Edexcel IGCSE physics radioacivity unit

1. presents
a production
Really
Radioactive
1

2. The nucleus
The tiny core of the atom is very dense. It consists of protons and
neutrons and is orbited by electrons. A proton is about the same
mass as a neutron but about 2000 times larger than an electron.
Images: PHET build an atom sim
Proton (aka atomic) number: 6 = 6 protons
Nucleon (aka mass) number:213 = 6 protons + 7 neutrons

3. Isotopes
Isotopes of an element are atoms which have the same
number of protons but different numbers of neutrons.
Images: PHET build an atom sim
3

4. Unstable nuclei
Certain nuclei are unstable. They decay spontaneously
and emit a particle (either alpha or beta) and the
nucleus forms an isotope of a new element.
Unstable = Radioactive
Decay = Disintegrate An unstable nucleus
Random = Spontaneous decays spontaneously
A radioactive
nucleus disintegrates
randomly
4

5. Alpha, beta, gamma
An alpha particle consists of 2 protons
and 2 neutrons (charge = +2). It is the
same as a helium nucleus. It leaves the
Image: ionactive
parent nucleus at a speed of 15 000 km/s.
A beta particle consists of one electron
(charge =-1; mass 1/8000 th of alpha). It
leaves the nucleus at speeds of up to nearly
300 000 km/s.
A gamma ray is NOT a particle. It is the
most energetic electromagnetic radiation
which is emitted by the nucleus at 300
thousand km/s,5the speed of light.

6. Penetration
Passes through ... Stopped by ...
few cm of air paper, skin
air, paper, skin few mm aluminium
few mm aluminium,
thick lead
most things
6

7. Ionisation of atoms in the
Nuclear radiation can knock electrons out
air or in your body.You can think of this as damage.
Image: remnet
causes a lot of ionisation
in a short distance
causes some ionisation
and travels further
When ionised, an
atom becomes an
causes very little ionisation ion pair
and is difficult to stop
7

8. Detecting radioactivity
The Geiger-Muller tube is one of many ways to do this.
One alpha or beta particle
entering the detector ionises
the gas within which creates
a voltage which is counted as
a ‘click’ by the ratemeter.
A photographic film will darken
when exposed to radiation
8 Image: darvill.clara.net

9. Background radiation
Medical
12% The Earth We are constantly
Cosmic rays
10%
14% exposed to low-
Other level radiation in
Food 2%
our environment.
12%
Most of these
sources are
Radon gas naturally occurring.
50%
9

10. Uses of radioactivity
Thickness control
Radiotherapy
Find
Leak out
tracer about
them Medical tracers
The smoke detector too 10

11. Carbon things After death, no
All living
dating
There is a fixed
contain the same more C-14 is
proportion of
fraction of C-14 as absorbed and it
carbon 14 in the
the atmosphere due decays with a
atmosphere.
to the food chain. half-life of 5730
years. The
fraction of C-14
remaining in old
organic material
can be used to
calculate its age.
Images: Answersingenesis
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12. Hazards of radioactivity
Find
out
about
them
12

13. Radioactive decay
Unstable nuclei decay
randomly. They change
into other elements. The
decay is exponential
which means that the
number of nuclei always
reduces by half in a given
time.
The rate of decay falls as well, but it
never gets mathematically to zero.
13

14. Half-life
The activity of a sample is measured in Bq (Bequerels =
decays per second). The activity decays exponentially with
time, just like the number of nuclei.
A sample of 24 million
nuclei has a half-life of 3 Solutions
years. After how many over the
years will there be 3 page
million nuclei left?
What is the half-life of
a sample whose activity
falls from 640 Bq to
40Bq in 30 minutes?
14

15. Nuclear equations
9 years
7.5 min
Parent Daughter
nucleus nucleus
A X−4 4 Alpha can be
XZ ⇒ Y Z−2 + α 2 also written
as He
A X 0 Beta can be
XZ ⇒ Y Z+1 + β also written
−1 as e
The particles in the nucleus are
not changed by gamma decay.
15

16. Nuclear transformations
The daughter nucleus is 2 elements lower
than the parent on the Periodic table.
The daughter nucleus is one element higher
than the parent on the Periodic table.
The daughter nucleus is identical to
the parent nucleus.