Fred B. Bateman

Dielectric relaxation behavior and its relation to microstructure in relaxor ferroelectric polymers: High-energy electron irradiated poly(vinylidene fluoride–trifluoroethylene) copolymers

The dielectric behavior of the high-energy electron irradiated poly(vinylidene fluoride–trifluoroethylene) 68/32 mol % copolymer was characterized in the dose range from 0 to 175 Mrad. It was found that as the dose increases, the copolymer evolves from a normal ferroelectric to a relaxor ferroelectric and then to a simple relaxor. Correspondingly, the crystalline morphology changes from a two-phase coexistence between a polar and nonpolar phases at low doses (<50 Mrad) to a macroscopically uniform nonpolar phase for doses beyond 50 Mrad, as revealed by x-ray data. Interestingly, it was observed that in the whole dose range, the Vogel–Fulcher (V–F) relationship could describe the measured relationship between the frequency and the peak temperature of dielectric constant well. For the copolymers irradiated with dose near 75 Mrad, which show a typical relaxor ferroelectric response, the V–F behavior is a result of freezing of the relaxation time (into polar glass state). For the copolymers irradiated with do...

Giant electrocaloric effect in ferroelectric poly(vinylidenefluoride-trifluoroethylene) copolymers near a first-order ferroelectric transition

We present directly measured electrocaloric effect (ECE) from the poly(vinylidenefluoride-trifluoroethylene) 65/35 mol. % copolymer. The data reveal a large difference in the ECE between that measured in applying and in removing the electric field. The difference is significantly reduced by modifying the copolymer with 20 Mrad of high energy electron irradiation. Moreover, an isothermal entropy change ΔSint = 160 J kg−1 K−1 and an adiabatic temperature change ΔTint = 35 °C can be induced in the irradiated copolymer. These results demonstrate the promise of achieving a significant ECE in ferroelectric polymers near first-order ferroelectric-paraelectric transition where multiple intermediate phases can exist.

Effects of electron irradiation on the ferroelectric properties of Langmuir-Blodgett copolymer films

The effect of irradiation on the ferroelectric properties of Langmuir-Blodgett films of the copolymer poly(vinylidene fluoride-trifluoroethylene) was investigated using 1‐MeV electrons for doses from 0.16to1.10MGy, where 1Gray(Gy)=100rad. Irradiation causes a systematic decrease in the phase-transition temperature, crystallinity, and spontaneous polarization of the films. The crystallinity and spontaneous polarization of the films decreased by amounts proportional to the dose, both tending toward zero near a dose of 1.30MGy. The ferroelectric-paraelectric phase-transition temperature, however, was only reduced by about 12%, indicating that the primary effect of irradiation was to convert a crystalline ferroelectric material to a noncrystalline dielectric.

A backscatter-suppressed beta spectrometer for neutron decay studies

We describe a beta electron spectrometer for use in an upcoming experiment that will measure the beta-antineutrino correlation coefficient (a coefficient) in neutron beta decay. Electron energy is measured by a thick plastic scintillator detector. A conical array of plastic scintillator veto detectors is used to suppress events where the electron is backscattered. A Monte Carlo simulation of this device in the configuration of the a coefficient experiment is presented. The design, construction, and testing of a full-scale prototype device is described. We discuss the performance of this spectrometer with respect to its suitability for the experiment.

Measurements and standards for bulk-explosives detection

Recent years have seen a dramatic expansion in the application of radiation and isotopes to security screening. This has been driven primarily by increased incidents involving improvised explosive devices as well as their ease of assembly and leveraged disruption of transportation and commerce. With global expenditures for security-screening systems in the hundreds of billions of dollars, there is a pressing need to develop, apply, and harmonize standards for x-ray and gamma-ray screening systems used to detect explosives and other contraband. The National Institute of Standards and Technology has been facilitating the development of standard measurement tools that can be used to gauge the technical performance (imaging quality) and radiation safety of systems used to screen luggage, persons, vehicles, cargo, and left-behind objects. After a review of this new suite of national standard test methods, test objects, and radiation-measurement protocols, we highlight some of the technical trends that are enhancing the revision of baseline standards. Finally we advocate a more intentional use of technical-performance standards by security stakeholders and outline the advantages this would accrue.

aCORN: An experiment to measure the electron-antineutrino correlation coefficient in free neutron decay

We describe an apparatus used to measure the electron-antineutrino angular correlation coefficient in free neutron decay. The apparatus employs a novel measurement technique in which the angular correlation is converted into a proton time-of-flight asymmetry that is counted directly, avoiding the need for proton spectroscopy. Details of the method, apparatus, detectors, data acquisition, and data reduction scheme are presented, along with a discussion of the important systematic effects.

The aCORN Backscatter-Suppressed Beta Spectrometer

Backscatter of electrons from a beta spectrometer, with incomplete energy deposition, can lead to undesirable effects in many types of experiments. We present and discuss the design and operation of a backscatter-suppressed beta spectrometer that was developed as part of a program to measure the electronantineutrino correlation coefficient in neutron beta decay (aCORN). An array of backscatter veto detectors surrounds a plastic scintillator beta energy detector. The spectrometer contains an axial magnetic field gradient, so electrons are efficiently admitted but have a low probability for escaping back through the entrance after backscattering. The design, construction, calibration, and performance of the spectrometer are discussed.

A Backscatter Suppressed Electron Detector for the Measurement of "a".

A new method of measuring the electron-antineutrino angular correlation coefficient, little “a”, from neutron decay—to be performed at the National Institute of Standards and Technology—will require an electron spectrometer that strongly suppresses backscattered electrons. A prototype consisting of six trapezoidal veto detectors arranged around a plastic scintillator has been tested with an electron beam produced by a Van de Graaff accelerator. The results of this test and its implications for the little “a” measurement are discussed.

SU-G-TeP2-05: Development of a Thimble Calorimeter for Absorbed Dose to Water Characterized in MV Photons

PURPOSE To develop a thimble sized polystyrene calorimeter for use from kV to MV photons, as a primary reference standard for applications from diagnostic CT imaging to therapy beam dose determination. METHODS A polystyrene calorimeter about 1.5 cm diameter embedded with small thermistors was characterized in a 6 MV photon beam from a clinical accelerator at 5 nominal dose rates from 0.8 to 4 Gy/min. Irradiations were delivered with beam on/off cycles first at 60 s and then at 20 s. Two sets of phantom conditions were evaluated: 1) in a 30 cm diameter polyethylene cylinder, and 2) in 10 cm depth of a 30 cm water phantom. The temperature waveforms were recorded and analyzed for temperature rise, arriving at a dose to polystyrene. This value is compared with the result of measurements under identical conditions using an ionization chamber calibrated for absorbed dose to water. Monte Carlo simulations were performed on the measurement systems to estimate such a ratio. RESULTS The ratio of the dose determined by the calorimeter to the dose reported by the ionization chamber was aggregated from all 5 dose rates. The 60 s results show a much elevated response in both phantoms compared to their respective expected results based on simulation. This deviation was reduced when the on/off cycles were shortened to 20 s. This behavior was possibly due to the heat conduction effects in the small calorimeter body. Finite element modeling is being conducted to simulate this effect. CONCLUSION A small solid plastic calorimeter offers the convenience of a portable absorbed dose standard based on direct measurement of energy deposition, but comes at the expense of heat transfer complications which need to be characterized. This work offers preliminary evidence of the behavior and quantitative assessment of the issues to be resolved in future investigations.

TU‐A‐BRB‐10: Ultrasonic Imaging of Dose to Water from a 1.5 MeV Electron Beam

Purpose: To use an ultrasonicsensor array to obtain thermal images produced by absorbed ‐dose distributions in a water phantom. Method and Materials: An array of 128 PVDF ultrasonictransducers arranged in a circle with radius 177.8 mm is used to image the heating of water in a phantom irradiated by a vertical, 1.5 ‐MeV electron beam capable of delivering dose rates ranging from Gy/min to kGy/min. Fan ‐beam sampling and reconstruction are managed by a double ‐multiplexer system that selects one transducer to emit a short pulse and scans the corresponding transducers across the tank to receive the propagated pulse. Time ‐of ‐flight measurements between transmitter/receiver pairs are then converted to temperature and passed to a filtered back projection algorithm to reconstruct the desired thermal distributions. Since dose from the beam is deposited mostly within the top cm layer of water, the array is located just below the water surface. A small, 4 ‐cm diameter Al mask is placed just above the water surface to aperture the beam. Results: Preliminary measurements involving therapy ‐level radiation were obscured by system noise, but higher doses, of order 1000 Gy/min, produced detectable images within seconds. Conclusion:Imaging of absorbed dose to water has been achieved for the first time, although at dose rates that are considerably above therapy level. Planned improvements in signal processing are expected to bring the detection limit into the therapy regime. Other uses of the array, e.g. triangulation of thermoacoustic disturbances caused by pulsed radiation sources, are also being investigated.