Activity, Dosimetry and Half-Life
In this section we will discuss the calculations that you need to be able to apply in this unit. We will calculate activity of radioactive substances, calculate the dose absorbed to our bodies and also discuss the dangers of absorbing ionising radiation.
We need to be able to protect ourselves to the best of our ability when radiation is being utilised. Workers in nuclear power plants need to constantly be aware of how much radiation exposure they have. Medical professionals need to be very carful not to be exposed to too much radiation when X-rays are involved in procedures and medical imaging.
Side note: I had a lot of fun drawing the wee guy in the hazmat suit.
Activity
Activity is the number of nuclear disintegrations in a period of time.
So if a radioactive substance has a high number disintegrations within a a small period of time, it is very active, and therefore emits a high amount of ionising radiation. A high amount of ionising radiation being emitted from a substance can be very dangerous.
Activity can be measured using a Geiger-Muller tube, also called a Geiger counter.
This is the equation to calculate activity:
Activity = number of disintegrations/time.
Activity is measured in Becquerels (Bq)
Number of disintegrations has no units
Time is measured in seconds (s)
Although not harmful, there is constant BACKGROUND RADIATION present all around us, that is produced by natural and man-made sources. This activity is picked up by Geiger counters but is very low (around 1Bq). Therefore, when measuring the activity of a substance, this background radiation level must be measured prior, and subtracted from the activity value of the measured substance.
Absorbed Dose
As radiation exposure can be dangerous, we need to be able to calculate the amount of energy that has been absorbed to the body.
This is called the Absorbed Dose.
This calculation relies on the mass of a material or tissue that absorbs the energy, as well as the amount energy emitted by the radioactive substance.
Absorbed dose is therefore calculated with this equation:
Absorbed Dose = Energy/Unit mass of material
Absorbed Dose is measured in Grays (Gy)
Energy is measured in Joules (J)
Mass is measured in kilograms (kg). (remember to convert grams to kilograms)
Therefore, one Gray is 1 Joule per Kilogram.
Equivalent Dose
Many factors determine the biological effect that radiation has to our bodies. These include:
Type of Radiation
The body tissues and organs exposed to the radiation
The absorbed dose
The damaging/ionising effect of different radiation types is quantified using the a factor called the Radiation Weighting Factor. Each type of radiation (Alpha, Beta, Gamma), is given a radiation weighting factor based off how damaging it can be when absorbed by tissue.
Equivalent Dose can be calculated to determine the biological effect of radiation on a particular tissue. This can be done using the following equation:
Equivalent Dose = Absorbed Dose x Radiation Weighting Factor.
Equivalent Dose has symbol H and is measured in Sieverts (Sv)
Absorbed Dose is measured in Grays (Gy)
Radiation weighting factor has no units, and the weighting factor for different radiation types will always be given in exam questions, unless you are asked to calculate it with a given equivalent dose and absorbed dose.
Equivalent Dose Rate
Equivalent dose rate allows us to monitor the biological effect of radiation over time. This is very important for safety in the workplace if people are exposed to radiation for a prolonged period of time. The equivalent dose rate can be determined by calculating the equivalent dose over a period of time:
H = H/t
Equivalent Dose rate = Equivalent Dose/time
Equivalent dose rate is measured in Sieverts/given time (may be seconds, hours or days depending on the question context.
Half Life
As a radioactive substance ages, its activity (no. decays/time) decreases. Each radioactive substance has a very specific time in which its activity is halved. For example, one isotope’s activity may take 3 years to reduce by half. The time that a radioactive substance takes for its activity to reduce by half is called half-life.
This is a very common question in exams, and it can be easy to drop marks on these calculations.
Uses of Radiation
Radiation has many uses, including in the production of energy, as well as in the medical field.
In the medical field radiation is used in the form of X-rays for imaging/diagnosing patients. X-rays can also be used therapeutically, helping in surgical procedures. Gamma radiation is utilised in radiotherapy for certain cancers and also used to sterilise equipment for surgeries.
Knowledge of radiation has allowed us to determine the ages of old carbon-based compounds. This is called carbon-dating and by determining the half-life of a carbon-based compound we are able to calculate the age of the substance. Radiation is therefore used in archaeology.
Radiation can also be used for energy production, space exploration and in smoke detectors.
Key Points!
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Activity
Activity is the number of nuclear disintegrations in a period of time.
Activity can be measured using a Geiger-Muller tube, also called a Geiger counter.
Activity = number of disintegrations/time.
Understand the term: “Background Radiation”
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Absorbed Dose
Absorbed dose definition
Absorbed Dose = Energy/Unit mass of material
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Equivalent Dose
Equivalent Dose can be calculated to determine the biological effect of radiation on a particular tissue. This can be done using the following equation:
Equivalent Dose = Absorbed Dose x Radiation Weighting Factor.
Many factors determine the biological effect that radiation has to our bodies. These include:
* Type of Radiation
* The body tissues and organs exposed to the radiation
* The absorbed dose
Equivalent dose rate allows us to monitor the biological effect of radiation over time.
H = H/t
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Half Life
The time that a radioactive substance takes for its activity to reduce by half is called half-life.
Be able to answer problem solving questions surrounding Half-life
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Radiation Uses
Know various uses of radiation including applications in the medical and energy industries.