High Count Rate Neutron Spectrometry With Liquid Scintillation Detectors

Liquid scintillation detectors are widely used in nuclear/high-energy physics and nuclear fusion for spectral measurements in mixed radiation fields due to their compactness, fast response and neutron/gamma discrimination capabilities. The use of response functions evaluated for the specific system and of appropriate methods of data analysis allows such systems to be used as broadband spectrometers for photons and neutrons. System stability and ability to reach high throughput count rates are key challenges for several applications (e.g., neutron spectrometry for nuclear fusion devices), but standard analog electronics limits the operation of liquid scintillation neutron spectrometers to low count rates ( ~ 3 ldr 10⁴ s^-1) . The count rate capabilities of a liquid scintillation neutron spectrometer (NE213 detector) from the Physikalisch-Technische Bundesanstalt (PTB) has been extended up to ~ 4.2 ldr 10⁵ s^-1, by coupling it to a digital acquisition system developed at ENEA-Frascati. Measurements have been carried out at PTB using gamma sources and accelerator-produced 2.5 MeV and 14 MeV neutrons. For 14 MeV neutron measurements, digital pulse height spectra (PHS) obtained at high count rates have been compared to PHS recorded with standard analog electronics. The results show that stable PHS (within 1%) can be obtained at high count rate despite the high sensitivity of the gain of photomultiplier tubes to count rate variations.