pulsed plasma soft x-ray source in biomedical research

Czech Technical University in PragueFaculty of Biomedical EngineeringDepartment of Natural Sciences

PULSED PLASMA SOFT X-RAY SOURCEIN BIOMEDICAL RESEARCH

by

PETR BRUZA

Thesis Supervisor: Doc. RNDr. Vlastimil Fidler, CSc.Ph.D. Programme: Biomedical and Clinical Technology

Thesis statement submitted tothe Faculty of Biomedical Engineering,Czech Technical University in Prague,

in partial fulfilment of the the degree Doctor of Philosophy.

Prague, February 2014

iii

The Doctoral thesis was produced in the full-time Ph.D. study atthe Department of Natural Sciences of the Faculty of BiomedicalEngineering of the CTU in Prague.

Candidate: Ing. Petr BruzaCTU in PragueNam. Sıtna 3105, 272 01 Kladno, Czech Republic

Supervisor: Doc. RNDr. Vlastimil Fidler, CSc.CTU in PragueNam. Sıtna 3105, 272 01 Kladno, Czech Republic

Brown University324 Brook Street, Providence, RI 02912, USA

Opponents: Doc. Ing. Ladislav Pına, DrSc.CTU in PragueBrehova 7, 115 19 Praha, Czech Republic

Ing. Karel Polak, CSc.Insitute of Physics, AS CRCukrovarnicka 10/112, 162 00 Praha, Czech Republic

The Doctoral thesis statement was distributed on . . . . . . . . . . . . . . . . . . . . .

The defence of the Doctoral thesis will be held on . . . . . . . . . . . . . . . . . . . . .at . . . . . . . . . . . . in front of the Board for the Defence of the Doctoral The-sis in the Doctoral programme Biomedical and Clinical Technology inthe room No.. . . . . . of the Faculty of Biomedical Engineering of the CTU,Nam. Sıtna 3105, 272 01 Kladno, Czech Republic. Those interested mayget acquainted with the Doctoral thesis concerned at the Dean Office ofthe Faculty of Biomedical Engineering of the CTU in Prague, Nam. Sıtna3105, 272 01 Kladno, Czech Republic.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .prof. Ing. Peter Kneppo, DrSc.

Chairperson of the Board for the Defence of the Doctoral ThesisFaculty of Biomedical Engineering of the CTU in Prague

Nam. Sıtna 3105, 272 01 Kladno, Czech Republic

Contents

1 Introduction 1

2 Goals 2

3 State of the art 3

4 Methods 6

5 Results and Discussion 8

6 Summary 16

List of Publications 17

References 20

7 Resume 25

iv

Introduction

This work presents the applications of the laser-driven soft X-ray plasmasource in two biomedical-related research directions. The common coreof both research directions lies in the improvement of medical soft- andhard X-ray imaging by both the development of the novel multi-modalX-ray imaging scheme and by research of the new scintillators for X-raydetectors.

The first direction pursues a multi-modal X-ray imaging scheme calledSpatial Harmonic Imaging (SHI). SHI permits the computational de-composition of the images acquired with a single exposure into an x-rayabsorption image, an isotropic scattering image, and a differential phasecontrast image. This technique is capable of greatly enhancing the im-age information obtained by e.g. the medical X-ray imaging systemsand soft X-ray microscopy. Our contribution to the research lies in themathematical description and verification of the SHI technique. The ver-ification was allowed exclusively by using the soft X-ray radiation. Wealso implement the SHI method in the lens-less soft X-ray microscopyand demonstrate its unique capabilities.

The second direction is oriented toward the measurements of lumines-cence kinetics of scintillators, which are excited by pulsed soft X-ray radi-ation. Scintillators as photon energy converters constitute the foremostX-ray detection component in a wide variety of medical X-ray imag-ing systems. Thus, their properties are crucial for performance of suchdevice. The high-speed or coincidence event based medical imaging sys-tems set severe requirements on temporal response of the scintillator.Thus, the investigation of luminescence kinetics is of main importancein the development of new scintillation materials. Our contribution tothis research consists in development and application of a new spec-troscopic technique, which would allow us to measure the luminescencedecay within a very wide temporal range (> ms), with nanosecond tem-poral resolution and very high signal-to-noise ratio (> 105). We meetthese requirements by exploiting a combination of an intense nanosec-ond soft X-ray excitation pulse, a very short absorption length of thesoft X-rays, and a fast photomultiplier-based detection.

1

Goals

1. Soft X-ray source

• To characterize the emission properties of the laser-driven plasmasoft X-ray source

2. Soft X-ray Spatial Harmonic Imaging (SHI)

• To describe how the absorption, scattering and phase shift in theobject are encoded into the harmonic image;

• To derive the optical transfer function of a scattering object;

• To derive and experimentally verify the theoretical framework ofSpatial Harmonic Imaging;

• To describe the SHI reconstruction of absorption, scattering andphase contrast images;

• To implement and demonstrate the image enhancement of the SHItechnique in projectional soft X-ray microscopy.

Soft X-ray excited luminescence spectroscopy of scintillators

• To develop and characterize a novel spectroscopic technique forhighly sensitive measurements of spectrally resolved luminescencekinetics;

• To study the emission spectra and luminescence kinetics of ZnO:Gananopowder scintillator;

• To study the luminescence kinetics of doped LiCaAlF6 crystals forprospective dosimetric systems in boron neutron capture therapy;

• To propose appropriate theoretical models and perform the re-convolution analysis of the experimentally measured luminescencedecay profiles;

• To infer the possible types of decay pathways and various scintil-lator defects.

2

State of the art

3.1 Soft X-ray Spatial Harmonic Imaging

The development of the new X-ray imaging techniques is focused onenhancement of the obtained information about the object alongsidewith the reduction of the dose deposited to the object (patient). Thegrating-based interferometric imaging methods [1] have demonstratedquantitative differential phase contrast [2, 3] and scattering contrast [4]using conventional x-ray sources. The neccesity of taking several expo-sures with a simultaneous precise displacement of the analyzer grid ischaracteristic to all of the so-called phase-stepping techniques [2]. Suchneccesities, however, make the image acquisition lengthy and elaborate.In the work [5, 6], the phase-stepping was omitted. Nevertheless, thesetechniques rely on very precise period and phase matching of the detectorpixels to the interference pattern, which render them less robust.

The Spatial Harmonic Imaging (SHI) overcomes the aforementioned dis-advantages. The first theoretical approach on SHI method was presentedin [7, 8]. More rigorous theoretical concept of SHI contrast formationwas formulated relatively recently [9]. In the same work the mathemati-cal model was found in fundamental agreement with measured profiles inhard X-ray regime. The published calculations [9] of scattering and dif-ferential phase contrast images show a strong heterodyne enhancementof phase contrast features induced by the boundary of the material ofcomplex refractive index with vacuum. However, the analytical descrip-tion of the SHI image contrast formation without the neccesity of electricfield calculation was found after consideration of several approximations,valid only for hard X-ray radiation. The initial work of SHI used in invitro biomedical imaging was published in [7], later in [10, 8, 11, 12]. In[13], the gold nanoparticles were successfully used as a contrast agentfor in vitro SHI imaging of hepatocellular carcinoma cells. In [14], theSHI was suggested as a time-resolved, chemically sensitive x-ray imagingvariant to various scanning x-ray diffraction or absorption spectroscopysystems. Recently, the SHI method was used for imaging of the scatter-ing in the optical regime [15].

3

CHAPTER 3. STATE OF THE ART 4

3.2 Luminescence Spectroscopy of scintillators

The knowledge of luminescence properties of scintillation crystals is cru-cial for determining their performance and provides feedback for theirchemical composition in preparation phase. Conventional γ-ray or pi-cosecond X-ray tube excitation in combination with time correlated sin-gle photon counting (TCSPC) method is routinely used for excellentemission decay measurements even in sub-nanosecond time range [16].However, the performance of TCSPC systems substantially drops in thelonger time range due to the limited count rate. The synchrotron radia-tion can also be used as a source for measurements of the time-resolvedscintillator response [17]. However, to detect components with decaytime above several μs a sophisticated high-speed shutter system [18]must be used due to the high repetition rate of synchrotron pulses. An-other approach was realized with pulsed UV excitation of UV-positionedband edge materials, such as PbWO4 [19]. However, in the majority ofscintillators the band edge is situated in VUV, where nanosecond pulseexcitation sources with low repetition frequency are not readily available.Moreover, to achieve excitation conditions similar to practical applica-tions, the excitation photons with energy of at least a few hundreds of eVare necessary to develop well the initial conversion stage [20]. To our bestknowledge, there is no experimental work describing the time-resolvedluminescence spectrometry with a dedicated table-top soft X-ray excita-tion source.

Scintillators in Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is a biochemically targeted ra-diotherapy based on the nuclear capture and fission reactions that occurwhen non-radioactive 10B isotope, is irradiated with epithermal neu-trons (0.414 eV < E < 10 keV) yielding alpha particles and recoilinglithium-7 nuclei. The fission products subsequently damage the sur-rounding cancerous tissue [21]. Besides the aspects of specificity of thebiochemical label, the success of the neutron therapy relies on a detailedquantification of the spectra and temporal stability of the neutron flux[22]. Only a good quantification of the neutron flux properties can yielda precise physical dosimetric characterization. The current dosimetrictechniques include the ionization chamber dosimeters, 3He proportionalcounters, polymerized plastic scintillators, thermoluminescent dosime-ters, germanium semiconductor detectors and few others [23]. However,

CHAPTER 3. STATE OF THE ART 5

these methods either do not allow for on-line monitoring of neutron flux,have poor n/γ resolution and efficiency (plastic scintillators) [24], orare not scalable due to scarcity of the essential material (3He). Anothergroup of solid neutron scintillators is based on neutron capture at 6Li iso-tope, following the reaction 6

3Li +10 n −−→ 3

1H(2.05MeV)+α (2.73MeV).Recently, a novel rare-earth doped, 6 Li enriched neutron scintillatorLiCaAlF6 (LiCAF) has been developed [25, 26, 27] and patented [24].The applicability of Ce- and/or Eu-doped LiCAF for thermal neutrondetection was discussed in [28] and demonstrated in [29, 30]. A goodscintillation response of Eu-doped LiCAF under thermal neutron expo-sure was presented in [31].

Scintillators in Time-of-flight Positron Emission Tomography

The Positron Emission Tomography (PET) is a functional imagingmodality, based on coincident detection of pairs of γ-photons emittedafter positron annihilation in neutron-defficient radionuclides (e.g. 11C,15O, 18F)[32]. The γ-photons are emitted in opposite direction from thepoint of annihilation, creating a so-called line of response (LOR). The2D and 3D tomographical reconstruction is allowed by detection of manyLORs by a ring of detectors, surrounding the patient. The detectors arethe key component of the system and comprise of a scintillator blockscoupled to UV/VIS-sensitive photodetectors (photomultiplier tubes, sil-icon photomultipliers, photodiodes). The Time of Flight Positron Emis-sion Tomography (TOF PET) modality uses the difference of arrivaltimes of the gamma photons on the ring detector to further enhancethe spatial resolution. However, it imposes severe requirements on theluminescence kinetics of the scintillator. While the scintillator geometry[33] and electronics [34] is equally important, we focus on the investiga-tion of an optimal TOF PET scintillator. The ideal scintillator shouldexhibit a picosecond rise time, sub-nanosecond, single-exponential decay(no delayed emission), high density and low self-absorption. A recentprogress in manufacturing of ZnO scintillating transparent ceramics [35]and glass ceramics [36] reestablished the interest in this material for de-manding applications such as TOF PET [37]. The temporal response ofthe ZnO:Ga was estimated to be an order of magnitude faster than thecurrent TOF PET detectors (LSO, LFS, LuAP etc.) [32]. In our studywe use the ZnO:Ga nanopowder, prepared by the patented UV inducedprecipitation of solid phase from aqueous solutions [38].

Methods


The typical experimental setup for in-line soft X-ray SH imaging andthe SHI reconstruction procedure is depicted in Fig. 4.1.

10

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Algebraic manipulationFourier Analysis Fourier Synthesis

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x

y

x

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Figure 4.1: The typical experimental setup for in-line SH imaging (top); the funda-mental steps of the discrete two-dimensional spatial harmonic analysis [7], performedon the image of the grid with object (bottom).

The process of spatial harmonic analysis consists of three consecutivesteps: 1) Fourier analysis , i.e. the forward Fourier transform of theexperimental images, yielding the complex spatial frequency spectra; 2)Fourier synthesis , i.e. the inverse Fourier transform of the selected(n,m)-th components of the spatial frequency spectra yielding the com-plex zero- and higher-order components of the image and 3) Algebraicmanipulation of the individual complex components to obtain the ab-sorption image A, scattering image S, or phase contrast image φ.

6

CHAPTER 4. METHODS 7

4.2 Luminescence spectroscopy of scintillators

We designed and built a table-top instrumentation, dedicated to themeasurements of the soft X-ray excited, spectrally resolved luminescencekinetics of scintillation materials [A.1, A.2]. The schematic setup of thespectrometer is depicted in Fig. 4.2. A nanosecond pulse of soft X-rayradiation is used to initiate the scintillation process. The 400 nm Tifoil was used as a soft X-ray bandpass filter. The majority of the totalSXR pulse energy is deposited in the first μm in the sample due tothe very short absorption length of the SXR radiation [39, A.1]. Theapproximate irradiance at the the scintillator is 109 ph/cm2 in singlepulse. Such high irradiance allows us to measure the luminescence decayprofiles with signal-to-noise ratio as high as 105. Owing to the shortduration of the SXR pulse (5 ns) and analog-mode detection (as will bedescribed later), the measurements can be performed with nanosecondresolution over millisecond time range. The high signal-to-noise ratio andwide temporal range of the detected profile are crucial for measurementsof the delayed luminescence signal.

IR laser, 7 ns1064 nm, 0.7 J

�� X-rays, 5 ns420 eV

Ar/N2 Plasma

Scin��tor

Ti ��ter

monochromator

photomul�lier I

t

digital storageoscilloscope

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Figure 4.2: The drawing of the developed time-resolved spectrometer, dedicated tothe luminescence decay measurements of scintillation materials.

The collected luminescence is spectrally resolved by a Czerny-Turnermonochromator (Jobin Yvon H20), and detected by a photomultipliertube (Hamamatsu R7056) detector in the direct current mode [40].The signal from the PMT is amplified by a wide-bandwidth amplifier(AD847) working as a current-to-voltage converter. The amplified signalis then digitized by a digital storage oscilloscope (Agilent DSO7104).

Results and Discussion


Verification of the SHI framework

The mathematical model of the SHI was proposed and verified by thecomparison of the calculated and measured differential phase contrast(Fig. 5.1) and scattering intensity (Fig. 5.2) profile, obtained from theSHI radiograph of the Mylar foil edge.

0 100 2000.0

0.5

1.0

1.5

Norm.Intensity(a.u.)

x (�m)

Measured ISimulated ISimulated I � LSF

Figure 5.1: The comparison of the calcu-lated (solid line) and measured (dashedline) differential phase contrast (DPC)profile of the edge of Mylar foil.

1000 1500 20000.0

0.5

1.0

S(x)

x (�m)

Simulated S(x)Experimental S(x)

Figure 5.2: The comparison of thecalculated (solid line) and measured(dashed line) scattering intensity pro-file of the edge of Mylar foil.

The graph shows an excellent match of the simulated and measured pro-file with the sample Pearson correlation coefficient ρ = 0.98 and ρ = 0.87,respectively, indicating a strong correlation. The strong correlation leadsus to conclusion that mathematical model of SHI contrast formation rep-resents well the experimental conditions in our imaging setup. The smalldiscrepancies in the peripheral SHI fringes in the Fig. 5.2 might origi-nate most likely from the non-uniform opening ratio of the used grid, orfrom the limited signal-to-noise ratio of the measured intensity profiles.We also tested the ability of the spatial harmonic imaging to encode thephase shift, induced by the object in the transmitted wave. The firstderivative of the wavefront shape (Fig. 5.3a) was calculated from the

8

CHAPTER 5. RESULTS AND DISCUSSION 9

experimentally obtained image, and further cumulatively integrated toobtain the shape of the wavefront in the area around the projected edgeof the Mylar object (Fig. 5.3b).

Figure 5.3: a) the experimentally obtained profile of the first derivative of the wave-front. The Mylar object is located on the left side of the fringes. b) the integratedwavefront profile. The location and theoretical phase shift induced by the object isrepresented by the red dashed line.

Comparing the absolute phase shift of the wavefront that passed theobject with respect to the uninteracted wavefront, we get the phase shiftof 2.6± 0.1 rad. The experimentally obtained phase shift is comparableto the theoretical phase shift of 2.7 rad, proving that the SHI method isable to faithfully reconstruct the phase shift in a well defined object.

Soft X-ray SHI microscopy

To demonstrate the capabilities of the lens-less SXR SHI microscopy, weimaged a dried, free standing cryo-microtome slice of the tendo calcaneusof Norway rat. The thickness of the cut was approx. 5 μm in native state.

Figure 5.4: Left: SXR images of tendon sample with superimposed 2000 lpi grid andits corresponting spatial frequency spectrum. Right: image of the grid only and itsspatial frequency spectrum. Width of the scale bar 100 μm.


The measured sample-with-grid image and grid-only image (see Fig. 5.4)were subject to the SHI Fourier analysis. Fig. 5.5 shows the resultingscattering images and differential phase contrast images for both vertical(0,1) and horizontal (1,0) directions.

Figure 5.5: Postprocessed a) horizontal and b) vertical scattering images S calculatedfrom (0,1) and (1,0) orders of frequency spectra, respectively. The postprocessed c)horizontal and d) vertical components of the spatial phase shift of the grid image,calculated from (0,1) and (1,0) orders of frequency spectra, respectively. The widthof the scale bar: 200 μm.


The capabilities of SHI imaging are best shown in Fig. 5.6 as comparisonof the simple projection radiograph (Fig. 5.6a) and a scattering image(Fig. 5.6b), using the same experimental conditions. To distinguish theangular-dependence of structured scattering, the (0,1) and (1,0) SHIimages were color-coded and mutually combined.

Figure 5.6: Comparison of the a) projectional SXR radiograph of the tendon sampleand b) the color coded SFHI image. Color circle represents the 2D lookup table withradius corresponding to linear intensity and color coded angle of scattering. Widthof the scale bar 100 μm.

Fig. 5.6 clearly shows the enhanced subjective visibility of collagen fibersin the SHI image. The main scattering features in the sample are thecollagen fibrils circumscribing the cavities (so called lacunae) createdby the chondrocytes. The apparently accentuated visibility and size oflacunae relative to those observed in the stained histological sections[41] points to the structural changes of the collagen fibers. The changesoccured most probably during the cryofixation and drying of the sample.Such deterioration and disintegration of the collagenous structures is,however, inevitable in cryopreparation process [42].

Owing to the directional sensitivity of the SHI imaging, we can resolvethe preferential orientation of the collagen fibers. Since we used the or-thogonal square grid, we obtained the orthogonal pairs of the scatteringand phase contrast images. Comparing the histograms of the scatteringintensity of both horizontal and vertical scattering images, we observedan enhanced signal in the horiontal image (calculated from the (1,0)spectral component). This led us to conclusion that a greater fractionof the collagen fibers are aligned in the horizontal direction.


5.2 Luminescence spectroscopy of scintillators

Spectrally resolved luminescence decay kinetics of ZnO:Ga

We measured the soft X-ray excited luminescence spectra and decay ki-netics of two materials with input Ga dopant concentration of 3.24×10−5

mol/dm3 and 1.50× 10−5 mol/dm3 (sample annotation 6-2g 1000R and6-2b 1000R, respectively). The decay profiles of both samples measuredat emission wavelength λem = 390 nm are shown in the Fig. 5.7. The 800ps characteristic decay time of ZnO:Ga [43] is shorter compared to ourIRF with estimated FWHM of 7.4 ns. We deduced that the measuredluminescence decay of ZnO:Ga might represent an actual instrumentresponse function with less than 1% error.

10-8 10-7 10-6 10-5 10-410-6

10-5

10-4

10-3

10-2

10-1

100

NormalizedIntensity(a.u.)

Time (s)

ZnO:Ga 6-2b-1000RZnO:Ga 6-2g-1000R

Figure 5.7: The luminescence decay F (t, λ) of ZnO:Ga nanopowder samples 6-2g1000R (red) and 6-2b 1000R (black), measured at emission wavelength λem = 390nm. The peak signal around 400 ns is an instrumental artifact. The increased noise ofthe averaged 6-2g 1000R curve is due to lower number (128) of repeated acquisitions.

We did not observe any effects of different Ga dopant concentrations onthe luminescence kinetics of ZnO:Ga.

The normalized, time-integrated emission spectrum of the 6-2g 1000Rsample is shown in the Fig. 5.8. For a comparison, the typical spec-


tral response of the TOF PET silicon photomultiplier (SiPM) detector(Hamamatsu S10362-11-025U) is also shown in the Fig. 5.8.

Figure 5.8: The time-integrated emission spectrum F (λ) of ZnO:Ga nanopowder,sample no. 6-2g 1000R (black dots); the typical spectral sensitivity of the TOF PETSiPM detector Hamamatsu S10362-11-025U (blue line). The emission spectrum ofZnO:Ga was corrected to the spectral sensitivity of the detector.

The central wavelength of the emission band is crucial to a lossless cou-pling with the detector. According to Fig. 5.8, the measured centralemission wavelength of 380 nm is well-matching the spectral response ofthe majority of suitable ultrafast semiconductor APD or SiPM detectors[44], peaking usually around 420 nm [45]. Moreover, we did not observeany delayed components that could lead to increased unwanted randomevent registration and longer system dead time. Thus, the ZnO:Ga hasfavourable emission properties for applications relying on the fast detec-tion and short dead time.


Scintillation characteristics of LiCaAlF6-based single crystals

The measured, soft X-ray excited decay kinetics of undoped, and Ce-and Eu-doped LiCaAlF6 single crystals are shown in the Fig. 5.9.

Figure 5.9: Scintillation decay of (a) undoped LiCAF, (b) Ce-doped LiCAF, and (c)Eu-doped LiCAF under nanosecond soft X-ray excitation at RT. The solid line is thefit I(t) of Eq. (1) to the data. The dashed line represents the instrument responsefunction. The parameters used in the fit are listed in Table I. The peak at about 300ns in (b) is an experimental artifact.

The kinetic model of the LiCAF-based crystal scintillators was approx-imated by a model function, containing two exponential terms and oneinverse power-law term. The exponential terms represent the time evo-lution of level population at two excited state levels or at one excitedstate and one shallow trap level. The inverse power-law term representsan additional tunneling [46] and/or other recombination process comingfrom the transfer stage of scintillation mechanism [A.3, 47].


Despite the very low light yield of the undoped LiCAF, we were able tomeasure a spectrally unresolved luminescence decay kinetics. The twoexponentials indicate most likely the presence of two different emissioncenters, which would also be consistent with previous findings of twoemission bands [48]. In previous study [48], a very fast luminescencecomponent was observed and attributed to either the excitonic emissionor to intraband transitions of hot carriers. In the case of SXR excita-tion, we observed this fast component but only with very low overallcontribution of few per cent to the total light yield. The inverse power-law component (55% contribution to the total light output) can indicatea tunneling process involved in the self-trapped exciton creation underhigh-energy excitation [A.3].

The scintillation response of LiCAF:Ce is dominated (88% of total in-tensity) by an exponential component with a decay time of 34.7 ns. Thiscomponent is characteristic for the perturbed Ce3+ centers, as reportedelsewhere [49]. The minor exponential component with contribution of5% to total intensity and the decay time of 15 μs can be present due totrapping of the migrating electrons at shallow traps [A.3]. The inversepower-law (7% of total intensity) can be present due to the tunnelingprocess present in the self-trapped exciton creation under high-energyexcitation. The fact that the major part of scintillation light is releasedin the first few tens of ns might lead to conclusion that the scintillationis very efficient. However, we observed a relatively low light yield. Itmight be caused either by trapping of the migrating charge carries indeep traps or by nonradiative recombination on crystal defects. We pro-pose the former case as more likely, considering also the findings in thethermoluminescence measurements [A.3].

The observed light yield of LiCAF:Eu was significantly higher than thatof LiCAF:Ce, which is in agreement with the literature [50]. We foundthat the exponential decay component dominates the luminescence ki-netics of the LiCAF:Eu with 98% of total emission intensity. The expo-nential decay time of 1.67 μs is the actual photoluminescence decay timeof Eu2+ in the LiCAF host [48]. Thus, we can conclude that the scin-tillation mechanism is dominated by the prompt capture of migratingcharge carriers at the Eu ion and their subsequent radiative recombina-tion process. In comparison with LiCAF:Ce, LiCAF:Eu shows a muchlower relative percentage of delayed recombination processes in the scin-tillation decay. The charge carriers are captured by Eu2+ centers moreefficiently than on Ce3+ centers.

Summary

This work has introduced the applications of the laser-driven soft X-rayplasma source a) in the novel, multi-modal X-ray imaging scheme calledSpatial Harmonic Imaging, and b) the spectroscopic technique dedicatedto the measurements of soft X-ray excited luminescence kinetics of scin-tillators. All of the defined goals were accomplished and the results werepublished in the listed publications.

Prior the imaging and spectroscopy experiments, we measured the emis-sion spectra, spatial profile and temporal profile of the soft X-ray emis-sion from the laser-driven Ar/N2 plasma source.

We have shown how the absorption, scattering and phase shift in theobject are encoded into the harmonic image. We also derived the opti-cal transfer function of a scattering object. Thus, we are able to infera meaningful material constant from the reconstructed scattering sig-nal. Further, we have derived and successfully verified the theoreticalframework of Spatial Harmonic Imaging. Theoretical simulations andexperimental results were found to be in a good agreement. We im-plemented the SHI technique in projectional soft X-ray microscopy andshown its unique capabilities on the biological object. SHI was demon-strated to deliver more detailed sample information than conventionalin-line X-ray absorption imaging.

We also developed and characterized a spectroscopic technique withpulsed soft X-ray source for highly sensitive measurements of spectrallyresolved luminescence kinetics. We measured the emission spectra andsoft X-ray excited luminescence kinetics of the ZnO:Ga powder scintilla-tor. We also measured the luminescence kinetics of undoped LiCaAlF6,LiCaAlF6:Ce and LiCaAlF6:Eu single crystal scintillators. We proposeda theoretical model including two excited state levels or one excited stateand one shallow trap levels given by exponential terms, and an additionaltunneling and/or other recombination process coming from the transferstage of scintillation mechanism given by the inverse power term. Thereconvolution analysis of the measured kinetic curves yielded the func-tion parameters of the best fit and also the relative contribution of eachterm to the total light yield. Finally, we discuss the possible types of lu-minescence decay pathways as well as origins of the scintillator defects.

16

List of Publications

[A.1] P. Bruza, V. Fidler, M. Nikl. Table-top instrumentation for time-resolved luminescence spectroscopy of solids excited by nanosecondpulse of soft X-ray source and/or UV laser. Journal of Instrumen-tation 6 (09), P09007, 2011. doi:10.1088/1748-0221/6/09/P09007,IF=1.656 (2012).

[A.2] P. Bruza, D. Panek, V. Fidler, P. Benedikt, V. Cuba, T. Gbur, P.Bohacek, and M. Nikl Applications of a Table-Top Time-ResolvedLuminescence Spectrometer With Nanosecond Soft X-ray Pulse Ex-citation. IEEE Transactions on Nuclear Science 61 (1), 2014. ISSN:0018-9499. doi:10.1109/TNS.2013.2279546, IF=1.219 (2012).

[A.3] M. Nikl, P. Bruza, D. Panek, M. Vrbova, E. Mihokova, J.A. Mares, A. Beitlerova, N. Kawaguchi, K. Fukuda, and A.Yoshikawa. Scintillation characteristics of LiCaAlF6-based singlecrystals under X-ray excitation. Applied Physics Letters 102 (16),2013. doi:10.1063/1.4803047, IF=3.794 (2012).

[A.4] P. Bruza, D. Panek, M. Vrbova, V. Fidler, and C. Rose-Petruck.Spatial frequency heterodyne imaging in the soft x-ray water win-dow. Under review: Applied Physics Letters, rating “very good”,2014. IF=3.794 (2012).

[A.5] P.W. Wachulak, A. Bartnik, A. Baranowska-Korczyc, D.Panek, P. Bruza, et al. Study of crystalline thin films andnanofibers by means of the laser-plasma EUV-source basedmicroscopy. Radiation Physics and Chemistry 93, 2013.doi:10.1016/j.radphyschem.2013.02.019, IF=1.302 (2012).

[A.6] P.W. Wachulak, A. Bartnik, H. Fiedorowicz, D. Panek, P. Bruza.Imaging of nanostructures with sub-100 nm spatial resolution usinga desktop EUV microscope. Applied Physics B 109 (1), 105-111,2012. doi:10.1007/s00340-012-5125-3, IF=1.782 (2012).

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LIST OF PUBLICATIONS 18

[A.7] F. Krejcı, J. Jakubek, M. Kroupa, P. Bruza, D. Panek. Pixeldetector Timepix operated in pile-up mode for pulsed imagingwith ultra-soft X-rays. Journal of Instrumentation 7 (12), 2012.doi:10.1088/1748-0221/7/12/C12013, IF=1.656 (2012).

[A.8] P. Vrba, M. Vrbova, P. Bruza, D. Panek, F. Krejcı, M. Kroupa,J. Jakubek. XUV radiation from gaseous nitrogen and argon tar-get laser plasmas. Journal of Physics: Conference Series 370 (1),012049, 2012. doi:10.1088/1742-6596/370/1/012049

[A.9] F Krejcı, M Kroupa, J Jakubek, P. Bruza, D. Panek Detectionof soft X-rays with the pixel detector Timepix operated as a highlysensitive dark-current free CCD-like camera. Nuclear Science Sym-posium and Medical Imaging Conference (NSS/MIC), IEEE, 2011.doi:10.1109/NSSMIC.2011.6154665

[A.10] P. Bruza XUV imaging and spectroscopy in biomedicine. Ph.D.Research Study, Faculty of Biomedical Engineering, Kladno, CzechRepublic, 2012.

Conference records

P. Bruza, D. Panek, V. Fidler, V. Cuba, L. Prochazkova, V. Jary, P.Bohacek, M. Nikl. Novel table-top instrument for time-resolved lumines-cence spectroscopy of scintillators using the nanosecond soft X-ray pulseexcitation. SCINT 2013, Shanghai, China (2013).

Bruza, P., V. Fidler, V., Nikl, M., Panek, D., Rose-Petruck, C., M.Vrbova, M., CTU Soft X-ray Imaging and Spectroscopy. SWISS 2012,Bern, Switzerland (2012).

S. Vondrova, D. Panek, P. Bruza, M. Vrbova, P. Vrba, P. Wachulak, F.Krejcı, J. Jakubek. Diagnostics and Modeling of Gas Puff Target LaserPlasma Radiation Source. 2012 International Workshop on EUV andSoft X-ray Sources, Dublin, Ireland (2012).

LIST OF PUBLICATIONS 19

Vrba, P., Vrbova, M., Bruza, P., Panek, D., Krejcı, F., Kroupa M., andJakubek, J., XUV radiation from gaseous nitrogen and argon target laserplasmas. 14th Latin American Workshop on Plasma Physics, J. of Phys.:Conference Series 370 (2012).

Krejcı, F., Jakubek, J., Kroupa, M., Bruza, P. and Panek, D. Detec-tion of soft X-rays using hybrid semiconductor pixel detector Timepixoperated as a high sensitive dark-current free CCD-like camera. IEEENuclear Science Symposium Conference Record (2011).

Bruza, P. Ongoing development of FBME Compact Soft X-Ray Micro-scope. Conference proceedings, Instruments and Methods for Biologyand Medicine (2011). FBMI, CVUT, Kladno (2011). Abstract p. 127.

Matejka, R., Bruza, P., Malakova, S.: Automation of biomedical spec-troscopy and imaging methods using Virtual Instrumentation. Confer-ence proceedings, Instruments and Methods for Biology and Medicine2011. FBMI, CVUT, Kladno (2011). Abstract p. 89.

Bruza, P. Fidler, V. and Nikl, M. Table-top instrumentation for time-resolved luminescence spectroscopy of solids excited by nanosecond pulseof soft X-ray source and/or UV laser. Second International Conferenceon Transient Chemical Structures in Dense Media, Paris, France (2010).

Publications cited by:

Lucchini, M., K. Pauwels, M. Pizzichemi, R. Chipaux, F. Jacquot, H.Mazue, H. Wolff, P. Lecoq, and E. Auffray. Response of Inorganic Scin-tillators to Neutrons of 3 and 15 MeV Energy. Trans. on Nucl. Sci. 61(1), 2014.

Y. Huajun, F. Peng, Q. Zhang, C. Guo, C. Shi, W. Liu, G. Sun etal. A Promising High-density Scintillator of GdTaO4 Single Crystal.CrystEngComm 16, 2013.

Yoshikawa, A., V. Chani, and M. Nikl. Czochralski Growth and Proper-ties of Scintillating Crystals. Acta Physica Polonica, A. 124.2 (2013).

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Resume

Tato doktorska prace prezentuje aplikace laserem buzeneho zdroje mek-keho rentgenoveho zarenı (SXR) ve dvou biomedicınckych vyzkumnychsmerech. Prınos prvnı casti spocıva v matematickem popisu, exper-imentalnı verifikaci a implementaci nove rentgenove zobrazovacı tech-niky “Spatial Harmonic Imaging (SHI)” s vyuzitım zdroje SXR zarenı.Technika SHI umoznuje vypocetnı rozklad puvodnıho obrazu, zıskanehobehem jedne expozice, na klasicky absorpcnı obraz, obraz mıry rozptyluzarenı a obraz diferencialnıho fazoveho kontrastu. Technika SHI jevyuzitelna v rentgenove mikroskopii a v medicınske ci prumyslove ra-diografii. Verifikace je vyhradne umoznena pouzitım zdroje mekkehorentgenoveho zarenı. Potvrzujeme shodu teoretickych simulacı s experi-mentalne namerenymi daty. Dale je predstavena implementace technikySHI v rentgenove projekcnı mikroskopii. Jako testovacı objekt je vyuzittenky histologicky rez slachy potkana. Dokazujeme, ze SHI technikaje schopna dodat vıce informacı o objektu, nez bezne absorpcnı zobra-zovanı. Dıky smerove citlivosti SHI techniky je napr. mozne odvoditpreferencnı orientaci kolagennıch vlaken v histologickem rezu.

Druhy vyzkumny smer se zabyva merenım kinetiky luminiscence scin-tilatoru, excitovanych pulsy mekkeho rentgenoveho zarenı. Vyzkumkinetiky luminiscence je dulezity pro vyvoj novych typu scintilatoru.Prınosem teto prace je vyvoj a aplikace nove spektroskopicke techniky scasovym rozlisenım, ktera umoznı merit kinetiku luminiscence ve velmisiroke casove skale (> ms) s nanosekundovym casovym krokem a velmivysokym odstupem urovne signalu k sumu (> 105). Tato technikaje vyuzita pros studium novych scintilacnıch materialu s velkym po-tencialem pro vyuzitı v medicınskem zobrazovanı - ZnO:Ga nanocastica dopovanych LiCaAlF6 monokrystalu. V prıpade ZnO:Ga studujemeprıtomnost pomalych luminiscencnıch procesu a vliv rozdılne koncen-trace Ga dopantu na kinetiku luminiscence. V prıpade dopovanychLiCaAlF6 scintilatoru studujeme vlastnı emisi LiCaAlF6 matrice a daleprenos energie z matrice na Ce3+ a Eu2+ dopanty behem scintilacnıhoprocesu. Z vyhodnocenı dat jsou vyvozeny zavery, popisujıcı nejpravde-podobnejsı typy zarivych a nezarivych procesu a defektu, ktere ovlivnujıprenos energie z matrice scintilatoru na emisnı centra.

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pulsed plasma soft x-ray source in biomedical research

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