Alpha Radiation [was Re: The Extinction Challenge (fwd)]

Robert J. Bradbury (
Mon, 2 Aug 1999 19:48:05 -0700 (PDT)

> John Clark <> wrote:

Ok, " interlocks are now off..."

    >> An Alpha particle will kill or mutate any cell it hits, a rare neutrino that hits
    >> would be almost as deadly.

     >Robert J. Bradbury <> On Monday, August 02, 1999
     >John, I'm almost certain this is incorrect.

>From Radiation Biophysics, E. L. Alpern, Academic Press, 1990, pg 42:

Properties of Alpha Decay:

  1. The energy of the alpha particle emitted is closely related to the half-life of the parent. In general, the shorter the half-life, the higher the energy of the emitted alpha parrticle.
  2. Alpha decay is restricted to the very heavy nuclides, Z greater than 83, except for two important exceptions ^8Be and ^8B.
  3. All of the alpha particles emitted for a particular nuclide decay pathway have identical energies. There may be several different energy alpha particles, however, since alternate decay schemes are common (refer to branching decay earlier in this chapter).
  4. The *range* of the alpha particle is unique since its energy is unique.

*...* my emphasis.

> A few inches of air or the thinnest tissue paper will stop an
> Alpha particle that's why Plutonium, a moderately strong emitter of
> Alpha particles, is pretty harmless as long as it's in one big lump
> (but not too big!) outside the body; only the surface material can
> gives off free particles and even those are stopped after a short distance.

Accurate. External plutonium isn't very harmful since it has to go through "dead" skin where there is no DNA to be mutated.

> Plutonium only becomes the most deadly poison on the planet when
> it's in the form of a fine powder and is ingested into the body,
> either by eating it or breathing it into the lungs, because every Alpha
> particle then emitted will hit a living cell someplace and kill it, or far worse,
> mutate it.

Yes, because once inside the lung or gut it has direct access to dividing epithelial cells whose DNA can be mutated. If a plutonium atom rests on the surface of a cell, the alpha particle may only have about 5 microns to travel before it hits the DNA. But the alpha particle itself probably isn't the real source of damage. Free radicals are probably the real problem. The free radicals may be generated by the alpha particle, *or* by your immune system responding to the damage which may be caused by the alpha particle. Your immune system sees the damage and responds by trying to "kill" the presumed bacteria with reactive oxygen species. Of course these have no effect on the radioactive particles and only serve to aggravte an already bad situation.

Now, from "Health Effects of Exposure to Low Levels of Ionizing Radiation", BEIR V, National Research Council, National Academy Press, 1990. [BEIR is the Comittee on the Biological Effects of Ionizing Radiation].

Under Alpha Particles, pg 144:

"Robertson et al. (Ro83) showed that the RBE [Relative Biological Effectiveness] for transformation by plutonium-238 alpha particles in Balb/3T3 cells was substantially *higher* than that for cell lethality."

So, plutonium *transforms* before it kills.

"At equivalent doses, alpha particles were substantially more cytotoxic than gamma rays and were more efficient in inducing oncogenic transformation."

This is what I was saying -- the cross section of alpha particles was higher and so they are more damaging than gamma-rays (or neutrinos). Gamma-rays or neutrinos are more likely to pass through a cell without interacting with a nucleus.

"Previous studies by Lloyd et al. (Ll79) showed that at a dose correspinding to a surviving fraction of 37%, about 14 particles traversed the nucleus for each cell killed. The fact that on average 13 particles may traverse a cell nucleus without killing the cell may explain the high efficiency with which high-LET particles induce transformed loci."

So, cells *can* survive alpha particles and in most cases it takes a number of alpha particles to transform or kill cells. Obviously if 13+ particles traverse the nuclei, then many more particles are traversing other areas of the cell.

The critical factor here is "LET", or "Linear Energy Transfer", which is the rate at which energy is transferred from the radiation to the atoms through which it is traversing. X-rays & gamma-rays are low-LET. Alpha particles are high-LET (again because the cross sectional size or "interaction probability" of the alpha particles is high).

"Hence, if the transformation frequencies for each type of high-LET particle are plotted against the corresponding survival values, the curves obtained cannot be superimposed. This suggests that there is a real difference in the RBE between cell killing and transformation (He88, Ya 85) and also indicates that there is a significant frequency of transformation at doses of high-LET radiations that have very little effect on cell survival." (pg 143).

There is a very simple explanation for this. High-LET radiation (e.g. alpha particles) will transfer most of its energy to water molecules (the most abundant molecules in the cell), with a significant probability of producing a hydroxyl radical which can attack and *mutate* DNA bases (e.g. converting Guanosine to 8-hydroxy-guanosine which cannot be copied correctly). The energy of these particles or their ionized byproducts/radicals *is not* high enough to break the DNA backbone. Only a *very* energetic particle has enough energy to interact with the DNA backbone and produce a double strand break. Double strand breaks are very bad because eukaryotic cells have a limited ability to repair them. Sufficient double strand breaks (~5+?) will cause the cell to activate pathways that deplete essential cell nutrients and eventually result in cell death. It isn't clear whether this is an adaptive mechanism (better to be dead than cancerous) or simply faulty programming.

> No cell can escape the massive damage of an alpha particle.

It depends on how close the alpha particle is, how good the free radical defenses are and how *reactive* your immune system is (a genetic quality). But clearly, cells *can* escape from single alpha particles!

So regarding your original statement:

>> An Alpha particle will kill or mutate any cell it hits, a rare neutrino that hits >> would be almost as deadly.

... Return safety interlocks to normal position....

John, you should assume in any discussions that if the material relates to biology and/or nanotechnology that I am likely to have in my personal library the relevant sources. If I suggest that you may be inaccurate, it is because I have good sources to back up those opinions. My education includes computer science, chemistry, microbiology, biochemistry and some physics and astronomy. I will willingly and freely admit when I am wrong or on swampy ground. However, if you make a claim without being able to back it up, I will slice you and dice you and nail you to the wall.

I can do no less, because it is very important for the "hear-say memes" to have a short lifespan before they turn into reality.