Radiological Science Laboratory

To start, what is radiation? Even though people casually talk about the threats of radiation, and the general public has its own impression about radiation, few people know that they live under the steady influence of radiation, and they “survive” thanks to radiation in a broader sense.

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Radiation, in its broader definition, refers to any particle or wave that has energy to travel through a medium. This includes virtually any elementary or composite particles, such as bosons and fermions, hadrons, leptons, photons, etc. – anything you can think of. I would not categorize elementary and composite particles here (You can just look up Wikipedia for this); however, they belong to the big family of radiation. What the public calls radiation refers in general, to the “ionizing radiation,” particles (waves) which have the ability to ionize an atom; thus, creating charged particles in consequence.

 

Nuclear radiation, which comes as an outcome of nuclear decay process or nuclear reactions, such as alpha particles, beta particles, gamma rays and neutrons falls under this category. And this is what most people are concerned with. Atomic radiation, notably characteristic x-rays from an excited atom, are often utilized for medical applications, as is well-known. Even though the boundary between the two in their energy range becomes somewhat ambiguous at some point, there are also non-ionizing radiation, which includes a wide spectrum of electromagnetic waves of lower frequency (or energy, since E = hν) – ultraviolet (UV), visible light, infrared, microwaves, radio frequency waves, thermal waves. I have categorized neutrinos under the non-ionizing category, because there is not yet a verified ionizing process from the neutrino interaction with matter, as shown in the schematic below.

 

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Let’s look into the instrumental side of these types of radiation. There are devices developed specific to a certain wavelength range of radiation. For instance, photodetectors or photodiodes are developed to optimally respond to the photonic wavelength range of, mainly visible light, infrared and UV. Measures to identify longer wavelength particles (microwaves and RF waves) utilize conductive sensors in the form of a radar or an antenna. However, what we pay attention to here in nuclear engineering are the categories of nuclear radiation and atomic radiation. We, as nuclear engineers, would like to “sense” the particle in a relatively higher energy range, where it would be capable of ionizing atoms either directly or indirectly. The "sensors" should be able to detect and measure each individual particle, thus we call them "(ionizing) radiation detectors."

 

 

The (ionizing) radiation detector is a major part of nuclear instrumentation as shown in the figure below. The field of nuclear instrumentation involves such area studying and utilizing a) accelerators, b) various source facilities, c) detectors, and d) molecular or ion beam lines. However, there is no clear distinction between each boundary, because many of these instruments are often systemically combined with each other to compose an integrated facility. Amongst these, I, in particular, pay a little more attention to detectors.

 

 

 

 

 

 

 

 

 

 

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When we say detector in general, this usually refers to any type of detectors, not only limited to those used within the field of nuclear radiation – photodetectors, chemical and biological sensors, and even smoke detectors. (Actually, smoke detectors do utilize nuclear radiation.) However, when people say radiation detectors, this usually refers to ionizing (or nuclear) radiation detectors which are a part of nuclear instrumentation.

 

In my nanocrystalline (NC) semiconductor detector research, I am not trying to cover the whole area of detectors, just trying to concentrate only on a little portion of the detector category which is normally used in the nuclear science and technology field. Technically speaking, the nanocrystalline (NC) semiconductor detector developed and investigated in my Ph.D. dissertation can be exploited as an ionizing radiation detector. However, since we paid little attention to x-rays and haven’t really tested for an x-ray source in the experiments, I would like to confine the scope of these NC detectors in this research within the boundary of nuclear radiation (α’s, β’s, and γ’s). It should be noted that most nuclear radiation detectors are ionizing radiation detectors, and, thus, can sometimes be used for x-ray or optical photon detection with a few adjustments in the set up.

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- Written by Prof. Geehyun Kim