CCD Primer

Binning
Bracket Pulsing
CCD Grading
Cosmic Rays
Dark Current
Deep Depletion CCD
Detection Modes
Dual Capacity Mode
Dual Readout Mode
Dynamic Range
Etaloning in CCDs
eXcelon CCD-EMCCD
UV Extension
Fiber Optics
Flat Fielding
Full Well Capacity
Gain
Image Calibration
Imager Architectures
Image Intensifiers
ITO CCD
Kinetics Mode
Linearity
Matching Resolution
MPP Mode
Noise Sources
On-chip Multiplication Gain
Open Poly CCD
Optical Window
PVCAM
Quantum Efficiency
Readout vs Frame Rate
Reducing Dark Current
Saturation/ Blooming
Signal to Noise Ratio
Spurious Charge
XP Cooling

 

Cosmic Rays

Liquid nitrogen cooled CCD cameras are able to integrate for up to 6 hours. Realistically integration times are reduced to about 60 minutes. Longer integrations are limited by the influence of cosmic rays. Our spectroscopy software program offers a cosmic ray filter to reduce their influence. Cosmic rays look like hot pixels and are randomly distributed over the entire image. The nature of cosmic rays makes it impossible to provide cosmic ray shielding for the camera housing or the CCD itself.

What are cosmic rays ?
Cosmic rays are subatomic particles arriving from outer space, which have high energy as a result of their rapid motion.

    The three key properties of a cosmic ray particles:

  1. Electric charge
  2. Rest mass
  3. Energy

About 87 percent of cosmic rays are protons (hydrogen nuclei) and about 12 percent are alpha particles (helium nuclei). Heavier elements are also present, but in greatly reduced numbers. For convenience, scientists divide the elements into light (lithium, beryllium and boron), medium (carbon, nitrogen, oxygen and fluorine) and heavy (the remainder of the elements). The light elements compose 0.25 percent of the cosmic rays. Because the light elements constitute only about 1 billionth of all matter in the universe, it is believed that light element cosmic rays are formed by the fragmentation of heavier cosmic rays that collide with protons, as they must do in traversing interstellar space. From the abundance of light elements in cosmic rays, it is inferred that cosmic rays have passed through the material equivalent of a layer of water 4 cm (about 1.5 in) thick. The medium elements are increased by a factor of about 10 and the heavy elements by a factor of about 100 over normal matter, suggesting that at least the initial stages of acceleration to the observed energies occur in regions enriched in heavy elements. The energy of cosmic ray particles is measured in units of giga (billion) electron volts (GeV) per proton or neutron in the nucleus. The distribution of the proton energy of cosmic rays peaks at 0.3 GeV, corresponding to a velocity two thirds that of light and falls toward higher energy, although particles up to 1011 GeV have been detected through showers of secondary particles created when they collide with atmospheric nuclei. On average, about 1 electron volt of energy per cubic centimeter of space is invested in cosmic rays in our galaxy. Even an extremely weak magnetic field deflects cosmic rays from straight line paths; a field of 3 10-6 gauss, such as is believed to be present throughout interstellar space, is sufficient to force a 1-GeV proton to gyrate with a radius of 10-6 light-year. A 1011-GeV particle gyrates with a radius of 105 light years, about the size of the galaxy. Thus, the interstellar magnetic field prevents cosmic rays from reaching the earth directly from their points of origin, accounting for the directions of arrival being isotropically distributed at even the highest energies.

Source is still not certain
The sun emits cosmic rays of low energy at the time of large solar flares, but these events are far too infrequent to account for the bulk of cosmic rays. If other stars are like the sun then they are not adequate sources either. Supernova explosions are responsible for at least the initial acceleration of a significant fraction of cosmic rays, as the remnants of such explosions are powerful radio sources, implying the presence of energetic electrons.