Neutron pumping of active medium formed by gadolinium isotopes 155Gd and 156Gd pair: A feasibility study.

The possibility of creating technical means for controlling the processes of accumulation and conversion of the energies of thermal and epithermal neutrons into the energy of monoenergetic photons due to neutron pumping of an active medium consisting of nuclei with long-lived isomeric states was studied in this work. The system under study consisted of an external pulse-periodic source of deuterium-tritium neutrons (PSN) and a subcritical blanket, which included a variable neutron-collimation beam-shaping assembly (vBSA) and an active medium. The vBSA was composed of moderating blocks and selective plates designed to trap and shape a pulsed neutron flux with subsequent conversion of a millisecond signature into monoenergetic photon emission. Gadolinium oxide enriched in 155Gd isotope was used as the active medium, where the heavier one could be at different excited states, the de-excitations of which were accompanied by photon emission. In this research, the possibility of using the conjugate system (i.e., blanket - PSN - vBSA) for converting excess neutron energy accumulated in the inverse state of 156Gd nuclei into photon emission was demonstrated in detail.

Gamma-ray energy spectrum unfolding of plastic scintillators using artificial neural network.

In this study, the unfolding of the plastic scintillator spectrum was undertaken using the artificial neural networks tools of MATLAB. To this purpose, the response matrix of the plastic scintillator was generated for 145 energy groups and in 512 pulse-height channels using the MCNPX2.6 code. The results confirmed that the relative error in the gamma-ray energy unfolding with artificial neural networks is less than 3.8%.

Gamma-ray spectroscopy with anode pulses of NaI(Tl) detector using a low-cost digitizer system.

Recently, digital gamma-ray spectroscopy employing low-cost and publicly available (Commercial off the shelf) digitizers has been frequently used in different studies worldwide. In this paper, we considered the digital methods for gamma-ray spectroscopy in which the anode pulses of the photomultiplier tube (PMT) output in a NaI(Tl) scintillation detector were immediately digitized by a PC sound card. We introduced and developed the methods for gamma-ray spectroscopy of microCurie gamma-ray sources by a sampling rate of 96 kHz. First, at low count rates, the pulse arrival time was determined directly by the raw waveform, and the gamma-ray spectrum was obtained by summing the corresponding values in the samples per pulse. In addition, the gamma-ray spectrum was obtained by an enhanced sampling rate waveform and the pulse arrival time was determined by employing the digital constant fraction discrimination (DCFD) method, where each pulse area was achievable by summing the corresponding values of pulse samples. On the other hand, fitting the appropriate model function on the pulses and obtaining the fitted pulse area were undertaken for gamma-ray spectroscopy. To this end, a non-iterative algorithm to fast fit the Gaussian model function was improved. Moreover, the pile-up correction was performed at different count rates employing the Maximum Likelihood Estimation (MLE) method and Gaussian model function. Also, an approximate method for solving the high run time challenge was identified in the MLE method for long-time waveforms. To reject the pile-up events, a method was introduced based on the calculation of the full-width at half maximum pulses. By applying the proposed rejection method, we achieved an energy resolution of 6.2% at 663 keV gamma-rays and a count rate of 5.3 kcps.

Characterizing the coaxial HPGe detector using Monte Carlo simulations and evolutionary algorithms.

A standard procedure for characterizing the high-purity germanium detector (HPGe), manufactured by Canberra Industries Inc., is performed directly by the company using patented methods. However, the procedure is usually expensive and must be repeated because the characteristics of the HPGe crystal changes over time. In this work, the principles of a technique for use in obtaining and optimizing the detector characteristics based on a cost-effective procedure in a standard research laboratory were developed. The technique required the geometrical parameters of the detector to be determined as precisely as possible by the Monte Carlo method in parallel with the optimization process based on evolutionary algorithms. The development of this approach facilitated the modeling of the HPGe detector as a standardized procedure. The results would be also beneficial in the development of gamma spectrometers and/or their calibrations before routine measurements.

A multi-moderator neutron spectrometer for use in BNCT studies of the Tehran research reactor.

The Tehran Research Reactor is the only appropriate and available neutron source in Iran for clinical boron neutron capture therapy (BNCT). One of the requirements for BNCT is to carefully evaluate and measure the therapeutic neutron beam (epithermal neutrons) as well as the fast and thermal neutron components for successful treatment. In this research, a multi-moderator neutron spectrometer (MMNS) with LiI(Eu) scintillator as neutron counter was proposed for these measurements in the range of 10-11 eV to 15 MeV. The results confirmed promising precision of the designed MMNS for the epithermal spectrum; however, the angular dependency of the therapeutic beam due to any probable change in the beam-shaping assembly should be considered.

Neutron beam preparation for soil moisture measurements: Nedis-PHITS and artificial neural networks study.

The angular distribution of thermal neutrons scattered from the surface of a soil sample was used to determine soil moisture content. All simulations were performed assuming optimal dimensions of the designed collimator and AmO2-Be neutron source. Moreover, an assembly consisting of five Amersham X.l4 type capsular neutron sources was studied to obtain faster, more accurate data. The results showed promising agreement with previously published data and provided insights into potential applications for the construction of biological shielding of nuclear reactors.

Sparsity-based pulse-processor for digital α-particle spectroscopy with Si-PIN-Diode detector.

Nowadays, digital methods are used in nuclear detection instruments which are more popular than conventional signal processing. In this work, a digital α-particle spectrometer, compatible with the low-cost digitizers, was designed and constructed. Similar to the analog pulse-height analyses, the digital signals must also be carefully shaped. Here, the charge pulses induced by alpha particles were measured in a silicon PIN-diode detector. Then, a digital sparsity-based pulse-processor was introduced for shaping the charge signals that were digitized by a low sampling rate digitizer. To this end, a charge-sensitive pre-amplifier was designed and fabricated for coupling to the digitizers. To perform off-line analysis, the detector pulses were stored through a combination of a fast-digital oscilloscope and the LabVIEW software. Finally, the pulse-height spectrum was obtained by a digital multi-channel analyzer block. The performance of the designed filter was examined for Ra-226 alpha spectrum by comparing its output with those of other conventional filters. Our results show the advantages of the sparsity-based filter in high-resolution measurements. The designed filter achieved an energy resolution of 12.3% at 4.78 MeV alpha particles.

Angular distribution of scattered neutrons as a tool for soil moisture measurement: A feasibility study.

The conceptual design of a soil-moisture measurement instrument using a rectangular soil sample and an almost collimated 241Am-9Be source was proposed. Unlike previous studies and in a different simulation approach, the soil moisture was determined using the angular distribution of thermal neutrons using MCNPX2.6 Monte Carlo code, where a cylindrical BF3 proportional counter located at different polar angles was responsible for thermal neutron detection. Both Monte Carlo library least-squares method (MCLLS) and artificial neural networks (ANN) were used to calculate the soil moisture based on BF3 count rates with small relative error, about 2% and 10% maximum relative errors, respectively.

Nedis-Serpent simulation of a neutron source assembly with complex internal heterogeneous structure.

The hybrid use of Nedis-2m and Serpent 2.1.30 codes to predict the radiation characteristics (i.e., neutron yield and energy spectrum) of an Am-Be source with a fine-grained mixture of americium dioxide (AmO2) and beryllium (Be) core was studied with a focus on the grain size influence on the simulation results. The study showed that the fine-grained structure of the source core would decrease the number of alpha particles participating in the nuclear reactions with 17,18O and 9Be nuclei, which softened the neutron energy spectrum and reduced the neutron yield. The simulations also confirmed that the source core made of the stable crystals of AmBe13 intermetallic alloy would improve the neutron yield to maximum 50% compared to the core made of AmO2. Moreover, a source with a variable neutron yield was proposed with a heterogeneous core of AmO2 rods embedded in Be. The neutron energy spectrum of heterogeneous source resembled the energy spectrum of Deuterium-Tritium (D-T) neutrons which were generated in a long magnetic trap with high-temperature plasma. The subcritical irradiation facility assembled from the nth number of heterogeneous Am-Be source can be used to study the properties of materials and the equipment operating in the epithermal and fast neutron spectra. The use of a heterogeneous Am-Be assembly, as a basic element of an irradiation installation, simplifies the handling and operation procedures because it is easily disabled by removing the Be layer, or by inserting a sheet of the appropriate size and material between the Be and Am rod.