MeV Fusion Neutrons Research Articles

Observation of high-energy deuterium–tritium fusion gamma rays using gas Cherenkov detectors

As fusion ignition conditions are approached using the National Ignition Facility (NIF), independent high-bandwidth gamma-ray fusion-burn measurements become essential complements to neutron-based methods. Time resolution ∼20 ps (10–30 GHz), energy discrimination, and significant stand off distance may be needed for credible burn history measurements. The 16.75 MeV gamma rays that accompany deuterium–tritium (d+t) fusion provide a high-bandwidth alternative to 14 MeV fusion neutrons for d+t burn-history measurements. A thresholding detector, based on the Cherenkov effect, resulting from Compton and pair production electron interactions in gaseous carbon dioxide, offers a means of separating the high-energy fusion gamma rays from other background sources. When coupled to appropriate streak camera recorders, a gas Cherenkov system response of 15 GHz is feasible. As a first step toward this goal, a gas Cherenkov system, employing a 1 GHz photodiode detector, was fielded at the Omega laser facility to demonstrate unambiguous detection of fusion gamma rays from high-yield d+t implosions. With the carbon dioxide Cherenkov production threshold at 12 MeV, the observed high-energy gamma-ray signal is proportional to the observed 14 MeV fusion neutron yield and independent of neutron-induced gamma rays.

Neutron images recorded with high-resolution bubble detectors

Neutron images have been successfully recorded with “gel-type bubble detectors.” 14 MeV fusion neutrons emitted from laser-irradiated inertial confinement fusion targets formed coded images in proof-of-principle experiments conducted at the OMEGA laser facility. In the detector, small (∼50 μm diameter) bubbles form when high-energy neutrons collide with the detector material. Each collision produces only one bubble whose position can be determined with a precision of better than 25 μm. Additionally, bubble detectors are insensitive to x-ray and gamma-ray background. For these ride-along experiments, bubble detectors were simply inserted between the imaging aperture and the scintillation detector of the primary experiment. Recorded images for high-yield (6×1013) targets show the bubble detector’s potential for high resolution. More complex “liquid” bubble detectors are expected to have efficiencies of ∼1% (versus 0.001% for gel-type) while maintaining the desirable characteristics of the gel-type units. This article discusses the bubble detector, the characteristics that make them an attractive recording device for neutron imaging, and initial bubble detector images. Simulation of the image statistics and the potential impact of bubble detectors on image system design are also discussed.

Nuclear fusion in gases of deuterium clusters heated with a femtosecond laser

Recent experiments on the interaction of intense, ultrafast pulses with large van der Waals bonded clusters have shown that these clusters can explode with substantial kinetic energy. Producing explosions in deuterium clusters with a 35 fs laser pulse, deuterium ions were accelerated to sufficient kinetic energy to drive deuterium–deuterium (DD) nuclear fusion. By diagnosing the fusion yield through measurements of 2.45 MeV fusion neutrons, over 104 neutrons per laser shot were measured when 100 mJ of laser energy is used.

Nuclear fusion driven by coulomb explosions of large deuterium clusters

Recent experiments on the interaction of intense, ultrafast laser pulses with large van der Waals bonded clusters have shown that these clusters can explode with substantial kinetic energy. By driving explosions in deuterium clusters with a 35 fs laser pulse, we have accelerated ions to sufficient kinetic energy to produce DD nuclear fusion. By diagnosing the fusion yield through measurements of 2.45 MeV fusion neutrons, we have found that the fusion yield from these exploding clusters varies strongly with the cluster size, consistent with acceleration of deuterons via Coulomb explosion forces.

Defect structure near edge dislocations in neutron-irradiated Cu—0.3 at.%Ge

Cu—0.3 at.%Ge bulk specimens were irradiated with 14 MeV fusion neutrons at 473 K. Dislocation loops were formed on one side of preexisting edge dislocations, whereas they were hardly formed in the dislocation-free matrix. A neutron irradiated sample was subsequently irradiated with 1 MeV electrons in a high voltage electron microscope. The behavior of the edge dislocation and the dislocation loops was simultaneously examined by in situ observation. The large dislocation loops of more than about 8 nm grew, while the smaller loops shrank and disappeared, and the dislocation loops were identified to be of interstitial type. The edge dislocation climbed toward the group of the dislocation loops. It was found that the loops were formed at the side of dilatation strain field of the edge dislocation. The direct effect of the cascade on the preferential nucleation and growth of the dislocation loops in the vicinity of edge dislocation was discussed.

Particle dependence of the gallium vacancy production in irradiated n-type gallium arsenide

The relative introduction rate of the gallium vacancy in n-type GaAs irradiated with /sup 60/Co /spl gamma/ rays, 7 MeV electrons, fusion (14 MeV) and fission (1 MeV) neutrons, protons (0.6 to 200 MeV), deuterons (1 to 10 MeV), /spl alpha/ particles (2.5 to 10 MeV), lithium (5 to 20 MeV) and oxygen ions (10 to 30 MeV) has been determined. Effects of annealing are reported. The measured introduction rates obtained with proton irradiation for energies up to 10 MeV, and for the heavy ions agree reasonably well with Rutherford scattering, NIEL calculations and the TRIM simulation. The results for electron irradiation also agree with the corresponding NIEL calculations.

Neutron and plasma irradiations of fusion reactor materials using fusion plasma neutron sources

A compact and steady-state DT plasma with parameters relevant to a fusion-reactor, the FEF (fusion engineering facility), has been considered as a neutron source for the development of fusion reactor materials providing both. (1) 14 MeV fusion neutron irradiation testing, and (2) high charged particle flux for testing of plasma-facing materials during neutron irradiation. The 14 MeV neutron wall loading is about 1–2 MW/m 2, with a test section area of 1 m 2. One version of the test section is near the edge of the plasma in the vacuum chamber. Another is for high heat flux testing of plasma facing materials, where the charged particle flux leads to heat loads of several 10 kW/cm 2. New versions of FEF, with localized high neutron fluxes, are also proposed by introducing a sloshing ion distribution into the plasma.

Neutron irradiation damage in transition metal carbides

Effects of neutron irradiation on the physical properties of light transition metal carbides, TiC x , VC x and NbC x , were examined, emphasizing the characterization of irradiation induced defects in the nonstoichiometric composition. TiC x irradiated with 14 MeV (fusion) neutrons showed higher damage rates with increasing C/Ti ( x) ratio. A brief discussion is made on “cascade damage” in TiC x irradiated with fusion neutrons. Two other carbides ( VC x and NbC x ) were irradiated with fission reactor neutrons. The irradiation effects on VC x were not so simple, because of the complex irradiation behavior of “ordered” phases. For instance, complete disordering was revealed in an ordered phase, “V 8C 7”, after an irradiation dose of 10 25 n/m 2.

Fusion-fission-light ion correlation of mechanical property change in a TZM alloy

In order to study the correlation of damage among fusion neutron-fission neutron-light ion irradiation at low fluence levels, mechanical properties and microstructures in a molybdenum alloy, stress-relieved TZM, were studied. Miniature tensile specimens and TEM disks were irradiated to fluences of 10 −4−10 −2 dpa by 14 MeV neutrons at RTNS-II, by fission neutrons at Kyoto University Reactor and by 16 MeV Deuterons at Tohoku University Cyclotron. Irradiation hardening was observed at all irradiation conditions. The ratio of the increase in yield stress at the fluence of 1 × 10 22/ m 2 was 4:3:2 for 16 MeV deuterons, fusion neutron and fission neutron, respectively. The magnitude of irradiation hardening was not expressed by the same curve even in the dpa scale. Small defect clusters were observed uniformly in specimens irradiated with any particles.

Observation of small vacancy detect clusters produced by high- energy collision recoils

The observation and analysis of small defect clusters produced by energetic particle irradiation is of importance in understanding the basic mechanisms of radiation damage. Renewed interest in fundamental studies stems from the need to understand very high energy recoil damage typical both of 14 MeV fusion neutrons and spallation neutrons produced around targets in high energy particle accelerators.The defect clusters may be of vacancy or interstitial type. Vacancy clusters occur -as vacancy loops, stacking fault tetrahedra, or three dimensional void embryos. Interstitial clusters occur as interstitial loops or three dimensional clusters. For irradiations carried out at temperatures below that which vacancies are predominantly immobile, analysis of the vacancy component of the total defect cluster population may be used to document the cascade collision processes produced by high energy recoils. The information required is defect cluster type, defect size distribution, and defect density. Since the defects of interest are generally in the size range 0.5 - 5 nm, electron microscopy imaging and analysis techniques are currently being re-assessed.

A prototype microstrip gas detector

The generation of fusion power in controlled thermonuclear fusion experiments is assessed by recording neutron emission from the plasma. The neutron diagnostic systems of present tokamaks routinely use gaseous detectors which can operate only within a limited dynamic range, from two up to three decades in the pulse counting mode. The MicroStrip Gas Chambers (MSGCs), due to their higher rate capability, can play an important role as new neutron detectors with larger operational range (four or more decades). The application of MSGCs for the measurement of the neutron emission on the Frascati Tokamak Upgrade (FTU) is presented. A new method for detecting 14 MeV fusion neutrons in the next-stage tokamak devices by means of a MSGC and a Pd converter is also discussed.

Short−range order in Au−Fe radiation−enhanced diffusion and the effectiveness of 14−MeV neutrons

Solute clustering in a Au−Fe (17% Fe) alloy has been demonstrated by Mössbauer effect measurements of the magnetic ordering temperature. Neutron irradiation at room temperature produces clustering by means of radiation−enhanced diffusion, while annealing at high temperatures is required to produce clustering thermally. The radiation−enhanced diffusion effect was used to compare directly the efficiency of mobile defect production by 14−MeV fusion neutrons and reactor neutrons. The 14−MeV neutrons are more effective by about a factor of 10 than the reactor neutrons (E≳0.1 MeV) in producing mobile lattice defects. Other compositions failed to respond to radiation−induced clustering, suggesting that Au−17−Fe is in a critical composition region regarding the response of the magnetic ordering temperature to variations in short−range order.