Askaryan effect in salt: SLAC T460, June 2002

T460 rock-salt target

• 4lb high-purity synthetic rock-salt bricks (density=2.07) – 6 tons of it.

• + some filler from local grocery store…• Makes & contains the shower

• Embedding 21 dual antennas (and lots of cable)

• Polarization axis parallel & perpendicular to shower axis

• Deaden reflections with anechoic foam, etc.

Detector layout; EGS simulation

• Bremsstrahlung beam from 28.5 GeV e-

Lateral distributions

• Beam exit point shown above• good indicator of transverse

size of shower! -- at 15 radiation lengths.

3cm

• Front Face: synchrotron radiation damage

• Shower Max. . .

Coherence

• Much wider energy range covered than previously: 1PeV up to 10 EeV• Coherence (quadratic rise of pulse power with shower energy) observed

over 8 orders of magnitude in radio pulse power• Differs from actual EeV showers only in leading interactions & shower

length ==> radio emission characteristics almost unaffected

Bow tie pulses: time and frequency domains

- Summed pulses (in phase)

- Fourier transform: (not quite linear)

Absolute pulse field strength

• Shows rise with frequency expected for Cherenkov

• Loss of coherence roll-off at ~5-10GHz also likely

Side antennae: horns & log-periodic

Cherenkov polarization tracking

• Radio Cherenkov: polarization measurements are straightforward

• Two antennas at different parts of cone:– Will measure different

projected plane of E, S

– Intersection of these planes defines shower track

Cherenkov radiation predictions:• 100% linearly polarized• plane of polarization aligned with plane containing Poynting vector S and particle/cascade velocity U

USE

Cherenkov Polarization tracking

• Antennas both on- and off-axis used

• Plane of polarization in main pulse tracks the projected direction of the particle shower

• First direct observation that we know of

Polarization tracking

Transition Radiation

Roadmap to a large-scale salt detector

1. Verify Askaryan process: silica sand, SLAC T444, 20002. Identify radio-transparent natural salt structures 2001

• Ground Penetrating Radar tests from 1970’s give strong indications

• Hockley salt dome tests (Gorham et al. 2002) confirm La>250m3. Extend accelerator results to rock salt 2002

– SLAC T460: salt behaves as predicted!

4. Cosmic-ray testbed for antenna development/signal characterization 2002• In progress since 2003

5. Site studies and selection 2005-20066. Detector construction & deployment 2007-2010