Radioactivity - San Diego Miramar Collegefaculty.sdmiramar.edu/.../Radioactivity.pdf · 8 Radioactivity February 8, 11 Rates of Radioactive Decay • Radioactive substance possess

February 8, 11 1 Radioactivity

Radioactivity

Dr. Fred Omega Garces Chemistry, Miramar College


Radionuclides - can spontaneously emit particles and radiation which can be expressed by a nuclear equation.

Spontaneous Emission: Mass and charge are conserved.

Alpha emission

Beta emission

Gamma emission

Positron emission

Electron capture

Nuclear Equation: Emission

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92238U → 90

234 Th + 24 He

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53131 I → 54

131Xe + -10e

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01n → 1

1p + -10e

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00γ

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611C → 5

11B + 10e

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11p → 0

1n + 10e

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11p + -1

0e → 01n

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+10e + -1

0e → 00γ

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-10β

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24α

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10e

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-10e


Subatomic Particles Nomenclature Particle Charge Mass (g) Nomenclature alpha 2+ 6.64e-24 4 He 4 α

2 2

beta 1- 9.11e-28 0 e 0 β -1 -1

gamma 0 0 0 γ proton 1+ 1.673 e-24 1 H 1 p

1 1 neutron 0 1.675 e-24 1 n

0 electron 1- 9.11e-28 0 e

-1 position 1+ 9.11e-28 0 e

+1


Belt of Stability: A closer look

A plot of neutrons vs. protons for the stable nuclides. A plot of N vs. Z for all stable nuclides gives rise to a narrow band that veers above N/Z=1 shortly beyond Z=10. The N/Z values for several stable nuclides are given. The most common modes of decay for unstable nuclides in a particular region are shown: nuclides with a high N/Z ratio often undergo β decay; those with a low ratio undergo e- capture or position emission; heavy nuclei beyond the stable band (and a few lighter ones) undergo α decay.

The blue box in the larger diagram is expanded to show the stable and many of the unstable nuclides in that area. Note the modes of decay; α decay decreases both N and Z by two; b decay decreases N and increases Z by one; position emission and e- capture increase N and decrease Z by one.

Neu

tron

s (N

)


Radioactive Series Predicting Nuclear Stability -

Radionuclides sometimes go through a series of emission (Radioactive series) before becoming a stable nuclei.

Nuclear disintegration series for U-238 under goes α-emission (blue arrows) and β-emission (red arrows) until it forms stable Pb-206 (which is an isotope within the belt of stability.


Transmutation - Change of nuclear identity by artificially striking nucleus with a particle. • Many medical chemotherapy isotopes are formed by transmutation. • Many new elements are discovered by transmutation

Nomenclature: Target (bombard, ejected) product 17O + 1H g 14N + 4He In this example, The target is 17O, the product is 14N, the bombarding particle is a proton 1H (or p) and the ejected particle is the alpha particle 4He (or α). The nomenclature is therefore

17O (p,α) 14N

Example: solve the following. The answer is in the next slide.

i) 238U + 1n g 239Np + 0β ( see next slide for answer) ii) 238U (n,γ) 239U ( see next slide for answer) iii) 18O (n,β) 19F ( see next slide for answer)

Nuclear Transmutation


Calculation of Age Based on t1/2 TURIN, Italy -- Almost everything about the Shroud of Turin is mysterious- its age, its authenticity, and the identity of the bearded man with deep-set eyes whose image is imprinted on the 14-foot length of yellowing linen, still believed by many Christians to be the burial cloth of Jesus. ....as carbon testing done on tiny swatches of the shroud concluded in 1988 -- or to the time of Jesus, the centuries-old fascination with the shroud....

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t12

= ln2k

t12

= 5730 yr

k = ln2t

12

=ln2

5730 yr=

0.6935730 yr

= 1.21•10-4 yr−1

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12 C14 C

=1012

1

14 C[ ]= 50.000 ppt based on 12C

14 C[ ]= 46.114 ppt today

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Ao = 50.000A = 46.114

" # $

= lnAoA

= kt

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ln50.00046.114

1.21• 10−4 yr−1 = t

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t = 668.6 yr

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1990 -669 = 1321 ±50 AD

The Shroud of Turin is a linen cloth over 4 m long. It bears a faint, straw-colored image of an adult male of average build who had apparently been crucified. Reliable records of the shroud date to about 1350, but for these past 600 years it has been alleged to be the burial shroud of Jesus Christ. Numerous chemical and other tests have been done on tiny fragments of the shroud in recent years. The general conclusion has been that the image was not painted on the cloth by any traditional method, but no one could say exactly how the image had been created. Re-cent advances in radiochemical dating methods, however, led to a new effort in 1987–1988 to estimate the age of the cloth. Using radioactive 14 C, the flax from which the linen was made was shown to have been grown between 1260 and 1390 A.D. There is no chance that the cloth was made at the time of Christ.


Rates of Radioactive Decay • Radioactive substance possess special rates of decay and half

lives, t1/2 . • Radioactive decay is a first-order kinetic process. ln {N/No} = -k t t1/2 = 0.693/ k

Half-life - The time in which

a substance will decay to one-

half its original mass.


Rates of Radioactive Decay Example: Iodine-131, a beta emitter, has a half-life of 8.0 days.

i) Write the nuclear equation for the beta-decay of I-131 ii) If you start with 12.0 g sample of I-131, what mass (g) would remain after 40 days iii) How much time (days) would passed if 48 g of I-131 decayed to 3.0 grams of I?


Rates of Radioactive Decay Example: Gold-198, is a beta emitter.

i) If you have 18.000 lb of gold but decrease to 1.125 lb in 10.8 days, what is the half life of gold-198 ? ii) How long will it take before 100grams of gold decay to 0.78 grams?

How much money would you have lose if the original 100 g of gold had a value of 5,300 $ ($1500 per oz)


Radioisotope Half-Lives Each isotope has its characteristic half-life unaffected by external conditions, i.e., temp, pressure, chemical nature.

Half-Lives and Type of Decay for Several Radioisotopes. Isotope Half-life (yr) Type of Decay

Natural radioisotopes 238U 4.5 •109 Alpha

235U 7.1•108 Alpha

232Th 1.4•1010 Alpha

40K 1.3•109 Beta

14C 5,730 Beta

Synthetic radioisotopes 239Pu 24,000 Alpha

137Cs 30 Beta

90 Sr 28.8 Beta

131I 0.022 Beta


Medical Application

Radiation can be used as a diagnostic tool to non-invasively probe the body


Biological Effects of Radiation

• Penetrating power of radiation is a function of mass.

• γ -radiation penetrates further than β-radiation which penetrates further than α-radiation.

• Radiation absorbed by tissue causes excitation (non-ionizing radiation) or ionization (ionizing radiation).

• Ionizing radiation is much more harmful than non-ionizing radiation

• Most radiation result in radical formation.

• Free radicals undergoes a chain reaction to form other radicals.


Benefits and Risks A graph of the sources of the average annual exposure of the US population to high-energy radiation. On average ~300mrem/yr sea level, but 400 mrem/yr Denver.

Radioactivity measurement:


Summary Radioactivity Process Type of Radiation α, β, γ, position Nuclear Equation -Nucleons are conserved Nuclear Stability -Greater Z, the higher n:p ratio Transmutation -Changing atomic identity Rates of Decay -Radioactivity follows first order Kinetics

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Activity 13: Basics of Radioactivity Objective: The purpose of this exercise is to become familiar with the properties of nuclear processes, to write nuclear equations. and

to understand how radiation exposure affects health.

Discussion: Chemical reactions occur when valence electrons of atoms interact with each other. In nuclear processes, however, the

nucleus of an atom is responsible for the process. Henri Becquerel is given credit for the discovery of radiation when he noted that a

photographic plate was exposed from a piece of uranium rock. Marie Sklodowska Curie completed much of the pioneering work on

nuclear chemistry after Becquerel's discovery. She was the first woman to win a Nobel Prize, and the first person to win two Noble

Prizes.

In 1902, Frederick Soddy theorized that "radioactivity is the

result of a natural change of an isotope of one element into an

isotope of a different element." Nuclear reactions involve changes

in particles in an atom's nucleus resulting in a change in the atom

itself. All elements heavier than bismuth (Bi), and some lighter

ones, exhibit natural radioactivity and thus can "decay" into lighter

elements. Unlike normal chemical reactions that form molecules,

nuclear reactions result in the transmutation of one element into a

different isotope or a different element altogether. The table to

the right shows the properties and nomenclature of some

subatomic particles involved in radiation activities.

Table A13.1 Nomenclature for subatomic particles

There are five types of radiation and nuclear changes; the three most common are:

1) Alpha (α) decay is the emission of an alpha particle from an atom's nucleus. An α particle contains two protons and two neutrons (and

is similar to a 42He nucleus). When an atom emits an alpha particle, the atom's atomic mass decreases by four units (because two protons

and two neutrons are lost) and the atomic number (z) will decrease by two units. The element is said to "transmute" into another element

that is two z units smaller. An example of this type of transmutation takes place when uranium decays into the element thorium (Th) by

emitting an alpha particle, as depicted in the following equation:

(an alpha emission)

2. Beta decay (β) is the transmutation of a neutron into a proton and an electron resulting in the emission of the electron from the

atom's nucleus. When an atom emits a β particle, the atom's mass does not change (since there is no change in the total number of

nuclear particles), but the atomic number will increase by one because the neutron transmutates into a proton. An example of this is the

decay of the isotope of iodine-131 to xenon-131.

(an beta emission)

3. Gamma radiation (γ) involves the emission of electromagnetic energy (similar to light energy) from an atom's nucleus. No particles

are emitted during gamma radiation, and thus gamma radiation does not cause the transmutation of atoms. However, γ radiation is

often emitted during, and simultaneous to, α or β radioactive decay. X-rays, emitted during the beta decay of cobalt-60, are a common

example of gamma radiation. Another example is the gamma emission and alpha emission of polonium-215 to lead-211.

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92238U → 90

234 Th + 24α

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53131 I → 54

131Xe + -10β

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(an alpha and gamma emission)

Table A13.2 The table below summarizes these radiation processes and the properties of the subatomic particles.

Radionuclides sometimes go through a series of emissions (a

radioactive decay series) before becoming stable nuclei. The radium

series starts from one isotope of uranium, the actinium series from

another isotope of uranium, and the thorium series from thorium.

The final product of each series, after ten or twelve successive

emissions of alpha and beta particles, is a stable isotope of lead. The

nuclear disintegration series for U-238 involves a-emissions (blue

arrows) and b-emissions (red arrows) until it forms stable Pb-206.

As mentioned previously, radioactive decay is the disintegration of an

unstable atom with an accompanying emission of radiation. As a

radioisotope decays to a more stable atom, it emits radiation only

once for each step. Several disintegration steps may be required to

change from an unstable atom to a completely stable atom. Radiation

will be given off at each step. However, once the atom reaches a

stable configuration, no more radiation is given off. For this reason,

radioactive sources become weaker with time. As more and more

unstable atoms become stable atoms, less radiation is produced and

eventually the material will become non-radioactive.

Figure A6.1 Radiation Decay of Uranium-238 to Lead-206

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84215Po → 82

211Pb + 24α + 0

0γ

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The decay of radioactive elements occurs at a fixed rate.

The half-life of a radioisotope is the time required for one

half of the amount of unstable material to degrade into more

stable material. For example, a source will have an intensity

of 100% when new. After one half-life, its intensity will be

cut to 50% of the original intensity. After two half-lives, it

will have an intensity of 25% of a new source. After ten

half-lives, less than one-thousandth of the original activity

will remain. Although the half-life pattern is the same for

every radioisotope, the length of a half-life is different. For

example, Co-60 has a half-life of about 5 years, while Ir-192

has a half-life of about 74 days.

Figure A6.2 The decay process of a radioisotope involves a decrease

of its original mass to half its value after one half-life. After four

half-lives, the isotope will decay to 1/16 of its original mass.

Ionizing radiation comes from both natural and artificial sources. The energy absorbed from exposure to radiation is called a dose.

Absorption of a dose changes can change the state of a specifically-tuned device. These changes provide measures of the dosages

received. Such devices are called dosimeters. Physical, chemical, and biological changes are used as the bases for dosimeters. Radiation

effects depend on the type of radiation, and various units are used for dosages.

Radioactive sources emit alpha, beta, or gamma rays. Each type has a unique effect on the health of living beings. Strengths of sources

are measured in the SI unit Bq (becquerel), which is the number of disintegrations per second, or decay rate. However, the cgs unit

curie (=3.700 x 1010 Bq) is still used in medical and technical practices. For convenience, modifiers have been used for the unit Ci. Decay

rates say nothing about energies or types of particles emitted. When neutron and other particles are the source, the intensity is either

expressed as the total number of particles per unit time or the number of particles per unit time per unit area. However, these numbers

do not contain information about the energy of the beam. For electromagnetic radiation such as a laser, the rate of energy emission

(watt) of the beam is often specified. No particular unit is used for intensity of X-rays, but the rate of photon emission is similar to the

rate of gamma ray emission.

The radiation effect depends on the amount of energy and the type of radiation to which a person is exposed. The amount of energy a

subject is exposed to differs from that absorbed. One sievert equals 100 rem, an older unit of measurement still in widespread use.

One sievert carries with it a 5.5% chance of eventually developing cancer. Doses greater than 1 sievert received over a short time

period are likely to cause radiation poisoning, possibly leading to death within weeks.

Table A6.3 Radiation measurements and activity levels

Some Units of Radiation Measurements

Measurement Common Unit SI Unit Relationship

Lethal Dose Radiation (Life-Forms)

Life-Form50 (rem) LD50 (rem) Activity curie (Ci) = 3.7•1010 becquerel (Bq) = 1 Ci = 3.7•1010 Bq disintegration/s 1 disintegration/s

Absorbed rad gray (Gy) tissue = 1 Gy = 100 rad Dose 1 J/kg

Biological rem = rad x factor 1 sievert (Sv) 1 Sv = 100 rem Damage

Insect 100,000

Bacterium 50,000

Rat 800

Human 500

Dog 300

Everyone is exposed to some level of radiation. At sea level, an average person is exposed to about 300 mrem/yr. Higher altitudes

receive higher radiation exposure. In Denver for example, an average person is exposed to about 400 mrem/yr. Usually, high-dosage

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exposures cause symptoms to develop immediately, and low dose exposures have delayed effects. The worldwide average total exposure

from all radiation sources is about 360 mrem/yr (http://www2.lbl.gov/abc/wallchart/chapters/appendix/appendixd.html ).

High-dose Radiation Exposures: From experiences in industrial and laboratory accidents, the atomic bomb explosions in Hiroshima and

Nagasaki, atomic and thermonuclear testing grounds, and miscalculated and accidental medical exposures of patients, we have learned

the consequences of high-dose radiation exposures.

Injuries due to radiation in the past have led medical professionals to divide radiation clinical cases into four categories. From these

categories, we learn to appreciate the level of danger when a whole body is exposed to various doses.

Low dosage: less than 1 Sv (100 rem). Patients under radiological treatments with a one-time whole-body exposure of 14-100 rem

showed no particular radiation syndromes, and they all recovered well. Very few cases showed nausea and vomit. Symptoms and harmful

effects vary due to different health conditions of individuals. Data for delayed effects are not reliable.

Medium low dosage: 1-2 Sv (100 - 200 rem). Victims receiving 100-200 rem showed nausea and occasional vomiting on the day of

exposure or the day after (onset of radiation sickness). Itching and burning were felt on the skin after exposure, and these sensations

subsided in few days. Two weeks later, however, dermatitis (skin inflammation), itching, burning and pain were severe. More serious cases

showed epilation (loss of hair), erythema (abnormal redness due to inflammation), necrosis (tissue death), wet desquamation (peel off),

followed by weeping, crusting, and ulceration (open sores). Some cases recovered if infections were prevented by medical treatment.

Medium high dosage: 2-5 Sv (200-450 rem). All victims receiving 200-450 rem showed anorexia (loss of appetite), fatigue, nausea and

vomiting, and some had diarrhea. These symptoms might persist for months, but some may show signs of recovery. However, the patients

in this group were susceptible to infection. Hemorrhaging (discharge of blood) in various tissues may happen, and chances of recovery are

limited to only a few.

High dosage exposure: more than 5 Sv (500 rem and more). The human lethal dose (LD50) is generally believed to be 400 to 500 rem,

and even lower if the dose is received in a short time period. Clinically, survival for victims who receive more than 500 rem is impossible.

Higher doses resulted in quick death. Victims went through stages of disorientation and shock due to injuries to the central nervous

(CN) and cardiovascular systems. On the other hand, some victims overcome infections, and they survived after bone marrow

transplants.

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Activity 13a: Basics of Radioactivity; Part 1

_____ / ____Score Name (last)____________________(first)____________________

Lab Section: Day _________ Time _______

I. For each of the following, determine the missing particle, and check mark the type: alpha-emission, beta-emission, positron emission, or electron capture.

Nuclear Equation Type of radiation event (Check mark event)

a) 230Th ® + 226Ra __Alpha emission __Beta emission __Positron emission __ Electron capture

b) 201Hg + ® 201Au __Alpha emission __Beta emission __Positron emission __ Electron capture

c) 234Th ® + 234Pa __Alpha emission __Beta emission __Positron emission __ Electron capture

d) 205Pb + ® 205Tl __Alpha emission __Beta emission __Positron emission __ Electron capture

e) 38K ® + 38Ar __Alpha emission __Beta emission __Positron emission __ Electron capture

ii. For each of the following, write the complete nuclear reaction for the indicated event in the space below. Write the atomic numbers in all your reactions.

a. beta-emission from 214Bi

Write complete reaction here:

b. alpha-emission from 237Np

c. electron capture by 195Au

d. positron emission from 11C

e gamma emission between 10e and -10e

annihilation

iii. For each, i) state whether the element’s identity will change, ii) how the atomic mass changes (+ value), iii) how the atomic number

changes (+ value), iv) the name of the event (positron emission, alpha emission, gamma emission, absorption of light, ionization...).

Process isotope undergoes i) Change Identity? Yes or No

ii) Atomic mass

iii) Atomic number

iv) Event name

a) release of an alpha particle

b) release of a negative particle with no mass

c) release of a gamma ray

d) loss of an electron in the valence orbital

e) promotion of electron from lower to higher energy level

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Activity 13b: Basics of Radioactivity, Part 2 _____ / ____Score Name (last)____________________(first)____________________

Lab Section: Day _________ Time _______

iv. Use the following data to answer the questions below: 1 curie (Ci) = amount of radioactive substance that undergoes 3.70 • 1010 disintegrations per second (dps)

1 roentgen (R) = deposition of 9.33 • 10-6 J/g of tissue. 1 rem = dose of radiation that has the effect of 1 roentgen Total annual radiation exposure from all sources 360 mrem http://www.lbl.gov/abc/wallchart/chapters/appendix/appendixd.html

a) The safe level of radon gas in homes is 4.00 • 10-12 Ci/L or less. If you measure radon levels in your basement (the most likely place for radon to build up), what is the maximum number of Ci that could be safe to find in a basement with dimensions of l2.0 ft x 15.0 ft x 6.00 ft?

b) If you were exposed to all sources of radiation from natural sources and human activities, how many years would it take for you

to absorb 500.0 rem (a fatal dose) of radiation?

c) If you smoke 1.5 packs of cigarettes a day, you will be exposed to 9000.0 mrem per year. Recalculate the number of years in part "b", assuming you also smoke 1.5 packs of cigarettes a day (1 pack contains 20 cigarettes).

v. It takes 1 hour and 17 minutes for a 1.000 g sample of a certain isotope to decay to 0.0625 g. What is the half-life of this isotope?

vi. A radioactive decay series begins with uranium-235 and undergoes the following sequence of emissions: alpha, beta, alpha, beta,

alpha, alpha, alpha, alpha, beta, beta, and alpha.

Show the series of steps below and determine the stable isotope that remains.

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Radioactivity - San Diego Miramar Collegefaculty.sdmiramar.edu/.../Radioactivity.pdf · 8 Radioactivity February 8, 11 Rates of Radioactive Decay • Radioactive substance possess