Nuclear RadiationChapter 3

1. Atoms consist of electrons, protons, and neutrons.

2. Atoms of elements are distinguished by the number of protons in the nucleus (the atomic number).

3. Isotopes of an element have different numbers of neutrons but the same number of p+ and e-.

4. Isotopes of elements react identically (in most chemical reactions).

5. Traditional chemical reactions focus primarily on interactions in the outer valence electrons of atoms.

Review:

• A nucleus with a specified number of protons and neutrons is a nuclide.

E

A – mass number (number of p+ + number of n)

Z – atomic number

• Together, protons and neutrons are called nucleons.

A

Z

Review of Nucleus

Nucleus: Particle Properties Protons, neutrons, and electrons are all fermions (spin 1/2). Protons and neutrons are “heavy” baryons (composed of 3

quarks).[proton = up, up, down quarks and neutron = up, down, down]

Electrons are “light” leptons.

Particle Charge amu Spin

Proton +e 1.007276 1/2 +2.79N

Neutron 0 1.008665 1/2 – 1.91N

Electron –e 5.4858×10-4 1/2 +1.00

The NucleusLet’s take a look at the nucleus, where the protons and neutrons reside.

Breaking these apart can cause a large release of energy.

Protons

What is Radioactivity?Elements that are RADIOACTIVE are UNSTABLE because they have too many nucleons or too much energy. In an attempt to become STABLE, they give up particles or energy and this is…….

RADIOACTIVITY

From Latin radioto (to radiate)

Radioactivity

C126

Stable

C136

Stable

C146

UnstableRADIOACTIVE

Radioactivity

C146

UnstableRADIOACTIVE

The nucleus of this atom is very heavy because it contains two extra NEUTRONS

In order to become stable it needs to get rid of some excess weight

RadioactivityBecause this atom is unstable a NEUTRON begins to break down

Neutron Breakdown

C146

UnstableRADIOACTIVE

Neutron BreakdownNeutrons are made up of positively and negatively charged particles

C146

UnstableRADIOACTIVE

The positive part of the NEUTRON is actually a PROTON

The negative part of the NEUTRON is called a BETA particle

An anti-neutrino is also released

Neutron Breakdown

C146

UnstableRADIOACTIVE

= Neutron breaking down

The Negative Beta Particle is released

This energy is RADIOACTIVE

Having released the particle the NEUTRON now becomes a PROTON

An anti-neutrino is also released

Neutron Breakdown

C146

UnstableRADIOACTIVE

N147

StableNITROGEN

Lose NEUTRON

Gain PROTON

Neutron Breakdown Is Just one way a element can become stable

Accompanied by Beta emission And the conversion of a NEUTRON to a PROTON Carbon-14 is a BETA emitter

There are other ways that an element can obtain stability and this results in different types of RADIATION

Nuclear Chemistry Introduction Most chemical changes deal with the valence electrons Nuclear chemistry deals with changes in the nucleus, often

accompanied by the release of a large amount of energy Unstable nucleus spontaneously emits a particle or energy.

Radiation comes from the nucleus of an atom. Radiation is energy in transit in the form of high speed

particles and electromagnetic waves Radiation cannot be tasted, felt, or smelt, but has the potential to do a

great deal of damage Radioactivity, or radioactive decay, is the spontaneous change of the

nuclei of certain atoms, accompanied by the emission of subatomic particles and/or high-frequency electromagnetic radiation.

There are five principal particles or waves of radiation we will learn about: Alpha ( or 4He2+) Beta ( or e-) Positron () Gamma () Neutrons (n)

Summary Of Decay Types

e

e

symbolparticle He

01

0 1-

particleinneutronsandprotonsparticle in

(NOT Charge here!)

protons 42

Main Types Of Radioactive Decay• An alpha () particle has the same composition as a

helium nucleus (42He): two protons and two neutrons.

• Beta () particles are electrons (-10e).

• Gamma () rays are a highly penetrating form of electromagnetic radiation (0

0). • Positrons are particles having the same mass as electrons

but carrying a charge of 1+ (+10e). A positron and an

electron can annihilate each other upon colliding, producing energy as photons:

-10e + +1

0e 2 00

• Other forms of radioactive decay:• Proton emission • Neutron emission • Electron capture (EC) is a process in which the nucleus

absorbs an electron from an inner electron shell, usually the first or second, thus converting a proton into a neutron, along with the release of an X-ray.

Radioactivity: Historical Overview 1896: Becquerel accidentally discovered that uranyl crystals

emitted invisible radiation onto a photographic plate. 1898: Marie and Pierre Curie discovered polonium (Z=84) and

radium (Z = 88), two new radioactive elements. 1903: Becquerel and the Curie’s received the Nobel prize in

physics for radioactive studies. 1911: Marie Curie received a 2nd Nobel prize (in chemistry) for

discovery of polonium and radium. 1938: Hahn (1944 Nobel prize) and Strassmann discovered

nuclear fission - Lisa Meitner played a key role! 1938: Enrico Fermi received the Nobel prize in physics for

producing new radioactive elements via neutron irradiation, and work with nuclear reactions.

Three Main Types of Radiation

Radioactivity

• All elements have at least one radioactive isotope.

• All isotopes with atomic number greater than 83 are radioactive.

• Artificial and Natural sources exist.• Radioactive isotopes have same chemical

properties as non-radioactive isotopes.

Stable and Unstable Nuclei

Everyday Radiation Exposure

Alpha () Particles Symbol: 2

4He or ; Equivalent to the Helium Atom It is composed of 2 protons, 2 neutrons, has a mass of 4 amus and a charge of

2+

Since they are so large they can cause great damage if they strike tissue.

But, they cannot travel very far because of their weight and they’re low energy

Travel 3-4 inches in air and can be blocked by a sheet of paper Cannot penetrate the epidermal layer of the skin

More of an internal hazard than an external hazard Once ingested they are usually within 3-4 inches of a vital organ

4 inches

Emission of Alpha Particle

3-4 inches

Because Alpha particles are so large, they are the most damaging. The probability of them coming into contact with other particles is great

Beta () Particles Symbol: -1

0e or ; A high energy electron Can be either positively or negatively charged Usually given off when a neutron is converted to a

proton or when protons convert to neutrons. Very small and can travel up to 100 feet in air Can penetrate the skin Can be stopped by a thin piece of metal or 2-3

inches of wood Since they are so small the likelihood of them striking

biological tissue is much less than an Alpha particle.

100 Feet

If particle strikes damage will occur

Particle may pass through without touching any matter

A neutron in the nucleus breaks down1 1 0 n H + e0 1 -1

Gamma () Particles and X-Rays For all practical purposes Gamma and X-rays are identical.

Gamma particles are produced by atomic disintegration X-rays are produced by machines and Electron Capture Both are pure energy and travel at the speed of light 3 x 108 m/s

Can travel great distances without striking other particles. If collision takes place, damage will occur

Because it is electromagnetic radiation, it is deeply penetrating Takes several feet of concrete or many inches of lead to stop them It has no mass or charge Very high energy There are very few pure gamma emitters, although gamma radiation

accompanies most and decay In radiology one of the most commonly used gamma emitters is Tc

43

99mTc →4399Tc +

A gamma decay will have no change in the atomic number or atomic mass

Much energy will pass through without any effect on biological matter

Some energy may cause ionization

Neutron Radiation

Symbol:01n

It has a mass of one, no protons, and no charge Very rare but very lethal Generated in the explosion of nuclear

weapons Neutron Bombs

Since this type of radiation is so specialized it is not usually discussed in lectures such as this

Types of nuclear radiation

RadiationType ofRadiation

Mass (AMU)

Charge Shielding material

Alpha Particle 4 2 Paper, skin, clothes

BetaParticle 1/1836 ±1 Plastic, glass, light metals

Gamma Electromag-netic Wave 0 0

Dense metal, concrete, Earth

NeutronsParticle 1 0 Water, concrete

From: http://www.physics.isu.edu/radinf/properties.htm

Nuclear PhysicsGeneral Rules:

1) emitted to reduce mass, only emitted if mass number above 209

2) emitted to change neutron into proton, happens when have too many neutrons

3) emitted (or electron capture) to change proton into neutron, happens when have too few neutrons

4) emitted to conserve energy in reaction, may accompany or .

5) Neutrons and protons emitted due to bombardment…

Bombardment Reaction Bombardment reaction-bombarding 2 stable atoms together,

creating a radioisotope

All of the known elements whose atomic number is greater than 92 were created from bombardment reactions

Nuclear Equations

Basic principle in writing a nuclear equation :

charge, mass number, and atomic number must be conserved in a nuclear reaction.

The two sides of a nuclear equation must have the same totals of atomic numbers and mass numbers.

Balancing Nuclear Eqns: reactants’ and products’

Atomic numbers must balance

and

Mass numbers must balance

Alpha decay

Beta decay

234Th 234Pa + 0e 90 91 1

beta particle

Gamma radiation

No change in atomic or mass number

11B 11B + 0

5 5 0

boron atom in a

high-energy state

Learning Check NR1

Write the nuclear equation for the beta emitter Co-60.

Solution NR1

Write the nuclear equation for the

Beta emitter Co-60.

60Co 60Ni + 0 e

27 28 -1

Producing Radioactive Isotopes

Bombardment of atoms produces radioisotopes = 60 = 60

59Co + 1n 56Mn + 4H e 27 0 25 2

= 27 = 27

cobalt neutron manganese alpha atom radioisotope particle

Learning Check NR2

What radioactive isotope is produced in the following bombardment of boron?

10B + 4He ? + 1n

5 2 0

Solution NR2

What radioactive isotope is produced in the following bombardment of boron?

10B + 4He 13N + 1n

5 2 7 0

nitrogen

radioisotope

Half-Life of a Radioisotope

The time for the radiation level to fall (decay) to one-half its initial value

decay curve

8 mg 4 mg 2 mg 1 mg

initial

1 half-life 2 3

Half–Life (t1/2)

The half-life (t1/2) of a radioactive nuclide is the time required for one-half the nuclei in a sample of the nuclide to decay.

The shorter the half-life t1/2, the larger the value of (decay constant) and the faster the decay proceeds.

The time required for one-half of the unstable nuclei to decay. (t1/2) A0

A = -------- 2n

A0 = original amountn = number of elapsed half lives

1 half life 1/2 original amount left (50%)2 half lives 1/4 original amount left (25%)3 half lives 1/8 original amount left (13%)4 half lives 1/16 original amount left (6.3%)

Selected Nuclide Half-lives

Learning Check NR3

The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 26 hours?

Solution NR3

t1/2 = 13 hrs

26 hours = 2 x t1/2

Amount initial = 64mg

Amount remaining = 64 mg x ½ x ½

= 16 mg

Radiocarbon Dating

Carbon-14 is formed at a nearly constant rate in the upper atmosphere by the bombardment of nitrogen-14 with neutrons from cosmic radiation. The carbon-14 is eventually incorporated into atmospheric carbon dioxide.

Carbon-14 in living matter decays by ¯ emissions at a rate of about 15 disintegrations per minute per gram of carbon.

When the organism dies, no more carbon-14 is integrated into the system. Ratio of 14C to 12C tells how long the item has been dead.

The half-life for carbon-14 is 5,730 years. This dating method works well if an object is between 5,000

and 50,000 years old.

Radiocarbon DatingRadiocarbon DatingRadioactive C-14 is formed in the upper atmosphere Radioactive C-14 is formed in the upper atmosphere

by nuclear reactions initiated by neutrons in cosmic by nuclear reactions initiated by neutrons in cosmic radiationradiation

1414N + N + 11oon ---> n ---> 1414C + C + 11HH

The C-14 is oxidized to COThe C-14 is oxidized to CO22, which circulates through , which circulates through

the biosphere.the biosphere.

When a plant dies, the C-14 is not replenished.When a plant dies, the C-14 is not replenished.

But the C-14 continues to decay with tBut the C-14 continues to decay with t1/21/2 = 5730 years. = 5730 years.

Activity of a sample can be used to date the sample.Activity of a sample can be used to date the sample.

NUCLEAR vs. CHEMCIAL REACTIONS

Nuclear reactions Chemical reactions

1. Atomic numbers may change 1. Atomic numbers do not change

2. Isotopes of an element have 2. Isotopes of a given element different properties behave almost identically.

3. There is a small but significant 3. There is no significant change mass change; matter is in the total quantity of matter converted to energy. in the reaction

4. Individual atoms are usually 4. Mole quantities are usually used in calculations used in calculations.

Summary

The five types of radioactive nuclides involve emission of alpha () particles, beta () particles, gamma () rays, positrons, and electron capture.

All known nuclides with Z > 83 are radioactive, and many of them occur naturally as member of four radioactive decay series.

In the formation of an atomic nucleus from its protons and neutrons, a quantity of mass is converted into energy.

Synthetic Nuclides For centuries, alchemists tried - without success - to change one

element into another – alchemy – turn lead into gold. The process of changing one element into another is called

transmutation. Modern scientists have learned to do this. Rutherford, in 1919, was able to convert nitrogen-14 into oxygen-17

plus some extra protons by bombarding the nitrogen atoms with particles. This is a naturally occurring isotope of oxygen and is not radioactive.

147N + 4

2He 178O + 1

1H Phosphorous-30 was the first synthetic radioactive nuclide. Since its discovery, scientists have synthesized over a thousand others.

Transuranium Elements

In 1940, the first of the transuranium elements - elements with a Z > 92 - was synthesized by bombarding uranium-238 nuclei with neutrons. This first element is plutonium.

23892U + 1

0n 23992U

23992U 239

93Np + 0-1e

23993Np 239

94Pu + 0-1e

Nuclear Stability

About 160 stable nuclides have an even number of protons and an even number of neutrons.

About 50 stable nuclides have an even number of protons and an odd number neutrons.

About 50 stable nuclides have an odd number of protons and an even number neutrons

Only four stable nuclides have an odd number of protons and an odd number of neutrons.

The magic numbers of protons or neutrons for nuclear stability are 2, 8, 20, 28, 50, 82, and 126.

Stability of Nuclides

All the stable nuclides lie within the belt of stability (as do some radioactive ones). Nuclides outside the belt are radioactive. Their modes of radioactive decay are indicated.

Energetics Of Nuclear Reactions

While working out the details of the theory of special relativity, Einstein derived the equation for the equivalence of mass and energy: E = mc2.

In a typical spontaneous nuclear reaction, a small quantity of matter is transformed into a corresponding quantity of energy.

Nuclear energies are normally expressed in the unit MeV (megaelectronvolt).

1 u = 931.5 MeV : one atomic mass unit contains energy equivalent to 931.5 megaelectronvolts.

1 amu = 1 u

Nuclear Binding Energy The energy released in forming a nucleus from its protons

and neutrons is called the nuclear binding energy and is expressed as a positive quantity.

Alternatively, nuclear binding energy is the quantity of energy necessary to separate a nucleus into individual protons and neutrons.

This explains why there is a mass loss of 0.0304 u in the formation of a helium nucleus from the two protons and two neutrons which comprise it. This quantity is called the mass defect of the nucleus.

Nuclear Binding Energy For Helium

Average Binding Energies

Nuclear Fission

Fission

large nuclei break up

235U + 1n 139Ba + 94Kr + 3 1n +

92 0 56 36 0

Energy

Fission

Nuclear Fusion

Fusion

small nuclei combine

2H + 3H 4He + 1n +

1 1 2 0

Occurs in the sun and other stars

Energy

Learning Check NR4

Indicate if each of the following are

(1) Fission or (2) Fusion or both:

A. Nucleus splits

B. Large amounts of energy released

C. Small nuclei form larger nuclei

D. Hydrogen nuclei react

Energy

Solution NR4

Indicate if each of the following are

(1) Fission (2) fusion

A. 1 Nucleus splits

B. 1 + 2 Large amounts of energy released

C. 2 Small nuclei form larger nuclei

D. 2 Hydrogen nuclei react

Geiger Counter

Used to detect radioactive substances

Exposure vs. Contamination

Exposure

Contamination

Exposure Your body has been subjected to

some type of radiation: Alpha Beta Gamma X-ray Neutrons

The amount of damage done depends on the type of radiation received, the amount of time exposed, and the amount of radiation.

•It does cause damage to your body•Exposure to radiation does not make you radioactive

Contamination Radioactive material has attached itself to you body

Internally Externally

You are also exposed as long as you are contaminated You are “a source”of radioactivity to others

Factors to Reduce Exposure

Time

• Distance

• Shielding

Time If you decrease the time exposed to a given isotope you

will decrease the dose of that exposure If an isotopes gives off 1 Rad/hour in .5 hours you

receive .5 Rads

Distance

Inverse Square Law If you double the distance between you and a

radioactive source you reduce the amount of exposure by ¼

Mathematically I=Io/R2

• I=Intensity at Distance R

• Io=Original Intensity

• R=Distance from Source

Application of Inverse Square Law

At a distance of one foot from a 14C source you receive an exposure dose of 1 RAD. What would be your exposure if you moved 10 feet from the source?

I=Io/R2

I=1RAD/102

I=1RAD/100

I=.01 RAD

By increasing the distance 10 times you decrease the dose 100 fold

Shielding(Barrier between you and the source) Type needed depends on type of radiation

produced Alpha

Air Paper

Beta Metal Wood Plexiglass

Gamma Concrete Lead

Penetrating Power

Alpha particles are most ionizing, but have the least penetrating power. Skin is adequate protection.

Beta particles are more penetrating but can be shielded with paper or thin foil.

Gamma radiation is the most penetrating. A lead barrier is needed for protection from them.

Types of Radiation

Non-Ionizing Ionizing

Non-Ionizing Radiation Waves of energy that do not have the

strength to break chemical bonds or alter the arrangement of atoms Lasers Microwaves Ultraviolet Light

Ionizing Radiation Energy is strong enough to break or

alter chemical bonds

Sources of Ionization

Alpha, beta and gamma rays from radioactive materials

Cosmic rays and the solar wind (lots of protons and neutrons)

Any charged particles with high energy passing through materials can strip electrons from atoms

Ionization & Biology

The ionization can disrupt the structure of crystals in solids

Can rip up proteins and other tissue molecules Tends to be bad news for living things

Since ionizing radiations often start as charged particles with energies in MeV range and electron binding is in eV range, one incoming particle can create lots of problems

Neutron Radiation Ionization

Neutrons cause ionization indirectly They primarily interact with nuclei and cause

nuclear reactions These reactions change the identity of the

atoms and thus the chemistry, disrupting important molecules

So, similar kinds of damage to tissue

Radiation & Biology

If individual protein molecules are damaged, most cells have plenty of protein and can recover

However, too much radiation can destroy too much protein and kill the cell

Worse, may change DNA and wreak all kinds of havoc Now you can start producing defective cells

Radiation & Biology

Radiation is classified as somatic or genetic Somatic damage kills cells and can affect the

functioning of systems Genetic damage is that affecting the

reproductive system and can result in defective offspring All radiation carries risk of damage!

Measure Radiation Amounts

We want to deal with measuring the amount of radiation received by a biological system

Just like medications, we refer to the dose Start with the radiation source How many particles (disintegrations) per

second does the source emit? Historical measure is the Curie

Amount & Energy of Radiation

Amount of Radiation (Activity):

Curie (Ci) = 3.7 x 1010dps

Becquerel (Bq) = 1 dps = 2.8 x 10-11 Ci

Energy of Radiation:

Roentgen (R) = 2.1 x 109 charges/cm3

= 2.58 x 10-4 coulomb/kg

Radiation Dosages

Dose (Amount + Energy)

rad = radiation absorbed dose – absorbed radiation energy per kg of material

(also called gray (Gy) = 100 rad)

rem = radiation equivalent man

(also called sievert (Sv) = 100 rem)

Activity The “radioactive strength” of an isotope Measured in units called Curies (Ci) One Curie = 3.7 x 1010 disintegrations/sec

Relatively speaking a Ci is a large unit so we usually deal in fractions of a Ci

Millicurie (mCi) = 0.001 Ci OR Microcurie (Ci) = 0.000001 Ci

This is the strength of 1 gram of Radium Many now use a new unit, the Becquerel which is

one disintegration per second Manufacturers specify the activity of a radioactive

source at the time of manufacture Of course, we need to know the half-life to calculate

present strength

Absorbed Energy Amounts? We need to be concerned with how much energy is

actually being absorbed by a target There has been a historical progression of units used to

measure the effect of radiation The first was the Roentgen

One Roentgen produces 1.6 x 1012 ion pairs in dry air at room temperature

The modern unit is the rad which is the amount of radiation which deposits energy at a rate of 10-2 J/kg in any absorbing material. The RAD is the measure of absorbed radiation energy in

any type of material A new SI unit is the gray which is 100 rad

Biological Effect of Absorbed Radiation?

Finally, we need to ask if there is any difference in tissue damage between the various possible types of radiation

The answer is that there is a BIG difference, so we had better take that into account as well

Alpha rays cause 10 to 20 times more damage than beta rays Since they are fat and move slowly, they confine their

damage to a smaller area and cause greater disruption in a single location

RBE We account for these differences by figuring out the

relative biological effectiveness or quality factor of the radiation The quality factors vary from one to twenty depending

on type of radiation and energy of the particlesRadiation Type RBE (Relative

Biological Effectiveness

)

X-rays 1

Gamma () rays 1

Beta () rays 1

Thermal (Slow) Neutrons 1n 3

Fast Neutrons 1n and Protons 1p Up to 10

Alpha () particles and heavy ions Up to 20

Measurement of Dosage: the REM The REM (Roentgen Equivalent Man)

Unit used to measure the effect that radiation (the number of RAD’s) will have on human tissue.

This is done by applying a correction or “quality factor” (RBE == relative biological effectiveness) to the RAD based on the type of particle the material emits

rem = rad x rbe This is known as the effective dose The latest SI unit for this is the Sievert which is

gray x quality factor or 100 rem

Units of Radiation dose

rad = radiation - absorbed dose the quantity of energy absorbed per kilogram of tissue 1 rad = 1 x 10-2 J / kg

rem = roentgen equivalent for man, the unit of radiation dose for a human:

1 rem = 1 rad x RBE

RBE = Relative Biological Effectiveness RBE = 10 for RBE = 1 for x-rays, -rays, and ’s

Sample Dosage ProblemA man working in a nuclear power plant has received an accidental exposure. The particular isotope that he was working with emitted 30 RADS of gamma radiation, and 3 RADS of fast neutron radiation. What was the worker’s total dose equivalent it REMS?

REM = RAD x Quality Factor

30 RAD’s gamma x1 = 30 REM

3 RAD’s fast neutron x 10 = 30 REM

TOTAL DOSE = 60 REM

Maximum Permissible Dose

Occupational Workers in mRem

Type of Exposure Yearly Exposure

Whole Body 5000

Lens of the Eye 15000

Hands and Feet50000

Pregnant Women 500 (Dose to Fetus)

Minors 10% of Adult Dose

Non-Occupational Worker in mRem

100 mRem any body part

Radiation Rates and Radiation Amounts

Note that Activity (in Bq or Ci) is a rate. It tells how fast something is decaying with respect to time.

Note that Exposure (in roentgens), Absorption (in rads or Grays), and Effective doses (in rems or Sieverts) are all amounts. They do not tell how fast this is occurring with respect to time.

Radioactive Events are RandomUnpredictable

Collision with biological tissue

Passes harmlessly through body

Biological Effects of Radiation on Living Tissue

Somatic Effects Non-stochastic (immediate)

Skin burns Ulcers Loss of hair Blood changes Vomiting Diarrhea

Stochastic (delayed) Formation of cancers and cataracts

Biological Effects of Radiation on Living Tissue

Genetic Effects Causes damage to chromosomes

Causes mutations in future generations May take many years to determine

• Examples: Hiroshima, Nagasaki, Chernobyl

Teratogenic Effects Damage to developing fetus or embryo

Dosages Required for Certain Immediate Effects

0-100 REM’s Survival certain No obvious symptoms Maybe some clinical signs if lab tests are done

100-200 REM’s Survival probable Begins signs of light radiation sickness

Nausea Vomiting Listlessness

200-1000 REM’s Survival questionable. Some will survive, some won’t. Severe radiation sickness Radiation burns

Over 1000 REM’s Survival impossible

Natural or Background Radiation We are all being exposed daily to a variety of radiation We receive about 100 mREM/year from background

The average non-occupational worker receives about 200 mREM/year of chronic radiation exposure

Present at all times as a result of radiation naturally present in the environment Cosmic rays Uranium, thorium and radon in soil] Building materials

We receive an additional 100 mREM/year from Medical and dental x-rays Smoke detectors Dials on watches, etc.

Differs depending on geographical location

Sources of Background Radiation

54%

8%

11%

11%

3%1%4%

8%

Other RadonMedical X-raysExternalTerrestrialCosmicNuclear MedicineConsumer Products

Radioactivity: Summary of Units Activity: Becquerel (Bq) = 1 decay / s

1 curie (Ci) = 3.7×1010 decays / s (or Bq)(disintegration rate of 1g of radium)

Ion Dose: Ionizing behavior of radiation is most damaging to us!

Roentgen = 2.6×10–4 C/ kgair (or 0.0084 j/kg)

Energy Dose: rad = 0.01 j/kg

Energy Dose for Human Health Considerations:rem = # rads × RBE

Dosages: 0.5 rem / yr = natural background5 rem / yr = limit for nuclear power plant workers500 rem = 50% die within a month750 rem = fatal dose (5000 rem = die within 1

week)

Radiation Exposure

Standard medical x-ray dosage is about 0.04 rem Recommended maximum annual dosage is 0.5

rem per year from all sources Occupational limits are 5 rem/year

These folks have to constantly monitor with film badges or pocket dosimeters to limit exposure to prescribed levels

Radiation Exposure

1000 rems are fatal 400 rems and half die 400 rems over an extended time you will

probably live, but not be in good shape Most hard data from Japanese exposed at the

end of WW II Some data from Chernobyl accident

Effects of RadiationEffects of Radiation

Applications in Nuclear Medicine Imaging

Gamma or positron emitting isotopes 99mTc, 111In, 18F, 11C, 64Cu

Visualization of a biological process Cancer, myocardial perfusion agents

Therapy Particle emitters Alpha, beta, conversion/auger electrons

188Re, 166Ho, 89Sr, 90Y, 212Bi, 225Ac, 131I Treatment of disease

Cancer, restenosis, hyperthyroidism

Nuclear Medicine: Imaging consumption of Na131ISource: Visuals Unlimited

Normal Thyroid An Enlarged Thyroid

Radiation Therapy

Radiation is used to deal with cancer and also for diagnostics (imaging)

Rapidly growing cells hurt more by radiation (same as chemotherapy exploits) Cells that divide quickly are:

Cancerous cells Hair follicles (loss of hair) Digestive tract epithelial cells (nausea)

Try to localize radiation to the tumor

Radiation Therapy MethodsThe goal is to minimize damage to surrounding tissue by limiting exposure.

Can achieve the same goal by implanting “seeds” directly into tumors.

Used for prostate cancers. Use body’s natural processes for other cancers. Iodine concentrates in thyroid, so inject hot “iodine.”

Latest Cancer Research

Carbon nanotubules (nanotechnology) attached to folate molecules, which are only found on most cancerous cells

Sent into cancerous cells using this process Near-infrared light radiation is then used to kill

the cells The nanotubules heat up and kill them Later, might attach nanotubules to antibodies to

target specific cancer cells

Tracer Studies

Tag molecules and introduce to the body and then watch natural processes occur

You can monitor for the presence of the radiation to see where it goes

Label a chemical with technetium-99 with 6 hour half life. Want to look at an organ? Pick a molecule that heads there and tag it with Tc-99 Often done for bone scans to look for cancer

Emission Tomography

Again, inject radioactive substance Positron emission tomography is interesting

tomo = slice or section; graphy = writing or imaging Use a positive beta emitter to tag a molecule The positron annihilates with an electron to form

two gamma rays Detect the gammas on an imaging basis as in CT

scans

Emission Tomography

Coincidence of signal detection establishes the originating location.

Nuclear Magnetic Resonance

Protons have an intrinsic angular momentum called spin

You can think about this for the time being as the proton is like a little planet that rotates on its axis.

Since the proton has charge, this means we have a rotating charge

We can consider the rotating charge to be a tiny current

Nuclear Magnetic Resonance

We learned that a current going in a loop generates a little magnetic field.

When we place the loop in an external magnetic field, the magnetic field in the loop tends to line up with the external magnetic field

You can convince yourself by considering the forces on the charges in the current as they circulate in the loop

Nuclear Magnetic Moment

Long story short, the axis of spin of the proton wants to line up with the magnetic field that we apply externally

The proton’s energy is lowest when the proton’s magnetic field points in the same direction as the external magnetic field

It is higher when it points in the opposite direction

Nuclear Magnetic Resonance

The proton’s energy level splits into two states depending on whether its spin is up or down. This is just like the Zeeman effect for electrons. The energy difference between these states corresponds to hf such that the frequency is about 40 MHz if the field strength is 1T. The energy is proportional to the field strength.

Nuclear Magnetic Resonance

If the magnetic field is modified by the presence of other things like electrons in the neighborhood, then the frequency will be slightly different. By measuring the frequency of energy absorption by the protons we can deduce its electron environment.

Nuclear Magnetic Resonance

This environment will depend on the chemical composition of the neighborhood. The changing frequency in different chemical environments is called the chemical shift. Chemists use this idea to study the structure of molecules.

Nuclear Magnetic ResonanceFor our purposes, we want to form images, so we need to sense the chemical shifts as a function of position in the body. Since different body structures have different chemical environments, we can map the structures by mapping the chemical shifts. This is MRI.

Nuclear Magnetic ResonanceTo get position information, we apply a magnetic field with a gradient (change in intensity from one location to another. Then we can carefully determine position.

Low Level Effects of RadiationThe effects of low level radiation are

hard to determine. There are no directly measurable biological

effects at the background level. Long term effects of radiation may include

heightened risk of cancer, but many different things have been related to long term heightened risk of cancer. Separating out the different effects and accounting for the different amounts of low level radiation make this very difficult to determine.

Low Level Effects of Radiation

At the cellular level, a dose of 100 millirems of ionizing radiation gives on average 1 "hit" on a cell. (So the background radiation gives about 2 hits per year to each cell.)

There are five possible reactions to a “hit”.1. A "hit" on a cell can cause DNA damage that

leads to cancer later in life. Note: There are other causes of DNA damage,

a relatively large amount from normal chemical reactions in metabolism.

Low Level Effects of Radiation2. The body may be stimulated to

produce de-toxifying agents, reducing the damage done by the chemical reactions of metabolism.

3. The body may be stimulated to initiate damage repair mechanisms.

Low Level Effects of Radiation4. The cells may kill themselves (and

remove the cancer risk) by a process called apoptosis, or programmed cell death (a regular process that happens when the cell determines that things are not right).

5. The body may be stimulated to provide an immune response that entails actively searching for defective cells - whether the damage was done by the radiation or by other means.

Low Level Effects of Radiation

There are two main theories:1. Linear Hypothesis: A single radiation “hit”

may induce a cancer. Therefore, the best amount of radiation is zero, and any radiation is dangerous. The more radiation, the more the danger.

This says effect #1 is always more important than effects 2-5.

Low Level Effects of Radiation2. Hormesis Hypothesis: A small

amount of radiation is actually good, but a large amount of radiation is certainly bad.

Many chemicals behave this way - for example B vitamins: we need some to live, but too much is toxic. Vaccines are also this way: we make ourselves a little sick to build up our defenses against major illnesses.

This theory says that at low levels, effects 2-5 are more important than effect 1.

Radiation TreatmentsIf high doses of radiation do bad things to

biological systems, can radiation be used as a treatment?

Ask yourself this: does a knife do harm to biological systems? If if does, why do surgeons use scalpels?

Fast growing cancer cells are more susceptible to damage from radiation than normal cells. For cancer treatment, localized (not whole-body) doses regularly exceed 10,000,000 mrems.

Food Food IrradiationIrradiation

•Food can be irradiated with Food can be irradiated with rays from rays from 6060Co or Co or 137137Cs.Cs.•Irradiated milk has a shelf life of 3 mo. Irradiated milk has a shelf life of 3 mo.

without refrigeration.without refrigeration.•USDA has approved irradiation of meats USDA has approved irradiation of meats

and eggs.and eggs.

Measurement of RadioactivityDevices

Film badges

photographic film exposed to radiation

Geiger Counter

number of disintegrations

Scintillation counter

large number of samples in lab

Where does chemical energy come from?

If chemicals are bound, then breaking the bonds does not release energy. It requires external energy. This energy can come from the formation of stronger bonds between the atoms, such as when you burn some sort of fuel. The fuel bonds break, but stronger bonds are formed with oxygen for a net release of energy. It can also come from the thermal energy of its surroundings, such as when you break the ionic bonds in salt by dissolving it in water

Those are sources of net energy change, however. At the site of the bond itself, this energy comes from the electromagnetic force (although there is some KE of the electrons in addition to the electrical PE). The charges (electrons and the nuclei) in chemicals are not perfectly evenly distributed, causing net electrostatic fields.

When bonds are broken or formed, this motion of charges in the fields (which exert a force on the charges) either absorbs or releases energy because the charges are being pulled or pushed by the electric fields of all the other charges present. If you want to think of it as an exchange of something, think of it as an exchange of photons (leave virtual photons out of this, that's for much faster processes involved in particle physics), which carry the electromagnetic force.

How about nuclear energy source? The nucleus has its own forces, AND the electromagnetic force. Typical nuclear reactions

are dictated by the strong force, which holds the nucleus together. The weak nuclear force predominantly causes beta decays.

But the queston is about the force between the parts of a nucleus, what holds it together, and that is the strong force. The exact form of the strong nuclear force is still a mystery. We have what we believe to be relatively exact models for the other 3 forces (electromagnetism, the weak force, and gravitation), although there's some debate about the extent of the knowledge we have about those. But the exact nature of the strong nuclear force remains unknown. It results from the exchange of a zoo of virtual particles (gluons and mesons), and it depends on too many things (such as the spins of the protons and neutrons which are bound together in the nucleus) to go into here. But it's just another force like gravitation and electromagnetism. The reason you don't feel it personally is that it's very short-range, it essentiall ends at the boundaries of the nucleus itself. Electromagnetic forces (like what holds magnets on your refrigerator) and gravitation are long-range, so we're more familiar with them because they do operate on objects which are of lengths that we can see and touch.

So, just like gravity pulls a rock towards the center of the Earth and makes it take energy to roll uphill (or pick up energy as it rolls downhill), adding nucleons (protons and neutrons) to or removing them from a nucleus requires energy. Think of the nucleus as a pit into which nucleons fall, pulled down by a strange type of gravity that suddenly gets really strong right next to and inside the hole. Now if you add a nucleon to a nucleus, it will generally just scatter unless there's some way to convert this strong force into energy that can be released. This can be in several forms, such as photons (gamma-rays are photons emmited by such processes which have very high energies) or other particles with high energies (say a proton fuses with a nucleus and a neutron is ejected). If the incoming nucleon has enough energy, that energy can be converted into new (generally unstable) particles. The nucleus is a complex place, so there's no single answer to that aspect of your question.

Final Nuclear Notes

Mostly the energy released is in the form of kinetic energy of the products of the reaction. For example, in the proton-proton chain that powers the sun:

proton + proton -> deuteron + positron + neutrino + KE of products The mass of the deuteron + positron + neutrino is less than the mass of

the two protons; this excess mass was converted to energy, in the form of kinetic energy of the deuteron, positron, and neutrino.

Usually, if there is electromagnetic radiation involved, it is listed explicitly (as a gamma ray), as in the next step in that chain:

proton + deuteron -> helium-3 + gamma + KE of products So you get both electromagnetic radiation (the gamma ray) and energy,

in the form of kinetic energy of the helium-3 nucleus and the gamma ray.

Similarly, in fission reactions, the excess energy is in the form of kinetic energy of the nuclear fragments.

In the following figure, this energy is referred to as "heat energy"; however, heat energy on an atomic scale is just kinetic energy

Measuring Health EffectsGamma rays (high energy photons) are very

penetrating, and so generally spread out their ionizations (damage).

Beta rays (high speed electrons) are less penetrating, and so their ionizations are more concentrated.

Alphas (high speed helium nuclei) do not penetrate very far since their two positive charges interact strongly with the electrons of the atoms in the material through which they go.

Measuring Health Effects

This difference in penetrating ability (and localization of ionization) leads us to create an RBE (radiation biological equivalent) factor and a new unit: the rem. The more localized the ionization, the higher the RBE.

# of rems = RBE * # of rads . This is called an EFFECTIVE dose.

RBE for gammas = 1; RBE for betas = 1 to 2; RBE for alphas = 10 to 20.

Levels of Radiation and Health EffectsIn addition to our own radioactivity (and our

food), we receive radiation from:a) space in the form of gamma rays; the

atmosphere does filter out a lot, but not all;

b) the ground, since the ground has uranium and thorium;

c) the air, since one of the decay products of uranium is radon, a noble gas. If the Uranium is near the surface, the radon will percolate up and enter the air.

Nuclear Physics

size of atoms: take water (H2O) density = 1 gm/cc, atomic weight = 18 gm/mole, (alternately, get mass of one molecule from mass spectrograph)

Avogadro’s number = 6 x 1023/mole(1 cm3/gm)*(18 gm/mole) / (6x1023molecules/mole)

= 3 x 10-23 cm3/molecule, so

datom = V1/3 = 3 x 10-8cm = 3 x 10-10 m.

Mass Defect & Binding Energy

By definition, mass of 6C12 is 12.00000 amu.

The mass of a proton (plus electron) is 1.00782 amu. (The mass of a proton by itself is 1.00728 amu, and the mass of an electron is 0.00055 amu.)

The mass of a neutron is 1.008665 amu.

Note that 6*mproton+e + 6*mneutron > mC-12 .

Where did the missing mass go to?

Mass Defect & Binding EnergySimilar question: The energy of the

electron in the hydrogen atom is -13.6 eV. Where did the 13.6 eV (amount from zero) go to in the hydrogen atom?

Answer: In the hydrogen atom, this energy (called the binding energy) was emitted when the electron “fell down” into its stable orbit around the proton.

Mass Defect & Binding Energy

Similarly, the missing mass was converted into energy (E=mc2) and emitted when the carbon-12 atom was made from the six protons and six neutrons:

m = 6*mproton + 6*mneutron - mC-12 =

6(1.00782 amu) + 6(1.008665 amu) - 12.00000 amu

= .099 amu; BE = m*c2 =

(0.099 amu)*(1.66x10-27kg/amu)*(3x108m/s)2

= 1.478x10-11J*(1 eV/1.6x10-19J) = 92.37 MeV

Mass Defect & Binding EnergyFor Carbon-12 we have: BE = m*c2 = 92.37 MeVIf we consider the binding energy per

nucleon, we have for carbon-12: BE/nucleon = 92.37 MeV /12 = 7.70

MeV/nucleon.

The largest BE/nucleon happens for the stable isotopes of iron (about 8.8 MeV/nucleon).

Mass Defect and Binding EnergyBe careful: The fact that isotopes of iron have the

highest binding energy per nucleon is NOT related to iron being a hard metal. The fact of being a metal is determined by the ELECTRONIC shells, NOT the nuclear binding.

Note: Chemical binding energies (ionization energies) are on the order of several eV. Nuclear binding energies are on the order of several MeV. Nuclear energies are thus an order of a million times stronger than electrical binding energies!

Plot of energy versus the separation distance

Nucleus: Particle Potential Wells Electron is only bound with negative total energy, and can never escape. Nucleon can be bound with positive total energy, and can escape by

tunneling through the Coulomb barrier nuclear decay processes. Leads to radioactive processes.

Electron Coulombic PotentialNucleon Nuclear Potential

En

erg

y

Radius r

Radiation Processes: – Decay (e– Emission) Parent nucleus decays to daughter nucleus plus electron and

anti-neutrino. Anti-neutrino is 3rd particle that explains range of electron kinetic

energies. If atom (Z) has greater mass than its right neighbor (Z+1), then

– decay is possible. Free neutron can decay into a proton.

t1/2 = 10.8 min, Q = 939.57 – (938.28 + 0.511) = 0.78 MeV

2

1

1

*electron mass included in daughter nucleus

( )

A AZ

A AZ Z

ZX D

Q MeV Ma

e v

ss X Mass D c

Positron Emission Tomography (PET) – A new and Important Tool in Imaging Research

In the technique of positron Tomography, a positron emitting isotope Is included into a molecule that is incorporated into a chemical reaction.

The positron emitted during the decay of the isotope will analite with anElectron and emit two 511 kev gamma rays that can then be detected, and the location of the decaying isotope isolated accurately.

B+ + e- Energy Two Gamma rays at 180o

e- + B+511 kev 511 kev

Common Positron emitting Isotopes: 15O, T1/2 = 122s ; 18F, T1/2 = 1.83 hr

11C, T1/2= 20.3 min , 13N, T1/2 = 9.97 min , ETC

The two gammarays come awayat 180o.

Positron Emission Tomograph

The Tomograph is aninstrument that is a ringof gamma ray detectors that react very fast togamma rays, and by measuring the time eachdetector receives the signalone can locate the point oforigin of the gamma ray to a precision of + 1 cm in ahuman being or any other physical object, with out any in vivo investigation. The detectors must have acapability of measuring up to + 250 ps per pulse.

_

_

Four Known Forces Two familiar kinds of interactions:

gravity (masses attract one another) and electromagnetism (same-sign charges repel, opposite-sign charges attract)

What causes radioactive decays of nuclei ? Must be a force weak enough to allow most atoms to

be stable. What binds protons together into nuclei ?

Must be a force strong enough to overcome repulsion due to protons’ electric charge

Previously, we peered inside the atom

We recalled that electrons orbit the atom’s massive nucleus and determine an element’s chemical behavior.

We explored the proton and neutron content of nuclei and the phenomena of radioactivity, fission, and fusion they make possible. Today we’ll look inside

the nucleons themselves.

Fundamental particles in the Standard Model are: Leptons Quarks Intermediate Gauge Bosons

Anti-matter

Each kind of elementary particle has a counterpart with the same mass, but the opposite electric charge, called its “anti-particle”. Electron: m= .0005 GeV, charge = +1, symbol e-

Positron: m = .0005 GeV, charge = -1, symbol e+

The anti-particle has a bar over its symbol: Anti-proton is written , anti-neutrino is

Anti-matter is rare in the explored universe It’s created in cosmic rays and particle accelerators

and some radioactive decays. When a particle and its anti-particle collide, they

“annihilate” one another in a flash of energy.

p

v

Where do the elements come from?

Stability diagramHeavy elements can fissioninto lighter elements.

Light elements can undergofusion into heavier elements.

Elements from helium to iron were manufactured in the cores of stars by fusion. Heavier elements are metastable and were made during supernovae explosions.

Fission: Chain Reaction Use neutrons from fission process to initiate other fissions! 1942: Fermi achieved first self-sustaining chain reaction.

For nuclear bomb, need more than one neutron from first fission event causing a second event.

For nuclear power plant, need less than one neutron causing a second event.

Chain reaction

For reaction to be self-sustaining, must haveCRITICAL MASS.

Figure 21.11: Upon capturing a neutron, the 235U nucleus undergoes fission to produce two lighter nuclides, free neutrons (typically three), and a large amount of energy.

Figure 21.12: Representation of a fission process in which each event produces two neutrons, which can go on to split other nuclei, leading to a self-sustaining chain reaction.

Figure 21.13: If the mass of the fissionable material is too small, most of the neutrons escape before causing another fission event; thus the process dies out.

Nuclear reactors

Fusion Light nuclei are more stable when

combined Tremendous energy released Hydrogen bombs and Fusion

power?

Schematic diagram of a cyclotron

Physicist works with a small cyclotron at the University of California at Berkeley.

Source: Corbis

CERN, the world's largest particle accelerator, lies at the foot of the Jura Mountains near Geneva, Switzerland.

Diagram of a linear accelerator

Accelerator tunnel at Fermilab, a high-energy particle accelerator in Batavia, Illinois.

Source: Fermilab Batavia, IL

Units used for Nuclear Energy Calculations

electron volt - (ev) The energy an electron acquires when it moves through a potential difference of one volt:

1 ev = 1.602 x 10-19J

Binding energies are commonly expressed in units of megaelectron volts (Mev)

1 Mev = 106 ev = 1.602 x 10 -13J

A particularly useful factor converts a given mass defect in atomic mass units to its energy equivalent in electron volts: 1 amu = 931.5 x 106 ev = 931.5 Mev