> Hal wrote:
> What does this mean, "traverse"? Does that refer to alpha particles
> which passed through the nucleus without interacting with any atoms there
> (i.e. alpha particles which don't "hit")? Or does it mean alpha particles
> which hit and interacted with atoms but without fatal effects?
>
Traverse, from my perspective, means to travel across without interacting in a detectable way. You can compute the number of alpha particles being produced from a # of atoms based on the half-life. You can then compute the volume of the nucleus and know how many alpha particles go through it.
The only interactions you used to be able to detect were: (a) double-strand breaks in sufficient quantity to kill the cell and (b) mutations in *exactly* the right bases to cause transformation (i.e. cancer). Now there are more sophisticated methods that can do chemical detection of mutated bases (i.e. 8-OHdG) that have no "significant" effects (perhaps because most of the DNA is junk or you can make many changes to DNA without having any effect [because the code is redundant]).
> I have the impression that the alpha particle travels for some distance
> without interacting with any atoms, then at some point it does pass near
> an atom's nucleus and causes a large energy deposit there. This is what
> we are calling a "hit".
Ok, but the way the statements were being made (from my reading) there seemed to be an implication that all alpha particles *must* produce a "hit" and that simply isn't accurate.
>
> John seemed to be saying that *if* an alpha particle "hits", that is,
> it is absorbed and interacts with atoms in the cell, it causes so much
> damage that it is virtually certain to kill or mutate the cell it "hits".
> That is consistent with the statement above if we assume that the 13
> particles traversed the nucleus without interacting with any atoms there,
> and the 14th killed the cell.
This is what I found objectionable -- there are many more atoms in a cell than just DNA and many atoms in DNA that can be hit without causing an effect. Only *iff* an alpha particle interacts with the *right* atoms in the DNA, will it produce a double strand break (and cell death) or a "critical" mutation.
>
I suspect this is a semantic problem. In physical terms, a "hit" is
an interaction with any atom. The way John (or Hal?) seem to be using
it is to imply a "hit" is *only* an interaction that kills or mutates
a cell. Since the killing/mutating potential/probability varies with
the energy/LET of the radiation/particle in my book this is overloading
the term "hit". This is especially true if "hits" that do not kill
or mutate the cell cause damage that causes an immune system reaction
that produces free radicals that later produce mutations or cell death.
The question is *what* is hit and how sensitive is your body to that?
A "hit" to a lipid molecule might produce lipid peroxidation that will
eventually go away (as the lipids get recycled). A "hit" to a protein
molecule that causes mutations that are "seen" by the immune system
can have cascading effects. A "hit" to a DNA moleucle (in the right
place) can cause mutations that lead to cancer or cell death.
> > 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.
> >
> > - Many alpha particles can pass through a cell with no effect.
>
> As noted, this is consistent with John's statement, as absence of
> interaction does not count as a "hit". The alpha particle would then
> hit another cell.
>
> > - It takes many more low-LET neutrinos passing through a cell to cause
> > damage corresponding to that of a high-LET (but low energy) alpha particle.
> > So neutrinos *are not* as deadly as alpha particles.
>
> I think John meant, per interaction. Obviously a neutrino, which John
> has said can pass through light-years of lead, will go through a cell
> more easily than an alpha particle which is stopped by a sheet of paper.
> The question is, for these high-energy supernova neutrinos, *if* one
> interacts with a cell, how would the damage compare with that caused by
> an alpha particle which "hits".
As I tried to say in my original message, I think this depends on the energy of the neutrino/alpha particle (in MeV). The more MeV you have, the greater your proability of breaking bonds of specific strengths. The thread (as I read it) seemed to imply that all alpha particles & neutrinos are created equal which isn't true (in contrast to a gamma/X-ray where you know exactly how many MeV a photon has).
If you don't qualify the energy of the particle and don't qualify the probability of interaction with atoms/bonds belonging to specific fractions of cell material, then I don't see how you can make statements that are as strong as "*will* kill or mutate *any* cell".
>
> Does this kind of rhetoric move the discussion forward?
>
In some respects. I'm not going to argue radiation transfer and
carcinogenesis with a psychologist because they don't have the
background for it. My general feeling is that John's physics
background is stronger than my physics background, so I expect
the discussion to be of a high quality. However, when
he makes a statement that I suggest may be inaccurate, and he
comes back with a comment that is essentially the same, I want him
to know that I will come back with hard data. The discussion might
have gone faster if he had tried to qualify the statements, as Hal
tried, to reflect what may have been a semantic misunderstanding.
My general impression is that John's statements (and the entire disucssion of radiation) were being made from a physics perspective without the large number of qualifications that must be made when dealing with the biological realities. Since I'm interested in the impact of radiation on both biological systems and hard nanotechnology systems I feel it is a disservice to oversimplify these things. I'm hoping that Robert Freitas will go into much greater depth on this subject in one of the upcoming volumes of Nanomedicine than Eric gave it in Nanosystems. My gut feel, is that if there is a "weak link" in nanotechnology, it involves the problem of radiation damage, so it *is* an important problem.
Robert