GNEPGNEPAlan J Hays

Genoa Central High School

Arkansas

Before you can intellectually discuss nuclear energy, one

must understand nuclear physics.

Challenges to discussing Nuclear Physics

• Homer Simpson• The “stigma” of “physics”

too difficult, therefore is to be avoided at all costs

• Society issuestoo difficult, therefore is to be avoided at all costs

• Religious issues young earthers and other controversies, therefore is to be avoided at all costs

• Mathematics is language of physics, therefore

is too difficult, therefore is to be avoided at all costs

Misconceptions• All radioactive elements are extremely

dangerous to living things.• There is no way to protect oneself from

radioactivity.• There are very few naturally occurring

radioactive elements.• Radioactive elements only occur in

nuclear power plants and nuclear weapons.

It’s all Earth Science to me!

Most all of the earth’s rock material contains uranium. Ratio of U-238 to U-235 0.7%

U-238 →Pb T1/2 4.5 bil yrs.

U-235 T1/2 700 million yrs.

@t = 0 yrs [U-238] = [U-235]

Ratio of T1/2U-238 to T1/2U-235 7

Age of earth 8 U-235 half-lifes 5.6 bil yrs

Lab Sequence

• What’s hot and what’s not

• Background radiation

• Radiation and Distance

• Shielding and absorption

• Half-Life

What’s Hot and What’s Not

• A discovery lab

• Students are given a brief on GM tube use

• Given a bag of miscellaneous materials.

• Observations

Background Radiation

• Count the “ticks” for a specified time• From a class get 20 – 30 recordings• Sample data: counts per 30 seconds.

 C                                        

                                         

                                         

                                         

                                         

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Background radiation: 40 cps

Radiation and Distance• Familiar “inverse square law”• Sample data: Background Radiation = 58cpm

Dist=cm Counts/30s B-R corrected

5 3283 6508

6 2086 4114

7 1376 2694

8 1047 2036

9 793 1528

10 676 1294

11 559 1060

12 533 1008

13 469 880

14 445 832

Radiation and Distance

• Graphing

y = 144651x-2.0132

0

1000

2000

3000

4000

5000

6000

7000

0 5 10 15

The exponent of x -2; data function very closely follows the inverse square law

2

1

xky

y = 144651x-2.0132

Shielding

• Beta source• Data:

Type Counts %

Unshielded 4388  

Paper 3860 88%

Thin Plastic 528 12%

Absorption

• Beta source and aluminum shielding• Sample data

Al thickness cpm

0.020 4673

0.040 2757

0.060 1109

0.080 553

0.100 171

0.120 39

y = 16545e-47.095x

0

1000

2000

3000

4000

5000

6000

7000

0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140

y = 16545e-47.1x

Absorption

• Gamma source and lead shielding• Sample Data

Pb thickness cpm

0 3762

0.032 3504

0.064 3478

0.125 3148

0.250 2486

y = -4966.7x + 3743.5

2000

2200

2400

2600

2800

3000

3200

3400

3600

3800

4000

0 0.05 0.1 0.15 0.2 0.25 0.3

y = -4967x + 3744

Half-Life• Ba-137m Isogenerator

Cs-137 Parent nuclide; half-life(t1/2) 30yrs

This decays to a metastable form(excited state) of barium(Ba-137m).

Ba-137m decays to Ba-137(ground state) by gamma emission. t1/2 2.5min.

• Freshness of samples is critical to good data

Half-Life

• Sample data:time Counts(N) Ln(N)

1 15 2.708

2 11 2.398

3 8.4 2.128

4 6.1 1.808

5 5 1.609

6 3.5 1.253

7 2.7 0.993

8 2 0.693

y = -6.3935Ln(x) + 15.188

0

2

4

6

8

10

12

14

16

0 2 4 6 8 10

Time vs Counts

y = -6.39ln(x) + 15.2

Half-life

• Mathematical modeling: teNN 0

teNN 0

tNN

0

ln

tNN 0lnln

bmxy

NtN

0lnln

Linear!

N = # of nuclei present at time t.

N0 = # of nuclei present at t = 0.

= decay constant (it’s constant!)

Half-life

• Linear graph Slope =

= - 0.2852

693.02ln

21 Ty = -0.2852x + 2.9821

0.000

0.500

1.000

1.500

2.000

2.500

3.000

0 2 4 6 8 102852.0693.0

2852.02ln

21 T

minT 43.221

Ln(N) vs time

y = -0.2852x + 2.9821

Physics Mantras• Don’t memorize; understand• To know physics is to do physics• A single good observation is worth a century of bad

philosophy• Process is more important than results• You only arrive at the right answer after making all

possible mistakes.• A theory is accepted only when the last of its

opponents dies off.• Pollyannas who believe anything is possible should be

subjected to a course in thermodynamics.