Re: Annealing of Radiation Damage

From: Gregory L. Hansen (glhansen_at_steel.ucs.indiana.edu)
Date: 10/13/04 

Date: Wed, 13 Oct 2004 01:10:57 +0000 (UTC)

In article <2NZad.32$45.12180@news.uchicago.edu>,
 <mmeron@cars3.uchicago.edu> wrote:
>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'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.

I sure would appreciate that.

>>>
>>>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.

Probably not. I assume it's polycrystalline, but we know nothing about
the grain size, crystal structure, etc. I'm sure knowledge of the crystal
structure exists, but we don't know it.

The reason the magnesium is in there at all is because lithium metal has a
phase change at 37 K. 25% Mg cuts that. Lead does, too, except it also
backscatters Debye-Scherrer cones with our 5 A neutrons, and that's just
too messy a systematic correction to make if we don't have to. The LiMg
seems to scatter isotropically at 5 A.

And writing that, I realize that if we use a different Li/Mg ratio, we'll
have to repeat the backscattering measurement, because that could change.

>
>>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:-)

Especially when you lose stuff. The LiMg target was removed five years
ago, and now nobody can find it! In that time we've had to move our
offices to the other end of the building, had to move all the apparatus so
the floor could be painted, a lab was moved, another one taken away from
us, and people have vague recollections of putting it somewhere safe, but
nobody knows where. And the person most likely to know has no idea
because everything has moved at least once since she's been involved.
Ugh. 

-- 
"Will we be suturing the anus?"