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Topic Review
Brief History of Gel Dosimetry
Advances in radiotherapy technology have significantly improved both dose conformation to tumors and the preservation of healthy tissues, achieving almost real-time feedback by means of high-precision treatments and theranostics. Therefore, developing high-performance systems capable of coping with the challenging requirements of modern ionizing radiation is a key issue to overcome the limitations of traditional dosimeters. In this regard, a deep understanding of the physicochemical basis of gel dosimetry, as one of the most promising tools for the evaluation of 3D high-spatial-resolution dose distributions, represents the starting point for developing new and innovative systems. 
  • 470
  • 02 Nov 2022
Topic Review
Reference Point Indentation
Reference Point Indentation (RPI) refers to a specialized form of indentation testing. RPI utilizes a unique method of measurement by establishing a relative reference point at the location of measurement. This unique capability makes it possible to measure materials that are in motion, oddly shaped, visco-elastic, or that may be coated or covered by another, softer material. Unlike traditional indentation testing, RPI testing uses the location of measurement as the relative displacement reference position. Indentation itself is perhaps the most commonly applied means of testing the mechanical properties of materials. The technique has its origins in the Mohs scale of mineral hardness, in which materials are ranked according to what they can scratch and are, in turn, scratched by. The characterization of solids in this way takes place on an essentially discrete scale, so much effort has been expended in order to develop techniques for evaluating material hardness over a continuous range. Hence, the adoption of the Meyer, Knoop, Brinell, Rockwell, and Vickers hardness tests. More recently (ca. 1975), nanoindentation techniques have been established as the primary tool for investigating the hardness of small volumes of material. However, even more recently (ca. 2006), interest in measuring functional roles of biomaterials drove the development of the Reference Point Indentation technique. New research in field such as biomaterials has led scientists to begin considering materials as complex systems that behave differently than the constituent parts. For example, materials like bone are hierarchical and made of many components including calcium, collagen, water, and non-collagenous proteins. Each of these components has unique material properties. When combined to form bone, the function of the tissue is different than any one constituent. Understanding this mechanical system is becoming a new field of research called Materiomics. RPI specifically aims to aid materiomics researchers understand the functional capabilities of these types of materials at a relevant length-scale.
  • 493
  • 01 Nov 2022
Topic Review
FFAG Accelerator
A Fixed-Field Alternating Gradient accelerator (FFAG) is a circular particle accelerator concept on which development was started in the early 50s, and that can be characterized by its time-independent magnetic fields (fixed-field, like in a cyclotron) and the use of strong focusing (alternating gradient, like in a synchrotron). Thus, FFAG accelerators combine the cyclotron's advantage of continuous, unpulsed operation, with the synchrotron's relatively inexpensive small magnet ring, of narrow bore. Although the development of FFAGs had not been pursued for over a decade starting from 1967, it has regained interest since the mid-1980s for usage in neutron spallation sources, as a driver for muon colliders and to accelerate muons in a neutrino factory since the mid-1990s. The revival in FFAG research has been particularly strong in Japan with the construction of several rings. This resurgence has been prompted in part by advances in RF cavities and in magnet design.
  • 1.6K
  • 01 Nov 2022
Topic Review
Jerk
In physics, jerk or jolt is the rate at which an object's acceleration changes with respect to time. It is a vector quantity (having both magnitude and direction). Jerk is most commonly denoted by the symbol j and expressed in m/s3 (SI units) or standard gravities per second (g0/s).
  • 1.0K
  • 01 Nov 2022
Topic Review
ADM Formalism
The ADM formalism (named for its authors Richard Arnowitt, Stanley Deser and Charles W. Misner) is a Hamiltonian formulation of general relativity that plays an important role in canonical quantum gravity and numerical relativity. It was first published in 1959. The comprehensive review of the formalism that the authors published in 1962 has been reprinted in the journal General Relativity and Gravitation, while the original papers can be found in the archives of Physical Review.
  • 851
  • 01 Nov 2022
Topic Review
Conjunction
In astronomy, a conjunction occurs when two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude, usually as observed from Earth. When two objects always appear close to the ecliptic—such as two planets, the Moon and a planet, or the Sun and a planet—this fact implies an apparent close approach between the objects as seen in the sky. A related word, appulse, is the minimum apparent separation in the sky of two astronomical objects. Conjunctions involve either two objects in the Solar System or one object in the Solar System and a more distant object, such as a star. A conjunction is an apparent phenomenon caused by the observer's perspective: the two objects involved are not actually close to one another in space. Conjunctions between two bright objects close to the ecliptic, such as two bright planets, can be seen with the naked eye. The astronomical symbol for conjunction is (Unicode U+260C ☌). The conjunction symbol is not used in modern astronomy. It continues to be used in astrology.
  • 1.2K
  • 01 Nov 2022
Topic Review
Venus (Planet)
Venus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. After Earth's Moon, it is the brightest object in the night sky, reaching an apparent magnitude of −4.6. As an inferior planet from Earth it never appears to venture far from the Sun, and its elongation reaches a maximum of 47.8°. Venus reaches its maximum brightness shortly before sunrise or shortly after sunset, and is often referred to as the Morning Star or as the Evening Star. A terrestrial planet, it is sometimes called Earth's "sister planet", as the two are similar in size and bulk composition. The planet is covered with an opaque layer of highly reflective clouds and its surface cannot be seen from space in visible light, making it a subject of great speculation until some of its secrets were revealed by planetary science in the 20th century. Venus has the densest atmosphere of the terrestrial planets, consisting mostly of carbon dioxide, and the atmospheric pressure at the planet's surface is 90 times that of the Earth. Venus' surface has been mapped in detail only in the last 20 years. It shows evidence of extensive volcanism, and some of its volcanoes may still be active today. In contrast to the constant crustal movement seen on Earth, Venus is thought to undergo periodic episodes of plate tectonics, in which the crust is subducted rapidly within a few million years separated by stable periods of a few hundred million years. The planet is named after Venus, the Roman goddess of love, and most of its surface features are named after famous and mythological women. The adjective Venusian is commonly used for items related to Venus, though the Latin adjective is the rarely used Venereal; the now-archaic Cytherean is still occasionally encountered.
  • 1.0K
  • 01 Nov 2022
Topic Review
Names of the Days of the Week
The names of the days of the week in many languages are derived from the names of the classical planets in Hellenistic astrology, which were in turn named after contemporary deities, a system introduced by the Roman Empire during Late Antiquity. In some other languages, the days are named after corresponding deities of the regional culture, either beginning with Sunday or with Monday. In the international standard ISO 8601, Monday is treated as the first day of the week.
  • 35.3K
  • 01 Nov 2022
Topic Review
AIDA (Mission)
The Asteroid Impact and Deflection Assessment (AIDA) mission is a proposed pair of space probes which will study and demonstrate the kinetic effects of crashing an impactor spacecraft into an asteroid moon. The mission is intended to test and validate impact models of whether a spacecraft could successfully deflect an asteroid on a collision course with Earth. The original plan called for a European spacecraft, the Asteroid Impact Mission (AIM), to operate in synergy with a large NASA impactor called Double Asteroid Redirection Test (DART) and observe the immediate effects of the impact. AIM was cancelled in 2016 when Germany was unable to fund its portion, and after some backlash within ESA, AIM was replaced in 2018 with a smaller spacecraft called Hera that will launch five years after DART to orbit and study the crater on the asteroid. Hera will also deploy Europe's first CubeSats in deep space for close-up asteroid surveying: Juventas and Milani. DART is currently planned to impact in 2022 in the small moon of asteroid 65803 Didymos, while Hera will arrive at Didymos in 2027, five years after DART's impact.
  • 593
  • 01 Nov 2022
Topic Review
ΔT
In precise timekeeping, ΔT (Delta T, delta-T, deltaT, or DT) is a measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of atomic time. Formally it is the time difference obtained by subtracting Universal Time (UT, defined by the Earth's rotation) from Terrestrial Time (TT, independent of the Earth's rotation): ΔT = TT − UT. The value of ΔT for the start of 1902 was approximately zero; for 2002 it was about 64 seconds. So the Earth's rotations over that century took about 64 seconds longer than would be required for days of atomic time. As well as this long-term drift in the length of the day there are short-term fluctuations in the length of day (Δτ) which are dealt with separately.
  • 897
  • 01 Nov 2022
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