by μ–veto
‘Isolation’ cut : no triggers 50 μs before and 100 μs after the positron candidate

10. Accidental coincidence background
Before subtraction
Distance between positron and neutron
Background
Time between positron and neutron
After subtraction
Fake one of the IBD products by uncorrelated triggers
Examples: single decays, neutrons passed through the passive shielding (few)
Background events from data: search for a positron candidate where it can not be present – 50 μs intervals far away from neutron candidate (5, 10, 15 etc ms)
Enlarge statistics for accidentals by searches in numerous non-overlapping intervals
Mathematically strict procedure
Cuts for the accidental coincidence exactly the same as for physics events
Any distribution – for physics events, background events and their difference

11. Additional cuts
Positron cluster coordinate
X-projection
Vertical projection of the distance between positron cluster and neutron capture point
Comparison of the distributions for the events which passed the muon and isolation cuts, with similar for those rejected by these cuts (enriched by backgrounds)
Positron cluster position: 4 cm from the edges
Vertical projection of the distance: <40 cm
Annihilation Compton scatterings: <9
Totally up to 6 cuts of similar kind
Reject background 3 times, but only 15% of the events
Number of positron signals out of the ionization cluster

12. Positron spectrum Top (7 days exposure) Middle (15 days)BG
Top (7 days exposure)
Middle (15 days)
Bottom (5 days)
3 detector positions
Pure positron kinetic energy (annihilation photons not included)
About 5000 neutrino events/day in detector fiducial volume of 78%
Pessimistic background estimate 5% based on 95% μ–veto geometrical coverage

13. Детектор DANSS на этапе сборки
Summary
The crew on board
DANSS detector recorded first data in April 2016
Preliminary analysis show the possibility to reach at least antineutrino events per day with less than 5% cosmic background
Data taking continues after two month of shutdown for upgrades in summer
μ-veto improved for better geometrical coverage
Additional shielding to be installed to reduce the backgrounds in the detector room
More interesting results follow soon
Детектор DANSS на этапе сборки
The work is partially supported by the State Corporation «RosAtom» through the state contract Н.4х.44.9Б
Thank you !

14. Детектор DANSS на этапе сборки
Backups
Детектор DANSS на этапе сборки

15. Cosmic muon event in the setup
Top μ-veto signals (position not to scale)
SiPM energy deposit
PMT energy deposit
Y-projection
X-projection
Color energy scale, ADCu

16. Suppression factors Event frequency Suppression factor Trigger rate
1000 --
Correlated pairs selection
0.7
1.5 * 103
Muon and isolation cuts 4
Accidental background subtraction 3
Additional cuts
0.06
>3
Total
0.05
8 * 104