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F.D. Brooks, University of Cape Town

M. Drosg, University of Vienna

F.D. Smit, iThemba LABS, Somerset West

A New Detector for Electron

Antineutrinos

Colloquium - UCT - 11 April 2012.

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F.D. Brooks, University of Cape Town, South Africa

M. Drosg, University of Vienna, Austria

F.D. Smit, iThemba LABS, Somerset West, South Africa

A Direction-Sensitive Detector

for Electron Antineutrinos

Eleventh International Conference on Applications of Nuclear Techniques

Crete, Greece, 12-18 June, 2011.

(published in AIP Conf. Proc. 1412 (2011) 177-184)

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with thanks to:

Rob de Meijer (EARTH)

Nieldane Stoddart (iTL)

Kerwin Ontong (UCT)

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Interest in νe detection from:

Weak-interaction physics

Astrophysics (next supernova)

Geophysics (geoneutrinos)

Nuclear material surveillance

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Evidence of neutrino oscillations – Lasserre & Sobel (2006)

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Evidence for geoneutrinos – Boroexino data

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Average no. of antineutrino interactions per year in a 1 kiloton detector

(estimated, from all known nuclear reactors)

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Production of plutonium during a typical reactor fuel cycle

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Figure 16: Left: a cutaway view of the plastic scintillator/Mylar

sandwich detector SONGS2. Mylar sheets, covered with a Gd-

doped paint, are interleaved between the 2 centimeter thick plastic

scintillator blocks. Light collection is accomplished with 4

photomultiplier tubes. Right: initial data from the plastic detector

showing sensitivity to the antineutrino signal.

SONGS2 antineutrino detector

for monitoring nuclear reactors

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Neutrinos/antineutrinos

● Postulated by Pauli (1930). (Pauli “apologized” to

physicists for “inventing” a particle that “could not” be

detected!)

● Fermi’s theory, incorporating Pauli’s hypothesis, was

successful in explaining features of beta decay (1934).

● Antineutrino detected by Reines, Cowan et al (1953)

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First detection of antineutrino – Reines and Cowan (1953)

Liquid scintillator

CH, Cd-doped. Built-

in large proton target.

75 cm x 75 cm diam.

90 PM tubes.

νe from Hanford

nuclear reactor

Double-pulse

“signature”.

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νe signature in a Cd-doped scintillator (1)

νe + p → e+ + n (Q = - 0.8 MeV σ ~ 10-43

cm2)

e+ + e- → γ + γ (Q = 1.02 MeV)

sum of pulses from e+ and 2 γ

2) Delay of 1-2 μs while the neutron thermalizes

3) Pulse #2 from neutron capture by Cd:

113Cd + nth → 114Cd* (Q ~ 9 MeV σ = 20 kb)

114Cd* → 114Cd + γ-cascade (Σ Eγ ~ 9 MeV)

sum of pulses from 4-6 γ

1) Pulse #1 from inverse beta decay of the neutron:

Delayed

coincidence

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An improved νe signature (2)

νe + p → e+ + n (Q = - 0.8 MeV σ ~ 10-43

cm2)

e+ + e- → γ + γ (Q = 1.02 MeV)

coinc. of pulses from e+ and 1 or 2 γ

2) Delay of 1-2 μs while neutron thermalizes

3) Pulse #2 from neutron capture by Cd: 113Cd + nth → 114Cd* (Q ~ 8 MeV)

114Cd* → 114Cd + γ-cascade (Σ Eγ ~ 9 MeV)

coinc. of pulses from 2 or more γ

1) Pulse #1 from inverse beta decay of neutron:

Delayed

coincidence

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Reines, Cowan et al (1956): detector #2 (schematic)

2m x 1.5m x 7.5cm deep

solution of CdCl2 in water.

2m x 1.5m x 60cm deep

liquid scintillator.

6 banks of 55 p.m. tubes

2 slabs of Cd solution between

3 slabs of liquid scintillator.

Inverse beta decay at 1 leads to 2

annihilation quanta detected at a.

γ’s from neutron capture by Cd at

2 are detected ~ 1 μs later, at b.

Delayed coincidence (1 followed

by 2) is the signature of νe.

Note: The e+ is not detected at 1.

However, since events 1 and 2

are coincidences, discrimination

against random coincidence

backgrounds is improved.

1 2

a

a

b

νe

e+ n γ e-

b b

b

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Used to detect νe from the

reactor at Savannah River.

CdCl2-in-water solution

Liquid scintillator

Fig. 6: End view

Fig. 7: Light detector (55 p.m. tubes)

at end of liquid scintilltor.

Photographs of antineutrino detector #2 (Reines et al, 1956)

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A large antineutrino detector – KamLand (1980)

1 kiloton of

liquid scintillator

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Super KamioKanda – 50 kilotons water (Cerenkov detector)

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Super KamioKande (Japan)

Photographed in 2000 while being

filled with water (50 kilotons).

11,200 PM tubes detect Cerenkov

radiation from νe νe (elastic

scattering) and other interactions

that release relativistic charged

particles. Note the technicians on

the raft, who are cleaning the PM

tubes as the tank fills up.

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The proposed νe detector is expected to have the following features:-

1) The scintillator will be 10B-doped instead of Cd-doped or Gd-doped.

2) Pulse shape discrimination will be employed in the selection of

double-pulse events.

3) A modular design that can be replicated easily and then assembled

into a large detector will be used.

4) Each module will consist of 19 or more independent scintillators, in

order to obtain a more selective antineutrino signature.

5) It is envisaged that semiconductor-based light sensors will soon

become available, to be used instead of photomultipliers.

6) Data capture will be accomplished by electronics based on up-to-date

techniques such as flash ADCs rather than NIM electronics.

7) Data reduction will be carried out by compact on-line computers.

8) Directional-sensing capability (FWHM ~ 15°).

Initial development and testing is now being undertaken at UCT/iTL,

using neutrons to mimic the antineutrino signature. Further tests are

planned at Koeberg nuclear power station, using antineutrinos.

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νe signature in a 10B-doped scintillator

νe + p → e+ + n (Q = - 0.8 MeV σ ~ 10-43

cm2)

e+ + e- → γ + γ (Q = 1.02 MeV)

coinc. of pulses from e+ and 2 γ

2) Delay of ~ 0.5 μs while neutron thermalizes

3) Pulse #2 from neutron capture by 10B:

10B + nth → 7Li* + α (Q ~ 2.3 MeV)

7Li* → 7Li + γ (0.48 MeV)

coinc. of pulses from 7Li, α and γ

1) Pulse #1 from inverse beta decay of neutron:

Delayed

coincidence

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30-50 cm

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PMT

35 cm

6 cm

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PMT B-doped scintillator

“honeycomb”

single cell

Al-foil

19-cell scintillator module (schematic)

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Container filled with 10B-doped liquid

scintillator NE311

0.2 mm Al foil to

separate cells.

19-cell unit

15-30 cm long cells

2 PM tubes per cell

6 cm

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νe event (1st pulse)

νe + p → e+ + n

e+ + e- → γ + γ

νe

e+

n

γ (0.5)

e-

neutron goes forward

En ≤ 100 keV

Vertex 1

Vertex position is

determined from

cell position (x,y)

and pulse height

division (z)

y

x

z

Ee = Eν – 1.8 MeV

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νe event (2nd pulse)

10B + n → α + 7Li*

7Li* → 7Li + γ

7Li*

α

n

γ (0.5)

e-

Vector from vertex 1

to vertex 2 gives

direction of νe

Vertex 2

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n-event (1st pulse)

n + p → p + n

p

n

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n-event (2nd pulse)

10B + n → α + 7Li*

7Li* → 7Li + γ

7Li*

α

n

γ (0.5)

e- same as for νe event

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Det A

Det B

Pulse #: 1 2

530 ns

2500 ns

Experiments with neutrons

and two small scintillators

n

p α

γ

e-

7Li*

Am-Be or 252Cf

neutron source

Iron

B (NE213) A (boron-doped)

Event 1

Event 2

Pulse #: 1 2

A

B

Discrim

& Coinc

Digital

Scope Trig

A

D

A Laptop

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-10 0 400 800 (ns) 1200 1600

0 400 800 (ns) 1200 1600

40

20

0

(mV)

40

20

0

-10

(mV) Lf L

30 0 240 ns

np (n, α)

γe

after

pulse

Double-pulse events

from a 252Cf source (Inverted pulses from

Detector A). T = 776 ns

T = 520 ns

Lf = fast component (0-30 ns)

L = total light (0-240 ns)

S = “Shape” = 100Lf / L

T = time difference

(1) hadron

(2) lepton

30 0 200 400 600 800 1000

0 200 400 600 800 1000

L1 (keVee)

L1 (keVee)

30

50

100

130

S1

(%)

2

1

0

T

(µs)

PSD from pulse 1

L1 vs S1

2240 events in 2 hr

L1 vs T

( T = time difference)

After-pulses

hadrons

leptons

S1 window

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0 200 400 600 800 1000 L2 (keVee)

2

1

0

T

(µs)

0 100 200 300 400 500 L2 (keVee)

100

50

0

S2

(%)

L2 vs T

PSD from pulse 2

L2 vs S2

After-pulses

After-pulses

n

n

L2-S2 cut

S2 window

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0 200 400 600 800 1000 L2 (keVee)

2

1

0

T

(µs)

L2 vs T after imposing:

window S1;

window S2; and

the L2-S2 cut.

204 events selected

from 2240

n

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Tests using reactor antineutrinos at Koeberg? (proposed)

43 m

Two reactors of 950 Mwe (2700 Mwth). At full power each

reactor produces ~ 1020 antineutrinos per second into 4π.

The test position is directly below reactor 2, 17 m from the

centre of the core, with 6 m of concrete in between. The

estimated antineutrino fluence at this point is 9 x 1012 per

cm2 s. This should produce about 5 inverse beta decays per

day in a detector of volume 1 litre. Thus for a module of

volume 30 litres ~ 150 events/day is estimated.

1 2

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SUMMARY

These tests show that the 10B-doped detector:

should select the antineutrino signature efficiently; and

should reject neutron-induced backgrounds efficiently.

Other tests have shown that the detector should have sufficient

position resolution to achieve a direction-sensing capability.

The next step will be to carry out further tests using larger

detectors with antineutrinos from the Koeberg nuclear reactors.

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SUMMARY

These tests show that the 10B-doped detector:

should select the antineutrino signature efficiently; and

should reject neutron-induced backgrounds efficiently.

Other tests have shown that the detector should have sufficient

position resolution to achieve a direction-sensing capability.

The next step will be to carry out further tests using larger

detectors with antineutrinos from the Koeberg nuclear reactors.

ευχαριστώ !

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