H. Henry Stroke

Bolometers as Particle Spectrometers

A spectrometer based on low-temperature calorimetry is under development since 1983. The present detector, capable of recording individual alpha and beta particles and X-ray photons, is based on a composite diamond-germanium bolometer. The advantage of a composite bolometer is that it separates the absorption and detection functions. Diamond, as an absorber, is of particular advantage because of its low heat capacity and high thermal diffusivity. The goal is a theoretical energy resolution of a few eV at 0.1/sup 0/K. Our initial experiments at 1.3/sup 0/K and 0.9/sup 0/K, which give resolutions in the keV range, are still noise limited. High-resolution applications, such as in X-ray astronomy and nuclear physics - in particular neutrino mass measurements - are forseen.

Micromachining of silicon for thermal and position-sensitive nuclear detector applications

As part of a programme aiming at the development of small nuclear radiation detectors, for example thermal detectors and position sensitive mosaic structures of surface barrier type, a technique for micromachining the detector bodies in silicon has been developed. The technique is based on an anisotropic etching property of a solution, mainly consisting of KOH. The etch rate is strongly orientation dependent with a speed in the 〈100〉 direction about 400 times faster than in the 〈111〉 direction. The major steps in the etching procedure are described and some examples of deep etching in Si are shown.

Atomic Physics with Radioactive Atoms

Abstract A review is given of atomic spectroscopy experiments with radioisotopes and their impact on atomic and nuclear structure studies. These range from dispersive and interference techniques to radiofrequency and laser spectroscopy. An extensive study has been made of spectra of actinides. Availability of beams of isotopes produced at accelerators and used “on line” permit studies of nuclides with sub-second half-lives. The atomic properties of francium could thus be obtained. Systematic measurements by these atomic techniques over a range of isotopes can yield data on nuclear charge and magnetization properties not readily available by nuclear spectroscopic methods.

Einstein and the atomic clock

The anniversary of Einsteins death on 18 April 1955 brings back the memory of a chat we had with him shortly before he died. It is a tale that began just before the US entered the Second World War, when I I Rabi and collaborators at Columbia University developed a method known as atomic beam magnetic resonance (ABMR). The high precision of ABMR immediately suggested a fundamental application: the laboratory measurement of gravitational red-shift, which Einstein had predicted back in 1907. The idea was to compare the times kept by these atomic clocks at two points in a varying gravitational field. As Norman Ramsey, who was then one of Rabis graduate students, recollects, the plan was to put one clock on the top of a mountain and one at the bottom of a mine shaft. I know what will happen – Ill inherit the bottom of the mine shaft while my colleagues, Jerrold Zacharias and Rabi, are up on the mountain-top enjoying the view!

Photoelectric Measurements of Doublet Intensity Ratios in Cesium

The effects of the spin-orbit perturbation on the alkali fine-structure doublets have a number of manifestations that have been reviewed recently by zu Putlitz.1 These include the non-zero minimum, as a function of energy, of the photoionization cross sections, polarization of the photoelectrons obtained near this cross-section minimum, and the departure, in the discrete spectrum, from the value 2 of the doublet intensity ratio. The value 2 is expected simply on the basis of the ratio of the statistical weights of the excited 2P3/2 and 2P1/2 states. The interest in these phenomena was renewed after the theoretical prediction by Fano,2 based on the perturbation effects by the spin-orbit interaction, that circularly-polarized light could orient the spins of electrons ejected in the photoionization process in cesium. The potential application of the phenomenon to make polarized electron sources was to be exploited soon thereafter.3,4 The two experimental efforts that were thus made to produce polarized electrons, and to measure the degree of polarization, relied on related, but different, techniques. The photoionization results, extrapolated into the discrete spectrum, made different predictions for the behavior of the resonance-line doublet intensity ratios: in one case3 the ratio would pass through a maximum, as the principal quantum number, n, of the excited P states increases toward infinity; in the other4 the ratio would continue to increase as the ionization potential is approached.