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In our last lesson, we studied the Geiger counter. It's an instrument that applies our knowledge of radioactivity. But, one of its limitations was that it couldn't gauge the dosage. That's not very helpful for repeated exposure. For example, we studied radioactive tracers and their use in medicine. We use radioactive tracers to diagnose, locate, and fight cancer. So, radiologists are exposed daily to radioactive decay. How do we keep these professionals safe from radiation exposure? We use a device called a dosimeter.
What is a dosimeter?
A dosimeter is a portable instrument that measures radioactivity. Remember, the Gieger counter was best at detecting alpha, beta, and gamma radiation. But, because it utilizes the formation of ions, it can't detect uncharged particles. So, it would completely miss neutrons or neutrinos. But, unlike a Gieger counter, dosimeters are sensitive to all types of radiation. Even better, they can gauge the radiation's energy. So, we can estimate an effective dose. There are a variety of different dosimeters. Each kind uses a unique mechanism for computing radiation and dosage. Let's study some of the most common types.
Geiger-Muller Tubes (Modified)
Previously, we saw that Geiger counters were unable to measure the dosage. That's true unless the instrument is modified. Typically, the manufacturer does some calibration testing. They'll have the instrument measure a radioactive element. The amount of radioactivity will have a known dosage. They record the counts. Then they program the device to associate the counts with the dosage. So, the electronics become able to convert between counts and dosage. But, since the conversion comes from observation, it differs between manufacturers.
Personal Dosimeters
A personal dosimeter is a small electronic device worn on a shirt pocket. It's ideal for professionals who get exposed to low dosages of radiation. The manufacturer can set the device to specific limits depending on the application. If the limit gets exceeded, it has an alarm that sounds.
But, how do they measure radiation? They use an exceptional electronic device called a MOS transistor. MOS stands for metal-oxide semiconductor. First, what is a semiconductor? They are materials whose conductivity is between a conductor and an insulator. Conductors allow electrons to move freely through them. For example, metals are full conductors because of their metallic bonding. We've also seen conductive carbon allotropes such as graphene because of the delocalized pi-bonding. Insulators stop the flow of electrons. They have significant ionization energies. It's difficult to remove the electron so it can travel. A semiconductor's conductivity is in between the two. It's challenging but not impossible to get electrons to flow. As semiconductors are heated, their resistivity lowers. So they conduct more easily at higher temperatures.
The most common semiconductor in dosimeters is silicon dioxide. As the radiation strikes the dosimeter, it affects the semiconductor. The silicon dioxide starts to conduct better. The better conduction changes the instrument's current. The change in current is measured and converted to a dosage. Different types of radiation will affect the current differently. So, it's sensitive to all varieties of radioactive decay and even to x-rays.
Extended Range Dosimeters
Sometimes, emergency personnel respond to situations with radiation exposure. They use dosimeters specially designed for extended ranges. It measures both the exposure rate and total. The exposure rate is how many particles are hitting per second and varies with distance. But, the total exposure depends on the increment of time.
How do they measure radiation at great distances? They use a device called a scintillation counter. In a scintillation counter, the radiation hits a piece of an ionic crystal. The most common crystals are alkali metals bonded to the halogen iodine. Sometimes, we use zinc sulfide. The radiation excites the electrons. The electrons relax by emitting a low-energy photon. The photons travel down a photomultiplier tube. They use the photoelectric effect to amplify the signal. Finally, the detector measures the amplified signal and converts it to an exposure dosage. Often, extended range dosimeters combine the scintillation counter with a Geiger counter. The Geiger counter provides increased sensitivity at low radiation doses.
