Chris C. Lawrence

Low-temperature characterization and micropatterning of coevaporated Bi2Te3 and Sb2Te3 films

Thermoelectric (TE) properties of the coevaporated Bi2Te3 and Sb2Te3 films are measured from 100 to 300 K for Seebeck coefficient αS and from 5 to 300 K for electrical resistivity ρe, mobility μe, and Hall coefficient RH. For the low-temperature characterization of TE films, the conditions for coevaporation deposition of Bi, Te, and Sb to form Bi2Te3 and Sb2Te3 films are also investigated, including substrate material, substrate temperature Tsub, and elemental flux ratio (FR). The resublimation of Te occurring above 473 K significantly affects the film composition and quality. Our optimal deposition conditions for Bi2Te3 films are Tsub=533 K and FR=2.4, and those for Sb2Te3 films are Tsub=503 K and FR=3.0. The TE properties of both films are strongly temperature dependent, while Bi2Te3 films show a stronger temperature dependence than Sb2Te3 films due to different major scattering mechanisms. αS of both the coevaporated films are close to or higher than those of bulk materials, but ρe is much higher (due ...

Warhead verification as inverse problem: Applications of neutron spectrum unfolding from organic-scintillator measurements

Verification of future warhead-dismantlement treaties will require detection of certain warhead attributes without the disclosure of sensitive design information, and this presents an unusual measurement challenge. Neutron spectroscopy—commonly eschewed as an ill-posed inverse problem—may hold special advantages for warhead verification by virtue of its insensitivity to certain neutron-source parameters like plutonium isotopics. In this article, we investigate the usefulness of unfolded neutron spectra obtained from organic-scintillator data for verifying a particular treaty-relevant warhead attribute: the presence of high-explosive and neutron-reflecting materials. Toward this end, several improvements on current unfolding capabilities are demonstrated: deuterated detectors are shown to have superior response-matrix condition to that of standard hydrogen-base scintintillators; a novel data-discretization scheme is proposed which removes important detector nonlinearities; and a technique is described for ...

Time-of-flight measurement for energy-dependent intrinsic neutron detection efficiency

Shortage in the current 3He supply has prompted a search for potential alternatives to the neutron detectors currently used in many nuclear nonproliferation and safeguards applications. An alternative detector must be efficient in detecting fission neutrons, and in rejecting or discriminating against gamma-ray radiation. For characterization of numerous detector types, it is helpful to have a technique for evaluating these two characteristics which is relatively fast and easy to perform. Here, a bench-top time-of-flight technique is presented which is based on a coincidence measurement with two ‘independent’ liquid scintillators (no direct source tagging is employed). The neutron source used is 252Cf. The technique can be used to measure energy-dependent intrinsic neutron detection efficiency for incident neutron energies of 0.5–5 MeV, as well as gamma-neutron discrimination efficiency. Measurement results are presented for three 2×2-inch cylindrical liquid scintillation detectors: EJ309, EJ315, and an additional EJ315 with naphthalene added.

A multistage in-plane micro-thermoelectric cooler

This paper presents a mutli-stage thermoelectric micro cooler designed for integration with a wide variety of MEMS devices, including MEMS resonators, which benefit from decreased phase noise at low temperatures. The cooler is fabricated using a multilayer process that allows the device to achieve thermal isolation greater than 10,000 K/W while still maintaining mechanical robustness for practical applications. Co-evaporated thin film bismuth telluride and antimony telluride are used as the thermoelectric materials. The cooler is 9 x 9 mm2 and a preliminary prototype has achieved cooling of 3.8 K, comparable to values predicted using a one-dimensional mathematical model.

Deuterated liquid scintillators: A new tool for neutron measurements

The response of large (4×6) deuterated liquid scintillators (up to 10 cm diameter by 15 cm) to neutrons in the energy range from 0.5 MeV to 20 MeV has been studied using several nuclear reactions, including d(d,n), and 12C(d,n)13N, at the University of Notre Dame FN tandem accelerator. The latter two reactions utilized 9 MeV and 16 MeV deuteron beams, including a pulsed beam that also permitted time‐of‐flight (ToF) measurements. Combining pulse‐shape discrimination and (ToF) allows gating on specific neutron energy groups to determine the detector response to specific neutron energies. Newly‐obtained and optimized pulse shape discrimination using digitized pulse analysis from these detectors will be presented in this paper. These measurements confirmed the ability of these detectors to provide useful neutron spectra without ToF.

Characterization of cadmium capture-gated detector for nuclear nonproliferation applications

Almost all commercially applied neutron-counting systems employ 3He tubes for detection of thermal neutrons. However, due to a large number of 3He systems deployed in the field, there is currently a severe shortage of 3He gas. Therefore, novel neutron detection systems are desirable, especially because a large number of detection systems are needed to meet current security demands. One class of detectors that has been explored is capture-gated scintillation detectors which consist of a standard scintillation detector (plastic or liquid) that has been modified to include a neutron capturing isotope. The capture-gated neutron-spectroscopy principle is based on the fact that a single neutron that interacts in a capture-gated detector can create two pulses that are related in time. In this work, we present simulation and measurement results for a cadmium/plastic-scintillator capture-gated detector. The detector consists of 13, 1-cm thick BC-408 plastic-scintillator layers and 12, 0.1-mm thick natCd layers. This detector possesses unique detection characteristics that could find use in nuclear nonproliferation applications.