Theme 7 – Radioactive Decay
ASTR 101
Prof. Dave Hanes
Understanding Atoms
As we saw earlier, they are mostly empty space!
One implication: atoms can be crushed down to much
denser states – hence neutron stars, a trillion times as
dense as water! (Learn more in ASTR 102.)
Meet the Nuclear Players
The neutrons and protons are just about equally
massive (and about 2000x as heavy as the
distributed electrons).
[The actual structure is more complex than that,
and involves quarks that make up the neutrons
and protons. We will not need to explore that.]
Atomic Number (Z)
Z = number of protons in the nucleus.
Hydrogen = 1 proton
Helium = 2 protons
Carbon = 6 protons
Oxygen = 8 protons
Uranium = 92 protons
Why Z Matters Chemically
Z determines the number
of surrounding electrons,
and thus how it bonds
chemically with other
elements
That’s why hydrogen
behaves as hydrogen!
Atomic Mass Number
But nuclei can also contain neutrons
[The mass of the neutron ~ the mass of the proton.]
Number of protons + number of neutrons =
atomic mass number
Isotopes
[‘Iso’ = same]
Adding neutrons changes the mass but not the
fundamental type of atom. Thus:
Hydrogen has 1 proton
1 proton + 1 neutron = ‘heavy’ hydrogen (a.k.a.
deuterium). Found in ‘heavy water.’
1 proton + 2 neutrons = ‘even heavier hydrogen’
(tritium)
How Many Isotopes Can There Be?
Are there only three hydrogens (ordinary;
deuterium; tritium)? Or could there be,
say, a ‘super-heavy’ hydrogen, with 1
proton and 50 neutrons?
Or could there perhaps be an‘ultra-light’
uranium, with 92 protons and just 5 or 6
neutrons?
Apparently Not!
Nuclei typically contain more
neutrons than protons –
but not usually twice as many.
U238 (shown by the red arrow)
has 92 protons and 146 neutrons.
C14 (the blue arrow) has 6 protons
and 8 neutrons.
Radioactivity!
Some isotopes of some nuclei are unstable,
especially (but not only) the very heavy ones.
They spontaneously break up, spitting out
smaller pieces and releasing energy.
As we will see, this is
(a) a great tool for age dating
(b) very useful in certain medical applications
(c) a health hazard in some circumstances
(d) important in determining a planet’s ‘heat budget’
Energy is Emitted in Two Forms
1.
Kinetic energy (the
energy of motion of
the moving particles)
1.
Radiant energy (the
energy carried by
penetrating radiation –
gamma rays)
Generating Electricity by Heating
[welcome to Pickering!]
https://www.youtube.com/watch?v=7fXYFQmR2j0
Three Kinds of Radioactivity
Discovered by accident in the late 1800s,
but not understood for many years
So, called
α, β, γ - alpha, beta, gamma rays
[neutrons are sometimes also released]
α, β, γ
Different penetrating powers – but what are they?
Consider the Leftover Nucleus
What’s left behind is not the same as it was!
The nucleus transmutes to a different element or
isotope.
This helps us understand the bits that have come
out!
I. Alpha Particle Decay
Example:
U
238
 Th
234
+ α
When U238 transmutes into Th234, it emits an
‘alpha particle’
What has the U nucleus lost? 4 units of mass, of
which 2 are protons. (U has 92 p; Th has 90)
Conclusion: Alpha particles are helium nuclei!
Another Example:
Plutonium Turns into Uranium
Helium on Earth
…is mostly radiogenic, formed by radioactive rocks and
trapped underground
II. Beta Particle Decay
Example:
Th234  Pa234 + β
No mass is lost, but one extra proton
appears.
[Th has 90, Pa has 91.]
How on Earth??? Sounds like magic!
Electric Charge is Conserved
[Another Conservation Law]
Within the nucleus, a brand new proton appeared, with a
“+” charge. So there must be a new “-” to go with it!
Here’s what happens: one neutron spits out an electron,
and turns into a proton, in what is called beta decay.
n  p (+) + e (-)
(that is, n  p + β)
Conclusion: The emitted beta particles are electrons! – but
not part of the original ‘cloud’ surrounding the atom.
III. Gamma Rays
These are light (not visible to the human eye: of
much higher energy and shorter wavelength).
Gamma rays can cause tissue damage, disrupting
DNA in the chromosomes.
This can be good (if targeted, used to kill cancerous
tumour cells)
or bad (if uncontrolled, causing leukemia, radiation
sickness).
Radiation Therapy
A Series of Decays,
in a Cascade to a
Final Stable
Daughter Product
(U238 finally ends up as
stable lead Pb206, but
that’s the end!)
The Decays Take Place Over Time
The atoms don’t all decay
at once: a radioactive
substance only gradually
converts to a stable form.
If we understand the rules
that control this behaviour,
we can use it as a clock.