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Neutron time-of-flight (nTOF) detectors have been used on Sandia National Laboratories' Z-Machine for inertial confinement fusion and magnetized liner fusion experiments to infer physics parameters including the apparent fuel-ion temperature, neutron yield, the magnetic-radius product (BR), and the liner rho-r. Single-paddle, dual-paddle, and co-axial scintillation nTOF detectors are used in axial lines-of-sight (LOS) and LOS that are 12° from the midplane. Detector fabrication, characterization, and calibration are discussed. [ABSTRACT FROM AUTHOR]

In inertial confinement fusion experiments, the neutron yield is an important metric for thermonuclear fusion performance. Neutron activation diagnostics can be used to infer neutron yields. The material used for neutron activation diagnostic undergoes a threshold reaction so that only neutrons having energies above the threshold energy are observed. For thermonuclear experiments using deuterium (D) and tritium (T) fuel constituents, neutrons arising from D + D reactions (DD-neutrons) and neutrons resulting from D + T reactions (DT-neutrons) are of primary interest. Indium has two neutron activation reactions that can be used to infer yields of DD-neutrons and DT-neutrons. One threshold is high enough that only DT-neutrons can induce activation, the second reaction can be activated by both DD-neutrons and DT-neutrons. Thus, to obtain the DD-neutron yield, the contribution made by DT-neutrons to the total induced activity must be extracted. In DD-fuel experiments, DT-neutrons arise from secondary reactions, which are significantly lower in number than primary DD-neutrons, and their contribution to the inferred DD-neutron yield can be ignored. When the DD- and DT-neutron yields become comparable, such as when low tritium fractions are added to DD-fuel, the contribution of DT-neutrons must be extracted to obtain accurate yields. A general method is described for this correction to DD-neutron yields. [ABSTRACT FROM AUTHOR]

A proof-of-principle CR-39 based neutron-recoil-spectrometer was built and fielded on the Z facility. Data from this experiment match indium activation yields within a factor of 2 using simplified instrument response function models. The data also demonstrate the need for neutron shielding in order to infer liner areal densities. A new shielded design has been developed. The spectrometer is expected to achieve signal-to-background greater than 2 for the down-scattered neutron signal and greater than 30 for the primary signal. [ABSTRACT FROM AUTHOR]

We present a general methodology to determine the diagnostic sensitivity that is directly applicable to neutron-activation diagnostics fielded on a wide variety of neutron-producing experiments, which include inertial-confinement fusion (ICF), dense plasma focus, and ion beam-driven concepts. This approach includes a combination of several effects: (1) non-isotropic neutron emission; (2) the 1/r² decrease in neutron fluence in the activation material; (3) the spatially distributed neutron scattering, attenuation, and energy losses due to the fielding environment and activation material itself; and (4) temporally varying neutron emission. As an example, we describe the copper-activation diagnostic used to measure secondary deuterium-tritium fusion-neutron yields on ICF experiments conducted on the pulsed-power Z Accelerator at Sandia National Laboratories. Using this methodology along with results from absolute calibrations and Monte Carlo simulations, we find that for the diagnostic configuration on Z, the diagnostic sensitivity is 0.037% ± 17% counts/neutron per cm² and is ~ 40% less sensitive than it would be in an ideal geometry due to neutron attenuation, scattering, and energy loss effects. [ABSTRACT FROM AUTHOR]

Experiments on the Z accelerator with deuterium gas-puff implosions have produced up to 3.7×1013 (±20%) neutrons at 2.34 MeV (±0.10 MeV). Although the mechanism for generating these neutrons was not definitively identified, this neutron output is 100 times more than previously observed from neutron-producing experiments at Z. Dopant gases in the deuterium (argon and chlorine) were used to study implosion characteristics and stagnated plasma conditions through x-ray yield measurements and spectroscopy. Magnetohydrodynamic (MHD) calculations have suggested that the dopants improved the neutron output through better plasma compression, which has been studied in experiments increasing the dopant fraction. Scaling these experiments, and additional MHD calculations, suggest that ∼5×1014 deuterium-deuterium (DD) neutrons could be generated at the 26-MA refurbished Z facility. [ABSTRACT FROM AUTHOR]

A deuterium gas-puff load imploded by a multi-MA current driver from a large initial diameter could be a powerful source of fusion neutrons, a plasma neutron source (PNS). Unlike the beam-target neutrons produced in Z-pinch plasmas in the 1950s and deuterium-fiber experiments in the 1980s, the neutrons generated in deuterium gas-puffs with current levels achieved in recent experiments on the Z facility at Sandia National Laboratories could contain a substantial fraction of thermonuclear origin. For recent deuterium gas-puff shots on Z, our analytic estimates and one- and two-dimensional simulations predict thermal neutron yields ∼3×1013, in fair agreement with the yields recently measured on Z [C. A. Coverdale et al., Phys. Plasmas (to be published)]. It is demonstrated that the hypothesis of a beam-target origin of the observed fusion neutrons implies a very high Z-pinch-driver-to-fast-ions energy transfer efficiency, 5 to 10%, which would make a multi-MA deuterium Z-pinch the most efficient light-ion accelerator. No matter what mechanism is eventually determined to be responsible for generating fusion neutrons in deuterium gas-puff shots on Z, the deuterium neutron yield is shown to scale as Yn∼Im4, where Im is the peak current of the pinch. Theoretical estimates and numerical modeling of deuterium gas-puff implosions demonstrate that the yields of thermonuclear fusion neutrons that can be produced on ZR and the next-generation machines are sufficiently high to make PNS the most powerful, cost- and energy-efficient laboratory sources of 2.5-14 MeV fusion neutrons, just like plasma radiation sources are the most powerful sources of soft and keV x rays. In particular, the predicted deuterium-tritium thermal neutron-producing capability of PNS driven by the next-generation ZR and ZX accelerators is ∼5×1016 and ∼1018, respectively. [ABSTRACT FROM AUTHOR]

Experiments on the Z accelerator with deuterium gas puff implosions have produced up to 3.9×1013 (±20%) neutrons at 2.34 MeV (±0.10 MeV). Experimentally, the mechanism for generating these neutrons has not been definitively identified through isotropy measurements, but activation diagnostics suggest multiple mechanisms may be responsible. One-, two-, and three-dimensional magnetohydrodynamic (MHD) calculations have indicated that thermonuclear outputs from Z could be expected to be in the (0.3–1.0)×1014 range. X-ray diagnostics of plasma conditions, fielded to look at dopant materials in the deuterium, have shown that the stagnated deuterium plasma achieved electron temperatures of 2.2 keV and ion densities of 2×1020 cm-3, in agreement with the MHD calculations. [ABSTRACT FROM AUTHOR]

Progress in Z-pinch experiments at Sandia's Saturn facility have underscored a need for an absolute yield measurement for DD fusion neutrons. The technique chosen for making this absolute yield measurement was neutron activation of indium metal samples. To calibrate the technique, a 175-keV deuteron beam was allowed to impinge on a 3.0-µm-thick erbium deuteride target, producing neutrons through the ²H (d,n)³He fusion reaction. The neutron flux produced at 0° and incident on nominal 5-g indium samples was determined by the associated particle method. This method employed protons measured from the ²H(d,p)³H reaction to infer the neutron flux produced. After neutron irradiation, the activity of the indium samples was measured with a Ge gamma-ray detector. The total activity of the metastable state [sup 115m]In (336.23 keV) was measured, compared with the total incident flux, and a calibration factor (indium counts/neutron/gram of indium) determined. For completeness, a calibration factor for DT neutrons from the ³H(d,n)[sup 4]He fusion reaction was also obtained through the measured activity of the metastable state [sup 114m]In(190.29 keV). The experiment and the measured calibration factors for both reactions are described in the paper. [ABSTRACT FROM AUTHOR]

We have measured the efficiency (tracks per incident neutron) of pure CR-39 for detecting DD and DT neutrons. Neutrons having average energies of 2.9 MeV (DD) and 14.8 MeV (DT) were produced by a 200-keV electrostatic accelerator and the neutron yields were measured using the associated particle counting technique. All CR-39 samples irradiated by DD or DT neutrons were etched for 2 h in a 70°, 6.25-N NaOH bath. For bare CR-39, the efficiencies for detecting 2.9- and 14.8-MeV neutrons were found to be (1.3±0.4)×10[sup -4] and (5.0±1.8) × 10[sup -5], respectively. We also investigated using CR-39 and polyimide as proton radiators. For detecting 2.9-MeV neutrons, the radiators had no significant effect on efficiency; but for detecting 14.8-MeV neutrons the polyimide radiator increased the efficiency to (7.8±2.8) × 10[sup -5]. [ABSTRACT FROM AUTHOR]

A basis set expansion method [V. Dribinski, A. Ossadtchi, V. A. Mandelshtam, and H. Reisler, Rev. Sci. Instrum. 73, 2634 (2002)] is applied to recover physical information about plasma radiation sources from instrument data, which has been forward transformed due to the nature of the measurement technique. This method provides a general approach for inverse problems, and we discuss two specific examples relevant to diagnosing fast z pinches on the 20-25 MA Z machine [M. E. Savage, L. F. Bennett, D. E. Bliss, W. T. Clark, R. S. Coats, J. M. Elizondo, K. R. LeChien, H. C. Harjes, J. M. Lehr, J. E. Maenchen, D. H. McDaniel, M. F. Pasik, T. D. Pointon, A. C. Owen, D. B. Seidel, D. L. Smith, B. S. Stoltzfus, K. W. Struve, W. A. Stygar, L. K. Warne, J. R. Woodworth, C. W. Mendel, K. R. Prestwich, R. W. Shoup, D. L. Johnson, J. P. Corley, K. C. Hodge, T. C. Wagoner, and P. E. Wakeland, in Proceedings of the Pulsed Power Plasma Sciences Conference (IEEE, 2007), p. 979]. First, Abel inversion of time-gated, self-emission x-ray images from a wire array implosion is studied. Second, we present an approach for unfolding neutron time-of-flight measurements from a deuterium gas puff z pinch to recover information about emission time history and energy distribution. Through these examples, we discuss how noise in the measured data limits the practical resolution of the inversion, and how the method handles discontinuities in the source function and artifacts in the projected image. We add to the method a propagation of errors calculation for estimating uncertainties in the inverted solution. [ABSTRACT FROM AUTHOR]

The 350-keV Cockroft-Walton accelerator at Sandia National laboratory's Ion Beam facility is being used to calibrate absolutely a total DT neutron yield diagnostic based on the 63Cu(n,2n)62Cu(β+) reaction. These investigations have led to first-order uncertainties approaching 5% or better. The experiments employ the associated-particle technique. Deuterons at 175 keV impinge a 2.6 μm thick erbium tritide target producing 14.1 MeV neutrons from the T(d,n)4He reaction. The alpha particles emitted are measured at two angles relative to the beam direction and used to infer the neutron flux on a copper sample. The induced 62Cu activity is then measured and related to the neutron flux. This method is known as the F-factor technique. Description of the associated-particle method, copper sample geometries employed, and the present estimates of the uncertainties to the F-factor obtained are given. [ABSTRACT FROM AUTHOR]

The MIT Linear Electrostatic Ion Accelerator (LEIA) generates DD and D3He fusion products for the development of nuclear diagnostics for Omega, Z, and the National Ignition Facility (NIF). Significant improvements to the system in recent years are presented. Fusion reaction rates, as high as 107 s-1 and 106 s-1 for DD and D3He, respectively, are now well regulated with a new ion source and electronic gas control system. Charged fusion products are more accurately characterized, which allows for better calibration of existing nuclear diagnostics. In addition, in situ measurements of the on-target beam profile, made with a CCD camera, are used to determine the metrology of the fusion-product source for particle-counting applications. Finally, neutron diagnostics development has been facilitated by detailed Monte Carlo N-Particle Transport (MCNP) modeling of neutrons in the accelerator target chamber, which is used to correct for scattering within the system. These recent improvements have resulted in a versatile platform, which continues to support the existing nuclear diagnostics while simultaneously facilitating the development of new diagnostics in aid of the National Ignition Campaign at the National Ignition Facility. [ABSTRACT FROM AUTHOR]