Australia Telescope National Facility

Neutrino Telescopes in Antarctica
Jenni Adams, PASA, 17 (1), 13.

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RICE

RICE is optimised to detect the radio-frequency Cherenkov radiation produced when an ultra-high energy (> 10¹⁵ eV) electron neutrino undergoes a charged current interaction in the ice. Thus RICE complements AMANDA by extending into the electron neutrino sector and offering a detection strategy for ultra-high energies. For a 10¹⁵ eV neutrino induced cascade the effective volume offered by one radio receiver in the ice is comparable to one phototube. The ratio of the effective volume of a radio receiver compared with a phototube grows with energy such that a radio receiver offers an effective volume ten times larger than a phototube for 10¹⁸ eV neutrinos (Price 1996). However in many astrophysical sources the neutrino energy spectrum falls steeply at high energies. To compare the event rates per year requires knowledge of this neutrino energy spectrum. For example Price (1996) considered the AGN spectra of Stecker and Salamon (1996)(SS) and Szabo and Protheroe (1994)(SP). Considering single elements in a radio or optical array he found using the SS spectrum that a radio receiver would yield an event rate three times higher than a phototube. However using the most optimistic of the SP spectrum gave comparable event rates and the least optimistic SP spectrum yielded an event rate a factor of ten lower for radio receivers.

Above 10¹⁵ eV electron neutrinos will be readily absorbed by charged-current interactions in the earth. Thus, in contrast to AMANDA, RICE searches for downgoing neutrinos. Since atmospheric muons do not trigger RICE it is not necessary to use the earth as a shield as AMANDA does.

An ultra-high-energy $\nu_e$ that undergoes a charged current interaction in the ice will transfer most of its energy to the resulting electron and subsequent electromagnetic shower. A charge imbalance will develop as positrons are annihilated and atomic electrons are scattered into the shower. Monte Carlo calculations show that the net charge is about 20 percent of the total number of electrons (Zas, Halzen and Stanev 1992). The moving blob of net negative charge will produce coherent Cherenkov radiation at wavelengths larger than its own spatial extent ( $\sim 10$ cm), corresponding to radio frequencies ( $\nu \leq 1$ GHz). The detection geometry is displayed in figure 1, showing the Cherenkov cone produced by an electromagnetic shower being detected by suitably located radio receivers.

RICE currently consists of a 16 channel radio receiver array in the ice. In the 1995-96 austral summer the AMANDA collaboration graciously consented to allow cables and radio antenna modules to be dropped in two AMANDA holes. Two antennas plus associated receiver electronics were deployed at depths of approximately 250m and 140m. The objective of this pilot experiment were to demonstrate that a radio-based effort could be launched with minimum impact on AMANDA, and to establish the procedure which would be used for full deployment. In 1996-97 additional hardware was deployed. This consisted of seven dipole antennas, tuned to $\sim$ 275 MHz with 10% bandwidth. Four of the antennas were deployed as receivers and three as transmitters. Again these were deployed in AMANDA holes. In 1998-99 four dedicated RICE holes were dug using a standard mechanical hole-borer and seven receivers are located in these holes at depths of 120m and 170m. Three surface horn antennas were also deployed in 1998-99 which are used as a veto of surface-generated noise.

Much of the analysis to date has been concerned with measuring and understanding the noise. A frequency-dependent effective noise temperature has been measured and is filtered at low frequencies. Long duration continuous broadcast backgrounds are also easily filtered. Short duration ``burst'' noise backgrounds from the surface or AMANDA below have been the subject of recent investigation and elimination based on the timing sequence of hits on various receivers is proving successful.

Since January 30, 1999, RICE has accumulated $\sim$ 45 days of livetime. The overwhelming majority of the recorded triggers to date are consistent with surface-generated noise backgrounds; no clear neutrino candidates have been observed. Analysis is currently in progress; roughly, every 15 days of livetime corresponds to a sensitivity level comparable to 1% that of the Stecker and Salamon (1996) predictions for the incident ultra-high energy neutrino flux. This is based on Monte Carlo simulations of the current relatively small array. The RICE Monte Carlo simulations are described by Frichter, Ralston and McKay (1996). Monte Carlo investigation of an extended 25 element RICE array indicates that energy resolution of $\sim$ 20% and angular resolution of < 1-5 $^{\circ}$ depending on the signal geometry is achievable.

A recent review of the status of RICE with a detailed account of event characteristics and reconstruction is given by Frichter (1999). A full list of personnel is also included in this review.

**Figure 2:** The RICE concept. An ultra-high energy electron neutrino intiates an electromagnetic shower in Antarctic ice. The resulting radio pulse is detected by a buried array of radio receivers.
$\begin{figure} \begin{center} \mbox{\psfig{file=rice-concept.eps,height=8cm}} \end{center} \end{figure}$

Next Section: Acknowledgements
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Contents Page: Volume 17, Number 1

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