Senada Avdic

Measurement of the neutron current and its use for the localisation of a neutron source

Abstract It was suggested recently [Pazsit, Ann. Nucl. Energy 24 (1997) 1257] that in addition to the traditionally measured scalar neutron flux, the information contained in the higher angular moments of the neutron flux, such as the neutron current or the flux gradient, could be used for monitoring and diagnostics of reactor cores. To explore this possibility, a detector was constructed with which vector components of the partial currents, and thus also components of the net current vector, can be measured. The two-dimensional current vector was measured in an experimental system containing a neutron source in a water tank, and was compared with calculated values. As one simple application, it was shown how the position of the source can be determined from measuring the scalar flux and the neutron current in one single spatial point. The experiments demonstrate both the suitability of the detector construction for the measurement of the neutron current vector and the use of the current in diagnostics and monitoring.

The effects of shielding around the source on neutron energy spectra

The knowledge of neutron energy spectra contributes to unambiguous identification of neutron sources in the fields of nuclear safeguards and nuclear non-proliferation. Since a real scenario situation includes the presence of shielding around the source, we have investigated the influence of the potential shielding surrounding the source on the shape of energy spectra for a few neutron sources. We have applied the maximum-likelihood, expectation–maximisation (MLEM) method with one-step-late (OSL) algorithm for neutron spectra unfolding. The pulse height distributions used in the unfolding procedures were simulated with the high accuracy by using the MCNP-PoliMi code based on the Monte Carlo method. A possibility to identify the shielded neutron sources by using the unfolding method was examined with two continuous-in-energy sources, such as 252Cf and 241Am–Be in source-shielding configurations with lead (Pb) and polyethylene (PE) blocks. The results of calculations have shown that the identification of 252Cf and 241Am–Be sources with 2.5 cm of Pb and PE shield can be achieved successfully by using the MLEM method with the OSL algorithm. However, the unfolded results for 252Cf and 241Am–Be sources with 10 cm of PE shield significantly deviate from the reference spectra and the sources cannot be correctly identified on the basis of their unfolded energy spectra.

Identification of Neutron Sources by Spectral Analysis of Pulse Height Distributions

This paper proposes a neutron source identification method based on the spectral analysis of neutron pulse height distributions obtained with liquid scintillation detectors. The fact that shielded and unshielded neutron sources have clearly defined spectral components with specific locations and intensities offers the possibility of identifying the sources based on spectral features alone, without having to unfold the energy spectra. Analysis of simulated and experimental data confirms that this new identification method is promising, and that good resolution power can be achieved.

Transmission measurement of porosity with a neutron generator in geophysics applications

Abstract Fast neutron transmission measurements, based on a stationary neutron generator, were performed to determine the volume porosity of a sample in a model experiment. The free volume in the mock-up, consisting of a cylindrical holder containing glass rods and sometimes even sand, was determined from measurements of the transmitted fast neutron flux in dry and flooded (with water) condition. In order to extract a quantitative measure of the free volume from the measurement, the neutron attenuation through the sample was modelled by the first flight approximation as well as with more advanced MCNP calculations. The experimental results show good agreement with results of the advanced Monte-Carlo calculations within the estimated error. Porosities of samples that were evaluated from the measurement using MCNP calculations for calibration show an error between 3 and 14% in comparison with the true (known) values.

Classification of Two-Phase Flow Regimes via Image Analysis by a Neuro-Wavelet Approach

A non-intrusive method of two-phase flow identification is investigated in this paper. It is based on image processing of data obtained from dynamic neutron radiography recordings. Classification of the flow regime types is performed by an artificial neural network (ANN) algorithm. The input data to the ANN are some statistical functions (mean and variance) of the wavelet transform coefficients of the pixel intensity data. The investigations show that bubbly and annular flows can be identified with a high confidence, but slug and churn-turbulent flows are more often mixed up in between themselves.

Joint probability of density functions of energy-lifetime distributions for neutrons and gamma-rays from 252Cf source

In contrast to some traditional neutron detectors, liquid scintillators are suitable for measuring spontaneous fission produced on the nanosecond time-scales. The high multiplicity events in a short period of time are signatures for fissile materials. The neutron and gamma multiplicities have potential to enable extraction of the fissile material attributes. The simulations presented in this paper were done using the MCNPX–PoliMi radiation transport code based on the Monte Carlo method. It was demonstrated that the energy and time information depending on the number of neutrons and gamma-rays emitted in spontaneous fission events of 252Cf contribute additionally to characterize a fission source.