Proposed New Method for Separating Isotopes
RECENTLY1 a double mercury discharge vessel was used (Fig. 1), in which a large current running in the outer vessel irradiated the small current discharge in the inner vessel with 2537 Å resonance radiation. This irradiation caused the running voltage and electron temperature in the inner discharge to be reduced appreciably over the non-irradiated mode of operation. The explanation offered was that the neutral Hg atoms in the inner vessel absorbed the resonance radiation and entered the 63P1 state (see Fig. 2), and from this level the atom was ionized by electron impact. In addition the 63P0 and 63P2 metastable levels are populated from the 63P1 level by electron collisions and by cumulative absorption and radiative decay of higher levels. Because the radiation field is doing approximately half the ionization work, the electron temperature drops correspondingly.
Reversal of transplantation immunity by liver grafting
The transplantation of organs between individuals of a species normally has two main consequences: (1) the tissue is rejected unless the individuals are matched for identity of transplantation antigens, especially those encoded by the major histocompatibility complex (MHC), and (2) the recipient is sensitized to the transplantation antigens of the donor so that second-set grafts are rejected in a more violent manner1. These rules of transplantation are not obeyed by liver grafts. Orthotopic transplants of liver are never rejected by many strains of rat even though the MHC barrier is crossed2. We have recently shown that instead of sensitizing the recipient, these enduring liver grafts induce a state of donor-specific unresponsiveness in which subsequent grafts of other organs, such as skin, are accepted permanently. It is possible that many strains simply cannot mount a sufficiently vigorous destructive immune response to outstrip the liver's great capacity to repair immune damage and that the systemic unresponsiveness observed is due to diversion of alloreactive lymphocytes with donor specificity into the liver allograft. Manipulations which increase the vigour of the immune response might therefore tip the balance in favour of liver graft rejection and away from induction of unresponsiveness. We have previously measured the degree to which the immune response can be increased by previous exposure to MHC antigens in a quantitative adoptive transfer system3. In the strain combination DA (MHC haplotype RT-1a) grafted to PVG (RT-1c), the lymphocytes of immunized animals are 1,000 times more potent than those of non-immune animals in procuring graft destruction when transferred to irradiated hosts. Unexpectedly, we have now found that liver grafts transplanted into previously immunized rats not only fail to reject but convert the state of heightened reactivity to donor grafts characteristic of immune recipients into one of non-reactivity characteristic of tolerant animals.
Ionization in a Mass Spectrometer by Monoenergetic Electrons
Over the last few years, a method has been developed for obtaining ionization probability curves with essentially monoenergetic electrons. A retarding potential is applied to the electron beam to yield an energy distribution with a sharp low‐energy limit. By varying the retarding potential slightly, a new low‐energy limit of the distribution can be selected. The difference in the ionization produced in the two cases is ionization by those electrons with a small energy spread selected from the original distribution. By pulsing the electrons and ions, it is possible to eliminate the adverse effect of the ion‐drawout field on the electron energy. With this retarding potential difference (RPD) method, a detailed analysis of ionization probability curves is possible. A full description of this method is given in this paper with a discussion of its advantages and limitations. The mass spectrometer used in this series of studies is described, particular attention being given to a description of the ion source. T...