Neutron Production Mechanism Research Articles

Investigation of the Neutron Production Mechanism in a 20 kJ Plasma Focus Device

Neutron production mechanisms in a medium energy plasma focus device (20 kJ) were investigated. The time-resolved hard X-ray, neutron signals were obtained with two detectors based on a plastic scintillator–photomultiplier combination located at two different angles (0°, 90°) to the plasma chamber and 10 m from the top of the anode. Neutron signal intensities at 0° and 90° were recorded for comparison. Neutron intensity width results in a distribution of neutron energy in the 0°, direction greater than the energy of thermonuclear neutrons (2.45 MeV).

Observation of Two Phases of Neutron Emission in a Low Energy Plasma Focus

Neutron emissions in a low energy 3.3 kJ (15 kV) plasma focus are studied. The system is operated in deuterium and deuterium-argon admixtures. Enhancement of the neutron yield is obtained with a suitable amount of high Z admixture. Time resolved neutron measurements are made by using four detectors positioned at two different distances at both the end-on and side-on direction. Maxwellian pulse fitting techniques are employed to resolve a two phase neutron emission history. The neutron energy and anisotropy for the two phases are determined. The energy and anisotropy of the two phases are found to be different and that the second phase is of high anisotropy. These results confirm the presence of two phases of neutron emission that are possibly due to at least two different kinds of neutron production mechanisms in the low energy plasma focus.

BACKGROUNDS TO SENSITIVE EXPERIMENTS UNDERGROUND

▪ Abstract We summarize residual background sources encountered in experiments conducted deep underground. Physical mechanisms of production and methods of estimation for the dominant sources are considered, and comparisons of the calculations with underground measurements are discussed. Principal background sources discussed include primary interactions of cosmic rays, mechanisms of neutron production by cosmic rays and low energy backgrounds from neutrons, primordial and anthropogenic radionuclides, and secondary radioactivity from spallation.

Optimization of the high pressure operation regime for enhanced neutron yield in a plasma focus device

The average total neutron yield is measured, using an indium foil activation detector, at various combinations of filling gas pressures (including the higher pressure operation regime) of deuterium, capacitor bank charging voltages, anode lengths and insulator sleeve lengths to optimize the neutron yield from the NX2 Plasma Focus device. A remarkable six-fold increase in the average maximum total neutron yield, to a record value of (7 ± 1) × 108 neutrons per shot, compared to the similar energy UNU-ICTP Plasma Focus device is achieved for deuterium at a relatively much higher filling gas pressure of 20 mbar. The average peak neutron energy for the axial direction (0°), radial direction (90°) and backward direction (180°) is estimated to be 2.89 ± 0.25 MeV, 2.49 ± 0.20 MeV and 2.11 ± 0.12 MeV, respectively. The average forward to radial neutron yield anisotropy is found to be 1.46 ± 0.28. The neutron energy and anisotropy measurements suggest that the neutron production mechanism may be predominantly beam target.

Experimental Study of Plasma Materials' Interaction in Plasma Focus “Dena”

It is widely recognized that plasma material interaction in fusion devices is a critical issue that affects the overall machine performance. The process of material selection with a low degradation effect on the confined plasma and on the nuclear fusion production in tokamaks is of great importance. However, plasma materials' interaction studies in tokamaks have some limitations, so it is suggested that a new method for investigating this phenomenon be developed. The high capabilities of plasma focus devices allow one to simulate the interaction studies of tokamak plasma and first-wall materials. Our experiments were performed on a 90-kJ Filippov-type plasma focus, Dena. Experimental results clearly show the influence of materials on the neutron yield and neutron production mechanisms.

Imaging of fusion reaction zone in plasma focus

In a low energy (2.3 kJ) Mather-type deuterium plasma focus, neutron and charged particle emission is investigated by using time-resolved neutron detectors and time-integrated charged particle pinhole imaging camera. The time-integrated charged particle pinhole images demonstrate the varying influence of magnetohydrodynamic (MHD) instabilities vis-a-vis filling pressure. The neutron production mechanism at play strongly depends upon the pressure. At lower pressure, the plasma column is highly unstable due to MHD instabilities and the neutron emission is found to be low with fluence anisotropy exceeding 3.5. At optimum pressure (2.5 mbar for this system), an almost stable dense plasma of about 17 mm3 volume is formed about 5 mm away from the anode, with neutron emission at its highest and the fluence anisotropy lowest. At higher pressure, the plasma column is stable, although it moves away from the anode like a jet and may then be called a moving boiler. In this case, the neutron emission is lowered compared to its optimum value and fluence anisotropy is increased. The data suggest beam-target mechanism at low pressure, trapped gyrating particles at optimum pressure and a jetlike moving boiler at higher pressure.

A 14 MeV neutron spectrometer for the Joint European Torus deuterium-tritium experiments

A 14 MeV neutron spectrometer, based on the principle of the proton recoil telescope, has been installed as a deuterium–tritium (DT) plasma diagnostic at the Joint European Torus. The device comprises three identical telescope modules arranged one behind the other, to increase efficiency without worsening resolution. It views the plasma along a horizontal line of sight. The total efficiency is 7.05×10−5 counts per (neutron per cm2), and the calculated response function has a full width at half maximum of 2.2% at an incident neutron energy of 14.1 MeV. The spectrometer was operational throughout the recent DT experiment No. 1, and selected neutron spectra have been analyzed in terms of the contributions from the various neutron production mechanisms.

Solar neutron and proton production during the 1990 May 24 cosmic-ray flare increases

view Abstract Citations (51) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Solar Neutron and Proton Production during the 1990 May 24 Cosmic-Ray Flare Increases Debrunner, H. ; Lockwood, J. A. ; Ryan, J. M. Abstract A detailed analysis of the solar cosmic-ray event on 1990 May 24 has now led us to conclude that the first increase was due to direct solar neutrons and the second increase was produced by solar protons presumably accelerated by an extended coronal shock. The first increase due to solar neutrons lasted about 25 minutes, had an integrated neutron flux at the Earth of 2.5 x 10 exp 4 neutrons/sq cm for En equal to or greater than 100 MeV, and was produced by a time-extended neutron production mechanism at the sun which had approximately the amplitude-time profile of the 79.5-109.6 MeV gamma-ray emission. The second increase starting at about 2102.5 UT was observed by many neutron monitors around the world. It had a duration of about 6 hr and was caused by a highly anisotropic flux of solar protons with a rigidity spectrum proportional to P exp -5.5 at the time of maximum intensity (2115-2125 UT). We concluded that these solar protons were accelerated for about 60 minutes by a diffusive coronal shock, and the total integrated number of protons with P greater than 0.25 GV (Ep equal to or greater than 30 MeV) at the sun was estimated to be equal to or greater than 2 x 10 exp 34. This is less than 10 percent of the total number of protons required at the sun to produce the observed solar neutron fluence of 2.5 x 10 exp 4/sq cm at the Earth. These results are discussed in the context of recent calculations by Ryan & Lee (1991). Publication: The Astrophysical Journal Pub Date: June 1993 DOI: 10.1086/172712 Bibcode: 1993ApJ...409..822D Keywords: Solar Cosmic Rays; Solar Flares; Solar Neutrons; Solar Protons; Particle Acceleration; Solar Corona; Solar Physics; Solar Physics; ISM: COSMIC RAYS; SUN: FLARES; SUN: PARTICLE EMISSION full text sources ADS |

Subject index

This chapter discusses the neutron sources. All neutron sources are based on nuclear reactions that free neutrons bound in nuclei. These are categorized according to the mechanisms by which this is done: fission chain reactions, fusion reactions, e--bremsstrahlung-induced photoneutron and photofission reactions, charged-particle nuclear reactions, and spallation. The last three together termed as accelerator-based reactions. Muon-catalyzed fusion and the neutron focus represent other categories of neutron-producing reactions. Electron-bremsstrahlung sources produce evaporation neutrons with an average energy of a few mega-electron-volts, whether due to photofission or photonuclear reaction. Since the neutron production mechanisms involve first producing bremsstrahlung photons, which then interact to produce neutrons, these sources produce very large gamma-ray fluxes as well, which sometimes represent a troublesome background. Neutron shielding is like that for a reactor. Spallation sources also produce predominantly evaporation neutrons, which boil off in the process of “cooling off” of highly excited nuclei. The types of accelerators that provide the highest intensities are resonant accelerators, which all have inherent pulse structure. In cyclotrons, synchrotrons, and storage rings, bunches of particles circulate at frequencies of a few megahertz.

Double scatter neutron time‐of‐flight spectrometer as a plasma diagnostic

The energy and flux measurement of D–D and D–T fusion neutrons provides a nonperturbing probe of plasma performance. In particular, ion temperature distribution of the plasma can be determined. This information can distinguish neutron production mechanisms, i.e., knock‐on or thermonuclear in origin. To perform a fast neutron measurement, a high data rate double scatter spectrometer has been developed. This allows for a definitive measurement of the flux, energy, angle, and time distributions of neutrons from within the plasma. In addition, x‐ray discrimination can be achieved from the time‐of‐flight information. The specific design issues and performance of the spectrometer will be described.

Pulsed power applications to intense neutron source development

The use of conventional and near term pulsed power technology to generate intense fluxes of neutrons for use in fusion reactor materials studies is discussed. Two types of neutron production mechanism are considered. For the immediate future, the use of single pulse or high rep rate intense ion beam sources is proposed to provide high fluxes of neutrons from beam-target interactions. Farther along in time, the use of intense ion or electron beams to initiate inertially confined fusion reactions will lead to intense sources of thermonuclear neutrons.

Study of an electron-beam discharge into a vacuum diode with polyethylene anode

A 40-nsec-duration 2.5-MV 25-kA electron-beam discharge into a vacuum diode with polyethylene anode has been studied with time-resolved optical photography and soft x-ray diagnostics. The beam was observed to be initially uniform but to filament at 15 nsec and form a single powerful pinch at 25 nsec. The interaction of the pinched beam and the anode produces a plasma ≤ 75 μm in diameter with a temperature of ∼ 50 eV. Neutron and deuteron emission from the diode were observed and the intensity ratios are consistent with a beam target neutron production mechanism.

Similarity laws in turbulent focus plasma with beam-beam neutron production

The problem of scaling plasma focus devices for increased neutron output has recently been discussed by several authors. Imshennik et al. developed a similarity theiry, proceeding from the assumption that the ratio of collision mean free path to characteristic length of the device should remain constant throughout the scaling process. In the following note, it will be shown that the constancy of the Mach number to Reynolds number ratio can be argued from a kinetic treatment of the neutron production mechanism. Furthermore, the influence of turbulent resistivity will be considered throughout the similarity considerations, and for the current an equation yielding the dip due to collaps will be employed instead of using the maximum current value.