SAMPLING AND SURVEYING RADIOLOGICAL ENVIRONMENTS - CHAPTER 3 ppt

These emissions of radiation include various combinations of energetic electrons, protons, and neutrons alpha particles and beta particles and electromagnetic radiation gamma rays and X

Radiation and Radioactivity

The purpose of this chapter is to provide the reader with the fundamentals of radioactivity, radiation, and radiation detection Radiological contamination in the environment is of concern to human health because, if left uncontrolled, the con-tamination could lead to adverse health effects such as cancer The interactions between radiation and the human body are essentially collisions between radiation

“particles” and atoms These collisions produce damage mostly by knocking elec-trons from their atomic orbit or leaving atoms in an energized state resulting in additional radioactivity The trail of destruction produced by the radiation particle

is on the atomic scale but may be sufficient to damage or kill cells in human tissue The same interactions that produce adverse health effects may be used to locate and quantify radiological contamination in the environment That is, collisions between

a radiation particle and atoms could occur in a radiation detector leading to a response such as displacing a needle or producing an electronic pulse The magnitude of cell damage or the characteristics of the detector response depend on the type and origin (source) of the radiation

Radiation comes in many physical forms and from a range of sources The types

or forms of radiation of most interest originate as emission from an unstable nucleus

or an excited atom These emissions of radiation include various combinations of energetic electrons, protons, and neutrons (alpha particles and beta particles) and electromagnetic radiation (gamma rays and X rays) There are also more exotic radiation particles such as muons, pions, neutrinos, etc., that are less relevant when considering environmental contamination Sources of radiation include rock and soil (primordial sources); nuclear reactors, high-energy particle accelerators, manufac-tured material, etc (anthropogenic sources); and outer space (cosmic radiation) The type and source of radiation must be taken into consideration when planning envi-ronmental studies since they will influence the selection of the appropriate radiation detection instrumentation

Radioactivity occurs when some part of an atom is unstable The instability comes from having too many protons or too many neutrons in a nucleus, or when

a proton or neutron is in an excited state (has too much energy) The type of radiation (alpha, beta, gamma, or X ray) that is emitted depends on the location of the

42 SAMPLING AND SURVEYING RADIOLOGICAL ENVIRONMENTS

instability That is, alpha, beta, and gamma are only emitted from the nucleus, while

X rays are only emitted from the electrons orbiting the nucleus The energy of a radiation particle depends on the excited state of the nucleus or the orbiting electron For example, a proton in a highly excited state may de-excite (lose the extra energy)

by emitting a gamma particle that has a few million electronvolts of energy An electron that is only slightly excited in its orbit around the nucleus may lose its extra energy by emitting an X ray with only a few electronvolts

Radioactive materials may contain a number of discrete kinds of radioactive atoms To categorize these atoms, the material is first broken into its elemental components (e.g., pure water is two parts hydrogen and one part oxygen) Once a particular element is identified, that element may be further categorized by isotope Whereas an element is defined by the number of protons in its nucleus (all hydrogen atoms have one proton), an isotope of an element is defined by the number of neutrons in the nucleus A cylinder full of pure hydrogen may contain atoms with zero, one, or two neutrons in the nucleus The cylinder therefore contains three hydrogen isotopes The isotopes that are radioactive are called radioisotopes Hydro-gen atoms with two neutrons in their nuclei are radioactive and are therefore radio-isotopes All radioisotopes that have the same number of protons and neutrons in the nucleus have identical physical properties They are chemically identical, emit the same type of radiation, and emit the radiation at the same rate

Radiation particles may be viewed as packets of energy or particles that carry energy This energy is transferred during collisions with matter, producing tissue damage or a detector response The unit often used to describe radiation energy is the electronvolt (eV), where 1 eV is defined as the amount of kinetic energy that

an electron would gain if accelerated through 1 V of potential difference A radiation particle may be very energetic with energies in the thousands of eV (keV) or millions of eV (MeV), or may have only fractions of an eV in energy The more energetic particles are of most interest to an environmental study since these are the particles that produce the most damage in tissue and produce distinct detector responses For example, consider a radiation particle with 1 MeV of energy It takes

an average of about 30 to 34 eV to knock an electron from its orbit around a nucleus The 1 MeV alpha particle could potentially liberate approximately 30,000 electrons In tissue, these electrons could disrupt cellular chemistry, break bonds

in a DNA strand, and generally produce damage that could result in cell mutilation

or cell death In a radiation detector, the 30,000 electrons could be collected and used to characterize the radiation type and source If a radiation particle has only

a few electronvolts, there would be minimal tissue damage and little chance of a measurable detector response

Because radioactivity results from instability in the atomic/nuclear structure, there

is very little that can be done to change the radioactive properties Changing the physical properties of a material by burning, dissolving, solidifying, etc., may change the chemistry of a material but does not change the structure of a nucleus or the radioactive properties A material can be bombarded with neutrons or exposed in the beam of a high-energy particle accelerator to change the nuclear structure (and radio-active properties), but these methods are very expensive, creating new and possibly more hazardous materials, and are typically never considered in an environmental

RADIATION AND RADIOACTIVITY 43

cleanup effort The most reliable method to reduce the radioactivity of a material is

to let time pass

One property that all radioactive materials have in common is that the level of radioactivity decreases with time Some materials may be radioactive for only a fraction of a second These materials have relatively unstable nuclear structures that lose the excess energy quickly Other materials can be radioactive for billions of years These materials have slightly unstable nuclear structures that are not as anxious

to lose the excess energy The rate by which radioisotopes emit radiation or go through radioactive decay is defined by its half-life A half-life is the amount of time

it takes for one half of the radioactive atoms to decay For example, if there are 1000 atoms of a radioisotope with a half-life of 1 year, about 500 will remain (and about

500 will have decayed) after 1 year After another year, only about 250 will remain, about 125 in another year, etc., until all the atoms have decayed By using this example, it is easy to see that a radioisotope with a half-life of 1 billion years will

be around for a very long time In fact, only a very small fraction of these atoms will undergo decay during a human’s lifetime On the other hand, a radioisotope with a half-life of a few minutes or less will be effectively gone in an hour Sometimes when a radioisotope decays, the remaining nucleus is also radioac-tive The original radioisotope is called the parent and the remaining isotope is called the daughter or decay product This first decay product can then decay into a second decay product, which may decay into a third, etc., until a nonradioactive (stable) decay product remains Not all radioisotopes undergo a series of decays For exam-ple, a carbon atom with six protons and eight neutrons (carbon-14) will emit a beta particle leaving a stable nitrogen atom There are, however, three decay series found

in nature that make up the radionuclides at most radioactively contaminated sites: the uranium series, the thorium series, and the actinium series These series are shown in Tables 3.1, 3.2, and 3.3, respectively The parent/daughter relationships, the modes of decay, energies of the radiation particles, and the half-lives presented for these series are always the same When characterizing a site contaminated with uranium series radionuclides, the information presented in Table 3.1 should be used

to select the proper field instrumentation, sampling procedures, laboratory analytical procedures, and health and safety procedures considering the degree to which equi-librium of the series is expected

3.1 TYPES OF RADIATION

When considering environmental contamination, the most relevant forms of radiation include alpha particles, beta particles, X rays, and gamma rays Each of these radiation particles has distinct physical characteristics that impact the way it interacts with matter, including human tissue or radiation detectors Exotic forms of radiation and energetic neutrons may also be important under certain conditions, but rarely in an environmental setting The following discussion describes the physical characteristics of the relevant radiation particles and corresponding effect the particle would have during collision interactions

Table 3.1 Uranium Series

Major Radiation Energies (MeV) and Intensities a

Uranium I 4.468 × 10 9 year 4.15

↓

0.095 0.096 0.1886

2.7 6.2 18.6 72.5

0.0633 0.0924 0.0928 0.1128

3.8 2.7 2.7 0.24

↓

99.87% 0.13%

1.001 0.2070.59

Emax = 1.26

0.132 0.570 0.883 0.926 0.946

19.7 10.7 11.8 10.9 12

↓

↓

0.144

0.37 0.07 0.045

U

238

92

Th

234

90

Pa

234m

91

Pa

234

91

PaIT

234

91

U

234

92

Th

230

90

RADIATION AND RADIOACTIVITY

↓

Emanation

↓

0.73 1.03

48 42.5 6.3

0.2419 0.295 0.352 0.786

7.5 19.2 37.1 1.1

↓

6.7 6.757

6.4 89.9 3.6

5.51 0.0120.008 1.421.505

1.54 3.27

8.3 17.6 17.9 17.7

0.609 1.12 1.765 2.204

46.1 15.0 15.9 5.0

↓

99.979% 0.021%

↓

↓

Radium C ′

1.87 2.34

25 56 19

0.7997 0.2918 0.7997 0.860 1.110 1.21 1.310 1.410 2.010 2.090

0.010 79.1 99 6.9 6.9 17 21 4.9 6.9 4.9

Table 3.1 (continued) Uranium Series

Major Radiation Energies (MeV) and Intensities a

Ra

226

88

Rn

222

86

Po

218

84

Pb

214

82

At

218

85

Bi

214

83

Po

214

84

Ti

210

81

↓

↓

↓

↓

a Intensities refer to percentage of disintegrations of the nuclide itself, not to original parent of series Gamma % in terms of observable

emissions, not transitions.

Source: Shleien, The Health Physics and Radiological Health Handbook, Scinta, Incorporated, Silver Spring, MD, 1992.

Table 3.1 (continued) Uranium Series

Major Radiation Energies (MeV) and Intensities a

Pb

210

82

Bi

210

83

Po

210

84

Tl

206

81

Pb

206

82

RADIATION AND RADIOACTIVITY

Table 3.2 Thorium Series

Major Radiation Energies (MeV) and Intensities a

3.95 4.01

0.2 23 76.8

0.059 0.126 0.190.04

↓

↓

↓

1.014 1.115 1.17 1.74 2.08

7 6.6 3.4 32 12 8

0.338 0.911 0.969 1.588

11.4 27.7 16.6 3.5

(+ 33 more β s)

0.166 0.216

1.19 0.11 0.08 0.27

↓

↓

Emanation

↓

Th

232

90

Ra

228

88

Ac

228

89

Th

228

90

Ra

224

88

Rn

220

86

↓

0.334 0.573

5.2 85.1 9.9

0.239

1.620

1.0 11.8 2.75

↓

64.07% 35.93%

↓

↓

↓

Thorium C ′

1.52 1.80

25 21 50

0.277 0.5108 0.583 0.860

6.8 21.6 85.8 12

a Intensities refer to percentage of disintegrations of the nuclide itself, not to original parent of series Gamma % in terms of observable

emissions, not transitions.

Source: Shleien, The Health Physics and Radiological Health Handbook, Scinta, Incorporated, Silver Spring, MD, 1992.

Table 3.2 (continued) Thorium Series

Major Radiation Energies (MeV) and Intensities a

Po

216

84

Pb

212

82

Bi

212

83

Po

212

84

Tl

208

81

Pb

208

82

RADIATION AND RADIOACTIVITY

Table 3.3 Actinium Series

Major Radiation Energies (MeV) and Intensities a

Actinouranium 7.038 × 10 8 year 4.2– 4.32

4.366 4.398 4.5– 4.6

10.3 17.6 56 11.3

0.1438 0.163 0.1857 0.205

10.5 4.7 54 4.7

↓

0.287 0.304

15 49 35

0.0256 0.0842 14.86.5

↓

↓

Protoactinium 3.276 × 10 4 year 4.95

5.01 5.029 5.058

23 25.6 20.2 11.1

0.0274 0.2837 0.300 0.3027 0.330

9.3 1.6 2.3 4.6 1.3

0.044

10 35 44

0.070 0.100 0.160

0.017 0.032 0.019 98.62% 1.38%

↓

5.978 6.038

20.2 23.3 24.4

0.050 0.236 0.300 0.304 0.330

8.5 11.2 2.0 1.1 2.7

U

235

92

Th

231

90

Pa

231

91

Ac

227

89

Th

237

90

Fr

223

87

Eavg = 0.343

Emax = 1.097

↓

5.716 5.747

24.1 52.2 9.45

0.144 0.154 0.269 0.324 0.338

3.3 5.6 13.6 3.9 2.8

↓

Emanation

6.819

7.4 12.1 80.3

0.271

↓

~100% 0.00023%

↓

0.97 1.37

4.8 1.4 92.9

0.405 0.427 0.832

3.0 1.38 2.8

↓

Table 3.3 (continued) Actinium Series

Major Radiation Energies (MeV) and Intensities a

Ra

223

88

Rn

219

86

Po

215

84

Pb

211

82

At

215

85

Bi

211

83

RADIATION AND RADIOACTIVITY

0.273% 99.73%

0.898 0.540.52

↓

a Intensities refer to percentage of disintegrations of the nuclide itself, not to original parent of series Gamma % in terms of observable

emissions, not transitions.

Source: Shleien, The Health Physics and Radiological Health Handbook, Scinta, Incorporated, Silver Spring, MD, 1992.

Table 3.3 (continued) Actinium Series

Major Radiation Energies (MeV) and Intensities a

Po

211

84

Tl

207

81

Pb

207

82

52 SAMPLING AND SURVEYING RADIOLOGICAL ENVIRONMENTS

3.1.1 Alpha Particles

An alpha particle is basically an energetic helium nucleus, consisting of two protons and two neutrons Alphas are emitted from the nucleus of an atom typically with energies in the million-electronvolt range Because the energy levels are high,

an alpha particle can ionize a large number of atoms when interacting with matter producing a relatively large amount of damage in tissue or creating a relatively large detector response The two protons in an alpha particle create a +2 charge (neutrons have no charge) and the alpha particle is over 7000 times more massive than electrons with which it interacts Both of these facts help limit the range an alpha particle travels That is, because of the large mass and +2 charge (recall that electrons have

a –1 charge), an alpha will undergo multiple collisions over a short track producing

a high density of liberated electrons In fact, a typical alpha particle will only travel

a few inches in air, and cannot penetrate the dead layer of cells on the surface of skin A common analogy used to describe the collisions between an alpha particle and electrons is to imagine throwing a bowling ball (symbolizing the alpha particle) through a room full of Ping-Pong balls (symbolizing the electrons) The bowling ball may easily displace many Ping-Pong balls at first, but will quickly lose its energy and come to rest after traveling a short distance

Because the range of an alpha particle is short, detectors must be held close to the radiation source to make a measurement Also because alpha particles are easily attenuated (shielded), a contaminated surface covered in dust, dirt, or paint may preclude alpha detection Another important characteristic of alpha particles is that they are emitted from nuclei at discrete energies For example, the radioisotope uranium-238 shown at the top of Table 3.1 emits an alpha particle at approximately 4.2 MeV If a site is contaminated with uranium, field personnel could use detectors

to look for the 4.2 MeV alpha to determine where uranium-238 levels are elevated

3.1.2 Beta Particles

A beta particle is basically an energetic electron, but unlike electrons, beta particles can have a +1 or a –1 charge Beta particles are emitted from the nucleus

of an atom and can have energies in the million-electronvolt range Unlike alpha particles that are emitted with discrete energies, beta particles are emitted with energies ranging from 0 eV to a maximum value characteristic of the radioisotope Like alpha particles, the maximum beta energy may be used to identify the radio-nuclide Beta particles have the same mass as an electron and a +1 or –1 charge and interact with electrons in matter (e.g., in tissue or detectors) more like Ping-Pong balls colliding with other Ping-Pong balls Instead of producing a high density of liberated electrons over a short track, the beta bounces around changing directions many times until it loses all its energy through a series of collisions

The range of a beta particle is energy dependent and it may travel several feet

in air Beta particles can penetrate the dead layer of cells on skin, but cannot penetrate through thin layers of paper, aluminum, wood, etc To measure beta particles in the field it is best to hold the detector close to the contaminated surface As with alpha