9 The Neutrino is Discovered
65
process, release nuclear energy which gets converted to heat and subsequently
to electricity. The fission products go through subsequent decays. Many of
the decays are beta decays that according to Fermi’s theory produce antineutrinos by the reaction such as n → p + e
−
+ ¯
ν e . These neutrinos are
quite numerous. For example, in 240 grams of uranium, there are about ∼10
23
atoms and they result in many neutrinos after beta decay. In a one-gigawatt
nuclear reactor, about 2 × 10
20 (about two hundred million trillion) neutrinos
are emitted per second. They come out with different energies and can be
used to undergo inverse beta decay for their detection. The detection process
involves the reverse of the beta decay process ¯
ν e +p → e
+
+n, as suggested by
Pontecorvo in 1946. Both the positron (e
+ ) and neutron produced in inverse
beta decay give spectacular gamma-ray signals, which can be detected and
confirm a neutrino reaction event.
The first place chosen by Reines and Cowan was the Hanford reactor in
the state of Washington. However, an impediment to the plan of Reines and
Cowan was the cosmic ray background at Hanford, which produced particles
which would look like they came from the neutrino reaction, confusing the
whole situation. Thus, Hanford did not fill the bill for neutrino detection. So
they had to find another site for their experiment. They found that a more
ideal place would be the new Savanna River reactor in the state of Georgia,
where the cosmic ray background could be shielded out much better. Savanna
River reactor was about a four and a half hours’ drive from Atlanta. There,
they decided to look for the anti-neutrinos emitted by the so-called P reactor to
cause the inverse beta decay. They were successful in their effort and discovered
this reaction and the anti-neutrino in 1956. A particle that has been in the
mind of theorists for more than two and half decades was finally detected.
Reines immediately sent a telegram to Pauli informing him of their discovery.
Pauli responded saying “Everything comes to those who know how to wait”
(see Fig. 9.2). So began the saga of the neutrino, with a busy half-century
ahead of discovering its properties and uncovering the detailed nature of its
participation in the weak force. There was tremendous sense of excitement in
the air.
9.3 The Years That Followed the Neutrino
Discovery
The discovery of the neutrino was a relief and at the same time groundbreaking. It was a relief since it rescued the sacred principle of energy
conservation in physics. It was groundbreaking because it led not only to
65
process, release nuclear energy which gets converted to heat and subsequently
to electricity. The fission products go through subsequent decays. Many of
the decays are beta decays that according to Fermi’s theory produce antineutrinos by the reaction such as n → p + e
−
+ ¯
ν e . These neutrinos are
quite numerous. For example, in 240 grams of uranium, there are about ∼10
23
atoms and they result in many neutrinos after beta decay. In a one-gigawatt
nuclear reactor, about 2 × 10
20 (about two hundred million trillion) neutrinos
are emitted per second. They come out with different energies and can be
used to undergo inverse beta decay for their detection. The detection process
involves the reverse of the beta decay process ¯
ν e +p → e
+
+n, as suggested by
Pontecorvo in 1946. Both the positron (e
+ ) and neutron produced in inverse
beta decay give spectacular gamma-ray signals, which can be detected and
confirm a neutrino reaction event.
The first place chosen by Reines and Cowan was the Hanford reactor in
the state of Washington. However, an impediment to the plan of Reines and
Cowan was the cosmic ray background at Hanford, which produced particles
which would look like they came from the neutrino reaction, confusing the
whole situation. Thus, Hanford did not fill the bill for neutrino detection. So
they had to find another site for their experiment. They found that a more
ideal place would be the new Savanna River reactor in the state of Georgia,
where the cosmic ray background could be shielded out much better. Savanna
River reactor was about a four and a half hours’ drive from Atlanta. There,
they decided to look for the anti-neutrinos emitted by the so-called P reactor to
cause the inverse beta decay. They were successful in their effort and discovered
this reaction and the anti-neutrino in 1956. A particle that has been in the
mind of theorists for more than two and half decades was finally detected.
Reines immediately sent a telegram to Pauli informing him of their discovery.
Pauli responded saying “Everything comes to those who know how to wait”
(see Fig. 9.2). So began the saga of the neutrino, with a busy half-century
ahead of discovering its properties and uncovering the detailed nature of its
participation in the weak force. There was tremendous sense of excitement in
the air.
9.3 The Years That Followed the Neutrino
Discovery
The discovery of the neutrino was a relief and at the same time groundbreaking. It was a relief since it rescued the sacred principle of energy
conservation in physics. It was groundbreaking because it led not only to