G. Passler

Determination of the first ionization potential of nine actinide elements by resonance ionization mass spectroscopy (RIMS)

The high sensitivity of RIMS enables the precise determination of the first ionization potential of actinide elements with a sample size of ≤1012 atoms. By multiple resonant laser excitation, the actinide atoms under investigation are ionized in the presence of an electric field, and the ions are mass-selectively detected in a time-of-flight spectrometer. The first ionization potential is obtained by scanning the wavelength of the laser used for the last excitation step across the ionization threshold Wth—indicated by a sudden increase of the ion count rate—at various electric field strengths. Extrapolation of Wth to electric field strength zero leads directly to the first ionization potential. The first ionization potentials (IP) of Am, Cm, Bk, Cf and Es were determined for the first time as IPAm=5.9736(3) eV, IPCm=5.9914(2) eV, IPBk=6.1979(2) eV, IPCf=6.2817(2) eV, IPEs=6.3676(5) eV with samples of 1012 atoms. Furthermore, the ionization potentials of Th, U, Np and Pu were remeasured.

Ground-state properties of neutron-deficient platinum isotopes

The hyperfine structure splitting and the isotope shift in the λ=266 nm transition of Pt isotopes within the mass range 183 ≦A≦ 198 have been determined by Resonance Ionization Mass Spectroscopy (RIMS) in combination with Pulsed-Laser Induced Desorption (PLID). The Pt isotopes were obtained at the on-line isotope separator ISOLDE-3/CERN as daugthers of the primarily produced Hg isotopes. Magnetic moments, quadrupole moments, and changes in the mean-square charge radii are deduced and compared with results of a particle-triaxial rotor model and mean field calculations. Good agreement with experimental data (including nuclear level schemes and transition probabilities) can only be obtained if triaxial shape is admitted. The calculations yield a smooth transition in the shape of odd-A Pt nuclei from a slightly deformed, nearly oblate195Pt via triaxial197-187Pt to a strongly deformed nearly prolate177Pt.

Trace analysis of plutonium in environmental samples by resonance ionization mass spectroscopy (RIMS)

Abstract Resonance ionization mass spectroscopy (RIMS) is well suited for trace analysis of long-lived radioisotopes in environmental, biological and technical samples. By multiple resonant laser excitation and ionization of the elemental atoms under investigation, an extremely high element selectivity can be achieved. In addition, isotope selectivity is obtained by subsequent mass analysis. The excellent sensitivity results from the large atomic cross-sections in the excitation–ionization process and the good detection efficiency for ions. The element selectivity of RIMS allows a simplified procedure for the chemical preparation of the samples compared to the requirements of thin sources for α-spectroscopy. Various samples have been determined by RIMS with respect to their content and the isotopic composition of plutonium in the ultra-trace regime. A detection limit of 10 6 to 10 7 plutonium atoms has been achieved for all isotopes, independent of their half-life and decay mode. For 239 Pu, this value is distinctly below the radiometric detection limit.

Determination of the first ionization potential of actinide elements by resonance ionization mass spectroscopy

Abstract Resonance ionization mass spectroscopy (RIMS) in the presence of an external static electric field has been used for the determination of photoionization thresholds. Extrapolation of the thresholds obtained with different electric field strengths to zero field strength directly leads to the first ionization potential (IP). The ionization potentials of the transplutonium elements americium, curium, berkelium and californium could be measured for the first time. Due to the high sensitivity of RIMS, samples of only 1012 atoms have been used. The results are: IPAm = 5.9738(2)eV, IPCm = 5.9915(2)eV, IPBk = 6.1979(2)eV and IPCf = 6.2817(2)eV. The same technique was applied to thorium, neptunium and plutonium, yielding IPTh = 6.3067(2)eV, IPNp = 6.2655(2)eV and IPPo = 6.0258(2)eV. Plotted as a function of the number of electrons N, the actinide ionization potentials can be approximated by two straight lines joining at the half-filled shell when normalized to ionization from the lowest fN s2 level to the lowest fN s level.

Resonance ionization spectroscopy of fermium (Z=100)☆☆☆

Laser spectroscopy has been applied for the first time to measure resonant transition frequencies of fermium (Zs 100). A number of 2.7=10 atoms was electrodeposited on a Ta filament and covered with a 1 mm Ti layer. Fm 10

Quadrupole moments and nuclear shapes of neutron-deficient gold isotopes

Abstract The hyperfine structure splitting and the isotope shift of the neutron-deficient gold isotopes 194−191Au in the optical transition 5 d 9 6 s 2 2 D 3 2 → 5 d 10 6 p 2 P 1 2 (627.8 nm) have been determined by collinear laser spectroscopy at the on-line isotope separator ISOLDE. The nuclear magnetic moments, spectroscopic quadrupole moments and changes in the mean square charge radii are deduced. Experimental electromagnetic moments and deformation parameters are compared with results of a particle-triaxial-rotor model calculation for both odd-A and odd-odd gold nuclei. Very good overall agreement between calculation and experimental data has been achieved. We find, for the first time, consistent evidence for a relatively large contribution from hexadecapole deformation (ϵ4 = 0.06, β4 = −0.05) to odd-A gold isotopes ground states in the region of 183 ≤A ≤ 197. Calculated values of quadrupole deformation are consistent with the prolate-to-oblate shape change between ground states of 186Au and 187Au. In addition, our results suggest another shape change between 190Au (oblate) and 191Au (triaxial). The former change is accompanied by a considerable change in quadrupole deformation, the latter occurs at almost constant deformation.

Fast, low-level detection of strontium-90 and strontium-89 in environmental samples by collinear resonance ionization spectroscopy

Environmental assessment in the wake of a nuclear accident requires the rapid determination of the radiotoxic isotopes 89Sr and 90Sr. Useful measurements must be able to detect 108 atoms in the presence of about 1018 atoms of the stable, naturally occurring isotopes. This paper describes a new approach to this problem using resonance ionization spectroscopy in collinear geometry, combined with classical mass separation. After collection and chemical separation, the strontium from a sample is surface-ionized and the ions are accelerated to an energy of about 30 keV. Initially, a magnetic mass separator provides an isotopic selectivity of about 106. The ions are then neutralized by charge exchange and the resulting fast strontium atoms are selectively excited into high-lying atomic Rydberg states by narrow-band cw laser light in collinear geometry. The Rydberg atoms are then field-ionized and detected. Thus far, a total isotopic selectivity of S > 1010 and an overall efficiency of ξ = 5 × 10−6 have been achieved. The desired detection limit of 108 atoms 90Sr has been demonstrated with synthetic samples.

Determination of the first ionization potential of einsteinium by resonance ionization mass spectroscopy (RIMS)

Abstract The first ionization potential of einsteinium (IP Es ) was determined by resonance ionization mass spectroscopy (RIMS) using samples with ≤10 12 atoms of 254 Es ( T 1/2 =276 days). This method is based on the measurement of photoionization thresholds as a function of applied electric field strength, followed by extrapolation to zero field strength to yield IP Es . An atomic beam of Es was created by heating a filament on which einsteinium was electrodeposited from an aqueous solution onto a tantalum backing and covered with titanium metal. Es atoms were ionized via a three-step excitation scheme, and the ions mass-selectively detected in a time-of-flight (TOF) mass spectrometer. The excitation scheme used included a previously unknown EsI level at 32 924.9 cm −1 . Furthermore, an autoionizing state at 51 447.3 cm −1 was also found. The first ionization potential of Es was determined to be 6.3676(5) eV (≡51 358(5) cm −1 ).

Quadrupole moments of radium isotopes from the 7p2P3/2 hyperfine structure in Ra II

The hyperfine structure and isotope shift of221–226Ra and212,214Ra have been measured in the ionic (Ra II) transition 7s2S1/2–7p2P3/2 (λ=381.4 nm). The method of on-line collinear fast-beam laser spectroscopy has been applied using frequency-doubling of cw dye laser radiation in an external ring cavity. The magnetic hyperfine fields are compared with semi-empirical and ab initio calculations. The analysis of the quadrupole splitting by the same method yields the following, improved values of spectroscopic quadrupole moments:Qs(221Ra)=1.978(7)b,Qs(223Ra)=1.254(3)b and the reanalyzed valuesQs(209Ra)=0.40(2)b,Qs(211Ra)=0.48(2)b,Qs(227Ra)=1.58(3)b,Qs(229Ra)=3.09(4)b with an additional scaling uncertainty of ±5%. Furthermore, theJ-dependence of the isotope shift is analyzed in both Ra II transitions connecting the 7s2S1/2 ground state with the first excited doublet 7p2P1/2 and 7p2P3/2.

On-line laser spectroscopy by resonance ionization of laser-desorbed, refractory elements☆

Resonance ionization mass spectroscopy was applied at the on-line isotope separator ISOLDE-3/CERN, Geneva to the refractory elements Au and Pt. The hyperfine structures and isotope shifts of gold and platinum isotopes were studied in long isotopic chains in the range from the stable isotopes down to the neutron-deficient ones with A = 183. Spins, magnetic moments, quadrupole moments and the differences in mean-square nuclear charge radii were determined. A mass-selected Hg ion beam was implanted into a graphite target. After the decay to Au or Pt, a pulsed atomic beam was formed by pulsed laser desorption. The atoms were ionized by three-colour, three-step resonant excitation. The photoions were detected mass-selectively by a time-of-flight mass spectrometer. An overall efficiency of ϵ ⋍ 10−5 was achieved defined as the number of atoms detected in the optical resonance to the number of Hg atoms implanted into the target. The background was as low as 1 event per 1000 laser shots. The publication gives a detailed description of the applied method, the experimental setup and its performance.