Which of the following could be a source of X-ray radiation? (A) Thermal motion of atoms and molecules. (B) Alternating electric currents. (C) High-speed electrons impacting on a metal plate. (D) Decaying atomic nuclei. (E) None of the answers is correct.
Before we start, let’s quickly recall that the electromagnetic spectrum is a continuous range of radiation that we can classify according to properties like energy and wavelength. And as far as the spectrum goes, X-ray radiation is quite high energy as characterized by very short wavelengths around 10 to the negative 10 meters or a ten billionth of a meter.
Now let’s begin with option (A), the thermal motion of atoms and molecules. Well, any object with a temperature above absolute zero has some energy in the form of molecular thermal motion. This thermal motion emits radiation in the form of infrared waves, which have wavelengths about a hundredth of a millimeter long. Here, we wanna identify a source of waves thousands of times shorter than that, so answer choice (A) is incorrect. Thermal motion emits infrared waves, not X-rays.
Moving on to option (B), recall that electromagnetic radiation is essentially caused by the acceleration of electric charge. In an alternating current, charge is accelerated over and over again, which emits electromagnetic radiation in the form of microwaves. However, microwave radiation is fairly low energy, with wavelengths about a centimeter long. So alternating electric currents are not a source of X-ray radiation.
Next, (C) says high-speed electrons impacting on a metal plate. It turns out smashing high-speed electrons into metal does accelerate electrons in the metal, and electromagnetic radiation is emitted. The resulting electromagnetic waves are quite high energy and fall within the range of X-ray radiation. In fact, this is actually how X-rays are created for medical imaging. X-rays have wavelengths so short they pass right through the body’s soft tissues. Therefore, option (C) looks good, so we can eliminate (E).
But let’s still go ahead and check out option (D). Decaying atomic nuclei refers to the nucleus of an atom splitting apart. This process, called nuclear fission, can release so much energy that it’s used in power plants. Nuclear fission emits gamma rays, which are categorically the highest energy type of electromagnetic radiation, with wavelengths thousands of times shorter than even those of X-rays. Therefore, decaying atomic nuclei is not a source of X-ray radiation.
Thus, we have found that answer choice (C) is correct. High-speed electrons impacting on a metal plate is a possible source of X-ray radiation.