Charles C. Chamberlain

Study of hard x-ray emission from intense femtosecond Ti:sapphire laser–solid target interactions

Interaction of intense Ti:sapphire laser with solid targets has been studied experimentally by measuring hard x-ray and hot electron generation. Hard x-ray (8–100 keV) emission spectrum and Kα x-ray conversion efficiency (ηK) from plasma have been studied as a function of laser intensity (1017–1019  W/cm2), pulse duration (70–400)fs, and laser pulse fluence. For intensity I>1×1017 W/cm2, the Ag ηK increases to reach a maximum value of 2×10−5 at an intensity I=4×1018 W/cm2. Hot electron temperature (KTh) and ηK scaling laws have been studied as a function of the laser parameters. A stronger dependence of KTh and ηK as a function of the laser fluence than on pulse duration or laser intensity has been observed. The contribution of another nonlinear mechanism, besides resonance absorption, to hard x-ray enhancement has been demonstrated via hot electron angular distribution and particle-in-cell simulations.

Dependence of hard x-ray yield on laser pulse parameters in the wavelength-cubed regime

Conversion efficiency and electron temperature scaling laws are experimentally studied in the wavelength-cubed (λ3) regime, where a single-wavelength focus allows low energy pulses incident on a Mo target to produce x rays with excellent efficiency and improved spatial coherence. Focused intensity is varied from 2×1016 to 2×1018 W/cm2. Conversion efficiency and electron temperature are best described by a power law for energy scaling while an exponential law best describes the scaling of these parameters with pulse duration.

Physical, Performance, and Dosimetric Characteristics of the Δ-Scan 50 Whole-Body/Brain Scanner

The Δ-Scan 50 computed tomographic (CT) scanner has the ability to determine x-ray attenuation coefficients with a precision ranging from 0.3 to 2.6% depending upon such factors as patient size and scan diameter. Resolution is 20–80% greater than the pixel width, with values ranging from 1.2 to 2.1 mm. The collimating system determines certain characteristics of the image and affects the dose to the patient: doses for typical procedures are on the order of 1 rad, and gonadal doses are in the millirad range. Potential improvements are discussed.

High resolution hard x-ray spectroscopy of femtosecond laser-produced plasmas with a CZT detector

We present measurement of characteristic Kα emission from Mo, Ag, and La targets irradiated by a 60 fs, 600 mJ, 10 Hz Ti:sapphire laser pulse at 1017–1019 W/cm2. These x-ray emissions can potentially be used in applications from laser-based hard x-ray sources to x-ray mammography so detailed knowledge of the spectra is required to assess imaging of the figure of merit. We show here that high resolving hard x-ray spectroscopy can be achieved, with resolving powers (E/ΔE) of 60 at 18 keV, with cadmium–zinc–telluride detection system. The Kα conversion efficiency from the laser light to the Kα photon was optimized thanks to this diagnostic and values as high as 2×10−5 were obtained.

Laser-based intense hard x-ray source for mammography

Characteristic Kα emissions from Mo, Ag and La targets irradiated by 60 fs, 600 mJ, 10 Hz Ti: Sapphire laser pulse at 1017 W/cm2 - 1019 W/cm2 can be potentially used in x-ray mammography. We have investigated x-ray spectra created by this novel x-ray source in this context. All the obtained spectra exhibited a dominating narrow emission lines with only a small portion of x-ray emission in Bremsstrahlung. Such spectra might be very usful in mammography and might improve contrast and dose utilizaion, as compared to a conventional mammographic x-ray tube. The effective focal spot size was of the order of 50 μm, i.e. significantly smaller than in conventional mammography. In contradiction to conventional mammography the effective x-ray focal spot size and the effective dose remained constant across the field of view. Kα conversion efficiency, from laser light to x-rays, was optimized and values as high as 2 x 10-5 have been obtained.

Experimental and theoretical optimization of laser-produced x-ray spectra for vascular imaging

Experimental and theoretical studies of image quality using iodinated contrast agent and x-ray spectra generated by laser- based x-ray source were performed. A TableTop Terawatt (T3) laser (intensity: 1017 - 1019 W/cm-2, pulse duration: 150 fs or 450 fs, with or without controlled pre-pulse) was used to crate x-ray source. Infrared and/or green beams were utilized. Ba, La, Ce, Nd, and Gd laser targets were used. For each target, a number of suitable filters was utilized to produce optimized x-ray spectra for a specific imaging task. The MTF function due to the focal spot was obtained. A simple theoretical model of x-ray detector response was developed. An index of image quality (Detective Image Quality) as well as a figure of merit for dual energy imaging FOM(DESA) were defined and optimized via x-ray spectrum manipulation. The optimum, for a specific imaging task, technique parameters such as: target/filter combination, focal spot size, laser-light wavelength and surface power density, laser pulse duration, pre-pulse delay and contrast ratio, and hot electrons temperature were obtained experimentally and confirmed theoretically. We found that an optimized laser-based x-ray source can outperform conventional x-ray tube-based source in application to vascular imaging in terms of contrast resolution and spatial resolution.

Development of novel ultrafast-laser-based micro-CT system for small-animal imaging

We investigated the performance of an ultrafast-laser-based X-ray source as a possible replacement of a microfocal X-ray tube in a micro-CT system for small-animal imaging. Using a number of solid targets (Ge, Mo, Ag, Sn, BaF/sub 2/, La, and Nd) with matching filters, we optimized conditions for X-ray generation and measured X-ray spectra, conversion efficiency, X-ray fluence, and X-ray focal-spot size. We obtained images of small animals. X-ray spectra created by ultrafast laser are advantageous for micro-CT imaging because most of the emission is in narrow characteristic lines. The spectra could be rapidly changed and matched to the imaging task (e.g. animal thickness and density). This novel X-ray source can be also easily applied in dual-energy micro-CT for small-animal imaging with suitable contrast agent (e.g. I-, Ba-, or Gd-based) and matching targets and filters for low- and high-energy beams. We have established that the effective X-ray focal-spot size can be smaller than 5 /spl mu/ and that the average power can surpass the power delivered by a microfocal X-ray tube with 5 /spl mu/m focal-spot size.

Hard x-ray emission from intense femtosecond laser-solid target interactions

Hard x-ray (8-100 keV) spectrum emission from plasma produced by femtosecond laser solid target interactions and Kα x-ray conversion efficiency have been studied as a function of laser intensity (1017 W/cm2 ~ 1019 W/cm2), pulse duration (70 fs ~ 400 fs), laser pulse fluence and laser wavelength (800 nm and 400 nm). The Ag Kα x-ray conversion efficiency produced by a laser pulse at 800 nm with an intensity I = 4x1018 W/cm2 can reach 2x10-5. We discuss the behavior of Kα conversion efficiency scaling laws as a function of the laser parameters. We found that the Kα x-ray conversion efficiency is more dependent on laser fluence than on pulse duration or laser pulse intensity. The conversion efficiency exhibits a similar value at I ~ 1x1018 W/cm2 when we work with a high contrast laser pulse at 400 nm or with a low contrast laser pulse at 800 nm, but in the first case it presents a higher scaling law. Consequently, the use of 400 nm laser pulses could be an effective method to optimize the Kα x-ray emission via vacuum heating mechanisms.

Experimental and theoretical studies of dual energy subtraction angiography (DESA) performed using laser-based x-ray source

Two types of x-ray sources for dual energy subtraction angiography (DESA), laser-based and conventional, were investigated. A Tabletop Terawatt laser was used to create x-ray source with Ba, La, Nd, Gd, and Ce targets. A theoretical model of image quality was developed. A Figure of Merit, FOM equals SNR./(integral dose)1/2, was obtained. Images of an angiographic contrast detail phantom were obtained using laser-driven x-ray source in DESA regime and a standard angiography unit in DSA regime. The log-signals due to Iodine contrast agent in the images were measured and compared with the theoretical model predictions. The integral dose was estimated. We found that the La and Ba lines extracted by a monochromator are optimal for imaging Iodine contrast with laser-based DESA. In this case, SNR exhibits three- to five-fold improvement, as compared to SNR expected for a tube-based DESA system. Consequently, dose utilization, as defined by FOM, improves by factor of two to three, depending on patient thickness and scatter conditions. When only filters are used, SNR and FOM due to laser-based system are comparable to those due to tube-based DESA. In this case, preferable target/filter combination for the laser system is Ba/I and Ce/Nd for the low- and high-beam, respectively.