When people talk about Radiation, they often describe a substance that is scary, dangerous, and not well understood. This is generally a fair assumption. You can’t see, hear, taste, feel, or smell radiation, yet is large doses it can be dangerous to you health. This does make it a scary substance. After all, you won’t know when you are being exposed to it.
So when are you being exposed to radiation? …. You are being exposed to radiation right now! It is all around you!
We’ve talked about this before- there is background radiation around you all the time. The source of this background radiation varies from the rocks and soils of the earth, through to cosmic radiation from outer space, oh and the radiation originating from inside your body as well.
So far we have talked about only one type of radiation- ionizing radiation. This is where radiation is not well understood. Ionizing radiation is the radiation that has enough energy to ionize atoms- to strip electrons of the atoms. This is how radiation can do damage to your cells and DNA.
This is what people often mean when they talk about radiation- they are talking about ionizing radiation. There are other types of radiation such as light and radio-waves- like the ones used for WiFi and you microwave oven, that are non-ionizing. These are not bad for you health.
It is when people confuse non-ionizing radiation and ionizing radiation that scare mongering can occur. Claiming that all radiation is bad it not true, and besides in Radiotherapy ionizing radiation is used in a way to help people be cured of cancer. That is a good use of radiation.
So if you can’t see, hear, taste, feel, or smell radiation, then how do you know where the radiation is?
This is a great question. In radiotherapy you need to accurately know where the radiation is and how much you are delivering to the patient’s cancer. So how do you do that? We will need to get into the physics of ionizing radiation first before I can tell you how you measure it.
When ionizing radiation hits something it ionizes the material- I told you this earlier. When something is ionized it releases electrons- electrons are the bases of electrical energy. So you can relate the amount of electrical energy to the amount of radiation present.
Radiation detectors convert this electrical energy created by the radiation into an electrical signal. This electrical signal can then be measured by an electrometer connected to the radiation detector. The electrometer is measuring a current created in the material created by the radiation present. And if measuring over time, rather than a rate, they measure an electrical charge.
Before you can determine how much radiation dose is being produced you need to calibrate the detector. This is done by sending the detector to a standards laboratory. When in the standards laboratory they compare you’re detector to their detector. Measuring the same radiation source and comparing the signal created by both. This will give your detector a calibration factor to relate the charge measured to the dose of radiation.
There is a worldwide chain of secondary and primary standards laboratories that can do this for your detectors. They give you a calibration factor to convert you charge into a unit of radiation dose called the Gray, name after Louis Harold Gray. These standard labs compare amongst themselves to check that they are all measuring the same unit of radiation dose. This is how you can go to any part of the world and receive the same amount of radiation dose in any radiotherapy center.
Simple- but there is more. Every detector will have further correction factors that need to be applied to truly measure the correct amount of radiation dose. This will depend on the detector characteristics and the type of radiation beam being measured. There are different types of detectors made for specific types of applications. For instance diagnostic radiology will use different detectors than radiotherapy, or nuclear medicine.
There are three main types of radiation detectors based on measuring the electrical charge created by the radiation. These are the ionization chamber, the semiconductor detector, and the diamond detector.
Let’s look further into and ionization chamber- as these are by far the most common type of detector. An ionization chamber is basically a volume of gas contained inside some chamber/container. There are two electrodes on either side of this chamber. These electrodes are connected to a power source which applies a voltage of between 100 to 1000 volts.
When the chamber is placed inside a radiation field, the radiation ionizes the gas and releases electrons. This creates positively and negatively charged ions, which will be attracted to the positive and negative electrodes on either side of the ionization chamber. Once they reach the electrodes they are converted to a current inside the wires.
An electrometer is attached to these electrode which can measure either the current created, or the total charge collected over time. This give you a measure of the instantaneous dose, or total does that the chamber receives.
All the equipment used for measuring radiation has to be super sensitive as the currents and charges measured are very small!. To give you an idea, you’re house is usually has a 10 to 15 amp wall power supply, and something like a cell phone charger usually uses 1 Amp. When we are measuring radiation we measure currents between 1 micro Amp through to 1 pico Amp. That is we measure amps between 0.000001 Amps to as small as 0.000000000001 Amps. This is one trillionth time smaller than what you’re cellphone uses. Quite small!
Measuring radiation take it out of the realm of being a scary substance that we can’t see. Using ionization chambers is one of the most common method in measuring it.