neutrino nucleus coherent scattering - prospects of future … · 2015-12-08 · neutrino nucleus...

Neutrino nucleus coherent scattering - Prospects of future reactorexperiments with germanium detectors

Marco Salathe1, Thomas Rink2

8 December 2015Applied Antineutrino Physics - Virginia Tech

[email protected], [email protected]

Marco Salathe, Thomas Rink (MPIK) Point contact germanium detectors 8 Dec. 2015 1 / 12

Coherent neutrino-nucleus scattering at a nuclear reactor with germaniumdetectors

What does it require?

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Energy [eV]

Measurement at shallow depthFit to distribution caused by noise fluctuations

Expected signal from CNNS (2 × 1013 νe/s/cm2)

• Measurement with point contact germanium detector (noise resolution of 300 eV, measuredwith a pulser measurement) at shallow depth with little shielding

• Earlier efforts: CoGeNT, TEXONO, GEMMA




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Reactor anti-neutrino spectrum

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ion

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Energy [MeV]

235U239Pu238U

241Putotal

Measured range

???

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x(fi

ssio

n)

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W/s

/cm

2]

Distance between reactor and detector [m]

• Typical reactor composition1: 50% 235U,30% 239Pu, 7% 238U, 5% 241Pu

• Average energy release: 205.2 MeV/fission

• Approximately 6 νe/fission are released

• Spectral distribution has been extractedfrom measured β-spectra (2-8 MeV)2,3

• Parametrization available for this range2,3

• Neutrinos beyond this range are below thetypical threshold or below the typicalbackground

Assumption: 3 GW reactor, 15 m distance⇒ flux: ∼ 2× 1013 νe/s/cm2

1Beda, A. G. et al. First result for the neutrino magnetic moment from measurements with the GEMMA spectrometer.English. Phys. At. Nucl. 70, 1873–1884 (2007).

2Huber, P. Determination of antineutrino spectra from nuclear reactors. Phys. Rev. C 84, 24617 (Aug. 2011).3Haag, N. et al. Experimental Determination of the Antineutrino Spectrum of the Fission Products of 238 U. Phys. Rev. Lett.

112, 122501 (2014).


Coherent neutrino-nucleus scattering (CNNS)

Z0

neutrinoneutrino

• Neutrinos (of all flavor) interact simultaneously with allnuclei in the atom:

dσ

dΩ=

G2F

16π2Q2

WE2ν(1 + cos θ)F 2(Q2)

QW =[N − (1− 4 sin2 θW )Z

]• The coherence condition (the de Broglie wavelength of the

momentum transfer is large compared to the size of theatom) is satisfied for neutrinos of Eν < 50 MeV

• 4 sin2 θW ≈ 0.944 ⇒ number of neutrons in atom isimportant (N=40.6 for germanium)

• Semi-classical treatment (Recoil energy T mass ofnucleus mN):

T = Q2/2mN = E2ν(1− cos θ)/mN

⇒ Maximal recoil energy Tmax = 2E2ν/mN

⇒ Differential cross section:

dσ

dT=

G2f

4πQ2

WmN

(1−

mNT

2E2ν

)Drukier, A. & Stodolsky, L. Principles and applications of a neutral-current detector for neutrino physics and astronomy.

Phys. Rev. D 30, 2295–2309 (1984).


Recoil spectrum of reactor-antineutrino in germanium and quenching

• Flux of ⇒ 2× 1013 νe/s/cm2 at15 m distance

• Germanium atom has largemass (mN = 68.2 GeV/c2), thusthe recoil energy T will be low(Tmax = 2E2

ν/mN)

• Only fraction of energy isconverted into ionization signal(Quenching factor ε = Eion/T )

• Lindhard model:

ε =k · g(ε)

1 + k · g(ε)

k = 0.133Z 2/3A−1/2

g(ε) = 3ε0.15 + 0.7ε0.6 + ε

ε = 11.5TZ−7/3

• Germanium: Z=32, A=72.6⇒ k=0.157 ⇒ ε ∼ 15− 20%

• At ∼ 0.1 cts/kg/keV/day levelendpoint of neutrino spectrumonly marginally of importance

Recoil Energy (keV)1 10

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ien

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model

Collar

Lindhard k0.1

Lindhard k=0.159

Lindhard k=0.2

Jones

Barbeau

TEXONO

CDMS

Messous

Chasman

Shutt

Sattler

Baudis

Simon

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Recoil signal (no cut off)Ionization signal (no cut off)

Recoil signal (cut off at 8 MeV)Ionization signal (cut off at 8 MeV)

Top figure and model description from: Barker, D. & Mei, D.-M. Germanium detector response to nuclear recoils in searchingfor dark matter. Astropart. Phys. 38, 1–6 (Oct. 2012).


Point contact germanium detectors

• First proposed by Luke et al.1 from Lawrence Berkeley National Laboratory in 1989 as a ’lowcapacitance large volume shaped-field germanium detector’

• Geometry:• A cylindrical shape• A small read out electrode embedded in a base of the cylinder (point contact)• A large high voltage electrode covering most of the remaining surface

• Features:• Small capacitance (a few pF) ⇒ Low noise properties• Unique time profile of signals that allows to distinguish certain types of events

• Example: Broad EnergyGermanium (BEGe) detector,produced by Canberra

0.00.20.40.60.81.0

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Slow pulse Fast pulse

1Luke, P. N. et al. Low capacitance large volume shaped-field germanium detector. Nucl. Sci. IEEE Trans. 36, 926–930 (Feb.1989).


Energy threshold - simple model

• The number of accidentally triggered events above a discrimination level d is described by1,2:

N(d) = N0 exp

(−d2

2σ2N

),

dn

dE(E) =

N0

σ2N

E exp

(−E2

2σ2N

)

• σN is the electronics noise. N0 is thetrigger rate if the threshold level is setto zero. For a RC-CR shaperN0 = 1/(2πTs), with Ts being theshaping time of the shaping filter3

• Assumption: Gaussian noisedistribution + Gaussian temporalcorrelation

• Fit to data (cusp filter, with 2.4µsshaping time): N0 ∼ 1.3× 1010 cts/day

• Simple formula (RC-CR shaper):N0 ∼ 0.6× 1010 cts/day

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MeasurementFit

1Rice, S. O. Mathematical Analysis of Random Noise. Bell Syst. Tech. J. 23, 46–156 (1944).2Rice, S. O. Mathematical Analysis of Random Noise. Bell Syst. Tech. J. 24, 46–156 (1945).3Spieler, H. Semiconductor Detector Systems. Oxford University Press (2005).


Energy threshold - reality

• Different filter used for triggeringdata acquisition and final dataanalysis⇒ Additional lower cut off

• Non-Gaussian noise distribution,non-Gaussian time correlation(shaping filter)⇒ Real noise distribution needsto be evaluated for each situationindividually

• The actual distribution can beeasily extracted from signals onlyconsisting of noise fluctuations

• The simple model works close tothe onset of the peak created byaccidentally triggered events

• The energy threshold is definedthrough the electronics noisecontribution (pulser resolution).

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Reconstructed energy [keV]

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)

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)

Data


Sensitivity study

• Antineutrino flux: 2× 1013 νe/s/cm2 (15 m distance from core)

• The ionization spectrum is smeared by the electronics noise

• At a fixed background level the electronic noise entirely defines the sensitivity

• The region of interest is limited by the distribution of accidentally triggered events and thebackground level

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Energy [eV]

CNNS FWHMN = 260 eVNoise FWHMN = 260 eV

Bkg level at 0.1 cts/kg/keV/day

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Sensitivity study

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• Antineutrino flux: 2× 1013 νe/s/cm2 (15 mdistance from core)

• Small region of interest for high electronicnoise

• Little improvement by reduction ofbackground level

• Background levels of the order of0.1-1 count/kg/keV/day required

• A pulser resolution of less than ∼ 100 eVrequired for being sensitive to the CNNS

• A significance of more than 5σ can bereached with roughly 2 years·kg exposure


Feasibility: Electronics noise

Example: Canberra Lingolsheim

• Large volume (∼ 1 kg) with low noise characteristics (< 100 eV FWHM noise resolution)available

Figure from: V. Marian, HPGe detector fabrication at CANBERRA, CANBERRA Specialty Detectors (Lingolsheim), FinalSymposium of the Sino-German GDT Cooperation, Ringberg Castle, October 2015,https://indico.mpp.mpg.de/getFile.py/access?contribId=18sessionId=3resId=0materialId=slidesconfId=3121


Feasibility: Background level

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]-1

y-1

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-1co

un

ts [

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il.

• New shallow depth (15 m w.e.) material screening facility at Max-Planck-Insitut furKernphysik, Heidelberg

• High muon veto efficiency (> 99%), good neutron moderation, low radioactivity ofcomponents

• Background levels below the required 1 count/kg/keV/day are observed

• A similar setup at a similar depth near a nuclear reactor is realistic and can produce similarbackground suppression.

Heusser, G. et al. GIOVE - A New Detector Setup for High Sensitivity Germanium Spectroscopy At Shallow Depth, 17 (July2015).


Conclusion

• Close to a reactor core high antineutrino fluxes can be observed (∼ 2× 1013 νe/s/cm2 at15 m distance of a 3 GW reactor)

• Background levels of the order of 0.1-1 count/s/keV/kg/day are necessary, but can beachieved

• The energy threshold is entirely defined by the noise resolution of the detector (can bemeasured with a pulser measurement)

• Large volume detectors (∼ 1 kg) are required as the rate of accidentally triggered signals isindependent of the detectors mass.

• A sufficiently low energy threshold can be achieved with a point contact germanium detectorof a noise resolution below 100 eV. Some manufactures claim that they can reach such a lownoise level

• A significance of more than 5σ can be reached with roughly 2 years·kg exposure

CNNS can be measured at a reactor site with modern point contact germanium detector systemsand state-of-the-art background suppression/shielding methods

Important notes:

• The spectrum < 1 keV most likely is not flat: Lines from neutron activation (cosmic andreactor) of the germanium are to be expected, but also lines from external backgrounds(Uranium, Thorium)

• Reactor neutron flux must be controlled and sufficiently suppressed

• Reactor on/off measurements are crucial to understand these effects


neutrino nucleus coherent scattering - prospects of future … · 2015-12-08 · neutrino nucleus...

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