The Study On Fission Of _Λ～(209)Pb Associated With K～+

In the investigation of the nonmesonic decay of hypernuclei, the hypernucleus-lifetime measurement is one of the focus. The lifetime of a hyperon bounded in a hypernucleus is affected by the nuclear environment. The lifetime changing tendency as a hypernucleus becomes heavier is sensitive to the weak interaction range, especially for heavy hypernuclei. So many experiments had been done to measure the lifetime of light hypernuclei, and their results agreed with the theoretical calculation pretty well. As for heavy hypernuclei, the lifetime can not be measured by the same way as the light hypernuclei due to the complex production caused the nonmesonic decay. However, in heavy hypernuclei the nonmesonic-decay-released energy (about 176 MeV) is above the fission barrier, thus fission becomes a signature of the decay of hypernuclei. In other words, the lifetime is determined by nonmesonic decay time, while the nonmesonic decay can be identified by detection of fragments from a time delayed fission. Through detecting the fission fragments, some measurements (indirect) in recoil shadow method had been performed by COSY. But their results were far from satisfaction for the defects of recoil shadow method. E02-017 did the first attempt to measuring heavy hypernuclei lifetime directly using reaction ²⁰⁹Bi(e, e'K⁺)_Λ²⁰⁹ at Jefferson Laboratory. In the experiment, the newly installed high resolution kaon spectrometer (HKS) and time-zero fission-fragment chamber (FFC) were used. HKS did a good job in separating kaons and reconstructing the target time, and FFC picked up fission fragments ignoring the light ions and particles. Although the decay time was not reconstructed, the real coincidence peak between kaons (strangeness) and fissions was extracted unambiguously for the first, and the kaon associated fission rate was obtained, which proved the feasibility of such direct measurement method. Accordingly theÏ€⁺ and p⁺ associated fission rate were also given. The non-strangeness fission rates are far lower than that associated with kaon, but the number of fission events associated withÏ€⁺ and p⁺ is event larger than that associated with kaon, becauseÏ€⁺ and p+ had much larger yield than kaon. Therefore, the including of the non-strangeness fission in COSY experiments is one of the causes of the large error in their results. At the same some technique parameters, such as the efficiency of the FFC detector planes, were also given. It will be very helpful for the direct measurement of heavy hypernuclei lifetime in future.