We report on the single and double multiphoton ionization of ground state Sr atoms observed in an atomic beam experiment with laser pulses of ∼5 ns duration, maximum intensity ∼4 × 10 11 W cm −2 and within the 710-740 nm wavelength range. The Sr + spectrum consists of two strong lines originating from three-photon resonant four-photon ionization of bound states, a number of weak autoionizing resonances and a broad line due to four-photon excitation of the doubly excited 5p 2 1 S 0 state. The latter, along with a strong, broad and structured spectral feature, is also evident in the wavelength dependence of the doubly charged Sr 2+ ion. A weakly evident but reproducible inflection point ('knee' structure) appears in the intensity dependence of the Sr 2+ yield at the location of the 5p 2 1 S 0 resonance. A complementary fluorescence experiment revealed the accumulation of population in the 5p 1/2 , 3/2 , 6s 1/2 and 5d 3/2 , 5/2 excited Sr + states during the laser pulse. All fluorescence signals depend on laser wavelength in a manner similar to the recorded Sr + spectrum. The population accumulation in the 5p 1/2 , 3/2 ionic states unambiguously proves the absorption of two photons above the first 5s 1/2 atomic threshold while that of the 5d 3/2 , 5/2 and 6s 1/2 ones suggest the absorption of at least two more. Since under our laser pulse duration and intensity the absorption of such a number of photons in an unstructured continuum is highly improbable, it may be concluded that the process is mediated by dense manifolds of near-resonant doubly excited states, their role in the dynamics of laser-atom interaction under our conditions being far more crucial than in studies conducted using intense ultrashort pulses.

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Gamma and conversion electron radiations following the decay of 1.7 s 123mC~ and 5.9 min lZsCs have been studied in singles and coincidence with mass-separated sources at the ISOLDE facility. Evidence for a 61.7 keV E3 transition, de-exciting the yisomeric state of I2ks, has been found. A scheme for the 123Cs ground-state decay including 11 new low-spin levels has been constructed. Log ff values have been calculated; limits on possible spins are deduced which invalidate in particular the previous J = { assignment for the 180.6 keV level in '"Xe. The results are discussed in the frame of odd-mass neutrondeficient Cs and Xe systematics. RADIOACTIVITY 123mC~, 123Cs (from La(p, 3p14n)), E,= 600 MeV, on-line massseparated sources; measured T1/2, E,, I,, E,, I,, yy coin, y ce coin; L23Cs, L23Xe; deduced levels, J , n, log fl. Ge(Li), Si(Li) and plastic detectors.

The intercomparison is the best tool for the evaluation of the quality of a standard, and the validity of the estimation of its uncertainty. It is also the only way to detect possible systematic errors, experimental errors, missing items in the uncertainty budget or mistakes in calculation. Since 1975, many intercomparisons have been organized in the whole pressure range, all over the world. It would be a very large and difficult task to list them. Only the most significant are given as examples. The most recent are presented in other papers at this seminar. A review of the most commonly used transfer standards and their characteristics is given. The more significant data obtained from the intercomparisons (technical data and direct exchanges) are analysed.