Misplaced Pages

#339660

59-529: On December 4, 2013, a cobalt-60 radioactive source stolen from a truck two days earlier in Tizayuca, Hidalgo , was recovered there, as well as the heavy truck itself; the decommissioned cobalt therapy machine had been en route from Tijuana, Baja California , to proper disposal at a radioactive waste storage centre in the nearby municipality of Temascalapa . Federal police and military units established an armed cordon approximately 500 metres (1,600 ft) around

118-401: A t o m , {\displaystyle N_{\rm {A}}={\frac {V_{\rm {m}}}{V_{\rm {atom}}}},} where V atom = ⁠ V cell / n ⁠ and n is the number of atoms per unit cell of volume V cell . The unit cell of silicon has a cubic packing arrangement of 8 atoms, and the unit cell volume may be measured by determining a single unit cell parameter,

177-417: A half-life of 5.2714 years. It is produced artificially in nuclear reactors . Deliberate industrial production depends on neutron activation of bulk samples of the monoisotopic and mononuclidic cobalt isotope Co . Measurable quantities are also produced as a by-product of typical nuclear power plant operation and may be detected externally when leaks occur. In the latter case (in

236-701: A Canadian-built CANDU at Embalse Nuclear Power Station . India has a number of CANDU reactors at the Rajasthan Atomic Power Station used for producing Co. India had a capacity of more than 6  MCi of Co production in 2021; this capacity is slated to increase with more CANDU reactors being commissioned at the Rajasthan Atomic Power Station. Heavy-water reactors are particularly well suited for production of Co because of their excellent neutron economy and because their capacity for online refueling allows targets to be inserted into

295-559: A missile nuclear warhead in the 1970 film Beneath the Planet of the Apes . Atomic mass unit The dalton or unified atomic mass unit (symbols: Da or u , respectively) is a unit of mass defined as ⁠ 1 / 12 ⁠ of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest . It is a non-SI unit accepted for use with SI . The atomic mass constant , denoted m u ,

354-489: A private medical company and subsequently dismantled for lack of personnel to operate it. The radioactive material, Co, ended up in a junkyard, where it was sold to foundries that inadvertently smelted it with other metals and produced about 6,000 tons of contaminated rebar . These were distributed in 17 Mexican states and several cities in the United States. It is estimated that 4,000 people were exposed to radiation as

413-410: A proton is 1.007 276 466 5789 (83) Da , the mass of a free neutron is 1.008 664 916 06 (40) Da , and the mass of a hydrogen-2 (deuterium) atom is 2.014 101 778 114 (122) Da . In general, the difference (absolute mass excess ) is less than 0.1%; exceptions include hydrogen-1 (about 0.8%), helium-3 (0.5%), lithium-6 (0.25%) and beryllium (0.14%). The dalton differs from

472-571: A radiation dose of 0.4 Sv. Some studies have found that this large group did not suffer a higher incidence of cancer mortality, as the linear no-threshold model would predict, but suffered a lower cancer mortality than the general Taiwan public. These observations support the radiation hormesis model, however other studies have found health impacts that confound the results . In August 2012, Petco recalled several models of steel pet food bowls after US Customs and Border Protection determined that they were emitting low levels of radiation, which

531-440: A relative standard uncertainty of 1.3 × 10 . In practice, the atomic mass constant is determined from the electron rest mass m e and the electron relative atomic mass A r (e) (that is, the mass of electron divided by the atomic mass constant). The relative atomic mass of the electron can be measured in cyclotron experiments, while the rest mass of the electron can be derived from other physical constants. where c

590-517: A result of this incident. In the Samut Prakan radiation accident in 2000, a disused radiotherapy head containing a Co source was stored at an unsecured location in Bangkok , Thailand and then accidentally sold to scrap collectors. Unaware of the danger, a junkyard employee dismantled the head and extracted the source, which remained unprotected for a period of days at the junkyard. Ten people, including

649-419: A significant mass difference between Ni and Co: 0.003  u . This amounts to nearly 20 watts per gram, nearly 30 times larger than that of Pu. The diagram shows a simplified decay scheme of Co and Co. The main β-decay transitions are shown. The probability for population of the middle energy level of 2.1 MeV by β-decay is 0.0022%, with a maximum energy of 665.26 keV. Energy transfers between

SECTION 10

#1732858179340

708-467: Is a unit of amount of substance used in chemistry and physics, such that the mass of one mole of a substance expressed in grams is numerically equal to the average mass of one of its particles expressed in daltons. That is, the molar mass of a chemical compound expressed in g/mol or kg/kmol is numerically equal to its average molecular mass expressed in Da. For example, the average mass of one molecule of water

767-424: Is about 18.0153 Da, and the mass of one mole of water is about 18.0153 g. A protein whose molecule has an average mass of 64 kDa would have a molar mass of 64 kg/mol . However, while this equality can be assumed for practical purposes, it is only approximate, because of the 2019 redefinition of the mole . In general, the mass in daltons of an atom is numerically close but not exactly equal to

826-406: Is affected by the binding energy of the nucleons in its atomic nuclei, as well as the mass and binding energy of its electrons . Therefore, this equality holds only for the carbon-12 atom in the stated conditions, and will vary for other substances. For example, the mass of an unbound atom of the common hydrogen isotope ( hydrogen-1 , protium) is 1.007 825 032 241 (94) Da , the mass of

885-681: Is capitalized. The name was endorsed by the International Union of Pure and Applied Physics (IUPAP) in 2005. In 2003 the name was recommended to the BIPM by the Consultative Committee for Units , part of the CIPM , as it "is shorter and works better with [SI] prefixes". In 2006, the BIPM included the dalton in its 8th edition of the SI brochure of formal definitions as a non-SI unit accepted for use with

944-447: Is carbon-13. The molecular masses of proteins , nucleic acids , and other large polymers are often expressed with the unit kilo dalton (kDa) and mega dalton (MDa). Titin , one of the largest known proteins, has a molecular mass of between 3 and 3.7 megadaltons. The DNA of chromosome 1 in the human genome has about 249 million base pairs , each with an average mass of about 650 Da , or 156 GDa total. The mole

1003-420: Is defined identically, giving m u = ⁠ 1 / 12 ⁠ m ( C) = 1 Da . This unit is commonly used in physics and chemistry to express the mass of atomic-scale objects, such as atoms , molecules , and elementary particles , both for discrete instances and multiple types of ensemble averages. For example, an atom of helium-4 has a mass of 4.0026 Da . This is an intrinsic property of

1062-443: Is described as an attempted theft, but some of the rods are reportedly still missing. In 1957, Chien-Shiung Wu et al. discovered that β-decay violated parity , implying nature has a handedness. In the Wu experiment , researchers aligned Co nuclei by cooling the source to low temperatures in a magnetic field. Wu's observation was that more β-rays were emitted in the opposite direction to

1121-495: Is lethal for humans, and can cause death (potentially in less than an hour from acute exposure). After entering a living mammal (such as a human), assuming that the subject does not die shortly after exposure (as may happen in acute exposure incidents), some of the Co is excreted in feces . The rest is taken up by tissues, mainly the liver , kidneys , and bones , where the prolonged exposure to gamma radiation can cause cancer. Over time,

1180-415: Is not known whether the thieves wanted the truck (which included a crane), the cobalt-60, or both. 19°54′N 99°05′W  /  19.900°N 99.083°W  / 19.900; -99.083 This article about a location in the state of Mexico is a stub . You can help Misplaced Pages by expanding it . Cobalt-60 Cobalt-60 ( Co) is a synthetic radioactive isotope of cobalt with

1239-447: Is presently the case can be understood from the table below (2018 CODATA). Silicon single crystals may be produced today in commercial facilities with extremely high purity and with few lattice defects. This method defined the Avogadro constant as the ratio of the molar volume , V m , to the atomic volume V atom : N A = V m V

SECTION 20

#1732858179340

1298-429: Is that it is a high-intensity gamma-ray emitter with a relatively long half-life, 5.27 years, compared to other gamma ray sources of similar intensity. The β-decay energy is low and easily shielded; however, the gamma-ray emission lines have energies around 1.3 MeV, and are highly penetrating. The physical properties of cobalt such as resistance to bulk oxidation and low solubility in water give some advantages in safety in

1357-584: Is the speed of light , h is the Planck constant , α is the fine-structure constant , and R ∞ is the Rydberg constant . As may be observed from the old values (2014 CODATA) in the table below, the main limiting factor in the precision of the Avogadro constant was the uncertainty in the value of the Planck constant , as all the other constants that contribute to the calculation were known more precisely. The power of having defined values of universal constants as

1416-527: The Avogadro number in honor of physicist Amedeo Avogadro . The discovery of isotopes of oxygen in 1929 required a more precise definition of the unit. Two distinct definitions came into use. Chemists choose to define the AMU as ⁠ 1 / 16 ⁠ of the average mass of an oxygen atom as found in nature; that is, the average of the masses of the known isotopes, weighted by their natural abundance. Physicists, on

1475-472: The International Union of Pure and Applied Chemistry (IUPAC), which had absorbed the ICAW, adopted a new definition of the atomic mass unit for use in both physics and chemistry; namely, ⁠ 1 / 12 ⁠ of the mass of a carbon-12 atom. This new value was intermediate between the two earlier definitions, but closer to the one used by chemists (who would be affected the most by the change). The new unit

1534-415: The law of definite proportions in terms of the atomic theory of matter implied that the masses of atoms of various elements had definite ratios that depended on the elements. While the actual masses were unknown, the relative masses could be deduced from that law. In 1803 John Dalton proposed to use the (still unknown) atomic mass of the lightest atom, hydrogen, as the natural unit of atomic mass. This

1593-399: The number of nucleons in its nucleus . It follows that the molar mass of a compound (grams per mole) is numerically close to the average number of nucleons contained in each molecule. By definition, the mass of an atom of carbon-12 is 12 daltons, which corresponds with the number of nucleons that it has (6  protons and 6  neutrons ). However, the mass of an atomic-scale object

1652-563: The SI . The name was also listed as an alternative to "unified atomic mass unit" by the International Organization for Standardization in 2009. It is now recommended by several scientific publishers, and some of them consider "atomic mass unit" and "amu" deprecated. In 2019, the BIPM retained the dalton in its 9th edition of the SI brochure, while dropping the unified atomic mass unit from its table of non-SI units accepted for use with

1711-420: The SI, but secondarily notes that the dalton (Da) and the unified atomic mass unit (u) are alternative names (and symbols) for the same unit. The definition of the dalton was not affected by the 2019 revision of the SI , that is, 1 Da in the SI is still ⁠ 1 / 12 ⁠ of the mass of a carbon-12 atom, a quantity that must be determined experimentally in terms of SI units. However,

1770-536: The absence of added cobalt ) the incidentally produced Co is largely the result of multiple stages of neutron activation of iron isotopes in the reactor's steel structures via the creation of its Co precursor. The simplest case of the latter would result from the activation of Fe . Co undergoes beta decay to the stable isotope nickel-60 ( Ni ). The activated cobalt nucleus emits two gamma rays with energies of 1.17 and 1.33 MeV , hence

1829-476: The absorbed cobalt is eliminated in urine. Cobalt is found in steel . Uncontrolled disposal of Co in scrap metal is responsible for the radioactivity in some iron products. Circa 1983, construction was finished of 1700 apartments in Taiwan which were built with steel contaminated with cobalt-60. About 10,000 people occupied these buildings during a 9–20 year period. On average, these people unknowingly received

Hueypoxtla - Misplaced Pages Continue

1888-670: The atomic mass unit was first obtained indirectly by Josef Loschmidt in 1865, by estimating the number of particles in a given volume of gas. Perrin estimated the Avogadro number by a variety of methods, at the turn of the 20th century. He was awarded the 1926 Nobel Prize in Physics , largely for this work. The electric charge per mole of elementary charges is a constant called the Faraday constant , F , whose value had been essentially known since 1834 when Michael Faraday published his works on electrolysis . In 1910, Robert Millikan obtained

1947-520: The atomic weight of silver, then the Faraday constant is given by: The NIST scientists devised a method to compensate for silver lost from the anode by mechanical causes, and conducted an isotope analysis of the silver used to determine its atomic weight. Their value for the conventional Faraday constant was F 90  = 96 485 .39(13) C/mol , which corresponds to a value for the Avogadro constant of 6.022 1449 (78) × 10  mol : both values have

2006-414: The bomb explodes, neutrons from the nuclear fission would irradiate the cobalt and transmute it to Co. No country is known to have done any serious development of this type of weapon. Co does not occur naturally on Earth in significant amounts, so Co is synthesized by bombarding a Co target with a slow neutron source. Californium-252 , moderated through water, can be used for this purpose, as can

2065-414: The case of a containment breach over some other gamma sources such as caesium-137 . The main uses for Co are: Cobalt has been discussed as a " salting " element to add to nuclear weapons , to produce a cobalt bomb , an extremely "dirty" weapon which would contaminate large areas with Co nuclear fallout , rendering them uninhabitable. In one design, the tamper of the weapon would be made of Co. When

2124-440: The definition of a mole was changed to be the amount of substance consisting of exactly 6.022 140 76 × 10 entities and the definition of the kilogram was changed as well. As a consequence, the molar mass constant remains close to but no longer exactly 1 g/mol, meaning that the mass in grams of one mole of any substance remains nearly but no longer exactly numerically equal to its average molecular mass in daltons, although

2183-404: The exposed radiation source in the empty lot where it had been removed from its protective shielding and abandoned. Classes were suspended for two days at a neighbourhood kindergarten named for Marie Curie . Six people showing signs of possible radiation exposure from the orphan source were later detained. The source's level of radioactivity was reported as 3000 curies (111 terabequerels ). It

2242-411: The first measurement of the charge on an electron, − e . The quotient F / e provided an estimate of the Avogadro constant. The classic experiment is that of Bower and Davis at NIST , and relies on dissolving silver metal away from the anode of an electrolysis cell, while passing a constant electric current I for a known time t . If m is the mass of silver lost from the anode and A r

2301-427: The incident. On 13 September 1999, six people tried to steal Co rods from a chemical plant in the city of Grozny , Chechen Republic. During the theft, the suspects opened the radioactive material container and handled it, resulting in the deaths of three of the suspects and injury of the remaining three. The suspect who held the material directly in his hands died of radiation exposure 30 minutes later. This incident

2360-412: The isotope and all helium-4 atoms have the same mass. Acetylsalicylic acid ( aspirin ), C 9 H 8 O 4 , has an average mass of about 180.157 Da . However, there are no acetylsalicylic acid molecules with this mass. The two most common masses of individual acetylsalicylic acid molecules are 180.0423 Da , having the most common isotopes, and 181.0456 Da , in which one carbon

2419-533: The length a of one of the sides of the cube. The CODATA value of a for silicon is 5.431 020 511 (89) × 10  m . In practice, measurements are carried out on a distance known as d 220 (Si), which is the distance between the planes denoted by the Miller indices {220}, and is equal to a / √ 8 . The isotope proportional composition of the sample used must be measured and taken into account. Silicon occurs in three stable isotopes ( Si, Si, Si), and

Hueypoxtla - Misplaced Pages Continue

2478-577: The natural variation in their proportions is greater than other uncertainties in the measurements. The atomic weight A r for the sample crystal can be calculated, as the standard atomic weights of the three nuclides are known with great accuracy. This, together with the measured density ρ of the sample, allows the molar volume V m to be determined: V m = A r M u ρ , {\displaystyle V_{\rm {m}}={\frac {A_{\rm {r}}M_{\rm {u}}}{\rho }},} where M u

2537-497: The neutron flux in a nuclear reactor . The CANDU reactors can be used to activate Co, by substituting the control rods with cobalt rods. In the United States, as of 2010, it is being produced in a boiling water reactor at Hope Creek Nuclear Generating Station . The cobalt targets are substituted here for a small number of fuel assemblies. Still, over 40% of all single-use medical devices are sterilized using Co from Bruce nuclear generating station . Exposure to Co

2596-536: The nuclear spin. This asymmetry violates parity conservation . Argentina, Canada, India and Russia are the largest suppliers of Co in the world. Both Argentina and Canada have (as of 2022) an all- heavy-water reactor fleet for power generation. Canada has CANDU in numerous locations throughout Ontario as well as Point Lepreau Nuclear Generating Station in New Brunswick, while Argentina has two German-supplied heavy water reactors at Atucha nuclear power plant and

2655-447: The other hand, defined it as ⁠ 1 / 16 ⁠ of the mass of an atom of the isotope oxygen-16 ( O). The existence of two distinct units with the same name was confusing, and the difference (about 1.000 282 in relative terms) was large enough to affect high-precision measurements. Moreover, it was discovered that the isotopes of oxygen had different natural abundances in water and in air. For these and other reasons, in 1961

2714-410: The overall equation of the nuclear reaction (activation and decay) is: 27 Co + n → 27 Co → 28 Ni + e + 2 γ Given its half-life, the radioactive activity of a gram of Co is close to 42  TBq (1,100  Ci ). The absorbed dose constant is related to the decay energy and time. For Co it is equal to 0.35  mSv /(GBq h) at one meter from

2773-400: The reactor core and removed after a predetermined time without the need for cold shutdown . Also, the heavy water used as a moderator is commonly held at lower temperatures than is the coolant in light water reactors , allowing for a lower speed of neutrons, which increases the neutron cross section and decreases the rate of unwanted (n,2n) "knockout" reactions. Co is the material encasing

2832-471: The relative standard uncertainty of 4.5 × 10 at the time of the redefinition is insignificant for all practical purposes. Though relative atomic masses are defined for neutral atoms, they are measured (by mass spectrometry ) for ions: hence, the measured values must be corrected for the mass of the electrons that were removed to form the ions, and also for the mass equivalent of the electron binding energy , E b / m u c . The total binding energy of

2891-471: The scrap collectors and workers at the junkyard, were exposed to high levels of radiation and became ill. Three junkyard workers later died of their exposure, which was estimated to be over 6  Gy . Afterward, the source was safely recovered by Thai authorities. In December 2013, a truck carrying a disused 111 TBq Co teletherapy source from a hospital in Tijuana to a radioactive waste storage center

2950-409: The six electrons in a carbon-12 atom is 1 030 .1089 eV  = 1.650 4163 × 10  J : E b / m u c  = 1.105 8674 × 10 , or about one part in 10 million of the mass of the atom. Before the 2019 revision of the SI, experiments were aimed to determine the value of the Avogadro constant for finding the value of the unified atomic mass unit. A reasonably accurate value of

3009-545: The source. This allows calculation of the equivalent dose , which depends on distance and activity. For example, 2.8 GBq or 60 μg of Co, generates a dose of 1 mSv at 1 meter away, within an hour. The swallowing of Co reduces the distance to a few millimeters, and the same dose is achieved within seconds. Test sources, such as those used for school experiments, have an activity of <100 kBq. Devices for nondestructive material testing use sources with activities of 1 TBq and more. The high γ-energies correspond to

SECTION 50

#1732858179340

3068-415: The three levels generate six different gamma-ray frequencies. In the diagram the two important ones are marked. Internal conversion energies are well below the main energy levels. Co is a nuclear isomer of Co with a half-life of 10.467 minutes. It decays by internal transition to Co, emitting 58.6 keV gamma rays, or with a low probability (0.22%) by β-decay into Ni. The main advantage of Co

3127-575: The unified atomic mass unit, with that name and symbol "u", was adopted by the International Bureau for Weights and Measures (BIPM) in 1971 as a non-SI unit accepted for use with the SI . In 1993, the IUPAC proposed the shorter name "dalton" (with symbol "Da") for the unified atomic mass unit. As with other unit names such as watt and newton, "dalton" is not capitalized in English, but its symbol, "Da",

3186-429: The unit of mass in the system of atomic units , which is the electron rest mass ( m e ). The atomic mass constant can also be expressed as its energy-equivalent , m u c . The CODATA recommended values are: The mass-equivalent is commonly used in place of a unit of mass in particle physics , and these values are also important for the practical determination of relative atomic masses. The interpretation of

3245-459: Was determined to be from Co that had contaminated the steel. In May 2013, a batch of metal-studded belts sold by online retailer ASOS were confiscated and held in a US radioactive storage facility after testing positive for Co. A radioactive contamination incident occurred in 1984 in Ciudad Juárez , Chihuahua , Mexico , originating from a radiation therapy unit illegally purchased by

3304-413: Was hijacked at a gas station near Mexico City . The truck was soon recovered, but the thieves had removed the source from its shielding. It was found intact in a nearby field. Despite early reports with lurid headlines asserting that the thieves were "likely doomed", the radiation sickness was mild enough that the suspects were quickly released to police custody, and no one is known to have died from

3363-399: Was made before the discovery of isotopes in 1912. Physicist Jean Perrin had adopted the same definition in 1909 during his experiments to determine the atomic masses and the Avogadro constant . This definition remained unchanged until 1961. Perrin also defined the "mole" as an amount of a compound that contained as many molecules as 32 grams of oxygen ( O 2 ). He called that number

3422-598: Was named the "unified atomic mass unit" and given a new symbol "u", to replace the old "amu" that had been used for the oxygen-based unit. However, the old symbol "amu" has sometimes been used, after 1961, to refer to the new unit, particularly in lay and preparatory contexts. With this new definition, the standard atomic weight of carbon is about 12.011 Da , and that of oxygen is about 15.999 Da . These values, generally used in chemistry, are based on averages of many samples from Earth's crust , its atmosphere , and organic materials . The IUPAC 1961 definition of

3481-519: Was the basis of the atomic weight scale . For technical reasons, in 1898, chemist Wilhelm Ostwald and others proposed to redefine the unit of atomic mass as ⁠ 1 / 16 ⁠ the mass of an oxygen atom. That proposal was formally adopted by the International Committee on Atomic Weights (ICAW) in 1903. That was approximately the mass of one hydrogen atom, but oxygen was more amenable to experimental determination. This suggestion

#339660