After-dinner talk at the Northern California Chapter of Health Physics Society meeting - 2018.01.18

1. NEUTRINOS:
The Chameleon in the
Elementary Particle Zoo
Alan Poon (awpoon@lbl.gov)
Nuclear Science Division
1

2. Steve Chu at Stuart Freedman Memorial Symposium (UC Berkeley, 2014)

3. I was at this point two years ago
Steve Chu at Stuart Freedman Memorial Symposium (UC Berkeley, 2014)

4. A conference summary talk this summer
Steve Chu at Stuart Freedman Memorial Symposium (UC Berkeley, 2014)

5. Steve Chu at Stuart Freedman Memorial Symposium (UC Berkeley, 2014)
I am here
I thank you to give me a chance to redeem my reputation.

6. Matter
Atom
down quarkup quark
gluon
Nucleus
Electron
Proton

7. Standard Model (SM) of elementary particles
Graphics from ThoughtCo.com
In the SM,
neutrinos are massless
In the SM,
three neutrino flavors

9. Neutrino Sources
Most neutrino sources are natural
(also man-made from accelerators and reactors)
Figure from https://masterclass.icecube.wisc.edu/en/learn/detecting-neutrinos
• Huge flux
• Broad energy range

10. It’s hard to detect neutrinos!
Formaggio and Zeller, Rev. Mod. Phys. 84, 1307 (2012)
[10-27cm2]
“Probabilityofdetection”
“Cross section” is too small !

11. An analogy of “cross section”
Canucks TV - 2017/12/13 https://www.youtube.com/watch?v=FblN3Dg_Wdg

12. Shielding neutrinos
λ =
1
ρσ
σ ∼ 10−43
cm2 @1 MeV
ρ =
(82 e/atom)(11.3 g/cm3
)(6.02 × 1023
atom/mole)
(207 g/mole)
= 2.7 × 1024
e/cm3
λ ∼ 4 × 1016
m = 4 light years
attenuation
length
density of
“target”
cross section
• Similar considerations as radiation shielding (e.g. Pb
for shielding γ-ray). You want the radiation to interact with
the shield.
Ex: 1-MeV neutrinos scattering off atomic electrons in Pb:
To study neutrinos, we need
a huge neutrino source (shoot more pucks)
and a large target (more slots)

13. Our Sun: how does it generate energy?
• It takes thousands of years for light generated in the solar interior
to reach the solar surface (and then another 8 minutes to reach us).
• The Sun generates energy
in its core by fusing protons
into 4He through (mostly)
the proton-proton chain:
•To prove that the Sun and other main sequence stars are powered
by fusion, one can search for neutrinos νe (electron-type neutrinos).
4 p 4
He + 2 e + 26.7 MeV

14. Nuclear reactions: detecting solar neutrinos
• First detection of solar neutrinos in a gold mine by
Ray Davis, Jr. was a heroic effort.
• Davis won the Nobel Prize in Physics in 2002.
• Used this tank of cleaning fluid (615 t) C2Cl4 to
capture solar neutrinos
• Flushed tank once a month, collected a few (if
lucky) 37Ar atoms in a glass vial (proportional
counter) each time, look for its decay back to
37Cl…repeat for 3+ decades!
• Saw ~1/3 of νe (electron-type neutrinos) that was
predicted from model.

15. Nuclear reactions: detecting solar neutrinos
“Ray Davis tells me that the experiment is simple (`only plumbing’) and that the
chemistry is `standard.’ I suppose I must believe him, but as a non-chemist I am
awed by the magnitude of his task and the accuracy with which he can accomplish
it. The total number of atoms in the big tank is about 1030. He is able to find and
extract from the tank the few dozen atoms of 37Ar that may be produced inside by
the capture of solar neutrinos. This makes looking for a needle in a haystack seem
easy.” - J. Bahcall

16. Why going underground?
• High energy particles (mostly
protons) from galactic and extra-
galactic sources bombard our
Earth’s upper atmosphere.
• They interact with nuclei in the
atmosphere (nitrogen, oxygen...)
and create a shower of other
particles.
• A significant fraction of these
secondary particles are muons, a
heavier cousin of electrons.
• Dosage at higher elevation is
higher. The atmosphere is a
shielding.

17. Shielding from cosmic rays
Figure from: Ann.Rev.Nucl.Part.Sci. 67 (2017) 231-251
Solar neutrino experiments must be built underground

18. What happened to the solar neutrinos?
Nature video: https://www.youtube.com/watch?v=ZP5jyWJ5hNo&t=126s

19. Sudbury Neutrino Observatory (SNO)
6800’ underground
near Sudbury, ON, Canada
(hometown of Alex Trebek)

20. Mining for knowledge...
→ →
→
→ 70000 underground
showers...

21. Why cleanroom and showers?
• Rock / mine dust are “hot” for
solar neutrino experiments
(from natural U and Th).
• In fact, all construction
materials we used in the
experiment had to be
screened for low radioactivity.
• Extra precaution in handling
and process (e.g. used non-
thoriated welding rods for
welds)
• One teaspoon of mine dust
would ruin the whole
experiment.
Image from inhabitat.com

22. Natural Decay Chains
𝛼
𝛽

23. Sudbury Neutrino Observatory (SNO)
• Use 1000 tonnes of D2O
Ultra-pure heavy water:
Amount of U or Th ~ 0.1 - a few ppq

24. Sudbury Neutrino Observatory (SNO)
• Use 1000 tonnes of D2O
• Can detect separately:
– νe (electron-type neutrinos)
– νe,µ,τ (all active neutrinos)

25. Sudbury Neutrino Observatory (SNO)
• Use 1000 tonnes of D2O
• Can detect separately:
– νe (electron-type neutrinos)
– νe,µ,τ (all active neutrinos)
• Observed:
– ~1/3 νe
– ~2/3 νµ,τ

26. Sudbury Neutrino Observatory (SNO)
• Use 1000 tonnes of D2O
• Can detect separately:
– νe (electron-type neutrinos)
– νe,µ,τ (all active neutrinos)
• Observed:
– ~1/3 νe
– ~2/3 νµ,τ
= Solar Model Prediction+

27. Sudbury Neutrino Observatory (SNO)
• Use 1000 tonnes of D2O
• Can detect separately:
– νe (electron-type neutrinos)
– νe,µ,τ (all active neutrinos)
• Observed:
– ~1/3 νe
– ~2/3 νµ,τ
• This means:
– νe transforms to νµ,τ while traveling from the Sun to us
– Neutrinos must have mass
= Solar Model Prediction+

28. Baseball → Tennis ball → Ping Pong ball
Minute Physics (https://www.youtube.com/watch?v=7fgKBJDMO54)

29. Saw that in atmospheric neutrinos too!
Kajita
First demonstration of physics
beyond the Standard Model (neutrino
oscillations, hence neutrino mass)
Neutrino 1998 conference

30. Nobel Prize in Physics 2015
The Nobel Prize in Physics 2015 was awarded jointly to
Takaaki Kajita and Arthur B. McDonald “for the discovery of
neutrino oscillations, which shows that neutrinos have mass.”

31. Future neutrino oscillation studies
JUNO (20kt liquid
scintillator)
IceCube/DeepCore/PINGU
Supernovae
Now
2026
KM3Net-ORCA
DUNE (40 kt LAr)
Hyper-K (187 kt H2O)
INO
CHIPS
SuperK

32. Natural radioactivity: Do you know that...
• Natural radioactive decays “power” our Earth
• How do we know?
We have observed the
neutrinos from the beta
decays (“geo-neutrinos”) in
the chains.
Radiogenic heat ~ 16±6 TW

33. The mystery of neutrino mass
Atom
down quarkup quark
gluon
Nucleus
Electron
Proton
Graphics from Scientific America
Why is the mass of the neutrinos so small?
Strictly speaking, the figure on the right is incorrect as the
neutrino “flavor" states are not mass eigenstates. Therefore,
saying “the mass of electron-neutrino” is incorrect.
(More later)

34. m(elephant)
m(ant)
⇠
6000 kg
0.01 mg
⇠
m(top)
m(⌫)

35. Determining the neutrino mass from β decays
• The shape of the β
spectrum near the end point
depends on the neutrino
mass.
3
H 3
He + e + e

36. The Karlsruhe Tritium Neutrino Experiment
~140 scientists, 20 institutions in 6 countries
Tritium
Source
1011
e- / s
Tritium pumping
& e- transport
Spectrometer
10 m diameter
24 m length electron
detector
< 1 e- / s
~70 m beamline
Located in Karlsruhe, Germany

37. Working Principles
Mass sensitivity goal: ~0.2 eV (3.6x10-34 g)
Data-taking starts this year!

38. Working Principles
https://goo.gl/55nC7s

39. Delivery of the spectrometer

40. Delivery of the spectrometer

41. Thank you!
Neutrinos, they are very small
They have no charge and have no mass
And do not interact at all.
The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids down a drafty hall
Or photons through a sheet of glass.
They snub the most exquisite gas,
Ignore the most substantial wall,
Cold-shoulder steel and sounding brass,
Insult the stallion in his stall,
And, scoring barriers of class,
Infiltrate you and me. Like tall
And painless guillotines, they fall
Down through our heads into the grass.
At night, they enter at Nepal
And pierce the lover and his lass
From underneath the bed-you call
It wonderful: I call it crass.
- John Updike

42. http://goo.gl/aZ6HSJ

44. An explosive idea - Reines and Cowan
- Fred Reines
Reines and Cowan scrapped this idea.

45. Discovery of the (Anti-)Neutrinos
€
νefrom reactor
p
e+
511 keV γ
511 keV γ
e+e- annihilationn
γγ
γ’s from n
capture on
Cd
Cd
Liquid Scintillator
γ
Liquid Scintillator
H2O+CdCl2
• F. Reines & C.L. Cowan [Physical Review 117, 160 (1960)]
• Detection at Savannah River reactor plant
Inverse Beta Decay
¯νe + p → n + e+

46. “Saw” the coincidences!
neutrino
neutrino
noise
cosmic ray

47. Heusser
Annu. Rev. Nucl. Part. Sci. 45: 543 (1995)
Sea level
15 m.w.e.
Cosmic-ray Primaries & Secondaries
• Minimal overhead burden goes a long way in reducing backgrounds
induced by nucleonics:
• But next-generation experiments need to go below 4000’ or more.
The deeper the better

48. Inconvenient truths (for low-background experiments)
• Time scales:
• Long-lived radioactive isotopes from supernova explosions in the
past have been in our Earth since its formation.
• Radiogenic heat from the decays of 238U, 232Th and 40K is a main
component of our Earth’s internal heat.
• Half-lives:
Age of the universe 13.8 x 109 years
Age of the Earth 4.5 x 109 years
238U 4.468 x 109 years
232Th 14.05 x 109 years
40K 1.251 x 109 years
These primordial radioisotopes do not decay away quickly

49. Cherenkov radiation - seeing relativistic charged particles
49
Graphics from http://physicsopenlab.org/
cos θC =
1
βn
θC = 41∘
in H2O (β~1)
Event in SNO
Cherenkov light is emitted when a charged
particle passes through a dielectric medium
at a speed higher than the speed of light in
that medium.

50. The deuteron
50
CC
NC
ES
νe + d → p + p + e−
νx + d → p + n + ν′x
νx + e−
→ νx + e−
•2 km underground at Vale’s Creighton
mine near Sudbury, ON, Canada
•1000t of D2O in a 12-m φ acrylic sphere

51. Neutrino-electron scattering
51
νe
νe
e
e
θ
(Te, ⃗pe)
(Eν, ⃗pν)
Te =
2meE2
ν cos2
θ
(me + Eν)2 − E2
ν cos2 θ
• Electron emitted at very small angle
with respect to the neutrino direction Eeθ2
e ≤ 2me:
Solar neutrino image
Solar neutrinos