Cosmic Rays

[Jim Tittle]

My knowledge of 'alternative science', garnered during years of searching for cryptids in the forests and swamps of North America, gives me what some may call "a unique perspective". I'll keep the tinfoil hat on, thank you very much.

[Don Parrish]

You guys are the reason why marijuanna should not be legalized :)

[Alister Chapman]

Originally Posted by Alan Emery

I don't know if the chips in cameras or our typical computers are really very vulnerable (unless you happen to own a Cray), but it is within the realm of possible. Cosmic ray particles are detected in experiments (normally undertaken in space or high altitude from ballons so there is no interference from the atmosphere) by exposing plastic sheets, then bringing them back and "etching" them using sodium hydroxide which dissolves along the path of damage faster than undamaged plastic. So by extension it is also possible to damage plastic parts of a camera including sensors. The chances of significant visible damage are really small -- but theoretically present.


Alan

It is not just theory. It happens all the time. Do your research and you'll see that an SD CCD sensor can expect to have a pixel damaged every year or so and an HD sensor around 4 a year at ground level. That's why masking circuits are built into most cameras. When you fly the risk is greatly increased especially on Polar routes, which is why Sony won't air freight cameras on polar routes. CMOS sensors are affected as well although the masking circuits tend to hide them so you rarely see them.

[Alan Emery]

Hi Allister,

You are the expert in this field for sure and I don't question that. My intention was to clarify what cosmic rays are and how likely it is that visibly significant damage can be done by them on normal computers or normal (SD or HD) camera sensors. The most likely to be affected are CCD sensors.

What you describe as observed and in-practice policies perfectly fits the theoretical models for the way cosmic ray particles behave in our situation here on earth. I would agree further that a camera above most of the atmosphere and at the poles is more likely to encounter a full cosmic ray particle with its complete complement of photons, helium nuclei, and electrons. These can cause damage to a sensor for sure.

Again just for clarification, we can detect cosmic ray particle strikes on the earth (which are primarily muons, not the full primary cosmic ray particles) on average about 2 times per minute per square centimeter. The particles are so tiny that the number of times they would actually interact with some physical part of a sensor is quite small -- I accept your estimate of 4 hits per year on a pixel-crowded HD sensor given your expertise.

I may be quite wrong here, and I acknowledge that, but I understood that a cosmic ray particle (muon type) hitting a computer chip or a camera sensor causes a discharge of electrons (because it is ionizing radiation). In a computer, the discharge is equivalent to a signal asking for a computation. On a sensor, I understood a cosmic ray particle strike normally causes a discharge of electrons that would be indistinguishable from a photon particle strike and thus give a false reading for the pixel that was affected. Thus the picture quality (or accuracy) would be reduced in that instance.

If I understand you correctly, your experience suggests that the damage from a muon can be more than just the discharge of electrons messing up that pixel for that picture, but rather that the damage can be a permanent loss of a pixel. I assume that damage would in future pictures render a black pixel on the sensor and in the final image. Again taking your numbers, that would suggest that a four-year old HD sensor could have about 16 dead pixels scattered randomly on the sensor as a result of cosmic radiation from muons. Here again, I bow to your knowledge and experience in making spectacular video images. Can the human eye detect a single dark pixel in an image of about 2 million pixels (even in a pale background) even in unusual viewing situations such as a large projection screen in a theater?

Finally, and again, I am a novice in making videos, so as a result of your comments, I am now made unsure of my knowledge, but I understood the dark mask process for CCD sensors was to correct a wide array of sensitivity errors caused in the manufacture of the sensor (read noise, thermal noise, electronic noise from the camera, uneven silicon distribution resulting in different pixel sensitivities). In this dark masking process a series of comparative images are compared to a flat master frame and the differences from the flat (or evenly dark) image are then subtracted digitally when the camera is taking pictures. But perhaps you are referring to a different masking circuit? I made the assumption that this was the only correction that the camera makes while it is taking pictures. I was not aware that a normal camera such as we use could correct for random cosmic ray particle strikes.

GIven the difficulty (essentially impossible) of shielding against cosmic rays particles that actually make it to earth (muons), and given that electronic discharges resulting from those cosmic ray particles are therefore essentially unavoidable, but rare ( a few times a year), I speculated that the likelihood of visibly significant effects from cosmic rays would be really small -- but theoretically possible.

However, if your experience and observation suggests that cosmic ray particle strikes at earth level result in permanent damage at a rate of 4 dead pixels per year in an HD sensor and that a few (4 to 20) randomly distributed dark pixels in an image of over 2 million pixels of varying intensities are visible then I stand corrected, and am happy to learn. In addition if the current cameras have circuitry that can correct for the cosmic ray strikes then I am also happy to stand corrected and to be impressed by modern technology.

Alan

[Jim Tittle]

I'm happy, too! ; )

[Alister Chapman]

What happens to both CMOS and CCD sensors as well as flash memory is that the energetic particle punches a small hole through the insulator of the pixel or memory cell. In practice what then happens is that charge can leak from the pixel to the substrate or from the substrate to the pixel. In the dark part of an image the amount of photons hitting the sensor is extremely small, each photon (in a perfect sensor) gets turn into an electron. It doesn't take much of a leak for enough additional electrons to seep through the hole in the insulation to the pixel and give a false, bright readout. With a very small leak, the pixel may still be useable simply be adding an offset to to the read out to account for the elevated black level. In a more severe cases the pixel will be flooded with leaked electrons and appear white, in this case the masking circuits should read out the adjacent pixel.

For a computer running with big voltage/charge swings between 1's and 0's this small leakage current is largely inconsequential, but it does not take much to upset the readout of a sensor when your only talking of a handful of electrons. CMOS sensors are easier to mask as each pixel is addressed individually and during the camera start up it is normal to scan the sensor looking for excessively "hot" pixels. In addition many CMOS sensors incorporate pixel level noise reduction that takes a snapshot of the pixels dark voltage and subtracts it from the exposed voltage to reduce noise. A side effect of this is it masks hot pixels quite effectively. Due to the way a CCD's output is pulled down through the entire sensor, masking is harder to do, so you often have to run a special masking routine to detect and mask hot pixels.

It may not sound much getting a single hot pixel, but if it's right in the middle of the frame, every time that part of your scene is not brightly illuminated you see it winking away at you and on dark scenes it will stick out like a sore thumb, thankfully masking circuits are very effective at either zeroing out the raised signal level or reading out an adjacent pixel.

Flash memory can also experience these same insulation holes. There are two common types of Flash Memory, SLC and MLC. Single Level Cells have two states, on or off. Any charge means on and no charge means off. A small amount of leakage, in the short term, would have minimal impact as it could take months or years for the cell to full discharge, even then there is a 50/50 chance that the empty cell will still be giving an accurate ouput as it may have been empty to start with. Even so, long term you could loose data and a big insulation leak could discharge a cell quite quickly. MLC or Multi Level Cells are much more problematic, as the name suggests these cells can have several states, each state defined by a specific charge range, so one cell can store several bits of data. A small leak in a MLC cell can quickly alter the state of the cell form one level to the next, corrupting the data by changing the voltage.

SxS Pro cards (blue ones) are SLC, SxS-1 (Orange cards) use MLC as MLC works out cheaper as fewer cells are required to store the same amount of data. Most consumer flash memory is MLC. So be warned, storing data long term on flash memory may not be as safe as you might think!

[Alan Emery]

Hi Allister,

Many thanks! Much easier to understand now. I especially appreciated the explanation of the camera start-up masking system for the CMOS sensor.

Alan

[Alan Emery]

Hi Alister,

As requested, I checked on the cosmic ray question some more. Here is what I found -- for purposes of clarification.

Primary galactic cosmic ray particles are composed primarly of protons, some helieum nuclei, a few electrons and some other bits. These rarely hit earth as they are deflected by earth's magnetic field or collide with the atmosphere. When they hit the atmosphere they create a shower of particles, most of which decay quickly into muons, but there are a few neutrons and other particles as well.

Muons hit earth at a frequency of about 1 muon per square cm per minute. When a muon hits a sensor it causes a discharge of electrons indistinguishable from a photon particle. A simple test with a DSLR will show this result. Turn on the camera and click the shutter in complete darkness. Then set the camera for a time exposure of about 10 minutes in complete darkness. In that time the sensor will have collected about 10 muon strikes. Some will be tiny dots others might be a streak if the muon hit obliquely. Compare the first and second exposures to be sure you are not looking at something else on the sensor. The next exposure will reset your sensor. For a video camera, the muon significance is negligble because the exposure is short (usually 1/30 sec or less).

Neutrons are a different story. They cause permanent damage as you suggested. They are much rarer, however. I found direct observation results that suggest neutron damage depends on the energy of the strike. "Minor" blemishes (not noticeable but detectable) occur about three times as frequently as "major" blemishes (which I presume would result in a winking pixel as you described). The rates are as follows: Major blemishes: 2/3 inch sensor about 1 every three years, 1/2 inch sensor about 1 in four years. 1/3 inch sensor about 1 in 7 years. Minor blemishes would be about three times as common as major blemishes. This same study showed rates in airplanes over the poles to be about 100 times as frequent -- so a very significant issue.

If your suggestion that HD sensors will receive strikes four times as frequently as SD sensors is your observation, I accept that, but I was unable to find similar observations. According to the literature I could find, neutron blemishes are dependent on area, not pixel density.

Hope that helps. Thanks again for your insight!
Alan

[Alister Chapman]

The more pixels there are the more chance there is of the particle hitting a critical part as the sensor is more densely packed. What ever the numbers, you can expect to see bright pixels crop up on a regular basis, especially if you fly. I'm back and forwards to the US on a regular basis and it plays havoc with my cameras. I had to get my old Digibeta re-programmed by Sony as it had so many damaged pixels that there wasn't enough ram to store the masking data. The re-programming sorted it out. I see hot pixels appear almost monthly on my PDW-700.

[John Cummings]

I had to retire my Sony HDCam due to too many bright pixels to hide. My HDX has travelled to 13 countries in the last two years and it is starting to show the same issues. I have no doubt that all that air travel has hastened their demise. Might be the cost of doing business, but still painful nonetheless.

[Steve Strickle]

I'd be inclined to dismiss this as folklore and nonsense, but I guess the real life data doesn't lie.
Someone needs to make a flight case to help this...what would it have to be made of?

[Don Parrish]

I respectfully have to ask this, Why does it only effect cameras ? Most of the cockpits in todays aircraft are called glass cockpits. There are radar and stormscopes and displays and I imagine a few cameras in todays modern aircraft. It would seem that if it could strike and effect a sensor it would strike and effect other things as well and have a material that could shield it. Muons often penetrate mountains, is there actually any atmosphere stopping them? If the atmosphere protects us then the cameras on the space station would be unprotected ? Hubble ?

[Pete Bauer]

Originally Posted by Steve Strickle

I'd be inclined to dismiss this as folklore and nonsense, but I guess the real life data doesn't lie.
Someone needs to make a flight case to help this...what would it have to be made of?

The lower the atomic number the better, so hydrogen is actually the best element; polyethylene is cheap, easy to work with, and contains lots of hydrogen. But from an every-day standpoint, Galactic Cosmic Radiation (GCR) can't really be shielded because it either takes impractical amounts of mass and/or significant magnetic fields to make a difference.

Originally Posted by Brian McKenna

...the CCDs on NASA's Solar Dynamics Observatory seem to be doing just fine...

That's why these probes cost hundreds of millions of dollars. You can't go to your local Fry's and get the kind of mil spec or custom hardware they use to build satellites. For non-critical purposes, the ISS -- relatively protected in low earth orbit by the magnetosphere -- uses off-the-shelf cameras that gradually suffer lost photosites and laptops that occasionally suffer unexplained bit flips. Don, aircraft avionics also are built to a more robust standard than consumer goods.

[Don Parrish]

An interesting read from NASA etc.


Muons

[Pete Bauer]

Don,
Interesting particle physics info, but for the sake of truth in adverstising it should be noted that the web site is a Principal Investigator's site, not a NASA web site.