Fred Reines and Clyde Cowan at the Control Center of the Hanford Experiment (1953)
In 1930 Wolfgang Pauli proposed a solution to the missing energy in nuclear beta decays, namely that it was carried by a neutral particle This was in a letter to the Tubingen congress. Enrico Fermi in 1933 named the particle the "neutrino" and formulated a theory for calculating the simultaneous emission of an electron with a neutrino. Pauli received the Nobel Prize in 1945 and Fermi in 1938. The problem in detection was that the neutrinos could penetrate several light years depth of ordinary matter before they would be stopped.
In 1951 Fred Reines at Los Alamos thought about doing some real challenging physics problem. In a conversation with Clyde Cowan they decided to work on detecting the neutrino. Their first plans were to detect neutrinos emitted from a nuclear explosion. Realizing that nuclear reactors could provide a neutrino flux of 10^13 neutrinos per square centimeter per second, they instead mounted an experiment at the Hanford nuclear reactor in 1953. The Hanford experiment had a large background due to cosmic rays even when the reactor was off. The detector was then moved to the new Savannah River nuclear reactor in 1955. This had a well shielded location for the experiment, 11 meters from the reactor center and 12 meters underground.
The detection of the neutrino was as the initiator of the inverse-beta decay reaction of:
anti-neutrino + proton -> neutron + positron.
The target was water with CdCl_2 dissolved in it. The positron was detected by its slowing down and annihilating with an electron producing two 0.5 MeV gamma rays in opposite directions. The pair of gamma rays was detected in time coincidence in liquid scintillator above and below the water by photomultiplier tubes detecting the scintillation light. The neutron was also slowed by the water and captured by the cadmium microseconds after the positron capture. In the capture several gamma rays were emitted which were also detected in the scintillator as a delayed coincidence after the positron's annihilation gamma ray detection. The detector contained 200 liters of water in two tanks with up to 40 kg of dissolved CdCl_2. The water tanks were sandwiched between three scintillator layers which contained 110 5" photomultipliers each, and the whole experiment measured only about 2 meters in each direction.
At Savannah River, Reines and Cowan carried out a series of measurements to show that (see Reines' personal accounts below):
The rarity of neutrino capture is shown in their signal rate, which was about three events per hour in the entire detector. The signal to background ratio was about four to one. Thus in 1956 was born the rich and continually exciting field of experimental neutrino physics, as discussed in other articles in this newsletter. This discovery was recognized by honoring Frederick Reines with the Nobel Prize in 1995.
Original papers are:
The history is detailed in the following accounts by Fred Reines which were the sources for this article:
The history is also in the collected papers:
The most easily accessible material may be that linked to by our Physics and Astronomy Department World-Wide-Web site. The links include the excellent writeup by the Royal Swedish Academy of Sciences, historical photos, and current research projects in neutrino physics by the neutrino group.