An innovative neutron spectrometer for soil moisture measurements

Abstract

Soil moisture is a crucial variable in evaluating soil properties and its interaction with the atmosphere, yet none of the techniques currently employed is fully adequate for evaluating the water content in soil over an area of hectares and depth of tens of centimeters. In recent times, it has been shown how the water content over this volume can be accurately assessed measuring changes in the epithermal flux of cosmic neutrons, which is extremely sensitive to the moderation caused by hydrogen. The instruments employed for neutron flux measurements are usually neutron counters, covered with moderator coatings for enhancing their sensitivity in the epithermal energy range. On the other hand, the worldwide shortage of $${}^{3}$$ 3 He caused a considerable increase in the costs associated with the manufacturing of proportional counters based on this gas, which were widely employed for their great sensitivity and noise rejection capability. In this work, we developed a $${}^{3}$$ 3 He-free neutron spectrometer for performing these measurements, which detects neutrons in the energy range from 0.01 ev to 1 GeV. The reconstruction of the energy spectrum allows a more accurate evaluation of the epithermal neutron flux and provides other information which improves the quality of soil moisture measurements. Irradiations performed with neutron sources of $${}^{241}$$ 241 Am and AmBe allowed to evaluate the spectrometric capability of the instrument, whereas the measurements of cosmic neutrons were employed to assess its sensitivity to cosmic radiation. The sensitivity of the instrument is slightly less than the one of the neutron counters currently employed, yet the access to the spectrometric information should provide greater accuracy in the epithermal flux measurements.

Calibration of a large-size wide-range neutron spectrometer

“…The cylinder was divided in 4 sectors (each constituted of two adjacent segments) called thermal, epithermal, fast, and high energy. Each sector was covered with moderator layers to maximize its sensitivity over the corresponding range of the energy spectrum [5]. The response functions of the sectors were calculated employing the MCNP6.2 code and are given as an input to an unfolding algorithm implemented in Python and based on the GRAVEL algorithm [6].…”

Testing a novel sensor design to jointly measure cosmic-ray neutrons, muons and gamma rays for non-invasive soil moisture estimation

“…Initially, all the CRNS detectors were based on proportional gas tubes filled in with helium-3 or boron trifluoride 65 (Schrön et al, 2018;Zreda et al, 2012). Alternative sensors are now emerging that could also open the path to new and wider applications (Cirillo et al, 2021;Flynn et al, 2021;Patrignani et al, 2021;Stevanato et al, 2019;Stowell et al, 2021;Weimar et al, 2020;van Amelrooij et al, 2022). In this context, the scintillator-based neutron detector design showed a good capability to measure neutrons with different energies (Cester et al, 2016).…”