Safety

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Introduction
Conventional Generators
Other Sources
Choice of Radiation
Monochromatization and Collimation of X Rays
X-Ray Safety

Introduction In crystallographic studies the scattered intensities are increased when the wavelength of the radiation is similar to the distance between scattering centers. X-Ray photons with typical wavelengths of 0.5-1.8 Å are on the order of interatomic distances (0.8-3 Å). Other energy sources such as electrons and neutrons are also used to in diffraction studies. X Rays can be generated by conventional generators, by nuclear fission, by synchrotrons, and by plasma sources. Conventional generators are by far the most widely used sources of X rays in the laboratory. back to top

Conventional Generators X Rays are produced whenever a beam of particles, with sufficient energy collides with a target material. X Rays for crystallographic studies are typically generated by bombarding a metal target with an energetic beam of electrons. The electrons are accelerated towards the target by a large applied potential between the e- source and the target. When the beam of electrons hit the target (or anode) a variety of events occur. This rapid deceleration of electrons causes the emission of X-ray radiation, photo-electrons, Auger electrons, and a large amount of heat. Actually two types of X rays are emitted in this process. X Rays are emitted in a continuous band of white radiation as well as a series of lines that are characteristic of the target material.
White Radiation

Some of these collisions, or rapid decelerations of the electrons, result in the emission of a continuous spectrum of X rays called white radiation or Bremsstrahlung. If all of the energy carried by an electron is converted into radiation, the energy of the X-ray photon would be given by

E_max = hn_max= eV

where h = Plank's constant, n_max = largest frequency, e = charge of an electron, V = applied voltage.

hn_max = hc/l_min = eV

l_min = hc/eV = 12398./V (volts)

It is not really probable that all of the energy of an electron would be converted to X rays during a single collision. Usually the electron enters into multiple collisions with the atoms of the target material. The highest intensity of white radiation is obtained at a wavelength that is about 1.5 times the minimum wavelength (Duane-Hunt limit). The white radiation intensity curve may be fit to an expression of the form:

I_w = A i Z Vⁿ, n ~ 2

where i is the applied current, Z is the atomic number of the target, V is the applied voltage and A is a proportionality constant. The only type of diffraction experiment that uses white radiation is the Laue experiment.

Characteristic Radiation

When the energy of the electron beam is above a certain threshold value (called the excitation potential) an additional set of discrete peaks is observed superimposed on the white radiation curve. The energies of these peaks are characteristic of the type of target material. When a sufficiently energetic beam of electrons collides with the target atoms it causes the core electrons to be ejected from the target atoms. Then valence electrons in higher energy states of the target atoms fill the core-shell vacancies and, in the process, emit X-ray photons. These X-ray photons have discrete energies that are equal to the difference in energy between the valence and core energy levels.

The characteristic lines in an atom's emission spectra are called K, L, M, ... and correspond to transitions from higher energy states to the n = 1, 2, 3, ... quantum levels, respectively. When the two atomic energy levels are adjacent, the transitions are described as a lines (n = 2 to n = 1, or n = 3 to n = 2). When the two levels are separated by one or more levels, the transitions are known as b lines (n = 3 to n = 1 or n = 4 to n = 2). Because all K lines arise from a loss of electrons in the n = 1 state, the Ka and Kb lines always appear at the same time. The n = 2 and higher energy levels are actually split into multiple energy levels causing the a and b transitions to split into a variety of closely spaced lines at high resolution. Thus, the observed Cu Ka line can be resolved into Ka₁ and Ka₂ lines with separate wavelengths at high scattering angle. The Ka₁ line is about twice as intense as the Ka₂ line. At low resolution (low scattering angle) the Ka wavelength is considered as a weighted average of the Ka₁ and Ka₂ lines with l (Ka_ave) = 2(l [(Ka₁) + l (Ka₂)]/3. The Ka line is about 10 times as intense as the Kb line. The intensity of the Ka line can be calculated by

I_k = B i (V - V_k)^1.5

where i = applied current, V_k = excitation potential of the target material, V = applied voltage. It can be shown that the ratio I_k / I_w is a maximum if the accelerating voltage is chosen to be about 4 times the excitation potential of the anode. The notation for describing the characteristic X-ray lines described above was first described by Siegbahn.

In 1991, the International Union of Pure and Applied Chemists (IUPAC) recommended that X-ray lines be referred to by writing the initial and final levels separated by a hyphen, e.g. Cu K- L₃, rather than using the Siegbahn notation, e.g. Cu Ka₁, which is based on the relative intensities of the lines (R. Jenkins, R. Manne, J. Robin, & C. Senemaud (1991) Pure and Appl. Chem. 63 735-746). A table of the correspondence between IUPAC and Siegbahn notations is given in the "International Tables for Crystallography," Vol. C (1995) Kluwer: Boston, p 167. The Siegbahn notation remains common in the chemical and crystallographic literature.

The generation of X rays is very inefficient. In addition to white radiation and characteristic lines, laboratory sources also produce Auger electrons and photo-electrons. However, the vast majority of the power used in generating X rays produces heat. Less than 1% of the energy applied to the tube actually produces the characteristic radiation used in most diffraction experiments.

Sealed-tube X-ray generators use a stationary anode. These tubes are limited in the power that can be applied to the tube by the amount of heat that can be dissipated through water cooling. One way to increase the heat dissipating ability of the system, and thus increase the X-ray beam intensity, is to move or rotate the anode surface so that the beam of electrons continually hits a new region of the anode. Typically rotating-anode generators yield about 5 times the flux of X-rays as is routinely produced by normal-focus sealed-tube generators.

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Other Sources There are two other sources of X-ray photons that have special applications in the laboratory. Certain radioactive materials decay to produce photons with energies in the X-ray region (e.g., ⁵⁵Fe). The flux of photons of this radioactive material is so low that it is not used as a source of X-rays for diffraction experiments. However, small samples of ⁵⁵Fe are often used to test the functioning of X-ray detectors.
Synchrotrons produce the highest flux sources available. Unfortunately, because synchrotrons are very expensive to build and maintain, there are few such sources available throughout the world.
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Choice of Radiation Most X-ray tubes used for diffraction have targets (anodes) made of copper or molybdenum metal. The characteristic wavelengths and excitation potentials for these materials are shown below. Copper radiation is preferred when the crystals are small or when the unit cells are large. Molybdenum radiation is preferred for larger crystals of strongly absorbing materials and for very high resolution (high 2q) data. The scintillation detectors, typically used in small molecule diffraction, have somewhat higher quantum efficiencies for molybdenum radiation than for copper radiation. Thus many labs use molybdenum radiation exclusively. However, because a brighter incident beam of X-rays is produced from a copper tube than from a molybdenum tube at the same power level, very small crystals of even strongly absorbing materials will often yield better diffracted intensities from copper radiation than from molybdenum radiation. Occasionally, other types of radiation are used for diffraction such as Cr, Fe, W or Ag.
Selected X-ray wavelengths and excitation potentials.

Cr Fe Cu Mo ------------------------------------------------------ Z 24 26 29 42 Ka1 (Å) 2.28962 1.93597 1.54051 0.70932 Ka2 (Å) 2.29351 1.93991 1.54433 0.71354 Ka (Å) 2.29092 1.93728 1.54178 0.71073 Ka (Å) 2.08480 1.75653 1.39217 0.63225 b filter Ti Cr Ni Nb Resolution (Å) 1.15 0.95 0.75 0.35 Excit. Pot. (kV) 5.99 7.11 8.98 20.0
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Monochromatization and Collimation of X Rays Nearly all of the data collection methods to be discussed require that the energy of the X-ray radiation be limited to as narrow a band of energies (and hence wavelengths) as possible. Using a narrow wavelength band of X rays significantly reduces the fluorescent radiation given off by the sample and makes absorption corrections much simpler to perform. It has been noted that when the applied voltage for K excitation occurs, both the Ka and Kb lines as well as the white radiation curve are observed. Usually the Ka band is selected for diffraction experiments. Also, these data collection methods require that the incident beam be a parallel beam of photons. To assure that the beam is as parallel as possible, the incident beam path is collimated to produce an incident beam that is about 0.5 to 1 mm in diameter.
Filters

When the energy of a photon beam is just above the excitation potential (absorption edge) of a material, that material strongly absorbs the given photon beam. If another substance can be found that has an absorption edge between the Ka and Kb lines of the incident photon beam, this other substance can be used to significantly reduce the intensity of the Kb line relative to the Ka line. The absorption edges of elements with Z_Filter = Z_Target - 1 (or - 2) meet this requirement. The thickness of the filtering material is usually chosen to reduce the intensity of the Kb line by a factor of 100 while reducing the intensity of the Ka line by a factor of 10 or less.

The absorption of X rays follows Beer's Law:

I / I^o = exp(-m*x)

where I = transmitted intensity, I^o = incident intensity, x = thickness of material, m = linear absorption coefficient of material that depends on the substance, its density and the wavelength of radiation. Since m depends on the density of the absorbing material, it is usually tabulated as the mass absorption coefficient m_m = m / r.

Monochromators

An alternative way to produce an X-ray beam with a narrow wavelength distribution is to diffract the incident beam from a single crystal of known lattice dimensions. X-Ray photons of different wavelengths are diffracted from a given set of planes in a crystal at different scattering angles according to Bragg's Law. Therefore a narrow band of wavelengths can be chosen by selecting a particular scattering angle for the monochromator crystal. Crystal monochromators should have the following properties.

The crystal must be mechanically strong and stable in the X-ray beam.

The crystal must have a strong diffracted intensity at a reasonably low scattering angle for the wavelength of the radiation being considered.

The mosaicity of the crystal, which determines the divergence of the diffracted beam and the resolution of the crystal, should be small.

A variety of geometries are possible for crystal monochromators. Most monochromators are cut with one face parallel to a major set of crystal planes. These monochromators are then oriented to diffract Ka lines from this major set of planes. Some monochromators are cut at an angle to the major set of planes in order to produce a diffracted beam with a smaller divergence. By properly curving the monochromator crystal, the diffracted beam may be focused onto a very small area. This curving may be achieved either by bending or grinding or both bending and grinding. Curved monochromators are usually reserved for special applications such synchrotrons.

Graphite crystals cut on the [002] face are the most common crystal monochromators used in X-ray diffraction laboratories. Other special purpose monochromators include germanium and lithium fluoride. In all commercially available single-crystal instruments, the monochromator is placed in the incident beam path. Powder diffraction instruments with a point detector typically place a monochromator in the diffracted beam path to remove any fluorescent radiation from the sample. Crystal monochromators systematically alter the polarization of the incident beam, requiring different geometric corrections be applied to the intensity data.

Collimators

Collimators are objects inserted in the incident- or diffracted-beam path to shape the X-ray beam. Metal tubes are typically used in single-crystal experiments. The inside radius of the collimators is typically chosen to be somewhat larger than the size of the sample so that the sample may be bathed in the incident beam at all times. Incident-beam collimators are usually manufactured with two narrow regions. The region closest to the X-ray source carries out the collimation functions. The second narrow region has a larger diameter and is used to remove the "parasitic" radiation that takes a bent path due to interaction with the edge of the first narrow region of the collimator. Diffracted beam collimators only function to remove any stray radiation from hitting the detector.

When a very intense and very small point source is needed, such as in protein crystallography, X-ray mirrors may be used to shape the incident beam. Mirrors are sometimes made from materials which act as beta filters for the radiation in use. Mirrors are primarily used with very bright X-ray sources such as synchrotrons or rotating-anode generators.

Powder diffraction experiments usually require a line-shaped incident beam that is produced from a pair of parallel knife edges. A set of Soller slits are used in the beam path after the knife edges to remove parasitic radiation that scatters from the edges of the blades. Soller slits are a set of parallel thin foil sheets which absorb nearly all of the X rays not traveling parallel to metal sheets.

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X Ray Safety X-Ray radiation is a form of ionization radiation that is potentially very dangerous. The most intense and therefore dangerous part of the instrument is the incident X-ray beam. Thus care should always be exercised to know the expected path of the incident beam. Scattered radiation is typically of such reduced intensity that it poses a much reduced health risk to the researcher.
There are several properties of X rays that make this type of radiation particularly dangerous to use in the laboratory. X-Ray radiation cannot be sensed by a human! Some people can feel the presence of X rays on their skin as a kind of "tingling sensation." What these people are feeling are charged air particles produced by the interaction of the ionizing X rays with air.

X-Ray radiation is also hazardous because it can "bounce" off surfaces and appear to "bend" around corners. Thus it is important to survey each instrument for leaks after any modifications to the instrument. These surveys are conducted by the Crystallography Lab director in the CIC facility.

Although X-ray instruments have the potential to be lethal, when used improperly, modern analytical X-ray instruments pose no great risk to careful users. The manufacture and use of analytical X-ray instruments is carefully regulated by the federal and state governments. Current regulations require a variety of safety devices be built into X-ray instruments that make it very difficult for anyone to even accidentally expose themselves to the dangerous incident X-ray beam. The design of the instruments limits even accidental exposures to the hands, arms and facial areas.

Also, the types of radiation used in diffraction instruments (primarily Mo and Cu radiation) are considered "soft" radiation. These types of soft radiation generally will not penetrate more than 2-4 cm into the body. Accidental exposures to radiation from diffraction experiments will usually cause damage only to the skin and possibly bones near the surface of the body. Exposure of soft X rays to the eyes may cause cataracts to form. Because of the possibility of cataracts forming, it is recommended that glasses be worn in an X-ray lab.

There are three general rules to reduce a person's exposure to any type of ionizing radiation. These rules are part of a NRC program to reduce exposure to radiation known as ALARA, As Low As Reasonably Achievable.

Reduce the time you are exposed to the radiation source.

Increase the distance between yourself and the radiation source.

Increase shielding between yourself and the radiation source.

Personal radiation detection devices (TLD badges) are generally less important to the diffraction worker than to other radiation workers. Diffraction instruments use highly collimated sources that are less likely to expose the radiation badge than diffuse radiation sources. However, these devices often give the first warning of exposure to a radiation worker. Actual amounts of exposure can be approximated by changes in the white blood cell count before and after the suspected incident. TLD badges are not worn in our lab except by personnel that are modifying or aligning the instruments.

As with all types of ionizing radiation, X Rays cause the most damage to rapidly growing, undifferentiated cells. Thus, for women that are pregnant or suspect that they are pregnant, special care should be taken to protect the fetus especially during the first trimester.

A second serious hazard from the X-ray instrument is electrical shock. The X-ray generator, which is a highly regulated DC power supply, operates at voltages of 40 to 60 kV in order to achieve an optimum flux of X rays. These power supplies should only be serviced by trained electrical engineers. If any object should fall under the generator cabinet, ask the Crystallography Lab staff to help you retrieve the object--do not go after the object by yourself. back to top

Introduction	In crystallographic studies the scattered intensities are increased when the wavelength of the radiation is similar to the distance between scattering centers. X-Ray photons with typical wavelengths of 0.5-1.8 Å are on the order of interatomic distances (0.8-3 Å). Other energy sources such as electrons and neutrons are also used to in diffraction studies. X Rays can be generated by conventional generators, by nuclear fission, by synchrotrons, and by plasma sources. Conventional generators are by far the most widely used sources of X rays in the laboratory. back to top
Conventional Generators	X Rays are produced whenever a beam of particles, with sufficient energy collides with a target material. X Rays for crystallographic studies are typically generated by bombarding a metal target with an energetic beam of electrons. The electrons are accelerated towards the target by a large applied potential between the e- source and the target. When the beam of electrons hit the target (or anode) a variety of events occur. This rapid deceleration of electrons causes the emission of X-ray radiation, photo-electrons, Auger electrons, and a large amount of heat. Actually two types of X rays are emitted in this process. X Rays are emitted in a continuous band of white radiation as well as a series of lines that are characteristic of the target material. White Radiation Some of these collisions, or rapid decelerations of the electrons, result in the emission of a continuous spectrum of X rays called white radiation or Bremsstrahlung. If all of the energy carried by an electron is converted into radiation, the energy of the X-ray photon would be given by E_max = hn_max= eV where h = Plank's constant, n_max = largest frequency, e = charge of an electron, V = applied voltage. hn_max = hc/l_min = eV l_min = hc/eV = 12398./V (volts) It is not really probable that all of the energy of an electron would be converted to X rays during a single collision. Usually the electron enters into multiple collisions with the atoms of the target material. The highest intensity of white radiation is obtained at a wavelength that is about 1.5 times the minimum wavelength (Duane-Hunt limit). The white radiation intensity curve may be fit to an expression of the form: I_w = A i Z Vⁿ, n ~ 2 where i is the applied current, Z is the atomic number of the target, V is the applied voltage and A is a proportionality constant. The only type of diffraction experiment that uses white radiation is the Laue experiment. Characteristic Radiation When the energy of the electron beam is above a certain threshold value (called the excitation potential) an additional set of discrete peaks is observed superimposed on the white radiation curve. The energies of these peaks are characteristic of the type of target material. When a sufficiently energetic beam of electrons collides with the target atoms it causes the core electrons to be ejected from the target atoms. Then valence electrons in higher energy states of the target atoms fill the core-shell vacancies and, in the process, emit X-ray photons. These X-ray photons have discrete energies that are equal to the difference in energy between the valence and core energy levels. The characteristic lines in an atom's emission spectra are called K, L, M, ... and correspond to transitions from higher energy states to the n = 1, 2, 3, ... quantum levels, respectively. When the two atomic energy levels are adjacent, the transitions are described as a lines (n = 2 to n = 1, or n = 3 to n = 2). When the two levels are separated by one or more levels, the transitions are known as b lines (n = 3 to n = 1 or n = 4 to n = 2). Because all K lines arise from a loss of electrons in the n = 1 state, the Ka and Kb lines always appear at the same time. The n = 2 and higher energy levels are actually split into multiple energy levels causing the a and b transitions to split into a variety of closely spaced lines at high resolution. Thus, the observed Cu Ka line can be resolved into Ka₁ and Ka₂ lines with separate wavelengths at high scattering angle. The Ka₁ line is about twice as intense as the Ka₂ line. At low resolution (low scattering angle) the Ka wavelength is considered as a weighted average of the Ka₁ and Ka₂ lines with l (Ka_ave) = 2(l [(Ka₁) + l (Ka₂)]/3. The Ka line is about 10 times as intense as the Kb line. The intensity of the Ka line can be calculated by I_k = B i (V - V_k)^1.5 where i = applied current, V_k = excitation potential of the target material, V = applied voltage. It can be shown that the ratio I_k / I_w is a maximum if the accelerating voltage is chosen to be about 4 times the excitation potential of the anode. The notation for describing the characteristic X-ray lines described above was first described by Siegbahn. In 1991, the International Union of Pure and Applied Chemists (IUPAC) recommended that X-ray lines be referred to by writing the initial and final levels separated by a hyphen, e.g. Cu K- L₃, rather than using the Siegbahn notation, e.g. Cu Ka₁, which is based on the relative intensities of the lines (R. Jenkins, R. Manne, J. Robin, & C. Senemaud (1991) Pure and Appl. Chem. 63 735-746). A table of the correspondence between IUPAC and Siegbahn notations is given in the "International Tables for Crystallography," Vol. C (1995) Kluwer: Boston, p 167. The Siegbahn notation remains common in the chemical and crystallographic literature. The generation of X rays is very inefficient. In addition to white radiation and characteristic lines, laboratory sources also produce Auger electrons and photo-electrons. However, the vast majority of the power used in generating X rays produces heat. Less than 1% of the energy applied to the tube actually produces the characteristic radiation used in most diffraction experiments. Sealed-tube X-ray generators use a stationary anode. These tubes are limited in the power that can be applied to the tube by the amount of heat that can be dissipated through water cooling. One way to increase the heat dissipating ability of the system, and thus increase the X-ray beam intensity, is to move or rotate the anode surface so that the beam of electrons continually hits a new region of the anode. Typically rotating-anode generators yield about 5 times the flux of X-rays as is routinely produced by normal-focus sealed-tube generators. back to top
Other Sources	There are two other sources of X-ray photons that have special applications in the laboratory. Certain radioactive materials decay to produce photons with energies in the X-ray region (e.g., ⁵⁵Fe). The flux of photons of this radioactive material is so low that it is not used as a source of X-rays for diffraction experiments. However, small samples of ⁵⁵Fe are often used to test the functioning of X-ray detectors. Synchrotrons produce the highest flux sources available. Unfortunately, because synchrotrons are very expensive to build and maintain, there are few such sources available throughout the world. back to top
Choice of Radiation	Most X-ray tubes used for diffraction have targets (anodes) made of copper or molybdenum metal. The characteristic wavelengths and excitation potentials for these materials are shown below. Copper radiation is preferred when the crystals are small or when the unit cells are large. Molybdenum radiation is preferred for larger crystals of strongly absorbing materials and for very high resolution (high 2q) data. The scintillation detectors, typically used in small molecule diffraction, have somewhat higher quantum efficiencies for molybdenum radiation than for copper radiation. Thus many labs use molybdenum radiation exclusively. However, because a brighter incident beam of X-rays is produced from a copper tube than from a molybdenum tube at the same power level, very small crystals of even strongly absorbing materials will often yield better diffracted intensities from copper radiation than from molybdenum radiation. Occasionally, other types of radiation are used for diffraction such as Cr, Fe, W or Ag. Selected X-ray wavelengths and excitation potentials. Cr Fe Cu Mo ------------------------------------------------------ Z 24 26 29 42 Ka1 (Å) 2.28962 1.93597 1.54051 0.70932 Ka2 (Å) 2.29351 1.93991 1.54433 0.71354 Ka (Å) 2.29092 1.93728 1.54178 0.71073 Ka (Å) 2.08480 1.75653 1.39217 0.63225 b filter Ti Cr Ni Nb Resolution (Å) 1.15 0.95 0.75 0.35 Excit. Pot. (kV) 5.99 7.11 8.98 20.0 back to top
Monochromatization and Collimation of X Rays	Nearly all of the data collection methods to be discussed require that the energy of the X-ray radiation be limited to as narrow a band of energies (and hence wavelengths) as possible. Using a narrow wavelength band of X rays significantly reduces the fluorescent radiation given off by the sample and makes absorption corrections much simpler to perform. It has been noted that when the applied voltage for K excitation occurs, both the Ka and Kb lines as well as the white radiation curve are observed. Usually the Ka band is selected for diffraction experiments. Also, these data collection methods require that the incident beam be a parallel beam of photons. To assure that the beam is as parallel as possible, the incident beam path is collimated to produce an incident beam that is about 0.5 to 1 mm in diameter. Filters When the energy of a photon beam is just above the excitation potential (absorption edge) of a material, that material strongly absorbs the given photon beam. If another substance can be found that has an absorption edge between the Ka and Kb lines of the incident photon beam, this other substance can be used to significantly reduce the intensity of the Kb line relative to the Ka line. The absorption edges of elements with Z_Filter = Z_Target - 1 (or - 2) meet this requirement. The thickness of the filtering material is usually chosen to reduce the intensity of the Kb line by a factor of 100 while reducing the intensity of the Ka line by a factor of 10 or less. The absorption of X rays follows Beer's Law: I / I^o = exp(-m*x) where I = transmitted intensity, I^o = incident intensity, x = thickness of material, m = linear absorption coefficient of material that depends on the substance, its density and the wavelength of radiation. Since m depends on the density of the absorbing material, it is usually tabulated as the mass absorption coefficient m_m = m / r. Monochromators An alternative way to produce an X-ray beam with a narrow wavelength distribution is to diffract the incident beam from a single crystal of known lattice dimensions. X-Ray photons of different wavelengths are diffracted from a given set of planes in a crystal at different scattering angles according to Bragg's Law. Therefore a narrow band of wavelengths can be chosen by selecting a particular scattering angle for the monochromator crystal. Crystal monochromators should have the following properties. The crystal must be mechanically strong and stable in the X-ray beam. The crystal must have a strong diffracted intensity at a reasonably low scattering angle for the wavelength of the radiation being considered. The mosaicity of the crystal, which determines the divergence of the diffracted beam and the resolution of the crystal, should be small. A variety of geometries are possible for crystal monochromators. Most monochromators are cut with one face parallel to a major set of crystal planes. These monochromators are then oriented to diffract Ka lines from this major set of planes. Some monochromators are cut at an angle to the major set of planes in order to produce a diffracted beam with a smaller divergence. By properly curving the monochromator crystal, the diffracted beam may be focused onto a very small area. This curving may be achieved either by bending or grinding or both bending and grinding. Curved monochromators are usually reserved for special applications such synchrotrons. Graphite crystals cut on the [002] face are the most common crystal monochromators used in X-ray diffraction laboratories. Other special purpose monochromators include germanium and lithium fluoride. In all commercially available single-crystal instruments, the monochromator is placed in the incident beam path. Powder diffraction instruments with a point detector typically place a monochromator in the diffracted beam path to remove any fluorescent radiation from the sample. Crystal monochromators systematically alter the polarization of the incident beam, requiring different geometric corrections be applied to the intensity data. Collimators Collimators are objects inserted in the incident- or diffracted-beam path to shape the X-ray beam. Metal tubes are typically used in single-crystal experiments. The inside radius of the collimators is typically chosen to be somewhat larger than the size of the sample so that the sample may be bathed in the incident beam at all times. Incident-beam collimators are usually manufactured with two narrow regions. The region closest to the X-ray source carries out the collimation functions. The second narrow region has a larger diameter and is used to remove the "parasitic" radiation that takes a bent path due to interaction with the edge of the first narrow region of the collimator. Diffracted beam collimators only function to remove any stray radiation from hitting the detector. When a very intense and very small point source is needed, such as in protein crystallography, X-ray mirrors may be used to shape the incident beam. Mirrors are sometimes made from materials which act as beta filters for the radiation in use. Mirrors are primarily used with very bright X-ray sources such as synchrotrons or rotating-anode generators. Powder diffraction experiments usually require a line-shaped incident beam that is produced from a pair of parallel knife edges. A set of Soller slits are used in the beam path after the knife edges to remove parasitic radiation that scatters from the edges of the blades. Soller slits are a set of parallel thin foil sheets which absorb nearly all of the X rays not traveling parallel to metal sheets. back to top
X Ray Safety	X-Ray radiation is a form of ionization radiation that is potentially very dangerous. The most intense and therefore dangerous part of the instrument is the incident X-ray beam. Thus care should always be exercised to know the expected path of the incident beam. Scattered radiation is typically of such reduced intensity that it poses a much reduced health risk to the researcher. There are several properties of X rays that make this type of radiation particularly dangerous to use in the laboratory. X-Ray radiation cannot be sensed by a human! Some people can feel the presence of X rays on their skin as a kind of "tingling sensation." What these people are feeling are charged air particles produced by the interaction of the ionizing X rays with air. X-Ray radiation is also hazardous because it can "bounce" off surfaces and appear to "bend" around corners. Thus it is important to survey each instrument for leaks after any modifications to the instrument. These surveys are conducted by the Crystallography Lab director in the CIC facility. Although X-ray instruments have the potential to be lethal, when used improperly, modern analytical X-ray instruments pose no great risk to careful users. The manufacture and use of analytical X-ray instruments is carefully regulated by the federal and state governments. Current regulations require a variety of safety devices be built into X-ray instruments that make it very difficult for anyone to even accidentally expose themselves to the dangerous incident X-ray beam. The design of the instruments limits even accidental exposures to the hands, arms and facial areas. Also, the types of radiation used in diffraction instruments (primarily Mo and Cu radiation) are considered "soft" radiation. These types of soft radiation generally will not penetrate more than 2-4 cm into the body. Accidental exposures to radiation from diffraction experiments will usually cause damage only to the skin and possibly bones near the surface of the body. Exposure of soft X rays to the eyes may cause cataracts to form. Because of the possibility of cataracts forming, it is recommended that glasses be worn in an X-ray lab. There are three general rules to reduce a person's exposure to any type of ionizing radiation. These rules are part of a NRC program to reduce exposure to radiation known as ALARA, As Low As Reasonably Achievable. Reduce the time you are exposed to the radiation source. Increase the distance between yourself and the radiation source. Increase shielding between yourself and the radiation source. Personal radiation detection devices (TLD badges) are generally less important to the diffraction worker than to other radiation workers. Diffraction instruments use highly collimated sources that are less likely to expose the radiation badge than diffuse radiation sources. However, these devices often give the first warning of exposure to a radiation worker. Actual amounts of exposure can be approximated by changes in the white blood cell count before and after the suspected incident. TLD badges are not worn in our lab except by personnel that are modifying or aligning the instruments. As with all types of ionizing radiation, X Rays cause the most damage to rapidly growing, undifferentiated cells. Thus, for women that are pregnant or suspect that they are pregnant, special care should be taken to protect the fetus especially during the first trimester. A second serious hazard from the X-ray instrument is electrical shock. The X-ray generator, which is a highly regulated DC power supply, operates at voltages of 40 to 60 kV in order to achieve an optimum flux of X rays. These power supplies should only be serviced by trained electrical engineers. If any object should fall under the generator cabinet, ask the Crystallography Lab staff to help you retrieve the object--do not go after the object by yourself. back to top