QUESTIONS

1. What is the "A number" and what is the "Z number" of an atom?
    
2. What is the mass of a photon?
    
3. What is the difference between "corpuscular radiation" & "electromagnetic radiation?"
    
4. Describe the production of photons by the "general or Bremsstrahlung" method & then by the "characteristic" method.
    
5. When you talk about mAs in radiology, what are you talking about?
    
6. There are 3 types of scatter radiation.  Describe them.  (Okay, here's a hint.  They're called "coherent or classical scatter," "incoherent or compton scatter," & "photoelectric scatter.")
   
   
   
   
    

ANSWERS

1. The A number is the "mass number" and refers to the number of protons & neutrons possessed by the atom.  (We don't care about the mass of electrons as they don't really contribute much to an atom's mass.)  For example Iodine 131 and carbon 14 are isotopes we are pretty familiar with.  131 & 14 are A numbers.
   
   The Z number, on the other hand, is the "atomic number:"  the number of protons.  The number of protons possessed by an atom determines which element the atom is.  For example, carbon 12 (normal garden variety carbon) & carbon 14 (the radioactive carbon used in dating old things) both have the same Z number (6).  They both have 6 protons.  If  they didn't, they wouldn't be carbon anymore.  Carbon 14 has 2 extra neutrons making it unstable.  It will "decay" or spit off particles until it has reached a stable configuration.  Carbon 12 & carbon 14 are said to be "isotopes."
   
   
   
   
    
2. Did I fool you?  Photons have no mass.  They are not really particles.  They are "wave-particles" which means that sometimes they act like waves & sometimes they act like particles.  Photons make up light.  Some examples of light waves include:
   
      x-rays
      ultra-violet light (don't get sunburned)
      infra-red light
      visible light (look at the colors)
      microwaves (get the popcorn)
      radio waves (turn it up, it's my favorite song)
   
   
   
   
    
3. Corpuscular radiation is made of particles (I guess the word "corpuscular" kind of means little bodies).  These particles are beta particles (really fast electrons) or alpha particles (helium nuclei - units of 2 protons &2 neutrons).  Remember we said that an unstable isotope spits off particles until it reaches a stable configuration?  Well, these are the little guys that it is spitting out.
   
   Remember that relative to photons (which have no mass at all), alpha & beta particles are huge & when they get spit off, they crash into other nearby atoms.  An example we all know is iodine 131 which emits beta particles. Because of their size, they only travel a few mm out of the thyroid tissue the Iodine has homed to.  They thus destroy only the local thyroid tissue & do not effect other body tissues.  An example I wa taught in high school which is slightly more morbid is that if there was a nuclear explosion right now.  The alpha particles are so big that they couldn't even hope to penetrate the walls of the building I am in.  The betas might get in but couldn't really get through my skin.  The photons though (the electromagnetic dudes) have no mass & can go right into my body.
   
   Electromagnetic radiation (photons) travels at the speed of light.  The amount of energy carried by a photon is determined by the wavelength & frequency of its wave, not by how fast it's going. The shorter the wavelength the higher the energy level.  The energy level of the photon also determines whether it is an x-ray, a visible light photon, a radiowave or what.
   
   
   
   
    
4. General or Bremsstrahlung radiation
   
   Ok, say we have a bunch of electrons attempting to fly through an area that has alot of atoms in it.  As an electron (which is negatively charged) happens to fly near a nucleus which is positively charged, the electron will get attracted & will slow down. (Bremsen in german means to brake.)  Slowing down means a loss of kinetic energy.  Since energy is conserved, this loss of energy has to be expressed somehow.  It is expressed as a photon. That's right. A photon is born as the electron slows down.
   
   (But will this photon be an x-ray?  If the atom involved has a high Z number, the photon will probably be an X-ray photon.  Why?  Because a high Z number means protons aplenty.  Protons aplenty means strong positive charge.  Strong positive charge (much stronger than the -1 value of the electron) means lots of energy lost by the electron.  Lots of energy lost means a photon with lots of energy & that's an x-ray.
   
   Characteristic radiation
   
   Okay, we have the same bunch of electrons trying to fly through a bunch of atoms.  In characteristic radiation, the electron manages to sock right into one of the electrons orbiting around a nucleus.  Total head on collision.  Both electrons go flinging away.  This leaves the atom missing an electron in an orbital shell that should really be full.  (Recall that inner orbital shells must fill up before outer orbital shells fill up.) Well, to fill the inner shell, one of the outer electrons drops down.  This means the electron loses energy.  You guessed it.  Another photon is born.  This time, since we know the energy difference between different orbital shells, we can calculate the amount of energy for the photon produced.  The energy lost is "characteristic" for the drop from one particular orbital shell to another.
   
   
   
   
    
5. mA =  miliAmperes (a unit of current)  Current , in case you forgot, is a measure of electrons per second moving through a wire.  mAs = miliAmperes x Seconds or electrons per second x seconds or a number of electrons.  We are talking about the number of electrons we are about to send flying through the field of atoms.  The more electrons we sends, the more photons we're going to make.
   
   
   
   
    
6. Coherent or Classical Scatter
   
   Okay, now we have a bunch of photons rushing towards the dog on the x-ray table.  Well, more specifically, the photons are rushing towards the atoms making up the dog on the x-ray table.  In coherent radiation, the photon whips past the electrons possessed by the dog atoms.  The photon gets deviated slightly & the electrons get mildly excited.  No big deal. No clinical relevance.
   
   Incoherent or Compton Scatter (the most common kind)
   
   The photon is whipping through the dog atoms & WHAMMO! It rams right into a dog electron.  The photon gets deflected & the electron gets knocked God knows where.  Well, you know what happens when an electron gets knocked God know where.  An outer electron falls down to take its place & a new photon is born.  Yes, that's right, the dog is the source of scatter radiation.  This is why you wear all those lead clothes.  Not to protect you from the primary bean (which is hopefully very focussed) but to protect you from the scatter bouncing all over the place from the dog & table & anything else in the primary beam's way.  This also means you are being dumb if you think that laying gloves over your hands will protect you; scatter comes from below,too!
   
   The photoelectric effect
   
   Our pal Al Einstein got the Nobel prize for this one (and not for relativity like most people think).  The photon crashes into the electron just like before but instead of both getting knocked away, they sort of fuse into a photoelectron.  The photoelectron goes zooming away & again Characteristic radiation results as the hole left behind is filled.