# Presentation: 694243

## GNEP

- Alan 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 issues
- 		too 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

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

- y = 144651*x*-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 *= 16545*e*-47.1*x*

## 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 = -4967x + 3744*

## Half-Life

- Ba-137m Isogenerator
- Cs-137 Parent nuclide; half-life(*t**1/2**) * 30yrs
-              This decays to a metastable form(excited state) of barium(Ba-137*m)*.
-              Ba-137*m* decays to Ba-137(ground state) by gamma emission.  *t**1/2** * 2.5*min.*
- 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 |

- Time vs Counts

*y = -6.39ln(x) + 15.2*

## Half-life

- Mathematical modeling:

- Linear!

*N* = # of nuclei present at time *t.*
*N**0* = # of nuclei present at *t = 0.*
*** **= decay constant (it’s ****constant!****)*

## Half-life

- Linear graph

*Slope = ***
- = - 0.2852

- 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.