In-flight measurements of energetic radiation from lightning and thunderclouds

In the certification procedure aircraft builders carry out so-called icing tests flights, where the zero degree Celsius altitude is deliberately sought and crossed in or under thunderstorms. Airbus also used these flights to test ILDAS, a system aimed to determine lightning severity and attachment points during flight from high speed data on the electric and magnetic field at the aircraft surface. We used this unique opportunity to enhance the ILDAS systems with two x-ray detectors coupled to high speed data recorders in an attempt to determine the x-rays produced by lightning in-situ, with synchronous determination of the lightning current distribution and electric field at the aircraft. Such data are of interest in a study of lightning physics. In addition, the data may provide clues to the x-ray dose for personnel and equipment during flights. The icing campaign ran in April 2014; in six flights we collected data of 61 lightning strikes on an Airbus test aircraft. In this communication we briefly describe ILDAS and present selected results on three strikes, two aircraft initiated and one intercepted. Most of the x-rays have been observed synchronous with initiating negative leader steps, and as bursts immediately preceding the current of the recoil process. Those processes include the return stroke. The bursts last one to four micro-second and attain x-ray energies up to 10 MeV. Intensity and spectral distribution of the x-rays and the association with the current distribution are discussed. ILDAS also continuously records x-rays at low resolution in time and amplitude.Comment: 28 pages, 9 figure

On the origin of hard X-rays in the growth of meter long sparks

Meter-long laboratory sparks generate high-energetic radiation in a similar way as lightning: Bremsstrahlung generated in collisions between high-energy electrons and air molecules. This study aims to localize and characterize the X-ray source and to visualize the relevant processes. A Marx generator delivers a standardized lightning impulse voltage pulse of 1.2/50 µs rise/fall time of positive or negative polarity. The generator was loaded by a spark gap formed by two conical electrodes at about 1 m distance; one of the electrodes was grounded. Applied voltages were 1 MV, which lead to breakdown of the gap. The voltage was measured by a high-voltage divider. Both electrodes were equipped with current probes to determine the electrical characteristics of the discharge. Two La(Ce)Br3 scintillation detectors measured the X-rays; different distances and angles gave information on the spatial distribution around the spark gap. Lead collimators limited the field of view. Lead attenuators of different thicknesses helped to determine the energy distribution. An intensified CCD camera allows us to capture images of prebreakdown phenomena with ten-ns resolution. All diagnostics was synchronized to better than 1 ns. Many hundreds of discharges allowed statistical analysis. The X-ray emission area is concentrated in the vicinity of the cathode. The variation with detector position shows a 1/r2 dependence of the detection rate, characteristic of a point-like source of constant luminosity. The reduction with attenuators of variable thickness agrees with a characteristic X-ray energy of 200 keV. The X-rays never occur before there is any cathode current. The nanosecond-fast photography allowed us to follow all pre-breakdown stages of the discharge, from the formation of a first inception cloud, to the formation and propagation of streamers crossing the gap. At a later stage, some cold streamer channels developed into hot leaders which then lead to breakdown. For the X-ray production negative streamers were a necessary and sufficient condition, for positive and for negative generator voltage

Experimental study on hard x-rays emitted from metre-scale negative discharges in air

We investigate the development of metre long negative discharges and focus on their x-ray emissions. We describe appearance, timing and spatial distribution of the x-rays. They appear in bursts of nanosecond duration mostly in the cathode area. The spectrum can be characterized by an exponential function with 200¿keV characteristic photon energy. With nanosecond-fast photography we took detailed images of the pre-breakdown phenomena during the time when x-rays were registered. We found bipolar discharge structures, also called 'pilot systems', in the vicinity of the cathode. As in our previous study of x-rays from positive discharges, we correlate the x-ray emission with encounters between positive and negative streamers. We suggest that a similar process is responsible for x-rays generated by lightning leaders

Relativistic five-quark equations and negative parity pentaquarks

The relativistic five-quark equations are found in the framework of the dispersion relation technique. The solutions of these equations using the method based on the extraction of the leading singularities of the amplitudes are obtained. The five-quark amplitudes for the low-lying pentaquarks including the u, d, s- quarks are calculated. The poles of these amplitudes determine the masses of the negative parity pentaquarks with I = 0, 1 and spin 3/2-, 5/2-. The mass of the lowest pentaquark with I = 0 and spin 3/2- is equal to 1514 MeV.Comment: 18 pages, pdf, published versio

Meter-scale spark X-ray spectrumstatistics

X-ray emission by sparks implies bremsstrahlung from a population of energetic electrons, but the details of this process remain a mystery. We present detailed statistical analysis of X-ray spectra detected by multiple detectors during sparks produced by 1 MV negative high-voltage pulses with 1 $\mu$s risetime. With over 900 shots, we statistically analyze the signals, assuming that the distribution of spark X-ray fluence behaves as a power law and that the energy spectrum of X-rays detectable after traversing $\sim$2 m of air and a thin aluminum shield is exponential. We then determine the parameters of those distributions by fitting cumulative distribution functions to the observations. The fit results match the observations very well if the mean of the exponential X-ray energy distribution is 86 $\pm$ 7 keV and the spark X-ray fluence power law distribution has index -1.29 $\pm$ 0.04 and spans at least 3 orders of magnitude in fluence

Relativistic electrons from sparks in the laboratory

Discharge experiments were carried out at the Eindhoven University of Technology in 2013. The experimental setup was designed to search for electrons produced in meter-scale sparks using a 1 MV Marx generator. Negative voltage was applied to the high voltage (HV) electrode. Five thin (1 mm) plastic detectors (5 $\rm cm^2$ each) were distributed in various configurations close to the spark gap. Earlier studies have shown (for HV negative) that X-rays are produced when a cloud of streamers is developed 30-60 cm from the negative electrode. This indicates that the electrons producing the X-rays are also accelerated at this location, that could be in the strong electric field from counterstreamers of opposite polarity. Comparing our measurements with modeling results, we find that $\sim$300 keV electrons produced about 30-60 cm from the negative electrode are the most likely source of our measurements. A statistical analysis of expected detection of photon bursts by these fiber detectors indicates that only 20%-45% of the detected bursts could be from soft ($\sim$10 keV) photons, which further supports that the majority of detected bursts are produced by relativistic electrons