Summer Institute at NYU in 2002

History. The New York Schools Cosmic Particle Telescope Project is a collaboration led by Prof. G. Farrar. It aims to create a next-generation astrophysical observatory in New York City, distributed among hundreds or thousands of schools or other sites, to detect the large showers of charged particles produced by energetic cosmic rays. In 2002, faculty at NYU, Columbia, Barnard, CUNY and public and private school teachers and students sponsored a summer institute at NYU which was very successful. The array of detector envisioned for the schools was based on thin plastic scintillators, using an inexpensive logic module developed for use by schools (the quarknet card: see www.—–quarknet) for coincidence logic and data recording by digitizing pulse start and stop times in a form suitable for recording in a computer.

Current Effort. Recently, work on this project at NYU has concentrated on expanding the original concept by adding a Cherenkov detector array based on using the water tanks that are seen on top of many New York City buildings. (A Cherenkov detector consists of a tank full of water with some photomultiplier tubes to detect Cherenkov light produced by charged particles passing through the water.) By law, every building in the New York metropolitan area taller than 6 stories must have one or more emergency water tanks of at least 3500 gallons capacity to supply sprinklers and augment water pressure to hydrants. These tanks are similar in dimensions to those used as Cherenkov detectors in the Pierre Auger Observatory, the largest current cosmic ray detection array. (see www.auger.org ) There are more than 15,000 of these tanks in New York City alone, on buildings whose height ranges between 20 and 380m. Distribution of these tanks in New York City is shown in fig.1. A NewYork Times essay on these tanks can be seen at http://www.nytimes.com/slideshow/2009/04/07/nyregion/20090408lens_index.html

Figure 1
Our idea is to develop an inexpensive way of converting these tanks into Cherenkov detectors at low cost. If this can be done, it would allow construction of an array smaller in area but denser than the Auger array, with detectors distributed over a range of heights, making it more sensitive to lower energy cosmic rays and horizontal showers caused by penetrating particles, such as neutrinos. In combination with the scintillator array originally proposed, it would provide good particle identification. Such an array would supplement existing and planned arrays, with unique sensitivity to particular interesting events. Its advantages are explained in more detail in www.——— (grant proposal)

Recent progress. In July 2005, a wooden rooftop water tank at NYU was converted into a Cherenkov detector, while still continuing its function as a standpipe pressure tank. A diagram of the tank is shown in fig .2 To do this, it had to be provided with photomultiplier tubes, made light tight, lined with light-reflecting material, and have its water cleaned sufficiently to transmit the Cherenkov light to the phototubes.

Figure 2
Three 9″ phototubes were donated to us by the manufacturer, Photonis. They were mounted on Styrofoam floats, held in place by rope harnesses. Light-tightness gave a good deal of trouble at first, even after covering all the above-water parts of the tank with sticky roofing material. This problem was finally solved by providing a bonnet of opaque material over the roof of the tank, supplemented by a tarpaulin to protect it from rainwater. The sides and bottom of the tank were covered inside by the plastic Tyvek, familiar from FedEx envelopes and used as diffuse reflecting material in the Auger tanks.

The tanks are constantly topped up with tap water to make up for leakage and evaporation, and impurities in local tap water would absorb or scatter most of the light passing through the tank before it reached the photomultipliers. Therefore, it was necessary to filter the water continuously to keep it sufficiently transparent. For this, we installed a pump and a filter system recommended by a local water purification company and costing about $500. This made the tank water comparable in light transmission to commercial distilled water, transmitting five times as much light as a tap water sample from a light source 3 meters distant.

Tests of the system showed that it could produce a usable signal from a single cosmic ray muon, though with considerably less efficiency than the Auger tanks. Further tests were made with a scintillation hodoscope to define single vertical muon tracks. Initial tests were made using a computer and other electronics on the roof. We have now constructed a system that allows the signals from the tank to be monitored and photomultiplier voltages adjusted remotely.

Future plans. We would like to improve light collection efficiency by working out a way to float Tyvek reflecting material on the top of the tank. We have also made some preliminary tests of wavelength-shifting fibers connected to a small photomultiplier tube as substitutes for large photomultiplier tubes. We found that, as expected from the experience of other users, they were about 3% as effective as photomultipliers per unit exposed area. However, they allow a huge amount of area to be exposed throughout the tank. Cherenkov light would only have a small distance to travel before reaching a fiber. This would make the system far less sensitive to water cleanliness and dirt on reflecting surfaces, important considerations in a non-sealed system like ours. Cost of such a system would be roughly comparable with cost of large-area photomultipliers.

We have requested funding (www.—–grant proposal) to pursue these possibilities and set up a small demonstration system of a few converted tanks to get experience in solving problems involved in operating a city-wide array.

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