The Challenges of Detecting Coherent

Neutrino Scattering

Coherent Scattering Workshop Nov 12th, 2015

J. A. Formaggio MIT

What is to gain?

Fundamental challenges

Techniques at play

What is to gain?

Fundamental challenges

Techniques at play

Fundamental Coherent Interactions

VOLUME 55, NUMBER 1 PHYSICAL REVIEW LETTERS

Bolometric Detection of Neutrinos

1 JUL+ 1985

De arBlas Cabrera, Lawrence M. Krauss and Frank Wile ek

epartment ofPhysics, Stanford University, Stanford, California 94305yman aboratory ofPhysics, Harvard University, Cambridge Mas h 0123

ns i u e for eoretical Physics, University of California, Santa Barbara, California 93106(Received 14 December 1984)

Elastic neutrino scattering off electrons in crystalline silicon at 1—10 mK 1a — m resu ts in measurablera ure c anges m macroscopic amounts of material, even for low-energy ( ( 0.41MeV)

pp v's from the sun. We propose new detectors for bolometric measurement of low-teractions includin co

n o ow-energy v in-

lationing coherent nuclear elastic scattering. A new and more sen t h fin co ' . sensi ive scarc or oscil-

a ions of reactor antineutrinos is practical ( —100 kg of S') d ld Ii, an wou ay the groundwork for amore ambitious measurement of the spectrum of 78 d 88 1pp, e, an so ar v s, and supernovae an-where in our galaxy (—10 tons of Si).

p novae any-

PACS numbers: 13.10.+q, 14.60.Gh, 29.40.—n

could directly determine the solar-core temperature.The electron events produced by sB v's have a higherweighted average cross section with energies up to 13MeV, but do not appear in Fig. 1 because of the sub-stantially reduced flux.However, coherent scattering off nuclei for these

highest energy v's produces energy transfers up toabout 10 keV with large rates, resulting in a peakbelow the significant energy range for the pp electrons(see Fig. 1). The coherent cross section for vectorneutral-current scattering off nuclei is given by

cr„,= GF2E2(Z[1 —4sin20„] —N) /47r(independent of v type), where N is the number ofneutrons in nucleus, and Z is the nuclear charge.

10CP

CV

xLLx

2(a ). pp ( e)2(b). pp(v+e)

5(a). 8 (,e)10

y tialcross section [Eq. (1)] and the monochromatic natureof the electron-capture 7Be v's, their recoil electronspectrum has a sharp cutoff at its upper energy, mak-ing signal detection easier. In fact, the width of theBe v energy (—1 keV) is determined by the solar-core temperature ( —107 K). Measurement of theresultant rounding of the recoil electron-energy cutoff

10

5( b). B~ (~~elI

10

I

1MeVI

1(X)keVI

10keVl

1keV10 0.leeV

E t.ecoi I

FI~G. 1. Event rate vs recoil energy for solar-v spectra.

1985 The American Physical Society

The problems associated with the detection of low-ener neutrinos ar-energy neutnnos are both well known and numerous. For ex-spi e o i s c ear scientific importance and after two decades of heroic effort

sun has still not been measured. Tradit' ll 1

s o eroic e orts, the v spectrum from the

duced nuclear transmutatio ' I th' L pra i iona y, ow-energy neutrino detection has invinvolved measurement of in-

new practical detector for makinns. n is etter we explore a new bolomep, olometric method. This method can provide a

ica e ec or or making more sensitive measurements of reactor v's, and can lead to ameasuring the spectrum of neutrinos emitted from the solar core.

v s, an can ea to a detector for

Th d'ffe differential cross section for a v or v with ener E to scatergy o scatter elastically off an electron with recoil energy T

do/dT= (GF2m/27r)((C„+ C )2+ (C„—C ) [1—T/E]2 (C C ) /g ~eT E)p (1)where, for v, (v, ), C„=2sin20+ —,

' and C, = —,'[——')

he difference between v, and v„scattering arises be-cause charged and neutral currents contribute to theformer, but only neutral currents to the latter. The to-tal cross section is then given by integration of Eq. (1)from T= 0 to T~,„=2E2/(2E+ m).In order to determine rates for solar-v interactions,

we calculate cross sections for production of electronsin a given energy range weighted over solar-v spectr a t

ates are then calculated with use of the integratedfluxes of Bahcall, s and with use of the fact that thereare fourteen electrons per Si atom (withsin20„=0.25). Our results are presented in Table Iand Fig. 1.As seen from Table I, a detector of recoil electrons

is most sensitive to the pp and Be solar v's. The ppneutrinos produce recoil electrons with a frequency of

—1 —1

Total (&ee+ ~eN)—1 ton d for silicon, about 500 times the totalrate currently achieved with the 37CI detector. ' As

2 7shown in Fig. 1, they produce recoil energies bel ow 10

ofkeV. The Be neutrinos interact about half as

o ten, and produce recoil electrons with energies up to60 keV. As a result of the relativel flat differen '

• Coherent scattering has been proposed and schemed as a means of detecting neutrinos for many decades.

• Relies on the principle of coherence, provides enhancement of cross-section that scales as A2.

2

by either resorting to targets with low mass numbers–considerably lowering the cross-section amplitude and re-quiring large mass detectors–or by looking instead at thecharged current reaction using higher energy neutrinos.In this paper, we discuss a low energy threshold detectorbased on cryogenic bolometers that has the capability ofreaching recoil energy thresholds as low as 10 eV. Suchdetectors re-open the door to neutral current coherentscattering as a method for sterile neutrino detection.

Neutrino-nucleus interactions which are coherent incharacter have the advantage of scaling as A2, where Ais the mass number of the target nucleus. For a targetnucleus with atomic number Z and neutron number N ,the cross-section as a function of recoil kinetic energy isgiven by the expression [13]:

d�(⌫A ! ⌫A)

dT=

G2F

4⇡MAQ

2W (1� MAT

2E2⌫

)F (q2)2 (1)

where GF is the Fermi coupling constant, MA is the massof the nucleus, F (q2) is the nuclear form factor, and QW

is the weak charge, defined by the relation:

QW = N � Z(1� 4 sin2 ✓W ) (2)

In our study, we will mainly consider mono-energeticelectron capture sources, all of which have neutrino en-ergies below 1 MeV. The maximum momentum transferfor such sources is |qmax| 2E⌫ ⌧ 2 MeV. Since theform factor F (q2) ! 1 for cases where the scale of themomentum probe is much larger than the size of the nu-cleus, we can safely ignore this correction factor for ouranalysis.

The maximum kinetic energy imparted on the nuclearrecoil depends on the neutrino energy and the mass ofthe recoil target:

Tmax E⌫

1 + MA

2E⌫

(3)

For a silicon target at 1 MeV, that implies a maximumkinetic energy of about 50 eV. For a germanium targetthe maximum kinetic energy would be around 20 eV.Such low kinetic energies are why detection of the processhas been so elusive to date. The fraction of events that isdetectable by a given experiment depends crucially on theinherent threshold of the detector. For a monochromaticsource of energy E⌫ , the e↵ective cross-section can bewritten as:

�̄ =

Z Tmax

T0

d�

dT(E⌫) · dT (4)

�̄ = �0(E⌫) · f(E⌫ , T0) (5)

where �0(E⌫) ⌘ G2

F

4⇡ E2⌫Q

2W is the total integrated cross-

section assuming no energy threshold and f(E⌫ , T0) rep-resents the fraction of events above a given threshold en-ergy, T0. In the limit that E⌫ ⌧ MA, the fraction ofevents above threshold can be written as:

f(E⌫ , T0) = (1� T0

Tmax)2 (6)

Any detector hoping to detect such a signal with su�-cient statistics must achieve as low a recoil threshold aspossible.

THE 37Ar SOURCE

Oscillometry-based measurements benefit greatly fromthe use of mono-energetic neutrino sources, since it re-duces the measurement to a pure flux-versus-distanceanalysis. Low energy electron capture sources provide themost e↵ective and clean source of such neutrinos avail-able to date [14]. A number of such neutrino sources havebeen considered in the literature; a few of them are listedin Table I. Historically, two such high intensity sourcehave been produced for neutrino studies: a 51Cr source,used by the SAGE and GALLEX experiments [15, 16],and an 37Ar gaseous source used in conjunction with theSAGE experiment [5].The 37Ar source is perhaps the most ideal with respect

to a future coherent-scattering measurement, for a num-ber of reasons:

• 37Ar produces a very high-energy, near mono-energetic neutrino (90.2% at 811 keV, 9.8% at 813keV).

• With the exception of inner bremsstrahlung pho-tons, almost all the energy is carried away by neu-trinos, facilitating shielding and enabling the sourceto be extremely compact.

• Extremely high production yield per reactor target.

The SAGE collaboration successfully produced such asource with a total activity of about 400 kCi to be usedin conjunction with their gallium solar neutrino detec-tor. The source was also very compact, extending 14 cmin length and 8 cm in diameter, including shielding [17].Further reduction in size might be possible, even with in-creased activity, making 37Ar an ideal portable neutrinosource.Despite its clear advantages as a source and its his-

torical precedent, production of such sources is lessthan ideal. The reaction process by which it is gener-ated (40Ca(n,↵)37Ar) requires a high fast neutron fluxabove 2 MeV, an energy regime where few reactors op-erate [18, 19]. Production also requires large amounts of

2

by either resorting to targets with low mass numbers–considerably lowering the cross-section amplitude and re-quiring large mass detectors–or by looking instead at thecharged current reaction using higher energy neutrinos.In this paper, we discuss a low energy threshold detectorbased on cryogenic bolometers that has the capability ofreaching recoil energy thresholds as low as 10 eV. Suchdetectors re-open the door to neutral current coherentscattering as a method for sterile neutrino detection.

Neutrino-nucleus interactions which are coherent incharacter have the advantage of scaling as A2, where Ais the mass number of the target nucleus. For a targetnucleus with atomic number Z and neutron number N ,the cross-section as a function of recoil kinetic energy isgiven by the expression [13]:

d�(⌫A ! ⌫A)

dT=

G2F

4⇡MAQ

2W (1� MAT

2E2⌫

)F (q2)2 (1)

where GF is the Fermi coupling constant, MA is the massof the nucleus, F (q2) is the nuclear form factor, and QW

is the weak charge, defined by the relation:

QW = N � Z(1� 4 sin2 ✓W ) (2)

In our study, we will mainly consider mono-energeticelectron capture sources, all of which have neutrino en-ergies below 1 MeV. The maximum momentum transferfor such sources is |qmax| 2E⌫ ⌧ 2 MeV. Since theform factor F (q2) ! 1 for cases where the scale of themomentum probe is much larger than the size of the nu-cleus, we can safely ignore this correction factor for ouranalysis.

The maximum kinetic energy imparted on the nuclearrecoil depends on the neutrino energy and the mass ofthe recoil target:

Tmax E⌫

1 + MA

2E⌫

(3)

For a silicon target at 1 MeV, that implies a maximumkinetic energy of about 50 eV. For a germanium targetthe maximum kinetic energy would be around 20 eV.Such low kinetic energies are why detection of the processhas been so elusive to date. The fraction of events that isdetectable by a given experiment depends crucially on theinherent threshold of the detector. For a monochromaticsource of energy E⌫ , the e↵ective cross-section can bewritten as:

�̄ =

Z Tmax

T0

d�

dT(E⌫) · dT (4)

�̄ = �0(E⌫) · f(E⌫ , T0) (5)

where �0(E⌫) ⌘ G2

F

4⇡ E2⌫Q

2W is the total integrated cross-

section assuming no energy threshold and f(E⌫ , T0) rep-resents the fraction of events above a given threshold en-ergy, T0. In the limit that E⌫ ⌧ MA, the fraction ofevents above threshold can be written as:

f(E⌫ , T0) = (1� T0

Tmax)2 (6)

Any detector hoping to detect such a signal with su�-cient statistics must achieve as low a recoil threshold aspossible.

THE 37Ar SOURCE

Oscillometry-based measurements benefit greatly fromthe use of mono-energetic neutrino sources, since it re-duces the measurement to a pure flux-versus-distanceanalysis. Low energy electron capture sources provide themost e↵ective and clean source of such neutrinos avail-able to date [14]. A number of such neutrino sources havebeen considered in the literature; a few of them are listedin Table I. Historically, two such high intensity sourcehave been produced for neutrino studies: a 51Cr source,used by the SAGE and GALLEX experiments [15, 16],and an 37Ar gaseous source used in conjunction with theSAGE experiment [5].The 37Ar source is perhaps the most ideal with respect

to a future coherent-scattering measurement, for a num-ber of reasons:

• 37Ar produces a very high-energy, near mono-energetic neutrino (90.2% at 811 keV, 9.8% at 813keV).

• With the exception of inner bremsstrahlung pho-tons, almost all the energy is carried away by neu-trinos, facilitating shielding and enabling the sourceto be extremely compact.

• Extremely high production yield per reactor target.

The SAGE collaboration successfully produced such asource with a total activity of about 400 kCi to be usedin conjunction with their gallium solar neutrino detec-tor. The source was also very compact, extending 14 cmin length and 8 cm in diameter, including shielding [17].Further reduction in size might be possible, even with in-creased activity, making 37Ar an ideal portable neutrinosource.Despite its clear advantages as a source and its his-

torical precedent, production of such sources is lessthan ideal. The reaction process by which it is gener-ated (40Ca(n,↵)37Ar) requires a high fast neutron fluxabove 2 MeV, an energy regime where few reactors op-erate [18, 19]. Production also requires large amounts of

CNS as Probe

• Channel opens new doors for a variety of physics

• Physics of supernovae (and detection)

• Probe into the form factors of nuclei at very small Q2 that are otherwise difficult to probe.

• Sensitive to new couplings

• Renewed interest in nuclear proliferation monitoring (inverse beta decay still stronger probe in reactor tampering, but coherent scattering also explored).

Form factor

Understanding the structure of the nucleus

Form factor, F (Q2) is the Fourier transform of the density

distributions of protons and neutrons in the nucleus.

F (Q2) =1

QW

∫

[

ρn(r)− (1− 4 sin2 θW )ρp(r)] sin (Qr)

Qrr2dr

density distributions

⟨R2⟩1/2SGII = 3.405 fm

⟨R2⟩1/2G202 = 3.454 fm

22

The Case for Sterile Neutrinos

• A number of recent (and not so recent) results seem to indicate the possibility of sterile neutrinos.

• Evidence stems from a variety of sectors:

• Cosmology (somewhat diminished from most recent PLANCK data)

• Short-baseline (LSND/MiniBooNE)

• Reactor anomaly

• Gallex / SAGE Calibration source

• All suggestive, but no “smoking gun” accepted by the community at the moment.

Reactor Anomaly

MiniBooNE

The Argument for Coherent Scattering

•Coherent scattering allows to probe neutrinos using a neutral current channel; oscillation signature would be clear sign of active → sterile mixing.

•Previous evidence mainly in energy. Uses distance (oscillometry) instead, same detector:

•For Δm2 ~ 1 eV

•L ~ O(1 meter); Eν ~ O(1 MeV)

•Simpler if just source is monochromatic.

The Argument for Coherent Scattering

•Coherent scattering allows to probe neutrinos using a neutral current channel; oscillation signature would be clear sign of active → sterile mixing.

•Previous evidence mainly in energy. Uses distance (oscillometry) instead, same detector:

•For Δm2 ~ 1 eV

•L ~ O(1 meter); Eν ~ O(1 MeV)

•Simpler if just source is monochromatic.

Look for Oscillations in Coherent Scattering

What is to gain?

Fundamental challenges

Techniques at play

One big obstacle...

• The recoil energy is extremely low compared to other reactions. This often offsets the gains in rate from the coherent enhancement.

• This leaves people with essentially three optimization paths:

1.Be smart about the target

2.Be smart about the neutrino energy

3.Be smart about the recoil detection

Tmax

E⌫

1 + MA2E⌫

Tmax

Element 3 MeV 30 MeV

Si 650 eV 65 keV

Ar 500 eV 50 keV

Ge 250 eV 25 keV

Os 100 eV 10 keV

Neutrino Sources

Sources Pros Cons

Radioactive Sources (Electron Capture)

Mono-energetic, can place detector < 1m from source,

ideal for sterile neutrino search

< 1 MeV energies require very low (~10 eVnr) thresholds,

limited half-life, costly

Nuclear Reactors Free*, highest flux

Spectrum not well known below 1.8 MeV, site access can be

difficult, potential neutron background

Spallation/Decay at Rest

Higher energies can use higher detector thresholds, timing can

cut down backgrounds significantly

Prompt neutron flux; large shielding or distances needed

* Nothing is really free.

Neutrino Sources

0.5 1.0 5.0 10.0 50.0109

1011

1013

1015

1017

1019

1021

En @MeVD

Flux@nêHMe

VsLD

SNS:nmnmne

Reactors:MIT reactorH5 MWLAdvanced Testreactor H110 MWLSan Onofrereactor H3.4 GWL

EC Sources:37Ar H5 MCiL

• The variety of sources trade off flux, energy and knowledge of spectrum.

*from Tali Figueroa and Adam Anderson

Sources

• Spallation source provides well-timed high energy neutrino beam.

• Reactor source provides continuous, high intensity lower energy beam.

• Radioactive sources would provide clean mono-energetic neutrinos (ideal for oscillometry studies)

the neutrino flux, determined with electron-neutrino elastic scattering (⌫ee� ! ⌫ee

�)as measured by ultra-large water Cerenkov detector(s), will have a systematic un-certainty of 1% with dominant contributions from the cross section and energy scaleuncertainties [155]. The statistical uncertainty on the flux depends on the run period,but is expected to be on the order of 1% as well. The near accelerator site is envi-sioned at or near the surface of the laboratory with the other cyclotrons located manykilometers away. Note that the far sites will produce insignificant coherent rates dueto the 1/r2 dependence of the flux. However, the near accelerator can provide a sig-nificant event rate, during the 13% beam-on time, for detectors which are sufficientlyclose and large. Examples of other physics opportunities with this near acceleratorare discussed in Refs. [152, 159, 160].

In order to provide realistic calculations, we examine three dark matter experi-ments which are drawn from the designs of GEODM [161], LZ [129], and MAX [162].These experiments use germanium, xenon, and argon as their targets, respectively.Note that neon is also commonly considered as an alternative target medium in thenoble liquid detectors mentioned. We assume that the accelerator and beam dumpare located at or near the surface. As GEODM is proposed for the DUSEL 7400 ftlevel and LZ/MAX are proposed for the 4800 ft level, we simply consider baselinelengths of 2.3 km and 1.5 km, respectively. The rates for each target are calcu-lated for a ton·year fiducial exposure since the design of each detector is still underconsideration.

As discussed above, the coherent neutrino-nucleus interaction takes place at verylow recoil energies. Fig. 7-2 shows the distribution of recoil energies for a DAR sourcewith 20Ne, 40Ar, 76Ge, and 132Xe. The experimental rates will strongly depend uponthe recoil energy threshold for reconstructed events, Tmin. As the exact values of thiscut for the various detectors are unknown, we consider five possible values of Tmin.For the aforementioned targets, we find the rates given in Table I, where we assume100% efficiency for detecting events in the time-window above the threshold Tmin.

We note that the coherent event rates for dark matter detectors at their nomi-nal depths underground are in the 0-35 events/ton/year range depending on target,baseline, energy threshold, and unrealistically assuming 100% detection efficiency.This is too low to be competitive with presently used neutron sources for detectorcalibration. Neutrons are adequate for energy calibration, despite their propensity tomultiple scatter and activate the detector. However, demonstrating a measured excessbetween beam-on and beam-off times corresponding to the expected neutrino signal

Figure 7-1: Energy distribution of neutrinos in a DAR source, from Ref [155].

175

Events/ton/year For Tmin

at distance target 0 keV 5 keV 10 keV 20 keV 30 keV1.5 km 40Ar 11.1 9.1 7.5 4.9 3.1

132Xe 36.4 16.3 6.6 1.1 0.176Ge† 21.9 14.6 9.4 3.5 1.4

2.3 km 76Ge 9.3 6.2 4.0 1.5 0.6

Table 7.1: Coherent neutrino scattering events/ton/year (with the accelerator runningat 1 MW with a 13% duty factor) for various detector layouts and thresholds. Therates reported assume 100% detection efficiency. †The present plan is for the GEODM(76Ge-based) baseline to be 2.3 km–although 1.5 km is included for completeness.

Recoil energy (keV) 20 40 60 80 100 120

Even

ts p

er k

eV p

er y

r per

ton

-210

-110

1

Ne Ar Ge Xe

Figure 7-2: Recoil energy distributions for coherent scattering 1.5 km from a DARneutrino source for Ne, Ar, Ge, and Xe. The rates reported assume 100% detectionefficiency.

would be a valuable consistency check for any dark matter detector. Such a verifi-cation would certainly aid the corroboration of any new dark matter limit/detectionclaim between DUSEL dark matter experiments.

Detection at a Ton-Scale Dark Matter Experiment

Next, we consider one of the nuclear targets mentioned above (76Ge) in more detail.GEODM is a proposed ton-scale dark matter detector [161] based on the cryogenicGe crystal technology used in the CDMS experiment. The target design for GEODMis an array of 300 ⇠5 kg Ge crystals operated at 40 mK with a total target mass of⇠1500 kg. Interaction events in an individual crystal produce populations of athermalphonons and electron-hole pairs which are measured by various phonon and ionizationsensors lithographically patterned on the crystal surfaces. The ratio of ionization tophonon signals for an event is a powerful discriminator between electron and nuclearrecoils. The signals also enable precise determination of the position and energy ofeach event, which allow volume and energy cuts. This information is used to setthe number of electron recoils that can pass the cuts and pose as nuclear recoils.

176

Decay at rest sources

10 20 50 100 200 500 1000 20000.1

0.51.0

5.010.0

50.0100.0

Threshold Energy @eVnrD

IntegratedRate@ev

têHkgdayLD

CNS Integrated Rate at Various Reactors

MITR - SiATR - SiSONGS-SiMITR -GeATR -GeSONGS-Ge

Reactor sources

What is to gain?

Fundamental challenges

Techniques at play

Different Technological Approaches

• Much like the diversity seen in dark matter detectors, a variety of technologies are being pursued.

• Each optimizing on a particular facet of source, detector threshold, size, and background rejection.

• Larger masses typically imply larger thresholds.

• A higher energy (SNS) allows greater change for detection. Greater fluxes but at lower energies available at reactors.

P-Contact HPGe

C-4 (Cogent-4)

CsI(Na)

Liquid Xe

MITR

SNS

The challenge of low thresholds

Scintillation

Ionization

Tmax

E⌫

1 + MA2E⌫

• The recoil energy is extremely low compared to other reactions. This often offsets the gains in rate from the coherent enhancement.

• Methods involving e-h pair detection have very low (or zero) quenching factors at these energies.

• Likewise, energies of at least few eV required to produce scintillation photons. Would yield poor statistics.

• Pure phonon detection suffers from no quenching effects, but difficult to build large scale detectors.

The challenge of low thresholds

• The recoil energy is extremely low compared to other reactions. This often offsets the gains in rate from the coherent enhancement.

• Methods involving e-h pair detection have very low (or zero) quenching factors at these energies.

• Likewise, energies of at least few eV required to produce scintillation photons. Would yield poor statistics.

• Pure phonon detection suffers from no quenching effects, but difficult to build large scale detectors. 10 20 50 100 200 500 1000 20000.0

0.2

0.4

0.6

0.8

1.0

Phonon Energy @eVnrD

f n

Lindhard Theoretical Ionization Fraction

fn Si

fn Ge

fn =kg(✏)

1 + kg(✏)

*from Tali Figueroa

10 20 50 100 200 500 1000 2000

0.050.10

0.501.00

5.0010.00

Threshold Energy @eVnr or eVeeD

IntegratedRate@ev

têHkgdayLD

Si-Ionization

Si-Phonon

Event rates for phonon versus ionizationIonization readout requires much lower

thresholds for the same rates

Ionization

Phonons

*from Tali Figueroa and Adam Anderson

10 20 50 100 200 500 1000 2000

0.050.10

0.501.00

5.0010.00

Threshold Energy @eVnr or eVeeD

IntegratedRate@ev

têHkgdayLD

Ge-Ionization

Ge-Phonon

Ionization

Phonons

Ionization readout requires much lower thresholds for the same rates

Event rates for phonon versus ionization

*from Tali Figueroa and Adam Anderson

Backgrounds

• Requirements for detection similar in scope to those from dark matter

• Radiogenic backgrounds still need to be suppressed, discriminated, or shielded.

• Need levels comparable to 1-1000 counts keV-1 kg-1 d-1

• Current detectors pushing where backgrounds sufficiently in control to achieve favorable signal-to-noise levels.

[keV]recE0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

even

ts /

50 e

V / y

ear

210

310

410

Total signalOs coherent scatter (10 kg)Ge coherent scatter (10 kg)Total cosmic bkg. (10 kg)Total radiogenic bkg. (10 kg)Cosmogenic activation

Ricochet event rates

Ricochet

COGENT

Neutrons

• Even if electromagnetic backgrounds fully eliminated, recoils still share the same characteristics as recoil signals.

• Especially problematic for SNS and reactor experiments (less so for source experiments)

• Only real remedy is shielding, which can be costly.

Engineering Design of a Fission Converter-Based Epithermal Beam for Neutron Capture Therapy

1.00E+00

1.00E-01

1.00E-02

1.00E-03 I , 1 , 11.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02

Energy (MeV)

Figure 4.17: Neutron Spectrum at the Patient Position after

All Shutters are Closed

287

Neutron Mitigation

• Experiments have taken an active role in mapping and measuring neutron backgrounds, particularly higher energy neutrons.

• Guides shielding design and background estimates for sensitivity calculations.

The challenges for coherent neutrino detector share much in common with those of dark matter detection. As such, the technology has matured significantly that measurement is within sight.

Possible to leverage sources and detector technology to extract a potential signal.

The issue of backgrounds (particularly neutrons) is of fundamental importance. One that is not easy to remedy or dismiss. Any potential signal will require scrutiny to ensure it is not a neutron recoil background.

Thank you