Showing papers by "Yaron Danon published in 2007"

Enhanced neutron production from pyroelectric fusion

Abstract: The pyroelectric effect has been utilized as a means of producing x rays, electrons, positive ions, and neutrons. Pyroelectric sources have advantages over conventional sources, in that they are low cost, only consume a few watts of power, and are smaller than most sources. While pyroelectric x ray sources are already mature enough to be sold as commercial devices, the current generation of pyroelectric neutron sources is too low in intensity to be useful for commercial applications. This report demonstrates techniques which increase neutron production by a factor of 5.6 over previously published data.

Applications of pyroelectric particle accelerators

Abstract: The discovery of pyroelectric X-ray generation in 1992 by Brownridge has led to a recent surge of interest in the use of the pyroelectric effect as a means of producing useful radiation. By heating or cooling a pyroelectric crystal such as lithium tantalate (LiTaO3) in a vacuum, a potential on the order of 100 kV can be generated. This potential is great enough to eject electrons from the crystal for the production of characteristic or bremsstrahlung X-rays, or to cause field ionization near a tip mounted to the crystal. By using the combined fields of two polarized crystals, the acceleration potential can be doubled, with one crystal acting as a particle emitter and the other crystal serving as a target. Such a paired-crystal system was used to generate X-rays with energies of greater than 200 keV, and can be used to fluoresce the K shell of thorium (Z = 92). An alternative use of pyroelectric sources is the field ionization of a dilute gas. If the positively-charged crystal is used to ionize a deuterium gas, and the target crystal is coated with deuterated target, the deuterium ions can be accelerated into the target at high enough energy to cause D–D fusion. Results verifying the production of D–D fusion neutrons from a pyroelectric source will be presented. Future applications of pyroelectric accelerator technology, such as the use of the electron beam for materials testing, will also be discussed.

Measurements with the high flux lead slowing-down spectrometer at LANL

Abstract: A Lead Slowing-Down Spectrometer (LSDS) was recently installed at LANL [D. Rochman, R.C. Haight, J.M. O’Donnell, A. Michaudon, S.A. Wender, D.J. Vieira, E.M. Bond, T.A. Bredeweg, A. Kronenberg, J.B. Wilhelmy, T. Ethvignot, T. Granier, M. Petit, Y. Danon, Characteristics of a lead slowing-down spectrometer coupled to the LANSCE accelerator, Nucl. Instr. and Meth. A 550 (2005) 397]. The LSDS is comprised of a cube of pure lead 1.2 m on the side, with a spallation pulsed neutron source in its center. The LSDS is driven by 800 MeV protons with a time-averaged current of up to 1 μA, pulse widths of 0.05–0.25 μs and a repetition rate of 20–40 Hz. Spallation neutrons are created by directing the proton beam into an air-cooled tungsten target in the center of the lead cube. The neutrons slow down by scattering interactions with the lead and thus enable measurements of neutron-induced reaction rates as a function of the slowing-down time, which correlates to neutron energy. The advantage of an LSDS as a neutron spectrometer is that the neutron flux is 3–4 orders of magnitude higher than a standard time-of-flight experiment at the equivalent flight path, 5.6 m. The effective energy range is 0.1 eV to 100 keV with a typical energy resolution of 30% from 1 eV to 10 keV. The average neutron flux between 1 and 10 keV is about 1.7 × 10 9 n/cm 2 /s/μA. This high flux makes the LSDS an important tool for neutron-induced cross section measurements of ultra-small samples (nanograms) or of samples with very low cross sections. The LSDS at LANL was initially built in order to measure the fission cross section of the short-lived metastable isotope of U-235, however it can also be used to measure (n, α) and (n, p) reactions. Fission cross section measurements were made with samples of 235 U, 236 U, 238 U and 239 Pu. The smallest sample measured was 10 ng of 239 Pu. Measurement of (n, α) cross section with 760 ng of Li-6 was also demonstrated. Possible future cross section measurements include fission and (n, p) and (n, α) reaction in radioactive samples.

High-accuracy filtered neutron beam and high-energy transmission measurements at the Gaerttner Laboratory

Abstract: Recently a method for high accuracy total cross section measurement in the energy range of 24keV to 940keV using an iron filtered beam was developed at RPI. Measurements the total cross section of carbon and beryllium are discussed. A new neutron detection system was developed at RPI and the first measurement with this system is reported here.

Nuclear Data Measurements at the RPI LINAC

Abstract: Acc The Gaerttner LINAC Laboratory at RPI uses a 60 MeV electron LINAC as a pulsed neutron source. The neutrons are used for a variety of experiments primarily related to nuclear data measurements. Neutron targets and detectors are optimized for different experiments and several flight paths are available for time of flight measurements. The capabilities include neutron transmission and capture measurements in the energy range from 0.01 eV to 2 keV. An additional detection system is currently being designed and will enable neutron transmission measurements in the energy range of 1 eV to 200 keV. These capabilities allow high accuracy determination of resonance parameters. New detection systems have been recently installed which will enable neutron transmission and scattering measurements in the energy range from 0.4 to 20 MeV. Filtered neutron beams enable high accuracy measurements of smooth cross sections in the energy range from 24 keV to 905 keV The laboratory also hosts a 66 metric-ton lead slowing down spectrometer (LSDS) that is currently used for simultaneous measurements of fission cross section and fission fragments energy and mass distributions of small samples. Detectors for (n,alpha) and (n,p) measurements of nanogram samples with the LSDS are also being developed and tested.

Production and application of a novel energy-tunable X-ray source at the RPI LINAC

Abstract: The 60-MeV electron linear accelerator at Rensselaer Polytechnic Institute (RPI) is used to produce parametric X-rays (PXR). PXR is an intense, quasi-monochromatic, energy-tunable, and polarized X-ray source derived from the interaction of relativistic electrons with the periodic structure of crystal materials. Experiments were performed using highly oriented pyrolytic graphite (HOPG), LiF, Si, Ge, Cu, and W target crystal radiators. Smooth X-ray energy tunability is achieved by rotating the crystal with respect to the electron beam direction. Measured energy linewidths consistently agreed with predicted values except in cases using lower quality HOPG. When the predicted energy linewidth was narrower than our Si X-ray detector resolution (350 eV at 17.5 KeV), a near-absorption edge transmission technique that takes advantage of the PXR energy tunabilty was used to measure the PXR energy linewidth for example, Si(4 0 0) FWHM of 134 eV at 9.0 keV (2%). Per electron, the photon production efficiency of PXR is comparable to synchrotron radiation sources. A theoretical model that considers electron multiple scattering, electron divergence, and crystal mosaicity was used to broaden the PXR photon distribution in order to calculate the predicted PXR photon yield. Comparing measurements and calculations resulted in a typical relative error below 50%. In some cases with LiF, the differences between predicted and measured values were as low as 2% for LiF(4 0 0). Finally, this work reports for the first-time PXR imaging. This was achieved using LiF(2 2 0) interacting with 56 MeV electrons with electron beam currents up to 6 lA. The LiF and graphite PXR target crystals were compared for use in soft tissue imaging, e.g. mammography using energies 17–20 keV. Low Z materials like graphite and LiF were most suitable for PXR production because of their low Bremsstrahlung production, electron scattering, and photon absorption. Graphite was more efficient at producing PXR photons while the LiF energy line width was narrower. � 2007 Elsevier B.V. All rights reserved. PACS: 41.60.� m; 87.59.� e; 41.75.Fr

Deuterium ionization for pyroelectric crystal accelerators

Abstract: Researchers have reported that pyroelectric crystals may be used to achieve DD fusion. Current research is concentrated on increasing the neutron yield for potential homeland security applications. An important aspect in realizing pyroelectric fusion is to ionize and accelerate deuterium gas toward a deuterated target. This research includes development of a novel mass spectroscopy system to characterize D2 gas ionization due to a pyroelectric crystal-generated electric field. A lithium tantalate (LiTaO3) crystal with a 70 nm tip was used to ionize and accelerate D2 gas through a magnetic field. An ion detection system including a zinc sulfide screen and a Starlight Express® camera was used to measure the deflection of the ions through the magnetic field in order to resolve whether or D ions are being produced. Measurements were compared to expected deflection calculations to corroborate results. 2 D

Measurements of fission fragment properties using RPI's lead slowing down spectrometer

Abstract: A double sided Frisch-gridded fission chamber for use in RPI's lead slowing-down neutron spectrometer (LSDS) is being developed at Rennselaer Polytechnic Institute. Placing this fission chamber in the high neutron flux of the LSDS allows the measurement of the energy dependent, neutron induced fission cross sections, as well as the mass and kinetic energy of the fission fragments of various small samples. The fission chamber consists of two anodes shielded by Frisch grids on either side of a single cathode. The sample is located in the center of the cathode and is made by depositing small amounts of actinides dissolved in solution on very thin films. The chamber was successfully tested and calibrated using 0.4 ng of 252 Cf and mass distributions were compared to previous work. As a proof of concept, the chamber was then placed in the LSDS to measure, simultaneously, the neutron induced fission cross section and fragment mass and energy distributions of 24.9 µ go f 235 U. The mass distribution as a function of neutron energy was examined and it may be possible to see changes in the mass distribution as a function of neutron energy. This process will enable the measurement of isotopes that are not available in large enough quantities (sub-micrograms) or with small fission cross sections (microbarns).

Thin metallic crystals for Parametric X-ray (PXR) production

Abstract: INTRODUCTION Parametric X-rays (PXR) are produced from relativistic electrons interacting with targets with periodic structures [1]. The rotation of a crystal target in an electron beam smoothly varies the PXR energy. PXR is energy tunable, quasi-monochromatic, and directionally intense [2] making it an attractive novel x-ray source for medical imaging applications. Two LINAC facilities have demonstrated PXR imaging [3, 4]. In both of these studies, electron beam currents were increased to Alevels, and both the Si and LiF crystal targets cracked under thermal stress. For imaging applications, low Z target crystals such as Si, LiF, and HOPG (graphite) have advantages over high Z targets because they produce less unwanted Bremsstrahlung, electron energy deposition, and electron scattering [5]. PXR production is a function of the square of the target crystal electric susceptibility, . High Z metallic targets such as W and Cu have higher electric susceptibility, better thermal characteristics, and much smaller optimum thicknesses. At the Rensselaer LINAC, 60 MeV electrons produced 12.0 keV and 13.6 keV PXR, respectively, from the 1-mm thick Cu111 and the W222 crystallographic planes in a Bragg geometry, i.e., the planes were parallel to the target surface [7]. In those experiments, the target crystals thickness was much thicker than the optimum. This work seeks to parametrically study electron scattering, and its effect on broadening the PXR photon distribution from thin crystals of these two metallic crystals.

Comment on "Anomalous neutron Compton scattering from molecular hydrogen"

Abstract: We address a recent paper 1 containing a claim that our calculation of the neutron scattering cross section from H2O and from D2O in the 40 keV range reported in Ref. 2 is incorrect. In the above work, 1 the authors studied molecular H2 using the neutron Compton scattering NCS technique at epithermal neutron energies 10‐200 eV. In their study some evidence was presented for the occurrence of an anomaly in which the n-p scattering cross section is reported to be lower than the normal value by 30%. This report followed a series of papers using several H-containing samples and claiming the observation of a similar shortfall in the n-p scattering intensity in all the samples studied. In the same paper, 1 reference was given to our recent publication 2

Neutron cross section measurements of elemental molybdenum and resonance parameter analysis

Abstract: The purpose of this work was to measure the neutron cross sections of molybdenum accurately. The Rensselaer Polytechnic (RPI) LINAC facility was used to measure the neutron interaction cross sections of molybdenum. Neutron capture time-of-flight measurements were made at 25m with a sodium iodide multiplicity detector. Transmission measurements were performed at 25m flight with a 6 Li glass scintillation detector. Nine different thicknesses of elemental molybdenum metal samples ranging from 0.051mm (0.002in.) to 6.35mm (0.250in.) were measured in either capture or transmission. Data from two transmission and one capture measurement have been analyzed using the multilevel R-matrix Bayesian code SAMMY. Throughout the energy spectrum, 10- 2000eV, resonance widths have been attained. Between one and two keV, the width assignments of overlapping resonances were obtained and compared to ENDF/B-VII.0. ENDF/B-VII.0 nuclear radii fit the transmission data between resonances better than those of ENDF/B-VI.8. Below 600eV, the inclusion of capture data in the fit enhanced our ability to determine radiation widths compared to using transmission data alone.

Measurements of X-rays from nanotubes and nanorods

Abstract: INTRODUCTION A nanostructure-based X-ray cancer treatment device does not currently exist. To demonstrate the feasibility of X-ray production, we performed several preliminary experiments recently to measure X-rays using nanotube and nanorod samples [1,2]. The X-ray production is based on electron “field emission” cold cathode principle. These experiments were carried out to demonstrate the feasibility of using novel nano-emitters as a cold cathode in the X-ray source design.

Comment on: Anomalous neutron Compton scattering from molecular hydrogen. Authors' reply

Abstract: We address a recent paper 1 containing a claim that our calculation of the neutron scattering cross section from H2O and from D2O in the 40 keV range reported in Ref. 2 is incorrect. In the above work, 1 the authors studied molecular H2 using the neutron Compton scattering NCS technique at epithermal neutron energies 10‐200 eV. In their study some evidence was presented for the occurrence of an anomaly in which the n-p scattering cross section is reported to be lower than the normal value by 30%. This report followed a series of papers using several H-containing samples and claiming the observation of a similar shortfall in the n-p scattering intensity in all the samples studied. In the same paper, 1 reference was given to our recent publication 2