cooling of objects in vacuum chambers

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Sorry about the subject line, I was trying to be concise and cound't find  
better wording.

We have noticed a phenomenon when pumping down silicon particle detectors in  
vacuum chambers, and I had a possible explanation.

So, we put large-area silicon strip detectors in a vacuum chanber, and then  
notice the leakage current when reverse biased goes up for a CONSIDERABLE  
time, but eventually levels off.  The last detector was the biggest we've  
ever used, about 200 mm diameter, and 1 mm thick.

My theory is that this thing has an insane amount of surface area and very  
little mass.  Also, it has really poor thermal conductivity to the outside  
world.  It is mounted on a piece of PC board material that is then mounted  
to an inner metal box in the vacuuum chamber.  So, as the air is pumped out  
of the chamber, the air gets cold.  Massive objects are not going to cool  
much, but the things with the greatest surface area/mass are going to cool  
the most.  Then, the detector is going to warm up at a very slow rate.  So  
slow, in fact, that the leakage only stabilizes after more than 8 hours.

Anybody have any experience with this?  I know one could calculate the  
adiabatic cooling due to expansion of the air, but I have NO IDEA how one  
would calculate the cooling of objects in the chamber as the air is pumped  
out.  It seems there would be some worst-case rate of pumping.  Pump really  
fast and the air is all gone before much heat has left objects, pump REALLY  
slowly, and the air all maintains thermal equilibrium with the chamber  
walls.  Somewhere in the middle of those extremes is where the most heat is  
drawn out of objects.

And, of course, the adiabatic cooling heads toward absolute zero as the  
pressure drops to hard vacuum levels, so that is not a useful point, either.

Thanks in advance for any comments, crazy or experienced!

(I did an experiment about 20 years ago with thermal conductivity in vacuum  
between hard surfaces without any conduction medium like thermal paste, and  
found that it is essentially zero!  I had a piece of beryllium oxide heat  
conductor with a water loop thermal epoxied to it, in a vacuum chamber.  I  
clamped an AD 590J temp sensor to it, and turned on the water cooling loop.
The temp of the sensor changed a few degrees over tens of minutes.  Then, I  
did it with one tiny drop of thermal grease, and the sensor reached  
equilibrium in less than 10 seconds.  In air, with no grease, it took less  
than 30 seconds.)

Jon

Re: cooling of objects in vacuum chambers
On Wednesday, May 11, 2016 at 7:36:37 AM UTC+10, Jon Elson wrote:
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The thermal conductivity of gases is a bit weird. It's pressure independent
 until the men free path of the gas molecules becomes longer than the width
 of the gap over which you are measuring the thermal conductivity, then dro
ps in proportion to pressure. It all makes perfect sense if you think about
 the heat being transferred by individual gas molecules, but it is counter-
intuitive.

The electron beam testers I worked on a Cambridge Instruments used finely f
ocused electron beam to scan the surface of working integrated circuit to m
easure the voltage at the point scanned - by chopping the beam we made the  
images stroboscopic, so you could measure the voltage over a brief period -
 down to half a nanosecond - making the device the world's bulkiest and mos
t expensive oscilloscope probe.

The integrated circuit had to be unpackaged, or at least in an unlidded pac
kage, and it had to be running under vacuum for the electron beam to actual
ly get to the surface.

When we looked at an ECL memory chip - which ran hot, even in air - we had  
to squeeze a length of copper braid under the package, and bolt the other e
nd of the braid onto the metal wall of the vacuum chamber to keep the chip  
cool enough to work. A bit of vacuum grease at either end of the braid did  
help.

--  
Bill Sloman, Sydney


Re: cooling of objects in vacuum chambers
On Tue, 10 May 2016 17:47:56 -0700 (PDT), snipped-for-privacy@ieee.org Gave us:

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  You never told us about your gayness before.

Re: cooling of objects in vacuum chambers
On Wednesday, May 11, 2016 at 10:56:07 AM UTC+10, DecadentLinuxUserNumeroUno wrote:
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Unsurprisingly. That was a typo for "mean free path" as would have been obvious from the context to anybody who knew about ideal gas theory. Many don't.

--  
Bill Sloman, Sydney

Re: cooling of objects in vacuum chambers
On Tue, 10 May 2016 18:28:14 -0700 (PDT), snipped-for-privacy@ieee.org Gave us:

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  I knew what it was, lattice boy.  Don't need no primers or accusations
of lack of knowledge from you.

Re: cooling of objects in vacuum chambers
On Wednesday, May 11, 2016 at 11:35:10 AM UTC+10, DecadentLinuxUserNumeroUno wrote:
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You've just demonstrated a lack of knowledge in  your enthusiasm for getting into a lather about a typo. It's not knowledge you lack, so much as a sense of proportion.

Had you actually known you would have produced something more patronising, like "Surely you meant *mean* free path?".

Pity about your pretensions - you do make the shallowness of your claims to sophistication a bit too obvious.

--  
Bill Sloman, Sydney

Re: cooling of objects in vacuum chambers
On Wednesday, May 11, 2016 at 12:51:47 AM UTC-4, snipped-for-privacy@ieee.org wrote:

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If you were more polite, people would not relish pointing out your typo's.

                                   Dan


Re: cooling of objects in vacuum chambers
On Wednesday, May 11, 2016 at 10:48:30 PM UTC+10, snipped-for-privacy@krl.org wrote:
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een obvious from the context to anybody who knew about ideal gas theory. Ma
ny don't.
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ons
tting into a lather about a typo. It's not knowledge you lack, so much as a
 sense of proportion.
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ng, like "Surely you meant *mean* free path?".
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s to sophistication a bit too obvious.
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.

That's a hypothesis, but you probably need to take into account that there  
are lots of different people posting here, and some will point out every la
st typo in the output of the most polite poster. It seems to have more to d
o with how tight their knickers are at the time.

--  
Bill Sloman, Sydney


Re: cooling of objects in vacuum chambers
Chronic troll a.k.a. "AlwaysWrong"...

--  
DecadentLinuxUserNumeroUno <DLU1 DecadentLinuxUser.org> wrote in news:hn05jblbat0b314r631h8q1sk23v14hpl9 4ax.com:

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We've slightly trimmed the long signature. Click to see the full one.
Re: cooling of objects in vacuum chambers
On Tuesday, May 10, 2016 at 5:36:37 PM UTC-4, Jon Elson wrote:
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Hi Jon,  interesting question.  
(too bad the flame wars seem to have found it already.)

I don't know silicon strip detectors.  (first google hit.)
http://hepwww.rl.ac.uk/OpenDays98/Detectors/silicon.htm

So kinda a photodiode with lots of metal strips.  

Does the leakage goes down if it is cooled?  (that doesn't seem right to me.)  
Can you forward bias it and use the voltage drop (at known current) to measure the  
temperature of the device?  (or stick a thermal couple on it?)    
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Hmm... does leakage return to "normal" levels after 8 hours?  

Could it be picking up more ions in the gas, which now have a longer  
mean free path and can so have a better chance of hitting it.  
(If you can't tell i'm making things up.)  
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huh,  OK I'm not sure how to calculate the cooling.... temperature of the gas
I'm assuming this is only at lower pressure's once you've "roughed" out the  
chamber to ~100 mTorr or so?  
But I think I could take a stab at calculating the cooling.  Given a gas at  
a different temperature than a small object.  (find number of atoms/molecules  
hitting per second, assume atom arrives with average (gas) temperature and leaves    
with temperature of object... calculate energy loss per collision... at that  
would give a cooling rate.  

It seems there would be some worst-case rate of pumping.  Pump really  
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Before you go chasing the thermal idea, I'd like to be sure that it really is  
a temperature effect... perhaps you've already done that.

What's the magnitude of the leakage... maybe you are seeing more ions  
from cosmic rays?  

George H.  
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Re: cooling of objects in vacuum chambers
George Herold wrote:


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Well, a GIANT, unsupported photodiode, yes.
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Sure it does, just like any semi.  Remember, these things have huge area,  
and we bias the whole detector, although it is broken up into thousands of  
pixels, they are all biased in parallel.  The big round one we just set up  
was leaking 2 uA at 100 V reverse bias.

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Hmmm, that's a very interesting idea.  This is a $15,000 detector, so they  
may not want me to fool around with it too much.

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We don't actually know what "normal" is supposed to be.  What we did see was  
that leakage increased for a long time, then reached a plateau.
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We get down to really serious vacuum in 20 minutes or so, better than 5 x  
10^^-5 Torr.  The Alpha particles have no problem flying a few inches at  
that pressure.  Also, the current from the Alpha particles is too small to  
measure at the power supply.  It resolves something like 10 nA in the least  
significat digit.

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Oh, we are now sure it is indeed temperature-related.  The 8-hour asymptote  
was kind of the giveaway.
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Starts at 1.8 uA when we get good enough vacuum to turn on bias without risk  
of corona.  Ends at 2.1 uA or so at thermal equilibrium.  Cosmic ray leakage  
is absolutely TINY.  If it was a serious problem, we'd never see the Alpha  
particles from our source.  Even the Alpha particles cause a tiny current,  
must be a couple nA maximum with a strong source.  If they run this detector  
without a mask plate in front of it, I might try to see if I can get a  
reading, but I doubt it.  (The mask plate has small holes equal to about 20%  
of the area, to cut down the particle rate.)   Generally, the only way to  
see the current change suddely is by letting light into the vacuum chamber.

Jon

Re: cooling of objects in vacuum chambers
On Wednesday, May 11, 2016 at 1:37:46 PM UTC-4, Jon Elson wrote:
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Ughh. did I write that? ... I meant to write something else.  
Doesn't matter, I mis-understood your mechanism.  
Cooling and then a slow return to steady state.  Hotter = more leakage.  
You've also got ~2 uA @100 V, 200 uW of power, could it be heating from that?
(I can think of experiments, but a few hour time constant makes them all  
painful.)  


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Huh, so if I'm reading you correctly you are trying to see nA (or lower) of signal
in the presence of 2uA of leakage.. and the change in leakage is a pain.  
Seems like it would pay to try and cool the detector.
(If you could measure the Temp a control loop maybe?)  
A big hunk of cooled (LN2?) metal behind the detector and let  
radiation do it's thing?  (radiation, as in radiative cooling.)  

George H.  
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Re: cooling of objects in vacuum chambers
George Herold wrote:


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I really doubt 200 uW could heat a disc 8" in diameter by more than 1 C.  
That is a lot of surface area to radiate from, about 100 in^^2.  300 cm^^2,  
counting both sides.

Yes, with this long time constant, we SHOULD see a rise in current, if the  
bias was heating things.  But, we DON'T see that, only after the pump-down.
So, I think the 200 uW heating is NOT actually producing any measurable  
heating - which I would NOT expect.

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Well, this thing has 128 conductive rings on one side, 128 pie wedges on the  
other.  So, each of these rings or pies sees 1/128 of the total bias  
current.  We are actually seeing signal currents in the fA range, I think.  
We get away with this as the ASIC that looks at these signals is tuned to  
the response of a particle going through the Silicon, and it tries to  
extract the signal and filter out the noise.  These events last a couple  
hundred ns.  We are on our 5th generation of the ASIC chips.

Jon

Re: cooling of objects in vacuum chambers

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If there is any moisture in there, when the pressure goes down, that moisture EVAPORATES, and that absorbs a lot of heat.

Mark



Re: cooling of objects in vacuum chambers
snipped-for-privacy@yahoo.com wrote:

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A tiny amount of moisture might adhere to the surface, but it is glass  
passivated, so the moisture won't get into the Silicon.  (Other stuff in the  
chamber could absorb more moisture, but it is not able to cool the Silicon.

Jon

Re: cooling of objects in vacuum chambers
On Wednesday, May 11, 2016 at 3:01:53 PM UTC-4, Jon Elson wrote:
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Hmm, (It didn't register how BIG it is.)  
(500W/ m^2 at 300K is ~15 W for 300 cm^2, 301 K would be ~15.2 W  
Well unless you've got very very shinny walls. :^)  
  
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Oh so it's not really a problem, you are mostly trying to understand what  
causes the apparent cooling.  

George H.  




Re: cooling of objects in vacuum chambers
George Herold wrote:


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Well, **MY** theory is, it is cooling.

We have seen this phenomena a number of times before, and were puzzled by  
it.  There is a well-known phenomena in radiation detectors where damage to  
the crystal lattice and implaning of ions causes the leakage current to rise  
until the detector is no longer usable.  There is also another phenomenon  
where the leakage rises, and we send the detector back to the manufacturer,  
who removes the wire bonds, cleans the detector and then re-bonds it.  When  
we get it back, the leakage current has gone down significantly.  We think  
this is due to contamination of the surface with vacuum pump oils, but we  
are guessing.

In many cases, this rise in leakage current after pumping the chamber is  
muddled by hitting the detector with a lot of particles, so we can't  
distinguish radiation damage from other effects.

This time, we had a brand-new detector in a relatively benign environment  
for several days, so we could actually see this effect all by itself.  My  
boss was quite concerned until we saw there was an asymptote to the rise in  
current.  The huge size of this detector may have made this a more  
pronounced effect than we've seen before.

Yes, so now that we have observed this in relative isolation, we'd like to  
understand what went on.  We do this stuff all the time, and if we could  
KNOW that every time we pump out the chamber, we'd see, for instance, a 20%  
decrease in leakage current that would then go back to baseline over the  
next 8 hours, that would be a real help in telling the normal behavior from  
the abnormal.

I've convinced myself that I understand the mechanism, but this is all "gut  
feel" with no measurements to back it up.  What I'd have to do to actually  
find out is attach a temp sensor to a big sheet of aluminum and pump it.  
Even the small mass of an AD590 sensor is enough to prevent much of the  
cooling I am expecting is going on, here.  I may actually mock this up  
sometime when we are not working on serious stuff here.

Jon

Re: cooling of objects in vacuum chambers
On Wednesday, May 11, 2016 at 5:31:29 PM UTC-4, Jon Elson wrote:
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Wow! OK.  I was thinking about this and was going to say that,
you know, 99.999..% (add as many nines as you want) more than  
I do... some guy on usenet.  
Anyway that was great!!  
How to put numbers on your cooling mechanism,
my problem is that at the start the gas all leaves by flow,
like pouring water out of a bucket... all the molecules(m's) leave,
fast and slow.  Then later on pumping is all about the fast ones leaving.
With some transition region in between.  You most likely know where,
those regions are. (I can guess, or look at wiki.)  
So do some guesstimate, from this point on all the m's only leave  
by hitting a wall and bouncing out.. with "no" energy before they
hit the wall.  then count m's and ratio to the area, do the heat
 capacity... around room temp all the m's are pretty much equal.  

That's crude but will give some number.  
George H.  

Re: cooling of objects in vacuum chambers
On Thursday, May 12, 2016 at 11:03:35 AM UTC+10, George Herold wrote:
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If my memory what I was told about the process of pumping down electron mic
roscope chambers is anything to go by, getting the gas out happens fast and
 early, and the process that took the time was getting rid of the physicall
y adsorbed water.

https://en.wikipedia.org/wiki/BET_theory

talks about the difference between multi-layer adsorbtion and the adsorbtio
n of the final monolayer, where the bonding to the surface tends to be rath
er stronger.

There's not enough energy involved to make much difference to the temperatu
re of the adsorbing surface - the layer is only one molecule thick where it
 exists at all - but the equilibrium vapour pressure of water above that re
sidual final monolayer is quite a lot lower than above a layer several wate
r molecules deep.

We had hydrocarbon monolayers as well - where we hit them with an electron  
beam they'd turn into a thick (and eventually visible) insulating layer of  
polymerised hydrocarbon, unless the substrate was thin enough to be warmed  
by the electron beam, where you'd get a much less inconvenient layer of ele
ctrically conductive gunge.

--  
Bill Sloman, Sydney
  


Re: cooling of objects in vacuum chambers
Jon Elson wrote:

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OK, I did the experiment, and there really is significant cooling.  I  
propped a piece of thin aluminum sheet in the chamber, and taped an AD590  
temperature sensor to the plate, with a little dab of thermal compound under  
it.  I was easily able to show a temperature drop of 5 C or more while it  
was being pumped down.  So, that maybe does support my theory.  The detector  
in question was surrounded by a housing and a plate to protect it, so it was  
kind of enclosed in a Kelvin shield, that may have increased the effect.  
Here, I had a plate just propped in the chamber with no attempt at thermal  
isolation, and the sensor produces a tiny amount of heat, a couple mW.  
Well, coupled to that big plate, that heat ought to be negligible, but maybe  
once it gets down to real vacuum, it affects the measurement.

Jon

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