Long-term radiation damage to CMOS sensors?

Hi all,

 

I recently had to document some goopy work I was doing with epoxy, and I wasn't

about to pull out a good camera to do it. So I pulled out an ancient Olympus

first-generation 1.3MP digital. If I were to smear the case with epoxy, so

what?

 

Here's the interesting thing. The camera has hardly seen any use for several

years, only being pulled out for mucky jobs such as this one. Otherwise, it

remains stored in a cool, dry, dark closet where I keep all my other

photographic equipment. To my amazement, I had a number of "hot pixels" that

weren't there the last time I used it (which might have been a couple of years

ago). What happened between then and now?

 

When I thought about it, I realized that cosmic radiation was probably the

culprit. Slowly, steadily, we get bombarded by low levels of cosmic radiation

that is known to degrade semiconductor devices. It is this degradation that

necessitates "radiation hardening" of circuits used by the military. A CCD

sensor would certainly be no exception, and I am certain this is how I got my

hot pixels. Otherwise, the sensor was exposed to neither light nor electrical

current! The culprit could also have been radon exposure (with alpha particles

being emitted from radon gas that floats through the air), but the radon

exposure in my area is not particularly high. It was admittedly very high

where I once lived in the midwest, but the hot pixels did not occur during the

time period when the camera was in that environment.

 

I then started wondering about my EOS 10D, which has a bit of age on it now. I

notice that there is quite a bit more noise in the image than there once was.

Moreover, there seem to be very subtle horizontal and vertical streaks,

presumably representing rows and columns of pixels. I even have a few hot

pixels that weren't there when the camera was new.

 

I see that there is very little written on this subject. There are a few

academic papers on the subject, but they pertain mostly to image sensors on

satellites, which are exposed to much higher levels of radiation. So I'm left

wondering about the practical implications of what I've experienced. In

particular:

 

(1) Are CMOS sensors somehow less prone to radiation damage than CCD sensors?

Are there any differences in how this damage appears in the final image?

 

(2) Are sensors with larger pixel areas less prone or more prone to damage

than sensors with smaller pixels?

 

(3) How damaging are the various forms of radiation that a camera might

experience, in particular (a) cosmic radiation, (b) alpha particle radiation

from radon degradation, and © x-irradiation. There are obviously very

different approaches one might take to protect cameras from these sources --

radiation shielding (cosmic) vs. air-tight containers (radon) vs. causing more

headaches for airport screeners (x-rays).

 

(4) Has anyone tried to shield digital cameras from radiation while in

storage? If so, has it kept the sensors fresher? What approaches work best?

Lead-lined boxes?

 

Again, the degradation I saw with my old Olympus was very sobering, and the

subtle changes I've seen in my slightly less ancient 10D are a bit worrisome

too. If I can lessen this degradation process on present and future cameras, I

think I should do it.

 

Thanks for any advice anyone has to offer!

 

Peace,

Sarah Fox

Graphic Fusion

I think you're assuming too much. There could be myriad other causes for your hot pixels and other anomalies.

 

What about moisture in the climate where you live? Or lack thereof? Electronics are sensitive to this kind of stuff over time -- I think a lot more than cosmic rays. Unless you're living close to a leaky nuclear power plant, I doubt that rays are the culprit.

 

If rays had that kind of effect, my old computers would not have worked as long as they did.

Whatever the mechanisms of degradation it clearly happens, and manufacturers seem to be reluctant to say anything much about it. I doubt that digital cameras will be as long lived as well built film cameras. By the time the sensor has degraded to the point where you might consider replacing it, it is unlikely that replacement sensors will still be available for the model in question, even if they could be replaced at reasonable cost otherwise.

Did you remember last year , Manufacturers recalled digicams that use a certain sony ccd, due to manufacturing defect or ccds becoming defective over time due to heat? Who knows your oly isnt suffering from the same defect. Cause It was only after the defect was discovered and acknowledged that sony modified (I pressume :) thier maufacturing techiques.

 

For the 10d, Hey, what about vibrations, heat , and maybe intermitently defective battery? For sure , all things would decay over time , with or without cosmic rays, nutrinos , solar flares, rfi, black holes and etc... Some just decay faster than some.

That should say , faster than others, sorry.

Did you leave the batteries in the camera while in storage? I

wonder if this may add another variable to the diagnosis.

I noticed that my Canon A80 appeared to produce noisier images after I purchased a Rebel XT. Were the images more noisy? No. A comparison showed the same noise levels as before. But my mind had grown used to the low noise of the XT.

 

However, I don't doubt that over time pixels can (and do) fail. My A80, for example, did develop a stuck pixel (it's always blue).

 

Can you replicate any of your previous photos (shutter speed, light levels, etc) to do an as accurate test as possible?

 

 

B.t.w on the topic of sensor degradation comes this interesting page, only available on archive.org:

http://web.archive.org/web/20041022185230/http://hogsett.com/goodies/ccdburn.htm

 

Additionally, here's a pdf on failure mechanisms in semiconductors (not sure how old it is). There's a short section called "Color CCD Image Sensors Optical Reliability" that discusses this briefly. I think it was Mark U that originally pointed me to the document.

 

http://www.sony.net/Products/SC-HP/tec/catalog/pdf/qr_2.pdf

Radiation hardening is designed into devices intended for space flight or operation in nuclear blast environments. That's a hugely different environment than simple trickle thorugh cosmic radiation over time. I'd suggest that if there is a "radiation" source in the local environment that could be damaging your camera, the camera may be the least of your worries.

It's certainly true that we're constantly bombarded by cosmic rays, but they are more of a nuisance than the source of the degradation that you observed in your cameras. Cosmic rays introduce artifacts in the image during long exposures with special image sensors that are designed for extremely high sensitivity in the near infrared. The artifacts appear as straight lines when the sensor is struck by muons and have worm-like tracks when it interacts with Compton-scattered electrons. Eliminating this background noise is always a big issue for astronomers.<p>

The degradation in the image quality of your cameras are caused by changes in the sensor transistor characteristics (e.g., threshold voltage shifts) and active pixels (e.g., increased diode leakage currents). There are many causes for these changes but cosmic rays are not the main culprits. On your question about shielding against cosmic rays, researchers at the Lawrence Berkeley national labs have determine that if you bury the instrument 180 meters underground in a special lead vault, you can eliminate the effects of cosmic rays...

<p>

<center>

<img src="http://img230.imageshack.us/img230/6505/cosmicraysrx2.jpg"><p>

<i>Cosmic rays in 30-min exposure at -140C, CCD with thick sensitive regions</i>

</center>

I've tried responding to this thread, but my responses don't make the board, and my effort is lost! Here's another try, and I'll be VERY brief, because I'm getting tired of sending text into a void. (I'm not very optimistic here...)

 

(1) THANKS! for the wealth of information! I now know the culprit isn't cosmic radiation, and I won't be building a leaded box to store my digitals. Perhaps acquiring some replacement sensors to be stored in liquid helium... Hmmmm.... (kidding)

 

(2) The Olympus -- stored sans batteries. The pixel defects (especially the blazing hot pixels) are unavoidably recent. I'd have noticed these very easily. If I had seen them when I first got the camera, I'd have returned the camera to the retailer.

 

(3) The noise in the 10D might be my imagination. I'll do some careful comparisons and get back to y'all with the results (assuming I can post to this thread again).

 

Again, thanks! :-)

 

Peace,

Sarah

Hi Sarah,

thanks for the interesting post. I'm doing some research on the effects of Cosmic Radiation on digital sensors and every little bit helps. I just came across a technical paper that describes this topic but at the cost of $35 unfortunately I cannot afford it.

My camera sensor (Canon EOD 30D) was damaged just 2 weeks ago during a trip to Italy and I am quite certain that the cause is cosmic radiation. I see a lot of skepticism here and I am a little baffled by that, maybe people think that cosmic radiation is something ethereal or magical, but as someone with scientific background I see that as a very plausible explanation. I used my camera all morning shooting in the countryside and at night, around midnight, I took it to the shoreline to take photos of the waves in moonlight. Reviewing the photos the next day I noticed over 20 hot pixels in all my night shots, starting with the first one, no burned pixels in the 50+ photos I shot during the day. My night shots were taken with 30" exposure. I cannot offer any help to fix the damage done, but I'm working on some theories to protect the equipment at least a little bit more.

 

Luca

Hi Luca,

 

Very interesting! I hate to cast skepticism on my own theory (and yours), but a shutter blade isn't likely to shield the CMOS sensor from much cosmic radiation. I would think the other materials in your camera, including the body, the glass, and so forth, would provide much better shielding overall. I would also think radiation would be lighter at night, since solar wind is at a minimum.

 

On the other hand, when you take a 30 sec exposure, you keep your CMOS sensor active for quite a long time. It's possible that you could have had some weak pixels that burnt out in the process.

 

Pixel defects are also more apparent with longer exposures, as the response at a defective pixel has longer to drift high or low. Do you see the same hot pixels with shorter exposures?

 

Anyway, it's all very interesting! Your experience will make me a bit more reluctant to take lots of time exposures. I'm interested in your feedback re the last question!

 

Peace,

Sarah

[[My camera sensor (Canon EOD 30D) was damaged just 2 weeks ago during a trip to Italy and I am quite certain that the cause is cosmic radiation]]

 

I'm sorry, Luca, but you're simply wrong on this. Your sensor has not been damaged. Hot pixels are a function of long exposures, heat, and sensor design. The longer the exposure the more chance the sensor has to heat up the greater the likelyhood of the number of hot pixels.

 

I would recommend you spend some more time researching the topic of long exposures with CCD and CMOS sensors and the various affects heat and time can play on photon counting. Your sensor is behaving this way not because of damage but because within the current limits of human design, this is the only way for it to behave.

 

Incidentally, Canon provides a noise reduction option with your camera. The camera will take a second exposure of a time equal to the previous, without opening the shutter. This is called a "dark frame." It uses the information from the dark frame image to subtract away the hot pixels from the previous image, rendering your final output very clean.

[[possible that you could have had some weak pixels that burnt out in the process]]

 

Again, there is incorrect information here. A hot pixel is not the same thing as a dead pixel or a stuck pixel. These are three completely different phenomena.

 

A hot pixel shows incorrect color information as a function of heat, exposure time, and ISO.

 

A dead pixel shows no color information (black) all the time regardless of exposure time and ISO.

 

A stuck pixel shows only one color all the time regardless of exposure and ISO.

Rob,

 

By "burn out," I'm referring to a breakdown of the semiconductors, not their under/over/nonresponse to light, per se.

 

Geeesh!

 

Being repeatedly beaten over the nose with a rolled up newspaper and told I'm "wrong" is the reason I have been absent from this forum for many, many months and will probably never post anything ever again.

 

(Hint: From this point on, anyone addressing me in this forum will not get a response. Please don't take it personally.)

 

Please, now that I'm gone (again), feel free to tell me how wrong-headed I am!

 

Sarah

[[being repeatedly beaten over the nose with a rolled up newspaper and told I'm "wrong" is the reason I have been absent from this forum for many, many months and will probably never post anything ever again.]]

 

Your conclusions, seemingly based on an incomplete understanding of how digital cameras work, are simply wrong. If you cannot accept that then there's little anyone can do. It's a shame you would choose such a path.