G. Kohse

First dark matter search results from a surface run of the 10-L DMTPC directional dark matter detector

Abstract The Dark Matter Time Projection Chamber (DMTPC) is a low pressure (75 Torr CF4) 10 liter detector capable of measuring the vector direction of nuclear recoils with the goal of directional dark matter detection. In this Letter we present the first dark matter limit from DMTPC from a surface run at MIT. In an analysis window of 80–200 keV recoil energy, based on a 35.7 g-day exposure, we set a 90% C.L. upper limit on the spin-dependent WIMP-proton cross section of 2.0 × 10 − 33 cm 2 for 115 GeV/c2 dark matter particle mass.

Characteristics of Composite Silicon Carbide Fuel Cladding after Irradiation under Simulated PWR Conditions

Silicon carbide possesses a high melting point, low chemical activity, no appreciable creep at high temperatures, and a low neutron absorption cross section, making it an attractive material to investigate for use as fuel cladding in light water reactors. The cladding design investigated herein consists of three layers: an inner monolith of SiC, a central composite layer of SiC fibers infiltrated with SiC, and an outer SiC coating to protect against corrosion. The inner monolith provides strength and hermeticity for the tube, and the composite layer adds strength to the monolith while providing a pseudo-ductile failure mode in the hoop direction. The tube may be sealed by bonding SiC end caps to the SiC tube. A number of samples were irradiated in a test loop simulating pressurized water reactor coolant and neutronic conditions at the Massachusetts Institute of Technology research reactor. Postirradiation hoop stress testing via internal pressurization revealed 10% to 60% strength reduction due to physical properties mismatches between the three layers and corrosion. Weight loss measurements indicated that some irradiation-assisted corrosion occurred. Scanning electron microscope analysis allowed determination of the fracture mechanisms for specimens ruptured during hoop testing. The thermal diffusivities of the as-fabricated three-layer tube samples were measured to be roughly three times lower than those of the as-fabricated monolith layer. With irradiation, the thermal diffusivities decreased by factors of 14 and 8 for the monolith and three-layered samples, respectively. This change may be attributed to radiation damage and the formation of a silica layer on the sample surface. Anisotropic swelling of the bonded α-SiC blocks was sufficient to fail five of the six bond test specimens after a 1.5-month irradiation. Two of each of the calcium aluminate and Ti foil bonded samples failed. One of two TiC/SiC bond samples survived.

Irradiation performance of polytetrafluoroethylene (Teflon®) in a mixed fast neutron and gamma radiation field

Abstract Samples of polytetrafluoroethylene have been irradiated with a mixed field of fast neutrons and gamma rays using the MIT Research Reactor. Dose levels from ∼0.3 to ∼50×106 Gy for gamma and from ∼0.13 to 80×104 Gy for fast neutrons were used. Weight loss, fluorine loss, and swelling were measured quantitatively. Subjective mechanical property tests were also performed to assess embrittlement. Aside from high levels of embrittlement, no large changes, ≳1.5%, were observed in the properties which were assayed even at the highest doses.

A background-free direction-sensitive neutron detector

The detection and measurements of properties of neutrons are of great importance in many fields of research, including neutron scattering and radiography, measurements of solar and cosmic ray neutron flux, measurements of neutron interaction cross sections, monitoring of neutrons at nuclear facilities, oil exploration, and searches for fissile weapons of mass destruction. Many neutron detectors are plagued by large backgrounds from x-rays and gamma rays, and most current neutron detectors lack single-event energy sensitivity or any information on neutron directionality. Even the best detectors are limited by cosmic ray neutron backgrounds. All applications would benefit from improved neutron detection sensitivity and improved measurements of neutron properties. Here we show data from a new type of detector that can be used to determine neutron flux, energy distribution, and direction of neutron motion. The detector is free of backgrounds from x-rays, gamma rays, beta particles, and relativistic singly charged particles. It is relatively insensitive to cosmic ray neutrons because of their distinctive angular and energy distributions. It is sensitive to thermal neutrons, fission spectrum neutrons, and high energy neutrons, with detection features distinctive for each energy range. It is capable of determining the location of a source of fission neutrons based on characteristics of elastic scattering of neutrons by helium nuclei. A portable detector could identify one gram of reactor grade plutonium, one meter away, with less than one minute of observation time.

Mechanical property and conductivity changes in several copper alloys after 13. 5 dpa neutron irradiation

A scoping experiment in which 25 different copper materials of 17 alloy compositions were irradiated to ≈13.5 dpa at ≈400°C in a fast reactor is described. The materials include rapidly solidified (RS) alloys, with and without oxide dispersion strengthening, as well as conventionally processed alloys. Immersion density, electrical conductivity, and yield stress and ductility by miniature disk bend testing have been measured before and after irradiation. In general, the RS alloys are stable under irradiation to 13.5 dpa, showing small conductivity changes and little or no swelling. Reduction of strength and ductility, in post-irradiation tests at the irradiation temperature, are not generally observed. Some conventionally processed alloys also performed well, although irradiation softening and swelling of several percent were observed in some cases, and pure copper swelled in excess of 5%. Higher dose irradiations will be required to establish the limits of swelling suppression in these alloys.

Progress in developing DBTT determinations from miniature disk bend tests

Experiments to investigate the possibility of obtaining ductile-to-brittle transition temperature (DBTT) data using bend tests of 3 mm diameter disks are described. A disk specimen 0.40 mm thick with two V-shaped grooves, 0.10 mm deep at right angles to each other along diameters of the disk face, is found to be suitable. In high strain-rate bend testing of materials which exhibit a Charpy V-notch (CVN) DBTT, such specimens undergo a marked change in load/deflection behavior as temperature is lowered. The temperature at which this transition occurs is 145–178 K below the CVN 68 J (50 ft-1b) temperature for three materials tested. There is some evidence that the miniature test transition correlates more consistently with the temperature at which CVN energy reaches a low value such as 7 J. This test offers interesting possibilities for in-service monitoring of critical components such as reactor pressure vessels. Further testing to investigate more fully the relationship between the miniature test and Charpy V-notch results is required.

Effects of Neutron Irradiation on Transpassive Corrosion Behavior of Austenitic Stainless Steels

Abstract Irradiated and unirradiated austenitic stainless steels (SS) were exposed to a transpassive potential (1,050 mVSCE) in a sulfuric acid (H2SO4) solution, and the corrosion responses were studied. Applicability of the transpassive technique to evaluation of radiation-induced segregation (RIS) of impurities was considered. The charge density during the potentiostatic transpassive test and the corrosion morphology after the test were functions of both the material chemistry and irradiation. The charge density of the unirradiated materials was correlated successfully with the bulk impurity level, as defined in terms of the parameter (wt% silicon + wt% phosphorus × 10). The charge density increased steeply above an impurity level parameter of 0.5, which corresponded to the region where intergranular attack (IGA) became visible. The change in charge density after neutron irradiation also was a function of the impurity level. Irradiation resulted in a considerable increase in charge density and grain-bou...

Fast neutron resonance radiography for homeland security

We present new experimental evidence supporting the technique of fast neutron resonance radiography (NRR). Using a set of neutron attenuation images collected at several different neutron energies, the images can be transformed into a set of elemental maps, indicating the presence and relative quantity of a fixed set of basis elements. Here we report on the construction, calibration, and results from a prototype NRR imagining system. We discuss the utility of elemental maps for automated detection of materials as well as standoff determination of chemical formulas.

Microstructural evolution and swelling of high strength, high conductivity RS-PM copper alloys irradiated to 13.5 dpa with neutrons

Microstructures and swelling have been studied for a series of high-strength, high-conductivity rapidly solidified (RS) powder metallurgy (PM) alloys and oxide dispersion alloys which were neutron irradiated to 13.5 dpa at 400°C. Another article of these proceedings presents results for irradiation-induced changes in electrical conductivity, mechanical strength and ductility. The RS-PM alloys include Cu-Mg-Zr-Cr (MZC) and two oxide dispersion stabilized (ODS) alloys, Cu-Zr-ZrO 2 and Cu-Zr-Cr-ZrO 2 . Ingot materials and pure copper were also studied for comparison purposes. All alloys show no significant swelling after irradiation except OFHC pure copper, which exhibited 7.0% swelling. For the ODS alloys, TEM microstructures show stable fine oxide pinning of grain and subgrain boundaries as well as dislocations. In addition to fine oxides, a large amount of very fine metallic and intermetallic precipitation was observed within the grains.

Three online neutron beam experiments based on the iLab Shared Architecture

Students at MIT have traditionally executed certain experiments in the containment building of the MIT nuclear reactor as part of courses in Nuclear Engineering and the third year laboratory course for Physics majors. A joint team of faculty and research staff from the MIT Nuclear Reactor Laboratory (MIT-NRL) and MITs Center for Educational Computing Initiatives have implemented online versions of three classic experiments; (a) a determination of MIT reactor coolant temperature through measurement of thermal neutron velocity, (b) a demonstration of the DeBroglie relationship of the kinetic energy and momentum of thermal neutrons and study of Bragg diffraction through a single copper crystal at various orientations, and (c) a measurement of beam depletion using a variety of shielding filters. These online experiments were implemented using the LabVIEW® virtual instrumentation package and the interactive version of the iLab Shared Architecture (ISA). Initial assessment of the online experiments indicates that they achieve comparable educational outcomes to traditional versions of the labs executed in the reactor containment building.