1. The 21st Particle & Nuclei International Conference
1-5 September 2017, IHEP, Beijing, China
China National Convention Center
Measurements of the reactor antineutrino with solid state scintillation detector
Igor Alekseev*
for the DANSS collaboration: ITEP(Moscow) + JINR(Dubna)
*ITEP (Alikhanov Institute for Theoretical and Experimental Physics)

2. There are several indications in favor of existence of the 4th neutrino flavor - “sterile” neutrino seen in oscillations
Expected parameters (G. Mention et al. Phys. Rev D (2011):
DANSS: Measure ratio of neutrino spectra at different distance from the reactor core – both spectra are measured in the same experiment with the same detector. No dependence on the theory, absolute detector efficiency or other experiments.
Naïve ratio without smearing by reactor and detector sizes and the resolution
Igor Alekseev (ITEP)

3. e+ (n,) Inverse Beta-Decay (IBD) Ee ≈ Eν – 1806 MeV
Continuous ionization cluster
Fast (prompt) signal
Ee ≈ Eν – 1806 MeV
Delayed signal
Gamma flush in the whole detector e+
(n,)
T ~ tens us
Prompt
Delayed
Neutron thermalization and capture
Igor Alekseev (ITEP)

4. Detector design
10 layers = 20 cm
X-Module
1 layer = 5 strips = 20 cm
Y-Module
PMT
100 fibers
Scintillation strips 10x40x100 mm3 with Gd-dopped coating
Double PMT (groups of 50) and SiPM (individual) readout
Strips along X and Y – 3D-picture
2500 strips = 1 m3 of sensitive volume
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
Igor Alekseev (ITEP)

5. KNPP - Kalinin Nuclear Power Plant, Russia
WWER1000
reactor
KNPP - Kalinin Nuclear Power Plant, Russia
Below 3.1 GW commercial reactor
DANSS on a lifting platform
A week cycle of up/middle/down position
No flammable or dangerous materials – can be put just after reactor shielding
Reactor fuel and body with cooling pond and other reservoirs provide overburden ~50 m w.e. for cosmic background suppression
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 on-line
The top position corresponds to ~15000 IBD events per day for 100% efficiency
Cosmic muon suppression measured
Igor Alekseev (ITEP)

6. Specially designed DAQ system
PAs
HVDAC
WFD
Input amplifiers
ADCs
FPGAs
Power and VME buffers
Single pixel SiPM signal
Selftrigger
Exceptionally low electronic noise
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, 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)
1 pixel
2 pixels
4 pixels
3 pix.
SiPM noise
spectrum
Igor Alekseev (ITEP)

7. The crew on board
Igor Alekseev (ITEP)

8. Gd(n,γ) H(n,γ) 248Cm fission – neutron source 22Na – gamma + positron
60Co – 2 x gamma
Gd(n,γ)
Σ=2.297 MeV
Σ=2.506 MeV
H(n,γ)
µ-data: calibration
SiPM linearity and uniformity
Average
18.25 p.e./MeV
Igor Alekseev (ITEP)

9. Building Pairs Trigger = digital sum of PMT > 0.7 MeV
Total trigger rate ≈ 1 kHz
Veto rate ≈ 400 Hz
True muon rate ≈ 180 Hz
Positron candidate rate ≈ 170 Hz
Neutron candidate rate ≈ 30 Hz
IBD rate ~ 0.1 Hz
IBD event = two time separated triggers:
Positron track and annihilation
Neutron capture by gadolinium
SiPM noise cut:
Time window ± 15 ns
Single pixel hits require PMT confirmation
Building Pairs
Positron candidate: 1-20 MeV in continuous ionization cluster
Neutron candidate: 3-15 MeV total energy (PMT+SiPM), SiPM multiplicity >3
Search positron 50 µs backwards from neutron
Significant background by uncorrelated triggers. Subtract background events: search for a positron candidate where it can not be present – 50 μs intervals 5, 10, 15 ms etc. away from neutron candidate. Use 16 non-overlapping intervals to reduce statistical error. All physics distributions = events - accidental events/16
After subtraction
Before subtraction
Accidental Background
Time between positron and neutron: 2 – 50 µs
Thermalization
Capture
Igor Alekseev (ITEP)

10. ‘Muon’ cut : NO VETO 60 μs before positron
Muon Cuts
VETO ‘OR’:
2 hits in veto counters
veto energy >4MeV
energy in strips >20 MeV
Two distinct components of muon induced paired events with different spectra:
‘Instantaneous’ – fast neutron
‘Delayed’ – two neutrons from excited nucleus
‘Muon’ cut : NO VETO 60 μs before positron
‘Isolation’ cut : NO any triggers 45 μs before and 80 μs after positron (except neutron)
‘Showering’ cut : NO VETO with energy in strips > 300 MeV for 200 μs before positron
‘Muon’ cut: t > 60 µs
Instantaneous component
Delayed component τ≈10 µs
Igor Alekseev (ITEP)

11. Positron cluster position: 4 cm from all edges
Distance between positron cluster and neutron capture center, 3D case : < 55 cm
Positron cluster coordinate
X-projection : > 4 cm from edges
Comparison of the distributions for the events which passed the muon cut with similar for those accompanied by muons
Positron cluster position: 4 cm from all edges
Vertical projection of the distance: <40 cm
Multiplicity beyond positron cluster: <11
Totally 8 cuts of this kind
Reject cosmic background >3 times, but only 15% of the events
Prompt energy beyond positron cluster : < 1.8 MeV
Igor Alekseev (ITEP)

12. Reactor OFF Background Spectrum and Fit of Cosmic Fraction
5.5/day
28/day * VT
≤ 1.5/day
@1-7 MeV
Fast neutron tails: linearly extrapolate from high energy region and subtract separately from positron and visible cosmic spectra
Subtract fraction of visible cosmics based on VETO transparency
Amount of visible cosmics ~50% of neutrino signal
VETO transparency uncertainty:
2.5% from muon count in sensitive volume, missed by veto -- underestimate
5.5% from ‘reactor OFF’ spectra – poor statistics
Cosmic background fraction ≤3% of neutrino signal, subtracted
9Li and 8He background estimates: < CL
Igor Alekseev (ITEP)

13. Detector operation Points at different positions equalized by 1/r2
Oct. ‘16
May ‘17
Dec. ‘16 | Jan. ‘17
Normalization by 12 points
Low power data in good agreement
Points at different positions equalized by 1/r2
Normalization by 12 points in November-December
Adjacent reactor flux subtracted (0.45% of full power)
power, ~170 days:
7.8∙105 total
7.0 ∙105 in definite positions
6.75 ∙105 after QA
Full detector operation
Detector comissioning
Summer shutdown for cooling repair
2016
2017
Igor Alekseev (ITEP)

14. Positron spectrum 3 detector positions cycled weekly
Pure positron kinetic energy (annihilation photons not included)
About 5000 neutrino events/day in detector fiducial volume of 78% (‘Up’ position closest to the reactor)
Down/Up spectrum ratio does not contradict to straight line with current statistics
Igor Alekseev (ITEP)

15. !!! Preliminary !!! Positron spectrum - compare to the theory
MC simulated DANSS response use theoretical antineutrino spectrum by Huber and Mueller
Very preliminary – more work on calibration and simulations needed and planned
!!! Preliminary !!!
Igor Alekseev (ITEP)

16. Probing for sterile neutrino
Gaussian CLs method (X. Qian et al. arXiv: [hep-ex]) – a conservative estimate
Theoretical curves for each Δm2 and sin2(2θ) calculated based on:
Model neutrino spectrum from Huber and Mueller
Fuel burning profile from NPP
Detector size
Detector response including tails
Systematics studies include variations in:
Reactor burning profile
Detector energy resolution (+20%/√E)
Levels of cosmic backgrounds (veto and fast neutrons)
Energy intervals used in fit
Combinations of the above
The most conservative variant is used
Only ratio of positron spectra at different R is used → independent of neutrino spectrum, detector efficiency and so on.
Igor Alekseev (ITEP)

17. NEOS: Phys. Rev. Lett. 118, (2017)
DANSS data already excludes a large fraction of allowed parameter region
3ν: χ2=28 / NDF=27
Igor Alekseev (ITEP)

18. More results are coming !
DANSS recorded the first data in April 2016 and now takes statistics at full speed of about 5000 antineutrino events per day in the closest position. Fine detector segmentation reduce residual background to less than 1% level. We already have 2 more months of data collected this summer, awaiting the analysis. Plan to double statistics by the end of the year.
Preliminary analysis of the portion of data collected till May 2017 already excludes a large and most interesting fraction of available sterile neutrino parameter space, using only the ratio of positron spectrum at two distances (independent on neutrino spectrum and detector efficiency)
We took data during 40 days scheduled reactor shutdown this summer and hope for even better background estimates
In addition to collection of more data we plan:
Improve MC for perfect reflection of detector energy response
Refine detector calibration
Continue systematic studies
Elaborate more analysis methods for better sensitivity
More results are coming !
The work is partially supported by the State Corporation «RosAtom» through the state contracts Н.4х and Н.4х.44.9Б and by Russian Science Foundation, grant
Igor Alekseev (ITEP)

19. Thank you !
Igor Alekseev (ITEP)

20. Reactor
Reactor vertical burning profile for 100% power during the campaign
Fuel contribution during the campaign
Reactor center
Begin
Middle
End
235U
69%
58%
47%
238U 7%
239Pu
21%
30%
39%
241Pu 3% 5%
Igor Alekseev (ITEP)

21. Oct 2015
Section #4
Section #1

22. Front-end electronics
Nov 2015
1 of 50 PMTs
Front-end electronics

23. Average number of neutrons from Cm fission = 3.2
Igor Alekseev, ITEP

24. 9Li and 8He background compatible with zero
90% limit: 1.64 * * / 20 = 2.7 events/day
Eshower > 800 MeV
9Li lifetime ms
Igor Alekseev, ITEP

25. Accidental background subtraction
Before subtraction
Fiducial volume – cut 4 cm on each side of the detector for positron coordinate. We took the whole detector for neutron capture flush.
Distance between positron and neutron, 2D case: <45 cm
Accidental Background
After subtraction
Distance, cm
92 cm
92 cm
Igor Alekseev (ITEP)

26. Very preliminary Partial statistics Agree with 1/R2
MIDDLE
DOWN UP
The fiducial volume divided vertically into 3 parts
Partial statistics
Agree with 1/R2
Igor Alekseev (ITEP)

27. NEOS uses normalization on Daya Bay – systematic error ?
arXiv: [hep-ex]
Comparison of our preliminary results to NEOS, Bugey, Daya Bay
█ DANSS preliminary 90% CLs
― NEOS 90% CL
― Bugey-3 90% CL
― Daya Bay 90% CLs
NEOS uses normalization on Daya Bay – systematic error ?
Igor Alekseev (ITEP)

28. Igor Alekseev (ITEP)