Physics 12

Mr. JeanMay 23rd, 2014

The plan:

• Video clip of the day• Radio Activity• Decay models• Half Life modeling• Text book questions• Visiting the Relatives

RadioactivityAs the heavier atoms become more unstable, particles and photons are emitted from the nucleus and it is said to be radioactive. All elements with A > 82 are radioactive.

As the heavier atoms become more unstable, particles and photons are emitted from the nucleus and it is said to be radioactive. All elements with A > 82 are radioactive. Examples

are:Alpha particles a b- particles

(electrons)b+ particles (positrons)

Gamma rays g

b-

g

b+

a

The Alpha Particle

An alpha particle a is the nucleus of a helium atom consisting of two protons and two neutrons tightly bound.

Charge = +2e- = 3.2 x 10-

19 C Mass = 4.001506 u

Relatively low speeds ( 0.1c ) Not very penetrating

The Beta-minus Particle

A beta-minus particle b- is simply an electron that has been expelled from the nucleus.

Charge = e- = -1.6 x 10-19 C-

High speeds (near c)-

Mass = 0.00055 u-

Very penetrating-

The Positron

A beta positive particle b+ is essentially an electron with positive charge. The mass and speeds are similar.

Charge = +e- = 1.6 x 10-19 C

+

High speeds (near c)+

Mass = 0.00055 u+

Very penetrating+

The Gamma Photon

A gamma ray g has very high electromagnetic radiation carrying energy away from the nucleus.

Charge = Zero (0)

g

Mass = zero (0)g

Speed = c (3 x 108 m/s)g

Most penetrating radiation

g

Radioactive DecayAs discussed, when the ratio of N/Z gets very large, the nucleus becomes unstable and often particles and/or photons are emitted.Alpha decay results in the loss of two protons and two neutrons from the nucleus.

42

4 42 2

A AZ ZX Y energy

X is parent atom and Y is daughter atomThe energy is carried away

primarily by the K.E. of the alpha particle.

Example: Write the reaction that occurs when radium-226 decays by alpha emission.

4 42 2

A AZ ZX Y energy

226 226 4 488 88 2 2Ra Y energy

From tables, we find Z and A for nuclides. The daughter atom: Z = 86,

A = 222226 222 488 86 2Ra Rn energy

Radium-226 decays into radon-222.

Radium-226 decays into radon-222.

Beta-minus Decay

Beta-minus b- decay results when a neutron decays into a proton and an electron. Thus, the Z-number increases by one.

01 1

A AZ ZX Y energy

X is parent atom and Y is daughter atom

The energy is carried away primarily by the K.E. of the electron.

-

Beta-plus Decay

Beta-plus b+ decay results when a proton decays into a neutron and a positron. Thus, the Z-number decreases by one.

01 1

A AZ ZX Y energy

X is parent atom and Y is daughter atom

The energy is carried away primarily by the K.E. of the positron.

+

Radioactive MaterialsThe rate of decay for radioactive

substances is expressed in terms of the activity R, given by:

NR

t

Activit

y

N = Number of undecayed nuclei

One curie (Ci) is the activity of a radioactive material that decays at the rate of 3.7 x 1010 Bq or 3.7 x 1010 disintegrations per second.

One curie (Ci) is the activity of a radioactive material that decays at the rate of 3.7 x 1010 Bq or 3.7 x 1010 disintegrations per second.

One becquerel (Bq) is an activity equal to one disintegration per second (1 s-1).One becquerel (Bq) is an activity equal to one disintegration per second (1 s-1).

The Half-LifeThe half-life T1/2

of an isotope is the time in which one-half of its unstable nuclei will decay.

No

0

2

N

0

4

N

Number of Half-lives

Nu

mb

er

Und

eca

yed

N

ucl

ei

1 432

0

1

2

n

N N

Where n is number of half-

lives

Half-Life (Cont.)The same reasoning will apply to activity R or to amount of material. In general,

the following three equations can be applied to radioactivity:Nuclei

Remaining

0

1

2

n

N N

Activity R

0

1

2

n

R R

Mass Remaining

0

1

2

n

m m

Number of Half-lives:

12

tnT

To do: p.915 (4-6)

Please keep in mind:

• Visiting the relatives due on Monday – May 26th , 2014

• Chapter 20 & 21 assignment due next Friday– May 31st, 2014

• Final assessment on Chapter 17, 18 to 21 on Wednesday June 11th, 2014