Development of radioactive beams at ALTO: Part 2. Influence of the UCx target microstructure on the release properties of fission products

Abstract

Producing intense radioactive beams, in particular those consisting of short-lived isotopes requires the control of the release efficiency. The released fractions of 11 elements were measured on 14 samples that were characterized by various physicochemical analyses in a correlated paper (Part 1). A multivariate statistical approach, using the principal component analysis, was performed to highlight the impact of the microstructure on the release properties. Samples that best release fission products consist of grains and aggregates with small size and display a high porosity distributed on small diameter pores. They were obtained applying a mixing of gro und uranium dioxide and carbon nanotubes powders leading to homogeneous uranium carbide samples with a porous nanostructure. A modelling under on-line ALTO conditions was carried out using the FLUKA code to compare the yields released by an optimized and a conventional target. __________________________________________________________________________________________ Introduction:The research and development program at the ALTO facility (Accélérateur Linéaire et Tandem d'Orsay) of the Institut de Physique Nucléaire d'Orsay (IPNO) aims to provide new beams of exotic and neuton-rich nuclei, as intense as possible. At ALTO, neutron-rich nuclei are produced by photofission in thick uranium carbide targets. An efficient way to improve the intensities of the radioactive beams, in particular those consisting of short-lived isotopes, would be to develop dense and porous targets. These two properties are antagonistic but essential to increase the fission fragment quantity produced and diffuse them out of the target respectively.

Influence of density and release properties of UCx targets on the fission product yields at ALTO

“…For these elements, the release properties of sample No. 5 are better than those of samples Nos 1 and 10 [8]. These differences observed in the release are related to the microstructure of the samples.…”

“…Therefore in the following, all the simulations are carried out without converter. In our previous study [8], we modelled a uranium carbide target as a cylinder of 13 mm in diameter and 19.3 cm in length. These dimensions correspond to the standard dimensions of the targets used in both ISOLDE and ISOL-ALTO installations.…”

“…Consequently, as shown in ref. [8], almost 90 % of the fissions are created in the first 6 cm of the conventional target (sample No.8 PARRNe BP897 CP). By reducing the length of the target to 6 cm, the number of fissions produced remains almost the same (1.97×10 10 versus 2.22×10 10 , for a 10 µA beam intensity) whereas the target volume and consequently the amount of radioactive waste is divided by a factor 3.2.…”

“…8 as a function of the half-life of the isotope and for the elements studied in ref. [8], namely krypton (Kr), strontium (Sr), tin (Sn), antimony (Sb), tellurium (Te), iodine (I), cesium (Cs) and barium (Ba). For the long-lived isotopes, the ratio of the exit rates between the two targets (ΦNo.n / ΦNo.8) tends to the ratio of the number of fissions produced in the two targets (Nfission No.n / Nfission No.8).…”

“…In recent years, at the ALTO facility, we have conducted studies to understand and determine the factors that improve the release efficiency of ISOL targets. A systematic study on synthesis parameters (milling and mixing of precursor powders, pressing, carburation) led to the development of nanostructured-target synthesis protocols [3]- [8].…”