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Energy Traps in Atomic Nuclei
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accumulation 2015-3-10 10:41
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Making spin traps Firing a bullet into a punchbag can make the latter locally hot, but it will not move very much. The lightweight bullet may have high kinetic energy (0.5MV2), but only low momentum (MV) and low angular momentum (MVr), even if it hits the punchbag well offcentre (hereM is the mass of the bullet, V is its velocity, and r is the impact parameter). It is the same with nuclei. Firing a proton into 180 72Hf108 with enough energy to overcome the electrostatic repulsion makes 181 73Ta108 at relatively low angular momentum, but at sufficiently high excitation energy to `boil off ' two neutrons to become 179 73Ta106. g-ray emissions follow, corresponding to transitions between quantum states, in a sequence that takes 179Ta down to its ground state, but only through states with low spin. Fusion reactions. Heavier projectiles are needed to make high-spin isomers, with fusion-evaporation reactions of the type illustrated in Fig. 3a. The record for a K-trap has been set12 by colliding 48Ca into 130Te. After fusion, which forms 178Hf, the excitation energy favours the emission of four neutrons which would lead to states in 174Hf . Occasionally only three neutrons are `evaporated', resulting in a rapidly spinning 175Hf nucleus, which sometimes is trapped in an isomeric state with (57/2) ~ of spin at an excitation energy of 7.5MeV. However, the low probability of the formation path makes difficult the study of 175Hf at high spin with this reaction. (As a result, the 7.5-MeV isomer has yet to have its half-life measured.) To do better, an initial beam or target with more neutrons is needed, but for calcium and tellurium these heavier isotopes are all unstable, with half-lives shorter than days, so they are unavailable for beams and targets at present. Developments in radioactive-beam technology13 should soon remedy this. The heaviest isotope of hafnium that presently can be studied with fusion-evaporation reactions is 178 72Hf106, made by colliding 4He into 176Yb, and involving the evaporation of two neutrons. This reaction brings in, at the most, 16 units of spin, just enough14 to make the K ˆ 16 trap at 2.4MeV, which has a half-life of 31 years.
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