Re: Annealing of Radiation Damage

mmeron_at_cars3.uchicago.edu
Date: 10/12/04 

Date: Tue, 12 Oct 2004 23:29:02 GMT

In article <ckhhrc$2dq$1@hood.uits.indiana.edu>, glhansen@steel.ucs.indiana.edu (Gregory L. Hansen) writes:
>In article <rOWad.29$45.10731@news.uchicago.edu>,
> <mmeron@cars3.uchicago.edu> wrote:
>>In article <ckhavm$vjp$2@hood.uits.indiana.edu>,
>>glhansen@steel.ucs.indiana.edu (Gregory L. Hansen) writes:
>>>
>>>I've begun studying radiation damage in solids, specifically the energy
>>>that gets locked up in defects and doesn't appear as heat. In the TRIM
>>>docs they say (at least for the silicon example) that they assume 99% of
>>>the displacements are instantly annealed. If there's a significant amount
>>>of annealing in a short time frame, that will be important to us, but 99%
>>>seems unbelievably high. And presumably at room temperature, but we'll
>>>be cryogenic.
>>>
>>>I've looked around and found a few things to read, but I haven't found
>>>anything yet that seems to address that, at least at cold temperatures. I
>>>know there's a few radiation guys running around here (Mati? Socks?).
>>>Can anyone suggest some reading material?
>>>
>>Hmm, nothing immediately comes to mind. Some general thoughts,
>>however:
>>
>>1) It is definitely material dependent. I do recall documented cases
>>of radiation (x-ray) damage in amorphous materials (glasses),
>>following a prolonged exposure. On the other hand, I do not recall
>>significant damage being observed in hard single crystals (silicon,
>>diamond), even under an intense and prolonged exposure. Soft crystals
>>(organiz, for example) that's another story.
>
>I'm interested in LiMg alloy, the ratio is not yet determined and depends
>in part on the simulations. The professor thought more Mg might reduce
>the radiation damage. I was hoping there would at least be some rules of
>thumb based on the temperature and the displacement energy.

Nothing I'm aware of, right now. If I'll come upon something, I'll
let you know.
>
>>
>>2) The above notwithstanding, temperature for sure is an issue.
>>We're using at times silicon at cryogenic temperatures, but what we
>>call "cryogenic" is LN2 temperature, still hellishly hot on the scale
>>you're accustomed to.
>>
>>3) More important (IMO) is the nature of radiation. X-rays carry
>>lots of energy but precious little momentum. They knock electrons
>>around but nuclei sta in place (with an extremely high probability).
>>You, I understand, are using neutrons. These can kick a nucleus out
>>of its place in the lattice so, per same energy deposited I would
>>expect much more damage.
>
>Our neutrons are in the meV range, with a little "m". The damage comes
>from
>
> n + 6Li -> triton + alpha + 4.8 MeV
>
Well, that's a big kicker.

>And so we can use TRIM to estimate the damage if we include an externally
>generated file containing initial conditions for a collection of tritons
>and alphas. And TRIM will give us a number of vacancies and interstitials
>created, but doesn't seem to say much about what happens next. I would
>have assumed they'd all just stay where they were put, except for that
>comment in a TRIM tutorial about a 99% instant anneal rate in silicon. If
>it's that high at room temperature, I wonder if it could be significant in
>a LiMg alloy on time scales of about ten minutes, even at 1.8 K.
>
I don't know. Couldn't rule out, offhand, a persisting change.
Correct me if I'm wrong, but your target is amorphous, isn't it, no
definite crystalline structure. If that's so, it'll be less self
repairing than, say, a silicon crystal.

>We have results of neutron flux measurements with a 6LiMg target and a 3He
>target, and they differ significantly. We've assumed the 3He results were
>unreliable since the signal was 100 times smaller and 100 times messier,
>and there were some calibration issues. And I still think the 3He results
>are most likely at fault, but the whole point of the 3He was to use a
>target that can't store energy in lattice defects, and if the earlier
>simulations were too naive then maybe the significant difference between
>the results is exactly what it should be. But we need to get a new target
>started before I start reanalyzing things.
>
Remember, all the easy things have already been done:-)

Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"

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