Detection Of Fission Neutrons Research Articles

Matrix effect correction in active neutron interrogation for residual fissile mass assessment in radioactive waste drums

Metallic waste in the form of shells and nozzles remains following the reprocessing of spent fuel at La Hague plant (France). Before the final disposal, drums containing the waste are transported to the compaction facility where its volume is reduced by a factor of 5. With the objective of controlling the criticality/safety levels, an active neutron interrogation system at the entrance of the facility is used to assess the residual fissile mass remaining in the waste, which relies on prompt fission neutron detection (known as the differential die-away technique, DDA). The measured signal is proportionally linked to the fissile mass by a calibration coefficient. However, two effects can produce an inaccurate prediction. The number of induced fissions for the same fissile mass varies according to the waste matrix composition, particularly the neutron slow-down and absorption ratio. Secondly, the presence of fissile clusters can impact the neutron interrogation due to an increase in self-shielding effect. This work presents a matrix effect correction method based on the use of internal flux monitors (sensitive to the neutron absorption ratio) and the transmission signal (sensitive to the slow-down ratio). It relies on a numerical model of the whole measurement cell developed with the Monte-Carlo N-Particle transport (MCNP) code. The calibration coefficient of 72 different matrices, representative of the waste produced at La Hague, were simulated and a statistical model using a multilinear regression was then established. This simulated-based approach provides a more robust and comprehensive estimation of the residual fissile mass without impacting accuracy thanks to the flux monitors and the transmission signals.

Assaying of SNM using Simultaneous Detection of Fission Neutrons and Gammas by Employing a Novel Phoswich Detector

For the precise measurements of special nuclear materials (SNM) including Pu and Am isotopes, we have used phoswich detector combination of two single crystal scintillators of Gd3Ga3Al2O12:Ce and CsI:Tl. High detection efficiency and sensitivity along with high figure of merit for the discrimination of these phoswich detectors ensures the detection and discrimination of thermal neutrons and gammas from spontaneous fission of Pu and other isotopes in presence of high gamma background. Using this detector, the low energy gammas, which is stopped completely in 1mm thick disc of GGAG, can be also discriminated from high energies gamma and shows linearity in wide range of sample quantities. By changing only the appropriate shielding, the similar setup was used for thermal neutron detection and shows a very good linearity over wide range. The quantity of a test sample was also calculated accurately by using the measured calibrated plot.

Multilayer phoswich scintillators for neutron / gamma discrimination

MultiLayer Phoswich (MLP) scintillators have been studied for fission neutron detection by Monte-Carlo simulations and experimental tests. n/γ discrimination behavior of these systems and their angular dependencies have been characterized. The possibility to carry on a neutron counting system based of such an approach has been proven, and according to the results, an optimal thickness of layers for fission neutron counting was found around [100–150] μm. But, contrary to current liquid or solid-state discrimination scintillators, it appears that MLP-based neutron counting systems cannot directly classify neutron and gamma-ray interaction events from pulse shape analysis. To operate properly, it is therefore required to implement a compensation algorithm in the system readout.

Neutron Slowing Down Time Based Inspection Method

The neutron slowing down time based inspection system extends the well-established differential die-away analysis (DDAA) to higher neutron energies and earlier fission neutron detection times. Simulation tools, mainly MCNPX, validated previously by experiments are used to further extend the domain of DDAA, which relies solely on thermal neutron-induced fissions, into the high-sub-MeV neutrons. Detailed time–energy correlated behavior of slowing-down neutrons, following a narrow pulse of 14-MeV neutrons, is calculated for ideal moderators like polyethylene and beryllium as well as for generic hydrogenous and metallic cargo materials. The time evolution and energy spectra of neutrons escaping the cargo-containing special nuclear materials (SNMs) (such as 233U, 235U, 239Pu, and 241Pu) are shown to be very different and easily separable from the same quantities when an SNM is absent. The key to the technique is the “vanishing time”: the time when high-energy source neutrons are scattered down to energies below those of fission neutrons. If fissile material is present, high-energy neutrons (>1 MeV) reappear solely due to fissions induced by the slowed down source neutrons, with energy below 1 MeV. The fission neutrons, which escape cargo absorption, are then detected by a direct fast neutron detector. The detectors can be, in principle, organic or 4He recoil-based scintillators, which are inherently insensitive to neutrons with lower energy than the fission neutrons. In addition, a successful implementation of this SNM inspection technique requires an intense medium-to-high duty factor narrow-pulse fast neutron generator, e.g., (d, T) or (d, D), and fast two-parameter (energy and time) data digitizers.

Improved fission neutron energy discrimination with 4He detectors through pulse filtering

This paper presents experimental and computational techniques implemented for 4He gas scintillation detectors for induced fission neutron detection. Fission neutrons are produced when natural uranium samples are actively interrogated by 2.45MeV deuterium-deuterium fusion reaction neutrons. Fission neutrons of energies greater than 2.45MeV can be distinguished by their different scintillation pulse height spectra since 4He detectors retain incident fast neutron energy information. To enable the preferential detection of fast neutrons up to 10MeV and suppress low-energy event counts, the detector photomultiplier gain is lowered and trigger threshold is increased. Pile-up and other unreliable events due to the interrogating neutron flux and background radiation are filtered out prior to the evaluation of pulse height spectra. With these problem-specific calibrations and data processing, the 4He detector's accuracy at discriminating fission neutrons up to 10MeV is improved and verified with 252Cf spontaneous fission neutrons. Given the 4He detector's ability to differentiate fast neutron sources, this proof-of-concept active-interrogation measurement demonstrates the potential of special nuclear materials detection using a 4He fast neutron detection system.

Isotopic Fissile Assay of Spent Fuel in a Lead Slowing-Down Spectrometer System

Abstract A lead slowing-down spectrometer (LSDS) system is under development to analyze isotopic fissile content that is applicable to spent fuel and recycled material. The source neutron mechanism for efficient and effective generation was also determined. The source neutron interacts with a lead medium and produces continuous neutron energy, and this energy generates dominant fission at each fissile, below the unresolved resonance region. From the relationship between the induced fissile fission and the fast fission neutron detection, a mathematical assay model for an isotopic fissile material was set up. The assay model can be expanded for all fissile materials. The correction factor for self-shielding was defined in the fuel assay area. The corrected fission signature provides well-defined fission properties with an increase in the fissile content. The assay procedure was also established. The assay energy range is very important to take into account the prominent fission structure of each fissile material. Fission detection occurred according to the change of the Pu239 weight percent (wt%), but the content of U235 and Pu241 was fixed at 1 wt%. The assay result was obtained with 2∼3% uncertainty for Pu239, depending on the amount of Pu239 in the fuel. The results show that LSDS is a very powerful technique to assay the isotopic fissile content in spent fuel and recycled materials for the reuse of fissile materials. Additionally, a LSDS is applicable during the optimum design of spent fuel storage facilities and their management. The isotopic fissile content assay will increase the transparency and credibility of spent fuel storage.

Verification of threshold activation detection (TAD) technique in prompt fission neutron detection using scintillators containing 19F

In the present study ⌀ 5′′× 3′′ and ⌀ 2′′× 2′′ EJ-313 liquid fluorocarbon as well as ⌀ 2′′ × 3′′ BaF2 scintillators were exposed to neutrons from a 252Cf neutron source and a Sodern Genie 16GT deuterium-tritium (D+T) neutron generator. The scintillators responses to β− particles with maximum endpoint energy of 10.4 MeV from the n+19F reactions were studied. Response of a ⌀ 5′′ × 3′′ BC-408 plastic scintillator was also studied as a reference. The β− particles are the products of interaction of fast neutrons with 19F which is a component of the EJ-313 and BaF2 scintillators. The method of fast neutron detection via fluorine activation is already known as Threshold Activation Detection (TAD) and was proposed for photofission prompt neutron detection from fissionable and Special Nuclear Materials (SNM) in the field of Homeland Security and Border Monitoring. Measurements of the number of counts between 6.0 and 10.5 MeV with a 252Cf source showed that the relative neutron detection efficiency ratio, defined as ϵBaF2 / ϵEJ−313−5′′, is 32.0% ± 2.3% and 44.6% ± 3.4% for front-on and side-on orientation of the BaF2, respectively. Moreover, the ⌀ 5′′ EJ-313 and side-on oriented BaF2 were also exposed to neutrons from the D+T neutron generator, and the relative efficiency ϵBaF2 / ϵEJ−313−5′′ was estimated to be 39.3%. Measurements of prompt photofission neutrons with the BaF2 detector by means of data acquisition after irradiation (out-of-beam) of nuclear material and between the beam pulses (beam-off) techniques were also conducted on the 9 MeV LINAC of the SAPHIR facility.

A new approach to prompt fission neutron TOF data treatment

Abstract The prompt neutron emission in spontaneous fission of 252Cf has been investigated applying digital signal electronics along with associated digital signal processing algorithms. A new mathematical approach, applicable to single events, was developed for prompt fission neutron (PFN) time-offlight distribution unfolding. The main goal was to understand the reasons of the long existing discrepancy between theoretical calculations and the measurements of prompt fission neutron (PFN) emission dependence on the total kinetic energy (TKE) of the fission fragments (FF). Since the 252 Cf (sf) reaction is one of the main references for nuclear data the understanding of the PFN emission mechanism is very important both for nuclear fission theory and nuclear data. The experimental data were taken with a twin Frisch-grid ionization chamber and a NE213-equivalent neutron detector in an experimental setup similar to the well known work of C. Budtz-Jorgensen and H.-H. Knitter. About 2.5 × 10 5 coincidences between fission fragment (FF) and neutron detector response to prompt fission neutron detection have been registered ( ∼ 1.6 × 10 7 of total recorded fission events). Fission fragment kinetic energy, mass and angular distribution, neutron time-of-flight and pulse shape have been investigated using a 12-bit waveform digitizer. The signal waveforms have been analyzed using digital signal processing algorithms. The main goal of this work was a detailed description of the prompt fission neutron treatment.

Tensioned Metastable Fluid Detectors in Nuclear Security for Active Interrogation of Special Nuclear Materials―Part B

This paper (constituting Part B) addresses active interrogation for detecting Special Nuclear Materials (SN- Ms) and includes description of the transformational Tensioned Metastable Fluid Detector (TMFD) based method for optimal monitoring. One of the greatest difficulties in detection of SNMs by active interrogation is the task of distinguishing between the probing particles and the secondary particles that indicate the presence of SNMs. The TMFD’s selective insensitivity and γ photon blindness features are advantageous for alleviating this problem. The working principle of the TMFD is discussed along with its applications for security. The experimental work to date involving detection of small quantities of uranium with conventional detectors is discussed along with results of fission neutron detection. Statistically significant detection was achieved within 5 minutes of counting to ascertain and measure conclusive evidence for the presence of a 55g sample of uranium containing 235U. Results of simulations of three active detection techniques utilizing a TMFD system are presented. The process for using the TMFD to discriminate active source particles using timing and energy are described. These simulations indicate that it should be possible to utilize the TMFD system for optimal neutron-based interrogation of SNMs.

A comparison of performance between organic scintillation crystals and moderated 3He-based detectors for fission neutron detection

Direct detection of fast neutrons using organic scintillators is one alternative to moderated thermal neutron detectors deployed to detect fission neutrons - a relevant question in light of dwindling {sup 3}He supplies. Recent developments in materials science have demonstrated the capability to grow larger crystals in reasonable times. In light of these developments, this study compares the relative performance of a {sup 3}He-based neutron module from a commercially available portal monitor with a theoretical organic scintillator of similar overall size. Stilbene serves as a benchmark with its performance estimated from a combination of energy deposition modeled by radiation transport calculations and an assumption of the lowest neutron energy at which pulse shape discrimination can effectively separate neutron and gamma-ray events. Before intrinsic detection efficiencies on par with moderated detector systems can be achieved, the results point to the need for further advances including significant increases in detector size, especially thickness, and/or lower pulse shape discrimination thresholds.

Optimization of a neutron detector for pulsed photonuclear monitoring of fissile materials

The objective of the present work is optimization of the geometry of a multilayer detector used in the pulsed photonuclear method of remote monitoring of fissile materials. Numerical simulation of the conversion of an electron beam into bremsstrahlung and the interaction of the latter radiation with components of the facility and the contents of the object of monitoring, resulting in neutron emission, is performed. The spectral-time parameters of the neutron flux arriving at the detector are obtained and the probability of detecting neutrons with different energy is calculated as a function of the thickness of the layers for moderation and absorption. The construction of the detector giving high fission-neutron detection efficiency and low sensitivity to background neutrons is chosen. The data obtained were used to build the neutron detector used in the experimental model of a facility for detecting fuel-cycle materials by the photonuclear method based on the U-28 accelerator.

Low-background detection of fission neutrons produced by pulsed neutron interrogation

Measurements designed to detect shielded Special Nuclear Materials (SNM) have been carried out using a pulsed 8.5-MeV neutron source. Fission-neutron counts were detected as a function of time in the intervals between 100-μs neutron bursts at burst frequencies of 500, 1000, and 2000 Hz. The pulse timing sequences were chosen to optimize detection of fission neutrons produced by thermal-neutron-induced fission in the SNM. Fission neutrons were detected directly as proton, carbon, and silicon recoils in silicon carbide (SiC) semiconductor fast neutron detectors. SiC detectors recorded neutron counts during and immediately following the source neutron bursts, allowing detection of fission neutrons with short (120 μs) die-away times. The SiC detectors demonstrated excellent background discrimination with more than 2000 neutron counts observed in time intervals where zero background counts were detected.

Nondestructive fissile material assay by induced fission neutron correlation

An NDA method and interpretation model for the determination of small fissile material masses, in radioactive waste, is elaborated. The method uses a pulsed neutron source to interrogate a waste item located in the sample cavity. The sample cavity is lined with graphite to yield a long thermal neutron lifetime. For each pulse of source neutrons, the slowed down neutron population causes thermal fission in the fissile material of the sample, resulting in the emission of fast neutrons from the sample. Some of these fast neutrons escape the cavity and the cavity liner, and are slowed down in an external polyethylene moderator, designed to yield a short thermal neutron lifetime, and are subsequently detected in incorporated thermal neutron detectors. Signal groups, representing the detection of fission neutrons, are recorded in several observation intervals after the pulse of source neutrons has died away in the polyethylene moderator. Following many pulses of source neutrons, the frequency of such signal groups is accumulated. The first, second, and third factorial moment of such frequency distributions are the estimated value of signal singlets, doublets and triplets, respectively. These multiplets are expressed as function of the fissile mass, the neutron detection probability, instrumental and nuclear parameters, and a stationary neutron signal background, thus permitting an absolute determination of the fissile mass of the sample.

A neutron sensor based on single crystal CVD diamond

We report the first neutron data for a single crystal Chemical Vapor Deposition diamond sensor. Results are presented for 2.5, 14.1, and 14.9 MeV incident neutrons. We show that the energy resolution for 14.1 MeV neutrons is at least 2.9% (as limited by the energy spread of the incident neutrons), and perhaps as good as 0.4% (as extrapolated from high resolution α-particle data). This result could be relevant to fusion neutron spectroscopy at machines like the International Thermonuclear Experimental Reactor. We also show that our sensor has a high neutron linear attenuation coefficient, due to the high atomic density of diamond, and this could lead to applications in fission neutron detection.

An insight into neutron detection from polycrystalline CVD diamond films

Radiation detectors fabricated from chemical vapour deposited (CVD) diamond have now been fabricated addressing several photon and particle detectors in the frame of specific industrial applications. Regarding the nuclear industry needs, diamond-based detectors have been fabricated and this paper presents the various techniques that can be of interest for the detection of fission neutrons. The ability of diamond to withstand the high irradiation fluxes as encountered in nuclear reactors is one of the motivations. Of the few techniques that are presented here, one approach relies on the use of converting materials, i.e. elements on which a nuclear reaction gives rise to the creation of charge particles that could be detected. Boron and particularly 10B has been used successfully. Other techniques are also presented including detection in the current mode from the elastic scattering of fission neutrons in diamond, as well as an approach where the boron material is inserted directly inside the material in the form of a dopant. From the material irradiation in neutron fluxes from a nuclear reactor, the modification of the material property was characterised, and particularly the modification due to the progressive effect of transmutation of 10B dopant elements. This particular feature may enable the fabrication of passive (off-line detection devices). The approaches will be discussed in terms of the advantages and drawbacks of the techniques.

Measurement of the 239Pu( n, f) cross section from thermal up to 30 keV neutron energy

At GELINA measurements of the 239Pu fission cross-section were performed covering the neutron energy region from thermal up to 30 keV. Fission fragment as well as fission neutron detection techniques were used. Also for the neutron flux determination different methods were applied. From the σ f -data, several fission integrals were calculated and compared with other results.

The pulse separation spectrum for the detection of neutrons from a mixture of fissions and single-neutron events

A study is made of the effect of the presence of some correlated neutron detections from fissions on the pulse separation spectrum of the detections from a source of single neutrons. A model is created for the detections from a fission and the exact form of the first-order perturbation of the spectrum for low fission rate ϱ f is found for this model. Comparison of the model with the actual conditions of fission neutron detection show that it can represent the latter accurately to the second order in N, form of the basic perturbation term only, which is to the first order in ϱ f and the second order in N. A brief discussion of the possible use of this perturbation to measure ϱ f is also given.

A scintillation counter with neutron and gamma-ray discriminators

Certain organic scintillators, notably anthracene, stilbene and quaterphenyl crystals and oxygen-free liquid scintillators, show an effectively longer scintillation decay time for heavily ionizing particles such as alpha-particles or protons than for electrons. A scintillation counter is described which distinguishes fast neutrons from gamma-rays by means of the different decay times of recoil proton and Compton electron scintillations respectively. Measurements of the proton-electron resolution for different scintillators are described. It is found, for example, that using a one inch thick stilbene crystal, 2 MeV neutrons may be detected with 9.5% efficiency while the detection efficiency for 2 MeV gamma-rays is reduced by decay time discrimination to less than 0.007%. An application of the counter for fission cross section measurements by fission neutron detection is described. The decay time properties of the scintillators are discussed and the longer decay time for protons (or alpha-particles) is interpreted in terms of slow (10 −7 sec) ion recombination processes in the scintillators; these processes produce a slow component in the scintillation decay and in a proton scintillation the proportion of slow component is more than in an electron scintillation 4).

The artificial disintegration of light elements

Certain organic scintillators, notably anthracene, stilbene and quaterphenyl crystals and oxygen-free liquid scintillators, show an effectively longer scintillation decay time for heavily ionizing particles such as alpha-particles or protons than for electrons. A scintillation counter is described which distinguishes fast neutrons from gamma-rays by means of the different decay times of recoil proton and Compton electron scintillations respectively. Measurements of the proton-electron resolution for different scintillators are described. It is found, for example, that using a one inch thick stilbene crystal, 2 MeV neutrons may be detected with 9.5% efficiency while the detection efficiency for 2 MeV gamma-rays is reduced by decay time discrimination to less than 0.007%. An application of the counter for fission cross section measurements by fission neutron detection is described. The decay time properties of the scintillators are discussed and the longer decay time for protons (or alpha-particles) is interpreted in terms of slow (10−7 sec) ion recombination processes in the scintillators; these processes produce a slow component in the scintillation decay and in a proton scintillation the proportion of slow component is more than in an electron scintillation4).