Primary Beam Source Research Articles

Report of AAPM Task Group 155: Megavoltage photon beam dosimetry in small fields and non-equilibrium conditions.

Small-field dosimetry used in advance treatment technologies poses challenges due to loss of lateral charged particle equilibrium (LCPE), occlusion of the primary photon source, and the limited choice of suitable radiation detectors. These challenges greatly influence dosimetric accuracy. Many high-profile radiation incidents have demonstrated a poor understanding of appropriate methodology for small-field dosimetry. These incidents are a cause for concern because the use of small fields in various specialized radiation treatment techniques continues to grow rapidly. Reference and relative dosimetry in small and composite fields are the subject of the International Atomic Energy Agency (IAEA) dosimetry code of practice that has been published as TRS-483 and an AAPM summary publication (IAEA TRS 483; Dosimetry of small static fields used in external beam radiotherapy: An IAEA/AAPM International Code of Practice for reference and relative dose determination, Technical Report Series No. 483; Palmans et al., Med Phys 45(11):e1123, 2018). The charge of AAPM task group 155 (TG-155) is to summarize current knowledge on small-field dosimetry and to provide recommendations of best practices for relative dose determination in small megavoltage photon beams. An overview of the issue of LCPE and the changes in photon beam perturbations with decreasing field size is provided. Recommendations are included on appropriate detector systems and measurement methodologies. Existing published data on dosimetric parameters in small photon fields (e.g., percentage depth dose, tissue phantom ratio/tissue maximum ratio, off-axis ratios, and field output factors) together with the necessary perturbation corrections for various detectors are reviewed. A discussion on errors and an uncertainty analysis in measurements is provided. The design of beam models in treatment planning systems to simulate small fields necessitates special attention on the influence of the primary beam source and collimating devices in the computation of energy fluence and dose. The general requirements for fluence and dose calculation engines suitable for modeling dose in small fields are reviewed. Implementations in commercial treatment planning systems vary widely, and the aims of this report are to provide insight for the medical physicist and guidance to developers of beams models for radiotherapy treatment planning systems.

Secondary ion yields for vacuum-type electrospray droplet beams measured with a triple focus time-of-flight analyzer.

We previously developed a massive cluster ion beam gun for secondary ion mass spectrometry (SIMS) in which the primary beam source is a vacuum electrospray. The secondary ion yields produced by this method had not yet been measured with a commercial time-of-flight (TOF) secondary ion mass spectrometer, and the ionization performance was unknown. A vacuum-type electrospray droplet ion gun was connected to a triple-focus TOF analyzer. The flight time of the secondary ions was measured using a sample-bias pulsing method, because a short pulse of the electrospray droplet beam could not be obtained. The secondary ion yields of an amino acid sample produced by the electrospray droplet beams and atomic Ga ion beams were compared. TOF secondary ion spectra were measured for the amino acid and peptide samples with a mass resolution of ~500 using the sample-bias pulsing method. The secondary ion yield of the amino acid sample produced with the 10 kV vacuum-type electrospray droplet beams was much higher than that produced by 10 kV Ga ion beams. In addition, the secondary ion yields for the peptide sample and amino acid samples were almost similar. This is the first report on secondary ion yields produced with vacuum-type electrospray droplet ion beams and measured with a semi-commercial TOF analyzer. The enhancement of secondary ion yields, in particular for relatively high-mass molecules, would be very useful in the SIMS analysis of a wide variety of biological samples. Copyright © 2016 John Wiley & Sons, Ltd.

Effects of a proton-conducting ionic liquid on secondary ion formation in time-of-flight secondary ion mass spectrometry.

A protic ionic liquid, diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]), has low vapor pressure and high protonic conductivity even at room temperature. Since [dema][TfO] has a mobile proton in its salt structure, its primary beam is expected to enhance the formation of protonated molecular ions. However, mass spectrometric characteristics of [dema][TfO] are not well known. In order to develop an ionic-liquid primary beam source, it is necessary to investigate such characteristics. The first time-of-flight secondary ion mass spectrometry (TOF-SIMS) experiment using an Ar(+) primary ion beam was performed to analyze two samples: a neat [dema][TfO] sample and a mixed sample of arginine and [dema][TfO]. Beam characteristics of [dema][TfO] generated by vacuum electrospray were investigated using an apparatus for measuring transient responses of a beam current. The second TOF-SIMS experiment using a [dema][TfO] primary beam was performed to analyze three samples: arginine, a mixture of arginine and [dema][TfO], and poly(ethylene glycol) (PEG300). The [dema][TfO] primary beam was useful in generating protonated arginine; however, it was not helpful in detecting PEG300. The results were explained by considering gas-phase basicities and proton affinities of analytes and [dema][TfO] constituents. Projectile energy per nucleon of the [dema][TfO] beam was examined; it would be necessary to reduce m/z values of ionic-liquid charged droplets. In addition, a screening method was proposed to select ionic liquids suitable for primary ion beams. Since [dema][TfO] can act as a proton source, its primary beam can effectively generate protonated secondary ions of analytes. Consequently, proton-conducting ionic liquids such as [dema][TfO] are expected to have great potentials as primary ion beams in TOF-SIMS.

Time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) using an ionic‐liquid primary ion beam source

An orthogonal acceleration time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) system has been developed using a prototype vacuum‐electrospay primary beam source that is capable of producing continuous ion beam of an ionic liquid. As a primary ion beam, an imidazolium ionic liquid, 1‐ethyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl) amide (EMI‐TFSA), was electrosprayed at pressures around 2 × 10−4 Pa and a flow rate of 50 nL/min. Using the TOF‐SIMS system, three samples were analyzed: (i) EMI‐TFSA, (ii) an ammonium ionic liquid, N,N‐diethyl‐N‐methyl‐N‐(2‐methoxyethyl)ammonium bis(trifluoromethanesulfonyl) amide (DEME‐TFSA), and (iii) a polyethylene glycol (PEG300). Preliminary data showed that the vacuum electrospray of the ionic liquid was applicable to a primary ion beam source for SIMS. Nevertheless, irradiation of the ionic‐liquid beam caused deposition as well as sputtering. The deposition would result from larger m/z components of ionic‐liquid charged droplets. This is because projectile energy per nucleon of the larger m/z components seems too low to sputter materials. Because m/z values of charged droplets decrease with decreasing flow rate, it is required to reduce the flow rate of the ionic liquid so as to suppress the deposition of the ionic‐liquid beam. Copyright © 2014 John Wiley & Sons, Ltd.

Receiver-aperture averaging of annular beams propagating through turbulent atmosphere

For an annular beam incidence, the power scintillation index in a weakly turbulent atmosphere is derived at the receiver plane, which has a Gaussian aperture. Employing this derivation, the receiver-aperture averaging factor is evaluated. Annular beams are found more advantageous than the Gaussian beams when compared on receiver-aperture averaging basis. The analyses indicate that the effect of the aperture averaging increases as the propagation length increases. Increase in the structure constant and the secondary beam source size is observed to strengthen the effect of the aperture averaging when the primary beam source size is fixed.

Beam wander of dark hollow, flat-topped and annular beams

Benefiting from the earlier derivations for the Gaussian beam, we formulate beam wander for dark hollow (DH) and flat-topped (FT) beams, also covering the annular Gaussian (AG) beam as a special case. Via graphical illustrations, beam wander variations of these beams are analyzed and compared among themselves and to the fundamental Gaussian beam against changes in propagation length, amplitude factor, source size, wavelength of operation, inner and outer scales of turbulence. These comparisons show that in relation to the fundamental Gaussian beam, DH and FT beams will exhibit less beam wander, particularly at small primary beam source sizes, lower amplitude factors of the secondary beam and higher beam orders. Furthermore, DH and FT beams will continue to preserve this advantageous position all throughout the considered range of wavelengths, inner and outer scales of turbulence. FT beams, in particular, are observed to have the smallest beam wander values among all, up to certain source sizes.

Influence of hydrocarbons on element detection in ion images by SIMS microscopy

Ion microscopy on fresh frozen cryostat sections, 5–10 μm thick, is useful to determine the distribution of elements and low molecular organic compounds in the larger areas of the tissues. Fresh frozen cryostat sections of tree frog eyeball were examined. Secondary ion images of Na, Mg, Al, C 2H 3, K, Ca and C 3H 5 were observed by ion microscopy (IMS-6f) using O 2 + as the primary beam source at an energy of 15 keV. The primary beam current was 10 −10 A, the ion image magnification was varied from 300 to 1500 and the mass resolution was set between 300 and 3000. The areas of high intensity ion counts of the organic compounds generally showed low ion counts of elements. After long exposure to the primary ion beam, the intensity of the organic compound ions decreased, whereas the intensity of atomic ions of elements increased.

Development of a position-sensitive gamma detector based on a scintillating fiber for a 2DACAR study

A position-sensitive gamma detector based on a scintillating fiber is described. At the KEK PF slow-positron facility, an intensity of more than 10 8 e +/s has already been achieved at the target point of the sample chamber utilizing the PF 2.5 GeV electron linac as its primary beam source, and studies for producing a positron microbeam have been carried out. In parallel with this, we have tried to realize a position-sensitive gamma detector for a positron microbeam. We attempted to use a fiber-optic scintillator for a position-sensitive detector. The glass compositions were alkaline earth, gadolinium, and terbium loaded silicate glasses. The glass was used as the core glass to fabricate bundles of fibers, with each fiber surrounded by cladding glass. Fiber sizes of 6 μm were used, which allowed an image resolution of greater than 20 line pairs per mm. The glass-fiber optic scintillating plate was coupled to an image intensifier. The image-intensifier output image was observed by a CCD camera.

A FAB source for SIMS. Studies of a gas discharge type FAB source.

A gas discharge type FAB source has been studied as a primary beam source for SIMS. The source consists of an anode, two cathodes and Sm-Co ring magnets which produce axial magnetic field between the anode and the cathodes. A cold cathode discharge is generated between the anode and the cathodes after introducing oxygen gas into the source. Oxygen ions which are created in the glow of the discharge and then accelerated towards the cathode, become atoms again with high energy by capturing electrons through soft contact with oxygen molecules or by recombination with electrons. The fast oxygen atoms have kinetic energy of 400-3000eV and a flow of 1013-1014 atoms/sec. The neutralization coefficient of the fast atoms is less than 50%. SIMS spectra for Al are obtained by using the FAB source.