Photon Beam Size Research Articles

Monte Carlo calculation of detector perturbation and quality correction factors in a 1.5 T magnetic resonance guided radiation therapy small photon beams

Objective. With future advances in magnetic resonance imaging-guided radiation therapy, small photon beams are expected to be included regularly in clinical treatments. This study provides physical insights on detector dose-response to multiple megavoltage photon beam sizes coupled to magnetic fields and determines optimal orientations for measurements. Approach. Monte Carlo simulations determine small-cavity detector (solid-state: PTW60012 and PTW60019, ionization chambers: PTW31010, PTW31021, and PTW31022) dose-responses in water to an Elekta Unity 7 MV FFF photon beam. Investigations are performed for field widths between 0.25 and 10 cm in four detector axis orientations with respect to the 1.5 T magnetic field and the photon beam. The magnetic field effect on the overall perturbation factor (P MC) accounting for the extracameral components, atomic composition, and density is quantified in each orientation. The density (P ρ ) and volume averaging (P vol) perturbation factors and quality correction factors () accounting for the magnetic field are also calculated in each orientation. Main results. Results show that P vol remains the most significant perturbation both with and without magnetic fields. In most cases, the magnetic field effect on P vol is 1% or less. The magnetic field effect on P ρ is more significant on ionization chambers than on solid-state detectors. This effect increases up to 1.564 ± 0.001 with decreasing field size for chambers. On the contrary, the magnetic field effect on the extracameral perturbation factor is higher on solid-state detectors than on ionization chambers. For chambers, the magnetic field effect on P MC is only significant for field widths <1 cm, while, for solid-state detectors, this effect exhibits different trends with orientation, indicating that the beam incident angle and geometry play a crucial role. Significance. Solid-state detectors’ dose-response is strongly affected by the magnetic field in all orientations. The magnetic field impact on ionization chamber response increases with decreasing field size. In general, ionization chambers yield closer to unity, especially in orientations where the chamber axis is parallel to the magnetic field.

Time- and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser

Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of nonequilibrium electronic processes, transient states in chemical reactions, or the interplay of electronic and structural dynamics offers fascinating opportunities for future research. Combining valence-band and core-level spectroscopy with photoelectron diffraction for electronic, chemical, and structural analyses requires few 10 fs soft X-ray pulses with some 10 meV spectral resolution, which are currently available at high repetition rate free-electron lasers. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines free-electron laser capabilities together with a multidimensional recording scheme for photoemission studies. We use a full-field imaging momentum microscope with time-of-flight energy recording as the detector for mapping of 3D band structures in (kx, ky, E) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 Å-1 diameter in a binding-energy range of several eV, resolving about 2.5 × 105 data voxels simultaneously. Using the ultrafast excited state dynamics in the van der Waals semiconductor WSe2 measured at photon energies of 36.5 eV and 109.5 eV, we demonstrate an experimental energy resolution of 130 meV, a momentum resolution of 0.06 Å-1, and a system response function of 150 fs.

FEL performance achieved at PAL-XFEL using a three-chicane bunch compression scheme.

PAL-XFEL utilizes a three-chicane bunch compression (3-BC) scheme (the very first of its kind in operation) for free-electron laser (FEL) operation. The addition of a third bunch compressor allows for more effective mitigation of coherent synchrotron radiation during bunch compression and an increased flexibility of system configuration. Start-to-end simulations of the effects of radiofrequency jitter on the electron beam performance show that using the 3-BC scheme leads to better performance compared with the two-chicane bunch compression scheme. Together with the high performance of the linac radiofrequency system, it enables reliable operation of PAL-XFEL with unprecedented stability in terms of arrival timing, pointing and intensity; an arrival timing jitter of better than 15 fs, a transverse position jitter of smaller than 10% of the photon beam size, and an FEL intensity jitter of smaller than 5% are consistently achieved.

Symmetry-broken electronic structure and uniaxial Fermi surface nesting of untwinned CaFe2As2

We used angle-resolved photoemission spectroscopy to make direct measurements of the electronic structure of the untwinned uniaxial state of CaFe${}_{2}$As${}_{2}$, the parent compound of an iron-based superconductor. The very small photon beam size, combined with the relatively large single-domain area on the crystal surfaces, allowed us to obtain the intrinsic symmetry-broken dispersions and Fermi surface (FS) geometries along the orthogonal Fe-Fe bond directions without any mechanical or magnetic detwinning processes. Comparing the optimized local density approximation calculations, an orbital-dependent band shifting is introduced to obtain better agreement, which is consistent with the development of orbital ordering. More interestingly, unidirectional straight and flat FS segments are observed near the zone center, which indicates the existence of a unidirectional charge density wave order. Our results indicate strong electronic anisotropy in CaFe${}_{2}$As${}_{2}$ and put strong constraints on theories for the iron-pnictide system.

Comparison of Dw measurements by alanine and synthetic diamond dosimeters in photon beams with 1 cm × 1 cm field size

For photon beam sizes lower than 2 cm × 2 cm no absorbed-dose-to-water (Dw) primary standards are available for determining the possible influence of field size on the dosimeter response. In this work, Dw measurements performed in 1 cm × 1 cm photon beams at PTB using alanine and synthetic diamond dosimeters were compared in order to assess the reliability of the two dosimetric systems in such a small field size. The ratio of Dw values measured by diamond dosimeters to Dw values measured by alanine was 0.991 ± 1.5% (1 SD) and 1.007 ± 1.3% (1 SD) for 6 MV and 10 MV photon beams, respectively. This shows that both systems are capable of accurately measuring Dw in small fields.

A combined dose calculation and verification method for a small animal precision irradiator based on onboard imaging

Novel small animal precision microirradiators (micro-IR) are becoming available for preclinical use and are often equipped with onboard imaging (OBI) devices. We investigated the use of OBI as a means to infer the accuracy of the delivered treatment plan. Monte Carlo modeling of the micro-IR including an elliptical Gaussian electron beam incident on the x-ray tube was used to score dose and to continue photon transport to the plane of the OBI device. A model of the OBI detector response was used to generate simulated onboard images. Experimental OBI was performed at 225 kVp, gain∕offset and scatter-glare were corrected. Simulated and experimentally obtained onboard images of phantoms and a mouse specimen were compared for a range of photon beam sizes from 2.5 cm down to 0.1 cm. Simulated OBI can be used in small animal radiotherapy to determine if a treatment plan was delivered according to the prescription within an uncertainty of 5% for beams as small as 4 mm in diameter. For collimated beams smaller than 4 mm, beam profile differences remain primarily in the penumbra region of the smallest beams, which may be tolerable for specific preclinical micro-IR investigations. Comparing simulated to acquired OBI during small animal treatment radiotherapy represents a useful treatment delivery tool.

A high-pressure polarized 3He gas target for the High Intensity Gamma Source (HI [formula omitted]S) facility at Duke Free Electron Laser Laboratory

A new polarized 3He target has been developed for nuclear physics experiments utilizing the High Intensity Gamma Source (HI γ S) at the Duke Free Electron Laser Laboratory (DFELL). This target is designed for a broad program of physics employing high intensity polarized photons and a polarized 3He target as an effective polarized neutron target. We report the performance of this target based on the technique of spin-exchange optical pumping (SEOP). Due to the significantly larger photon beam size at the HI γ S facility than that of a typical electron beam at electron accelerator laboratories, the HI γ S 3He target contains a larger number of 3He nuclei than any polarized 3He targets reported previously. A target polarization on the order of 33–46% has been achieved from both a regular 3He–Rubidium (Rb) target and a hybrid target which contains a mixture of Rb and potassium. The target polarization has been determined within 3 % using both the Electron Paramagnetic Resonance (EPR) technique and the NMR–AFP (Adiabatic Fast Passage) technique.

Linearity tests of silicon photodiodes for EUV radiation

Photodiodes are used as easy-to-operate detectors in the extreme ultraviolet (EUV) spectral range. The Physikalisch-Technische Bundesanstalt (PTB) calibrates photodiodes with a 0.3% or better relative uncertainty for the spectral responsivity. These calibrations are based on the comparison of the photodiodes to a cryogenic electrical substitution radiometer as primary detector standard using monochromatized synchrotron radiation. The linearity of the diode signal with incident radiant power was studied with EUV radiation. The linearity of the photodiodes was tested with quasi-dc illumination for different photon beam spot sizes. A systematic and significant variation of the maximum external photocurrent of the diodes with the photon beam spot size was found. The maximum current in linear operation without bias voltage (less than 1% relative saturation) decreased from about 3 mA for a 6 mm photon beam diameter to 0.2 mA for a 0.25 mm diameter. The corresponding irradiance increased from 30 mW/cm 2 for the 6 mm aperture to about 2 W/cm 2 for the 0.25 mm aperture. This behaviour is attributed to a change in the effective serial resistance with the photon beam size. The values derived from the saturation measurement vary between 65 Ω for a 6 mm and 540 Ω for a 0.25 mm diameter beam. For spot sizes much smaller than the diode's active area, the serial resistance scales logarithmically with the spot size.

Construction and performance of a figure-8 undulator

The figure-8 undulator is a novel insertion device adopted as a light source at the soft x-ray photochemistry beamline of the Super Photon ring-8GeV. It was designed to produce linearly polarized soft x rays between 100 and 5000 eV with low on-axis power density. Magnetic fields of the constructed device were measured with compensation for the planar Hall effect using two different methods. After magnetic-field corrections, it was found that the first-harmonic intensity and the on-axis power density calculated using the measured fields were almost the same as ideal ones. After installation in the storage ring, a power-distribution pattern was observed by a fluorescent screen and it was found that the shape corresponded to expectation. The quantity of fluctuation of the photon-beam axis was estimated by measuring spectra at various observation angles and it was found to be small compared to the photon-beam size. In addition, it was also found that the measured spectrum at the gap of 77 mm agreed well with the calculated one.

An ultrafast photon beam size monitor utilizing the nonlinearity of photodiode detectors

This study presents a method capable of monitoring the variation of photon beam size at ultrahigh speed, e.g. one beam size measurement per microsecond. At high photon intensity, photodiode response nonlinearity occurs due to the saturation effect. The nonlinearity of the response of a fast photodiode is utilized to monitor the photon beam size variation at ultrahigh speed. Herein we present the theory, the computer simulation results and the experimental evidence for the method.

Properties of a submicrometer x-ray beam at the exit of a waveguide

This report discusses the properties of a 13-keV submicrometer x-ray beam exiting from a waveguide. Waveguides for this spectral regime can be constructed by enclosing a low-absorbing material between highly absorbant metals. Best performance is found for about 0.1 μm guiding layer thickness. Measurements of the photon beam size close to the exit and of the intensity distribution far from the exit will be presented. From these data one derives a beam size at the exit which is identical to the guiding layer thickness. This number being in the submicrometer range offers interesting perspectives for microscopy experiments in the hard x-ray range.

Photodesorption from stainless steel, aluminum alloy and oxygen free copper test chambers

Photodesorption yields due to synchrotron radiation (SR) from type 316 stainless steel, type A6063 aluminum alloy and high purity oxygen free copper (ASTM Class-1 OFC) test chambers were measured. SR with critical energy of 4 KeV from the 2.5 GeV storage ring of the Photon Factory at KEK was used. The photon beam size is 0.385 mrad in both vertical and horizontal directions. Seven test chambers which have the same shape were tested. The yields of OFC finished with machining is reduced to 9 × 10 −6 molecules photon −1 (N 2 equivalent) which is the lowest value throughout the experiments. Stainless steel treated with electrolytic polishing and 48h prebaking at 450°C shows the second lowest yield. The yield of aluminum alloy is somewhat higher than the others. The yields for other gas species were also measured.