Estimation and Measurement of Alpha Decay Pulses in Fission Detectors and Their Practical Application for Verifying Detector Health

Abstract

Signals generated by spontaneous alpha decay of uranium in fission chambers are very small and are buried in background noise. These signals are often simply discriminated by amplitude as unwanted background noise. However, if these signals can be distinguished from white noise, they could function as a checking source that is electric-noise-free and is embedded in the detector itself. To confirm our hypothesis, we estimated and measured the pulse height of alpha decay signals in the fission chambers of the Large Helical Device (LHD) at the National Institute for Fusion Science (NIFS). When background noise was sufficiently low, the plateau curves of alpha decay signals could be measured by using the maintenance function of the neutron monitoring system. When background noise was high, instruments such as an oscilloscope and a multi-channel analyzer (MCA) successfully caught alpha decay signals by using their high-performance trigger functions. Measuring alpha decay pulses is also an indicator of the health of the detector and its preamplifier because if something were wrong with either component, the alpha decay signals would prove difficult to measure. This practical application of alpha decay signals to verify neutron instrumentation systems that use the noise-sensitive Campbelling method will be beneficial for fusion facilities and fission reactors and help meet ALARA policy because this method does not require an external checking source.