Radiation and half life crib sheet (and practice questions).

Half-life and radiation factsheet:

General information:

Radioactivity happens because:

1.       Some atoms are unstable, and break apart or loose smaller particles to become more stable atoms – this called the ‘activity’.

2.       Activity is measured Becquerel’s.

3.       When atoms decay they give off radiation.

4.       The amount of time for half the radioactive atoms to decay into stable atoms is called the half life.

5.       Radiation ionises atoms ((knocks off an electron from them) which is damaging to DNA.

Types of radiation are:

Radiation	How penetrating?	What is it?	What’s its charge?	What’s a use?	Weighting factor
Alpha particle	Will be stopped by a few cm of air, or skin. Least penetrating.	Two protons and two neutrons bound together	+2	Used in smoke detectors	20
Beta  particle	Will go through air and thin metal, like aluminium, but not thick or very dense metal like lead.	An electron	-1	Measuring  the thickness of paper in industry	1
Gamma ray	Will go through thick metal sheets, concrete, or rock. The most penetrating.	A very high energy photon (an electromagnetic wave, like light or radio waves)	Neutral	Radiotherapy, to kill cancer cells	1

Natural sources of radiation are everywhere, most are natural radioactive atoms in the air (like radon gas), the rocks and soil (like Uranium ore), or dissolved in water. Some radiation comes from stars or other objects in space, and is called cosmic radiation.

Equations:

Activity:

Activity is defined as ‘The number of atoms to decay per second): the equation for this is:

 
Where A is activity in Becquerels, N is the number of atoms that decayed, and  t is the time in seconds their decay took.

Doses of radiation:

Radiation is measured in two ways:

Absorbed dose, which is measured in Greys (or more often milligreys). The equation for Absorbed dose :

Where D is absorbed dose in Greys, E is the amount of energy the radiation carried in Joules, and M is the mass of the thing absorbing the radiation in kg

Equivalent dose takes into account how damaging  each kind of radiation is, by multiplying the absorbed dose by a weighting factor (a unitless number picked by observing how much damage each type does). It’s equation is:

Where H is the equivalent dose in, W is the weighting factor, D is the absorbed  dose in Greys. Equivalent dose’s  unit is Sieverts.

Half-life and radioactivity practice questions:

 

1: Three kinds of radioactive source give off three different kinds of radiation.

·         Source A gives off radiation that goes through  aluminium sheet but is stopped by  a lead brick

·         Source B gives off radiation that is stopped  by aluminium sheet, but travels through several centimetres of air

·         Source C gives off radiation that is stopped by several centimetre of air.

Which source is giving of which type of radiation?

2: A chunk of radioactive material has a half-life of twenty three seconds. 

a)    How many whole half-lives does it go through before its activity has dropped to 6.25% of the original value? Hint:  1 whole half-life takes the activity to 50%, 2 half-lives takes the activity to  25%, and so on – you can draw a table of  half-lives vs % of activity if it helps.

b)    How many seconds before the activity level drops to 25%?

3: A 4 Kg cat gets hit with 50 joules of radiation.

a)    What is its absorbed dose?

b)    The radiation is alpha particles, with a weighting factor of 20. What is the cat’s equivalent dose?

4:  A radioactive source is made of 10,000,000 radioactive atoms. After 2 seconds 1,000,000 have decayed. What’s its activity in Becquerel’s?

Answers here