Randy M. Hill

Small-animal radionuclide imaging with focusing gamma-ray optics

Significant effort currently is being devoted to the development of noninvasive imaging systems that allow in vivo assessment of biological and biomolecular interactions in mice and other small animals. While physiological function in small animals can be localized and imaged using conventional radionuclide imaging techniques such as single-photon emission tomography (SPECT) and positron emission tomography (PET), these techniques inherently are limited to spatial resolutions of 1-2 mm. For this reason, we are developing a small animal radionuclide imaging system (SARIS) using grazing incidence optics to focus gamma-rays emitted by 125I and other radiopharmaceuticals. We have developed a prototype optic with sufficient accuracy and precision to focus the 27.5 keV photons from 125I onto a high-resolution imaging detector. Experimental measurements from the prototype have demonstrated that the optic can focus X-rays from a microfocus X-ray tube to a spot having physical dimensions (approximately 1500 microns half-power diameter) consistent with those predicted by theory. Our theoretical and numerical analysis also indicate that an optic can be designed and build that ultimately can achieve 100 μm spatial resolution with sufficient efficiency to perform it in vivo single photon emission imaging studies in small animal.

Optical and multilayer design for the first Kirkpatrick-Baez optics for x-ray diagnostic at NIF

At the Lawrence Livermore National Laboratory (LLNL) we are designing, developing and testing multiple Kirkpatrick-Baez (KB) optics to be added to the suite of x-ray diagnostic instruments for the National Ignition Facility (NIF). Each optic consists of four KB channels made of spherically super-polished x-ray substrates. These substrates are multilayer-coated to allow steep grazing angle geometry and wavelength filtering. These optics are customized for different experiments and will provide NIF with an alternative x-ray imaging technique to pinholes, improving both resolution and photon throughput. With this manuscript we describe KB optic requirements, specifications, optical and multilayer designs.

Reflective multilayer optic as hard X-ray diagnostic on laser-plasma experiment

A multilayer-based optic was tested for use as an X-ray diagnostic on a laser-plasma experiment. The multilayer optic was employed to selectively pass X-rays between 55 and 100 keV. An order of magnitude improvement in signal-to-noise ratio is achieved compared to a transmission crystal spectrometer. A multilayer response model, taking into account the source size and spectral content, is constructed and the outlook for application above 500 keV is briefly discussed. LLNL-JRNL-664311.

The MEL-X project at the Lawrence Livermore National Laboratory: a mirror-based delay line for x-rays

At the Lawrence Livermore National Laboratory (LLNL) in collaboration with the Linac Coherent Light Source (LCLS) we are developing a mirror-based delay line for x-rays (MEL-X) to enable x-ray pump/x-ray probe experiments at Free Electron Lasers (XFELs). The goal of this project is the development and deployment of a proof-of-principle delay line featuring coated x-ray optics. The four-mirror design of the MEL-X is motivated by the need for ease of alignment and use. In order to simplify the overlap of the pump and the probe beam after each delay time change, a scheme involving super-polished rails and mirror-to-motor decoupling has been adopted. The MEL-X, used in combination with a bright pulsed source like LCLS, features a capability for a high intensity pump beam. Its Iridium coating allows it to work at hard x-ray energies all the way up to 9 keV, with a probe beam transmission of 35% up to 8keV, and 14% at 9keV. The delay time can be tailored to each particular experiment, with a nominal range of 70 - 350 fs for this prototype. The MEL-X, combined with established techniques such as x-ray diffraction, absorption or emission, could provide new insights on ultra-fast transitions in highly excited states of matter.

Engineering precision relocation capability into a large-cantilevered telescoping diagnostic for a Kirkpatrick Baez x-ray Optic

The Kirkpatrick Baez Optic (KBO) diagnostic designed for the National Ignition Facility (NIF) requires very precise alignment between four pairs of mirrors that make up four x-ray imaging channels. Furthermore, the overlapping image axis of the four pairs must be aligned to within a 50 μm radius of the NIF target center. In order to achieve this the diagnostic utilizes a telescoping snout that when extended, locates the mirrors at the end of a Diagnostic Load Package (DLP), cantilevered more than three meters out from its bolted connection points. Discussed in this paper are the structural challenges and the mechanical design solutions that were implemented to achieve the ±50 μm pointing accuracy. During an Inertial Confinement Fusion (ICF) experiment, the KBO diagnostic will be 117 mm away from the extremely high impulse, target implosion shock wave, which requires a unique approach to protecting the sensitive optics which will also be discussed.

Characterization and simulation of soft gamma-ray mirrors for their use with spent fuel rods at reprocessing facilities

The use of a grazing incidence optic to selectively reflect K-shell fluorescence emission and isotope-specific lines from special nuclear materials is a highly desirable nondestructive analysis method for use in reprocessing fuel environments. Preliminary measurements have been performed, and a simulation suite has been developed to give insight into the design of the x ray optics system as a function of the source emission, multilayer coating characteristics, and general experimental configurations. The experimental results are compared to the predictions from our simulation toolkit to illustrate the ray-tracing capability and explore the effect of modified optics in future measurement campaigns.

Silicon single crystal as back-reflector for high-intensity hard x-rays

At the Lawrence Livermore National Laboratory (LLNL) we have engineered a silicon prototype sample that can be used to reflect focused hard x-ray photons at high intensities in back-scattering geometry.1 Our work is motivated by the need for an all-x-ray pump-and-probe capability at X-ray Free Electron Lasers (XFELs) such as the Linac Coherent Light Source (LCSL) at SLAC. In the first phase of our project, we exposed silicon single crystal to the LCLS beam, and quantitatively studied the x-ray induced damage as a function of x-ray fluence. The damage we observed is extensive at fluences typical of pump-and-probe experiments. The conclusions drawn from our data allowed us to design and manufacture a silicon mirror that can limit the local damage, and reflect the incident beam before its single crystal structure is destroyed. In the second phase of this project we tested this prototype back-reflector at the LCLS. Preliminary results suggest that the new mirror geometry yields reproducible Bragg reflectivity at high x-ray fluences, promising a path forward for silicon single crystals as x-ray back-reflectors.