Silicon carbide semiconductor material (SiC) is widely used in power electronics and in recent years, it has also been utilized as an alternative to $^3$He-based neutron gas detectors. SiC-based detectors are highly radiation-resistant, allow excellent dis- crimination between neutrons and gamma rays and offer an excellent signal-to-noise ratio. Neutron detection in semiconductor detectors involves the use of a material that interacts with neutrons, producing charged particles that can be detected in a semiconductor diode. This material is called a converter. The choice of converter depends on the energy spectrum of the neutrons of interest. For the detection of thermal neutrons, converters containing $^{6}$Li or $^{10}$B are used almost exclusively. This master’s thesis focuses on the investigation of converters for fast neutrons, consid- ering isotopes such as $^{40}$Ca, $^{39}$K, and $^{35}$Cl, which could enable sensitivity in the neutron energy range around 1 MeV, and the radioactive isotope $^{22}$Na.
In the master’s thesis, a series of experimental measurements of the SiC detec- tor response with different configurations of KCl converters for fast neutrons were conducted at the TRIGA reactor at the Jožef Stefan Institute. However, the ob- tained experimental results did not confirm the contribution of the converter to the detection of fast neutrons, mainly due to the intrinsic response of the detector.
The detector response was investigated computationally using particle transport simulations based on the Monte Carlo method with the MCNP program. Two approaches were used in the simulations with respect to the reaction rates of the selected isotopes, the direct approach and the two-step approach. The first step of the two-step approach was to define the neutron source and calculate the source of secondary charged particles resulting from neutron interactions in the converter. The second step was to define the source of secondary particles and calculate the detector response, i.e., the pulse spectrum due to the incident secondary particles.
Computational analysis showed an extremely high response for the $^{22}$Na con- verter at energies below 0.5 MeV and an order of magnitude higher response than for the KCl and CaCl$_2$ converters. The energy spectra of KCl and CaCl$_2$ are very similar, as Cl is part of both materials and the isotopes $^{40}$Ca and $^{39}$K have similar threshold microscopic cross sections for the (n,p) reactions. We compared compu- tational and experimental results for the pulse spectrum in the diode and found few similarities. The calculated average energy of the repulsive nuclei is 0.77 MeV.
We found that the intrinsic response of the detector is at least an order of magni- tude higher than the expected response with a fast neutron converter, which explains the experimental results. For further research on fast neutron converters, we con- ceptually designed a configuration of a heavily shielded detector and converter and computationally determined its shielding performance.
|
