HR: 0800h
AN: AE31A-0046    [Abstracts]
TI: X-ray Production in Laboratory Sparks in air
AU: * Rahman, M
EM: Mahbubur.Rahman@Angstrom.uu.se
AF: Division for Electricity, Uppsala University, Box 534, Uppsala, SE-751 21, Sweden
AU: Cooray, G V
EM: Vernon.Cooray@Angstrom.uu.se
AF: Division for Electricity, Uppsala University, Box 534, Uppsala, SE-751 21, Sweden
AU: Rakov, V A
EM: rakov@ece.ufl.edu
AF: Department of Electrical and Computer Engineering, University of Florida, P.O. Box 116130, Gainesville, FL 32611, United States
AB: Dwyer et al. [2005] recently observed X-ray bursts (in the 30 to 150 keV range) from high-voltage laboratory sparks (positive and negative; 5 cm - 2 m) in air. In this paper, we present results of an independent experiment conducted at a different high-voltage facility and using different instrumentation to confirm (or refute) the production of X-rays by laboratory discharges. The experiment was conducted at the high-voltage laboratory of Uppsala University, Sweden. A spark was created in air at atmospheric pressure applying a standard lightning impulse voltage (1.2/50 impulse; front time: 1.2 μs, time to half-value: 50 μs) to an 80-cm long rod-to-hemisphere air gap. In this experiment the generator was charged to 1 MV with negative polarity. The voltage across the gap at breakdown, the current in the spark, the visible optical radiation from the spark, and the emission of X-rays produced by the spark were measured. The emission of X-rays was detected by a barium fluoride (BaF₂) crystal scintillator which had a shape of frustum of a right cone (front diameter: 2.2 cm, rear diameter: 4 cm, and length: 4 cm). The scintillator was attached to a 51 mm PMT (Photonis XP2020/URQ, rise time 1.4 ns, spectral sensitivity 200-550 nm). The ungrounded, light-tight, and electromagnetically sealed metal cabinet having the complete X-ray detector, optical measuring system, oscilloscope and power supplies, was placed about 1 m away from the spark gap so that the center of the crystal was a few centimetres below the high voltage rod electrode. A total of 83 negative voltage impulses were applied across the gap and in 49 cases (59% of the events) the X-ray signals were detected. In 23 cases, the X-ray signal appeared before the collapse of the applied voltage across the gap and about 1 μs prior to onset of the main discharge current. In 26 cases, the X-ray signal appeared at around the main discharge current peak. A rough estimation of total deposited energy in the crystal was carried out based on measured signals from three X-ray sources, which are ¹³⁷Cs, ⁶⁰Co, and ²⁴¹Am. The lowest total deposited energy was calculated to be around 30 keV. The amplitudes and the width of the detected X-ray signals were different for different events and some signals were saturated. The average total deposited energy of the unsaturated signals was calculated to be around 170 keV. The total deposited energy in the case of saturated signals is in the order of few MeV. The detected X-ray signal is consistent with the response of the BaF₂ detector to a signal from a ¹³⁷Cs radioactive source placed on the aluminum-covered-window of the metal cabinet. Moreover, a comparison to the X-ray and optical signals shows clearly that the first recorded X-ray signal, which appears before the collapse of the applied voltage across the gap, is associated with the pre-discharge activity in the gap. The second recorded X-ray signal appears during the collapse of the applied voltage across the gap. There were 4 common cases out of 49 when the signals appeared simultaneously both before and during the collapse of the gap. No X-rays were detected from sparks with positive voltage impulse applied to the rod for the same gap geometry, although in this case the breakdown voltage was lower than that corresponding to the negative voltages.
DE: 3300 ATMOSPHERIC PROCESSES
DE: 3304 Atmospheric electricity
DE: 3324 Lightning
DE: 3359 Radiative processes
DE: 3394 Instruments and techniques
SC: Atmospheric and Space Electricity [AE]
MN: 2007 Fall Meeting