Alpha Beta Gamma Biological Impact Assume that alpha & beta particles and gamma photons each reach skin at the same energy. It's known that they penetrate most deeply in order by gamma, beta and alpha. How would they compare in terms of total biological impact? In other words, which would be most harmful?
 A: The thing you're trying to get at is the "quality factor" or "Q value" of the radiation.
Short short version


*

*photons, betas, muons: 1

*protons, pions, other light mesons: 2

*neutrons: 1--10 depending on energy

*alphas and fission fragments: 20


Note that alpha have a very high value, but it doesn't count starting from the surface of the skin, as at the usual energies they will be fully stopped before entering live tissue.
A: I am not sure this has been answered clearly enough, despite several good answers
There are 2 factors to take into account here - 
1) energy density and likelihood of deposition to tissue
2) penetration and likelihood of reaching tissue
So, to break that down:
Part 1)
The most energetic particles are alphas. 
That makes sense, as they are essentially helium nuclei (2p,2n). If you think e=mc2, then they are naturally going to have a higher average energy than beta negatives (electrons) or gammas (photons).
Beta negatives (electrons) are probably next, as they also have mass, and due to the likelihood they also arise with high energy often move relativistically (near light speed). 
Gammas, as a form of electromagnetic radiation, have a spectrum. Worth noting here that 'gamma' doesnt specify an energy level on the spectrum, but rather than it was generated by a nucelus and not an electron. They can be weaker than most xrays, or contain more energy than most betas. For example you can get a 600keV gamma and a 100keV beta.
Alphas are more likely to get even further, into the MeV range. The others can too, but it is not common.
So that is how energetic they are.
This also relates to how likely they are to deposit into tissue. The more massive a particle is, the more likely it will interact (poor oversimplification, but just accept that for now). 
This means an alpha, being super duper massive compared to the others, will react quickly. They lose all their energy in less than a mm of tissue. This means, if it is right next to a susceptible piece of tissue, it will give ALL the energy it has to that tissue. Very bad news, cell-killingly bad news. 
A beta will probably lose it's energy over several centimetres. This means the damage gets a bit more spread out, which means each section of tissue gets less of a dose.
A gamma will get a lot further. A lot of gammas can pass through your body barely interacting at all, and therefore giving each tissue only a fraction of the energy it holds.
So that is how they affect you physically. But then we have the other problem - can they affect you in real life ie. Can the damaging radiation actually reach the cells?
Part 2)
Penetration. 
This was partially touched upon at the end of the last bit, but is a slightly different topic. We have seen gammas penetrate, betas do less, and alphas barely get anywhere. This means, to tissue, alphas are the biggest baddie.
But can they get to tissue?
Lets look at an alpha. These really only come from supermassive nuclei like uranium, and are not relevant in Fukushima as Uranium is not decaying, the daughter products of uranium like cesium, iodine and stronium are (beta and gamma emitters).
But for alphas, becuase this is a good concept -
Alphas lose all there energy immediately. This is a bad thing for tissue, to get say 1MeV deposited in a smal(1mm3) space. 
But if the alpha generator is on your skin, then you are totally unharmed! This seems counter intuitive, but because the alpha is so damn good at passing it's deadly energy to tissue, it cannot get through your dead skin layer.
Betas are the next baddie, they can deposit a lot of energy in a small region. But again, for a lot of lower energy betas you can attenuate them out (shield them) with some thick clothes and the dead skin, with an air gap in between. You will only get a fraction of the radioactive dose that is on the outside of your body. To make things even better, your skin and muscle tissue is not very sensitive to radiation anyway, so it has less biological effect.
Gammas get through, but can of course pass the whole way through. You will end up with most of the radiation dose, fairly evenly spread throughout your tissues.
So that's that right? Gammas get you, high energy betas can, and low energy betas and all alphas are harmless?
No. 
The major population danger is not do to external radiation, but due to the ingestion (swallowing) of a radiation source. By this I dont mean you swallow an alpha or beta or gamma, but instead swallow radioactive cesium, iodine, stronium etc.
Inside your body, the alphas and betas can wreak havok. You have no defenses. All the cesium generated betas will hit small areas in your gut. 
The body also processes these compounds naturally whether they are radioactive or not. We use iodine in our thyroid, and stronium is similar enough to calcium that we will put it in our bones.
So when radioactive versions of these products are absorbed, and decay, the betas and gammas they release have a direct action on the thyoid (for iodine) and the bone marrow (for strontium).
Then add to that the problem that the thyroid and blood cells are actually sensitive to radiation, unlike the skin and muscle, and the effect of each Gy absorbed is multiplied by factors of 5 or more.
This is getting long, so I will just summarise:
CONCLUSION: If you can keep them on the outside of your body, alphas and a large percentage of betas are harmless. Gammas will still get you, even behind several centimeters of lead! 
If sources of alphas and betas get in your body, via contaminated food, breathing or something else, then they are far, far worse than gammas.
For the workers at the plant currently, they are worried about gammas, because they have oxygen tanks and shielded suits.
For the people of Japan, they are worried about alpha and beta generators, because they can get into the food chain, and do far, far more damage.
Hope that helps. Sorry for the no pictures!
A: Gamma is the most harmful - and it's hard to shield it. Alpha get absorbed in dead skin layer. Beta's are in between.
A: Well it's like this:
alpha: take a step back, happy days
beta: cover yourself with a sheet of paper
gamma: see me running try to keep up.
