ANTINEUTRINO DETECTOR FOR v OSCILLATION STUDIES

AT FISSION WEAPON TESTS AND AT LAMPF

Herald W. Kruse, Rosalie Loncoski, and Joseph M. Mack

Los Alamos Scientific Laboratory

Los Alamos, NM 87544

ABSTRACT

Two v oscillation experiments are planned,

incorporating large volume (4400 liter) liquid scintil-

lation detectors 1) at large distances (200-900 m) from

fission weapon tests and 2) at 33 m from LAMPF beam

stop where significant v events are detected only if

some oscillation operates, such as v +e

. Design cri-

teria, detector characteristics, and experimental

considerations are given.

PURPOSE OF EXPERIMENT

Interest in neutrino oscillation experiments has

heightened recently, following announcement that

oscillations may have been observed. We are planning

two types of experiments, utilizing the same detector,

to study v oscillations in unique ways.

At the Nevada Test Site we plan to observe ve

events at large distances (200-900 m) from fission

weapon tests. Oscillation may be observed by inspect-

ing departures of signal rates from llr2 dependence in

this range or, if the oscillation length is small, a

reduction in the energy integrated signal rates, in

comparison to computed values, would be apparent.

Initially, a prototype detector will be fielded and, if

successful, additional units will be constructed en-2

abling inspection of llr dependence simultaneously on

each event.

At Los Alamos Meson Physics Facility (LAMPF), v

interactions will be sought in a detector located 33 m

from the beam dump. In this case, a significant number

of such signals are expected only if oscillations are

present, such as v + v . This is the case since v

are not produced directly by the LAMPF beam; other

reactions from vp and ve are prohibited by energy

conservation considerations. Observation of a beam

associated signal purported to be a consequence of such

an oscillation, would lead to subsequent experiments at

other distances and perhaps with additional detector

units at different locations.

*Work performed under the auspices of the U.S.Department of Energy.

EXPERIMENT DESIGN

The inverse beta decay reaction

ve +p + + n

can well be used to study oscillations, because of its

relatively high cross section. The original detection

scheme, employed by Reines and Cowan in their identifi-

cation experiment, still appears to be an appropriate

choice since it incorporates a powerful method for

background reduction.

Figure 1 illustrates features of our detector,

which is under construction. An inner volume (1390liter) contains a liquid scintillator in which v

interact with protons. Resultant 6 deposits kinetic

and annihilation energy in the scintillator giving a

prompt pulse from photomultipliers (54 each 20-cm dia).The neutron moderates in a few Ps and is captured by

Gd, loaded into the inner scintillator volume. Neutron

capture gammas (8 MeV total) give rise to a second

pulse. This delayed coincidence of two events provides

excellent background discrimination.

l- 3.8 CM LEAD SHIELD

TRANSPARENT PLASTIC WINDOWTOP AND BOTTOM

..........X. <SCINTILLATOR COSMIC RAY "VETO"COUNTER (2300 LITER)SCINTILLATOR (3000 LITER)!00 0 l ~~- ALUMINUMSCINTILLATOR LOADED WITH

GD (1400 LITER)HOLLOW TUBE FOR INTRODUCING

41 RADIOACTIVE CALIBRATION SOURCESLIQUID FILL TUBES

:-:.:-.::-.:-.:-.:-.:- :::..lPURGE LINE TO KEEP SCINTILLA-..l....*: ....*.*... . OR FREE OF OXYGEN

20 CM DIA. PHOTOMULTIPLIER54 TUBES ARE USED

METER

Fig. 1. Neutrino detector features. The innermosttank is optional; Gd may be loaded into the entire 4400liter volume. The entire assembly weighs about 22 tons.

U.S. Government work not protected by U.S. copyright.880

The large volume scintillator (- 3000 liter)

surrounding the central target volume, was initially

intended to improve the detection efficiency for

reactions occurring in the central volume containing

Gd. If we are successful in obtaining economical

Gd-loaded scintillator, then we will eliminate the

inner tank and simply load Gd into the entire 4400liter.

A 3.8-cm lead shield surrounds the sensitive

volume to reduce gammas from the surrounding soil and a

41r anticosmic blanket (also liquid scintillator) sur-

rounds the entire detector. For the LAMPF study,

discrimination of 106 is desired against charged cosmic

events and an additional veto (anticoincidence) counter

is being constructed, containing liquid scintillator

viewed by photomultipliers. The inner veto counter has

limited discrimination imposed by the necessity for

apertures in the scintillator for signal cables, liquid

fill tubes, and mechanical supports. The outer veto

counter has no such apertures except below the detec-

tor. Another layer of lead (3.8 cm) is located between

these two veto counters to reduce muon associated

bremsstrahlung and other neutral events. This entire

assembly is to be located in a cased hole, drilled in

tuff and covered with 8 m of sand (see Fig. 2). This

sand provides important cosmic-ray background reduc-

tion, particularly at LAMPF, where high-energy cosmic

neutron may recoil and subsequently capture, thus

giving a simulated neutrino interaction.

For the weapon-test experiment, ground shock mit-

igation is incorporated into the design. The detector

will be encapsulated inside an iron cylinder; the

region between these inner and outer casings will be

partially filled with styrofoam and polyurethane foam,

designed to crush and provide shock mitigation to 2 g

acceleration. During such ground motion, we expect

photomultipliers to continue to be functional. Vibra-

tion tests performed to date, with operating high

voltage on the photomultiplier, indicate no gain change

up to 3 g at frequencies of 20 to 2000 Hz for the 20-cmdia tubes tested (EMI D340B).

The scintillator intended for this detector con-

tains a p-xylene solvent, chosen because of a need to

maintain high light output, long self absorption

length, solubility for Gd compound,3 ability to providePSD (pulse shape discrimination), and low cost. Fur-

ther testing and evaluation of the self-absorptionlength, the PSD characteristics, and photomultiplierlinearity are underway.

The xylene poses two potential threats to our

experiments because of its high flammability and

ability to dissolve our Lucite windows. We plan to

prevent the latter by coating such surfaces in contact

with xylene, with a transparent sheet of Teflon.

Various safety procedures regarding operational and

logistic concerns are being formulated.

DETECTION EFFICIENCY

Monte Carlo calculations of coupled electron/

photon transport (and appurtenant cascades) providedthe basis for estimating theoretical detector ef-

ficiencies. This approach combined condensed-historyelectron and conventional single-scattering photonMonte Carlo to simulate the transport of all genera-tions of particles from the source energies down to 10

keV. The electron transport included energy-lossstraggling, multiple elastic scattering and the pro-

duction of knock-on electrons, continuous bremsstrah-

lung, characteristic x-rays and annihilation radiation.

The physics of positron and electron transport were

assumed identical. Photon transport allowed photo-electric, Compton, and pair-production interactions and

possible subsequent generation and transport of cor-

responding secondary particles.

Fig. 2. Schematic representation of experimentalarrangement at LAMPF. The inner veto and Pb are alsocontained in Fig. 1. The sand above the detector is tobe contained in large buckets, permitting removal.Liquid fill tubes and signal cables are routed, asshown, in order to avoid leaks (loss of detectionefficiency) in the outer cosmic veto counter.

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Some of the relevant detection efficiencies have

been computed. For example, Fig. 3 illustrates such

efficiency for 45 MeV electrons for two cases: 1) the

configuration of Fig. 1 where the target volume con-

taining Gd is limited to - 1400 liter and 2) a large

volume containing Gd (4200 liter). This latter case

will be chosen if our self absorption length in the

liquid scintillator is sufficiently long. The larger

volume case gives increased signal production of a

factor of three at an expense in neutrino detection

efficiency of 1.5, due to the larger contribution of

edge effects for this geometry.

Probability for neutron capture has also been

computed by Monte Carlo, as in Fig. 4. The case of 1%

Gd is intended for weapon tests where a short capture

time is required to discriminate against accidental

coincidence events. At LAMPF, Gd percentage will be

lower, perhaps 0.3% in order to extend the capture

time, thus enabling rejection of undetected muons which

give decay electrons with - 2 Ps lifetime, another

correlated background.

Detection efficiency for neutron capture gamma

rays has been computed in Fig. 5. The complex gamma

spectrum has been approximated as a coincident, inde-

pendent set of four gamma rays, each having 2 MeV.

Overall detection efficiency will be determined by

actual operation conditions where various thresholds

and energy window limits will be compromised against

background rates. At this time, the following LAMPF

conditions are expected: 1% dead time assuming 25 Ps

"veto" gate time; neutron capture probability of .77,

assuming 0.3% Gd in capture time of 50 ps; positron

detection efficiency of 0.77 assuming 20 MeV threshold;

neutron capture gamma detection efficiency of 0.77 with

4 MeV threshold. The total detection efficiency for

these conditions is thus 0.45, assuming the full 4400

liter volume may be loaded with Gd.

C)

*c

.2

0

0 20 30Threshold (MeV)

'u Ou

Fig. 3. Calculated detection efficiency of 45 MeV

electrons for the volumes of liquid scintillatorindicated.

SIGNAL RATES

For a typical weapon test, we expect about ten v

events to be recorded (with no v oscillation) within atime interval of - 30 s, with good signal/ noise, - 10.

In order to improve the statistical uncertainty in sucha result, recording on several weapon tests is planned;

we also hope to construct several additional detector

units. If we can operate without the innermost tank

then these signal rates will double. There is also a

good chance that backgrounds can be reduced below the

levels estimated, thus permitting a recording of a

greater number of signal events. This results since

the signal rates, due to v associated with B decay ofe

fission fragments, are expected to decrease with time,

and a lower background preserves a high signal/noise to

a later time.

If the various background rates are as low as

estimated at LAMPF, then the signal/noise will be very

Time (p.s)Fig. 4. Calculated probability of neutron capture totime indicated in liquid scintillator loaded withvarious Gd percentages. Neutrons, 3.5 MeV, were dis-tributed uniformly throughout scintillator at timezero.

I

aL-s

.u 06

4-

20 .4

Cs0.2i

0 2 4 6 8Threshold Energy (MeV)

Fig. 5. Calculated detection efficiency for neutron

capture gamma rays in Gd. Four gamma rays each having2 MeV comprised each event. The events were distributeduniformly throughout the 4200 liter of liquidscintillator.

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1.4 A 4200 liter0 400 liter surrounded

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high and an appropriate sensitivity to the v oscil-

lation phenomenon results.4For each of these experiments, the prompt pulse

energy (within the energy window selected) will be

recorded, allowing rejection of certain background

events. Such information will also permit evaluation

of an oscillation length, if observed, for various Veeenergies. For weapon tests, this energy range is 1.8

to - 5 MeV while at LAMPF, the range is 20 to 53 MeV.

ACKNOWLEDGEMENTS

Photomultiplier shake table tests are continuing

with support from Donald Bartram, Jim Toevs, and Norman

Kernodle. Detector engineering and construction draw-

ings were made by Albert Bateman, Larry Rice, Charles

Linder, John Hahn, and Richard Reitman. Liquid scin-tillator tests were made by Sam Egdorf, EG&G, Eric

Austin, and Allen Brown. Helpful suggestions have been

made by Robert Smith, EG&G, Jim Toevs, Thomas Bowles,

and Robert Burman.

REFERENCES

1. F. Reines, H.W. Sobel, and E. Pasierb, "Evidencefor Neutrino Instability," American Physical So-ciety Meeting, Washington, D.C., April, 1980 andsubmitted to Physical Review Letters.

2. F. Reines, C.L. Cowan, Jr., F.B. Harrison, A.D.McGuire, and H.W. Kruse, "Detection of the FreeAntineutrino," Phys. Rev. V 117, 159 (1960).

3. The technique for obtaining a suitable Gd compoundwas developed by Earl Fullman, EG&G, Albuquerque,NM, and is intended for publication.

4. Calculated sensitivity to 6m2, characterizing theoscillation, is tentatively estimated at - 0. 1 eV2and is intended for future publication.

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