Neutral-current detection via 3He(n,p)3H in the Sudbury Neutrino Observatory

Progress in Particle and Nuclear Physics PERGAMON

Progress

in Particle

and Nuclear

Physics 40 (1998)

113-121

Neutral-Current Detection via 3He(n,p)3H in the Sudbury Neutrino Observatory for the Sudbury

R. G. H. ROBERTSON”

Neutrino

Observatory

Collaboration

“U!rIl’r!Yir~ 0/ Uirs/lin,qloU. Smfrlr. W./lYXIYS.1 .s..‘l. The rate of neutral-current comparison

neutrino

of the rate for electron

interactions

in SNO can be measured

elastic scattering

rate on deuterium,

b) neutron

c) neutron

in an array of 3He-filled proportional

capture

capture

on Cl ions added to the heavy water, and,

proportional

counters,

radioactivity

are essential to minimize photodiiintegration

for producing

the technique

ultra-pure

by: a)

to the pure charged-current

counters.

In the case of

described in this paper, extremely

nickel counter

low levels of

backgrounds.

A method

bodies by chemical vapor deposition

from

Ni(CO)k has been shown to yield purities with respect to U and Th of order 10-r’ by weight. between

Pulse-shape

3He(n,p)3H

digitization

affords a powerful

events and background

have been developed

for deriving

means for diitinguishing

alphas or electrons.

Novel methods

the position of events with singleended

readout

from counter strings up to 12 m in length.

1

The Sudbury Neutrino Observatory

The Sudbury

Neutrino

Observatory

to resolve the “solar neutrino pure heavy water Creighton

(SNO) [l] is a laboratory

problem.”

Deuterium’s

modes by which low-energy

d + u,

+

p+p+e--

d + uz

+

p+n+v,-2.22

d+p=

excavated

neutrinos (CC)

MeV

(NC)

-)

n + n + e+ - 4.03 MeV

(CA)

-+

e-+v,

(ES)

PlI: SOl46-6410(98)00015-5

0

1998

and

is 1000 tonnes of 99.92% isotopically 2070 m below ground in the INCO

make it ideal for the study of neutrino

1.44 MeV

0146~6410~98~$19.00+0.00

chamber

of neutrinos

Ontario.

unique nuclear properties

are four principal

e-+v,

The heart of the detector

(DsO) placed in a specially

nickel mine near Sudbury,

to study the properties

can interact

interactions.

with the heavy water:

Elsevier Science BV. All rights reserved. Printed In Great Britain

There

114

R. G. H. Roherrsort 1 Prog. Part. Nucl. Ph_w. 40 (1998)

The first of these reactions

proceeds

only, the second is the neutral-current probability

charged-current

interaction

(ES). While neutrinos

W-boson

SNO therefore

events produce

The resulting

diameter

in coincidence

are produced

One is to dissolve

Cerenkov

the third is the

from electrons,

the cross section

of neutrino

radiation

contribution

of

oscillations

largely indepen-

of the CC rate to the NC rate.

photomultipliers

that is detected surrounding

is the signal that

would identify strategies

on 75% abundant can be detected

in SNO by an array of 9,500

the acrylic sphere that holds the

a NC event has occurred a pe interaction).

for detecting

in the heavy water several tonnes

captures

of for

by the Sun as it burns its nuclear fuel, but

to reveal the presence

of possible

with equal

may reach the Earth as other flavors.

with a positron

and a number

MgC12. When a neutron

and can be initiated

(CA), and the fourth is the elastic scattering elastically

Cerenkov radiation

of a free neutron

moderator,

be devised.

neutrinos

via the direct comparison

R1408 204mm

or two neutrons

of deuterium

of electron neutrinos

6 times that for other flavors as a result of the additional

has the capability

Charged-current

DzO. The detection

antineutrinos

occur, then the neutrinos

dent of solar properties

(CC) interaction

(vZ = I+, v,, or Y,) and their antiparticles,

flavors can scatter

Only electron

oscillations

Hamamatsu

of electron

all active

is about

exchange.

if neutrino

excellent

(NC) disintegration

by any of the active neutrinos

neutrinos electron

by the charged-current

113-121

(detection

Heavy water is an

the thermalized

of a chloride

of one

neutrons

can

salt, such as NaCl or

35C1, it emits 8.6 MeV in gammas,

which shower.

by the PMT array in the same way CC events are

detected. The method

described

in the heavy water.

2 2.1

in this paper is to detect

the neutron

The NC and CC signals are completely

Neutral-Current

in 3He proportional

separate

in this approach

counters

placed

[2, 3, 41.

Detector Array

The Signal

SNO is sensitive reactions

to only the sB neutrinos

on deuterium,

sB are constrained

and the existence

by current

are reported

the corresponding

of a Cerenkov threshold.

observations

rate RES in terms of the effective

The results

from the Sun, as a result of the Q-values

as 2.44f0.06

?i$

The fluxes of active neutrinos

with the SuperKamiokande

CC and NC cross sections

detector

x lo6 v, cm- 2 s-l . If conversion

and Pinsomreault

from

[5]. Writing the total

a, and a,,,, and the fluxes & and 4,,,,,

flux is 17.2 x lo6 + + u, cme2 s-l, which provides

in SNO. The solar model of Bahcall

for CC and NC

[6] predicts

to uG + v, is complete,

an upper bound to the NC rate

a v, flux of 6.6 x lo6 v, cmd2 s-l,

R. G. H. Robertson / Prog. PUI.I. Nucl. Phy.

Table 1: Rates for neutral-current MeV induced

interactions

40 /199X/ 113-I-71

in SNO and charged-current

115

above T, = 5

interactions

by sB neutrinos.

FhlX lo6 cm-’ s-l

Detected

Species

CC rate

Total NC rate

Detected

NC rate

Y-1

Y-1

Y-1

NC/CC

2.44

“.Z

4170

2210

550

1.78

ve

3040

1610

400

4.82

VP + VT

0

4370

1090

6.6

v, + up + r+

3040

5980

1500

0.493

17.2

“fi + VT

0

15600

3900

00

yielding an intermediate those scenarios

scenario.

are summarized

The expected

0.132

rates in SNO for CC and NC neutrino

in Table 1. For simplicity,

interactions

the shape of the 8B spectrum

in

is taken to

be undistorted. In deriving

the detected

rates from the total rates in Table 1, the efficiency for neutron

(after cuts) is taken to be 25%, and the threshold energy.

Bahcall

and Lisi [7] integrate

for detecting

the Kubodera-Nozawa

electrons

is taken to be 5 MeV kinetic

cross sections

[8] over the sB spectrum

and find for the NC cross section 0.478 x 10e4’ cm2, and for the ratio of the CC interaction 5 MeV electron

energy to the total NC interaction

of the effect of instrumental sre estimated

2.2

resolution.

rate 1.882. The latter figure includes

Uncertainties

detection

in the NC and CC cross sections

rate above an estimate individually

to be S%, but only 1% in the ratio.

Physics Issues in Design of Array

Detection

of thermal

takes advantage applications counters

neutrons

of an enormous

in neutrino

physics

for SNO poses unusual

by the reaction

cross section,

[9, 10, 111. Nonetheless,

from naturally

be allowed to leach significantly permeates

readily

water at absolute

through pressures

art that has included

the design of an array of 3He proportional

problems.

The signal, from 4 to 43 neutrons by backgrounds

5330 b, and is a well developed

per day produced

occurring

radioactivity

in the heavy water, must not be overwhelmed in construction

into the heavy water and interfere

many otherwise

acceptable

materials.

materials,

nor can substances

with water purification. The counters

up to 3.5 atm for ten years or more, placing stringent

Helium

are to survive under demands on detector

longevity and stability. A counter array with high neutron efficiency is needed; but not at the cost of significantly obscuring the cerenkov light from CC events. These conditions severely constrain detector

R. G. H. Robertson/

116

design, the materials

employed

Prog. Pmt. Nucl. Phys. 40 (1998)

in constructing

the detector

113-121

array, and the methods

of handling

and

deployment. Backgrounds by gammas

above 2.22 MeV, as the neutrons

odisintegration energetic

of several kinds can be identified.

neutrons.

gammas.

At the bottom

In principle,

capture

of neutrons

cannot

of the natural

emerging

activities

events,

be distinguished

from neutrin-

also have high energy gammas,

78 days, to be relevant.

Energetic

particularly

from the wall of the proportional

in the gas as a 3He(n,p)T

Even very low-energy

directly

of deuterium

Th and U decay chains are two sufficiently

and by (cr,py) and (cu,nr) reactions,

Alpha particles of ionization

produced

many cosmogenic

one, 56Co, has a long enough halflife,

Most serious is photodisintegration

event, as can electrons

such as the decay of tritium,

photons

but only

can be produced

by

outside the SNO detector.

counter

can leave the same amount

from /3 decay and Compton can compromise

scattering.

the signal via random

summing. The principal materials

defense

and to assure

pulse-shape

discrimination

ion-current

profiles

against

these

a high degree

of cleanliness

and position

event by event.

backgrounds

encoding

during

breakdown

Radioactivity

and electromagnetic

must not only be minimized,

constraints

dictate

requiring

low-noise

interference

but in addition

assembly.

radioactivity Beyond

in construction

that,

techniques

are used, both of which require digitizing

the vessel at the ends of cables 8 - 15 m in length, high-voltage

is to minimize

that

design.

must be controlled.

it is essential

preamplifiers

for

the pulse are outside

Spurious

pulses from

Backgrounds

of all types

to have robust techniques

to messure

them

in situ.

2.3

Neutron-Capture

Because

of the effectiveness

neutron

capture

lattice with 1 m spacing

optical

on lattice

constant,

photons

calculations

gives a neutron

capture

via the charged are absorbed

subject

to systematic

For a given array geometry 3He used in the counters. thermal

neutrons.

(NCD) can be employed. on a square

efficiency

interference

current

(121 of 45 f5%

interaction.

and 3He partial

uncertainties

the neutron

for

with a total length of N 800 m arranged

approximations

Even at modest

The efficiency

detectors

pressure

capture

pressures

is highly sensitive

and different and differences

with tolerable

Approximately

12% of uniformly

of neutron are shown.

source is also shown ss a function

[12, 131 make use of certain

and are therefore

current

and the high cross-section

[?]. In Figure 1 the dependence

enrichment,

from a point

moderator

array of neutral counters

deuterium

of neutrons

sparse

as a neutron

proportional

light produced

generated

detection

of heavy water

on 3He a rather

An array of 5cm diameter

to Cerenkov

Efficiency

of radius. reference

capture

efficiency

The efficiency The Monte

for

Carlo

designs for detectors,

of order 10%.

efficiency is governed

by the partial

of l-3 atm, the counters

are essentially

to isotopic

As can be seen, neutrons

enrichment.

pressure

of

“black” to

R. G. H. Robertson / Prog. Purr. Nucl. Phw.

117

40 i 199X I I13~117l

60

.E 30 g w 0 20 ; E Q loo OA

1

I

1

I

I

0

1

2

3

4

5

01

6

’

’

98.4

1.0 8 0.8

i=/

.f 0.6 g W Q

Ll2

1 il

0.4

J

’

99.6

100.0

I ’

I

I

I

600

Radial Position of Source, cm

40

80

K

I

I I (a)

\

$0.2

400

’

’

r=5cm

_’

i?

200

’ 99.2

D, 0 isotopic Enrichment, %

3He Absolute Pressure, atm

0

c

98.8

i

I 1

120

Lattice Constant, cm

Figure 1: Neutron capture efficiency as a function of (a) 3He pressure, (b) isotopic enrichment of heavy water, (c) position of a point neutron source, and, (d) lattice constant of the square grid, with detector radius as a parameter (2 atm 3He). From ref. [12].

created near the wall of the vessel (at 600 cm), have a high probability of escaping rather than being captured, which reduces the detector’s sensitivity to backgrounds generated at or outside the vessel wall. For neutrons generated uniformly in the heavy water, Table 2 indicates their fates. For 99.85% enriched heavy water, the mean distance from the point of generation to the point of capture for thermal neutrons is 110 cm and the mean time to capture is 16 ms with the NCD array, compared with 48 cm and 4 ms, respectively, for 0.25% NaCl.

2.4

Gas Fill

The detectors are filled with a gas mixture of 85% 3He and 15% CFd at a total pressure of 2.5 atm to provide a good compromise between gas gain and stopping power (to mitigate “wall effect” wherein either the proton or the triton strikes the wall before the end of its range). A lower pressure, and

118

R. G. H. Robertson/

Prog. Part. Nucl. Phys. 40 (1998)

113-121

Table 2: Capture fraction by isotope, in %, uniform source: MCNP 4.2 calculation, 99.85% D20, 2.5 atm 3He, 0.5 atm CFI, O&mm thick Ni wall, 112 strings of detector with total length 808 m. “Acrylic” neutrons are generated at the vessel wall by (r,n) at E., = 2.6 MeV. Also shown are the fractions for 0.25% NaCl. 3He Detectors

0.25% NaCl

1Uniform

Acrylic

4.5

2.9

1.5

11.6

4.4

2.9

1.5

1.9

0.6 82.9

43.4

1.3

0.7

Material

Uniform

4crylic

3He

46.8

14.8

2H

11.8

‘H ~-6ZNi

r

33,37(-J

23Na IS-160

2.1

0.8

0.5

0.3

Acrylic

19.2

55.0

7.9

38.2

HsO (external)

5.2

19.7

2.5

15.8

1

therefore operating voltage, would simplify microdischarge management and increase drift speeds, but would require thick-walled counters to resist collapse. At the bottom of the SNO vessel, the absolute pressure is that due to a column of DzO of height 18 m plus 1.3 atmospheres of air pressure, for a total of 3.2 atmospheres. The copper anode wire of diameter 50 pm is low in radioactivity and ohmic losses. At 1800 V the gas gain is approximately 100. For higher gas gains positiveion space charge at the wire [4] causes the avalanche multiplication to depend increasingly on track orientation, ‘Ike

amounts of electronegative contaminants such as oxygen and water degrade severely the

performance of the gae in a sealed counter. Consequently, counter surfaces are etched in a solution of 4-M nitric acid, 0.17-M hydrofluoric acid for 50 minutes at room temperature, b&d

under vacuum,

and purged with boiloff N2 prior to fill. (The etch also removes surface debris stripped off the mandrel during the fabrication process.) The 3He gas from the Department of Energy facility in Savannah River contains a small amount of tritium, about 0.5 mCi/l, which is reduced to 5 nCi/l or lees by passage through a charcoal-loaded cold trap and by recirculation through a SAES St101 getter. At that level, random coincidences and pileup of tritium decay pulses are no longer a concern in the energy regime of interest. Some detectors, about 5% by length, are filled with 4He to provide a check on backgrounds.

R. G. H. Robertson / Prog. Parr. Nucl. Phj,s. 40 (1998)

2.5

119

113-121

Construction

The bodies of the proportional counters are made of ultra-pure nickel tubes fabricated at Mirotech (Toronto) Inc., by thermolysis of Ni(CO)d vapor at the surface of an anodized aluminum mandrel heated to 215 C. A limited number of elements (Pb, Ra, Th, and U not among them) react with CO to form carbonyls, and the metals formed by chemical vapor deposition (CVD) from this precursor can be expected to be free of the most troublesome radioactivities. Cobalt does form a carbonyl, and may be conveyed through this chemistry at a level that is not at present known. Analysis of the nickel deposit, which has properties very similar to conventional met8llurgic8l products, by radiochemical neutron activation analysis has shown [14] Th levels in the bulk material of order lo-i2

(1 ppt) by

weight or less. Nickel is a relatively inert metal and has been commonly used to provide corrosion protection for other metals. The exposed macroscopic surface 8rea in SNO is 125 m2. In pure (17.3MR)

water

the measured leach rate is 122 pg rnb2 d-‘, which is not expected to compromise the operation of the water purification plant. Endcaps are also made by CVD, in this case on stainless steel mandrels, and sre welded into the tubes with 8 Lumonics 50-W Nd-YAG laser welder. Insulators are Heraeus-Amersil Suprssil T-21 synthetic fused silica tubes. The insulators are internally coated with a layer of pyrolytic graphite at anode potential to eliminate electric fields inside them. They extend 2.5 cm into the gas volume to act 8s field tubes and prevent multiplication of electrons from regions where the electric field is distorted. Silica-nickel is 8 highly mismatched seal, so the design places the silica under compressive stress at working temperature8 to take advantage of the high compressive strength of that material. Techniques were developed in collaboration with IJ Research (Santa Ana), Inc. to metallize and solder the seals with a 965~3.5 eutectic Sn:Ag alloy. Counters are filled through copper tubes, which are then pinched off. All assembly is carried out in a Class 10-1000 cleanroom. A cross section of a counter is given in Hime [4]. The detectors have a wall thickness of 0.36 - 0.48 mm and 8re fabricated in unit lengths of 200, 227, and 272 cm in order to fill the sphere efficiently. ( A firm upper limit is set by the dimensions of the cage in the INCO Number 9 shaft, which permits 8n object 3.7 m long to be brought down without the need to suspend it beneath the cage.) The active length is 13 cm less than the mechanical length. A total mech8nical length of 750 m is to be deployed in the heavy water in the form of 96 “strings” up to 11 m long welded during deployment. Spaces of 35 to 50 cm are allowed between the string ends and the vessel. The bottom of each string is terminated with a 3Cknsopen-ended 415ohm delay line in aid of position reedout by pukreflection

timing. A single Ql-ohm cosxial cable, made

of copper and polyethylene by South Bay Cable (Idyllwyld, CA), Inc., carries signals from the top of each string up the neck of the vessel to preamplifiers. The cable is positively buoyant in heavy water.

R. G. H. Robertson/

120

Prog. Pm.

Nucl. Phys. 40 (1998)

Table 3: Photodisintegration

are installed

2.6

beta emitters.

y branch, %

(r,n) Probability

2.615

36%

i/470

(per Y)

3.26 (18%)

2.445

1.5%

l/750

(per 7)

ssco

1.8

var.

s2sAC

2.06 (11%)

Var.

234mPa

2.28 (100%)

4eK

1.3 (89%)

Isotope

Ep(,,), MeV E,, MeV

212Bi

1.8 (18%)

214Bi

Each string floats upward,

and high-energy

113-l.?/

is anchored

restrained

l/1125 0

89%

0 0

1.46 (EC)

to an attachment

point afhxed to the bottom

by a flexible braided

by a remotely

(per decay)

anchor attachment

of the acrylic vessel and

made of Vectran

fiber. The strings

vehicle (ROV) once the vessel is filled with heavy water.

operated

Backgrounds

In Table 3 are listed some of the decay properties light is the principal

means of quantifying

of light by other activities

components,

counting,

predictions

that

the photodisintegration

techniques

alpha counting)

quantitative

other backgrounds

isotopes.

Since detection

background

of Cerenkov

in situ, the production

is also important.

By mesns of a suite of radioassay direct gamma

of relevant

activation

applied to samples

identified

analysis, radiochemical

and to the complete

can be made concerning

will accompany

events can be unambiguously

(neutron

methods,

inventory

of smaller

the levels of photodiiintegration

the NCD array in SNO. About

half of all neutron

via track length ZISenergy as being distinct

and capture

from alpha particles

(for this reason the array efficiency is taken to be 25% rather than the nominal 45% capture efficiency). Electron

and Compton

backgrounds,

from neutron

events.

determination

and subtraction.

3

As a result,

and microdischarge photodisintegration

events, have topologies is the only background

separated

still further

requiring

a separate

Conclusions

In 1997, radioassay

indicates

that

approximately

130 neutrons

volume of the vessel by the NCD array, and a further components.

Initial

photodisintegration

56Co activity

produces

rate once the array

about

is deployed

per year will be produced

in the main

300 near the wall by cables and termination

200 per year. depends

The determination

primarily

on observing

of the actual the Cerenkov

Prog. Purr. Nd. Phys.40 (IYYN)113-121

R. G. H. Rohrrtsotll

spectrum water.

of the gammas

responsible,

and is expected

If the levels are as anticipated controlled

production

rate will be less than 8% at 6000 neutrons

flavor content

and independently

Neutrino

assayed,

Observatory

of the 8B neutrino

oscillations

after one year’s operation

the uncertainty

suited to make a determination

flux from the Sun, essentially data.

in the water can be in the NC

per year.

is uniquely

that flux within the range allowed by current of neutrino

by Th and U in the heavy

(5 5 fg/g of Th, 5 10 fg/g U), and “‘Rn

sdequately

The Sudbury

to be dominated

121

independent

of the neutrino

of the actual magnitude

By this means a model-independent

can be made, if they are the explanation

for the solar neutrino

of

determination problem.

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et al., “A Heavy Water Detector

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