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Novel possibilities to create cryogenic plasma states in liquid helium II below 2.16 K at 1 atm are proposed. Below 0.4 K, a mixture of positive and negative ions in the superfluid can be collision-free in the sense that the momentum relaxation collision frequencies between ions and neutral atoms, impurity atoms and excitons such as phonons, rotons and vortices are less than the plasma frequencies. In particular, positive ions in the plasmas are analogous to free electrons in conventional gaseous discharges at low pressures, although the plasma parameters are quite different from each other. A possible cryogenic plasma is proposed to be produced as a transient state. Preliminary experiments for ion mixture left after pulsed discharges in liquid helium II are demonstrated.

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Many long-lived non-gamma emitting radioactive nuclei such as 129 I, 14 C and 93 Zr are produced as wastes from nuclear fuel cycle facilities. They are called "difficult-to-measure" radio nuclei. Among them 129 I is a long-livedemitting isotope with a half-life of 1.57 x 10 7 years. Iodine compounds are mobile in the vadose zone and groundwater and increase a significant long-term risk. Transmutation of 129 I is a challenging issue in nuclear waste management and disposal. If 129 I is transmuted into 128 I (half life; 25 minutes), it can easily be measured by a Ge detector. However, the 129 I( , n) reaction cross section has not been measured so far. In this study we have measured the inclusive 129 I( , n) 128 I cross section by using bremsstrahlung photons. The bremsstrahlung photons were produced from a 30-MeV electron linac. Measured average activation cross section agrees with 12% deviation from the evaluated one in the IAEA photonuclear data library. Gamma and neutron fluxes for the ( , n) and the (n, ) reaction were also calculated by the EGS and the MCNP codes.