Y2 Neutrino Physics (spring term 2017)

Dr E Goudzovski eg@hep.ph.bham.ac.uk

http://epweb2.ph.bham.ac.uk/user/goudzovski/Y2neutrino

Lecture 8

Solar neutrino experiments (part I)

Previous lecture

1

Atmospheric energy spectrum: wide, GeV range; flavour

composition: /e2 at GeV energy, /e>2 at higher energy.

The leading atmospheric (and astrophysical) neutrino detection

technology: water and ice Cherenkov detectors.

Observables sensitive to oscillations: muon/electron ratio,

up-down asymmetries, zenith angle (z) and L/E distributions.

Atmospheric neutrino observations are consistent with

oscillations with m2=2.4×103 eV2 and

near-maximal mixing (atm 45o). [Oscillations established in 1998]

This lecture

2

The sun as thermonuclear reactor.

The Homestake (Cl-Ar) radiochemical experiment.

Solar neutrino detection with water Cherenkov detectors.

Gallium-Germanium (Ga-Ge) radiochemical experiments.

Super-Kamiokande solar neutrino results.

The solar neutrino problem.

Reading list:

B.R. Martin and G. Shaw. Particle physics. Chapter 2.3.2.

D. Perkins. Introduction to high energy physics. Chapter 9.7.1.

C. Sutton (chapter 6), N. Solomey (chapter 4).

“Solar neutrino experiments” in Soler et al.

Journal articles: see course webpage.

Sun as thermonuclear reactor

3

Net fusion reaction in the solar core:

Solar power: P = 3.8×1026 W = 2.4×1039 MeV/s.

Neutrino emission rate: F = 2P/Q 2×1038 s1.

Distance to source: LSun-Earth = 1.5×1011 km.

Neutrino flux on Earth: = F / (4L2Sun-Earth) 6.6×1010 cm2 s1.

NB: ATMOSPHERIC~10 cm2 s1; reactor (L=10m): REACTOR~1013 cm2 s1.

Average thermal energy produced:

Q (4mp mHe) <E(2e)> 26 MeV.

That is 26 MeV / (4×938 MeV) = 0.7% of the initial mass!

4He = = (ppnn)

Fusion is strongly suppressed:

(1) weak interaction; (2) Coulomb repulsion.

Mean proton lifetime in the core (~150 g/cm3, T~107 K): p~1010 years.

Extremely low power production density in the core: ~300 W/m3.

(the pp-cycle)

Main solar neutrino components

4

pp neutrinos: E<0.42 MeV pep neutrinos: E=1.44 MeV

(99.77%) (0.23%)

Be neutrinos: E1=383 keV; E2=862 keV Boron (8B) neutrinos: E<14.1 MeV

The most energetic of the

significant components

Highest energy; easiest to detect.

Many experiments measure only this component.

The dominant component

4

Deuterium/Helium production

Further up the chain: Beryllium and Boron

(then 2H 3He 4He,

without producing significant

neutrino fluxes)

3

A non-assessed problem:

compute the max neutrino energy;

assume the protons to be at rest

Solar neutrino spectrum

5

Flu

x a

t Eart

h s

urf

ace (

cm

2s

1M

eV1 o

r cm

2s

1)

pep: 1.44 MeV

Be: 0.862 MeV

Be: 0.383 MeV

Hep: Emax=18.8 MeV

Boron: Emax=14.1 MeV

~104 of total flux

pp: Emax=0.42 MeV

Cl-Ar Thresholds: Ga-Ge Super-Kamiokande SNO

Radial distribution of e production in core

~1 MeV energies:

~103 times lower

than atmospheric

neutrinos

Flavour

composition:

e

The Homestake experiment

6

Radiochemical (not “real time”) experiment

Detector: 380 m3 (615 tonnes) of C2Cl4

(tetrachloroethylene; “dry cleaning liquid”). 37Cl natural abundance: 24%.

Expected 37Ar production rate 1.4/day.

Energy threshold:

Homestake gold mine (South Dakota, USA)

Method:

Extraction of 37Ar every few months by flushing the tank with He.

Purification of 37Ar; mix with 36Ar; detection of beta decay

back into 37Cl in a proportional counter (half-life: 1/2 = 35 days).

Detector depth: 1,478 m.

Construction: 196566.

Data taking: 196894.

Not sensitive to pp-neutrinos (E<0.42MeV)

Not sensitive to solar muon or tau neutrinos

Principle (Bruno Pontecorvo, 1946):

detected by induced radioactivity

Homestake expected event rate

7

From the expected neutrino spectrum in slide 5,

Solar neutrino flux with energy above threshold (0.814 MeV):

~ 107 cm2s1.

That is 1.7 interactions / day

Rate of neutrino interactions per 37Cl atom:

The Homestake detector: 615 tonnes of C2Cl4.

Molar mass: =166 g/mol. Therefore = 4×615 t/166 g = 1.5×107 mol of Cl.

Natural abundance of 37Cl: 24%. Therefore =3.6×106 mol of 37Cl.

Approximate IBD cross-section at E = few MeV (see lecture 4): 1042 cm2.

Rate of IBD interactions (i.e. 37Ar production):

37Ar37Cl transition by K-capture

8

Auger electron

(Ekin=2.8 keV)

detected in the counter

K-capture 37Cl atom in an excited state

is forbidden by energy conservation

(ep ne)

e

e

is allowed

X-ray photon

The Homestake detector

9

Homestake detector construction

Volume: 380 m3, C2Cl4 mass: 615 tonnes

Homestake experiment: results

10

Measured 37Ar production rate:

SSM prediction:

SNU (Solar Neutrino Unit):

the unit of event rate in

radiochemical experiments

1 SNU =

1 event/s/(1036 target atoms)

Year

Standard Solar Model prediction

Nobel Prize 2002: Raymond Davis

(1036 atoms ~ 1012 moles

~106 tonnes)

Mostly boron (8B) neutrinos

First detection of Solar e; evidence for Solar e deficit

37Ar production rate measurements over 25 years

Average measured rate

e detection with the Cl method

11

forbidden by lepton flavour conservation

allowed

but 37S37Cl beta-decay lifetime is too small: (37S) = 305 s

forbidden by electric charge conservation

Cl experiments (by Davis and Harmer) at nuclear reactors in 1950s

(Brookhaven, Savannah River) failed to detect .

Combined with CowanReines discovery via ,

first evidence that neutrinos and anti-neutrinos are distinct particles.

(sulfur)

Detection of with the Chlorine method is impossible

Water Cherenkov detectors

12

Detection via elastic scattering (ES) :

Electron is detected via Cherenkov light.

High detection threshold (Eth5 MeV)

to suppress natural decays.

Sensitive to 8B neutrinos only.

e range in H2O (at 5 MeV): 2.5cm.

ES dominated by e scattering:

(e) 6.5() 6.5()

(all neutrons are confined in 16O: a doubly magic nucleus)

Solar neutrinos (E ~ few MeV) have insufficient energy

for the “atmospheric” reaction (inverse beta decay):

ES RATE ~ (e) + 0.15() + 0.15()

IS FORBIDDEN

Real-time solar neutrino detection

e e

e e

W

(ES-CC) (ES-NC)

e

Z0

x x

e

ES directionality

13

Water Cherenkov detectors were

the first neutrino telescopes

directly observing the Sun.

First direct evidence that

solar heat is generated

in a nuclear fusion reaction.

Birth of neutrino astronomy.

Directionality of elastic scattering

on electrons:

For Ee = 10 MeV typical for

this application,

Cf. Angular diameter of the Sun:

(+contribution from e multiple

scattering, ~250 at 10 MeV)

Kamiokande results

14

Target volume: 2,100 m3 H2O.

Instrumentation: 948 PMTs.

Data taking: 19871995.

flux: Data/SSM

Cosine of angle S wrt the sun

SSM expectation

Data

~25 times smaller predecessor of

the Super-Kamiokande

Solar neutrino (e) deficit confirmed

Y.Fukuda et al., Phys. Rev. Lett. 77 (1996) 1683

Background

from natural

radioactivity

cos S

Solar neutrinos observed: 39035.

Measured solar neutrino flux

above detection threshold

(assuming all to be e)

= (2.800.38)×106 cm2s1.

(558)%

of the SSM prediction

Solar neutrinos by 1990

15

Cl threshold Kamiokande

threshold

Solar neutrino deficit observed by

Homestake and Kamiokande.

However, high thresholds:

both experiments were not sensitive

to the main pp neutrino component.

Possible errors in the solar models?

The solar neutrino problem

Theory (SNU)

Homestake

experiment

Theory

Sola

r neutr

ino f

lux

Kamiokande

experiment

8B 8B

7Be

Gallium experiments

16

Energy threshold:

Very low! Compare to 0.814 MeV for the Cl-Ar method.

Sensitive to the dominant pp-neutrinos.

Experiments:

(1) SAGE (19902008, Baksan, Russia): 50 tons of liquid metallic Ga;

(2) GALLEX/GNO (19912003, Gran Sasso, Italy): 30 tons of Ga in GaCl3 solution.

71Ga natural abundance: 40%.

Method:

71Ge extracted every few weeks, converted chemically to GeH4 gas.

GeH4 injected into a proportional counter, electron capture detected:

e+71Ge e+71Ga (half-life 1/2 = 11.4 days; X-rays and Auger electrons).

Radiochemical method: (Vladimir Kuzmin, 1965)

Gallium laboratories

17

Baksan gorge,

Caucasus mountains, Russia Gran Sasso national park,

Apennines, Italy

Mountains provide natural shielding against cosmic rays

A non-assessed problem: compute the 71Ge production rate in these experiments,

estimating the neutrino flux from slide 5.

SAGE experiment (Russia)

18

Tunnel leading to Baksan Laboratory

Gallium tanks and

Germanium extraction system

Motors stirring liquid Gallium.

Melting point: 300C.

Gallium: used in microelectronics.

Drastic fall of price in the 1970s.

Gallium experiments: results

19

SAGE rate

168 extractions: 19902008

PRC80 (2009) 015807

GALLEX/GNO rates

19902003

PLB616 (2005) 174

SSM prediction:

SSM

SSM

~40% deficit of

solar neutrinos,

but this time

mainly pp-neutrinos

Gallex

GNO

Super-Kamiokande results

Full solar cycle (19962006)

Neutrino detection via elastic scattering:

(highly directional)

First observation of Earth’s orbit

eccentricity with neutrinos

Target volume: 50,000 m3 H2O.

Data sample: 19962008.

Phase I

Natural radioactivity

( decay)

Solar e

Phys. Rev. D73 (2006) 112001

Phys. Rev. D78 (2008) 032002

Seasonal variation of flux

Evidence for the solar origin

Data/(SSM expectation) = 0.450.02

another evidence for Solar deficit

Precision measurements of neutrino flux

and energy spectrum from

~20000 solar neutrino candidates.

Solar neutrinos by 2000

21

Deficit of both 8B and pp neutrinos,

but no “smoking gun” signal

for the origin of the deficit.

Is the Sun dying inside?

Does it produce dark matter?

Sola

r n

eutr

ino

flu

x

The Solar neutrino problem

Theory

Theory (SNU)

Radiochemical Radiochemical

8B 8B

8B

pp

7Be 7Be

Theory (SNU)

Cl Kamiokande Ga

Homestake

Kamiokande

Super-K

Summary

22

The Sun is a powerful e emitter (~1038/s). Most solar neutrinos

are in sub-MeV energy range; minor components extend to ~20 MeV.

By 2000, the solar neutrino flux was measured to ~5% precision

by water Cherenkov experiments [(Super-)Kamiokande]

and radiochemical experiments [Homestake, SAGE, GALLEX/GNO].

Detector energy threshold is a key issue; only Gallium experiments

are sensitive to the dominant (pp) solar neutrino component.

The solar neutrino problem by 2000: all experiments observed

~50% deficit of Solar e with respect to solar model predictions.

No convincing evidence for its origin.