Production of radioactive beams of francium

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Abstract

We describe the production of francium beams from the fusion–evaporation reaction Au197(O18,kn)215-kFr generated by a 100 MeV O6+18 beam on a thick gold target. The physics of the production process is discussed, together with estimates of expected production rates. The production target is heated to 1200K and kept at a potential of +3kV to enhance Fr diffusion and surface desorption, and to accelerate surface-ionized ions. Details are given on its design and construction. The performance of the target is measured as a function of primary beam energy and flux, target temperature and extraction voltage. Average production rates are 0.7×106ions/s for 210Fr with a primary beam flux of 1012particles/s, with peaks of 2×106ions/s. From these measurements, information on the efficiency of the release processes is inferred.

Introduction

A cold cloud of rare atoms is desirable in several fields. Doppler-free spectroscopy of poorly-known atomic levels and measurements of fundamental interactions in atomic systems are examples of studies pursued in laboratories around the world [1], [2].

Francium is particularly suitable because it is the heaviest alkali metal and it has several isotopes with relatively long lifetimes. Also, one expects enhanced parity-violating effects and reduced theoretical uncertainties from isotope comparisons [3], [4]. By accumulating rare atoms in a trap, one partially compensates for their scarcity. Our group and the one at SUNY Stony Brook, who pioneered the field [5], [6], [7], [8], [9], [10], are focusing on the isotopes with mass numbers in the range 208–211, and especially on 210Fr.

A production target, a beam transport line and a magneto-optical trap (MOT) are operating at LNL, the national laboratories of the Istituto Nazionale di Fisica Nucleare (INFN) in Legnaro, Italy. An overview of our work is given in Refs. [11], [12], [13], [14]. In this paper, we focus on the physics of francium production.

Section snippets

Production method

Francium has no stable isotopes. Its longest-lived isotope is 223Fr with a half-life T1/2=21.8min. It has to be produced either by nuclear decay [15], [16] or by induced nuclear transmutation.

We produce francium in the mass range 208A211 via fusion of 18O and 197Au and successive evaporation of neutrons from the compound nucleus: 197Au(18O,kn)215-kFr

Gold is chosen because it is mono-isotopic and chemically inert. Its melting point (1337 K) is relatively low, so that diffusion of nuclei

Estimates of expected yield

The observed yield Y depends on the fusion–evaporation production rate P and on the combined extraction efficiency ɛ of the following processes: diffusion in the target bulk (ɛd), surface desorption (ɛs), surface ionization (ɛi), and transport to a catcher foil where the ions are detected (ɛt): Y=P·ɛd·ɛs·ɛi·ɛtP·ɛ.

The contribution dP to the production rate in a small target thickness dt can be calculated from the incident 18O flux j, the number density n of the target nuclei, and the

Target assembly

The target operates at a relatively-high voltage (3 kV) and at a temperature near the melting point of gold (typically, 1200 K). It is necessary to heat it efficiently and to control its temperature at beam powers up to 25 W, while guaranteeing electrical insulation from the surrounding vacuum chamber.

The target is obtained by melting 1.5 g of gold (99.9999% purity) under vacuum (10-6mbar) on a 8.65 mm-diameter, 90 mm-long tungsten rod, on the end of which a dovetail joint is previously machined to

Measurements of production yield

We have performed experiments with several targets at different energies, fluxes, and temperatures. There is usually more than one data point for each target. When measurements taken in similar conditions are combined, the median is shown and the error bars reflect the dispersion of the data (from the 0.159 percentile to the 0.841 percentile, for a standard 68% coverage).

The production yield is measured by identifying the α decays of francium (Table 2). A retractable aluminum catcher foil

Conclusions

We have developed gold targets for the production of radioactive beams of francium in the mass range 208A211. The most abundant isotope is 210Fr with a maximum yield of 3×106ions/s with a primary 18O6+ 100 MeV beam flux of 1.5×1012particles/s.

The average overall extraction efficiency is estimated to be about 15% under normal operating conditions and 40% when the target is locally melting. We attribute most of the inefficiency to the process of desorption from the target surface.

Since the

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