Pecharsky V.K., Zavalij P.Y. Fundamentals of Powder Diffraction and Structural Characterization of Materials

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Experimental techniques

indicative of the relative biological-damage potential of the particular

type of radiation. The unit of dose equivalent is the rem or, more

commonly, rnrem (SI unit is the sievert,

Jlkg). Dose equivalent

may likewise be expressed as a rate with units of redh or mredh. For x-

ray exposures, the numerical value of the rem is essentially equal to that

of the rad.

Shielding. It is any material, which is placed around or adjacent to a

source of penetrating radiation for the purpose of attenuating the

exposure rate from the source. For shielding x-rays, materials composed

of high atomic number elements such as lead are highly effective.

Half-value layer. It is that thickness of a given material (i.e. shielding)

required to reduce the exposure rate from a source of gamma or x-rays to

one-half of its unshielded value.

3.4.2

Biological

effects

ionizing radiation

The energy deposited by ionizing radiation as it interacts with matter may

result in the breaking of chemical bonds. If the irradiated matter is living

tissue, such chemical changes may result in an altered structure or function

of constituent cells. The effects of ionizing radiation described at the level of

the human organism can be divided broadly into two categories: stochastic

and non-stochastic effects.

As implied from the name, stochastic, these are effects that occur by

chance. Stochastic effects caused by ionizing radiation consist primarily of

genetic effects and cancer. As the dose to an individual increases, the

probability that cancer or a genetic effect will occur also increases. However,

at no time, even for high doses, is it certain that cancer or genetic damage

will result. Similarly, for stochastic effects, there is no threshold dose below

which it is relatively certain that an adverse effect cannot occur.

addition,

because stochastic effects can occur in unexposed individuals, one can never

be certain that the occurrence of cancer or genetic damage in an exposed

individual is due to radiation.

Unlike stochastic effects, non-stochastic effects are characterized by a

threshold dose below which they do not occur.

addition, the magnitude of

the effect is directly proportional to the size of the dose. Furthermore, for

non-stochastic effects, there is a clear causal relationship between radiation

exposure and the effect. Examples of non-stochastic effects include sterility,

erythema (skin reddening), ulceration, and cataract formation. Each of these

effects differs from the other in both its threshold dose and in the time over

which this dose must be received to cause the effect

(i.e. acute vs. chronic

exposure).

282

Chapter

The effect of ionizing radiation upon humans or other organisms is

directly dependent upon the size of the dose received. The dose, in

turn,

dependent upon a number of factors including the strength of the source, the

distance from the source to the affected tissue, and the time over which the

tissue is irradiated. The ability of a given dose of ionizing radiation to cause

health effects is influenced by the rate at which the dose is imparted.

Because of various repair processes, which occur within the human body, a

given dose of radiation in excess of that needed to produce a particular

non-

stochastic effect may, in fact, not produce such an effect if the dose is

imparted over a relatively long period of time (i.e. chronic exposure). It is

conservatively assumed, however, that dose rate is not a critical factor in

controlling the probable occurrence of stochastic effects such as cancer or

genetic damage.

Long-term exposures to relatively low levels of ionizing radiation are

referred to as chronic exposures. Such exposures have little potential for

inducing non-stochastic health effects and are of concern because of their

potential for initiating cancer or genetic damage. Short-term exposures to

relatively high levels of ionizing radiation are referred to as acute exposures.

Such exposures have the potential for producing both non-stochastic effects

(e.g. erythema, epilation, and ulceration) as well as stochastic effects (e.g.

cancer) in the irradiated tissue.

The effectiveness of a given dose of external radiation in causing

biological damage is dependent upon the portion of the body irradiated. For

example, because of differences in the radiosensitivity of constituent tissues,

the hand is far less likely to suffer biological damage from a given dose of

radiation than are the gonads. Similarly, a given dose to the whole body has

a greater potential for causing adverse health effects than does the same dose

to only a portion of the body.

3.4.3

Exposure limits

Concern over the biological effects of ionizing radiation began shortly

after the discovery of x-rays. From that time to the present, numerous

recommendations regarding occupational exposure limits have been

proposed and modified by various radiation protection groups, the most

important being the International Commission on Radiological Protection

(10). These guidelines have, in turn, been incorporated into regulatory

requirements for controlling the use of materials and devices emitting

ionizing radiation.

general, the guidelines established for radiation exposure have had as

their principle objectives:

(1)

the prevention of acute radiation effects (e.g.,

erythema, sterility), and (2) the limiting of the risks of late, stochastic effects

Experimental techniques

283

(e.g., cancer, genetic damage) to "acceptable" levels. Numerous revisions of

standards and guidelines have been made over the years to reflect both

changes in the understanding of the risk associated with various levels of

exposure and changes in the perception of what constitutes an "acceptable"

level of risk.

Current guidelines for radiation exposure are based upon the conservative

assumption that there is no safe level of exposure.

other words, even the

smallest exposure has some probability of causing a late effect such as

cancer or genetic damage. This assumption has led to the general philosophy

of not only keeping exposures below recommended levels or regulatory

limits but of also maintaining all exposures "as low as is reasonably

achievable"

(ALARA).

This is a fundamental principle of current radiation

safety practice.

Many of the recommendations of the ICRP and other radiation protection

groups regarding radiation exposure have been incorporated into regulatory

requirements by various countries. For the

U.S.

Department of Energy

facilities, radiation exposure limits are found in Title 10, Part 835 of the

Code of Federal Regulations

(10CFR835).

Table

3.1

provides a summary of

the dose limits for occupational external exposures.

Table

3.1.

Summary of dose limits [lo

CFR

835.202, 1003(a)(l), (a)(2)]

Type of exposure Limit

General employee: whole body (internal

external)

redyear

General employee: lens of the eye (external) 15 redyear

General employee: hands, arms below the elbows, feet, legs below the 50 redyear

knee

General employee: any organ or tissue (other than lens of the eye) and skin 50 redyear

Declared pregnant worker: fetus/embryo (internal and external) 0.5 redges-

tation period

Minors: whole body (internal and external)

0.1

redyear

Minors: lens

the eye (external), skin and extremities 10

of general

employee limit

For whole body exposure, the non-occupational exposure limit is 100

mredyear. This is in addition to the 360 mremtyr received, on average, by

individuals in the

U.S.

from natural background radiation and manmade

radiation sources. The 100 rnrernlyear limit also applies to individuals under

age 18 who work in the vicinity of radiation sources.

3.4.4

Radiation hazards of analytical x-ray systems

Because they utilize extremely high intensity x-ray beams, analytical x-

ray systems have the potential for posing significant hazards if operated

inappropriately. Radiation exposures can result not only from the primary

284

Chapter

beam but also from scattered (or diffracted) rays and secondary radiation

from the sample or shielding material.

The x-rays utilized most commonly in analytical systems have

wavelengths in the range of 0.5 to 10 angstroms. These are often referred to

as soft x-rays because of the ease by which they are absorbed in matter.

While this characteristic enables soft x-rays to be readily shielded (generally

requiring only a few millimeters of lead or iron), it also makes them

particularly hazardous since they are highly absorbed even by soft tissue.

The primary beams from analytical x-ray systems are generally well

collimated with beam diameters of less than one centimeter. Because of their

intensity and their high degree of absorption in tissue, they can produce

severe and permanent local injury from exposures of only a fraction of a

second.

Exposure to the primary beam of powder diffraction units is a major

concern.

fact, the greatest risk of acute accidental exposures from

analytical systems occurs in manipulations of the sample to be irradiated by

the direct beam in diffraction studies. Exposure rates on the order of 10,000

lUs (-4x10~

R/h)

can exist at the tube housing port. At these levels,

erythema would be produced from exposures of only 0.03 second and

permanent injury could be inflicted in only 0.1 second. The fingers, of

course, are the part of the body most at risk from such high exposures.

For x-ray diffraction systems, the diffracted beam is also small and

focused with an intensity of up to 80 lUh (-22 rnR/s). Prolonged or repeated

exposures to a beam of this intensity could result in an individual exceeding

the annual dose limit for the particular tissue irradiated.

Through interactions with the sample and shielding material, the primary

beam in diffraction systems often produces diffuse patterns of scattered and

secondary x-rays in the environment around the equipment. Exposure rates

of 150

mlUh

near the shielded sample are not uncommon.

3.4.5

Hazard control measures for analytical x-ray systems

Requirements for controlling potential hazards associated with analytical

x-ray systems are specified in most states by law. For example, at Ames

Laboratory these requirements are detailed in the Radiological Protection

Program or Policy

#10202.003, "Analytical X-ray Safety". Portions of these

hazard control and safety measures are summarized below.

Equipment requirements.

Beam entry shut-off device. A device which prevents the entry of any

portion of an individual's body into the primary beam path or which

Experimental techniques

285

causes the beam to be shut off upon entry into its path shall be provided

on all open-beam configurations.

Shutters. On open-beam configurations, each port on the source housing

shall be equipped with a shutter that cannot be opened unless a collimator

or coupling has been connected to the port.

Housing interlock. Each x-ray tube housing shall be equipped with an

interlock that shuts off the tube if the tube is removed from the housing

or if the housing is disassembled.

Shielding provided by housing. Each x-ray tube housing shall be

constructed so that, with all shutters closed, the radiation, measured at a

distance of five centimeters from its surface, is not capable of producing

a dose in excess of

2.5 mrem in one hour.

Warning lights.

easily visible warning light with the words

"X-RAY

ON" shall be located near any switch that energizes an x-ray tube and

shall be illuminated only when the tube is energized.

Labeling. All analytical x-ray equipment shall be labeled with a readily

discernible sign bearing the radiation symbol and the words:

(1)

"CAUTION

HIGH INTENSITY

X-RAY

BEAM" on the x-ray source

housing; and

(2)

"CAUTION RADIATION

THIS EQUIPMENT

PRODUCES RADIATION WHEN ENERGIZED" near any switch that

energizes the x-ray tube.

Area requirements.

Radiation levels. The components

an analytical x-ray system shall be

arranged and shall be sufficiently shielded to ensure that radiation levels

near the components cannot result in an individual receiving a dose in

excess of 2 mrem in any one hour.

Surveys. Radiation surveys of all analytical x-ray systems sufficient to

show compliance with the radiation levels established in the previous

item must be performed: (1) upon installation of the equipment and at

least every twelve months thereafter; (2) following any change in the

number, type, or arrangement of components in the system;

(3)

following

any maintenance requiring the disassembly or removal of a system

component.

Posting. Each room containing analytical x-ray equipment shall be

conspicuously posted with a sign bearing the radiation symbol and the

words "CONTROLLED AREA." The interlocked barriers surrounding

the x-ray systems shall be posted as a "BUFFER AREA, and the area

immediately adjacent to all x-ray beam ports, whether open beam or

closed beam, shall be posted with a sign which reads "CAUTION [OR

DANGER]

HIGH RADIATION AREA".

286

Chapter

Operating Requirements.

Procedures. Normal operating procedures shall be written and available

to all analytical x-ray equipment workers.

Bypassing safety devices. No individual shall bypass a safety device or

interlock unless the individual has obtained written approval of the

radiation safety officer.

Altering system components. No operation involving the removal or

alteration of shielding materials, tube housing, shutter, collimators, or

beam stops shall be performed until it has been determined that the beam

is off and will remain off until safe conditions have been restored.

Personnel monitoring requirements. At the Ames Laboratory, the Health

Physics Group issues ring dosimeters to all users of x-ray diffraction

devices and, if requested by the user, whole body badges

(Figure

3.17) in

addition to the extremity dosimeters. Dosimetry is used to provide an

indication of the amount of external radiation exposure the wearer has

received, and must be worn at all times when using the equipment. The

dosimetry will not be issued to any individual until the required safety

training has been completed. Dosimeters do not protect from radiation

exposure

they are only used to determine the received dose, if any.

Each individual who wishes to be authorized to use analytical x-ray

equipment at the Ames Laboratory must first receive training concerning

the radiation hazards associated with the use of the equipment, the

function and importance of the equipment's safety devices, proper

operating procedures, and procedures to follow in the event of a

suspected radiation exposure.

Figure

3.17.

Whole body badge (top) and finger dosimeters (bottom) issued to all trained

analytical x-ray users at the

DOE'S Ames Laboratory.

Experimental techniques

287

3.5

Sample preparation

Many factors affect quality of powder diffraction data (e.g. see

Figure

3.1,

Figure

3.5 and

Figure

3.16), and the quality of the specimen used in a

powder diffraction experiment is one of them. It is difficult to

overemphasize the importance of proper sample preparation, especially

because it is always under the complete control of the operator

carrying out

the experiment. Poorly prepared samples will inevitably result in unusable

experimental data, which will require additional effort to repeat everything

from the beginning, thus both time and resources will be wasted. On the

other hand, a high quality sample for powder diffraction may take longer to

prepare, but this will be time well spent!

3.5.1

Powder requirements and powder preparation

The true powder diffraction pattern can only be obtained from a

specimen containing an infinite number of individual particles realizing an

infinite number of orientations in the irradiated volume (e.g. see

Figure

2.31,

Chapter

2).

other words, the particles in the specimen should have a

completely random distribution of crystallographic orientations of grains or

crystallites with respect to one another. Clearly, this ideal situation is

impossible to achieve. However, if one considers a 10 mm diameter and 0.1

mm deep cylindrical sample holder filled with 50 pm diameter spherical

particles, it is easy to estimate that it will hold nearly 9x

lo4

of such particles

at 74.05% packing density, i.e. assuming close packing of the spheres

(Figure

3.18).

When particle size is reduced to 30 pm, the same volume will

contain -4x105 particles, and when the particles are 10 pm in diameter, it

will take a total of -lxlo7 different particles to fill the same volume. These

are large numbers, which may be considered sufficient to approximate the

infinite quantity of particles required for collecting powder diffraction data.

It is, therefore, obvious that a nearly infinite number of particles in a

specimen is easily achieved by reducing average particle size. Another very

effective approach to increase both the number of particles in the irradiated

volume and the randomness of their orientations is to spin the specimen

continuously during data collection.'

The majority of materials, which are routinely examined by powder

diffraction, are initially in a state unsuitable for the straightforward

preparation of the specimen. Unless the material is already in the form of a

fine powder with the average particle size between 10 and 50 pm, its particle

size should be reduced.

Sample spinning is the simplest way to better randomize particle orientations. Whenever

possible it should be employed during data collection.

Chapter

100

Particle diameter,

(pm)

Figure

3.18.

The total number of spherical particles in a cylindrical specimen 10

rnrn

diameter and 0.1

deep as a function of particle size assuming close packing of the

spheres.

specimen diameter,

specimen depth and

particle diameter.

The most commonly used approach to reduce particle size is to grind the

substance using a mortar and pestle, but mechanical mills will also do a fine

job

(Figure

3.19).

Both the former and the latter are available in a variety of

sizes and materials. Mortars and pestles are usually made from agate or

ceramics. Agate is typically used to grind hard materials

(e.g., minerals or

metallic alloys), while ceramic equipment is suitable to grind soft inorganic

and molecular compounds. Mechanical mills require vials and balls, which

are usually made from hardened steel, tungsten carbide, or agate.

Dependent on the nature of the material it may take from several seconds

to several minutes of either manual or mechanical grinding. Prolonged

grinding should not be relied upon, as it usually results in the creation of an

excessive surface (its structure is different from the bulk) and in the

agglomeration of small particles. When aggregation effects are severe,

adding a chemically inert liquid, which does not dissolve the material under

examination, may help to prevent excessive conglomeration.

some cases,

prolonged mechanical grinding may cause serious degradation in the

crystallinity of the material. If the crystallinity is reduced due to induced

stresses and micro-deformations, it can be restored by a brief (10-30 min)

heat treatment at temperatures near

113

of the melting temperature of the

material on the absolute thermodynamic temperature scale.

Experimental

techniques

Figure

3.19.

The tools most commonly used in sample preparation for powder diffraction

experiments: agate mortar and pestle (left), and ball-mill vials made from hardened steel

(middle) and agate (right).

A mortar and pestle are normally the only grinding option when the

quantity of the material is limited, while mechanical mills usually require

much larger amount of material to begin with. It is important to keep milling

equipment clean to prevent cross-contamination of samples. One of the best

ways to clean a mortar and pestle before grinding a new sample is to use a

5050

(by volume) mixture of nitric acid (HN03) and water. When using a

hardened steel or a tungsten carbide ball-mill, the vial should

cleaned

using a blaster with fine

5pm) A1203 powder, and the previously used

balls should normally be discarded. Agate vials and balls can be cleaned

using

5050

mixture of water and nitric acid. At the very least, flashing

with a low-boiling point organic solvent (e.g. acetone or alcohol), wiping

and drying the grinding equipment should always be performed.

One of the biggest concerns when using mechanical grinding in a ball

mill is the possibility of contamination of the sample with the vial (balls)

material. This is particularly true when hard materials are subject to

mechanical processing. To ensure that no contamination occurred, it is

advisable to perform a chemical analysis of the material before and after ball

milling. When a mortar and pestle are used to grind hard materials, excessive

tapping on large pieces of a sample to break them should be avoided as

much as possible since small chips of agate or ceramic may cross-

contaminate the resulting powder.

Some materials, e.g. metallic alloys and polymers, may be quite ductile.

Grinding them using a mortar and pestle or a ball-mill is nearly always

unsuccessful, and the powders may be produced by filing. Needless to say,

the file should be clean (preferably a brand new tool) to prevent cross

Chapter 3

contamination. It is often the case that filings of ductile metallic alloys will

become contaminated by small particles broken off the file. The latter can be

easily removed from the produced powder by using a strong permanent

magnet

(e.g. Nd2Fe14B or SmCo5) provided the powder of interest is

paramagnetic or diamagnetic at room temperature. Powders produced by

filing usually must be heat treated before preparing a specimen for a powder

diffraction experiment to relieve the processing-induced stresses.'

Regardless of the method employed to produce fine particles, the

resultant powder should be screened using appropriate size

sieve(s). The

most commonly used sieves have openings from

ym. It is also

important to ensure that the sieve is clean before sifting the powder under

examination to eliminate cross-contamination. Sieves may be cleaned using

a pressurized gas

(e.g. nitrogen or helium from a high pressure cylinder),

andlor they can be washed in a low boiling point solvent (e.g. acetone or

alcohol) before drying by a high pressure gas. It may be problematic to sift

powders of low-density materials, but every effort should be made to do so.

Sifting not only eliminates large particles from the powder, but it also helps

to break down agglomerates that may have formed during grinding.

3.5.2

Powder

mounting

As mentioned at the beginning of this section, another important

requirement imposed on a high quality powder sample is the realization of

the infinite number of possible orientations of the particles with respect to

one another, i.e. complete randomness in their orientations. The reduction of

particle size is the necessary but not sufficient condition to achieve this.

reality, nearly ideal randomness in particle orientations is only feasible with

a large number of particles, which have spherical or nearly spherical

(isotropic) shapes.

many cases, grinding or milling produces particles with

far from isotropic shapes and, therefore, special precautions should be taken

when mounting powders on sample holders. The most severe cases of non-

random particle orientation distribution~ are expected when platelet-like or

needle-like particles are produced by grinding, see Figure 3.20.

When powder particles have thin platelet-like shapes, they will tend to

agglomerate, aligning their flat surfaces nearly parallel to one another

(Figure 3.20, left). As a result, the orientations of platelets are randomized

via rotations about a common axis normal to their largest faces, and such

samples are expected to have a uniaxial preferred orientation (or texture).

Since

113

of the absolute melting temperature is usually sufficient for an effective stress-

relief, some materials may self-anneal during the filing. One of the examples is lead (Pb),

which is a ductile metal and has melting temperature

601

K. Lead powder self-anneals at

room temperature

(-298

K), thus producing sharp Bragg peaks in the as-filed state.