CAPTURING COSMIC RAYS WITH A DIGITAL CAMERA

CAPTURING COSMIC RAYS WITH A DIGITAL CAMERA

Rudy E. Kokich

Cosmic Rays are extremely energetic subatomic particles which fly through the universe at speeds approaching the speed of light. They were discovered in the early 1900's by Wolf, Pacini, Hess, and Millikan, who carried out extensive research on ionizing radiation using shielded electrometers, which measure electric charge on a metal plate or a ball. They showed that ionizing radiation was present deep under ground and under water, that its levels were much higher at higher altitudes, and that the levels varied at different latitudes on the Earth's surface. The results suggested that this radiation originated in outer space, and was composed of charged particles which are influenced by the Earth's magnetic field. The extreme energy of this radiation was demonstrated by the fact that it penetrated without significant attenuation to electrometers enclosed in thick metal containers.

With the development in subsequent decades of high altitude air travel and space travel, research on cosmic rays, and their effects on the human body and delicate electronic circuits became ever more important. By comparing ground-based and space-based research, it was confirmed that cosmic rays are not electromagnetic radiation, but subatomic particles which can be divided into two types: primary and secondary.

Primary cosmic rays are charged subatomic particles arriving from outer space at nearly light speed velocities. 88% are protons, or nuclei of Hydrogen. 10% are nuclei of Helium composed of two protons and two neutrons, also known as alpha particles. 1% are nuclei of heavier, more complex elements in the periodic table, present in approximately the same relative abundance as in the solar system. And 1% are isolated electrons. It is not known why electrons are markedly less efficiently accelerated than atomic nuclei.

In particle physics, energy is measured in electronvolts (eV). One eV, equal to approximately 1.6 x 10^-19 joules (J), is the energy change on a single electron as it moves across the electrical potential difference of 1 Volt. On the average, a primary cosmic ray particle carries the energy of 4.8 x 10^-11 J, which corresponds to 3 x 10⁸ eV, or to the velocity of 2/3 the speed of light. However, few particles move at nearly the speed of light, and carry the energy of 3 x 10²⁰ eV, which is - incredibly - sufficient to light a 100W light bulb for 1 second. This is also equivalent to the kinetic energy of a baseball flying at 100 miles per hour. By comparison, the highest energy particles produced by the upgraded Large Hadron Collider are in the range of 13 x 10¹² eV, some 23 million times weaker. Such Very High Energy Cosmic Rays are fortunately very rare, with the estimated incidence of only a single event per square kilometer of Earth's surface per century.

Main sources of the primary cosmic ray particles are presumed to be shock waves from supernova events and magnetic fields around neutron stars, pulsars, and accretion discs around black holes in active galactic centers. Since the particles are charged, and deviated by strong galactic, interplanetary, and planetary magnetic fields, the direction of the original source can not be directly determined. The Sun also serves as an intermittent source of relatively low energy cosmic ray nuclei and electrons, which are accelerated by the shock waves in the solar corona and by solar flares. During periods of high solar activity, local density of charged particles may increase between 100 and 1 million times, and last up to several days.

When primary cosmic rays enter the Earth's atmosphere, they collide with atoms and molecules in the air resulting in Secondary Cosmic Rays: showers of high energy subatomic fragments which in turn decay into other particles and gamma rays. At the Earth's surface, the most common secondary particles are Muons, negatively charged Leptons, similar to electrons, but with 207 times greater rest mass. Only 3% of the cosmic rays on the surface are primary cosmic rays.

Muons are thought to be generated with the mean energy of 6 GeV (6 x 10⁹ eV) at the altitude of 15,000 meters. They interact very little physically with ordinary matter but, since they carry a negative charge, they do lose energy by ionizing atoms near which they pass. The longer the path through a material, the greater the energy loss. At sea level, where they constitute about half the natural background radiation, the mean energy of muons is around 4 GeV. This allows them to penetrate deep under water, and over 700 meters under ground. To fully shield from most secondary cosmic rays would require a wall of iron 2 meters, or 6 feet thick.

Like other elementary particles, the muon can exist as ordinary matter: called negative muon, or antimatter: called antimuon or positive muon. Positive muons decay 100% of the time into positrons, antineutrinos, and neutrinos. However, negative muons of ordinary matter may follow one of two paths. Some decay into electrons, neutrinos, and antineutrinos, while others may rejoin matter by capture into very tight orbits around atomic nuclei where they combine with a proton to produce a neutron and a neutrino.

Muons are unstable elementary particles with a half-life of 2.2 µs (microseconds). This means that every 2.2 millionths of a second half of all the muons spontaneously decay into more stable particles. During that time light covers only 660 meters. Virtually no muons would survive long enough to traverse the 15,000 meter trip through the Earth's atmosphere were it not for the relativistic effects of time dilation. Due to their velocity, which is near the speed of light, time on a muon passes about 40 times more slowly than in our frame of reference, and its half-life is proportionately longer. This allows it a sufficient life span to travel through the atmosphere.

Muons can be detected with cloud chambers, Geiger counters, and scintillator detectors, but can also be recorded with common digital camera CCD and CMOS chips which are sensitive to charged particles. Muon flux at the surface of the Earth averages approximately 1 particle per square centimeter per minute. The surface area of the APS-C camera sensor (22.3 x 14.9 mm) is 3.3 cm², which means that we can expect on average 3 muon strikes on the sensor during a 1 minute exposure.

The following methods were used to obtain images listed below:

- Camera: Canon T3i, lens removed and replaced with a light-tight body cap.

- Camera set to BULB for long exposures, with remote control shutter switch attached.

- Sensitivity set to ISO 1600. Higher sensitivities result in too much thermal noise. Less sensitivity results in less distinctive muon trails.

- Image quality in camera's Menu set to maximum resolution 18M (5184x3456), JPG format.

- In camera's Menu, go to Custom Functions, C.FnII:Image, turn on Long exp. noise reduction. This feature automaticaly takes an appropriate dark exposure to eliminate defective pixels in the original photo. In spite of this, some isolated defective or hot pixels may still appear on long exposure photos at high sensitivity settings. However, it is statistically extremely unlikely for several defective pixels to appear in a cluster or a row, adjacent to one another.

- On the Canon T3i camera, turning off Live View before taking a long exposure eliminates amplifier glow.

Examples of Amplifier Glow, and Defective or Hot Pixels and Thermal Noise on magnified images

- Since most cosmic rays arrive at high angles to the horizon, the camera is placed on its back so that the CMOS sensor is horizontal, and maximally exposed to the cosmic ray flux.

- The length of exposure should be 1 to 3 minutes. Longer exposures result in more thermal noise. Shorter exposures are less likely to capture cosmic rays. Keep in mind that the camera will then automatically take a "dark" exposure of equal duration.

- Images, which appear entirely black at low magnification, are magnified about 5x (approximately 20% crop) with image processing software, slowly scanned, and examined for "dots", "clusters", and "streaks". These are further magnified to reveal muon trails similar to those shown below.

-Image processing software used here for magnification and cropping was XnView, but a number of other excellent freeware programs are suitable.

-Pixel "clusters" are caused by muons striking the sensor at high angles, while "streaks" are created by muons striking at lower angles. There is no statistical confidence that single, isolated, illuminated pixels, or "dots", are caused by muon collisions rather than residual defective or "hot" CMOS pixels. Isolated "dots" should be excluded from the muon count.

Several interesting cosmic ray studies are available to amateurs and students.

The camera may be placed upright, with the sensor in the vertical position, which should, in theory, result in more "streaks", and fewer muon trails overall.

The camera may be covered with a thick metal pot, which should not result in the decrease in the muon flux since muons readily pass through matter.

Daytime and night-time muon flux may be compared during periods of known high and low solar activity.

It is reasonable to assume that the total number of illuminated pixels in a "cluster" or a "streak" is proportional to the energy level of the incident muon. With data from a large number of muon trails it is possible to draw a histogram with the number of illuminated pixels on the X-axis, and the number of muon trails on the Y-axis. Such a histogram would reflect the distribution of relative muon energy under the given experimental conditions.

There are several venues where tourists may visit deep under ground. One is the nickel mine in Sudbury, Ontario. Another is Ruby Falls in Chattanooga, Tennessee. If an opportunity presents to take measurements, muon flux and mean energy should be significantly lower deep under ground than on the surface.

Conversely, taking measurements during high altitude airplane flights should show more numerous and more energetic muon trails and, possibly, an occasional primary cosmic ray trail.