Transcript More Nuclear Physics - University of Dayton

More Nuclear Physics
Neutrons and
Neutrinos
More Nuclear Physics
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
More Nuclear Physics
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: ν (Greek letter nu)
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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.
IMB
Irvine-Michigan-Brookhaven detector
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 lightdetecting 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
Beta (β) Decay
The neutron is not a stable particle. A neutron will decay into a proton and
what was called a beta particle.
The β particle is now known to be an electron
n  p + e +ν
In this process, an anitneutrino is emitted.
Beta (β) Decay
The neutron is not a stable particle. A neutron will decay into a proton and
what was called a beta particle.
The β particle is now known to be an electron
n  p + e +ν
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+ν
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:
A E
Z
Beta (β) Particle Notation
In nuclear processes, the β particle is symbolized as
0
–1
β
The electron (β particle) is NOT a nucleon (A = 0) and it is the opposite
(electrically) of a proton (Z = -1).
Radioactive Decay
β emission: The beta particle is an electron ( 0-1 )
A
ZX

In beta decay, a neutron in the nucleus decays into a proton and an electron. The
proton stays in the nucleus because of the strong force, the electron drifts away
from the nucleus.
Radioactive Decay
β emission: The beta particle is an electron ( 0-1 )
A
ZX

?
?Y
+
0
-1
+ photon
In beta decay, a neutron in the nucleus decays into a proton and an electron. The
proton stays in the nucleus because of the strong force, the electron drifts away
from the nucleus.
Radioactive Decay
β emission: The beta particle is an electron ( 0-1 )
A
ZX
A

Z+1 Y
+
0
-1
+ photon
In beta decay, a neutron in the nucleus decays into a proton and an electron. The
proton stays in the nucleus because of the strong force, the electron drifts away
from the nucleus.
Radioactive Decay
β emission: The beta particle is an electron ( 0-1 )
A
ZX
A

Z+1 Y
+
0
-1
+ photon
In beta decay, a neutron in the nucleus decays into a proton and an electron. The
proton 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.
Radioactive Decay
β emission: The beta particle is an electron ( 0-1 )
A
ZX
A

Z+1 Y
+
0
-1
+ photon
Note: if you are monitoring the radiation provided by a gas of made
up of the parent element, the amount of radiation will be controlled by
the radioactive decay rate.
Radioactive Decay
Amount of
element X
Luminosity
Exponential
Decay
Exponential
Decay
Time
Time