A novel neutron detector for soil moisture estimation over large area

The main goal of my thesis work is to characterize a novel neutron detector for soil moisture estimation. Nowadays due to climate change and population increasing, the correct management of the water resources is extremely important. For this reason in agriculture is necessary to develop an instrument that are able to determine soil moisture over large area in order to optimize water consumption. Several techniques are proposed in the past from point-scale to remote sensing. In the last decade, a proximal geophysical method has been developed in order to fill the gap between point-scale and remote sensing approaches. The technique is called cosmic ray neutron sensing (CRNS) and it is based on the natural neutrons detected on the earth's surface that are mostly generated by cosmic rays, according to various processes. The fast neutron rate is anti correlated with the soil moisture content due to the great capacity of hydrogen present in water to thermalize fast neutrons. State-of-the-art probes used in CRNS are based on He3 proportional counter tubes. He3 is a nuclide produced almost entirely in artificial contexts as sub-product of the tritium decay, the current storage is depleting and the price is high and rising. In my thesis, a new solution is studied. I use a novel detector based on zinc- sulfide inorganic scintillator doped with lithium (Ej-426). The light generated by the scintillator is collected and send to the photo-multiplier tube (PMT) by a wavelength shifter (Ej-280). The readout is made of a 1 inch photo-multiplier from Hamamatsu, a fast digitizer DT5725 from CAEN, and low-cost, low-power, embedded computer BeagleBone black. In the first part of the work I focused my attention on discrimination algorithm in order to find the optimal filter that I can use to select correctly the event generated by neutrons and discard noise events. Then, I use this filter in a data acquisition, made in Casalserugo (-PD-) from August 2019 to December 2019. The results of this analysis show how the neutron counts is correlated with soil moisture content. After that, I study the response of the detector in function of the temperature by putting the scintillator and the photo-multiplier in a climate chamber. As I expect the neutron rate decrease when the temperature increase, this effects is due to the gain change of the PMT. The data acquired in the climate chamber are useful to correct the neutron rate in the field. Another important aspect of the CRNS is that the neutron rate must be correct by the incoming flux of cosmic ray, for this reason, near the neutron detector we put a plastic scintillator in order to detect cosmic muons and try to use the muon rate to correct the neutrons counts. In the future the system will be improved in order to determine also the biomass. The farmers can to carry out the appropriate treatments fertilization works and phytosanitary treatments if they have detailed information about crop growth.