Neutron Counting in Mixed Neutron-Gamma Fields with Common NaI(Tl) Detectors

Iodine-comprising scintillators as NaI(Tl) are capable of detecting thermal neutrons with surprising efficiency, even in the presence of ambient gamma radiation. This is due to a peculiarity of the 128 I level scheme. De-excitation cascades following neutron captures in 127 I, a constituent of NaI w...

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Bibliographic Details
Main Authors: Pausch, Guntram, Kreuels, Achim, Scherwinski, Falko, Kong, Yong, Kuster, Mathias, Lentering, Ralf, Wolf, Andreas, Stein, Juergen R.
Format: Conference Proceeding
Language:English
Subjects:
Crystals
Delay effects
Detectors
Fluctuations
Image resolution
Neutrons
Scintillators
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Summary:Iodine-comprising scintillators as NaI(Tl) are capable of detecting thermal neutrons with surprising efficiency, even in the presence of ambient gamma radiation. This is due to a peculiarity of the 128 I level scheme. De-excitation cascades following neutron captures in 127 I, a constituent of NaI with quite large neutron capture cross-section, often involve the 167.4 keV state of 128 I with 175 ns half-life, which feeds another isomeric level, the 137.9 keV state with 845 ns half-life. Strong transitions involving these levels are highly converted, they mostly generate signals in the crystal. A sequence of three pulses within a few microseconds, the second and third one corresponding to energy depositions around 30 keV and 138 keV, respectively, is a distinct feature of such a de-excitation cascade and a clear signal for a neutron capture in 127 I. The paper demonstrates thermal-neutron detection with a common ∅2"×2" NaI(Tl) scintillation detector by identifying such triple-pulse sequences in the digitized detector signals. Energy depositions and time delays of the (often piled-up) sub-pulses with respect to the event trigger reflecting a prompt part of the cascade are determined by consecutive stripping of the signal components. Random triple events, caused by ambient gamma radiation or statistical fluctuations exceeding the trigger threshold, are strongly suppressed by energy and time cuts defining valid delayed contributions. Another measurement using a 6 Li co-doped ∅2"×2" NaI(Tl) (NAIL™) scintillator allowed estimating the neutron detection efficiency attained with this novel technique.
ISSN:2577-0829
DOI:10.1109/NSS/MIC44867.2021.9875847