Pion capture

Temperature dependence of pion capture probability in water (Kachalkin et al. 1979] and ammonia (Horvath et al. 1982). The probability is normalized to that measured at room temperature. The temperature is expressed as = (T-Tm)/(Tc-Tm), where T, T, and Tc are the temperatures of measurement, melting point, and critical point, respectively. Ordinary temperature scales are also shown above the boxes. The curves represent fits using the bondbreaking (mixture) model of hydrogen-bonded liquids (Haggis et al. 1952 Luck 1979)... 

Spots" Arising from Pion Capture in Explosives Using Thermal Initiation Theory J.Chem. Phys. 40.(6).1736-1744. 

For historical reasons and for their general principles, pion therapy and the still experimental Boron Neutron Capture Therapy are also briefly discussed. 

The muon g — 2 value has been determined in a series of experiments at CERN [45,46]. The primary purpose of the new muon g — 2 experiment at Brookhaven National Laboratory is to improve the precision of the experiment by about a factor 20 and verify the presence of the electroweak effect which has been evaluated to two loop orders in the Standard Model. In this experiment, polarized muons from pion decays are captured in a storage ring with a uniform magnetic field and a weak-focusing electric quadrupole field. For a muon momentum of 3.09 GeV/c and 7 = 29.3 the muon spin motion is unaffected by the electric quadrupole field and the difference frequency uia is given by... 

Exotic atoms are produced by stopping a beam of negatively charged particles like muons, pions, or antiprotons in a target, where they are captured in the Coulomb potential of the atoms at high principal quantum numbers n. These systems deexcite mainly by fast Auger emission of electrons in the upper part of the atomic cascade and more and more by X-radiation for lower-lying states. 

At the ttE5 beam of the Paul-Scherrer-Institut (PSI), about 2% of the incoming pions (> 109/s) are stopped in the gas cell with a degrader set-up optimized for pionic atoms. Muons originating from pions decaying shortly before capture are slow enough to be stopped in the gas cell as well. With a set-up optimized for muons, the count rate for muonic atoms is about 4% of the one for pions. 

Nuclear and pion related 7-rays provide important information about the spectra of protons and ions accelerated in solar flares [e.g. Hua and Lingen-felter, 1987 Murphy et al., 1987 Lockwood et al., 1997 Hua et al., 2002], However, nuclear 7-ray lines probe the proton spectrum only up to 40 MeV, while 7-rays from pion decays are only observed in the most intense flares. In addition, any spectral break in the proton spectrum is likely to he below the pion production threshold. Neutrons produced at the solar surface over a wide range of energies may provide important information from the 50-300 MeV regime, complementing 7-ray observations. Due to the long neutron thermal-ization time ( 100 s) the 2.223 MeV neutron capture line is only a limited measure of neutron production. The spectrum of accelerated and interacting protons can be deduced more reliably from direct neutron measurements. 

An analysis of the hot-spot model was attempted by Cemy and Kaufman [50], who irradiated several explosives with tt" mesons (pions). The decay of mesonic atoms formed by the capture of n mesons can result in the emission of 12-17 charged particles from a single lattice site. It was estimated that a temperature of IO" °C would be produced over a 10-A radius for 10 " sec. The calculations indicated that the temperature would decrease rapidly, but that the radius of a hot-spot site would increase to meet the criterion of Bowden. 

Few of the pions formed in the annihilation process reach the earth s surface. They undergo radioactive decay (life-time about 10 s) to muons and neutrinos, or they collide with other particles in the atmosphere and are annihilated. The muons have properties similar to the electron, but are unstable, decaying with a life-time of about 2 x lO s to electrons and neutrinos. The collision reactions of the pions result in the formation of a large number of other particles such as electrons, neutrons, protons, and photons. Some of the electrons so formed are captured in a thick zone around the earth known as the inner van Allen belt. 

For the capture of a muon or pion by the atoms with and Zj in a binary system, the... 

The capture ratio of the muon has been measured for chlorides as shown in Fig. 14. Fermi and Teller assumed that the capture probability is proportional to the energy loss of the muon (or pion) near the corresponding atomic species. This leads to a relation between the capture ratio and Z as follows ... 

K X-rays are observed only for light nuclei, the yield of such X-rays (per pion stopped) decreases rapidly with Z. This is attributed to an increasing probability for nuclear capture from the 2p state and is in agreement with a known attractive interaction between a nucleon and a meson in a state. The yield of K X-rays for Z = 8 is down to about 3% of all pions that have stopped L X-rays have a yield of about 8% for Z = 26. 

Experimentally, the two-photon decay of neutral pions from the charge-exchange reaction following n capture in proton ... 

According to the original LMM model, the probability Wthat the pion is captured on the proton can be described as the product of three probabilities (O Fig. 28.9)y... 

Net atomic charges have been obtained from atomic Coulomb capture ratios of pions in 3-BN by applying the modified mesomolecular model, which accounts forthe Auger process in the cascade transitions from mesomolecular to atomic orbitals. The resultant atomic charges are +0.48 for the boron and -0.48 for the nitrogen atom [16]. 

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