Some fascinating physics is associated with these unusual but simple linear arrays or white transient streaks – Muon and Cosmic Ray impacts on my astrophotography camera CCD. The following is long-winded, but if interested, read on, if not, move on ! 😉

During astrophotography we take series of ‘dark frames’ at the end of each imaging session (with the cover on the telescope to block all light). The idea is for the pixels on the CCD chip to record only electronic noise inherent in the imaging system, and no outside light sources. These ‘Dark’ frames are subtracted from each image sub-frame to cancel/negate electronic noise, so yielding a cleaner sub-frame with a higher signal/noise ratio. The evenly distributed light grey speckled pixels across an entire digital image are this electronic noise in these highly zoomed in portions of the dark frames. But what are these white streaks !

These are actually secondary cosmic rays (muons and anti-muon sub-atomic particles) impacting on the CCD chip while each 1-minute dark frame is being recorded.

Most cosmic rays are the nuclei of common atoms that are stripped of their electron shells (so a form of ionising radiation) and travelling mostly at 43% and 99.6% of the speed of light (299,792 km/second). 89% of cosmic rays are single protons (hydrogen atom sans an electron), 9% are alpha particles (helium nuclei stripped of electrons) and 1% are solitary electrons (beta particles). Some nuclei of heavier atoms also exist as cosmic rays. Some of the most energetic cosmic rays are the nuclei of heavier iron atoms travelling at close to light speed. These sub-atomic particles are accelerated to near light speed by a range of potential processes including supernova, pulsars and effects near super massive black holes in active galactic nuclei.

Every inch of the earth’s surface, you and I, and my camera CCD chip is continuously rained on by a flux of muons (electron-like subatomic particles with a mass 207 times heavier than an electron) formed by collision of cosmic rays with atoms in the upper atmosphere which fragments them and creates cascading chains of muons (secondary cosmic rays) down through the atmosphere and also travelling at relativistic velocities. Time dilation of the muons due to their relativistic velocity allows them to reach the earth’s surface in greater quantities than they would otherwise do based on their extremely short half-life. I.e time dilation slows down their half-life. Their energies are prodigious, and they can pass through several hundred metres of solid rock.

The highest energies measured for individual cosmic rays is the equivalent of a single atom nucleus impacting with the force-equivalent of a baseball impacting at 96 km/hour. This results in cosmic rays being able to pass through substantial quantities of matter. As an example, the colloquially known “Oh-My-God particle” (Google that) had 320 EeV (Exa-electronvolts) which is 40 million times more energy than the fastest particles accelerated at CERNS’s Large Hadron Particle Collider in Switzerland.

Earth’s magnetosphere protects us from most of these cosmic ray particles, but outside of this protective shell cosmic rays become increasingly hazardous to space-farers. During the Apollo mission, Buzz Aldrin and Neil Armstrong noticed strange flashes of light in their ‘closed’ eyes in the darkened realm of the space capsule, that took on a variety of shapes and dimensions, and which have since been identified as high-energy cosmic rays transferring energy to light-detecting rod and cone cells in their eyes as the atomic particles transitted their retinas. In the words of Don Pettit from the International Space Station … “In space I see things that are not there. Flashes in my eyes, like luminous dancing fairies, give a subtle display of light that is easy to overlook when I’m consumed by normal tasks. But in the dark confines of my sleep station, with the droopy eyelids of pending sleep, I see the flashing fairies.”

Cosmic rays are also responsible for many DNA mutations that drive the evolution of life, as they transit the human body and damage DNA. Such cumulative DNA damage is also linked to mutated cells that can lead to a variety of cancers. Cosmic rays affect computers in space and to a lesser degree on earth, damaging electronics at a microscopic scale, and can lead to glitches and computer memory issues. They are also responsible for the higher radiation dose experienced by commercial airline pilots who spend a lot of time at altitude. Muon tomography using detectors of the natural atmospheric muon flux have also been used to discover hidden chambers within the Great Pyramid in Giza.

Being fascinated by the cosmos, and knowing that you can detect these transient hits on your camera CCD, I scanned 50 of my dark frames for these well-known cosmic ray and muon impacts, and found 15 strikes across 50 photographic frames (each frame was exposed for a minute). This means cosmic rays and their downward-cascading muon collision products hit the CCD of my astrophotography camera at a frequency of around 1 every 3.5 minutes. The rounder impacts are more direct hits whilst the linear streaks are from grazing impacts. These particles will not have been stopped by the camera walls, they simply injected some of their energy into the pixel wells on the CCD chip as they tunnelled through the pixel architecture on the camera CCD imaging chip and then tunnelled through some 300m up to 2km of earth and rock below my pool paving where I had set up the telescope with camera for the capture of the dark frames.

Finally, there is a citizen science project currently being developed that uses globally-distributed common cell phones to detect cosmic ray and muon transits on in-phone camera chips, based on a very clever APP plus in-phone data processing. The APP will be released soon for IOS phones as part of a project to understand the directionality of cosmic rays and their muon cascades.