4-9

Investigation of Chemical Reaction Processes Using Radioactive Atoms Produced by Neutron Irradiaton



Fig. 4-17 Cryostat for ultra-low temperature chemical experiments using neutrons

A cryostat which is a kind of vacuum bottle for low temperature experiments, has been installed in a neutron beam guide of JRR-3M. Very low temperatures of around -270degree Celsius(1.42-2.5K) are obtained using the heat of vaporization of liquid helium. The chemical species tritium is produced by a nuclear transformation of helium-3 through the absorption of neutrons.



Fig. 4-18 Analytical results for the reaction product T2 by radio-gas chromatography at -196degree Celsius

The product T2 is mostly populated in the ortho state. The arrow shows the nuclear spin orientation (spin direction) in T2.



Fig. 4-19 Comparison of the fraction of ortho-T2 formed at ultra-low temperature () with the equilibrium fractions found in nature at room and at experimental temperatures

he results show that almost all the T2 molecules are formed preferentially in the ortho-state, exceeding the fractions found in nature.


Nuclear transformation caused by neutrons permits the generation of a naturally low-abundance radioactive isotope - i.e. an element with different mass. The radioactive isotope generated can be detected with very high sensitivity through its radioactivity.
This capability of neutrons has led us to produce tritium, which is a radioactive hydrogen isotope and has very low natural abundance, through the nuclear transformation of helium-3 (3He) in the liquid phase at the Japan Research Reactor No. 3M (JRR-3M). This development has enabled us to succeed for the first time in observing the recombination reaction of tritium, T+TT2, at very low temperature (1.4-2.5K). Although the recombination reaction among hydrogen isotopes (hydrogen, deuterium and tritium [T]) is the most fundamental chemical reaction, its mechanism has not been well established up to now.
The nucleus of tritium has spin quantum number of 1/2, and since there are two spin directions, the T2 molecule formed by the recombination reaction (Fig. 4-17) has two spin isomers, that is, ortho-T2 (parallel nuclear spin) and para-T2 (antiparallel nuclear spin). From the ratio of ortho-T2 to para-T2 formed, it is possible to understand the mechanism of the recombination reaction.
Fig. 4-18 and 4-19 show strongly preferential formation of ortho-T2. Based on these experimental results and rotational selection rules, it is concluded that T2 molecules are formed through a metastable state with an odd rotational quantum number.
As a consequence of using neutrons to generate tritium, then, substantial progress has been made in clarifying the fundamental chemical reaction among hydrogen isotopes.


Reference
K. Iguchi et al., Non-statistical Formation of J=1 T2 (ortho-T2) in Recombination Reaction of T+T+MT2+M in Liquid Helium at 1.42-2.50K, Chem. Phys. Lett., 349, 421 (2001).

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Persistent Quest Research Activities 2002
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