Applications of radiation interactions in detectors
Click here to see what's new. The calibration method using a high-power halogen tungsten lamp as a calibration source has many advantages such as strong equivalence and high power, so it is very fit for the calibration of high-energy laser energy meters. However, high-power halogen tungsten lamps after power-off still reserve much residual energy and continually radiate energy, which is difficult to be measured. Two measuring systems were found to solve the problems. One system is composed of an integrating sphere and two optical spectrometers, which can accurately characterize the radiative spectra and power—time variation of the halogen tungsten lamp.
This measuring system was then calibrated using a normal halogen tungsten lamp made of the same material as the high-power halogen tungsten lamp. In this way, the radiation efficiency of the halogen tungsten lamp after power-off can be quantitatively measured. In the other measuring system, a wide-spectrum power meter was installed far away from the halogen tungsten lamp; thus, the lamp can be regarded as a point light source. The radiation efficiency of residual energy from the halogen tungsten lamp was computed on the basis of geometrical relations.
All the tested halogen tungsten lamps reached The measuring uncertainty of total radiation energy was 2. Simonds, and John H. Express 25 4 You do not have subscription access to this journal.
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Citation lists with outbound citation links are available to subscribers only. You may subscribe either as an OSA member, or as an authorized user of your institution. He computed the time for light to reach Earth from the Sun as 8 minutes 12 seconds. The first terrestrial measurements were made in by Fizeau and a year later by Foucault. Any measurement of velocity requires, however, a definition of the measure of length and of time. Current techniques allow a determination of the velocity of electromagnetic radiation to a substantially higher degree of precision than permitted by the unit of length that scientists had applied earlier.
In the value of the speed of light was fixed at exactly ,, metres per second, and this value was adopted as a new standard. Furthermore, the second —the international unit of time—has been based on the frequency of electromagnetic radiation emitted by a cesium atom. After a long struggle electromagnetic wave theory had triumphed. The Faraday - Maxwell - Hertz theory of electromagnetic radiation seemed to be able to explain all phenomena of light , electricity , and magnetism.
The understanding of these phenomena enabled one to produce electromagnetic radiation of many different frequencies which had never been observed before and which opened a world of new opportunities.
No one suspected that the conceptional foundations of physics were about to change again. The quantum theory of absorption and emission of radiation announced in by Planck ushered in the era of modern physics. Planck was led to this radically new insight by trying to explain the puzzling observation of the amount of electromagnetic radiation emitted by a hot body and, in particular, the dependence of the intensity of this incandescent radiation on temperature and on frequency.
The quantitative aspects of the incandescent radiation constitute the radiation laws. The Austrian physicist Josef Stefan found in that the total radiation energy per unit time emitted by a heated surface per unit area increases as the fourth power of its absolute temperature T Kelvin scale. In another Austrian physicist, Ludwig Boltzmann , used the second law of thermodynamics to derive this temperature dependence for an ideal substance that emits and absorbs all frequencies.
Such an object that absorbs light of all colours looks black, and so was called a blackbody. The wavelength or frequency distribution of blackbody radiation was studied in the s by Wilhelm Wien of Germany. It was his idea to use as a good approximation for the ideal blackbody an oven with a small hole. Any radiation that enters the small hole is scattered and reflected from the inner walls of the oven so often that nearly all incoming radiation is absorbed and the chance of some of it finding its way out of the hole again can be made exceedingly small.
The radiation coming out of this hole is then very close to the equilibrium blackbody electromagnetic radiation corresponding to the oven temperature. The decrease of the radiation output at low frequency had already been explained by Lord Rayleigh in terms of the decrease, with lowering frequency, in the number of modes of electromagnetic radiation per frequency interval. Rayleigh, following the principle of equipartition of energy , assumed that all possible frequency modes could radiate with equal probability. A possible way out of this dilemma was to deny the high-frequency modes an equal chance to radiate.
To explain quantized absorption and emission of radiation, it seemed sufficient to quantize only the energy levels of mechanical systems.
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Hertz discovered the photoelectric effect quite by accident while generating electromagnetic waves and observing their propagation. His transmitter and receiver were induction coils with spark gaps. He measured the electromagnetic field strength by the maximum length of the spark of his detector. In order to observe this more accurately, he occasionally enclosed the spark gap of the receiver in a dark case. In doing so, he observed that the spark was always smaller with the case than without it. He concluded, correctly, that the light from the transmitter spark affected the electrical arcing of the receiver.
He used a quartz prism to disperse the light of the transmitter spark and found that the ultraviolet part of the light spectrum was responsible for enhancing the receiver spark. Hertz took this discovery seriously because the only other effect of light on electrical phenomena known at that time was the increase in electrical conductance of the element selenium with light exposure. This was accomplished in by J. Lenard discovered that for a given frequency of ultraviolet radiation the maximum kinetic energy of the emitted electrons depends on the metal used rather than on the intensity of the ultraviolet light.
The light intensity increases the number but not the energy of emitted electrons. Moreover, he found that for each metal there is a minimum light frequency that is needed to induce the emission of electrons. Light of a frequency lower than this minimum frequency has no effect regardless of its intensity. This derivation and comparison made no references to substances and oscillators. At the end of this paper, Einstein concluded that if electromagnetic radiation is quantized, the absorption processes are thus quantized too, yielding an elegant explanation of the threshold energies and the intensity dependence of the photoelectric effect.
Convincing evidence of the particle nature of electromagnetic radiation was found in by the American physicist Arthur Holly Compton. While investigating the scattering of X-rays , he observed that such rays lose some of their energy in the scattering process and emerge with slightly decreased frequency. This so-called Compton effect can be explained, according to classical mechanics , as an elastic collision of two particles comparable to the collision of two billiard balls.
The recoiling electron was observed and measured by Compton and Alfred W. Simon in a Wilson cloud chamber. It has the value 0. During the mids the German physicist Gustav Robert Kirchhoff observed that atoms and molecules emit and absorb electromagnetic radiation at characteristic frequencies and that the emission and absorption frequencies are the same for a given substance.
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Such resonance absorption should, strictly speaking, not occur if one applies the photon picture due to the following argument. Since energy and momentum have to be conserved in the emission process, the atom recoils to the left as the photon is emitted to the right, just as a cannon recoils backward when a shot is fired. Because the recoiling atom carries off some kinetic recoil energy E R , the emitted photon energy is less than the energy difference of the atomic energy states by the amount E R. When a photon is absorbed by an atom, the momentum of the photon is likewise transmitted to the atom, thereby giving it a kinetic recoil energy E R.
The absorbing photon must therefore supply not only the energy difference of the atomic energy states but the additional amount E R as well. Accordingly, resonance absorption should not occur because the emitted photon is missing 2 E R to accomplish it. This is because for visible light the recoil energy E R is very small compared with the natural energy uncertainty of atomic emission and absorption processes. The situation is, however, quite different for the emission and absorption of gamma-ray photons by nuclei.
The recoil energy E R is more than 10, times as large for gamma-ray photons as for photons of visible light, and the nuclear energy transitions are much more sharply defined because their lifetime can be one million times longer than for electronic energy transitions. The particle nature of photons therefore prevents resonance absorption of gamma-ray photons by free nuclei.
In this case , there is a strong probability that the recoil momentum during absorption and emission of the gamma photon is taken up by the whole solid or more precisely by its entire lattice. This then reduces the recoil energy to nearly zero and thus allows resonance absorption to occur even for gamma rays. How can electromagnetic radiation behave like a particle in some cases while exhibiting wavelike properties that produce the interference and diffraction phenomena in others?
What Is Electromagnetic Radiation?
This paradoxical behaviour came to be known as the wave-particle duality. Bohr rejected the idea of light quanta, and he searched for ways to explain the Compton effect and the photoelectric effect by arguing that the momentum and energy conservation laws need to be satisfied only statistically in the time average. In he stated that the hypothesis of light quanta excludes, in principle, the possibility of a rational definition of the concepts of frequency and wavelength that are essential for explaining interference. The following year the conceptual foundations of physics were shaken by the French physicist Louis de Broglie , who suggested in his doctoral dissertation that the wave-particle duality applies not only to light but to a particle as well.
De Broglie proposed that any object has wavelike properties. In Clinton Joseph Davisson and Lester Germer of the United States observed diffraction and hence interference of electron waves by the regular arrangement of atoms in a crystal of nickel. That same year S. Kikuchi of Japan obtained an electron diffraction pattern by shooting electrons with an energy of 68 keV through a thin mica plate and recording the resultant diffraction pattern on a photographic plate.
The observed pattern corresponded to electron waves having the wavelength predicted by de Broglie. The diffraction effects of helium atoms were found in , and neutron diffraction has today become an indispensable tool for determining the magnetic and atomic structure of materials.
The interference pattern that results when a radiation front hits two slits in an opaque screen is often cited to explain the conceptual difficulty of the wave-particle duality. Often we deliver better than expected news and peace of mind, and no additional measures are recommended or the fix is more simple than you think. We are a group of scientists working to make your indoor spaces healthier. Subscribe to our e-mail newsletter to receive helpful updates and articles from Healthy Building Science. Request an Inspection Call us at or click the button below to schedule your building health assessment.
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