DT Neutron Generator Research Articles

Measurement and analysis of activation induced in titanium with fusion peak neutrons

The intense neutron flux densities in fusion reactor blankets produce activation in the blanket materials relevant to operational safety, decommissioning, etc. The aim of the present work is to check the European Activation System EASY-2007 for its capability to predict important gamma activities induced in titanium in a fusion neutron field. Many advanced low-activation materials for fusion applications contain titanium, most notably in the breeder material Li2TiO3. In the present work, a small sample of Ti was irradiated with the intense DT neutron generator of Technical University of Dresden. The gamma-radioactivity following irradiation was measured and nuclide activities were derived. For each of the measured gamma activities, the corresponding value was calculated with EASY, and calculation-to-experiment ratios (C/E) were determined. EASY predicted the induced gamma activities, isotopes of scandium, well with some overestimation for 47Sc. The results of this measurement together with available EXFOR and validated state-of-the-art activation libraries are discussed.

Test facility for a neutron flux spectrometer system based on the foil activation technique for neutronics experiments with the ITER TBM

An important aim of neutronics Test Blanket Module experiments in ITER will be to check the prediction accuracy of nuclear responses in an environment more closely resembling a future fusion reactor than could be achieved so far with existing facilities. Quantities to be measured and checked are for example spectral neutron fluxes, tritium production rates, and nuclear heating. KIT, in cooperation with Technical University of Dresden (TUD), is developing a test measurement system with the goal to allow automated measurements of time profiles of spectral neutron fluxes by means of activation foil packages. The system is currently being installed at the Neutron Laboratory of TUD. In the present state, irradiation in the TBM is simulated with the intensive DT neutron generator of TUD, and a high-purity Ge-Detector is utilized for collecting the gamma-ray spectra of the activated foil packages. The test facility is flexible to explore also other types of gamma-ray detectors such as CdTe or CZT in future investigations which might be found better suited for the final system to be used with the TBM. First test measurements are underway with sets of activation foils which are based on well-established dosimetry reactions while extension to short-living radioisotopes is foreseen for future work.

Measurement and analysis of neutron flux spectra in a neutronics mock-up of the HCLL test blanket module

Fast neutron and gamma-ray flux spectra and time-of-arrival spectra of slow neutrons have been measured in a neutronics mock-up of the European Helium-Cooled Lithium–Lead Test Blanket Module with the aim to validate nuclear cross-section data. The mock-up was irradiated with fusion peak neutrons from the DT neutron generator of the Technical University of Dresden. A well characterized cylindrical NE-213 scintillator was inserted into two positions in the LiPb/EUROFER assembly. Pulse height spectra from neutrons and gamma-rays were recorded from the NE-213 output. The spectra were then unfolded with experimentally obtained response matrices of the NE-213 detector. Time-of-arrival spectra of slow neutrons were measured with a 3He counter placed in the mock-up, and the neutron generator was operated in pulsed mode. Monte Carlo calculations using the MCNP code and nuclear cross-section data from the JEFF-3.1.1 and FENDL-2.1 libraries were performed and the results are compared with the experimental results. A good agreement of measurement and calculation was found with some deviations in certain energy intervals.

Development of a Fast-Neutron Detector With Silicon Photomultiplier Readout

A robust neutron detector was developed to monitor the neutron flux in fast-neutron/gamma radiography (FNGR) cargo scanners. The main features of the new neutron detector were an ability to handle high count rate for statistical precision and a high damage threshold for longevity when used close to intense fast neutron sources (up to 1010 n/s). The detector comprises a fast scintillator coupled to a shielded 100 pixel Hamamatsu 1 mm times 1 mm silicon photomultiplier (SiPM) via a 220 cm length of wavelength shifting (WLS) plastic fibre. The WLS fibre was glued onto two long sides of a 10 times 10 times 50 mm scintillator wrapped in Tyvek reflector. Scintillation pulses of up to ~40 photons reached the SiPM. With the detector mounted 60 mm from a Thermo A-325 DT neutron generator (up to 6 times 107 n/s) the count rate was over 4 times 104 counts per second. The SiPM gain was effectively stabilised against temperature variations by automatically trimming the bias voltage with SiPM temperature. Comparison data is provided for a photodiode-scintillator neutron detector placed 134 cm from the A-325 neutron generator and for a PMT-WLS-scintillator neutron detector.

Novel neutron spectroscopy and neutron monitoring using position sensitive detector

Neutron spectrometry is carried out using various methods, whereas the Bonner sphere method is widely used. The wide range of neutron energy measurements need to use multiple moderator spheres enclosing a neutron counting detector. We introduce the development of a novel type of neutron energy spectrometer using a high efficiency, one-dimensional position sensitive detector (PSD) and a cone shaped single moderator structure. This device is portable and useful to record data simultaneously. Such operations for rarely occurring events are not possible with multisphere device. Two different moderator structures with symmetrical and non-symmetrical shapes of variable thicknesses over the sensitive length of the PSD are designed and will be optimized for the best energy resolution. The neutron response along the PSD position is recorded using the compact module for pulse processing and digital division with an FPGA. The PSD response as a function of moderator thickness and neutron energy is observed for various neutron sources. We present the design concept of the spectrometer and results of the measurements carried out at DT neutron generator and using an AmF source. Neutron transmission and moderation data is simulated using the GEANT4 code. The measured data and energy response function is proposed to be used for the measurement of the neutron energy spectrum.

Blanket mock-up experiment with a LiAlPb assembly irradiated with 14 MeV neutrons in preparation of the HCLL-TBM neutronics experiment

Irradiation experiments with a LiAlPb assembly have been performed using the DT neutron generator of Technical University of Dresden. The main goal was to qualify LiF thermoluminescence detectors (TLDs) for tritium breeding experiments and to test a recently improved NE-213 spectrometer, both for application in a neutronics mock-up experiment of the European HCLL-TBM at ENEA Frascati and Technical University of Dresden (TUD) planned for 2008. The LiF-TLD were set up inside the LiAlPb assembly and irradiated with the DT neutron generator of TUD in continuous mode. A good agreement with TPR calculated with MCNP was found. Fast neutron spectra inside the assembly have been obtained with the NE-213 detector. The flux of the thermal and epithermal neutrons inside the assembly, which is important for the tritium production rate from Li 6 , was measured with a He 3 counter and the DT neutron generator operated in pulsed mode.

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.

Analysis of a shield design for a DT neutron generator test facility

Independent numerical simulations have been performed using the MCNP5 and SCALE5 radiation transport codes to evaluate the effectiveness of a concrete facility designed to shield personnel from neutron radiation emitted from DT neutron generators. The analysis considered radiation source terms of 14.1 MeV monoenergetic neutrons located at three discrete locations within the two test vaults in the facility, calculating neutron and photon dose rates at 44 locations around the facility using both codes. In addition, dose rate contours were established throughout the facility using the MCNP5 mesh tally feature. Neutron dose rates calculated outside of the facility are predicted to be below 0.01 mrem/h at all locations when all neutron generator source terms are operating within the facility. Similarly, the neutron dose rate in one empty test vault when the adjacent test vault is being utilized is also less then 0.01 mrem/h. For most calculation locations outside the facility the photon dose rates were less then the neutron dose rates by a factor of 10 or more.

Radiation fields from neutron generators shielded with different materials

As a general guide for assessing radiological conditions around a DT neutron generator numerical modeling has been performed to assess neutron and photon dose profiles for a variety of shield materials ranging from 1 to 100cm thick. In agreement with accepted radiation safety practices high-Z materials such as bismuth and lead have been found to be ineffective biological shield materials, owing in part to the existence of (n,2n) reaction channels available with 14.1MeV DT neutrons, while low-Z materials serve as effective shields for these sources. Composite materials such as a mixture of polyethylene and bismuth, or regular concrete, are ideal shield materials for neutron generator radiation because of their ability to attenuate internally generated photon radiation resulting from neutron scattering and capture within the shields themselves.

Design and construction of a facility for neutron activation analysis using the 14 MeV neutron generator at HMS Sultan

The potential for using a small, sealed tube, DT neutron generator for neutron activation analysis has been well documented but not well demonstrated, except for 14 MeV activation analysis. This paper describes the design, construction and characterization of a neutron irradiation facility incorporating a small sealed tube DT neutron generator producing 14 MeV neutrons with fluence rates of 2·108 s−1 in 4π (steady state) and 1011 s−1 in 4π (pulsed). Monte Carlo modeling using MCNP4c and McBend9 has been used to optimize the design of this facility, including the location of a thermal irradiation facility for conventional neutron activation analysis. A significant factor in designing the facility has been the requirement to conform with Ionising Radiation Regulations and the design has been optimized to keep potential radiation doses to less that 1 μSv/h at the external walls of the facility. Activation of gold foils has been used for flux characterization and the experimental results agree well with the modeling.

Improved in situ calibration technique for ITER ex-vessel neutron yield monitor

The ITER neutron diagnostic system needs to cover both the DD- and DT-phases of ITER experiments. To ensure the required accuracy for diagnosing both phases, it is preferable to calibrate the diagnostic system to both DD and DT neutrons. In this paper, the feasibility of two kinds of calibration measurements has been verified through MCNP simulations for the ITER ex-vessel neutron yield monitor. It is indicated that both calibration procedures using DD and DT neutron generators can be implemented with the required accuracies within a realistic measurement time. In the advanced DD-phase of the ITER experiment with high density D plasma producing high energy tritons, where a number of DT neutrons are produced, the independent calibration to DD or DT neutrons could lead to unacceptable measurement uncertainty. An effective method to reduce the measurement uncertainty for the two-component neutron source is discussed.

Portable active interrogation system

The system consists of a pulsed DT neutron generator (5×107n/s) and a portable but high intrinsic efficiency, custom-designed, polyethylene-moderated 3He neutron detector. A multichannel scaler card in a ruggedized laptop computer acquires the data. A user-friendly LabVIEW program analyzes and displays the data. The program displays a warning message when highly enriched uranium or any other fissionable materials is detected at a specified number of sigmas above background in the delayed region between pulses. This report describes the system and gives examples of the response of the system to highly enriched uranium and some other fissionable materials, at several distances and with various shielding materials.

Detailed design of ex-vessel neutron yield monitor for ITER

Taking into consideration the latest design of the International Thermonuclear Experimental Reactor (ITER) main units, we have made the detailed design consideration for an ex-vessel neutron yield monitor to meet the ITER requirements. The monitoring system is constructed of four detector modules consisting of several U235 fission chambers with different sensitivities and graphite (or beryllium) neutron moderator. We also selected possible spaces in the diagnostic ports to install them at appropriate distances and neutron shielding effects from the plasma. Through Monte Carlo neutron transport calculations, it has been confirmed that the present system can cover all the neutron yields encountered in the ITER experiments including the in situ calibrations with a time resolution around 200 μs without detector replacement over the whole ITER experiments. This system can also be calibrated with 10% of required accuracies in a realistic 50 h of accumulation time using a DT neutron generator.

NMIS plus gamma spectroscopy for attributes of HEU, PU and HE detection

A combined nuclear materials identification system–gamma ray spectrometry system can be used passively to obtain the following attributes of Pu: presence, fissile mass, 240/239 ratio and metal versus oxide. This system can also be used with a small, portable, DT neutron generator to measure the attributes of highly enriched uranium (HEU): presence, fissile mass, enrichment, metal versus oxide; and detect the presence of high explosives (HE). For the passive system, time-dependent coincidence distributions can be used for the presence, fissile mass, metal versus oxide for Pu, 240/239 ratio, and gamma ray spectrometry can also be used for 240/239 ratio and presence, allowing presence and 240/239 ratio to be confirmed by two methods. For the active system with a DT neutron generator, all relevant attributes for both Pu and HEU can be determined from various features of the time-dependent coincidence distribution measurements. Active gamma ray spectrometry would determine the presence of HE. The various features of time-dependent coincidence distributions and gamma ray spectrometry that determine these attributes are discussed with some examples from previous determinations.

3D Treatment Planning for 14 MeV Neutrons

Three-dimensional treatment planning is now widely used in radiotherapy with high energy photons. Special tools, for example beam's-eye view (BEV) and dose-volume histograms (DVH) are developed to simulate the shape and position of the beams and to evaluate the dose distribution in the target and the organs at risk. 3D visualisation is used to assess the dose distribution with respect to the target and the organs at risk. Standard blocks made from steel are used at our department to reduce the dose to critical organs. The ring model, originally developed for photon treatment, could be modified to be used with neutrons, too. This implies that the neutron dose is separated into its primary and secondary components, arising from concentric rings around the point of interest. A comparison between measurements in a water phantom and corresponding calculations demonstrates the attainable accuracy. For the DT neutron generator used for patients' treatment at our institution a commercially available treatment planning system was modified to enable the coplanar calculations of three-dimensional treatment plans. Furthermore, a visualisation system developed at our hospital is used to render the 3D dose matrix, together with anatomical data and the regions of interest. The mode of shaded surfaces and transparency simulates a three-dimensional impression of the rendered structures. A realistic case of a neutron treatment is used to clarify the different steps of the 3D treatment planning procedure and its implementation into clinical use.

Triton burnup measurements using scintillating fiber detectors on JT-60U

A new type of 14 MeV neutron detector, based on scintillating fibres, was developed for triton burnup measurements in JT-60U deuterium discharges. The detector consists of an array of scintillating fibres embedded in an aluminum matrix, coupled to a magnetic-resistant phototube with a high current output base, enabling count rates up to 100 MHz. The detector characteristics have been investigated using the Fusion Neutron Source (FNS) DT neutron generator. From the time-dependent measurement of 14 MeV neutron emission in neutral beam (NB) heated plasma discharges, the fast triton diffusivity has been evaluated to be 0.05–0.15 m 2 s −1 Loss of fast tritons caused by Toroidal Alfvén Eignmode (TAE) instability was observed in the peripheral region of the plasma.

First experimental attempts of129I direct measurements by neutron interrogation and gamma-ray spectrometry

As an altemative to129I measurement by X-ray spectrometry or ICPMS, we explored the possibilities of activation analysis using thermal or 14 MeV neutrons. Preliminary qualitative measurements were done with samples of about 5 mg. These samples were exposed to two neutron sources:252Cf and DT neutron generator. The most interesting reaction is the neutron capture which leads to useful signatures at 536.1, 668.5, and 739.5 keV, associated with a half-life of 12.36 h.

Calibration issues of the Tokamak Fusion Test Reactor multichannel neutron collimator

The calibration procedures for the detectors in the neutron collimator are reviewed. The absolute calibration was performed for the NE451 detectors, in situ, by moving a DT neutron generator in the Tokamak Fusion Test Reactor (TFTR) vacuum vessel across each sight line. This calibration was transferred to other detectors in the same channel. Four new sight lines have been installed at a different toroidal location, which view the plasma through the vacuum vessel port cover rather than through thinned windows. The new detectors are cross calibrated to the NE451 detectors with a “jog shot” procedure, where the plasma is quickly shifted in major radius over a distance of 30 cm. The jog shot procedure shows that scattered neutrons account approximately for 30% of the signal of the new central channels. Corrections for detector nonlinearity are discussed. The neutron source strength from the collimator agrees within 10% with the source strength from global neutron monitors in the TFTR test cell.

Scintillating-fiber 14 MeV neutron detector on TFTR during DT operation

A compact 14 MeV neutron detector using an array of scintillating fibers has been tested on the TFTR tokamak under conditions of a high gamma background. This detector uses a fiber-matrix geometry, a magnetic field-insensitive phototube with an active HV base and pulse-height discrimination to reject low-level pulses from 2.5 MeV neutrons and intense gammas. Laboratory calibrations have been performed at EG&G Las Vegas using a pulsed DT neutron generator and a 30 kCi 60Co source as background, at PPPL using DT neutron sources, and at LANL using an energetic deuterium beam and target at a tandem Van de Graaff accelerator. During the first high-power DT shots on TFTR in December 1993, the detector was 15.5 m from the torus in a large collimator. For a rate of 1×1018 n/s from the tokamak, it operated in an equivalent background of 1×1010 gammas/cm2/s (∼4 mA current drain) at a DT count rate of 200 kHz.

A neutron activation gamma ray spectrometer for planetary surface analysis

A pulsed DT neutron generator system, similar to that used in commercial well logging, offers the possibility of performing accurate elemental analyses to depths of tens of centimeters in a few seconds with the probe on the body's surface. Through time-phased measurements of the gamma-ray spectrum synchronized with the neutron pulses, concentrations of hydrogen, carbon and key mineral forming elements can be determined even with a low-resolution spectrometer. If a high resolution spectrometer is used, the number of elements measured and the sensitivity for measurement is increased. An implanted probe system, such as in a comet or ice cap penetrator, would offer the highest possible sensitivity. An inexpensive neutron probe system based on modifications of the Schlumberger well-logging system has been proposed for the Discovery/Venera/SAGE Mission to Venus and the Mars Polar Pathfinder. Preliminary experiments on a simulated Venusian surface indicate that high quality results can be obtained.