Microcontrollers exposed to X-Ray Beam

[Wouter van Ooijen]

> Anyone on the list have experience / knowledge of using PIC microcontrollers
> which may be exposed to X-ray beams?  

Sorry, no first-hand knowledge.

I think the effect will be cumulative, especially on FLASH/OTP program
memory. In addition you should expect (anticipate?) RAM single-cell-flips.

In such a situation I would
- choose the oldest technology (largest chip feature size) I could get
- probably prefer OTP over FLASH
- if at all possible design the code for 'one-shot run' and use an
external 555 or similar to restart the chip for each run

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Wouter van Ooijen

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[Alan B. Pearce]

> Anyone on the list have experience / knowledge of using PIC
> microcontrollers
> which may be exposed to X-ray beams?  A customer has an application for
> use
> in radiation therapy rooms, and the electronics package will occasionally
> be
> in the path of the therapy beam during patient exposure; due to the nature
> of the package, this is unavoidable.  The question is: will this
> eventually
> cause the electronics - specifically the microcontrollers - to fail?  Some
> empirical testing done Saturday in a Cancer Center showed that the
> microcontrollers will eventually fail, but only after extended exposure -
> approx. 1 1/2 hours in a continuous beam, something that will not happen
> with a patient.  If the package is exposed intermittently, will the
> radiation effects be cumulative?  Tried Microchip & Google with very
> little
> useful info uncovered.  Thanks,

I remember going to a conference a good many years ago, where one of the
papers described using a microprocessor based system to build a robot that
could be put under an x-ray machine to medical students how to recognise
various items on an x-ray image. He built the dummy with appropriate
internal parts, and had it working nicely.

Then came the day when they tried it under the x-ray. Every thing went
swimmingly for a while, then a limb would give a spasm, and then carry on
normally. Then another would do the same, and so it went as time passed,
until all the limbs were just behaving in a totally random mix of spasms. He
got the unit back to his engineering lab, and found that all the EPROMS had
been erased by the x-rays. The solution was to blow some bipolar PROMS,
which being non-erasable, didn't suffer the same fate.

As to how to deal with your problem, a starting point is to use some 1mm
tantalum sheet top and bottom of the PCB. This is what we do with
instruments on space craft, where chips have doubtful radiation performance,
but then that is shielding from particle damage rather than x-rays. I guess
for x-ray shielding, some similar thickness lead sheet would be the answer.

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[Apptech]

>>  (Hot tip: use an anti-static floor-wax on your plastics
>> and you
>>  shouldn't have to ever treat them again! Just dip your
>> plastic bins and
>>  trays in. Then let them drip drain and dry. Think of the
>> antistatic
>>  floor wax as an almost permanent spray.

I maent to add - antistatic sprays that are meant to make
conditoons acceptable for people may well not reduce ESD to
the level that works OK for all components. The general
consensus in years gone by [tm] was that they were not good
enough.

> How are you going to prove it works though? More to the
> point, how are
> you going to prove it's still working if it ever did?

A foil electrometer / electroscope is cheap and easy to make
(and maybe even to buy) and will give a good idea if ESD
protection is working.

OR here's a FET electroscope that can be adapted. Cheap and
easy

        http://www.eskimo.com/~billb/emotor/chargdet.html

    http://www.nfinity.com/~exile/electro.htm

    http://www.mos.org/sln/toe/simpleelectroscope.html

$17 - out of stock

    http://www.physlink.com/estore/cart/GoldFoilElectroscope.cfm

$32

   http://wardsci.com/product.asp?pn=IG0008328&bhcd2=1210072291




        Russell


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[Nate Duehr]

piclist@... wrote:
> So I read up on ESD and a few people stress that you do not actually want
> conductive storage like those foil bags or black plastic boxes.
>
> It makes sense.  If I have an open box with a part inside, and I reach in
> and touch a lead, any static charge on me can go through the chip, into
> the box and ground out.  A non-conductive box would not do that.  Of
> course grounding yourself before touching anything is better, but mistakes
> happen.

You shouldn't be "reaching in and touching a lead" before you're
adequately connected to the ESD workstation ground.

Nate
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[piclist-7]

On Tue, 6 May 2008, Nate Duehr wrote:
> You shouldn't be "reaching in and touching a lead" before you're
> adequately connected to the ESD workstation ground.

If I had a workstation for my projects that would be great. :-)

But you are right.  I do have a ground strap running along the side of my
desk for both working with computer parts as well as electronics.  

Most of my parts are in EDS bags or black conductive boxes.. chips stuck
in conductive foam.  I just wish I had ESD versions of the containers I
use for non-sensitive parts.  I like having things sorted so I know what
I have and also can find it.  A big bag of parts while safe in the
sheilding, is not very safe once I dump it onto my desk to find the
part I need. :-)

I don't think they even make EDS storage boxes with clear lids, which is
what I would really like to have a dozen or more of.  I guess the only way
to make plastic safe is to fill it with carbon or coat it in something.

--
Ian Smith
www.ian.org


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[Sean Breheny]

Hi Ian,

I am pretty sure they DO have ESD boxes with clear lids:

http://www.all-spec.com/1/viewitem/SM0882/ALLSPEC/prodinfo/w3path=cat

This particular one is conductive (I think you would want dissipative,
but I suspect that is available too).

Sean


On Wed, May 7, 2008 at 10:50 AM,  <piclist@...> wrote:

> On Tue, 6 May 2008, Nate Duehr wrote:
>  > You shouldn't be "reaching in and touching a lead" before you're
>  > adequately connected to the ESD workstation ground.
>
>  If I had a workstation for my projects that would be great. :-)
>
>  But you are right.  I do have a ground strap running along the side of my
>  desk for both working with computer parts as well as electronics.
>
>  Most of my parts are in EDS bags or black conductive boxes.. chips stuck
>  in conductive foam.  I just wish I had ESD versions of the containers I
>  use for non-sensitive parts.  I like having things sorted so I know what
>  I have and also can find it.  A big bag of parts while safe in the
>  sheilding, is not very safe once I dump it onto my desk to find the
>  part I need. :-)
>
>  I don't think they even make EDS storage boxes with clear lids, which is
>  what I would really like to have a dozen or more of.  I guess the only way
>  to make plastic safe is to fill it with carbon or coat it in something.
>
>  --
>  Ian Smith
>  www.ian.org
>
>
>
>
>  --
>  http://www.piclist.com PIC/SX FAQ & list archive
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>

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[Apptech]

> Most of my parts are in EDS bags or black conductive
> boxes.. chips stuck
> in conductive foam.  I just wish I had ESD versions of the
> containers I
> use for non-sensitive parts.  I like having things sorted
> so I know what
> I have and also can find it.

> A big bag of parts while safe in the
> sheilding, is not very safe once I dump it onto my desk to
> find the
> part I need. :-)

Get a sheet of Butyl rubber, such as I suggested before.
Take meter to place of sale. Stick meter probes into sheet
on megohms range. ANY ohms reading is OK. Some few types
I've met had no reading but almost all have some. Some are
very low R and are a hazard to PCB operation if an operating
cct is placed on the sheet.

Here local suppliers get material wrapped in Butyl rubber
outers and they sell them for staff Christmas party funds.
Much cheaper than buying new BUT even at new prices Butyl
rubber costs <<$ than "proper" electrostatic mats.

You can also line draws with BR sheet.

> I guess the only way
> to make plastic safe is to fill it with carbon or coat it
> in something.

OR read my prior post.
Zinc electrostatic shielding spray whiffed on lightly. Or,
perhaps, "cold galvanising" paint.



        Russell


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[Steve Moulding]

Steve Moulding wrote:
> Anyone on the list have experience / knowledge of using PIC
> microcontrollers which may be exposed to X-ray beams?  ...[snipped]

[PIC]  Thank you to everyone who contributed to the discussion.

Maybe as a summary, I can explain what I found out about this issue, partly
from our own testing, and partly from infornation provided by readers of
this list.

X-radiation will affect devices with FET transistors used to implement
logic/memory chip circuitry.  Our testing revealed that not only are memory
cells erased, but that on-chip digital control circuitry can be damaged to
the extent that it stops working.  Specifically, the Microchip PIC devices
we were testing were found to be readable after exposure, with some (but in
our case not all) memory locations erased.  But it was also found that the
chips could not be erased normally and re-programmed - obviously, some other
damage had occurred, rendering the devices unusable.

Also, the effects of radiation are cumulative.  Even to the extent that the
effect in FETs is made useful by manufacturing dosimeters that measure the
change in characteristics due to radiation and translating them into
exposure parameters.

Polyethylene can be used as a shield for X-radiation, but the thicknesses
needed for our application would be excessive.  Lead shielding is very
effective for shielding purposes, but there is some question about building
the shielding into the product and thereby violating RoHS directives.  Since
the device will be used in Europe as well as other parts of the world, this
may be a problem.  But there must be some exception for this type of medical
equipment, since lead aprons and other types of personal shielding are still
approved for use in radiation suites / therapy rooms.  The required lead
thickness may be prohibitive also - that remains to be seen.

We also discovered that the IR photo IC that we are using to detect the data
stream from the remote control will fail, but at much higher radiation
levels.  At this time we have no way of telling whether this is due to FET
failure in the circuitry, of if the detector diode is affected.  In any
case, it failed as part of the test.

Rad-hardened devices are available for some functions, but no
microcontrollers as far as I have been able to determine (this may be wrong,
however).  It seems that some rad-hardening techniques are classified and so
are not available to the general public.  Some are only available for
military and other government entities.  And all are prohibitively expensive
for our application, even if available.

Our testing leads us to believe that the inadvertant exposure of our device
to random X-radiation, coupled with the time factor related to exposure will
result in device failures on the order of one or less failures of the
equipment per year.  We are considering shielding options, but also making
the affected circuity modular, so that it can be treated as removable /
pluggable by the user should failure occur.  That won't make the customer
happy if failure is experienced, but a swap-out program may solve that
issue, since the cost factor is relatively small.

Again, thanks for the input; it is very much appreciated.

Steve



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[Chris Smolinski]

>Polyethylene can be used as a shield for X-radiation, but the thicknesses
>needed for our application would be excessive.  Lead shielding is very
>effective for shielding purposes, but there is some question about building
>the shielding into the product and thereby violating RoHS directives.  Since
>the device will be used in Europe as well as other parts of the world, this
>may be a problem.  But there must be some exception for this type of medical
>equipment, since lead aprons and other types of personal shielding are still
>approved for use in radiation suites / therapy rooms.  The required lead
>thickness may be prohibitive also - that remains to be seen.

Have you considered other materials? I've used tantalum and tungsten
in the past. Neither is as inexpensive as lead, unfortunately, and
machining the latter is a PITA.  What is the maximum X-ray energy (kV
on the tube)?

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Black Cat Systems
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[Steve Moulding]

Chris Smolinski wrote:

>> Polyethylene can be used as a shield for X-radiation, but the
>> thicknesses needed for our application would be excessive.  Lead
>> shielding is very effective for shielding purposes, but there is
>> some question about building the shielding into the product and
>> thereby violating RoHS directives.  Since the device will be used in
>> Europe as well as other parts of the world, this may be a problem.
>> But there must be some exception for this type of medical equipment,
>> since lead aprons and other types of personal shielding are still
>> approved for use in radiation suites / therapy rooms.  The required
>> lead thickness may be prohibitive also - that remains to be seen.
>
> Have you considered other materials? I've used tantalum and tungsten
> in the past. Neither is as inexpensive as lead, unfortunately, and
> machining the latter is a PITA.  What is the maximum X-ray energy (kV
> on the tube)?
>
> --
>
> ---
> Chris Smolinski
> Black Cat Systems
> http://www.blackcatsystems.com

Hi Chris,
Tantalum might be a possibility, especially as it is available in many
forms.  The machine we are dealing with is a therapy machine generating
X-rays at 6 MV.
Steve



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[Steve Moulding]

Steve Moulding wrote:

> Chris Smolinski wrote:
>>> Polyethylene can be used as a shield for X-radiation, but the
>>> thicknesses needed for our application would be excessive.  Lead
>>> shielding is very effective for shielding purposes, but there is
>>> some question about building the shielding into the product and
>>> thereby violating RoHS directives.  Since the device will be used in
>>> Europe as well as other parts of the world, this may be a problem.
>>> But there must be some exception for this type of medical equipment,
>>> since lead aprons and other types of personal shielding are still
>>> approved for use in radiation suites / therapy rooms.  The required
>>> lead thickness may be prohibitive also - that remains to be seen.
>>
>> Have you considered other materials? I've used tantalum and tungsten
>> in the past. Neither is as inexpensive as lead, unfortunately, and
>> machining the latter is a PITA.  What is the maximum X-ray energy (kV
>> on the tube)?
>>
>> --
>>
>> ---
>> Chris Smolinski
>> Black Cat Systems
>> http://www.blackcatsystems.com
>
> Hi Chris,
> Tantalum might be a possibility, especially as it is available in many
> forms.  The machine we are dealing with is a therapy machine
> generating X-rays at 6 MV.
> Steve

It just occured to me that maybe I haven't made it clear that this is not a
regular x-ray machine as most people think of them.  The device is a linear
accelerator, which accelerates electrons to the desired energy, smashes them
into a metallic target generating high-energy x-rays, and then collects,
focuses,and collimates the x-rays into a beam that is used to radiate
cancerous tumors accurately.  The procedure is usually referred to as
external beam therapy.  The beam that is generated is the one that will
subject our electronics to x-radiation after having passed through the
patient.

Try this url for an example (sorry about the length - I couldn't get TinyURL
to work for me).

http://www.medical.siemens.com/webapp/wcs/stores/servlet/ProductDisplay~q_catalogId~e_-11~a_catTree~e_100010,1008643,12757,1012165,1014065~a_langId~e_-11~a_productId~e_180106~a_storeId~e_10001.htm

Steve



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[Chris Smolinski]

>  > Hi Chris,
>>  Tantalum might be a possibility, especially as it is available in many
>>  forms.  The machine we are dealing with is a therapy machine
>>  generating X-rays at 6 MV.
>>  Steve

Ahh, 6 MeV. Yes, you'll need a lot of lead/tungsten/etc. The mfp of
Pb at 6 MeV is around 6 cm, for W closer to 3.5.  How about using DU
for shielding?  That should give the RoHS goons something to worry
about. Hey, at least it ain't lead. (The mfp at 6 MeV is 2.5 cm so it
is slightly better)


>It just occured to me that maybe I haven't made it clear that this is not a
>regular x-ray machine as most people think of them.  The device is a linear
>accelerator, which accelerates electrons to the desired energy, smashes them
>into a metallic target generating high-energy x-rays, and then collects,
>focuses,and collimates the x-rays into a beam that is used to radiate
>cancerous tumors accurately.  The procedure is usually referred to as
>external beam therapy.  The beam that is generated is the one that will
>subject our electronics to x-radiation after having passed through the
>patient.

Of course the vast majority of the x-rays are at lower energies, with
peaks at the characteristic energies for the target material. In
fact, chances are that most of the damage is being done by lower
energy x-rays anyway, as the higher energies are zipping through the
Si without causing damage. Well, most of them. You might find that
you only need to shield up to a few hundred kV?  Insert hand waving
here. Or are the lower energies already being attenuated by the
machine/patient?


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[Alan B. Pearce]

>We also discovered that the IR photo IC that we are using to detect the
>data stream from the remote control will fail, but at much higher radiation
>levels.  At this time we have no way of telling whether this is due to FET
>failure in the circuitry, of if the detector diode is affected.  In any
>case, it failed as part of the test.

LEDs and photodiodes also suffer significant degradation under radiation.
Using opto-isolators on spacecraft is a necessary evil that requires a fair
amount of thinking about degraded performance of opto devices.

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[Al Shinn]

Look into using bismuth for shielding. PeptoBismal is Bismuth subnitrate
so you can eat the stuff. It is dense (9.87gm/cc, vs 11.35 for lead) and
very easy to cast as it has a quite low melting point. Easy to machine
too. It is used in shotgun shells in places where lead has become
unacceptable and in fact that is an easy cheep way to get an evaluation
sample (just don't set the damn thing off) (or for just a little bit
email me and I'll mail you some)
--

Looking forward,
Al Shinn


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[Al Shinn]

I don't know if anyone mentioned that shielding metals can be in powder
form. You could mix say, tungsten powder with say, epoxy to and thus
avoid the difficulty of machining hard metals.
--

Looking forward,
Al Shinn


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