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---------------------- 2329 - Gamma-Rays and Cosmic Rays
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- Gamma-rays are the highest frequency electromagnetic radiation and therefore have the highest energy levels in the electromagnetic spectrum. Their wavelengths are 10^-2 to as small as 10^-14 meters. The smaller the wavelength the higher the frequency the greater the power.
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- Their frequency ranges from 10^10 to as high as 10^22 cycles per second (hertz). Their energy measured on a single photon ranges from 100,000 to 10,000,000 electron volts. In contrast visible light has an energy level of 2 to 3 electron volts.
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- The energy of a photon of electromagnetic radiation is given by:
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---------------------- Energy = Planck’s Constant * frequency of the radiation
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---------------------- E = h * f
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---------------------- Frequency = speed of light / wavelength
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---------------------- f = c / w
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---------------------- Planck’s constant = h = 6.624 * 10^-34 joule * seconds
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- The photon has zero mass at rest and in a vacuum always travels at the same speed, 670,633,500 miles per hour. Photons begin moving at the speed of light at the moment of creation and keep moving at that speed until the moment of absorption.
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- A photon is emitted by an atom in 10^-8 seconds. And, it need be in the vicinity of an atom for only about 10^-8 seconds to have a good chance of being absorbed. Ordinary light has a much longer wavelength than gamma-rays and takes longer to pass a single atom.
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- Light is therefore more readily absorbed and rarely penetrates more than a couple of dozen atom-thicknesses into a solid. In contrast, gamma-rays will penetrate 10 feet of lead before being absorbed.
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---------------------------------- Shortest Highest Highest
--------------------------------- Wavelength Energy Temperature
------------------------------- Nanometers, 10^-9 m Electron volts Kelvin
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------------------ Radio 10^7 10^-4 .03
------------------ Microwave 4 * 10^5 3 * 10^-3 30
------------------ Infrared 700 2 4100
------------------ Visible 400 3 7300
------------------ Ultraviolet 10 1000 3 * 10^6
------------------ X ray 10^-2 100,000 3 * 10^8
------------------ Gamma ray >10^-2 > 10^5 > 3 * 10^8
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----------------- Cosmic ray These are particles 10^9
----------------- High Energy Cosmic ray 10^20
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- Gamma-rays are produced in the core of star’s nuclear reactions. Our Sun produces gamma-rays when its hydrogen is fused into helium.
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- The Sun’s core fusion starts with six hydrogen nuclei and ends with one helium nuclei and two hydrogen nuclei. The mass of six hydrogen nuclei is 6 * (1.674 * 10^-27 kilograms) = 10.044 * 10^-27 kilograms.
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- The mass of one helium nuclei and 2 hydrogen nuclei is 6.643 * 10^-27 + 2 * (1.674 * 10^-27 kilograms) = 9.991 * 10^-27 kilograms.
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- The difference is 0.053 * 10^-27 kilograms which is converted into energy in the form of gamma rays, electromagnetic radiation, according to Einstein’s formula, E = m* c^2.
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---------------------- Energy = mass * (speed of light) ^2
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---------------------- Energy = 0.053 * 10^-27 kg * 8.98755 * 10^16 (m/sec)^2
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---------------------- Energy = .47 * 10^-11 joules
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---------------------- One Electron volt = 1.6 * 10^-19 joules
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---------------------- Energy = 0.3 * 10^8 electron volts
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- This amount of energy is multiplied 10^38 times every second to equal the energy output of the Sun. The luminosity of the Sun is 2.4 * 10^45 electron volts / second.
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- The Sun’s gamma-rays fight their way to the surface of the Sun bouncing, absorbing and emitting, through all the particles in the very dense interior. They finally reach the surface with much of their energy lost, emitting from the surface of the Sun with all the lower frequencies in the spectrum, including those of visible light.
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- The Sun’s electromagnetic radiation striking the Earth’s atmosphere, covers the entire electromagnetic spectrum. The longer wavelength radiation penetrates the atmosphere to reach the Earth’s surface. This includes radio waves, infrared, light, and some ultraviolet. The penetration of the atmosphere depends on the wavelength of the radiation in relation to the physical size of atoms and molecules that it encounters.
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- The shorter wavelengths hit particles more their own size. Gamma-ray has a wavelength of 10^-11 meters. The diameter of an atom is 10^-9 meters and its nucleus 10^-13 meters. Red and yellow light wavelengths ( 7 * 10^-7meters ) penetrate more easily than blue and ultraviolet.
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- Blue light ( 4 * 10^-7 meters ) is one half the wavelength of redlight and is the first of the shorter wavelengths to begin to scatter in the atmosphere. That is why the sky is blue.
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- Ultraviolet, with shorter wavelengths still, is even more attenuated by the atmosphere. That is why you have trouble getting a tan in late afternoon because the ultraviolet wavelengths have to pass through more atmosphere in order to reach you.
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- X-rays and Gamma-rays have such short wavelengths that very little radiation can ever make it through the atmosphere and reach the ground.
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- It is a good thing that celestial gamma-rays dissipate their energies 10 to 300 miles above our heads because gamma-rays can kill living cells. In fact they are used in medicine to kill cancerous cells in radiation therapy.
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- Marie Sklodowska Curie, the Polish-French chemist who discovered radiation and X-rays, died of leukemia a form of cancer caused by the overexposure to radioactive radiation (1934).
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- Research in celestial gamma-rays needs to use instruments held aloft in high altitude balloons or in satellites. The first gamma-ray telescope was carried into orbit in 1961. When this was first done astronomers were surprised to learn that great bursts of gamma-rays were being seen all over the cosmos. They are now being detected at the rate of about one per day. Supernova explosions, neutron stars, pulsars, and black holes are all sources of celestial gamma-rays.
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- Gamma-ray telescopes can not use lenses and mirrors because this radiation passes right through them. These telescopes use Compton scattering to detect the gamma-rays. The scattering is measured when the gamma-rays strike electrons and lose energy, much the same way they lose energy high in Earth’s atmosphere.
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- If the gamma-ray telescope looks at the moon it sees a round blob with no lunar features visible, but the blob is brighter than gamma-rays emitting from our quiet Sun. However, solar flares, from a not so quiet Sun, would out shine the moon.
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- The brightest gamma-ray objects in the sky would be spinning neutron stars and pulsars. The Crab Nebula pulsar would be a bright light in the sky. The gamma-ray bursts that occur at least once per day, last for only 0.2 seconds up to 40 seconds, popping off like cosmic flashbulbs from unexpected directions, flickering, and fading after briefly dominating the gamma-ray sky. The energy released in a burst of 10 seconds can be more than our Sun will emit in its entire 10,000,000,000-year lifetime.
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- We expect to see a gamma-ray burst in our Milky Way Galaxy once every few million years. So far, it appears that all of the bursts we have observed to date have come from far outside our Milky Way Galaxy.
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- After 25 years of study these gamma-ray bursts remain a mystery. The Compton Gamma-Ray Observatory was a satellite launched in 1991. Its gyro failed and it was de-orbited in June, 2000, after accumulating a large amount of data that is still being analyzed.
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- Gamma-rays are not to be confused with Cosmic Rays. Cosmic rays are actually bits of matter, normally, hydrogen nuclei, or protons. Sometimes, they are the positive ions, or nuclei from higher elements than hydrogen. They are charged pieces of atoms traveling at high speeds with enormous energies.
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- They are distinguished from gamma-rays because their path can be bent in a magnetic field. While gamma rays, like X-rays, are electromagnetic radiation that are massless and not affected by magnetic fields.
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- Normally, cosmic rays arrive with energy levels of 10^9 electron volts. Very few exceed 10^16 electron volts. However, nearly a dozen incidents have been recorded of cosmic rays with 3 * 10^20 electron volts.
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- This high level of energy can not be explained. The generators must be quasars, or gamma-ray bursters, or some other cataclysmic generators. It is thought that positive ions with energies above 5 * 10^19 electron volts would be attenuated by the cosmic background radiation that pervades space.
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- Quasars are billions of lightyears away. Cosmic rays should have lost most of their energy by the time they reach us. The most energy production we can imagine are colliding galaxy clusters and magnetars, superdense stars with extremely high magnetic fields, but even these may not be able to create jets as high as we need to explain high energy cosmic rays.
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- The plastic helmet worn by Jim Lovell during Apollo 8 lunar mission in 1968 bears microscopic pits, 1/50 of an inch long, left by bombarding cosmic rays. This alerted researchers to the need for better shielding for astronauts during missions into deep space.
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- We have much more to learn about high energy cosmic rays and high energy gamma-rays.
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- Footnote: Arthur Holly Compton was an American physicist born in Ohio in 1892. He got his Ph.D. at Princeton in 1916. In 1923 while studying X-rays he noted that scattering lengthened their wavelength. Arthur presumed that the photon of light struck an electron, which recoiled, subtracting some energy from the photon thereby increasing its wavelength.
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- It was Arthur who first named the “photon” to account for the light quantum in its particle aspect. He got the Noble prize in 1927 and worked on the Manhattan project in World War II. He was chancellor of Washington University in St. Louis until 1953.
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- April 9, 2019 26
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-------------------------- Tuesday, April 9, 2019 --------------------------
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