Theme 7 – Radioactive Decay

ASTR 101Prof. Dave Hanes

Understanding AtomsAs 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 protonHelium = 2 protonsCarbon = 6 protonsOxygen = 8 protonsUranium = 92 protons

Why Z Matters Chemically

Z determines the number

of surrounding electrons,

and thus how it bondschemically with otherelements

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)

2. 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, γDifferent 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.