Kristin L. Hertz

An integrated field emission array for ion desorption

Field emission arrays that are used for ion desorption must be capable of operating at high applied voltages. The large electric fields can lead to dielectric breakdown or electron emission from the gate, both of which may result in catastrophic failure. Methods were developed to fabricate tip arrays with integrated gate electrodes, separated from the substrate with sufficient dielectric to sustain high voltages. To suppress gate electron emission, processes were developed to fabricate geometries that favor high fields at the tip while minimizing the field at the gate.

Alpha-emitting radioisotopes for switchable neutron generators

Abstract Traditionally, radioisotopic neutron generators mix an alpha-emitting radioisotope with beryllium. The disadvantage of such an alpha–Be source is that they emit neutrons at a steady rate even when stored. These conventional generators are extremely awkward to use in many applications because of the neutron shielding required to prevent exposure to personnel and sensitive electronics. Recently, at our laboratory and others, the possibility of using switchable radioactive neutron sources has been investigated. These sources rely on a mechanical operation to separate the alpha-emitting radioisotope from the Be target, thus allowing the source to be switched on and off. The utility of these new switchable sources is critically dependent on the selection of the alpha-emitting radioisotope. In this paper we discuss issues that determine the desirability of an alpha-emitting source for a switchable neutron generator, and select alpha emitters that are best suited for use in this application.

Field ionization characteristics of an ion source array for neutron generators

A new deuterium ion source is being developed to improve the performance of existing compact neutron generators. The ion source is a microfabricated array of metal tips with an integrated gate (i.e., grid) and produces deuterium ions by field ionizing (or field desorbing) a supply of deuterium gas. Deuterium field ion currents from arrays at source temperatures of 77 K and 293 K are studied. Ion currents from single etched-wire tips operating under the same conditions are used to help understand array results. I-F characteristics of the arrays were found to follow trends similar to those of the better understood single etched-wire tip results; however, the fields achieved by the arrays are limited by electrical breakdown of the structure. Neutron production by field ionization at 293 K was demonstrated for the first time from microfabricated array structures with integrated gates.

The effect of coatings on deuterium retention and permeation in aluminum 6061-T6 APT tritium production tubes

Abstract The accelerator production of tritium project will utilize spallation neutrons incident on thousands of 3 He gas filled metal tubes to produce tritium by way of the exothermic 3 He(n,p) 3 H reaction. Tritons with energies up to 192 keV and protons with energies up to 576 keV are directly implanted into the tube walls. To minimize tritium retention in the tubes and permeation into the coolant surrounding the tubes, it is desirable to have the implanted tritium migrate back to the inner surface of the tubes and rapidly recombine to be released as T2 and HT. Aluminum alloy (Al 6061-T6) is the primary candidate material for fabrication of the tubes. Aluminum alloy samples implanted with deuterons and protons to fluences as high as 3×1022 D (and p)/m 2 were studied. Deuterium retention was measured by mass spectrometry during thermal desorption. Approximately 10% of the implanted deuterium was retained. Copper, nickel and anodized coatings on aluminum alloy were studied as possible methods of reducing retention and permeation of the tritium. In these experiments, the Cu and Ni coatings reduced the retention significantly, whereas retention increased in the anodized coated sample.

Field desorption arrays for neutron generator ion sources

The development of portable active neutron interrogation systems for field detection applications could be facilitated by the use of a novel microfabricated atomic deuterium ion source in compact, accelerator-driven neutron generators. This ion source uses field desorption from the tips of microfabricated field emitter-type arrays to produce atomic deuterium ions. The resulting generator design would have the unique characteristic of being easily scalable to provide the neutron outputs required to meet a wide variety of field detection needs.

A portable active interrogation system using a switchable AmBe neutron source

Active neutron interrogation is an effective technique used to locate fissionable material. This paper discusses a portable system that utilizes a AmBe neutron source. The AmBe source consists of an americium alpha source and a beryllium target that can be switched into alignment to turn the source on and out of alignment to turn the source off. This offers a battery operated backpack portable source. The detector system that has been fabricated for use with this source is a fifteen tube 3He neutron detector. The results of initial experiments with the detector and MCNP calculations are discussed.

Definition of energy-calibrated spectra for national reachback

Accurate energy calibration is critical for the timeliness and accuracy of analysis results of spectra submitted to National Reachback, particularly for the detection of threat items. Many spectra submitted for analysis include either a calibration spectrum using 137Cs or no calibration spectrum at all. The single line provided by 137Cs is insufficient to adequately calibrate nonlinear spectra. A calibration source that provides several lines that are well-spaced, from the low energy cutoff to the full energy range of the detector, is needed for a satisfactory energy calibration. This paper defines the requirements of an energy calibration for the purposes of National Reachback, outlines a method to validate whether a given spectrum meets that definition, discusses general source considerations, and provides a specific operating procedure for calibrating the GR-135.

A microfabricated electrostatic field desorption ion source

The use of an electrostatic field desorption (EFD) ion source would constitute a significant advance in the design and operation of neutron generators. The results would directly benefit the use of neutron generators for active interrogation in the search for special nuclear material and the replacement of radioisotopic sources, particularly in man-portable scenarios. The novel EFD approach uses high electrostatic fields to produce pure atomic deuterium ions from a conductive surface, rather than ions produced from deuterium plasma. This concept has the potential to surpass current state of the art sealed neutron tube designs in many key performance areas including lifetime, reliability, efficiency, and neutron yield. Over the past few years a thorough study of the ion production and neutron yield of fabricated devices has been conducted. Devices that are 1 mm2 consistently produce approximately 1000 n/cm2/s from the deuteron-deuteron reaction when operating in the dc mode. Electric fields of 20 V/nm are consistently achieved resulting in molecular deuterium ions from field ionization. Further increases in electric fields are necessary to reliably produce deuterons from field desorption. Both the modeling and experimental results to date are discussed.

7.1: A microsystems enabled field desorption source

Technologies that have been developed for microelectromechanical systems (MEMS) have been applied to the fabrication of field desorption arrays. These techniques include the use of thick films for enhanced dielectric stand-off, as well as an integrated gate electrode. The increased complexity of MEMS fabrication provides enhanced design flexibility over traditional methods.

Investigation of CaF2:Eu scintillator for D-D neutron interrogation

CaF2:Eu is an attractive radiation detection material because it is inert, non-hygroscopic, shock resistant, and can be less expensive than other radiation detection materials. A CaF2:Eu scintillation detector was constructed to identify whether energy dependent differences in (n,p) and (n,α) cross sections could be exploited to distinguish fission neutrons from D-D neutrons in an active interrogation system. Experimentally, the charged particles are difficult to distinguish from the significantly larger number of γ-rays produced in (n,γ) reactions. In addition, modeling results show that fission neutrons produce only slightly higher charged particle production rates than D-D neutrons. For charged particle production in CaF2:Eu to succeed in fission neutron detection, a superior γ-ray discrimination technique is required.