This article on radiation detection and measurement is part of the Science in Sci-fi, Fact in Fantasy blog series. Each week, we tackle medical or technical aspects of science fiction or a historical / world-building topic of fantasy with input from an expert. Please join the mailing list to be notified every time new content is posted.
The Expert: Sifa Poulton
By day, Sifa Poulton is a PhD candidate in Nuclear Physics (Radiation Detection and Measurement) at the University of Surrey, UK, and the National Physical Laboratory. By night she is a book blogger, specialising in SFF across all age ranges, and writes SFF books that combine her love of science and history. You should check out her blog and follow her on Twitter or Instagram.
Radiation Detection and Measurement in Science Fiction
The heroes enter the room and an irregular click click clicking noise can be heard. The frequency of the clicks increases as they slowly advance. That is the sound of a Geiger-Müller counter, ubiquitous in media and labs as a way of detecting ionizing radiation.
Ionizing radiation isn’t just present in the labs of villains, though. It constantly arrives on Earth from the sun and the rest of the universe as cosmic rays, radiates from rocks in the ground, and can be deliberately created in accelerators. Some of its many uses include medical imaging, cancer treatment, inspecting manufactured goods, and determining the age of historic objects.
What is ionizing radiation?
Rebecca Enzor has contributed a great piece on the subject. The basics for this post are that ionizing radiation is any type of radiation that can knock an electron from an atom, turning the atom into a positive ion. There are several different types of ionizing radiation, which can be split into two main categories: charged and uncharged. Charged particles comprise of: alpha particles and beta particles. Uncharged particles comprise of: x-rays, gamma rays, and neutrons. This article is going to use the term “particle” to mean any unit of ionizing radiation.
How does ionizing radiation interact with matter?
As the name suggests, ionizing radiation primarily interacts with atoms by transferring sufficient energy to remove an electron, ionizing it. This energy transfer happens in one of two ways; collisions and coulomb interactions. Both charged and uncharged particles can collide directly with the atom, but only charged particles, gamma rays, and x-rays can undergo coulomb interactions. Coulomb interactions are much more likely to occur than direct collisions.
Most detectors measure ionizing radiation through the collection of the electrons knocked free by interactions. A large voltage is applied across the detector, which accelerated any freed electrons towards the anode. It is usually detected as a current (as current is the movement of charge.) As the freed electrons are being accelerated towards the anode, they can knock off other electrons from other atoms. These in turn can knock off more, and so on. This is called a cascade. In gas and liquid detectors, the ionized atoms will drift towards the cathode and be collected there.
The ionizing radiation particle is unlikely to transfer all its energy in one interaction. It may interact several times within a detector before all its energy is lost, or it may only transfer some of its energy and then leave the detector. For detectors designed to measure the amount of energy the ionizing radiation has, radiation that only transfers some of its energy within a detector affects the efficiency of the measurement.
Ionizing radiation can also interact with matter through scintillation. There are two ways this can happen. The first is that an electron is freed from a lower energy state (if you simplify atomic structure to the nucleus as a sun orbited by electron planets, the closer to the nucleus the orbit is, the lower the energy.) A higher energy state electron “drops” down in the empty energy state, releasing a photon of energy equivalent to the difference of the two energy states.
The other is when the energy transferred from the radiation to the atom is insufficient to free an electron. Instead, an electron gains energy and excites. It then de-excites and emits a photon, usually within the visible spectrum. Some of the earliest detectors were simply people counting flashes of light!
As neutrons cannot transfer energy through the far more probably coulomb interaction, they are detected by the by-products of their interaction with a known material. The neutrons impact on a sheet of material in front of the detector and undergo a nuclear reaction (this simply means a reaction where the nucleus of one of more atom is changed.) The material is selected because of its well-known reaction and by-products – a combination of alpha and beta particles, and gamma rays, and x-rays.
The detectors themselves vary depending on what type of particle they are trying to detect and what information needs to be measured.
Gas Ionization chambers
As the name suggests, this type of detector involves chamber of gas. The gas becomes ionized when the ionizing radiation interacts with it. A Geiger-Müller counter is an example of a gas ionization chamber, with the gas chamber being the cylinder you point at the radiation. Geiger-Müller counters can only measure the amount of ionizing radiation detector, not the energy of the particles themselves. However, a different type of gas ionization chamber, called a proportional counter, can.
Semiconductor detectors
When atoms are ionized within these detectors, the excited electrons enter the conduction band – an energy region where the electrons are free to move. This allows them to be collected at the cathode. These detectors are usually made of silicon (good for detecting alphas and betas) or germanium (good for gamma and x-rays). The number of electrons collected at the cathode is proportional to the energy of the incident radiation, allowing for a spectrum to be recorded.
Scintillator detectors
Unlike with the previous two types of detectors, these detectors do not collect information by measuring a current of electrons or ions. Instead, these detectors (which are made from regularly repeating crystal structures) release light. In order to measure this light, a detector must be coupled to a photomultiplier tube, or PMT.
When the photon reaches the PMT, it interacts with a photocathode, which releases an electron through the same mechanism as ionizing radiation does. However, only one electron is released as the photon energy is insufficient to release more than one electron through multiple interactions.
The released electron is then accelerated and multiplied by a series of dynodes. A dynode an electrical conductor (like an anode or a cathode) within a vacuum that releases more electrons when the accelerated electron hits it. All the electrons produced and accelerated by the series of dynodes (usually ten to twelve) are then collected by an anode, and a current recorded. This current is also proportional to the energy of the original incident radiation.
Choosing which detector to use
The type of particle we want to detect, and what information we want to gain, determine the type of radiation detector we use.
If all we want to do is measure how much radiation there is, then a Geiger-Müller is a robust, portable detector. However, once we want to measure the energy of the radiation, then things get more complicated.
When measuring gamma-rays, the detector is chosen based on a compromise between efficiency (what percentage of the total radiation you want to measure), energy resolution (how accurate measure of energy is), and timing (how quickly the detector can measure and then ‘reset’ in order to detect the next event.) Germanium detectors have good energy resolution (and some versions of the crystal are very good at measuring the lower energy x-rays) but do not have good efficiency. LaBr3 and CeBr3 have better efficiencies and timings, but poorer energy resolutions.
The situation the detector will be used in also needs to be accounted for. A germanium detector must be cooled with liquid nitrogen to produce the cleanest signal, so it can only be used where there is sufficient space and immovability for liquid nitrogen tanks. LaBr3, CeBr3, and NaI do not require cooling, so can be taken out of the lab to measure in the field. There is also an added consideration about radiation arising from within the detectors themselves – this is particularly crucial for detectors such as LaBr3, which has a well-known internal radiation spectrum, which must be accounted for when analyzing the spectrum.
Calibration
Before any measurements with unknown sources can begin, a detector must first be calibrated. This is because every detector has its own unique response to radiation. The electronics paired with it to collect the data affects this response too, so the detector and electronics should not be changed after calibration (or recalibration must happen.)
When simply detecting the amount of radiation, a calibration source of known activity (number of emitted particles a second) is used.
When calibrating for energy, one or more sources are used that emit radiation of known energies over a range of energies. This is because a detector’s response varies according to energy. 152Eu is a common single calibration source for gamma-ray detectors as its spectrum contains twenty well known peaks over a large range, but sources made from multiple radioactive elements may also be used.
When calibrating for timing, a single source that emits two or more particles with a known time difference between them is used. 60Co is a common example.
In Summary
And now, the heroes, having selected and calibrated their radiation detectors, are ready to set off in search of the supervillain’s lair or begin their experiment that will change the course of humanity’s future forever…
Follow me and you'll never miss a post:
Please share this article:
Very nicely done.