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Nuclear Physics and SocietyPhysics Department

University of Richmond

Nuclear Basics

Motivation: Educate the Public and University communities about basic nuclear physics ideas and issues

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U.S. Department of Energy Workshop

July 2002, Washington D.C.

Role of the Nuclear Physics Research Community (universities and national laboratories) in Combating Terrorism

Education and Outreach

•Community

•Local PD and FD

DOE Workshop …

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Border Control/ US Customs

•1,000,000 visas/year

•422 ports of entry

•1700 flights / day

•290 ships / day

•60 trains / day

•1200 busses / day

•540,000,000 border entries / year

Time per primary inspection

8 seconds => 1 hour delay

Cargo Containers

10,000,000 per year … 10,000 per ship!

5 / minute @ L.A.

< 3% inspected

What the Course is/is not

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This is not a radiation workers course

This is not a course that will certify you for anything

We hope that we can introduce you to some basic facts about nuclear physics, about radiation, about detectors etc., which may be useful for you to know.

Who are We

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Con Beausang

Chairman & Associate Professor Physics Department

Jerry Gilfoyle

Professor, Physics Department

Paddy Regan

Professor Physics Department, University of Surrey, U.K.

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Monday April 13th

Lecture 1:The types of radiation, their properties and how these can be used to detect them. Some basic definitions. Introduction to radiation detectors.

Tuesday April 14th Laboratory Session: 12:15 3:30 pm

Environmental Radiation Laboratory experienceMeasurement of half-lifeDemonstration of shieldingFind the source

Lecture 2:The creation of the elements. Nuclear physics in the cosmos.

Wednesday April 15th

Laboratory Session 2: 12:15 3:30Repeat of Tuesdays experience

Lecture 3:Applications of Nuclear Physics: Nuclear weapons, nuclear power and nuclear medicine.

Thursday April 16th Lecture 4Some of the frontiers of modern nuclear physics research

Nuclear Physics and Society

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The Cookie Quiz

Alpha cookie

Beta cookie

Gamma cookie

Neutron cookie

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The Cookie Quiz

Alpha cookie

Beta cookie

Gamma cookie

Neutron cookie

Throw away

Put in pocket

Hold in clenched fist

Eat one … GOAL: Minimize your radiation exposure

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The Cookie Quiz: Answer 1

Alpha cookie

Beta cookie

Gamma cookie

Neutron cookieThrow away

Put in pocket

Hold in clenched fist

Eat one …

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The Cookie Quiz: Correct Answer

Alpha cookie

Beta cookie

Gamma cookie

Neutron cookie

Throw away

Put in pocket

Hold in clenched fist

Eat one …

GOAL: Minimize your radiation exposure

Mutiny at onceRetire from the navy and Toss ALL cookies away

… when I was young(er), I was curious

…

What are we made of ?

… sugar and spice and all things nice … that’s what little girls are made of

… snips and snails and puppy dogs tails … that’s what little boys are made of.

… ok mum, … so what are sugar, spice and snails etc. made of? … cells

… molecules… atoms

… nuclei

The Uncertainty Principle

Heisenberg (Quantum Mechanics)

(position) (momentum) > Constant

Beausang (Teaching)

(truth) (clarity) > Constant

Atoms … are made of …

Electrons… very light, but occupy most of the volume inside an atom

Nuclei … lie at the Core of Atoms… very heavy, very small, very compact…occupies almost none of the volume inside the atom

How do we know?

How to see the invisible?… size of your probe… scattering

Alpha-particle beam

DetectorZinc-sulfide

screen

Discovery of the nucleus~1910

The eyes of Geiger andMarsden

16-inch Battleship shells and

tissue paper

Think of atoms as being like a mini solar system

… The sun at the center is the nucleus, the electrons orbit the nucleus, like the planets orbit around the sun

Bohr Model

Electrons

•Very small

•Point-like particles (i.e.nothing inside an electron)

•Very light ~ 1/2000th of proton mass

•Negatively charged (-1 elementary charge)

•Electrons occupy almost all the space in the atom (orbiting the nucleus like the earth and other planets orbit the sun)

•Have almost none of the mass of the atom

•All of chemistry has to do with electrons from different atoms interacting with each other

The Nucleus

•Made up of protons and neutrons

•Almost all of the mass of the atom is concentrated in the nucleus.

• >99.9% of the known mass in the universe.

•Occupies almost none of the volume of the atom.

•Radius < 1/10,000•Volume < 1/1,000,000,000,000

•The nucleus is the source of almost all the things we commonly think of as being

radioactive.

The Nucleus Protons•Positively charged

(+1 elementary charge)•Size ~ 1 fm (10-15 m)•Mass 938 MeV/c2 = 1

Neutrons•Neutral

(0 charge)•Size ~ 1 fm (10-15 m)•Mass 939 MeV/c2 ~ 1

Neutrons are slightly more massive than the protons!!!This has huge consequences for us!

Delicate Balances

Laws of Physics

1) If it can happen … it will happen …

2) If some law forbids it to happen … it will happen more slowly …

3) If a process is really REALLY forbidden to happen … it just takes a long time …

Standard Model:Neutron and proton are very close relatives

quark structure… proton (uud)… neutron (udd)

Many laws allow neutrons to `change into’ into protons … change a d-quark into a u-quark (or vice versa)

… beta-decay

The half life of a free neutron (i.e., one not inside a nucleus) is only about 12 minutes!!!Mass Neutron = 939.565330 MeV/c2

Mass Proton = 938.271998 MeV/c2

But …Inside a nucleus … neutrons are stable

The half life of a free proton is > 1031 years Inside some nuclei protons can ‘decay’ into neutrons

Imagine … if they were not!Then in ~ 1-2 hours the entire universe would be made of Hydrogen

E = mc2

The Nucleus•Atoms are electrically neutral•The number of protons in a nucleus is equal to and determines the number of orbiting electrons

the chemistrythe element name

•Hydrogen (11H)

1 proton, 0 neutronsMass = 1

•Helium (42He) (Alpha-particle)

2 protons, 2 neutronsMass = 4

•Uranium (23892U)

92 protons, 146 neutronsMass = 238

The Nucleus

Many elements have several stable nuclei with the same number of protons but different numbers of neutrons …

same name same chemistrydifferent mass

Isotopes

The Periodic Table of the Elements

Chart of the Nuclei

1H 2D

3He 4He

6Li 7Li

n

9Be

3T

5He 6He

5Li

6Be 7Be 8Be8Li7He

9Li

10Be10Li 11Li

8He 9He

11Be 12Be

10B 11B9B

14Be

12B 13B 14B 15B8B7B

12C 13C 14C 15C 16C 17C11C10C9C8C

Z =

No.

of

Pro

tons

0

1

2

3

4

5

6

N = No. of Neutrons

0 1 2 3 4 5 6 7 8 9

Chart of the Nuclei

The Landscape~300 stable ~ 7000 unstable … radioactive.

Half Life

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Time taken for half of the substance to decay away

Example:

If you have 1000 radioactive nuclei

and

If their half life is 30 minutes

After 30 minutes 500 nuclei remain

After 60 minutes 250 remain

After 90 minutes 125 remain

After 120 minutes 62 remain

There is a huge variation in half lives of different isotopes …. From a tiny fraction of a second to roughly the age of the universe.

Some Isotopes & Their Half LivesSome Isotopes & Their Half Lives

ISOTOPEISOTOPE HALF-HALF-LIFELIFE

APPLICATIONSAPPLICATIONS

 Uranium billions of years

 Natural uranium is comprised of several different isotopes. When enriched in the isotope of U-235, it’s used to power nuclear reactor or nuclear weapons.

 Carbon-14  5730 y  Found in nature from cosmic interactions, used to “carbon date” items and as radiolabel for detection of tumors.

 Cesium-137  30.2 y  Blood irradiators, tumor treatment through external exposure. Also used for industrial radiography.

 Hydrogen-3  12.3 y Labeling biological tracers.

 Irridium-192 74 d Implants or "seeds" for treatment of cancer. Also used for industrial radiography.

 Molybdenum-99 66 h Parent for Tc-99m generator.

 Technicium-99m  6 h Brain, heart, liver (gastoenterology), lungs, bones, thyroid, and kidney imaging, regional cerebral blood flow, etc.

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The Amount of Radioactivity is The Amount of Radioactivity is NOT Necessarily Related to SizeNOT Necessarily Related to Size

• Specific activity is the amount of radioactivity found in a gram of material.

• Radioactive material with long half-lives have low specific activity.

1 gram of Cobalt-60has the same activity as

1800 tons of natural Uranium

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For Example: Suppose we have

1,000,000,000 atoms of material A with a half life of 1 second

and

1,000,000,000 atoms of material B with a half life of 1 year

(real sources have many more atoms in them)

Suppose they both decay by alpha emission.

In the First Second

Substance A: Half the nuclei will decay

… 500,000,000 alpha particles will come zipping out at you.

1 year = 365 days * 24 hours * 60 minutes * 60 seconds = 31,536,000 seconds

In the First Second for substance B

Only ~ 500,000,000 / 31,536,000 = 16 nuclei will decay

… only 16 alpha particles will come zipping at you

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On the other hand …

In 10 seconds … almost all of the radioactivity in substance A is gone away

But it takes years for the activity of substance B to go away!

Nuclear Bombs …

The fissile material (U or Pu) has a long half-life. Low specific activity. Not much activity on the outside.

Dirty Bombs …

The radioactive material wrapped around the explosive would probably have a much shorter half-life. Perhaps significant activity on the outside.

Types of Radioactivity

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Each type of radiation has different properties which affect the hazards they pose, the detection mechanism and the shielding required to stop them.

Five Common Types

Alpha Decay

Beta Decay

Gamma Decay

Fission

Neutron Emission

Each of the particles emitted in the decay carries a lot of kinetic energy. Damage can be caused when this energy is absorbed in a human cell.

Alpha Decay

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An alpha particle () is an energetic, He nucleus (4

2He2)

Alpha decay mostly occurs for heavy nuclei

Example

23894Pu 234

92U + 42He

Half-life: 88 years

Energy =5.56 MeV

Alpha Decay

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Very easy to shield

A sheet of paper, skin, or a few cm (~inch) of air will stop an alpha particle

External Hazard: Low

Internal Hazard: High

Alpha Decay238

94Pu144 23492U142 +

• Parent nucleus 23894Pu144

• Daughter Nucleus 23492U142

– Often the daughter nucleus is also radioactive and will itself subsequently decay.

– Decay chains or families (e.g. uranium, thorium decay chains). 36

Decay Chains

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23894Pu 234

92U + t1/2 = 88 yrs

23492U 230

90Th + t1/2 = 2.5 105 yrs

23090Th 226

88Ra + t1/2 = 8.0 104 yrs

22688Ra 222

86Rn + t1/2 = 1.6 103 yrs

22286Rn 218

84Po + t1/2 = 3.8 days

21884Po 214

82Pb + t1/2 = 3.1 min

21482Pb 214

83Bi + t1/2 = 27 min

21483Bi 214

84Po + t1/2 = 20 min

21484Po 210

82Pb + t1/2 = 160 s

Decay Chains

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21082Pb 210

83Bi + t1/2 = 22 yrs

21083Bi 210

84Po + t1/2 = 5 days

21084Po 206

82Pb + t1/2 = 138 days

20682Pb is STABLE

Decay Chains

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Pu

U

Th

Ra

Rn

Po

Pb

Hg

Au

Beta Decay

• The neutron and the proton are very similar to each other (very closely related).

• A neutron can ‘change into’ a proton, or vice versa.

• When this happens, an energetic electron (or positron) is emitted.

• This is called beta-decay

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A beta-particle is an electron (e) or its anti-particle the positron (e+)

n p + e- + p n + e+ +

Beta Decay

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In terms of nuclei beta-decay looks like

As in the case of alpha decay the daughter nuclei are usually radioactive and will themselves decay.

•Beta-particles are HARDER to stop

Since the electron is lighter than an alpha-particle and carries less charge.

•Therefore, the range of a beta-particle is greater and it takes more shielding to stop beta-particles (electrons or positrons) than alpha particles

~ few mm or 1 cm of lead

~ few feet of air

13755Cs82 137

56Ba81 + e- +

Beta-Decay

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•Beta-particles are HARDER to stop

Since the electron is lighter than an alpha-particle and carries less charge.

•Therefore, the range of a beta-particle is greater and it takes more shielding to stop beta-particles (electrons or positrons) than alpha particles

~ few mm or 1 cm of lead

~ few feet of air

Gamma-Decay

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•A beta-decay or alpha-decay typically leaves the daughter nucleus in a highly excited state.

•To get to the ground state the nucleus (rapidly … almost instantly) emits one or more gamma-rays

•Gamma-rays are a very energetic form of light. More energy and more penetrating than x-rays.

•No charge

•Much more penetrating than either alpha or beta.

•Few inches of Pb, many feet of air

Gamma-Decay

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•Gamma-ray energies are characteristic of the nucleus.

•Measure the energies … identify the nucleus.

(just like atoms or molecules give off characteristic colors of light).

Measuring the gamma-ray is by far the best and easiest way to measure what type of radioactive substance you are dealing with.

Fission

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What holds nuclei together?

•Protons repel each other (opposites attract, like

repel)

•Coulomb Force

Some other force must hold nuclei together

The STRONG FORCE

•Attractive and Stronger than the Coulomb Force

•But short range

Fission

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What happens if you have a lot of protons (i.e in a heavy nucleus)?

…Eventually the Coulomb repulsion will win

… and the nucleus will fall apart into two smaller (radioactive!!) nuclei.

FISSION

An enormous amount of energy is released.

This energy is utilized in power plants and in fission bombs.

Fission

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The heavy parent nucleus fissions …

into two lighter fission fragment nuclei …

Plus some left over bits … energetic neutrons

Example:

252Cf is a spontaneous fission source …

Sometimes this process happens spontaneously … sometimes you can ‘poke’ at the nucleus and induce it to fission

Fission …Fission Fragments

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Are emitted with a huge energy but stop very quickly (very short range).

Are all radioactive nuclei and will decay usually by beta-and gamma-decay

Mass

Pro

bab

ilit

y Heavy

fragment

Light fragment

They have a broad

range of masses

Induced Fission

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Some nuclei can be made to fission when struck by something …

Usually the something is a neutron

Example: 235U + n fission

Remember … in the fission process extra neutrons are released

If some of these strike other 235U nuclei … they can induce another fission

Induced Fission

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Chain Reaction

Controlled … nuclear power plant … exactly one neutron per fission induces another fission.

Uncontrolled … nuclear bomb … more than one neutron per reaction induces another fission

What is a “Dose” of Radiation?What is a “Dose” of Radiation?

• When radiation’s energy is deposited into our body’s tissues, that is a dose of radiation.

• The more energy deposited into the body, the higher the dose.

• Rem is a unit of measure for radiation dose.

• Small doses expressed in mrem = 1/1000 rem.

• Rad & R (Roentgens) are similar units that are often equated to the Rem.

51From Understanding Radiation, Brooke Buddemeier, LLNL

Typical DosesTypical Doses

Average Dose to US Public from All sources 360 mrem/year

Average Dose to US Public From Natural Sources 300 mrem/year

Average Dose to US Public From Medical Uses 53 mrem/year

Coal Burning Power Plant 0.2 mrem/year

Average dose to US Public from Weapons Fallout < 1 mrem/year

Average Dose to US Public From Nuclear Power < 0.1 mrem/year

Occupational Dose Limit for Radiation Workers 5,000 mrem/yr

Coast to coast Airplane roundtrip 5 mrem

Chest X ray 8 mrem

Dental X ray 10 mrem

Head/neck X ray 20 mrem

Shoe Fitting Fluoroscope (not in use now) 170 mrem

CT (head and body) 1,100 mrem

Therapeutic thyroid treatment (dose to the whole body) 7,000 mrem

52From Understanding Radiation, Brooke Buddemeier, LLNL

Types of Exposure & Health EffectsTypes of Exposure & Health Effects

• Acute Dose– Large radiation dose in a short period of time– Large doses may result in observable health effects

• Early: Nausea & vomiting• Hair loss, fatigue, & medical complications• Burns and wounds heal slowly

– Examples: medical exposures andaccidental exposure to sealed sources

• Chronic Dose– Radiation dose received over a long period of time – Body more easily repairs damage from chronic doses – Does not usually result in observable effects– Examples: Background Radiation and

Internal Deposition

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Inhalation

From Understanding Radiation, Brooke Buddemeier, LLNL

Dividing Cells are the Most RadiosensitiveDividing Cells are the Most Radiosensitive

• Rapidly dividing cells are more susceptible to

radiation damage.

• Examples of radiosensitive cells are

– Blood forming cells

– The intestinal lining

– Hair follicles

– A fetus

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This is why the fetus has a exposure limit (over gestation period) of 500 mrem (or 1/10th of the annual adult limit)

From Understanding Radiation,Brooke Buddemeier, LLNL

At HIGH Doses, We KNOW At HIGH Doses, We KNOW Radiation Causes HarmRadiation Causes Harm

• High Dose effects seen in:– Radium dial painters

– Early radiologists

– Atomic bomb survivors

– Populations near Chernobyl

– Medical treatments

– Criticality Accidents

• In addition to radiation sickness, increased cancer rates were also evident from high level exposures.

55From Understanding Radiation,Brooke Buddemeier, LLNL

Effects of ACUTE ExposuresEffects of ACUTE Exposures

Dose (Rads*) Effects

25-50First sign of physical effects

(drop in white blood cell count)

100Threshold for vomiting

(within a few hours of exposure)

320 - 360~ 50% die within 60 days

(with minimal supportive care)

480 - 540~50 % die within 60 days

(with supportive medical care)

1,000 ~ 100% die within 30 days

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* For common external exposures 1 Rad ~ 1Rem = 1,000 mrem

From Understanding Radiation,Brooke Buddemeier, LLNL

At LOW Doses, We PRESUME At LOW Doses, We PRESUME Radiation Causes HarmRadiation Causes Harm

• No physical effects have been observed

• Although somewhat controversial, this increased risk of cancer is presumed to be proportional to the dose (no matter how small).

The Bad News: Radiation is a carcinogenand a mutagen

The Good News: Radiation is a very weakcarcinogen and mutagen!

Very Small DOSE = Very Small RISK

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From Understanding Radiation

Brooke Buddemeier, LLNL

Radiation Detectors

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Range of Radiation

Alpha: Small. Shield with a piece of paper

Beta: Smallish Shield with a ½ inch or so of Pb

Gamma: Long Shield with a few inches of Pb

Neutron: Very long Shield with many inches of parafin

To detect the radiation it has to

a) Get to and b) Get into your detector

Radiation Detectors

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Almost all work on the same general idea

When an energetic charged particle passes through matter it will rapidly slow down and lose its energy by interacting with the atoms of the material (detector or body)

•Mostly with the atomic electrons

It will ‘kick’ these electrons off of the atoms leaving a trail of ionized atoms behind it (like a vapor trail of a jet plane)

Radiation detectors use a high voltage and some electronics to measure these vapor trails. They measure a (small) electric current).

Radiation Detectors

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Like a bullet going through something

A friction force will slow it down and stop it

Friction

More Charge More friction

More Massive More friction

More friction Shorter Range

Radiation Detectors

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It has to get into your detector

e.g. Alpha …. A few inches of air or a piece of paper stops it … if your detector is a few feet away, it will not detect the alpha …

e.g. Alpha … if the sides of the detector are too thick the alpha will not get in and will not be detected

Radiation Detectors

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Neutrons and gamma-rays are neutral

No charge … much less friction … much longer range

When they penetrate matter eventually they also will interact somehow (gamma-rays interact via Compton scattering, photoelectic effect or pair production, neutrons will collide with protons in the nuclei) and these interactions produce energetic charged particles.

The detectors are sensitive to these secondary particles.

Types of detector

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Alpha, Beta and Gamma radiation

Film Badges

Gas Counters (Geiger counters)

Scintillators

Solid State Detectors

Film Badges

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Will detect: beta, gamma and neutron

Need to send away and develop the film and then later will tell you what does you received

Used by radiation workers

TLC devices … similar idea but with real-time readout

Gas Counters

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e.g. Geiger Counters

Will Detect: Alpha, Beta, some gamma

No identification … just tells you something is there

With a thin entrance window GM-tube is sensitive to alphas

Scintillators

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Make a flash of light when something interacts

Sodium Iodide

Cesium Iodide

Will Detect: Alpha (with thin window), beta (with thin window) and gamma.

Gives moderate to bad energy information … some information on the type of radiation

Semiconductor Detectors

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Germanium

Silicon

Will Detect: Gamma rays (also beta and alphas in a laboratory, not in the field)

Excellent energy resolution: Can measure exactly was source you are looking at.

Spare Transparencies

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Radioactive Decay

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When can a nucleus decay? …

•When there is a lighter nucleus for it to decay into

•When this decay is allowed by certain conservation laws ….

•Conservation of energy

•Conservation of charge

•Certain other ‘quantum numbers’

When a physical process can happen … it will happen.When it is forbidden to happen … it just takes a little longer!

If a nucleus can decay … it will

Beta Decay

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Various laws must be obeyed, including1. Conservation of Energy

• E = mc2 … a heavy particle can decay into lighter one(s).

• The excess energy is turned into kinetic energy of the light particles

2. Conservation of Charge• An electron is produced

3. Conservation of Lepton Number• a very nebulous particle called a neutrino is

also produced

n p + e- +