Dosimetry and Safety

In the previous section, the activity of a source was found. This is a useful quantity to measure as it allows scientists to measure how much radiation is being given off, and can give a rough idea of how dangerous the source could be. 

In order to fully understand the damage caused by radiation, it is not the amount of radiation given out by a source that must be considered, but the amount absorbed by a person. 

Absorbed Dose

The first stage in measuring the biological damage caused by radiation is to measure how much is absorbed by a person. The more energy absorbed, the more damage caused. 

To calculate this, the following formula can be used:-

Where the unit for Absorbed Dose is Grays (Gy) 

Example 1 -

If 100 mJ of energy are absorbed by 3 kg of tissue, what is the Absorbed dose?

E = 100 mJ  =  100x10-3 J

m = 3

D  =  E / m  =  ( 100x10-3 ) / 3  =  0.033 Gy

D  =  3.3mGy 

Note - The absorbed dose gives a general  indication of damage, but does not take into account the type of radiation. This is important as the same absorbed dose of Alpha particles is much more damaging than the same dose of Gamma rays. 

Equivalent Dose

In order to accurately compare the damage caused by each of the radiation types, the absorbed dose value must be weighted to take into account the type of radiation absorbed. To calculate this, the following formula can be used:-

Where the Equivalent Dose (H)  is measured in Sieverts (Sv). 

Equivalent dose is a measure of the biological harm caused by the radiation. 

The weighting factor (WR) is a different value for each radiation type, allowing a true comparison between effects of radiation absorption. The following table gives some example values for WR:-

For example - 

A scientist is working with a radioactive substance. She determines that during the session she was exposed to 5 mJ by Gamma rays and 2mJ of Alpha particles. If she has a mass of 50kg, what is the equivalent dose she receives?

Step 1 - Calculate Absorbed Dose for each

Step 2 - Calculate the Equivalent Dose for each

Step 3 - Sum together to calculate the Total Equivalent Dose. 

Step 1 - 

DGamma  =  ( 5x10-3 ) / 50  =  1x10-4 Gy

DAlpha  =  ( 2x10-3 ) / 50  =  4x10-5 Gy

Step 2 - 

HGamma  =  1x10-4 x 1  =  1x10-4 Sv

HAlpha  =  4x10-5 x 20  = 8x10-4 Sv

Step 3 - 

HTotal  =  8x10-4 + 1x10-4  =  9x10-4 Sv

HTotal  = 0.9 mSv

Note - On average each person will receive an equivalent dose of ~ 2.7mSv due to Background Radiation sources.

Radiological Effects

It is clearly understood that exposure to radiation can cause severe health effects, even death, and that the effect on the human body is based on level of exposure. The diagram below shows the effects of increasing radiological exposure:-

Uses of Radioactivity 

There are lots of different uses for radiation used in everyday life, showing that not all radiation is dangerous:-

Treatment of Cancer

Sterilisation of Medical Instruments

Radioactive Tracers

Food Irradiation

Carbon Dating

Treatment of Cancer

One of the most widely known uses of radiation in medicine is the treatment of Cancer. 

High level of radiation will kill living cells, but if used correctly, can be used to target Cancerous cells, whilst doing minimal damage to surrounding tissue:-

The diagram above shows how by rotating the Radioactive source around the patient, the Cancerous cells can be kept at a high level of exposure ( at the centre of the circular path ), whilst reducing the exposure to other tissue. 

Radioactive Tracers

Radiation can be used to image inside the body, allowing doctors to track the flow of fluids throughout the Circulatory, Lymphatic or Digestive systems. By injecting or ingesting a liquid containing a low activity, short half life Gamma source, the flow of fluids around the body can be imaged. 

The image above shows a Gamma camera, the device used to detect the Gamma rays emitted by the tracer.

Note - A Gamma source must be used as it is the only type of radiation penetrative enough to leave the body so it can be detected. 

The image below shows a Gamma tracer scan of a healthy person. Note the bright patch when most of the radioactive tracer has already passed from the blood into the bladder for removal from the body:-

The image below show a Gamma tracer scan of a person with Non-Hodgkin Lymphoma (a cancer of the Lymphatic system). The main mass of the tumour can be seen on the right side of the image as a brightly coloured area of tissue:-

Note - The above images are actually in the wrong order and are of the same person. The second image is the pre-treatment image, and the first image is the result after extensive Chemotherapy and Radiotherapy to treat the Cancer. 

Sterilising Medical Instruments

Ensuring that all medical instruments are hygienic and safe for use is a very difficult and expensive task. It would be too costly to always bin instruments after one use, so Radiation can be used to Sterilise these instruments to ensure their safety:-

The equipment is washed multiple times, sealed in plastic, then exposed to high intensity Gamma rays. This kills all Bacteria that remained after washing, ensuring sterility. 

Food Irradiation 

Certain foods which spoil quickly (eg fruit or vegetables) can be irradiated with Gamma rays, which kills any fungus or Bacteria within the food, extending its shelf life.

Note - The two processes of irradiation above do not cause the objects to become radioactive themselves, therefore they remain safe to use or eat.

The video below shows a scene from the film "28 Days Later" showing the preservative effects of irradiated fruit...

As a Medical Physicist, you would research new techniques and develop new medical equipment for treatments in hospitals. You’d make sure specialist equipment is safe and works properly.

You’d work with equipment, like MRI scanners, ultrasounds and x-rays, which are used to investigate patients' conditions.

You could work on imaging techniques to track how organs are functioning and to help with image-guided surgery

If you work on radiation and radio therapies you could calculate dosages for beams and radioactive implants used in the treatment of cancers.

You could develop electronic instruments which take measurements or support damaged organs.

Or you could work on laser technology. It is used to reduce the need for invasive surgery, for example to break up kidney stones or treating eye disorders.

You would work closely with medical professionals such as doctors, radiographers and medical physics technicians.

Carbon Dating 

Up until the 1940s, there was no true scientific test allowing the age of Archaeological finds to dated with accuracy. The American Scientist Willard Libby developed  the process of Radiocarbon Dating, for which he was awarded the Nobel Prize for Chemistry. 

Carbon Dating uses the radioactive decay of a radioactive isotope of Carbon (14C). When an organism is living, it constantly takes in Carbon from its environment, replacing any radioactive Carbon lost through decay. This means that the level of radioactive Carbon with a living organism is at a constant value. 

When that organism dies, however, this 14C is not replaced, and therefore the older the sample, the less 14C will be present. By measuring the amount of 14C and comparing this to the half-life of 14C, an estimation of the age of the material can be found. As 14C has a long half-life (5730 Years), it is possible to date materials up to ~50,000 years old. 

The image below sums up the process of Carbon Dating:-