Large Natural Cherenkov Detectors: Water and Ice

10 Pages·0.22 MB·English

by  F. Halzen

University of Wisconsin - Madison
MADPH-97-1026
November 1997
∗
Large Natural Cherenkov Detectors: Water and Ice
Francis Halzen
aPhysics Department, University of Wisconsin, Madison, WI 53706,USA
In this review we first address 2 questions:
• why dowe need kilometer-scale muon and neutrinodetectors?
• what dowe learn from theoperating Baikal and AMANDAdetectors about theconstruction of kilometer-
scale detectors?
I will subsequently discuss the challenges for building the next-generation detectors. The main message is that
these are different, in fact less ominous, than for commissioning the present, relatively small, detectors which
must reconstruct eventsfar outside theirinstrumented volume in order to achieve large effectivetelescope area.
8
9
1. Why Kilometer-Scale Detectors? eightordersofmagnitudeinwavelength. Explor-
9
1 ing this energy region with neutrinos does have
High energy neutrino telescopes are multi-
the definite advantage that they can, unlike high
n purpose instruments; their science mission cov-
energy photons and nuclei, reach us, essentially
a
ers astronomy and astrophysics, cosmology, par-
J withoutattenuationinflux, fromthe largestred-
ticle physics and cosmic ray physics. Their de-
8 shifts.
ploymentcreatesnewopportunitiesforglaciology
The challenge is that neutrinos are difficult to

1 and oceanography, possibly geology[1]. The ob- detect: the small interaction cross sections that
v servations of astronomers span 19 decades in en-
enable them to travelwithout attenuation over a
9 ergy or wavelength,from radio-wavesto the high
Hubbleradius,arealsothereasonwhykilometer-
0
energy gamma rays detected with satellite-borne
0 scale detectors are required in order to capture
detectors[2]. Major discoveries have been histor-
1 them in sufficient numbers to do astronomy[5].
0 ically associated with the introduction of tech- There is nothing magical about this result —
8 niques for exploring new wavelengths. The im-
I will explain this next.
9 portant discoveries were surprises. In this spirit,
Cosmic neutrinos, just like accelerator neutri-
/
x the primary motivation for commissioning neu- nos,aremadeinbeamdumps. Abeamofacceler-
e trino telescopes is to cover uncharted territory:
ated protons is dumped into a target where they
- wavelengths smaller than 10−14 cm, or energies
p produce pions in collisions with nuclei. Neutral
e inexcessof10GeV.Thisexplorationhasalready pions decay into gamma rays and charged pions
h been launched by truly pioneering observations
into muons and neutrinos. All this is standard
:
v using air Cherenkov telescopes[3]. Larger cos- particle physics and, in the end, roughly equal
i mic ray arrays with sensitivity above 107 TeV,
X numbers of secondary gamma rays and neutri-
anenergywhere chargedprotonsmay pointback
nos emerge from the dump. In man-made beam
r
a at their sources with minimal deflection by the dumps the photons are absorbed in the dense
galactic magnetic field, will be pursuing similar
target; this may not be the case in an astro-
goals[4]. Could the high energy skies be devoid
physical system where the target material can
of particles? No, cosmic rays with energies ex-
be more tenuous. Also, the target material may
ceeding 108 TeV have been recorded[4]. Between
be light rather than nuclei. For instance, with
GeV gamma rays and the most energetic cosmic
an ambient photon density a million times larger
rays, there is uncharted territory spanning some than the sun, approximately 1014 per cm3, par-
∗Talk presented at the 5th International Workshop ticles accelerated in the superluminal jets associ-
on Topics in Astroparticle and Underground Physics atedwithactivegalacticnuclei(AGN),maymeet
(TAUP97),GranSasso,Italy,Sept.1997.
2
Table 1
in order to be detected, the neutrino has to in-
Cosmic Beam Dumps
teract within a distance of the detector which is
shorter than the range of the muon it produces.
Beam Target In other words, in order for the neutrino to be
cosmic rays atmosphere detected, the produced muon has to reach the
cosmic rays galactic disk detector. Therefore,
cosmic rays CMBR
R
AGN jets ambient light, UV Pν→µ ≃ λinµt ≃AEνn, (1)
shocked protons GRB photons
whereRµ isthemuonrangeandλint theneutrino
interaction length. For energies below 1 TeV,
where both the range and cross section depend
linearly onenergy, n=2. BetweenTeV and PeV
morephotonsthannucleiwhenlosingenergy. Ex- energies n = 0.8 and A = 10−6, with E in TeV
amples of cosmic beam dumps are tabulated in units. For EeV energiesn=0.47,A=10−2 with
Table 1. They fall into two categories. Neutrinos
E in EeV.
produced by the cosmic ray beam are, of course,
At PeV energy the cosmic ray flux is of order
guaranteed and calculable. We know the proper- 1 per m2 per yearandthe probabilityto detect a
ties of the beam and the various targets: the at- neutrino of this energy is of order 10−3. A neu-
mosphere,the hydrogeninthe galacticplaneand
trino flux equal to the cosmic ray flux will there-
the CMBR background. Neutrinos from AGN
foreyieldonlyafeweventsperdayinakilometer
and GRBs (gamma ray bursts) are not guaran-
squareddetector. At EeV energy the situation is
teed, though both represent good candidate sites worse. With a rate of 1 per km 2 per year and
for the acceleration of the highest energy cosmic
adetectionprobabilityof0.1,onecanstilldetect
rays. Thattheyarealsothesourcesofthehighest
severaleventsperyearinakilometersquaredde-
energy photons reinforces this association.
tectorprovidedtheneutrinofluxexceedsthepro-
In astrophysical beam dumps, like AGN and
ton flux by 2 orders of magnitude or more. For
GRBs,thereistypicallyoneneutrinoandphoton
the neutrino flux generated by cosmic rays inter-
produced per accelerated proton[1]. The accel-
acting with CMBR photons and such sources as
erated protons and photons are, however, most
AGNandtopologicaldefects[7],thisisindeedthe
likely to suffer attenuation in the source before
case. All above estimates are conservative and
theycanescape. So,ahierarchyofparticlefluxes
the rates should be higher because the neutrinos
emerges with protons < photons < neutrinos. A
escape the source with a flatter energy spectrum
generic neutrino flux can be obtained from this
than the protons[6]. In summary, the cosmic ray
relation by, conservatively,equating the neutrino
flux and the neutrino detection efficiency define
with the observed cosmic ray flux. The detector
the size of a neutrino telescope. Needless to say
size can now be determined[6] by taking into ac-
that a telescope with kilometer squared effective
countthe detectionefficiencyforneutrinoswhich
area represents a neutrino detector of kilometer
is much reduced compared to protons.
cubed volume.
Neutrino telescopes are conventional particle
detectors which use natural and clear water and
2. Baikal and the Mediterranean
ice as the Cherenkov medium. A three dimen-
sional grid of photomultiplier tubes maps the First generation neutrino detectors are de-
Cherenkov cone radiated by a muon of neutrino signed to reach a relatively large telescope area
origin. Nanosecond timing provides degree reso- and detection volume for a neutrino threshold
lutionofthe muontrackwhichis,athighenergy, of tens of GeV, not higher and, possibly, lower.
aligned with the neutrino direction. The proba- This relatively low threshold permits calibration
bilitytodetectaTeVneutrinoisroughly10−6[1]. of the novel instrument on the known flux of at-
It is easily computed from the requirement that, mosphericneutrinos. Itsarchitectureisoptimized
3
for reconstructing the Cherenkov light front ra- Table 2
diated by an up-going, neutrino-induced muon
rather than for detecting signals of TeV energy 0th generation 1st generation km3
and above. Up-going muons are to be identified (cid:214)
inabackgroundofdown-going,cosmicraymuons BAIKAL
(cid:214)
which are more than 106 times more frequent for IMB AMANDA ICECUBE(D)
MACRO, LVD NESTOR*
a depth of 1 kilometer.
The “landscape” of neutrino astronomy is . . .SUPER K . . .ANTARES*
sketched in Table 2. With the termination of the
pioneering DUMAND experiment, the efforts in Telescope area
water are, at present, spearheaded by the Baikal < 103 m2 2· 103 m2 104~105 m2 ~ >106 m2
experiment[8]. Operating with 144 optical mod-
ules (OM) since April 1997, the NT-200 detec-
Threshold
tor will be completed by April 1998. The Baikal
detector is well understood and the first atmo- 5 MeV 1 GeV tens of GeV ~ <1 TeV
sphericneutrinoshavebeenidentified;wewilldis-
EASY!
cuss this in more detail further on. The Baikal • timing • geometry
• energy
site is competitive with deep oceans although
• no background
the smaller absorption length requires somewhat
denser spacing of the OMs. This does however (cid:214) n -candidates
result in a lower threshold which is a definite ad- * R & D
vantage, for instance in WIMP searches. They
have shown that their shallow depth of 1 kilome-
terdoesnotrepresentaseriousdrawback. Byfar
the most significant advantage is the site with a ley. Thelatterhasmadeseminalcontributionsto
seasonal ice cover which allows reliable and in- the designofdigital OMs. Most ofthem havere-
expensive deployment and repair of detector ele- cently joined the AMANDA experiment and, as
ments. farasdeploymentsareconcerned,theireffortwill
Inthefollowingyears,NT-200willbeoperated proceed in ice.
as a neutrino telescope with an effective area be- The NESTOR collaboration[9], as part of an
tween 103 ∼ 5×103 m2, depending on the en- ongoing series of technology tests, has recently
ergy. Presumably too small to detect neutrinos deployed two aluminum “floors”, 34 m in diame-
fromAGNandotherextraterrestrialsources,NT- ter,to a depth of2600m. Mechanicalrobustness
200 will serve as the prototype for a larger tele- was demonstrated by towing the structure, sub-
scope. For instance, with 2000 OMs, a thresh- mergedbelow 2000m, from shore to the site and
old of 10 ∼ 20 GeV and an effective area of back. The detector will consist of 12 six-legged
5×104 ∼ 105 m2, an expanded Baikal telescope floors separated by 30 m.
would fill the gap between present underground The Antares collaboration[10]is in the process
detectorsandplannedhighthresholddetectorsof ofdeterminingthecriticaldetectorparametersat
cube kilometer size. Its key advantage would be a 2000 m deep, Mediterranean site off Toulon,
low threshold. France. A deliberate developmenteffortwill lead
The Baikal experiment represents a proof of to the construction of a demonstration project
concept for deep ocean projects. These should consisting of 3 strings with a total of 200 OMs.
have the advantage of larger depth and optically Forneutrinoastronomytobecomeaviablesci-
superiorwater. Theirchallengeistodesignareli- ence several of these, or other, projects will have
able and affordabletechnology. Three groups are to succeed. Astronomy,whether in the optical or
confronting the problem: NESTOR and Antares in any other wave-band, thrives on a diversity of
intheMediterraneanandagroupatLBL,Berke- complementaryinstruments,noton“asinglebest
4
instrument”. When the Soviet government tried
NT-96 Array
outthelattermethodbycreatinganationallarge
mirror project, it virtually annihilated the field.
3. First Neutrinos from Baikal
The Baikal Neutrino Telescope is being de-
ployedinLakeBaikal,Siberia,3.6kmfromshore
at a depth of 1.1 km. An umbrella-like frame
holds8strings,eachinstrumentedwith24pairsof
37-cm diameter QUASAR photomultiplier tubes
(PMT). Two PMTs in a pair are switched in co-
incidence in order to suppress background from
bioluminescence and PMT noise.
They have analysed 212 days of data taken in
94-95 with 36 OMs. Upward-going muon can-
didates were selected from about 108 events in
whichmorethan3pairsofPMTstriggered. After
quality cuts and χ2 fitting of the tracks, a sam-
ple of 17 up-going events remained. These are
not generated by neutrinos passing the earth be-
lowthedetector,butbyshowersfromdown-going
muonsoriginatingbelowthearray. Inasmallde-
tectorsucheventsareexpected. In2eventshow-
ever the light does not decrease from bottom to
top, as expected from invisible showering muons Figure 1. Candidate neutrino event from NT-96
belowthedetector. Adetailedanalysis[11]yields in Lake Baikal.
a fake probability of 2% for both events.
Afterthedeploymentof96OMsinthespringof
96, three neutrino candidates have been found in
a sample collected over18 days. This is in agree- 96–97. Itconsistsof300opticalmodulesdeployed
mentwiththeexpectednumberofapproximately atadepthof1500–2000m;seeFig.2. Anoptical
2.3. One of the events is displayed in Fig. 1. In moduleconsistsofan8inchphotomultipliertube
this analysis the most effective quality cuts are and nothing else. Calibration of this detector is
the traditional χ2 cut and a cut on the probabil- in progress, although data has been taken with
ity of non-reporting channels not to be hit, and 80 OM’s which were deployed one year earlier in
reporting channels to be hit (Pnohit and Phit, re- order to verify the optical properties of the ice
spectively). To guarantee a minimum lever arm (AMANDA-80).
for trackfitting, they wereforcedto rejectevents The performance of the AMANDA detector is
withaprojectionofthe mostdistantchannelson encapsulatedintheeventshowninFig.3. Coinci-
the track smaller than 35 meters. This does, of denteventsbetweenAMANDA-80 andfourshal-
course, result in a loss of threshold. low strings with 80 OM’s (see Fig. 2), have been
triggered for one year at a rate of 0.1 Hz. Ev-
ery 10 seconds a cosmic ray muonis trackedover
4. The AMANDA South Pole Neutrino
1.2 kilometer. The contrast in detector response
Detector
between the strings near 1 and 2 km depths is
Constructionofthe first-generationAMANDA dramatic: while the Cherenkov photons diffuse
detector[12]wascompletedintheaustralsummer on remnant bubbles in the shallow ice, a straight
5
30 m shown in Fig. 4 for the shallow and deep ice[13].
The distributions have been normalized to equal
ice surface level
areas; in reality, the probability that a photon
travels 70 m in the deep ice is ∼107 times larger.
There is no diffusion resulting in loss of informa-
tion on the geometry of the Cherenkov cone in
the deep ice.
190 m 5. Intermezzo: AMANDA before
and after
Depths The AMANDA detector wasantecedently pro-
posed on the premise that inferior properties of
810 m
ice as a particle detector with respect to water
couldbecompensatedbyadditionalopticalmod-
1000 m ules. The technique was supposed to be a factor
5∼10 more cost-effective and, therefore, compet-
itive. The design was based on then current in-
formation:
380 m • the absorptionlength at 370 nm, the wave-
length where photomultipliers are maxi-
1520 m mally efficient, had been measured to be
8 m,
• the scattering length was unknown,
• the AMANDA strategy was to use a large
numberofcloselyspacedOM’stoovercome
1950 m
the short absorption length. Muon tracks
35 m 60 m triggering6ormoreOM’sarereconstructed
with degree accuracy. Taking data with a
simple majority trigger of 6 OM’s or more
Figure 2. The Antarctic Muon And Neutrino at100Hzyieldsanaverageeffectiveareaof
Detector Array (AMANDA). 104 m2.
The reality is that:
• the absorptionlengthis 100m ormore,de-
track with velocity c is registered in the deeper
pending on depth[14],
ice. The optical quality of the deep ice can be
assessedby viewing the OM signalsfrom a single • the scattering length is 25∼30 m (prelimi-
muontriggering2stringsseparatedby79.5m;see nary),
Fig. 3b. The separationof the photons along the • because of the large absorption length OM
Cherenkov cone is well over 100 m, yet, despite spacings are similar, actually larger, than
some evidence of scattering, the speed-of-light that those of proposed water detectors.
propagationofthetrackcanbereadilyidentified. Also, a typical event triggers 20 OM’s,
The optical properties of the ice are quantified not 6. Of these more than 5 photons are
by studying the propagation in the ice of pulses not scattered. In the end, reconstruction is
oflaserlightofnanosecondduration. Thearrival therefore as before, although additional in-
times of the photons after 20 m and 40 m are formation can be extracted from scattered
6
m)
pth (m) Depth (
De
Times (ns) Times (ns)
Figure 3a. Cosmic ray muon track triggered by Figure 3b. Cosmic ray muon track triggered by
both shallow and deep AMANDA OM’s. Trigger both shallow and deep AMANDA OM’s. Trigger
times ofthe opticalmodulesareshownasafunc- times are shown separately for each string in the
tion of depth. The diagram shows the diffusion deepdetector. Inthiseventthemuonmostlytrig-
of the track by bubbles above 1 km depth. Early gersOM’sonstrings1and4whichareseparated
and late hits, not associated with the track, are by 79.5 m.
photomultiplier noise.
of muon neutrino origin. At the 10−6 level of
photons by minimizing a likelihood func-
the background, candidate events can be identi-
tionwhichmatchesmeasuredandexpected
fied; see Fig. 6. This exercise establishes that
delays[15].
AMANDA-80 can be operated as a neutrino de-
The measured arrivaldirections of background tector; misreconstructed showers can be readily
cosmic ray muon tracks, reconstructed with 5 or eliminated on the basis of the additional infor-
more unscattered photons, are confronted with mation on the amplitude of OM signals. Monte
their known angular distribution in Fig. 5. The Carlosimulation,basedonthisexerciseconfirms,
agreement with Monte Carlo simulation is ad- that AMANDA-300 is a 104 m2 detector (some-
equate. Less than one in 105 tracks is misre- what smaller for atmospheric neutrinos and sig-
constructedasoriginatingbelowthedetector[13]. nificantly larger for high energy signals) with 2.5
Visualinspectionrevealsthat the remainingmis- degrees mean angular resolution[15]. We have
reconstructed tracks are mostly showers, radi- verified the angular resolution of AMANDA-80
ated by muons or initiated by electron neutri- by reconstructingmuontracksregisteredincoin-
nos, which are reconstructed as up-going tracks cidence with a surface air shower array[16].
7
u) u)
(a. (a. 0.45
0.5
0.4
0.35
0.4
0.3
0.3 0.25
0.2
0.2
0.15
0.1
0.1
0.05
0 0
0 1000 2000 3000 4000 0 1000 2000 3000 4000
Photon arrival times (ns) Photon arrival times (ns)
Figure 4. Propagation of 510 nm photons indicate bubble-free ice below 1500 m, in contrast with ice
with some remnant bubbles above 1 km.
6. Towards ICE CUBE(D) no timing information is really required. Tim-
ing is still necessary to establish whether a track
A strawman detector with effective area in ex-
is up- or down-going, not a challenge given that
cess of 1 km2 consists of 4800 OM’s: 80 strings
the transit time of the muon exceeds 2 microsec-
spaced by ∼ 100 m, each instrumented with
onds. Using the events shown in Fig. 3, we have,
60 OM’s spaced by 15 m. A cube with a side of
in fact, already demonstrated that we can re-
0.8 km is thus instrumented and a through-going
ject background cosmic ray muons. Once ICE
muoncanbe visualizedbydoubling the lengthof
CUBE(D) has been built, it can be used as a
thelowertrackinFig.3a. Itisstraightforwardto
veto for AMANDA and its threshold lowered to
convince oneself that a muon of TeV energy and
GeV energy.
above, which generates single photoelectron sig-
With half the number of OM’s and half the
nalswithin50mofthemuontrack,canberecon-
price tag of the Superkamiokande and SNO solar
structed by geometry only. The spatial positions
neutrinodetectors,theplantocommissionsucha
of the triggeredOM’s allow a geometric track re-
detectorover5yearsis notunrealistic. The price
construction with a precision in zenith angle of:
tag of the default technology used in AMANDA-
300 is $6000 per OM, including cables and DAQ
OM spacing electronics. Thissignalcanbetransmittedtothe
angular resolution≃
length of the track surface by fiber optic cable without loss of infor-
≃15m/800m≃1degree; (2) mation. Given the scientific range and promise
8
104
103
102
10
1
s
t
n104
e
v
e

f
o103

r
e
b
m
102
u
n
10
1
104
103
102
10
1
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
cos(zenith)
Figure 5. Reconstructed zenith angle distribution of muons triggering AMANDA-80: data and Monte
Carlo. The relative normalization has not been adjusted at any level. The plot demonstrates a rejection
ofcosmicraymuonsatalevelof10−5whichisreachedbyonly2cuts: acutonthenumberofunscattered
photons and on Phit, a quantity introduced in the context of the Baikal experiment.
9
order, by noting that string spacings determine
12 the cost of the detector. The attenuation length
istherelevantquantitybecauseitdetermineshow
11
far the light travels, irrespective of whether the
photons are lost by scattering or absorption. Re-
13
9 member that, even in the absence of timing, hit
geometry yields degree zenith angle resolution.
NearthepeakefficiencyoftheOM’stheattenua-
tion length is 25–30 m, larger in deep ice than in
7
8 water below 4 km. The advantage of ice is that,
10 unlikeforwater,itstransparencyis notdegraded
14 for blue Cherenkov light of lower wavelength, a
propertywehope totakefurther advantageofby
5
using wavelength-shifter in future deployments.
4 The AMANDA approach to neutrino astron-
3 omy was initially motivated by the low noise of
sterile ice and the cost-effective detector technol-
2
ogy. These advantages remain, even though we
1
know now that water and ice are competitive
as a detector medium. They are, in fact, com-
plementary. Water and ice seem to have simi-
6
lar attenuation length, with the role of scatter-
ing and absorption reversed. As demonstrated
with the shallow AMANDA strings[17], scat-
tering can be exploited to range out the light
and perform calorimetry of showers produced by
electron-neutrinos and showering muons. Long
scattering lengths in water may result in supe-
riorangularresolution,especiallyforthesmaller,
first-generation detectors. This can be exploited
to reconstruct events far outside the detector in
order to increase its effective volume.
Figure6. Acandidateup-going,neutrino-induced
Acknowledgements
muon in the AMANDA-80 data. The numbers
indicate the time sequence of triggered OMs, the This research was supported in part by the
size of the dots the relative amplitude of the sig- U.S.DepartmentofEnergyunderGrantNo.DE-
nal. FG02-95ER40896 and in part by the University
of Wisconsin Research Committee with funds
grantedbytheWisconsinAlumniResearchFoun-
dation.
of suchan instrument, a kilometer-scaleneutrino
detectormustbeoneofthebestmotivatedscien-
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