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We report the application of a new nonoptical detection scheme in laser spectroscopy on ion beams of on-line isotope separators. The method consists of ground-state depopulation by optical pumping in collinear laser-ion-beam interaction, state-selective neutralization, and charge-state-separated fast-atom counting. Strontium isotope-shift measurements are extended into the strongly deformed region of the neutron-rich isotopes up to '^Sr. The radii are generally well described by the droplet model including empirical deformations obtained from BiE2) values.

Isotope shifts of the stable strontium isotopes Sr, Sr, Sr, and ' Sr and the radioactive isotopes Sr and Sr have been measured using laser spectroscopy on collimated atomic beams. The changes in mean square charge radii extracted from the isotope shifts show a pronounced shell effect at the closed neutron shell 1V =50. The experimental data are compared with the predictions of the droplet model and Hartree-Fock plus BCS calculations. The analysis suggests that the changes in mean square charge radii are due to a change in size, a change in the predominantly dynamic deformation, and a change in the diffuseness of the nuclear charge distribution.

Nuclear ground state spins of the odd-mass strontium isotopesbetween A = 79 and 97 were determined by measurements of the hyperfine structure in the ionic transition 5s 2SI/2 -5p 2P3/9-The spins of 93Sr and 97Sr'are revised To I = 5/2 and I = I/2, respectively, while assignments for the remaining isotopes are confirmed.The sequence of strontium isotopes ranges from the N : 50 shell closure for 88Sr into the regions of well-developed deformation for both neutron-deficient and neutron-rich isotopes. Laser spectroscopy measurements of the ground state moments and radii mainly cover the transitional region of the neutron-deficient branch [1,2]. We have recently started a more extended investigation of the nuclear ground state properties in strontium in an on-line experiment at the mass separator ISOLDE at CERN. In this communication we report the determination of the spins for the odd-A isotopes with 79 S A S 97 from the atomic hyperfine structure (hfs). This series of sDins includes two controversial cases, 93Sr and 97Sr, where first spin assignments from nuclear spectroscopy were revised later on [3 7]. Among these, 97Sr deserves special attention since the odd-mass isotones with N = 59 are considered to be an excellent testing ground for nuclear models because of their unique situation in the narrow range of neutron numbers where shape coexistence is indicated [7,8].Our hfs measurements have been performed 5s 2SI/2 -5p 2F3/2 transition in the (l = 407.8 nm) of the Sr § ion. We used the technique of cellinear fast-beam laser spectroscopy and the experimental procedure is similar to that one described in ref. [9]. * Present ad'dress: Fhysikalisch-Technisc~e Bundesanstalt,

The depth dose for electrons is sensitive to energy and the AAPM Task Group 24 has recommended that tests be performed at monthly intervals to assure electron beam energy constancy by verifying the depth for the 80% dose to within +/- 3 mm. Typically, this is accomplished by using a two-depth dose ratio technique. Recently, a new device, the Geske monitor, has been introduced that is designed for verifying energy constancy in a single reading. The monitor consists of nine parallel plate detectors that alternate with 5-mm-thick absorbers made of an aluminum alloy. An evaluation of the clinical usefulness of this monitor for the electron beams available on a Varian Clinac 20 has been undertaken with respect to energy discrimination. Beam energy changes of 3 mm of the 80% dose give rise to measurable output changes ranging from 1.7% for 20-MeV electron beams to 15% for 6-MeV electron beams.

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With the modern high-energy linear accelerators, the following beam characteristics have to be taken into account in the monitor unit (MU) calculation of a wedged treatment: (i) the field size dependence of wedge factors; (ii) the changes in depth dose and maximum build-up depth (dmax) induced by wedges; and (iii) the field size dependence of dmax. The incorporation of a field size specific wedge factor in an MU calculation is straightforward. Effects (ii) and (iii) however, often cause confusion and inconsistency in the choices of the reference depth for wedge factors and the normalization depth for wedged depth dose, and consequently can lead to inconsistent MU calculation formalism with additional efforts of up to 7% in the delivered dose. In this note, we illustrate a derivation of an exact central axis MU calculation for wedged treatments, which correctly accounts for the effects mentioned above.