We can’t feel, see, or hear it- Radiation!

Ionizing Radiation is strange! It’s not like many other things we come into contact in our day to day lives. We can’t hear it, we can’t see it, we can’t feel it, we can’t smell it, we can’t taste it. In fact we can’t detect it directly in many instances, instead we measure its effects. So radiation is an indirectly measurable quantity if we use physics terms. This is somewhat like measuring the height of a tree by measuring its shadow and calculating its height. When you really think about it though, it is not that uncommon to indirectly measure a quantity.

So in the post The prehistoric error I mentioned that Roentgen realized he was producing x-rays as he could see material around his cathode ray give off light, he saw film blacken, and gases where being ionized. So the indirect quantity that can be measured to determine the presence of radiation is the darkening of the film, and the amount of ionization occurring. These two methods are still some of the common methods we used to measure radiation today. Measuring radiation is one of the major tasks performed by a medical physicist and I will try and described some of the common methods to do this include; Fluorescence, semiconductors, MOSFETS, and calorimetry.

Ionization

It makes sense to measure the ionization to detect the presence of ionizing radiation. And in fact most of the methods to detect the ionization in different ways. So when an energetic photon or other particle is traveling through something it will strip electrons from the material. Leaving ions around, particles with a positive or negative charge. An ionization chamber is a device placed inside some medium to collect this charge. It does this by using voltage, or potential differences, to attract the charged particles. So an ion chamber will have a positive and a negative bit. The charged particles are accelerated to the opposite charge on the ion chamber. To measure the amount of charge we measure the current collected.

That all seems pretty straightforward doesn’t it. Add some voltage to a device and measure the current. There are quite a few difficulties in the measurement thought. One of the major one is the magnitude of the current generated. Just to give you some idea, normal household plus for electrical devices can run a max of around 10 to 15 amps, depending on the country you live in. Beyond this, it will switch off a circuit breaker or blow a fuse. When we measure current we are measuring in the nanoamp to pico amp range, that ~10 trillion times smaller. So we need very sensitive equipment to get an accurate answer.

Film

Measuring radiation using film is somewhat similar to ionisation. We are probably aware of film’s ability of measuring radiation, as this historically was the way you would get x-rays. If you have had an x-ray recently you will know that this typically doesn’t occur anymore as it has gone digital. Similar to photos going digital. Both meant that Kodak lost a huge market, and filing for bankruptcy in 2011.

So instead of attracting ions and measuring them as current, film uses a chemical process. The ions are absorbed in an active layer in the film. In doing so they interact with smaller molecules, or groups of atoms, called monomers and combine them with other monomers and form polymers. This darkens the film, so that the darker the film the more radiation that it has been exposed to. One problem with this is that there are only a limited number of monomers inside a layer of film. Hence you can only measure up to a certain amount of radiation. Once this level has been hit more radiation will not darken the film anymore. Film measurements are useful as they give 2 dimensional information, whereas most ionization chamber measurements only give one dimensional information.

Semiconductor and MOSFETs

These act in a similar way to ion chambers, except that they measure the charge collected in a solid- silicon, rather than in air as with an ion chamber. The voltage difference needed to attract the charges is formed through the process of making a semiconductor. So there is no need to plug an external voltage to them. The other nice thing about using semiconductors is that since silicon is much denser than air, more ionization is released from the radiation, giving a larger signal to read.

Fluorescence. Fluorescence is a method of measuring radiation which first stores the radiation energy inside a material, and then releases it later on to be measured. The ionization inside the material gives the electrons a little bite more energy than they are used to. This energy can then be released later with a bit of persuasion. The persuasion can be heating the material up, or firing a laser at the material. Either way the radiation is released as a light photon. The light photons released can then be measured using an optical camera. The amount of light being measured being proportional to the amount of radiation absorbed.

And the final method I am going to talk about it Calorimetry. Calorimetry is one of the most direct methods to measured dose since temperature change is the most direct effects of radiation in a material. Calorimetry is hard though, because the changes in temperature that we are trying to measures is tiny. Normal background head needs to be taken into account and the device used needs to be thermally isolated from everything around it. Although it is hard, it is one of the standard methods used around the world. To give you an idea about what temperature changed we are talking about a fatal dose of radiation is around 5 Sieverts delivered to your entire body. 5 Sieverts delivered to 1 kg of water would only increase the temperature by ~0.001 degrees.

So to wrap up, there are three different physical processes we use to measured radiation. These are electronic, chemical, and thermal. Electronic involves measuring the charge created when radiation ionizes a material. Chemical involves measuring the chemical changes lefts after the ionization has interacted with a material. And Calorimetry involves measuring the head transferred to a medium when radiation has interacted with some material

Human just did not evolve to be able to detect radiation. So in evolutionary terms it didn’t pose our species much danger, otherwise we would’ve evolved to sense it to protect ourselves from it. But with our modern technology we can produce radiation in much larger quantities using different types of devices such as accelerators. So we have also had to develop methods new technologies to measure radiation. And we have basically been using the same methods as in Roentgen’s time, but with much more precision and accuracy. We still use ionization as our gold standard, and film that is sensitive to radiation as the main method of measuring radiations presence.

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RRR

Ok so I said we can’t hear, see, feel, smell, or taste ionizing radiation. This is strictly true but in some instances we can hear, see, feel, smell, or even taste it, indirectly of course. We can hear it in certain circumstances when it is being produced by linear accelerators from the sound of high voltage and accelerating electrons. We can see it interacting with the atmosphere creating the northern lights. Or if we are playing with nuclear material which accidentally goes prompt critical it releases Cerenkov radiation, releasing a blue flash and a deadly dose of gamma rays. Feeling and tasting it is a bit more controversial. Obviously the delayed effect can be felt, but the instantaneous one are not as simple as feeling the heat from the sun on your skin. There are stories from scientists accidentally exposed to these prompt critical nuclear reactions who said they could feel a pulse of energy go through them. And there are patients who have said they can feel the radiation when being treated. But we don’t know if these are placebo like effects, since in both cases they have the knowledge that ionizing radiation is present.

Originally published at www.radicalradiationremedy.com.