H. Heather Chen-Mayer

High-resolution ultrasonic thermometer for radiation dosimetry

This paper describes recent developments in the area of high-precision ultrasonic thermometry with the potential to provide on-site direct determination of radiation doses administered for cancer treatment. Conventional calorimeters used for this purpose measure radiation-induced heating in a water phantom at one point in space by means of immersed thermistors and are subject to various thermal disturbances due to Ohmic heating and interactions of the radiation with the sensor probes. By contrast, the method described here is based on a high-resolution ultrasonic system that determines the change of the speed of sound due to small temperature changes in an acoustic propagation path in the radiation-heated water, thereby avoiding such undesired thermal effects. The thermometer is able to measure tens of microkelvin changes in the water temperature averaged over the acoustic path of about 60 cm at room temperature, with root-mean-squared noise of about 5 microK. Both incandescent and ionizing radiation heating data are presented for analog and digital implementations of a laboratory prototype. This application of the ultrasonic technique opens up possibilities for a new approach to performing therapy-level radiation dosimetry for medical clinics and standards laboratories.

Distribution of chlorine in quartz determined by neutron beam focusing prompt gamma activation analysis and micro-x-ray fluorescence

This paper describes two spatially resolved analytical techniques for chlorine distribution analysis in high-purity quartz glass. The first, prompt gamma activation analysis (PGAA), to which most of this paper is devoted,is emphasized because a new neutron focusing technique has made this study feasible. Despite the low concentration of chlorine, the neutron absorption cross section for Cl is about 200 times greater than for Si, making Cl in Si an ideal system for PGAA. The second technique described is micro-x-ray fluorescence using a laboratory-based spectrometer. The results from each technique and their ability to quantify low levels of chlorine (400-1800 μg Cl/g SiO 2 ) are described.

Time-resolved radiation beam profiles in water obtained by ultrasonic tomography

This paper presents a practical ultrasonic system for near real-time imaging of spatial temperature distributions in water caused by absorption of radiation. Initial testing with radiation from a highly attenuated infrared lamp demonstrates that the system is able to map sub-millikelvin temperature changes, thus making it suitable for characterizing dose profiles of therapy-level ionizing radiation beams. The system uses a fan-beam tomographic reconstruction algorithm to invert time-of-flight data derived from ultrasonic pulses produced and detected by a circular array of transducers immersed in water. Temperature dependence of the speed of sound in water permits the conversion of these measured two-dimensional velocity distributions into temperature distributions that indicate the absorbed radiation dose. The laboratory prototype, based on a 128-element transducer array, is used to acquire temperature maps of a 230 mm × 230 mm area every 4 s with sub-millikelvin resolution in temperature and about 5 mm resolution in space. Earlier measurements with a single-channel version of this prototype suggest refinements in signal-conditioning electronics and signal-processing algorithms that would allow the present instrument to resolve temperature changes as low as a few microkelvin. Possible applications include real-time intensity profiling of radiation beams and three-dimensional characterization of the absorbed dose.

Standard Reference Materials for Medical CT

NIST Standard Reference Materials 2087 and 2088 for medical computed tomography (CT) have become available. The materials allow CT reconstructions to be tied to the International System of Units in length, density, and mass attenuation coefficient.

Determination of Hydrogen in Niobium by Cold Neutron Prompt Gamma‐Ray Activation Analysis and Neutron Incoherent Scattering

The presence of trace amounts of hydrogen in niobium is believed to have a detrimental effect on the mechanical and superconducting properties. Unfortunately, few techniques are capable of measuring hydrogen at these levels. We have developed two techniques for measuring hydrogen in materials. Cold neutron prompt gamma‐ray activation analysis (PGAA) has proven useful for the determination of hydrogen and other elements in a wide variety of materials. Neutron incoherent scattering (NIS), a complementary tool to PGAA, has been used to measure trace hydrogen in titanium. Both techniques were used to study the effects of vacuum heating and chemical polishing on the hydrogen content of superconducting niobium.

Convergent beam neutron crystallography

Applications of neutron diffraction for small samples (<1mm3) or small fiducial areas are limited by the available neutron flux density. Recent demonstrations of convergent beam electron and x-ray diffraction and focusing of cold (λ>1 Å) neutrons suggest the possibility to use convergent beam neutron diffraction for small sample crystallography. We have carried out a systematic study of diffraction of both monoenergetic and broad bandwidth neutrons at the NIST Research Reactor and at the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory. Combining convergent beams with time-of-flight Laue diffraction is particularly attractive for high efficiency small sample diffraction studies. We have studied single crystal and powder diffraction of neutrons with convergence angles as large as 15° and have observed diffracted peak intensity gains greater than 20. The convergent beam method (CBM) shows promise for crystallography on small samples of small to medium size molecules (potentially even for proteins), ultra-high pressure samples, and for mapping of strain and texture distributions in larger samples.

Characterization of hydrogen in concrete by cold neutron prompt gamma-ray activation analysis and neutron incoherent scattering

A combination of cold neutron prompt gamma-ray activation analysis (PGAA) and neutron incoherent scattering (NIS) has been used for nondestructive characterization of hydrogen as a function of position in slabs of wet concrete of different composition. Hydrogen was determined by PGAA by scanning each sample across of 5 mm diameter neutron beam in 10 mm increments, and measuring the 2223 keV prompt gamma ray. NIS measurements were performed by scanning the samples across a 5 mm diameter neutron beam at 5 mm increments and detecting scattered neutrons. The measurements demonstrate the feasibility of the techniques for 2D compositional mapping of hydrogen and other elements in materials, and indicate the potential of these methods for monitoring the uniformity of drying concrete.