Acoustic detection of Ultra-High-Energy Cosmic-Ray Particles

Understanding the highest energy cosmic radiation represents one of the most challenging fields of modern physics. While abundant high quality data are routinely collected up to energies of several tens of GeV, the study of higher energies is limited by the very low fluxes available. Yet the region around 10²⁰ eV should be particularly interesting since on one hand we do not well understand the acceleration mechanisms that would produce such high energies, while on the other we expect that, as Greisen, Kuzmin, and Zatsepin (GZK) pointed out in 1964, the flux should drastically fall off because pion photo-production on the cosmic background radiation should absorb protons traveling from cosmological distances. Hence ultra high energy (UHE) cosmic rays (CR) present intriguing ties between particle physics, astrophysics and cosmology.

The experimental situation is somewhat confused as illustrated by the figure below where data from the AGASA collaboration in Japan are compared to the recent results from the HiRes experiment in the US. While AGASA seems to see events (and, maybe, even increasing flux) above 10²⁰eV, HiRes [1] data follow the prediction of the GZK model (black curve).

Essentially only two techniques have been used until now to detect cosmic rays at the highest energies. In the air-fluorescence (AF) detectors the shower produced in the atmosphere by the incoming particle is detected by observing the fluorescence it produces on N₂ molecules with a special telescope looking from a hill-top towards the horizon. In this way the shower is sampled by observing the transverse profile. The absolute energy measurement has to rely on the knowledge of the atmosphere light absorption properties at the time the event was recorded. The measurement of the distance of the shower is substantially improved by binocular vision when two telescopes are available. The other technique in use consists in sampling the part of the shower that survives the atmosphere by means of sparse scintillator or Cherenkov counters installed on a large area of the earth surface. In this case energy and shower shape (primary particle identification) have to be inferred, using simulations from the transverse sampling of the shower tails. While HiRes belongs to the first type of detector and AGASA to the second, a larger system, based on a combination of the two techniques is being built by the Auger Collaboration [2].

Although the GZK cutoff would imply a flux of neutrinos around 10²⁰ eV the detection of neutrinos and other weakly interacting particles at these high energies is still in its infancy. In this case the sensitivity would roughly scale as the total mass of the detector (as opposed to the surface as in the case of (strongly interacting) cosmic rays.) While at lower energies dedicated “neutrino telescopes” are operating (Amanda and Baikal) or are being constructed (Antares, Nestor and km³) the size of these detectors will probably not be sufficient to reach the flux sensitivity needed for interesting measurements at and above 10²⁰eV. A number of imaginative techniques are being explored including the use of Cherenkov radio emission in lunar rocks, Antarctic ice and salt formations. Until now no neutrino event around 10²⁰eV has been observed. The best limit, obtained by searching with the NASA Goldstone Antenna for fast radio pulses that would indicate Cherenkov emission in the lunar regolith [3], is still quite far from the region where interesting signals maybe (or in some case should be) observed (see figure above).

The importance of developing alternative techniques for the detection of UHE particles in cosmic radiation should be clear from our brief introduction and from the fact that even for the much more familiar case of photon-based astronomy, our current understanding of the universe derives from the use of a multitude of different devices and techniques. All these methods complement each other, each making accurate measurements of different parameters at different photon energies. The range of instruments goes from radiotelescopes to bolometers, from optical telescopes to gamma-ray detectors. This suite of instruments is based at sea level, on high mountains, on large airplanes and in space. Even seemingly technical details such as the introduction of adaptive optics brought a revolution to our ability to observe and understand the universe.

The detection of particles using acoustic techniques was proposed for the first time by G.A. Askaryan in 1957 [4]. Precise calculations of the expected pressure pulse amplitude from ionization events in water can be found in [5]. The dominant effect (at temperatures sufficiently different from 4°C) is the instantaneous heating of the water that then expands producing a bipolar pressure wave like shown in the figure to the left. This mechanism was experimentally demonstrated by detecting the pressure wave with a hydrophone in a tank of water (and other liquids) when bombarded with an intense beam produced a various accelerators [6]. Apart from some data collected with one hydrophone mounted on one of the photomultiplier strings in the Baikal detector, no work was done on practical cosmic ray detection until our activity at AUTEC was started about two years ago. As discussed in our paper [7] the acoustic technique in water may present advantages over other techniques. First, as shown in the figure to the left, the attenuation length of sound in sea water is larger than 1 km for frequencies around 10 kHz where most of the power for our signal is supposed to be. Hence one may be able to use a sensor array that would be sparser than what would be needed for Cherenkov detection (the light attenuation length in water being shorter than 50 m). Second, low-noise hydrophones can be found at a cheaper price than large photomultiplier tubes. Third, the technique is more intrinsically calorimetric and hence one may hope that it would be a more reliable energy measurement. In general the additional information that would derive from acoustic readout would certainly complement and extend a large water- (or ice-) based Cherenkov detector.

We propose to continue and expand the study of acoustic detection in sea water with the existing AUTEC hydrophone array. Our group has pioneered this work in the last two years and NSF support at this critical time would enable us to finish the analysis of the data collected and implement a second generation data acquisition system. At the same time this project will explore the more speculative possibility of equipping with vibration sensors an object outside of the earth’s atmosphere for cosmic-ray detection. In fact acoustic detection requires a large energy to be deposited in a small region of the medium. Hence only neutrinos and other hypothetical weakly interacting particles would be able to penetrate the atmosphere and give a detectable signal in the ocean. An acoustic detector using as target an object outside the earth atmosphere would, on the other hand, be sensitive to any UHE particle.

Only a few existing hydrophone arrays have a bandwidth that matches well the needs of particle detection. The largest of them (at least in the US) is at the AUTEC facility that the US Navy operates on Andros Island in the Bahamas. The array comprises about 52 hydrophones with 50 kHz bandwidth, mounted on a hexagonal lattice 4 m above the ~1600 m deep ocean floor. The distance between phones is generally about 3 km, although two fine tracking regions with denser phones also exist. A general layout of the array is shown in the figure at the bottom of the previous page. Since two years our group has been using one of the fine tracking regions with 7 phones (shown to the right of the figure) to perform initial studies. Our very simple agreement with the Navy has allowed us to collect data with 208 days of live-time between summer 2001 and winter 2002. The amount of data collected is shown as function of time in the figure below, where we also show that our efficiency is essentially 100% at night, dropping somewhat during the day when the Navy may run their test program and, by agreement, turn our system off. Remarkably this data-taking period has demonstrated that 70% overall efficiency is possible over long periods of time, a much larger efficiency than one can hope to achieve with many of the existing UHE CR detection techniques. We already have on disk the largest –by far— acoustic data set ever collected for cosmic ray physics.

Our simple data-acquisition system (DAQ) is composed of a National Instruments digitizer card installed on the bus of a 1.7 GHz Pentium-4 PC. The card is configured to digitize 7 channels, each at a rate of 179 kS/s. Signals from each of the phones are passed though a matched filter and compared to an adaptive threshold. Pulses accepted by this triggering algorithm are then written to disk for off-line analysis. Every month data is transferred onto an external hard drive and shipped to Stanford (the internet bandwidth available at AUTEC is insufficient for our massive data transfer requirements). We also calibrated the amplitude response of the system by producing pulses similar to the ones from UHE neutrinos with implosions of light bulbs dropped in the water attached to an appropriate weight. Bipolar neutrino-like events can be separated from most of the backgrounds found at AUTEC by analyzing the scatter plot between number of cycles and duration for each event. Good events are expected to occur in regions 4 and 5 in the figure above. Subsequent analysis, to further reject background and accurately measure the energy of the neutrino candidates, requires the study of signals that are in the appropriate time coincidence at four or more phones. Such coincidence is, for the time being, formed off-line because of the very limited resources of our online system. When studying the time coincidences and reconstructing the original event energies, the variations of the sound speed in the water column have to be properly accounted for. The detector acceptance calculation is complicated by the fact that acoustic radiation is expected to be emitted by the high energy showers in a “pancake” geometry [7]. This is due to the fact that there is destructive interference between radiation emitted by the two ends of the elongated (~0.2 m diameter, ~20 m long) showers, resulting in a cylindrical wave.

The AUTEC data collected until now and being analyzed, while probably not sufficient to improve the present sensitivity [3] to UHE neutrinos, is providing a wealth of information on the technique and we expect to perfect the analysis and be able to publish a new flux measurement based on our data alone. This measurement would be achieved within the scope of the work proposed here and would be the first such measurement published on the subject.

The experience already acquired with this data-set allows us to envisage a second phase of our work at AUTEC that would represent the core of the activities funded by this proposal. We now describe the upgrades that, using better hardware and software, would permit us to undertake an improved data collection phase that would probably last for more than one year. It is important to realize that the timely continuation and extension of our activity at AUTEC is essential to maintain the momentum established and to keep the interest of the Navy in our work.

At present the adaptive threshold algorithm is limited in its ability to quickly adapt to the variable noise conditions found at AUTEC. We found that the noise level can change by two orders of magnitude in minutes, because of weather variability and boating traffic. A possible new algorithm would continuously measures the noise distribution and directly reset thresholds to the predicted ideal value. We are confident that such an optimized threshold algorithm could achieve a factor of two reduction in effective thresholds, decreasing by a similar amount the present ≈5·10²⁰ eV energy threshold, while, at the same time, increasing by about an order of magnitude (2³) the present acceptance at high energy.

A second improvement would derive from the possibility of performing coincidences on-line using a much more powerful 2-lever trigger/DAQ scheme. In the current configuration each of the phones triggers independently. Multiple-phone coincidences cannot be formed on-line because of the low speed of sound and the limited buffer length and computing power of the single PC used. We believe that the multi-phone combinatorics can be analyzed and coincidences formed in a second level of trigger that would be run on a second, very powerful computer common to all phones. A 100 baseT connection between the first and the second levels of computers would be sufficient for the data-rates of interest here. Online coincidences would enable us to substantially improve the background rejection and, in turn, permit a further reduction of the energy threshold.

We also plan to greatly increase out detector surface and mass. The 7 phones that are currently read-out span an area of 7 km². The entire available array (52 phones) spans ~250 km², a factor of 36 larger. Because the cables from the full array come to shore in two different locations separated by more than 20 km it is not clear at present how easy it will be to readout and merge the data from the two different sub-arrays. Hence we plan to initially readout only the north section that includes the two fine tracking clusters (and hence slightly more that half of the phones). We envisage installing five 2.4 GHz Pentium 4 machines, each with a digitizer card configured to acquire six phones. The slightly lower number of phones per computer and faster CPUs should be sufficient for the improved first level of trigger. The data would then be passed-on to a high-end 3GHz dual-CPU machine that would perform the function of second level trigger and event builder, reconstructing multi-phone coincidences. A GPS receiver would supply accurate time to stamp the events so that interesting signals could be correlated in time with other detectors. Should the full 52-phone array be eventually read-out, the accurate time stamp could also be used for off-line data re-synchronization between the two cable terminals.

The planned increase in the size of the detector is also expected to substantially improve our tracking accuracy, since most of the events triggered-on with the present system lay outside of the sub-array used. In addition we will be able to test background conditions in different regions of the “Tongue Of The Ocean” where the array is located and with different phone spacing.

Finally we plan to increase our off-line data storage capacity. Currently off-line analysis is performed on the 10-CPU Linux cluster that is used for generic computing by our group. The data-set we already have accumulated has a size of about 500 GB, severely stressing the other activities of the group. We will acquire a dedicated 1.2 TB RAID disk array to be added to the current 1 TB one. We note that while other computing activities in the group are in part performed on off-site computers, our local computer is the only resource available for this work.

The data collected until now makes our group the pioneer in this field. Since we started this activity several other groups, particularly in Europe, have come forward with plans to study acoustic radiation as a means to detect UHE particles. Several scientists in the Antares, Nestor and Amanda/km³ collaborations are developing phones to be installed, on a trial basis, on some of their photomultiplier strings. The possibility of continuing and expanding our work as described here will allow us to maintain a leadership position in this emerging and exciting field. We have also taken the initiative to organize the first workshop on acoustic detection techniques that will be held at Stanford on September 13 and 14, 2003. The workshop, which will be sponsored by the newly established Kavli Institute for Particle Astrophysics and Cosmology, will foster interaction between all the people active in acoustic detection around the world.

Our work at AUTEC would also have the capability to collect data for tracking cetaceans in a large and deep region of sea. In particular sperm whales are known to visit the “Tongue Of The Ocean” and current data-sets consist of single-phone observations (no 3D tracking) without a specific trigger (hence with extremely poor efficiency). Our present trigger rejects most of the signals likely to be from marine mammals but the expanded DAQ system may have an efficient enough online reduction to be able to accept and store such signals without substantial burden for the data storage and transfer system. We are in the process of researching a connection with scientists in the field of marine mammals for what could be a very interdisciplinary use of our data. To this end an abstract from our group has been selected for oral presentation at the large conference Oceans 2003 that covers all aspects of oceanography (San Diego Sept 22-26, 2003). We hope to take advantage of this opportunity and learn more about underwater sound tracking while connecting with scientists with interests and expertise in the possible biological aspects of our data.

As mentioned, in addition to the work in water at AUTEC we intend to begin the preliminary exploration of the concept of sensing the vibrations produced in solid objects outside of the earth atmosphere. One possible target would be the moon that, despite its “dusty” appearance, is known to have a large mechanical Q (at least an order of magnitude larger than that of the earth). This is because, in the absence of water, the different grains of dust behave like they were welded together for small amplitude vibrations. Other bodies in the asteroid belt may, however, present advantages. In particular “M-type” asteroids are thought to be possibly composed of Fe-Ni magnetic alloys [8], making the implantation of sensors easier. A future space mission could involve the deployment of hundreds of micro-probes, each consisting of a micro-machined vibration sensor, a device to attach or embed itself on the surface of the target body and a radio to relay the vibration data back to the mother spacecraft that would be hovering or orbiting above the target body for the duration of the mission. Interest for exploring the concept of a mission of this kind has been expressed by Dr. Stanley Hunter of NASA Goddard Space Flight Center. Such a mission would be very interdisciplinary in nature, as it would collect data that would be of great interest for cosmic-ray physics as well at planetary science/geophysics. In addition it would include the study of engineering issues related to the management of a large network of semi-independent probes.

Much of the work proposed for the development of this concept would involve theoretical studies to understand: 1) the details of sound production and propagation from high energy showers that are specific to solids; 2) the background from impacts on different possible target bodies and 3) the investigation of properties of different types of target bodies (silicate vs metallic asteroids, asteroids vs moon). We also envisage performing a small set of experiments to engineer the best miniaturized sensor and its mechanical connection with the target body. This experimental work may be extended, at a later stage, in a test beam, possibly at SLAC. A block of material with properties consistent with a realistic target body could be instrumented to then study the vibration signals induced by the impacts of high energy particles.

The PI has substantial experience in discussing science with lay people. Along with many lectures on neutrino physics for a broad audience, in October 2001 the PI was invited to lecture at a meeting of the American Association of Science Writers. It is our experience from the work we have done until now at AUTEC that the program proposed here will enable us to embark in many activities that will help bring the excitement of science to the larger public. Undergraduate students have been the very core of the activities at AUTEC until now and will maintain a very prominent role in this program. In addition the working relationships we have developed with the US Navy have allowed us to describe our project to a number of people outside of our field. We have been able to receive substantial support from the Navy personnel thanks to the excitement that they feel for being part of a science project that will further our knowledge of topics like astrophysics and particle physics. The idea of detecting neutrinos by the sound they make in water does greatly excite common people and spurs them to try to learn more. In the two years we have worked at AUTEC three articles have appeared in the broader press regarding our work [9], demonstrating the appeal of this project to large audiences. The connections we hope to make with the marine biology and underwater acoustics communities will further connect our activities with the scientific community at large. The study of the possibility of mounting a space mission to instrument the moon or an asteroid for the purpose of cosmic ray detection should allow us to expand the ability to bring the excitement of science to the public.