The last two subjects I want to briefly cover are also
usually classified as high-energy astrophysics, and that is cosmic ray
and neutrino astrophysics. First, cosmic rays (liberally borrowed from
"Cosmic rays are high energy charged particles, originating in outer space, that travel at nearly the speed of light and strike the Earth from all directions. Most cosmic rays are the nuclei of atoms, ranging from the lightest to the heaviest elements in the periodic table. Cosmic rays also include high energy electrons, positrons, and other subatomic particles. The term "cosmic rays" usually refers to galactic cosmic rays, which originate in sources outside the solar system, distributed throughout our Milky Way galaxy. However, this term has also come to include other classes of energetic particles in space, including nuclei and electrons accelerated in association with energetic events on the Sun (called solar energetic particles), and particles accelerated in interplanetary space."
Discovery and Early Research: Cosmic rays were discovered in 1912 by Victor Hess, when he found that an electroscope discharged more rapidly as he ascended in a balloon. He attributed this to a source of radiation entering the atmosphere from above, and in 1936 was awarded the Nobel prize for his discovery.
For some time it was believed that the radiation was electromagnetic in nature (hence the name cosmic "rays"), and some textbooks still incorrectly include cosmic rays as part of the electromagnetic spectrum. However, during the 1930's it was found that cosmic rays must be electrically charged because they are affected by the Earth's magnetic field.
From the 1930s to the 1950s, before man-made
particle accelerators reached very high energies, cosmic rays served as
a source of particles for high energy physics investigations, and led
to the discovery of subatomic particles that included the positron and
muon. Although these applications continue, since the dawn of the space
age the main focus of cosmic ray research has been directed towards
astrophysical investigations of where cosmic rays originate, how they
get accelerated to such high velocities, what role they play in the
dynamics of the Galaxy, and what their composition tells us about
matter from outside the solar system. To measure cosmic rays directly,
before they have been slowed down and broken up by the atmosphere,
research is carried out by instruments carried on spacecraft and high
altitude balloons, using particle detectors similar to those used in
nuclear and high energy physics experiments.
Cosmic Ray Energies and Acceleration: The energy of cosmic rays is usually measured in units of MeV, for mega-electron volts, or GeV, for giga-electron volts. (One electron volt is the energy gained when an electron is accelerated through a potential difference of 1 volt). Most galactic cosmic rays have energies between 100 MeV (corresponding to a velocity for protons of 43% of the speed of light) and 10 GeV (corresponding to 99.6% of the speed of light). The number of cosmic rays with energies beyond 1 GeV decreases by about a factor of 50 for every factor of 10 increase in energy. Over a wide energy range the number of particles per m2 per steradian per second with energy greater than E (measured in GeV) is given approximately by N(>E) = k(E + 1)-a, where k ~ 5000 per m2 per steradian per second and a ~1.6. The highest energy cosmic rays measured to date have had more than 1020 eV, equivalent to the kinetic energy of a baseball traveling at approximately 100 mph!
It is believed that most galactic cosmic rays derive
their energy from supernova explosions, which occur approximately once
every 50 years in our Galaxy. To maintain the observed intensity of
cosmic rays over millions of years requires that a few percent of the
more than 1051 ergs released in a typical supernova
explosion be converted to cosmic rays. There is considerable evidence
that cosmic rays are accelerated as the shock waves from these
explosions travel through the surrounding interstellar gas. The energy
contributed to the Galaxy by cosmic rays (about 1 eV per cm3)
is about equal to that contained in galactic magnetic fields, and in
the thermal energy of the gas that pervades the space between the
Cosmic Ray Composition: Cosmic rays include
essentially all of the elements in the periodic table; about
89% of the nuclei are hydrogen (protons), 10% helium, and about 1%
heavier elements. The common heavier elements (such as carbon, oxygen,
magnesium, silicon, and iron) are present in about the same relative
abundances as in the solar system, but there are important differences
in elemental and isotopic composition that provide information on the
origin and history of galactic cosmic rays. For example there is a
significant overabundance of the rare elements Li, Be, and B produced
when heavier cosmic rays such as carbon, nitrogen, and oxygen fragment
into lighter nuclei during collisions with the interstellar gas. The
isotope 22Ne is also overabundant, showing that the
nucleosynthesis of cosmic rays and solar system material have differed.
Electrons constitute about 1% of galactic cosmic rays. It is not known
why electrons are apparently less efficiently accelerated than nuclei.
Cosmic Rays in the Galaxy: Because cosmic
rays are electrically charged they are deflected by magnetic fields,
and their directions have been randomized, making it impossible to tell
where they originated. However, cosmic rays in other regions of the
Galaxy can be traced by the electromagnetic radiation they produce.
Supernova remnants such as the Crab Nebula are known to be a source of
cosmic rays from the radio synchrotron radiation emitted by cosmic ray
electrons spiraling in the magnetic fields of the remnant. In addition,
observations of high energy (10 MeV - 1000 MeV) gamma rays resulting
from cosmic ray collisions with interstellar gas show that most cosmic
rays are confined to the disk of the Galaxy, presumably by its (micro-Gauss)
field. Similar collisions of cosmic ray nuclei produce lighter nuclear
fragments, including radioactive isotopes such as 10Be,
which has a half-life of 1.6 million years. The measured amount of 10Be
in cosmic rays implies that, on average, cosmic rays spend about 10
million years in the Galaxy before escaping into inter-galactic space.
Very High Energy Cosmic Rays: When high
energy cosmic rays undergo collisions with atoms of the upper
atmosphere, they produce a cascade of "secondary" particles that shower
down through the atmosphere to the Earth's surface. Secondary cosmic
rays include pions (which quickly decay to produce muons, neutrinos and
gamma rays), as well as electrons and positrons produced by muon decay
and gamma ray interactions with atmospheric atoms. The number of
particles reaching the Earth's surface is related to the energy of the
cosmic ray that struck the upper atmosphere. Cosmic rays with energies
beyond 1014 eV are studied with large "air shower" arrays of
detectors distributed over many square kilometers that sample the
particles produced. The frequency of air showers ranges from about 100
per m2 per year for energies >1015 eV to only
about 1 per km2 per century for energies beyond 1020
eV. Cosmic ray interaction products such as neutrinos are also studied
by large detectors placed deep in underground mines or under water.
Most secondary cosmic rays reaching the Earth's
surface are muons, with an average intensity of about 100 per m2
per second. Although thousands of cosmic rays pass through our bodies
every minute, the resulting radiation levels are relatively low,
corresponding, at sea level, to only a few percent of the natural
background radiation. However, the greater intensity of cosmic rays in
outer space is a potential radiation hazard for astronauts, especially
when the Sun is active, and interplanetary space may suddenly be filled
with solar energetic particles. Cosmic rays are also a hazard to
electronic instrumentation in space; impacts of heavily-ionizing cosmic
ray nuclei can cause computer memory bits to "flip" or small
microcircuits to fail.
One big goal of future missions (e.g., Pamela) is to find
anti-matter CRs which could be produced using rather exotic means:
Why high-energy neutrino astronomy?
The reason for high-energy neutrino astronomy is to open up all
wavelengths for astronomy and to peer into sources that would
be opaque to photons and protons.
At high energies photons (gamma-rays) and protons are not viable probes.
With a high-energy neutrino detector we can open up a new window on the Universe. New windows have usually meant new discoveries.The fundamental scientific motivation for high-energy neutrino astronomy is that of transparency, as compared to photons and CRs:
A measurement of the flux, energy spectrum, angular
timing of high-energy neutrinos is a fundamental observation of the
But those were too difficult. Nowadays it is the detection of the interaction and interaction products. When a neutrino interacts with a proton or neutron (or an electron) it will produce a cascade of particles. This cascade of particles will generally build up to a substantial number of particles and then die out on the scale of 10 meters. If it is a charged-current reaction,
These particles and particle cascades, when in water (or ice), will produce a lot of light primarily through the Cerenkov effect. This Cherenkov light comes out as a cone-shaped shock wave at about 40-degrees from the particles' trajectory.
The detector concept is an array of optical modules (OMs) which detects the time of arrival (to roughly one nanosecond) and intensity of the Cherenkov light. By comparing the arrival timing and intensity of this light one can attempt to reconstruct the event geometry and energy deposition. Amanda:
IceCube is the follow-on experiment:
You can even look at the "spectrum" (19 events!):