Sealed Tube Neutron Generator Research Articles

Characterisation of neutron fields at the n-lab, a fast neutron facility at the University of Cape Town

The n-lab is a fast neutron facility based in the Department of Physics, University of Cape Town, offering collimated neutron beams produced by an MP 320 deuterium–tritium sealed tube neutron generator, and a 220 GBq americium–beryllium radioisotopic source. Characterisations of the spatial and energy distributions of the fast neutron beams were performed using an EJ-301 organic liquid scintillator detector and digital data acquisition system. Neutron energy spectra were obtained through unfolding analyses with MAXED, and a Monte Carlo approach to the propagation of uncertainties was implemented. Measurements of fluence and neutron emission rates were determined through the neutron activation of copper foils and subsequently used to validate the scaling of the unfolded neutron energy spectra.

Evaluation of the scintillation screens used for low energy pulsed ion beam profiling

For a compact pulsed sealed-tube neutron generator using vacuum arc as ion source, an ion beams diagnostic system based on scintillation screen was established. And the transverse intensity distribution of the deuterium ions on the target of the pulsed neutron generator was obtained. In order to validate the accuracy of the diagnostic system, the ion irradiation effect of the Al2O3, Quartz and YAP scintillation screens were studied. Steady pulsed deuterium ion beam with energy and intensity comparable to the pulsed ion beam used in the neutron generator was let continuously irradiated on the scintillation screens, and the light yield and spot size changes of the screens were recorded. Results showed that, YAP screens own the worst radiation hardness among the three. For Al2O3 and Quartz screens, though their light yield degraded with the ion radiation, the measured beam profile almost not changed for integrated beam intensity of up to 2.5 × 1014 ions/mm2. Therefore, the diagnostic system can provide accurate results for the ion beam diagnostic for neutron generators.

Note: Self-biased voltage to suppress secondary electrons by a ZnO varistor in a compact pulsed neutron generator.

A large amount of back streaming electrons will bring about a part of current drain on power supply, cause sparking or high-voltage breakdowns, and affect the neutron yield and waveform for a compact sealed-tube pulsed neutron generator. A novel idea which uses a ZnO varistor to provide a constant self-biased voltage to suppress the secondary electrons is introduced. The I-V curve for the ZnO varistor was measured in the experiment. The effects of suppressing the secondary electrons were investigated using a ZnO varistor, linear resistors, and an independent power supply, respectively. The results show that the secondary electrons are suppressed effectively by the compact ZnO varistor, while not increasing the size and the component of the device. It is a promising design for compact sealed-tube neutron generators.

Optimization of fast neutron flux in an irradiator assembly using Monte Carlo simulations

When carrying out experimental work related to neutron interactions with the matter, it is important to get an accurate knowledge about the angular distributions of neutron beam. Descriptions of the neutron source assembly as well as the irradiator design are discussed. A sealed tube neutron generator is used to produce 14 MeV neutrons through deuterium (2H) - tritium (3H) reactions. Monte Carlo simulations have been carried out for optimizing 14 MeV neutron flux distribution around a tritium target. Simulation was also tested by experimental measurements using a foil activation method. The discrepancy between two methods is due to sample preparations, irradiation facility and geometry.

Simulating transients in the subcritical reactor by using a sealed-tube neutron generator

Research on transients is one of the main focuses in the field of accelerator driven subcritical reactor (ADSR). But inducing the transients by accelerator is inconvenient and costly. Simulating transients by sealed-tube neutron generator, through improving the driver of Penning ion source and modulating trigger signals of pulsed neutron, is a fast, accurate, reliable and low-cost method. This method has been used in simulating transients on the physics test platform of ADSR in China Institute of Atomic Energy (CIAE). And this method also can be applied in PGNAA.

Ion Beam Profile Diagnostic Methods for Vacuum Arc Ion Source in Sealed-Tube Neutron Generator

The pulsed ion beam from a vacuum arc source in a compact sealed-tube neutron generator owns many special features, such as multiparticle mixture beams, high-intensity beams, short pulsewidth beams, transporting in a single gap acceleration system, and eccentric beams. Beam profile and composition are useful information for designing a high-intensity sealed-tube neutron generator, especially for the insulation withstand of the device, the life of the tritium target, the neutron yield, and so on. An improved traditional online diagnostic method that uses an aluminum-coated scintillator screen and a novel offline diagnostic method that is based on secondary ion mass spectrometry are introduced, respectively. The preliminary experimental results can show the intensity distribution of the ion beam profile and the composition with high resolution. The results from the two methods can fit each other well.

Development of a Time-tagged Neutron Source for SNM Detection

Associated particle imaging (API) is a powerful technique for special nuclear material (SNM) detection and characterization of fissile material configurations. A sealed-tube neutron generator has been under development by Lawrence Berkeley National Laboratory to reduce the beam spot size on the neutron production target to 1mm in diameter for a several-fold increase in directional resolution and simultaneously increases the maximum attainable neutron flux. A permanent magnet 2.45GHz microwave-driven ion source has been adopted in this time-tagged neutron source. This type of ion source provides a high plasma density that allows the use of a sub-millimeter aperture for the extraction of a sufficient ion beam current and lets us achieve a much reduced beam spot size on target without employing active focusing. The design of this API generator uses a custom-made radial high voltage insulator to minimize source to neutron production target distance and to provide for a simple ion source cooling arrangement. Preliminary experimental results showed that more than 100 μA of deuterium ions have been extracted, and the beam diameter on the neutron production target is around 1 mm.

Neutron generator burst timing measured using a pulse shape discrimination plastic scintillator with silicon photomultiplier readout

An EJ-299-34 plastic scintillator with siliconphotomultiplier (SiPM) readout was used to measure the fast neutron outputof a pulsed Thermo-Fisher A-325 Deuterium-Tritium sealed tube neutrongenerator (STNG). The SiPM signals were handled by a prototype digital pulseprocessing system, based on a free-running analogue to digital converterfeeding a digital signal processor (DSP). Pulse shape discrimination wasused to distinguish between detected fast-neutrons and gammas. Pulsedetection, timing, energy and shape were all processed by the DSP inreal-time.The time-dependency of the neutron output of the STNG was measured forvarious pulsing schemes. The switch-on characteristics of the tube stronglydepended on the operating settings, with the delay between pulse turn-on andthe production of neutrons ranging between 13 μs to 74 μs for thetested pulse rates and duty cycles. This work will facilitate theoptimization and modeling of apparatus that use the neutron generator'spulsing abilities.

Monitoring of concrete structures by using the 14 MeV tagged neutron beams

The maintenance and repair of reinforced concrete structures, especially those submerged in the sea-water require effective inspection and monitoring techniques for assessing the state of corrosion in the reinforcement material. An underwater inspection system was developed which is able to monitor the corrosion of the reinforcement. The system is composed of a remotely operating vehicle (ROV) equipped with the sealed tube neutron generator (NG). By rotating the NG and by using the associated alpha particle technique it is possible to measure the concrete cover thickness together with the reinforcing bar diameter. The possibility of estimating the carbon and chloride contents in the concrete was investigated. Iron plates of different thickness, covered by 6 cm thick concrete block, were successfully detected and the thickness of the concrete cover was estimated. In addition, reinforcing bar of one and 3 cm in diameter were identified and measured. All measurements could be performed without cleaning the concrete surface from fouling material.

Inspecting Minefields and Residual Explosives by Fast Neutron Activation Method

As an upgrade of a robotic mobile system for anti-personnel landmine clearance, a fast neutron probe has been considered for the detection of mines and explosive residues. Laboratory tests were made by using the 14 MeV $6\times 10^{7}$ neutrons/sec beams with the associated alpha particle detection and with a ${\rm LaBr}_{3}$ gamma-ray detector. Simulant of the anti-personnel mine was used as a target. Several measurements were made with the target buried into the soil at different depths. For each depth minimal time measurement was estimated for false negative 0.4% and false positive equal to 10%. Tests showed that it is possible to detect buried landmines as well as residual explosives; however, in order to reach the optimal speed of 10 cm/s of the demining vehicle it is necessarily to use several sealed tube neutron generators and few dozen of ${\rm LaBr}_{3}$ gamma-ray detectors.

The Underwater Detection of TNT Explosive

Tagged neutron inspection system was used for the underwater detection of explosives. Neutrons were produced in the d+t nuclear reaction with API 120 sealed tube neutron generator. Five kilograms of TNT explosive were measured in a pool filled with water. Measurements were done in target in, target out configuration. Also, a sandwich made of 6 mm thick iron plates with explosive placed between two layers of sea sediments was measured under the same conditions. Sediments were collected in the Adriatic Sea. Measurements were repeated with TNT explosive replaced by rocks of similar size and shape. Neutron tube was rotated in order to find the position of explosive (rocks) in the sandwich. It was found that TNT can be detected underwater and that is possible to distinguish TNT from the sea rocks placed between 6 mm thick iron plates or between layers of sediments by their ratios of carbon and oxygen.

Radiation Fields in the Vicinity of Compact Accelerator Neutron Generators

Intense pulsed radiation fields emitted from sealed tube neutron generators provide a challenge for modern health physics survey instrumentation. The spectral sensitivity of these survey instruments requires calibration under realistic field conditions while the pulsed emission characteristics of neutron generators can vary from conditions of steady-state operation. As a general guide for assessing radiological conditions around neutron generators, experiments and modeling simulations have been performed to assess radiation fields near DD and DT neutron generators. The presence of other materials and material configurations can also have important effects on the radiation dose fields around compact accelerator neutron generators.

Simple Penning ion source for laboratory research and development applications

A simple Penning ion generator (PIG) that can be easily fabricated with simple machining skills and standard laboratory accessories is described. The PIG source uses an iron cathode body, samarium cobalt permanent magnet, stainless steel anode, and iron cathode faceplate to generate a plasma discharge that yields a continuous 1 mA beam of positively charged hydrogen ions at 1 mTorr of pressure. This operating condition requires 5.4 kV and 32.4 W of power. Operation with helium is similar to hydrogen. The ion source is being designed and investigated for use in a sealed-tube neutron generator; however, this ion source is thoroughly described so that it can be easily implemented by other researchers for other laboratory research and development applications.

Portable fast-neutron radiography with the nuclear materials identification system for fissile material transfers

The nuclear materials identification system (NMIS) has historically been used with a 252Cf source for bulk interrogation of highly enriched uranium and plutonium. Recently, a portable associated-particle sealed-tube neutron generator (APSTNG) has been added as an interrogation source. This paper reports results from using the NMIS with the neutron generator to perform transmission radiography. In particular, the use of associated-particle techniques allows low-dose imaging to be performed simultaneously with traditional measurements, without physical collimation, and in high background environments.

Development of a field desorption ion source for neutron generator applications

Field desorption is a process in which external electric fields are used to promote the ionization and ejection of atoms chemisorbed on the outer surface of an electrode. This process occurs in the presence of a high electric field (10–40V/nm) and can be used to desorb hydrogen or its isotopes from a metal substrate, producing 100% atomic ions in the high field region surrounding the emitting surface. Electrostatic modeling and simulations have been performed to study design parameters for nanoscale metal desorption tips intended to be used in a high density emitter arrays as a field desorption ion source within a sealed tube neutron generator. The radius of the emission tip and the proximity of the emitting cone to the edge of the gate hole are of particular importance; for maximal desorption region fields the optimal position of the emission tip is within the plane of the gate film. An important design challenge is to ensure fields at the tip are sufficient for desorption while ensuring fields at the gate are not large enough for electron emission.

Progress in tagged neutron beams for cargo inspections

The use of neutron beams produced via the D+T reaction and tagged by the associated particle technique has been recently applied to cargo container inspections. In the EURITRACK project, a portable sealed-tube neutron generator has been designed and built to deliver 14MeV neutron beams tagged by a matrix of 64 YAP:Ce alpha-particle detectors read by a multi-anode HAMAMATSU H8500 Photomultiplier Tube. The performances of this alpha-particle detector have been determined as a function of the count rate at the Rudjer Boskovic Institute, Zagreb (Croatia). Moreover, tests of the final detector operated inside the sealed-tube neutron generator are fully satisfactory.

EURITRACK tagged neutron inspection system design

The EURITRACK project aims at developing a non-destructive measurement system, using an associated particle sealed tube neutron generator, to detect explosives or other threat materials concealed in cargo containers. Chemical composition of the suspect item is determined by coincidence measurements between alpha particles and photons resulting from neutron interactions in the inspected voxel of the container. We present the design and the performances of the measurement system obtained by Monte Carlo calculations. Selected gamma detectors are clusters of 5’’×5’’×10’’ and 5’’×5’’ sodium iodide scintillators, and a block of 100 kg of TNT located in a container filled with a metallic matrix having a density of 0.2 g/cm3 is shown to be detectable in 10 minutes.

An alpha particle detector for a portable neutron generator for the Nuclear Materials Identification System (NMIS)

A recoil alpha particle detector has been developed for use in a portable neutron generator. The associated particle sealed tube neutron generator (APSTNG) will be used as an interrogation source for the Nuclear Materials Identification System (NMIS). With the coincident emission of 14.1MeV neutrons and 3.5MeV alpha particles produced by the D–T reaction, alpha detection determines the time and direction of the neutrons of interest for subsequent use as an active nuclear materials interrogation source. The alpha particle detector uses a ZnO(Ga) scintillator coating applied to a fiber optic face plate. Gallium-doped zinc oxide is a fast (<1ns), inorganic scintillator with a high melting point (1975°C). One detector has been installed in an APSTNG and is currently being tested. Initial results include a measured efficiency for 3.5MeV alphas of 90%.

Development of a thermal neutron sensor for Humanitarian Demining

A thermal neutron sensor prototype for Humanitarian Demining has been developed, trying to minimize cost and complexity of the system as required in such application. A 252Cf source or a sealed-tube neutron generator is employed to produce primary fast neutrons that are thermalized in a moderator designed to optimize the neutron capture reaction yield in buried samples. A description of the sensor, including the performances of the acquisition system based on a Flash ADC card and final tests with explosive simulants are reported. A comparison of the sensor performance when using a radioactive source to that when employing a sealed-tube neutron generator is presented. Limitations and possible applications of this technique are discussed.

Neutron spectral distribution from a sealed-tube neutron generator by multiple-foil activation unfolding method

Abstract The spectral distribution of 14 MeV neutrons from a KAMAN sealed-tube neutron generator has been determined using threshold-energy monitor reactions in combination with the SULSA unfolding code, in which no input ‘guess spectrum’ is required except measured reaction rates of threshold monitor reactions. Result indicates that value of mean neutron energy determined in the present study compares well with the value obtained using the Nb–Zr ratio method.