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Neutrons and

Neutrinos

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Neutrons and Neutrinos

Nucleon – particles that can be found in the nucleus of an atom.

There are two types of nucleons:

* Protons

* Neutrons

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Neutrons

When investigating the masses of nuclei, it was determined that nuclei could not contain only protons.

To account for this observation, Rutherford proposed that there be a particle in the nucleus that had mass but no charge.

He called this particle the neutron, and imagined the neutron to be a closely paired proton and electron.

The neutron was eventually discovered by Chadwick.

Common symbol: n

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Neutrons

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Neutrinos

Predicted in 1931 by Wolfgang Pauli.

Based his prediction on the fact that energy and momentum did not appear to be conserved in certain radioactive decays. He predicted that the missing energy might be carried off, unseen, by a neutral particle which was escaping detection.

In 1934, Enrico Fermi produced a comprehensive theory of radioactivity, which included this hypothetical particle.

Fermi called the particle a neutrino (Italian for “little neutral one”).

Common symbol:

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Neutrinos

No particles were detected in the reactions where energy did not balance.

Implication: Neutrinos do NOT interact with matter very easily.

As a result, neutrinos are extraordinarily hard to detect.

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Neutrinos

First detection occurred in 1956

Utilized neutrinos (technically antineutrinos) predicted to be produced by nuclear reactions in the nuclear reactor at Savannah River, South Carolina.

Reines and Cowan experiment consisted in using a target made of around 400 liters of a mixture of water and cadmium chloride. The anti-neutrino coming from the nuclear reactor interacts with a proton of the target matter, giving a positron and a neutron. The positron annihilates with an electron of the surrounding material, giving two simultaneous photons and the neutron slows down until it is eventually captured by a cadmium nucleus, implying the emission of photons some 15 microseconds after those of the positron annihilation. All those photons are detected and the 15 microseconds identify the neutrino interaction.

underground neutrino detector in salt mine on the shore of Lake Erie

8,000 ton Water tank

[USA]

IMB

Irvine-Michigan-Brookhaven detector

The United States neutrino detector is 2000 feet underground in a salt mine near Fairport, Ohio (slightly east of Cleveland). The detector is the collaborative effort of the Proton Decay Group of the University of Michigan, the University of California (Irvine) and the Brookhaven National Laboratory. The detector is 10,000 metric tons of highly purified water. In this pool are 2048 extremely sensitive light-detecting photo multiplier tubes. These tubes uniformly cover the walls, floor and ceiling of the totally enclosed pool of ultra-pure water that measures approximately 80' x 70' x 70'.

A neutrino travels through water faster than light travels through water. This gives rise to an optical shock wave (analogous to a sound wave's sonic boom) that is perceived as a blue light, so-called Cerenkov radiation. The array of photo multiplier tubes senses this light and a sophisticated computer system quantifies the amount of light, its location within the tank and the time that the light flashes occurred. Physicists then interpret the meaning of the observed light patterns. The entire complex is 2,000 feet underground so that the mass of the earth shields the detector from stray cosmic, as well as earth-born radiation.

IMB

Irvine-Michigan-Brookhaven detector

IMB is an acronym for a neutrino observatory located under Lake Erie. It is run by a group of American institutions headed by the University of California at Irvine, the University of Michigan, and the Brookhaven National Laboratory (hence the acronym).

IMB consists of a roughly cubical tank about 20 meters on a side, full of water and surrounded by 2048 photomultiplier tubes. IMB detects neutrinos by picking up the Cerenkov radiation generated when a neutrino collides with either a proton or an electron (both of which are plentiful in water). IMB is thus able to estimate the direction of the neutrino by analyzing the spatial arrangement of the tubes that detected radiation.

The efficiency of IMB is quite low: if 100 trillion neutrinos pass through the detector, on average only one will be detected.

IMB

Irvine-Michigan-Brookhaven detector

IMB

Irvine-Michigan-Brookhaven detector

n p + e +

Beta () Decay

The neutron is not a stable particle. A neutron will decay into a proton and what was called a beta particle.

The particle is now known to be an electron

In this process, an anitneutrino is emitted.

n p + e +

Beta () Decay

The neutron is not a stable particle. A neutron will decay into a proton and what was called a beta particle.

The particle is now known to be an electron

Gives credibility to Rutherford’s hypothesis that the neutron was, in fact, a tightly bound proton and electron. This explains the numbers in the table in slide 4 (previous).

Electron Capture

A proton can also capture an electron, forming a neutron while emitting a neutrino.

p + e n +

Radioactive Decay

Atomic Number (Z): Number of protons in the nucleus.

Atomic Mass Number (A): Number of nucleons in the nucleus.

A nucleon can be either a proton or a neutron.

Chemical elements: AZ E

Beta () Particle Notation

In nuclear processes, the particle is symbolized as

The electron ( particle) is NOT a nucleon (A = 0) and it is the opposite (electrically) of a proton (Z = -1).

0 –1

Radioactive Decay

emission: The beta particle is an electron ( 0-1 )

AZ X A

Z+1 Y + 0-1

In beta decay, a neutron in the nucleus decays into a proton and an electron. The electron stays in the nucleus because of the strong force, the electron drifts away from the nucleus.

The daughter nucleus has moved up one position on the periodic table.