Radioactivity

RadioactivityHistory

Discovered (accidentally) ByHenri Becquerel

“He carries thus his uranium salts under the sun, places them close to photographic plates covered with black strong paper. The development of the plates shows that uranium salts radiates X rays, the only known radiation capable of impressing the plates through black paper. The following week, the sky of Paris is grey and covered No way to expose uranium salts to the sun. Becquerel stows his covered photographic plates and his uranium salts in a drawer. By "chance", the sun is absent during many days and the plates are left in the drawer during many days.

Finally, Henri Becquerel decides nevertheless to develop his unexposed plates. Very surprised, he remarks that the plates are impressed. A new type of radiation emitted by non fluorescent uranium has gone through the strong black paper.

RadioactivityThe shadow of the copper cross that Becquerel had placed between uranium and the covered plates is visible: the new radiation has not gone through it”

RadioactivityTypes of RadioactivityIt soon became clear that this radioactivity came in three different types:-

alpha

beta

gamma

Radioactivity

alphaBig

Positive charge

Easily stopped by paper (not penetrating)

Radioactivitybeta

Small

Negatively charged

Can be stopped by aluminium foil

Radioactivity gamma

Very small (no mass)

No charge

Stopped by 5cm lead

Radioactivity

alphabeta

gamma

paper aluminium lead

Radioactivity

Radioactivity comes from the NUCLEUS of an atom.

This can change the nucleus.

U238

92

Symbol ( U – Uranium)

Mass number (total no. of protons and neutrons)

Atomic number

(no. of protons)Uranium 238 is an alpha emitter

Radioactivity

U238

92

234

90Th

Thorium 90 is a beta emitter

Mass number goes down by 4.

Atomic number goes down by 2

Radioactivity

Th234

90

234

91

PaMass number stays the same

Atomic number goes up by 1

Radioactivity

Co60

27

Co

60

27

Mass number stays the same

Atomic number stays the same

Radioactivity

Radioactivity is a random event. It is impossible to say when an individual radioactive atom will decay.

RadioactivityBut, in a certain time, called the half- life, half of the total number of atoms will decay.

RadioactivityR adi oac ti ve Dec ay

0

100

200

300

400

500

600

0 5 10 15 20 25 30 35

T ime / min sOne half life

RadioactivityBackground Radiation

Radioactivity is all around us.

Many rocks, especially granite, contain small amounts of Uranium. This produces Radon gas.

The sun produces radiation, some of which can reach the Earth’s surface.

Medical procedures can produce radioactivity.

Fall-out from weapons also produces radioactivity.link

RadioactivityBackground Radiation

RadioactivityDangers of Radioactivity

Alpha and beta particles cannot penetrate very deeply into human bodies. As they stop they lose kinetic energy, this generates heat which leads to burns. If the radioactive material is inside the body (e.g. in lungs) this can be very dangerous.

RadioactivityDangers of Radioactivity

All types of radiation cause ionisation in molecules. This can interfere with biological reactions, or damage genes leading to possible cancers. Alpha sources are more likely to damage genes than beta or gamma.

RadioactivityUsing RadioactivityIN THE HOME

Smoke detectors contain Americium 241. This is an alpha source. It has a long half life.

alpha source detector

BeepBeep

RadioactivityUsing Radioactivity

IN INDUSTRY

Thickness measurement in paper mills uses beta sources with long half lives.

detector

Beta source

RadioactivityUsing RadioactivityIN INDUSTRY

Tracers in industry often use gamma sources with short half lives.

Detector DetectorBeep

RadioactivityUsing RadioactivityIN MEDICINE

Gamma radiation can be used to diagnose problems in lungs or circulatory system.

In Nuclear Medicine the patient is given a radioactive tracer agent either by mouth (capsules) or intravenous injection. The tracer goes to the target organ to be studied and allows it to be imaged with a gamma camera. A gamma camera takes pictures of the radiation photons emitted by the radioactive tracer in the organ and allows the radiologist to determine the nature and extent of the problem.

RadioactivityGamma Camera

How it works Image showing

tumour

RadioactivityUsing RadioactivityIN MEDICINE

Gamma radiation can be used to treat some tumours. These contain cells that are rapidly growing. Rapidly growing cells are easily destroyed by radiation. Care must be taken to avoid too much damage to healthy cells.

RadioactivityUsing Radioactivity

In Archaeology

Cosmic ray protons blast nuclei in the upper atmosphere, producing neutrons which in turn bombard nitrogen, the major constituent of the atmosphere . This neutron bombardment produces the radioactive isotope carbon-14. The radioactive carbon-14 combines with oxygen to form carbon dioxide and enters into the cycle of living things.The carbon-14 forms at a rate which appears to be constant, so that by measuring the radioactive emissions from once-living matter and comparing its activity with the equilibrium level of living things, a measurement of the time elapsed can be made.

RadioactivityUsing Radioactivity

RadioactivityUsing Radioactivity

RadioactivityUsing Radioactivity

RadioactivityUsing Radioactivity

If the amount of C14 in a wooden beam has fallen to 40% of its original value, how do you work out the age of the beam?

40%

7162 years