Neutrino
A
neutrino (
Italian pronunciation: [neuˈtriːno], meaning "small neutral one";
English pronunciation: /njuːˈtriːnoʊ/) is an
elementary particle that usually travels close to the
speed of light, is
electrically neutral, and is able to pass through ordinary
matteralmost undisturbed. This makes neutrinos extremely difficult to detect. Neutrinos have a very small, but nonzero
rest mass. They are denoted by the Greek letter ν (
nu).
Neutrinos are similar to the more familiar
electron, with one crucial difference: neutrinos do not carry electric charge. Because neutrinos are electrically neutral, they are not affected by the electromagnetic forces which act on electrons. Neutrinos are affected only by a "weak" sub-atomic force of much shorter range than electromagnetism, and are therefore able to pass through great distances in matter without being affected by it. As neutrinos have mass, they also interact gravitationally with other massive particles. Gravity, however, is by far the weakest of the four known forces.
[1]Most neutrinos passing through the Earth emanate from the Sun. Every second, in the region of the Earth, about 65
billion (
6.5×1010)
solar neutrinos pass through every square centimeter perpendicular to the direction of the sun.
[2]
Properties and reactions
The neutrino has half-integer
spin (½ħ) and is therefore a
fermion. Neutrinos interact primarily through the
weak force. The discovery of
neutrino flavor oscillations implies that neutrinos have mass. The existence of a neutrino mass strongly suggests the existence of a tiny neutrino magnetic moment
[11] of the order of
10−19 μB, allowing the possibility that neutrinos may interact electromagnetically as well. An experiment done by
C. S. Wu at
Columbia University showed that neutrinos always have left-handed
chirality.
[citation needed]It is very hard to uniquely identify neutrino interactions among the natural background of radioactivity. For this reason, in early experiments a special reaction channel was chosen to facilitate the identification: the interaction of an antineutrino with one of the hydrogen nuclei in the water molecules. A hydrogen nucleus is a single proton, so simultaneous nuclear interactions, which would occur within a heavier nucleus, don't need to be considered for the detection experiment. Within a
cubic metre of water placed right outside a nuclear reactor, only relatively few such interactions can be recorded, but the setup is now used for measuring the reactor's plutonium production rate.
Neutrino sources
[edit]Artificially produced neutrinos
Nuclear reactors are the major source of human-generated neutrinos. Anti-neutrinos are made in the beta-decay of neutron-rich daughter fragments in the fission process. Generally, the four main isotopes contributing to the anti-neutrino flux are
235U,
238U,
239Pu and
241Pu (i.e. the anti-neutrinos emitted during
beta-minus decay of their respective fission fragments). The average nuclear fission releases about
200 MeV of energy, of which roughly 4.5% (or about
9 MeV)
[34] is radiated away, as anti-neutrinos. For a typical nuclear reactor with a thermal power of
4,000 MW, meaning that the core produces this much heat, and an electrical power generation of
1,300 MW, the total power production from fissioning atoms is actually
4,185 MW, of which
185 MW is radiated away as anti-neutrino radiation and never appears in the engineering. This is to say,
185 MW of fission energy is
lost from this reactor and does not appear as heat available to run turbines, since the anti-neutrinos penetrate all building materials essentially tracelessly, and disappear.
[35] The anti-neutrino energy spectrum depends on the degree to which the fuel is burned (plutonium-239 fission anti-neutrinos on average have slightly more energy than those from uranium-235 fission), but in general, the
detectable anti-neutrinos from fission have a peak energy between about 3.5 and
4 MeV, with a maximal energy of about
10 MeV.
[36] There is no established experimental method to measure the flux of low energy anti-neutrinos. Only anti-neutrinos with an energy above threshold of
1.8 MeV can be uniquely identified (see
neutrino detection below). An estimated 3% of all anti-neutrinos from a nuclear reactor carry an energy above this threshold. An average nuclear power plant may generate over
1020 anti-neutrinos per second above this threshold, and a much larger number which cannot be seen with present detector technology.
Some
particle accelerators have been used to make neutrino beams. The technique is to smash
protons into a fixed target, producing charged
pions or
kaons. These unstable particles are then magnetically focused into a long tunnel where they decay while in flight. Because of the
relativistic boost of the decaying particle the neutrinos are produced as a beam rather than isotropically. Efforts to construct an accelerator facility where neutrinos are produced through
muon decays are ongoing.
[37]Such a setup is generally known as a
neutrino factory.
High energy cosmic neutrinos
The energy of supernova neutrinos ranges from a few to several tens of MeV. However, the sites where
cosmic rays are accelerated are expected to produce neutrinos that are at least one million times more energetic, produced from turbulent gaseous environments left over by supernova explosions: the
supernova remnants. The origin of the cosmic rays was attributed to supernovas by
Walter Baade and
Fritz Zwicky; this hypothesis was refined by
Vitaly L. Ginzburg and
Sergei I. Syrovatskywho attributed the origin to supernova remnants, and supported their claim by the crucial remark, that the cosmic ray losses of the Milky Way is compensated, if the efficiency of acceleration in supernova remnants is about 10 percent. Ginzburg and Syrovatskii's hypothesis is supported by the specific mechanism of "shock wave acceleration" happening in supernova remnants, which is consistent with the original theoretical picture drawn by of
Enrico Fermi, and it is receiving support from observational data. The very high energy neutrinos are still to be seen, but this branch of neutrino astronomy is just in its infancy. The main existing or forthcoming experiments that aim at observing very high energy neutrinos from our galaxy are Baikal,
AMANDA,
IceCube, Antares, NEMO and
Nestor. Related information is provided by very high energy
gamma rayobservatories, such as
HESS and
MAGIC. Indeed, the collisions of cosmic rays are supposed to produce charged pions, whose decay give the neutrinos, and also neutral pions, whose decay give gamma rays: the environment of a supernova remnant is transparent to both types of radiation.
Still higher energy neutrinos, resulting from the interactions of extragalactic cosmic rays, could be observed with the
Pierre Auger Observatory or with the dedicated experiment named
ANITA.
Neutrino detection
Because neutrinos are very weakly interacting, neutrino detectors must be very large in order to detect a significant number of neutrinos. Neutrino detectors are often built underground in order to isolate the detector from
cosmic rays and other background radiation.
Antineutrinos were first detected in the 1950s near a nuclear reactor.
Reines and
Cowan used two targets containing a solution of cadmium chloride in water. Two scintillation detectors were placed next to the cadmium targets. Antineutrinos with an energy above the threshold of
1.8 MeV caused charged current interactions with the protons in the water, producing positrons and neutrons. The resulting positron annihilations with electrons created photons with an energy of about
0.5 MeV. Pairs of photons in coincidence could be detected by the two scintillation detectors above and below the target. The neutrons were captured by cadmium nuclei resulting in gamma rays of about
8 MeV that were detected a few microseconds after the photons from a positron annihilation event.
Sinar beta merupakan radiasi partikel bermuatan negatif. Sinar beta merupakan berkas elektron yang berasal dari inti atom. Partikel beta yang bemuatan -1e dan bermassa 1/836 sma. Karena sangat kecil, partikel beta dianggap tidak bermassa sehingga dinyatakan dengan notasi 
. Energi sinar beta sangat bervariasi, mempunyai daya tembus lebih besar dari sinar alfa tetapi daya pengionnya lebih lemah. Sinar beta paling energetik dapat menempuh sampai 300 cm dalam udara kering dan dapat menembus kulit.
Sinar gamma adalah radiasi elektromagnetek berenergi tinggi, tidak bermuatan dan tidak bermassa. Sinar γ dinyatakan dengan notasi 
. Sinar gamma mempunyai daya tembus. Selain sinar alfa, beta, gamma, zat radioaktif buatan juga ada yang memancarkan sinar X dan sinar Positron. Sinar X adalah radiasi sinar elektromagnetik.
