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NEUTRINO DETECTOR (Autumn 1955)

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NEUTRINO DETECTOR (Autumn 1955) GLORIOUS EXPERIMENTS IN PHYSICS Luigi Di Lella Physics Department, University of Pisa, Italy WARNING The experiments discussed in these lectures represent only a small sample of “glorious experiments”. This is a personal choice based on the following criteria: . Impact on the understanding of Weak Interactions and Neutrinos . Clever experimental ideas and methods 24th Indian - Summer School of Physics, Prague, September 2012 Experiments discussed in these lectures: . First direct neutrino detection at a location far from production . Observation of parity violation in the π µ e decay chain . Precise measurements of the muon magnetic anomaly (g – 2) . Measurement of the neutrino helicity . Discovery of the second neutrino . Discovery of the violation of CP symmetry . First observation of neutrino Neutral-Current interactions . First observation of production and decay of the weak intermediate bosons W± and Z A short historical introduction to each experiment will be included. First neutrino detection Historical introduction A puzzle in β – decay: the continuous electron energy spectrum First measurement by Chadwick (1914) 210 Radium E: Bi83 (a radioactive isotope produced in the decay chain of 238U) [ If β − decay is (A, Z) → (A, Z+1) + e–, then the emitted electron is mono-energetic ] Several solutions to the puzzle proposed before the 1930’s (all wrong), including violation of energy conservation in β – decay December 1930: public letter sent by W. Pauli to a physics meeting in Tübingen Zürich, Dec. 4, 1930 Dear Radioactive Ladies and Gentlemen, ...because of the “wrong” statistics of the N and 6Li nuclei and the continuous β-spectrum, I have hit upon a desperate remedy to save the law of conservation of energy. Namely, the possibility that there could exist in the nuclei electrically neutral particles, that I wish to call neutrons, which have spin ½ and obey the exclusion principle ..... The mass of the neutrons should be of the same order of magnitude as the electron mass and in any event not larger than 0.01 proton masses. The continuous β-spectrum would then become understandable by the assumption that in β-decay a neutron is emitted in addition to the electron such that the sum of the energies of the neutron and electron is constant. ....... For the moment, however, I do not dare to publish anything on this idea ...... So, dear Radioactives, examine and judge it. Unfortunately I cannot appear in Tübingen personally, since I am indispensable here in Zürich because of a ball on the night of 6/7 December. .... W. Pauli 14 6 . “Wrong” statistics of the N7 and Li3 nuclei: in 1930 (before the neutron discovery by Chadwick in 1932) nuclei (A,Z) were believed to consist of A protons and A-Z electrons 14 6 N7 and Li3 nuclei were believed to consist of 21 and 9 fermions, respectively expect half – integer spin but measure integer value . Pauli’s “neutron” is a light particle . As everybody else at that time, Pauli believed that if radioactive nuclei emit particles, these particles must exist in the nuclei before emission Theory of β-decay (E. Fermi, 1932-33) β− decay: n → p + e− + ν + + 14 14 + β decay: p → n + e + ν (e.g., O8 → N7 + e + ν) ν: the particle proposed by Pauli (named “neutrino” by Fermi) ν: its antiparticle (antineutrino) Fermi’s theory: a point interaction among four spin ½ particles, using the mathematical formalism of creation and annihilation operators invented by Jordan ⇒ particles emitted in β – decay need not exist before emission – they are “created” at the instant of decay Prediction of β – decay rates and electron energy spectra as a function of only one parameter: Fermi coupling constant GF (determined from experiments) Energy spectrum dependence on neutrino mass µ (from Fermi’s original article, published in German in Zeitschrift für Physik, following rejection of the English version by Nature) Measurable distortions for µ > 0 near the end-point (E0 : max. allowed electron energy) 1934: Bethe and Peierls use Fermi’s theory to estimate the cross-section for processes in which “a neutrino hits a nucleus and a positive or negative electron is created while the neutrino disappears and the charge of the nucleus changes by 1” (Nature 133, 532) −44 2 σ ≈10 cm “corresponding to a penetrating power of 1016 km in solid matter. It is therefore absolutely impossible to observe processes of this kind with the neutrinos created in nuclear transformations. With increasing energy, σ increases (in Fermi’s model for large energies as E2) but even if one assumes a very steep increase, it seems highly improbable that, even for cosmic ray energies, σ becomes large enough to allow the process to be observed. ........................ One can conclude that there is no practically possible way of observing the neutrino.” H. BETHE, R. PEIERLS Physical Laboratory, University, Manchester February 20th, 1934 Late 1930s: discovery of Uranium fission 1940 - : applications (mostly military) 1952: Frederick Reines and Clyde Cowan (Los Alamos) begin thinking of methods of neutrino detection using large volumes of liquid scintillator First idea: install detector near a test atomic explosion. Expect detection of few events before detector destruction. Second idea: install detector near a nuclear reactor (a strong isotropic source of ν from β – decay of fission fragments) 1954: Install a 300 liter liquid scintillator detector near one of the Hanford reactors (a military reactor used for Plutonium production) Observe a reactor – independent background signal from cosmic rays DESIGN A NEW DETECTOR TO BE INSTALLED IN AN UNDERGROUND LOCATION 11 m FROM THE CENTER OF THE SAVANNAH RIVER 700 MW REACTOR THE ANTINEUTRINO DETECTOR AT THE SAVANNAH RIVER REACTOR (Autumn 1955) 200 liter tanks containing H2O - CdCl2 solution 2 meters 800 liter tanks containing liquid scintillator + photomultipliers ANTINEUTRINO DETECTION METHOD + ν + p → e + n on the free protons of H2O . Detect two simultaneous γ – rays from e+ annihilation at rest . Detect the late (~ 10 µs) γ – rays from neutron capture in Cadmium First results published in July 1956 (Science 124, 103) Final results: Reines, Cowan, Harrison, McGuire, Kruse, Phys. Rev. 117 (1960) 159 Reactor – associated event rate : 2.9 ± 0.2 events / hour Demonstration that signal is due to ν + p → e+ + n : . Insert Pb layers of variable thickness between H2O + CdCl2 target and scintillator tanks signal is reduced as expected for 0.5 MeV γ – rays produced inside target; . Remove CdCl2 from target signal from late neutron capture in Cd disappears; . Replace 47% of the target protons by deuterons using the appropriate mixture of heavy and ordinary water in the target: + 1 + [Expect σ(ν + d e + n + n) ≈ 15 σ(ν + p e + n) for reactor antineutrinos] measure reduction of reactor – associated signal as expected Violation of Parity (P) Invariance in the π µ e Decay Chain Historical introduction . In the early 1950s, two mesons with strangeness +1 were discovered: τ+ π+π+π- (final state P = -1) ; θ+ π+π0 (final state P = +1) with equal mass and mean lifetime values the τ – θ puzzle . Following a critical review of all experimental data then available, Lee and Yang proposed that the weak interactions may not be invariant under Parity (T.D. Lee and C.N. Yang, Phys. Rev. 104 (1956) 254) and suggested experiments to test this hypothesis . The first experiment to observe the violation of P symmetry in β – decay was a study of the decay 60Co 60Ni* + e− + ν using polarized 60Co nuclei (C.S. Wu et al., Phys. Rev. 105 (1957) 1413) p θ J (60Co spin) Measured electron angular distribution consistent with the form σ ⋅ p p I(θ ) ∝1+α =1+α cosθ with α = −1 E E σ : unit vector along J ; p , E : electron momentum, energy Observations of the Failure of Conservation of Parity and Charge Conjugation in Meson Decays: the Magnetic Moment of the Free Muon R.L. Garwin, L.M. Lederman and. M. Weinrich, Phys. Rev. 105 (1957) 1415 176 MeV/c beam from the Nevis synchrocyclotron contains 10% muons from π µ decay in flight Pions stop in Carbon absorber Muons stop in the Carbon target; Magnetic field B perpendicular to figure induces muon spin precession with angular precession rate e ωs = g µ B 2mµ c gµ : gyromagnetic ratio mµ : (g = 2 to lowest order muon mass for a spin ½ fermion) If Parity is not conserved in the π µ e decay chain one may expect: . muon polarization (Pµ ) along momentum; . electron angular asymmetry with respect to the Pµ direction: I(θ) = 1 + α Pµ cosθ Muon spin precession B – dependent modulation of the electron time distribution : τ ≈ 2.2 µs : mean muon decay time t θ : angle between detected electron I(t)∝ exp− [1+αPµ cos(θ -ω t)] τ s and incident beam direction Results with π+ beam Rate of detected electrons Full curve is computed with 0.75 < t < 2.0 µs under the assumptions versus precession field gµ = 2 ; αPµ = − ⅓ . Clear evidence for parity violation in both π µ ν and µ e ν ν decay: αPµ = -1 / 3 (estimated error 10%) . First measurement of the muon magnetic moment : gµ = 2.00 ± 0.10 . Data with negatively charged beam show reduced asymmetry: (αP)µ− ≈ 0.15 (αP)µ+ Measurement of the muon anomalous magnetic moment e Muon magnetic moment gµ 2mc 2 For a spin ½ particle obeying the Dirac equation g = 2 Quantum fluctuations of the electromagnetic field around the muon modify this value: gµ − 2 gµ = 2(1+ aµ ) aµ = 2 Anomalous magnetic moment aµ ≈ 1 / 850 Inject longitudinally polarized muons with momentum p into a uniform magnetic field B e B ω = − muon angular velocity ( ωc / 2π “cyclotron
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