Radioactivity

AP Physics: M. Blachly

Review

14 17 1 N ___ Hn

14 14 17 6 1 N C Hn

Balance the nuclear reaction:

History

Roentgen on Dec 22 1895, "photographed" his wife's hand

History: Becquerel

In 1896 put his wrapped photographic plates away in a darkened drawer with some crystals containing uranium.

the plates were exposed during storage. 

History: Becquerel

This was found to be the case with all compounds containing uranium

History: The Curies

Marie and Pierre Curie worked in Becquerel’s lab

Worked on isolating other elements which show activity

All three shared Nobel Prize for their work with radioactivity

History: The Curies

Had no idea radiation was harmful

Marie kept it by her bed because it glowed.

Pierre carried a chuck in his pocket to show people he met.

History: The Curies

Pierre is killed by horse-cart

Marie isolated the most radioactively potent element, named radium

Wins Nobel prize again by herself.

History: Rutherford

Used a radioactive source (Pb) to investigate the types of radioactivity

Radiation Types

Alpha Particles: heavy, positive charge

Beta Particles: lighter, negative charge

Gamma: no detectable mass, no charge

Radiation Types

History: Rutherford

History: Rutherford

Discovered that almost entire mass must be centered in a “nucleus” that was unimaginably small

History: Other Particles

1930 Paul Dirac predicts the existence of a positron

Positron found in 1932.

More Particles

1932 Chadwick shot -particles at Be.

4 9 122 4 6He Be C X

Radioactivity

Is a random probability event

The decay rate is proportional to the number of radioactive nuclei

( )N

R t Nt

Radioactivity

The decay rate can be given by

0( ) tR t R e

Radioactivity

The amount of radioactive nuclei left at any time is given by the function

0( ) tN t N e

Activity

SI unit of activity: Becquerel

• 1 Bq = 1 decay/sec

Traditional Unit: The Curie

• Activity of 1 gram of Radium

• 1 Ci = 3.7 x 1010 Bq

Half Life

Time it takes for half of the radioactive nuclei to decay

01/ 2( )

2

NN T

0

50

100

150

200

250

300

350

400

0 5 10 15 20

Time (Years)

Ra

dio

ac

tiv

e N

uc

lei (

gra

ms

)

Half Life

1/ 2

ln 2T

1/ 20 0

1

2TN N e

Half-life

Determine the activity of a 1 g sample of Ra-226, which has a half-life of 1600 years.

Half-life

23

21

1 mole 6.02 10 nuclei1 g

226 g 1 mole

2.66 10 nuclei

N

10

1/ 2

10

1/ 2

ln 2, , 5.045 10 sec

ln 2 decays3.66 10

sec

R N TT

NR

T

Half-life Example

C-14 has a half life of 5730 years. A 50 gram bone sample shows 200 decays/min. Living bone gives an activity of 15 decays/minute/gram. How old is the bone?

Half-life Example Solved

Bone is about 10,900 years old

Activity Levels

Radioactivity Level, in Bqsource for medical therapy 100 T1 kg high-level nuclear waste 10 Tmedical diagnosis 70 M1 kg uranium 25 M1 kg low level radioactive waste 1 M1 smoke detector 30 k1 adult human 7 k1 kg fertiliser 5 k1 kg of granite 1 k

Your turn

A scrap of paper taken from the Dead Sea Scrolls was found (using a mass spec) to have a 14C/12C ratio of 0.795 times that found in plants living today. Estimate the age of the scroll.