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