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OCR for page 87
Radio Telescopes as the Detectors of
Super-High-Energy Neutrinos
R. D . DAGKESAMANSKY, Lebedev Physics
I.M. ZHELEZNYKH, Institute for Nuclear Research
The registration of super-high-energy neutrinos is a very difficult and
also very important problem that requires construction of detectors with
large effective target masses. In 1961, Aska~yan pouted out me possibility
of registering cascades in dense media by the Cherenkov radioemission of
an excess of negative charges in the cascades which arose in interaction
between high energy particles and the atoms of medium. The total energy
emitted is proportional to E2, where E is the primary particle energy
(Gusev and Zheleznyl~ 1983; Markov and Zheleznykh 1986~:
WE 10 (E(eY)/10 ~ [ergs].
(1)
Most of this energy will be emitted by radio band at frequencies f < fma~
~ 109 Hz, where the cascade's radioem~ssion will be coherent. At f < fmax
the spectrum of radioemission will be proportional to f:
Pfdf = (2W/fm=)f f
(2)
The corresponding "telescopes" for cosmic high-energy neutrino de-
section by radioe~ssion of cascades induced underground, but whose
development continues in the atmosphere were proposed in Markov and
~heleznykh (1986~; Dedenko et al. (1981~; Markov and Zheleznykl~ (1979~.
The effective target masses of such detectors could be ~ 109 tons and
more. The properties of Cherenkov radioemission of cascades and the
properties of ice in the Antarctic Region (very weak radiowave absorption
at low temperature) make it possible to propose Radio Antarctic Muon
and Neutnno Detection ~AND): antennas should be placed on the
ice surface of ~ 10 km2 to search for radio signals from neutrino (muon)
87
OCR for page 88
88
AMERICAN AND SOVIET PERSPECTIVES
cascades of energy 1014-1015 eV and higher in a volume of ~ 101 ° m3
(Gusev and Zheleznykh 1983~.
Ib search for astrophysical neutrinos of energies higher than 1019
eV, observations with radio telescopes were proposed (Zheleznylth 1988~.
Indeed, by using available high sensitive radio telescopes we can hope
to detect the coherent Cherenkov radioemission that has arisen in very
distant targets. For example, we can fly to observe such radioemission
from cascades in the surface layer of the Moon (the surface area is about
107 km!.
The energy W is emitted as a very short pulse (Atp ~ 10-8 s) because
of the short lifetime of the cascade at a relatively dense medium. The
emission is concentrated within the "thick hollow cone" with the opening
angle ~ ~ 50° and Me solid angle Q ~ 0.5 steradians. Therefore, the peak
flux density measured by observers inside the "thick hollow cone" will be:
Speak = f2 QD2^tp ra˘~^lvv rn Liz
_~1~-23~-~2r ~_-2~--1l (3)
~in,
where D is the distance to the Moon, fg is frequency in GHz and E20 is
the neutnno energy in 102°eV.
On the other hand, the sensitivity of radio telescope is
Muslim = 2kTs~Ae-fI(~fT)-ll2
(4)
where Toys is the telescope system noise temperature' Aeff is the effective
area of the radio telescope, k is Boltzman's constant, Of is the frequency
bandwidth of the radiometer and T is integration time (T > 1/~.
There is some restriction however. The receiver's frequency bandwidth
has to be equal or less than Af~i~p = 10-9 ~ the largest possible
bandwidth defined by the frequency dispersion of signals in the ionosphere.
Therefore, the integration time cannot be less than T`li~p = 1/^f~li5p, and
so we shall observe usually spreaded pulses. On the other hand, it will be
possible to use multichannel receivers with the total (effective) bandwidth
l\feff = n ~fai5p ~ 0.3f. In this case we can write effective sensitivity
relative to the pulse-type signal as following:
ASeff = 2kTay5(`Aeff~tp`)~~(~^/disp ~feff)~~/2 = (5)
= 1.5 · 1O-~3T3y5(Aeff · ~tp)-if-2tW - m-2 · Hz-~]
Figure 1 shows the dependences of Speak and ~ Seff from frequency
f calculated for the three values of neu~no energy and for the three
large radio telescopes. It is evident from Figure 1 that the largest radio
OCR for page 89
HIGH-ENERGY ASTROP~ICS
\
104 4
103
1o2
101
89
art.
-
-
-
-
-
~ \\
\ \
\
-
~'<
L ~
0.1 0.2
frequency (GHz)
0.5
FIGURE 1 Frequency dependencies of expected cascade's peak flux densities (Speak) and
of effective sensitivities (A Serf) of several large radio telescoped
telescopes give us already today the opportunity for registartion of the
cascades induced by neutrinos with the energies E > 102° eV.
Now, it will be interesting to estimate the expected rate of such events.
Unfortunately, at present there is no good prediction of high-energy neutri-
nos fluxes. However, many modern models of the universe, for example the
models with superconductive cosmic strings or with supermassive particles
(monopoles, maximons), predict super-high-energy neutrinos (E ~ 102°-
1022 eV or even 1025 - 1028 eV).
Some speculative estimates of high-energy neutrinos dudes were made
by Hill et al. (1987) using the extrapolation of Fly's Eye limit on deeper
penetrating particles. From this estimate one can conclude only that an
OCR for page 90
go
AMERICAN AND SOVIET PERSPECTIVES
upper limit to the rate of the events induced by neutrinos in the five-meter
Moon surface layer would be about 105 year~i for neutrinos with E >
102° eV (here we used for the neutrino-nucleon cross-section In ~ 10-3i
cm-2, see for example Butkevich ef al. (19~. In most models of the
universe with superconductive strings (see also HiD et al. 1987) it may
be expected from 10: to 105 observable cascades per year for the same
neutrino energies.
It should be noted also that the same cascades and pulses of radioe-
mission could be raised (and may be much more often) by interaction of
high energy protons with the Moon. However, proton interactions will be
observed only near the edge of a lunar disk, whereas neutrino interactions
will occur at every part of the disk with approximately the same probability.
There is additional difficulty concerning the registration of such rel-
ativel`,r rare sporadic radio pulses in the presence of different kinds of
interference. 1b increase the reliability of detection we can try lo use the
specific features of signal The delay of pulses at lower frequencies on
account of the ionospheric dispersion is one of such specific features. For
example, the delay will be about 0.4p sec. at 300 MHz and about 4p sec.
at 100 MHz. Therefore, the corresponding digital filter will be very useful
for detection of the pulses.
Another way to increase the reliability of detection is the simultane-
ously use of two or more radio telescopes for Moon monitonug. It should
be noted however, that none of the radio telescopes currently has suitable
receivers to make these observations. Moreover, the beam width of large
radio telescopes, at decimeter wavelengths are usually much less than the
lunar disk's diameter. Consequently, there is the necessity of construction
of a special kind of feed antenna (perhaps a matrix feed antenna) that will
permit sunultaneous reception of pulses from most parts of the lunar dish
We hope that all these difficulties can be overcome in the near future.
RE~:RENC:ES
Askaryan, GN 1961. Zh. Eksp. Tear. Fin (Soy. Phys. JETP) 41: 616-618.
Gusev, G4, and I.M. Zheleznykh. 1983. Pis'ma Zh. Eksp. Tear. Fiz. (JETP Lett.)
38:505-507.
Markov, MA, and I.M. Zhelznyth. 1986. Nucl. Instr. Meth. Phys. Res. A248:242-251.
Dedenko, LG., MN Markov, and I.M. Zheleznykh. 1981. In: Cense, R. E. Ma, and
Roberts (eds.~. Proc 1981 International Conference on Neutrino Physim and
Astrophysics. Maui, Hawaii. 1:292.
Markov, M^, and I.M. Zbeleznykb. 1979. Page 177. In: Learned, J. (ed.~. DUMAN
Workshop at Khabarovsk and Lake Baikal. Honolulu.
Zheleznykh, I.M. 1988. Prom "Neutnno 88" (in press) Boston.
~ill, Cl:, D.N. Schramm, I:P. Walker. 1987. Phys. Rev. D. 36: 1007-1016.
Buttevich, ~V., NB. Kaidarov, PI. K~astev, AV. L~onov-Vendrovski, and I.M. Zheleznykh.
1988. ~ Phys. C - Particles and Fields. 39 241-250.
Representative terms from entire chapter:
feed antenna
