| Budget Amount *help |
¥3,100,000 (Direct Cost: ¥3,100,000)
Fiscal Year 1998: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 1997: ¥2,400,000 (Direct Cost: ¥2,400,000)
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| Research Abstract |
The purpose of this study is to realize the Bose-Einstein condensation (BEC) of the helium gas by means of laser cooling.For BEC, helium atoms have to be trapped at a high density and a low temperature, and the most significant obstruction to BEC is the Penning collision between trapped He atoms, which lie in the metastable excited triplet state.Therefore, to realize BEC, it is important to investigate this ionization process.In this study, though BEC has been unsuccessful, the Penning ionization rate at an ultralow temperature has been investigated in detail.In particular, it has experimentally and theoretically been found, for the first time, that the ionization rate at an ultralow temperature is much different between ^4He and ^3He.
For Penning collisions between He atoms cooled and trapped by lasers, there are two possible cases : one is a collision between two He(2s^3S) atoms (S-S collision), and the other is the one between He(2s^3S) and He(2p^3P) (S-P collision).For S-S collisions, the Penning collision rate coefficient at 0.5 mK has been found to be (3.8*1.1)*10^<-10>cm^3/s for ^4He and (1.1*0.4)*10^<-9>cm^3/s for ^3He, and the latter is as much as three times larger than the former.From our theoretical analysis, this large isotopic difference has been found to result from the fact that at such a low temperature the lowest order partial wave alone contributes to Penning collisions.In such a collision caused by a single partial wave, a difference in the quantum statistical symmetry explicitly affects the collision cross sections.On the other hand, for S-P collisions, though the ionization rate coefficients for both isotopic species are two orders of magnitude larger than for S-S collisions, only a small isotopic difference has been found.
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