Do they differ in just frequency and wavelenght ? Or there is more about it
[Physics] the difference between xrays and ultra violet rays
electromagnetic-radiationx-rays
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Photons interact with matter if the matter offers quantum transitions that match, or nearly match, the photon's energy in the inertial frame of the matter. Ordinary matter such as wood, stone, etc. offers several groups of possible quantum transitions.
- Rotation of molecules (if they are free to rotate, i.e., not condensed matter)
- Vibration of molecules - bending, quivering actions
- Electronic excitations
- Nuclear excitations (there being various kinds, ignored here for simplicity)
Microwaves have such low energy they can't do much, though they might excite some types of vibrations on larger floppier molecules - however, any type of molecule that could be described as "floppy" probably isn't good for construction materials. Rotational modes aren't possible in a strong material made of crosslinked polymers or silicates. So microwaves mostly fly right through.
Near-infrared and visible light can kick electrons into higher molecular orbitals. Even if the energies aren't a match, just close, there is interaction, as Heisenberg lets them cheat temporarily. Also, having more energy, visible light photons can stir up a greater variety of vibrational modes. There's nothing in common wall materials to prevent that, and in fact, the interaction with photons is so strong that the material, if not super-thin (microns), will be opaque. Of course, glass is an exception.
Gamma rays are of such high frequency, electrons (or ions, or polarized ends of molecules) can't keep up due to inertia - so no interaction, or only a little. At the right frequencies, gamma photons can interact with nuclei, but for a randomly chosen source of gammas, its photons are unlikely to match closely enough with any of the available nuclear excitations, and can't really do much at the molecular level - therefore, the material is almost transparent.
All this is so oversimplified...
There is not a lot of "straight up physics" here, but it's an interesting question nonetheless.
There is a lot of clinical (not just economic) value in fast CT scans. Three in particular that are worth noting:
- if you want to do a contrast-enhanced (flow) scan, you need to see the contrast "everywhere" before it becomes too diluted. The faster you can scan the whole body, the better; otherwise you may need to give the patient multiple doses of contrast, which is undesirable (not without risk)
- Suppress respiratory motion: for many scans the patient is asked to hold their breath; if you have a sick patient, they cannot hold their breath for long and a fast scan is preferred. The more axial FOV you cover in one rotation, the shorter the time to scan the entire chest.
- Stop cardiac motion: if you can image the entire heart with the detector, and your gantry rotates fast enough, you can "stop the heart" and produce stunning 3D images of the heart, including detail of the coronary arteries.
Of course there is a cost to having larger detectors, and fast rotation speeds. Some of the complexities include:
- You need collimation on the detector side to reject scatter: this requires very precise alignment as the pixels on the detector become smaller
- As the axial extent of the FOV increases, you run into a problem called the "heel effect": since the anode is tilted at an angle of about 8° (so the apparent focal spot size is much less than the actual size of the spot made by the electron beam on the anode), as you change the angle at which you look at the anode, the size of the spot (and the apparent intensity) changes. At one end, this makes the spot look small and you are count starved; at the other end, the spot is large and the intensity of Xrays is greater. This is circumvented by making the anode angle larger - this makes the flux smaller, and the thermal problems bigger... just when faster rotation requires MORE flux.
- As the gantry rotates faster, the acceleration of components increases as the square of the velocity. For a gantry rotating at 0.25 seconds (so it covers 180° + fan angle of detector in under 0.2 second, enough to "freeze" the heart), an object at a radius of rotation of 50 cm experiences a centripetal acceleration of about 32 g... and one of those components is the CT tube that contains an anode with a mass of several kg, at a temperature of 1200 °C (white hot), rotating at about 5000 rpm on vacuum (i.e. unlubricated) bearings. That's an engineering challenge...
Yes - multi detector CT scanners are more expensive; but their rapid adoption after initial introduction demonstrates they have value. But they are very challenging to design.
See also this article for some cardiologist thoughts on multi slice scanners.
Best Answer
They have various properties that differ, but the differences are quantitative, not qualitative, and there is no sharp boundary. The differences occur because of the difference in frequency. A wave that is a gamma ray in one frame of reference could be an x-ray if observed in a different frame.
An example of their different properties is that gamma rays are more penetrating, and are more likely to undergo Compton scattering rather than the photoelectric effect. The reason x rays and gammas were given different names is that originally nobody knew they were both part of the electromagnetic spectrum. The name "gamma" was simply a label used to classify them by how penetrating they were -- more penetrating than alphas or betas.