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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
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
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
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.