Time-Domain Measurement of Broadband Coherent Cherenkov Radiation

We report on further analysis of coherent microwave Cherenkov impulses emitted via the Askaryan mechanism from high-energy electromagnetic showers produced at the Stanford Linear Accelerator Center (SLAC). In this report, the time-domain based analysis of the measurements made with a broadband (nominally 1-18 GHz) log periodic dipole array antenna is described. The theory of a transmit-receive antenna system based on time-dependent effective height operator is summarized and applied to fully characterize the measurement antenna system and to reconstruct the electric field induced via the Askaryan process. The observed radiation intensity and phase as functions of frequency were found to agree with expectations from 0.75-11.5 GHz within experimental errors on the normalized electric field magnitude and the relative phase; 0.039 microV/MHz/TeV and 17 deg, respectively. This is the first time this agreement has been observed over such a broad bandwidth, and the first measurement of the relative phase variation of an Askaryan pulse. The importance of validation of the Askaryan mechanism is significant since it is viewed as the most promising way to detect cosmogenic neutrino fluxes at E > 10^15 eV.Comment: 10 pages, 9 figures, accepted by Phys. Rev.

Observation of the Askaryan Effect: Coherent Microwave Cherenkov Emission from Charge Asymmetry in High Energy Particle Cascades

We present the first direct experimental evidence for the charge excess in high energy particle showers predicted nearly 40 years ago by Askaryan. We directed bremsstrahlung photons from picosecond pulses of 28.5 GeV electrons at the SLAC Final Focus Test Beam facility into a 3.5 ton silica sand target, producing electromagnetic showers several meters long. A series of antennas spanning 0.3 to 6 GHz were used to detect strong, sub-nanosecond radio frequency pulses produced whenever a shower was present. The measured electric field strengths are consistent with a completely coherent radiation process. The pulses show 100% linear polarization, consistent with the expectations of Cherenkov radiation. The field strength versus depth closely follows the expected particle number density profile of the cascade, consistent with emission from excess charge distributed along the shower. These measurements therefore provide strong support for experiments designed to detect high energy cosmic rays and neutrinos via coherent radio emission from their cascades.Comment: 10 pages, 4 figures. Submitted to Phys. Rev. Let

Correlation between microstructure and magnetotransport in organic semiconductor spin valve structures

We have studied magnetotransport in organic-inorganic hybrid multilayer junctions. In these devices, the organic semiconductor (OSC) Alq$_3$ (tris(8-hydroxyquinoline) aluminum) formed a spacer layer between ferromagnetic (FM) Co and Fe layers. The thickness of the Alq$_3$ layer was in the range of 50-150 nm. Positive magnetoresistance (MR) was observed at 4.2 K in a current perpendicular to plane geometry, and these effects persisted up to room temperature. The devices' microstructure was studied by X-ray reflectometry, Auger electron spectroscopy and polarized neutron reflectometry (PNR). The films show well-defined layers with modest average chemical roughness (3-5 nm) at the interface between the Alq$_3$ and the surrounding FM layers. Reflectometry shows that larger MR effects are associated with smaller FM/Alq$_3$ interface width (both chemical and magnetic) and a magnetically dead layer at the Alq$_3$/Fe interface. The PNR data also show that the Co layer, which was deposited on top of the Alq$_3$, adopts a multi-domain magnetic structure at low field and a perfect anti-parallel state is not obtained. The origins of the observed MR are discussed and attributed to spin coherent transport. A lower bound for the spin diffusion length in Alq$_3$ was estimated as $43 \pm 5$ nm at 80 K. However, the subtle correlations between microstructure and magnetotransport indicate the importance of interfacial effects in these systems.Comment: 21 pages, 11 figures and 2 table

Incidence of insulin-requiring diabetes in the US military

The aim of the study was to determine age- and race-related, and overall incidence rates of insulin-requiring diabetes in adults in the US military. Electronic records for admissions to US military and Tricare hospitals during 1990–2005 and visits to military clinics during 2000–2005 were identified using the Career History Archival Medical and Personnel System at the Naval Health Research Center, San Diego, CA, USA. Population data were obtained from the Defense Manpower Data Center and Defense Medical Epidemiology Database. In men there were 2,918 new cases of insulin-requiring diabetes in 20,427,038 person-years at ages 18–44 years (median age 28 years) for a total age-adjusted incidence rate of 17.5 per 100,000 person-years (95% CI 16.4–18.6). Incidence rates were twice as high in black men as in white men (31.5 vs 14.5 per 100,000, p < 0.001). In women there were 414 new cases in 3,285,000 person-years at ages 18–44 years (median age 27 years), for a total age-adjusted incidence rate of 13.6 per 100,000 (95% CI 12.4–14.9). Incidence rates were twice as high in black women as in white women (21.8 vs 9.7 per 100,000, p < 0.001). In a regression model, incidence of insulin-requiring diabetes peaked annually in the winter–spring season (OR 1.46, p < 0.01). Race and seasonal differences persisted in the multivariate analysis. Differences in incidence rates by race and season suggest a need for further research into possible reasons, including the possibility of a contribution from vitamin D deficiency. Cohort studies using prediagnostic serum 25-hydroxyvitamin D should be conducted to further evaluate this relationship

Radio Detection of High Energy Particles: Coherence Versus Multiple Scales

Radio Cherenkov emission underlines detection of high energy particles via a signal growing like the particle-energy-squared. Cosmic ray-induced electromagnetic showers are a primary application. While many studies have treated the phenomenon approximately, none have attempted to incorporate all the physical scales involved in problems with time- or spatially- evolving charges. We find it is possible to decompose the calculated fields into the product of a form factor, characterizing a moving charge distribution, multiplying a general integral which depends on the charge evolution. In circumstances of interest for cosmic ray physics, the resulting expressions can be evaluated explicitely in terms of a few parameters obtainable from shower codes. The classic issues of Frauhofer and Fresnel zones play a crucial role in the coherence.Comment: 25 pages, 10 figure

FORTE satellite constraints on ultra-high energy cosmic particle fluxes

The FORTE (Fast On-orbit Recording of Transient Events) satellite records bursts of electromagnetic waves arising from near the Earth's surface in the radio frequency (RF) range of 30 to 300 MHz with a dual polarization antenna. We investigate the possible RF signature of ultra-high energy cosmic-ray particles in the form of coherent Cherenkov radiation from cascades in ice. We calculate the sensitivity of the FORTE satellite to ultra-high energy (UHE) neutrino fluxes at different energies beyond the Greisen-Zatsepin-Kuzmin (GZK) cutoff. Some constraints on supersymmetry model parameters are also estimated due to the limits that FORTE sets on the UHE neutralino flux. The FORTE database consists of over 4 million recorded events to date, including in principle some events associated with UHE neutrinos. We search for candidate FORTE events in the period from September 1997 to December 1999. The candidate production mechanism is via coherent VHF radiation from a UHE neutrino shower in the Greenland ice sheet. We demonstrate a high efficiency for selection against lightning and anthropogenic backgrounds. A single candidate out of several thousand raw triggers survives all cuts, and we set limits on the corresponding particle fluxes assuming this event represents our background level.Comment: added a table, updated references and Figure 8, this version is submitted to Phys. Rev.

Radio-Frequency Measurements of Coherent Transition and Cherenkov Radiation: Implications for High-Energy Neutrino Detection

We report on measurements of 11-18 cm wavelength radio emission from interactions of 15.2 MeV pulsed electron bunches at the Argonne Wakefield Accelerator. The electrons were observed both in a configuration where they produced primarily transition radiation from an aluminum foil, and in a configuration designed for the electrons to produce Cherenkov radiation in a silica sand target. Our aim was to emulate the large electron excess expected to develop during an electromagnetic cascade initiated by an ultra high-energy particle. Such charge asymmetries are predicted to produce strong coherent radio pulses, which are the basis for several experiments to detect high-energy neutrinos from the showers they induce in Antarctic ice and in the lunar regolith. We detected coherent emission which we attribute both to transition and possibly Cherenkov radiation at different levels depending on the experimental conditions. We discuss implications for experiments relying on radio emission for detection of electromagnetic cascades produced by ultra high-energy neutrinos.Comment: updated figure 10; fixed typo in equation 2.2; accepted by PR

Observations of the Askaryan Effect in Ice

We report on the first observations of the Askaryan effect in ice: coherent impulsive radio Cherenkov radiation from the charge asymmetry in an electromagnetic (EM) shower. Such radiation has been observed in silica sand and rock salt, but this is the first direct observation from an EM shower in ice. These measurements are important since the majority of experiments to date that rely on the effect for ultra-high energy neutrino detection are being performed using ice as the target medium. As part of the complete validation process for the Antarctic Impulsive Transient Antenna (ANITA) experiment, we performed an experiment at the Stanford Linear Accelerator Center (SLAC) in June 2006 using a 7.5 metric ton ice target, yielding results fully consistent with theoretical expectations

Ultra-Relativistic Magnetic Monopole Search with the ANITA-II Balloon-borne Radio Interferometer

We have conducted a search for extended energy deposition trails left by ultra-relativistic magnetic monopoles interacting in Antarctic ice. The non-observation of any satisfactory candidates in the 31 days of accumulated ANITA-II flight data results in an upper limit on the diffuse flux of relativistic monopoles. We obtain a 90% C.L. limit of order 10^{-19}/(cm^2-s-sr) for values of Lorentz boost factor 10^{10}<gamma at the anticipated energy E=10^{16} GeV. This bound is stronger than all previously published experimental limits for this kinematic range.Comment: updated to version accepted by Phys. Rev.