INSTITUTE OF PHYSICS PUBLISHING PHYSICA SCRIPTA Phys. Scr. T125 (2006) 180–181 doi:10.1088/0031-8949/2006/T125/040 Beta decay half-life of 231 Ra M J G Borge 1 , R Boutami 1 , L M Fraile 1 , K Gulda 2 , W Kurcewicz 2 , H Mach 3 , T Martínez 4 , B Rubio 4 , O Tengblad 1 and for the IS322-Collaboration 1 Instituto de Estructura de la Materia, CSIC, Serrano 113 bis, E-28006 Madrid, Spain 2 Department of Physics, University of Warsaw, Pl-00 681 Warsaw, Poland 3 Department of Radiation Sciences, Uppsala University, SE-75121 Uppsala, Sweden 4 Instituto de Física Corpuscular, CSIC—University of Valencia, Apdo. 22805, E-46071 Valencia, Spain E-mail: borge@iem.cfmac.csic.es Received 20 July 2005 Accepted for publication 1 September 2005 Published 28 June 2006 Online at stacks.iop.org/PhysScr/T125/180 Abstract The low-energy structure of 231 Ac has been investigated by means of γ , conversion electrons, γ –γ and γ –e - spectroscopy following the β - -decay of 231 Ra. Here, we report on the precise determination of the 231 Ra β -decay half-life. PACS numbers: 21.10.Tg, 23.20.Lv, 27.90.+b Since the observation of low-lying K π = 0 - bands in doubly even radium nuclei [1] the possibility that some nuclei can be described by mean field with broken reflection symmetry has been considered [2]. Numerous experimental and theoretical discoveries were made in the 1980s providing extra evidence of reflection asymmetric octupole deformation around A = 225 [3]. Many of these nuclei have been studied at ISOLDE in β -decay experiments [4]. An important feature of static octupole deformation in odd- A nuclei is the observation of parity doublets, i.e. rotational bands with the same intrinsic parameters and spins, but opposite parity lying close in excitation energy. The study of the upper border of this octupole deformed region is of great relevance in order to understand the interplay of octupole and quadrupole collectivities and to reveal the exact mechanism by which the octupole deformation disappears in the presence of a well-developed quadrupole field. Experimentally one should observe an enhancement of the B(E1) transition strength for the parity pair partner bands. With this aim the IS322 collaboration at CERN carried out a systematic investigation of the heavy transitional Fr–Th nuclei. These experiments provided the first information on the absolute values of B(E1) in this octupole transitional region. Relatively large B(E1)-rates have been measured indicating the presence of octupole correlations in 227 Fr, 229 Ra and 229 Th. Much weaker but still noticeable are the correlations in the heavier isotopes 231 Ra and 231 Th. Here, the study of their isobar 231 Ac is reported, for which only scarce information exists [5–7]. The structure study of 231 Ac is based on two sets of measurements performed at the PSB-ISOLDE(CERN) on- line mass separator. In both experiments a 1 GeV proton beam bombarded a UC 2 -C target, producing via spallation reactions the A = 231 isobars. In the first experiment, a W-surface ionizer was used inhibiting the ionization of 231 Fr. In the second experiment, the use of a Ta(Re) surface ion source of lower working function favoured the production of 231 Fr. Therefore the excited structure of 231 Ac was populated directly from the β -decay of 231 Ra in the first run with an estimated production of 4 × 10 3 atoms per μC, and through the decay chain of 231 Fr → 231 Ra→ 231 Ac in the second where the production was similar, 5.6 × 10 3 atoms per μC[8]. The produced ions were accelerated and mass-separated before being collected onto a magnetic tape transport system which connected two independent but simultaneously operated measurement stations. The radioactive beam was deposited in the centre of the specially designed fast timing station to measure the half-life of excited states in the nano- and pico-second range by the Advanced Time Delayed (ATD) βγγ (t) method [9]. The effective dynamic range of the β -gated γγ coincidences was 25–1500keV. The study of the γγ coincidences allowed the determination the level scheme. Several half-lifes of states in 231 Ac could be derived, and the deduced B(E1) rates are used to infer the possible presence of octupole correlations, which is discussed in detail in [10]. The radioactive sample was transported a metre away by the tape to the conversion electron station for γ identification and conversion electron measurements. 0031-8949/06/125180+02$30.00 © 2006 The Royal Swedish Academy of Sciences Printed in the UK 180