A New Instrument for Measuring Energetic Radiation From Triggered Lightning

In this presentation, we describe a new instrument built at the Florida Institute of Technology for measuring x-rays and gamma-rays from rocket-triggered lightning. During the summer of 2002, the instrument was placed less than 25 m from the 11 m tall launch tower at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida. The instrument consists of a 5" by 3" cylinder of NaI(Tl) scintillator attached to a 5" photomultiplier tube (PMT) detector, plus a control detector, identical in every way but with no scintillator attached to the PMT. Great care was taken to reduce false signals from the electrically noisy environment. Both detectors are battery operated and placed inside a thick aluminum, watertight box, sealed to prevent RF noise and light leaks. Data are transmitted to a National Instruments PCI 5102 data acquisition card and PC via fiber optic links. The output signals from the preamplifiers attached to each PMT are digitized with 8 bit resolution every 1 microsecond, and the data acquisition is triggered externally by the current measured (threshold = 4.5 kiloamps) at the launch tower, which usually corresponds to the occurrence of the return strokes. For each trigger, a total of 1 second of data are acquired simultaneously for both PMTs with 0.1 seconds of pre-trigger data. This technique allows individual signals to be analyzed to distinguish real x-rays from noise or other false signals. On 2002 July 20 and 25, 5 rockets were launched from the tower under thunderstorm conditions, resulting in a total of 5 triggered lightning flashes containing at least 24 return strokes terminated on the launch tower. During these lightning events, large amounts of energetic radiation (>>10 keV) in the form of x-rays and gamma-rays and/or energetic electrons were observed just prior to 20 of the 24 recorded return strokes, depositing on average tens of MeV into the detector per stroke. We shall present an overview of the design of the instrument and its performance in the laboratory and field environments.