## dosimetry

This week, we’ve looked at calculating radiation doses.  The absorbed dose D, measured in Grays (Gy), takes into account the energy E absorbed and the mass m of the absorbing tissue.

$D = \displaystyle {E \over m}$

The higher the energy, the greater the absorbed dose.  If you are wondering why the absorbing mass is important, consider the different masses of tissue involved in a dental x-ray and a chest x-ray….

We also learned about equivalent dose in Sieverts (Sv). The equivalent dose H gives an indication of the potential for biological harm by considering the absorbed dose D and a weighting factor $W_R$.

$H=DW_R$

Different types of radiation have different weighting factors, e.g.

type of radiationweighting factor
gamma1
x-ray1
beta1
alpha20

The more damaging forms of radiation have a larger weighting factor.

Absorbed dose and equivalent dose are usually expressed in smaller units; μGy, mGy, μSv, mSv.

In the UK, the population receives an average equivalent dose of 2.2mSv per year due to background radiation produced by cosmic rays, radon gas and materials dug up from the Earth’s crust, such as rocks and soil. In addition to this exposure to background radiation, the Government has set a further equivalent dose of 1mSv per year for members of the public.  This limit can be increased to 20mSv for people who work in the nuclear industry, certain medical occupations (such as radiographers) and airline pilots – all of whom will exceed the public limit in the course of their job.

This occupational increase for some individuals can be justified on the grounds that workers are not as vulnerable to the effects of radiation exposure since they are neither children (high rate of cell division so more chance of dna damage being copied) or elderly (reduced ability to repair damage).  In many cases, these workers will also be screened on a regular basis by occupational health staff at their place of work.

Here is a poster from the excellent xkcd site that explores examples of the different levels of equivalent dose.

Click on the picture for a larger version.

###### source: XKCD

Notice that the scale changes as you move through the poster from blue to green to red.

The dosimetry topic is comprehensively covered at BBC Bitesize.

## national 4 – waves & radiation revision

Here are some summary notes to help with your revision for the waves and radiation unit assessment.  Your test is scheduled for Friday 24th April.
Extra revision materials and questions are available on BBC Bitesize.

## x-rays

X-rays are a form of electromagnetic radiation.  They have a much higher frequency than visible light or ultraviolet.  The diagram below, taken from Wikipedia, shows where x-rays fit into the electromagnetic spectrum.

###### image by Materialscientist

Wilhelm Röntgen discovered x-rays and the image below is the first x-ray image ever taken.  It shows Mrs. Röntgen’s hand and wedding ring.  The x-ray source used by Röntgen was quite weak, so his wife had to hold her hand still for about 15 minutes to expose the film.  Can you imagine waiting that long nowadays?

This was the first time anyone had seen inside a human body without cutting it open.  Poor Mrs. Röntgen was so alarmed by the sight of the image made by her husband that she cried out “I have seen my death!” Or, since she was in Germany, it might have been

that she actually said.

Röntgen continued to work on x-rays until he was able to produce better images. The x-ray below was taken about a year after the first x-ray and you can see the improvements in quality.

Notice that these early x-rays are the opposite of what we would expect to see today. They show dark bones on a lighter background while we are used to seeing white bones on a dark background, such as the x-ray shown below.  The difference is due to the processing the film has received after being exposed to x-rays.

In hospitals, x-rays expose a film which is then developed and viewed with bright light.  X-rays are able to travel through soft body tissue and the film behind receives a large exposure.  The x-rays darken the film. More dense structures such as bone, metal fillings in teeth, artificial hip/knee joints, etc. block the path of x-rays and prevent them from reaching the film.  Unexposed regions of the film remain light in colour.

Röntgen’s x-ray films would have involved additional processing steps.  The exposed films were developed and used to create a positive.  In creating a positive, light areas become dark and dark areas become light.  So the light and dark areas in Röntgen’s x-rays are the opposite of what we see today.  Our modern method makes it easier to detect issues in the bones as they are the lighter areas.

Röntgen was awarded the first ever Nobel Prize for Physics in 1901 for his pioneering work in this field of physics.

Medical imaging has come a long way since Röntgen’s discovery of x-rays.  This promotional video from German company Siemens outlines the advances that have been made since the early 20th century.

I have attached a recording of a short BBC radio programme about the first x-ray and what people in the Victorian era thought of these new images.  Click on the player at the end of this post or listen to it in iTunes.

At the beginning of this week, we looked at the physics of ultraviolet radiation.

image courtesy of sonrisaelectrica

The section of the electromagnetic spectrum with wavelengths ranging from 10nm to 400nm is called ultraviolet radiation (uv for short).  Sunlight contains uv rays and it’s those uv rays that are responsible for the suntan you get during the summer holidays.  This Australian animation shows how the ultraviolet in sunlight causes our skin to tan and explains why too much uv will damage our skin.  The SunSmart page has loads of information on staying safe in the sun.

The damage that uv can do to cells is put to good use in some sterilisation equipment, such as this bottle for safe drinking water and the toothbrush sanitiser shown below.

The Nobel Prize for Medicine was awarded to Niels Rydberg Finsen in 1903 for his research into the effects of ultraviolet on the bacteria that cause tuberculosis.

We used uv banknote checkers in class to view some of the security features built into British banknotes. This image of a Clydesdale Bank £10 note shows part of the pattern that can only be seen under uv light.

image from Science Photo Library

There is a Bank of England leaflet (pdf) with further information on the security features in our banknotes.

Remember that whenever something glows under a uv light, we’re not seeing the uv radiation itself because our eyes can’t detect ultraviolet.  Instead, we see the fluoresence; visible light given out in response to the uv falling on the material.

Some hair gels fluoresce under uv light.  Here is someone with some of the uv gel in his hair.

but we don’t see anything until we turn on the uv light.

Cool, eh?

You can even buy genetically modified tropical fish that glow under uv light.