Lab 12: Radiation
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Nuclear reactions involve changes in the nucleus of an atom. Chemical reactions involve changes in the
electron structure around the atom.
Chemical reactions the same atoms are involved:
Na   Cl   NaCl
Nuclear reactions the nucleus changes, resulting in an entirely new atom:
4
U  234
Th

90
2 He
238
92
There are three types of nuclear reactions we will talk about:
1.
Alpha Decay ( - decay) – A reaction whereby a parent atom releases an  particle. An  particle
consist of 2 protons and 2 neutrons. An  particle is basically a Helium nucleus
atomic
weight
isotope
#
Atomic
#
# of
protons
A
Z
X  ZA42Y  24He
210
84
4
Po206
Pb

82
2 He
2. Beta Decay ( - decay) – A reaction whereby a parent atom releases a -particle.  particles are
simply free electrons. Since electrons have a charge of –1 proton a beta decay
will have the following form:
A
Z
X Z A1Y  10
In lab today we will look at the  - decay of Strontium – 90:
90
38
Sr  Y  
90
39
0
1
3. Gamma Decay ( - decay) – A reaction whereby a parent atom releases a  -particle.  - decay is
usually a result of an atom decaying from a higher energy state to a lower one.
The emission of a  - particle is a mechanism by which an atom can get rid of
excess energy. The neat thing about  - particles is that they are massless, it
is a very high energy electromagnetic wave. Because of the high energy of the
 - particle it also removes an electron from an atom.
A
Z
X Y  
A
Z 1
0
1
In lab today we will look at the  - decay of Cobalt – 60:
Co Ni    
60
27
60
28
0
1
As these atoms undergo the nuclear reactions they will emit the radiation. Today we will measure
this radiation using the Geiger counters.
Also, we will look at how this radiation is affected by shielding. When dealing with radiation
shielding we talk in terms of half-thickness – which is the thickness of a certain material that
will stop half of the incident radiation.
counts
N
N/2
x1/2
thickness of shielding
The equation that relates the intensity of the transmitted radiation to the thickness of the
material is:
I  I 0e
 x 

 ln 2 
 x1 / 2  where, I0 is the unshielded intensity, x is the thickness and x1/2 is the
half-thickness
We can rearrange this equation:
I 
ln 2
ln    
x
x1/ 2
 I0 
Similar to the half-thickness there is a time when the original atom has been reduce by half.
This is called it’s half-life.
Let’s say I start with 100 g of some radioactive material with a half-life of 100 years.
Time
Time
100 g
0
0
50 g
1 half-life
100 years
25 g
2 half-lives
200 years
12.5 g
3 half-lives
300 years
6.25 g
4 half-lives
400 years
remaining amount
Remaining mass
N
N/2
t1/2
time
Mass – Energy:
Let’s say we start with 100 grams of C-14, after one half-life it decays through the following
reactions:
C 147N  10
14
6
How much C-14 do you have? 50 grams
How about N-14?
It turns out that you will wind up with less than 50 grams of N-14 because some of this mass
is converted to energy