Upgrade of the IGN-14 neutron generator for research on detection of fusion-plasma products
Introduction
The deuterium and tritium nuclear reaction, t(d,n)α, commonly abbreviated as D–T, is the most promising reaction for energy production from a thermonuclear fusion. This opinion is reflected in the decisions taken by the European Commission on construction of the fusion reactor ITER (International Thermonuclear Experimental Reactor) [1].
This information indicates that the demand for research in the field of diagnostics for D–T nuclear reaction, which is the main component of the thermonuclear synthesis, should be intensively developed. 14 MeV neutron sources of medium neutron yield play an important role in this field of research. There are many issues related to the study of fusion neutrons for which the existing neutron sources are sufficient. For example, laboratory research and preliminary tests of related systems as well as methods for monitoring and spectrometric measurements of neutrons and resultant reaction products (mainly alpha particles) need to be performed prior to their final applications in large tokamaks.
Most of neutron sources based on nuclear reactions require accelerated particles and produce fast neutrons with energies of several MeV [2] in addition to classic fission nuclear reactors that are efficient source of neutrons with energies up to about 2 MeV. A number of nuclear reactions are used as the basis for the construction of radioisotopic and apparatus neutron sources, e.g.: 9Be(γ,n)2α (Q=−1.67 MeV), 9Be(α,n) 12C (Q=5.91 MeV), d(d,n) 3He (Q=3.29 MeV), t(d,n)α (Q=17.6 MeV).
The first two reactions are used in radioisotopic neutron sources, where gamma and alpha radiation are obtained from other radioactive nuclides. There are other radioisotopic sources like Po–Be, Pu–Be and Am–Be with the broad neutron peak around few MeV. The spontaneous fission of 252Cf also provides a widely used radioisotopic fast neutron source with the maximum yield at 1 MeV [3]. All of these sources do not meet the significant condition of research onthe D–T nuclear fusion – neutron spectra of these sources do not reach energy of 14 MeV.
The D–D and D–T reactions are used in neutron generators (apparatus neutron sources). Both of them are exothermic, thus requiring relatively low energy particle beams (100–500 keV). The D–T reaction has a resonance at about 100 keV with the cross-section of 5 barn, while the D–D reaction has its weaker resonance at about 2 MeV (about 0.1 barn) [4]. The cross-section of the D–T reaction is much higher than that of reaction D–D, which results in a neutron yield of about two orders of magnitude higher.
Neutron generators are a small-sized high-energy beam-target neutron sources still attractive for researchers for its compactness and controllability compared to other neutron sources such as nuclear reactors, accelerators, and radioisotopes. Apart from industrial applications of neutron generators (e.g. sealed neutron tubes) from time to time new reports appear in the literature about new technical solutions for neutron generators, focusing primarily on increasing the neutron yield (e.g. [5], [6], [7]).
Among a variety of applications of the neutron generators, a special interest is focused on experimental research for the thermonuclear D–T plasma. Only the D–T neutron generator is a convenient flexible laboratory device that produces an energy neutron spectrum with a peak around 14 MeV, analogous to the neutron spectrum produced in the D–T fusion plasma [8]. Although such a neutron spectrum can be produced in a plasma-focus devices [9], when working with a deuterium–tritium mixture as a working gas, that solution is not practical because of the difficulty in maintaining the safety of tritium treatment. Another solution may be a deuterium–lithium converter from thermal-to-high energy neutrons in the form of a container placed in neutron flux of a fission reactor [10]. The container walls are filled e.g. with 6LiD. As a result of the thermal neutron reaction with 6Li a fast neutron flux is produced inside the converter. It is a two step reaction: the first is 6Li(nth,t)α (Q=4.79 MeV), and next tritium reacts with deuterium and 6Li: d(t,n)α (Q=17.58 MeV), 6Li(t,n) 8Be (Q=16.02 MeV), producing high energy neutrons. Such solutions are very rare, due to a strong disturbance of neutron field in the reactor.
A small sealed neutron tube can be applied as a 14 MeV calibration source in large tokamaks like JET or ITER working with the D–T plasma. It is a much more reliable calibration, than that with the use of an isotopic since it reproduces the shape of the neutron energy spectrum from D–T plasmas. Neutronic experiments, related to ITER and fusion power plant research, with mock-up of the European helium-cooled lithium-lead (HCLL) test blanket module (TBM) are realized at a D–T neutron generator [11] as well as other irradiation experiments in 14 MeV neutron fields (e.g. [12], [13], [14], [15] ).
In this paper the D–T pulsed neutron generator at the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN), in Kraków (Poland) is described. We focus on the possibility to use this device for testing new systems of neutron and other charged particle detection in view of application for thermonuclear reactors.
Section snippets
IGN-14 − fast neutron generator at the IFJ PAN
The neutron generator in use at the Institute of Nuclear Physics is a linear accelerator where the deuterium ion incident beam induces the nuclear reaction t(d,n)α in the tritium target. This is a flexible laboratory device which is fully adjustable and its parameters can be controlled according to the needs of the experiments. The device can operate in either continuous or pulsed beam regime with control on both the neutron yield and the pulse parameters. For the basic use (production of 14 MeV
Nuclear reactions and mixed radiation field at the IGN-14 neutron generator
The main nuclear reaction in the D–T operational regime is reaction (1). In the D–T fusion plasma research, both reaction products, neutrons and alpha particles are deeply investigated for diagnostic reasons in terms of rate and energy distribution:In the D–T fusion plasma the nuclear reaction between two deuterium nuclei is also possible. In the beam-target regime of the neutron generator, after some time of the operation the deuterium concentration increases due to
Measuring vacuum chamber
Alpha particles are easily and strongly energy downgraded in their interaction with materials and ultimately absorbed and their accurate measurement is possible under vacuum conditions. In the case of the fusion reactor, alpha particles are to be measured inside the vessel, where their source, i.e. the fusion plasma is formed. In the case of a neutron generator, they can be measured inside the ion-guide pipe near the tritium target (Fig. 1).
The measuring chamber was designed as a replaceable
Vacuum system of the IGN-14 neutron generator
The vacuum system of the IGN-14 neutron generator consists of two parts. The first one maintains vacuum in both the ion source and the acceleration system while the second one in the measuring chamber. The main vacuum system consists of a backing pump and a high vacuum pump and is connected to the acceleration part. The measuring chamber is equipped with the backing pump only. The oil-free vacuum should be of the order of 10−6 Torr or less in the whole system during the acceleration of the
Auxiliary equipment and data acquisition systems
The IGN-14 neutron generator is equipped with two monitor linesand data acquisition systems. Monitor lines measure the average neutron yield. One line is based on the BF3 detector surrounded with paraffin and the second one is based on a fast neutron scintillating probe type FN-1 (Saint Gobain). The shape of the deuteron current pulse is continuously displayed on an oscilloscope.
A control system based on the LABVIEW software checks the average pulse rate in a selected detection line and if it
Mixed particle field in the measuring chamber
The whole measuring chamber and the tritium target, where the source of mixed particles is formed, have been numerically modeled for an estimation of space and energy distribution of the reaction products. Detailed results of the calculations are presented in the paper by Wiącek and Dankowski [17]. Here, only the alpha and proton particle field distributions are quoted as an example to show the possibility of measurements in the experimental vacuum chamber (Fig. 4, Fig. 5). The scales for
Summary
The IGN-14 neutron generator is a laboratory multi-purpose experimental device. The main role of this unit is the production of neutrons in a beam-target D–D or D–T reaction. Fast neutron flux can be used to a variety of research in the field of neutron interactions with the matter i.e., activation analysis. After slowing-down in a moderator a spatial thermal neutron field is formed which is useful to study neutron diffusion phenomena in bulk materials.
Moreover, the generator is a source of the
Acknowledgments
The authors would like to thank Prof. K. Drozdowicz and M.Sc. Eng. J. Dankowski for profitable discussions on the subject of the paper.
Work was performed within the strategic research project “Technologies supporting the development of safe nuclear power” financed by the National Centre for Research and Development (NCBiR, Poland). Research Task “Research and development of techniques for the controlled thermonuclear fusion”, Contract no. SP/J/2/143234/11.
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