We ask for 4 new shifts (to be combined with 2 shifts left for IS386 from 2005) of radioactive beam of 229Ra in order to search for the alpha decay branch from the 3.5 eV state in 229Th into the states in 225Ra and to measure its half life by alpha spectroscopy.

Why do we want to study properties of the 3.5 eV state in 229Th? A: It is a VERY special state of strong fundamental interest ! 229Th is unique having an exceptionally low first excited state at only a few eV thus comparable to the excitation energies on the atomic scale. Various ideas have been considered utilising the low energy of this state (229Th is very long lived, almost stable) including a new standard for optical time clock or a nuclear transistor, yet basically it would be an exciting and unique laboratory case for the intersection of atomic and nuclear processes. By now there are about 20 publications on various ideas. Fascinating physics experiments have been proposed for this nucleus, including studies of hydrogen like 229Th or laser spectroscopic experiments, provided the energy of this state and its level lifetime are known. However, their determination is a very challenging experimental problem and various international teams have tried to solve it.

Long Term Strategic Goal: The strategic goal (Jan Zylicz, ISOLDE Workshop 2000) is to perform an experiment at the GSI storage ring with the hydrogen-like ions of 229Th for the following reasons: (i) to verify theoretical predictions of the mixing of the states in 229Th of different spin (the ground state 5/2+ and the isomeric 3/2+) expected to occur in the extremely strong magnetic field generated by a single electron in the K shell, (ii) in the same experiment, to determine with high precision the energy of the level, which is now known with a large error: 3.5(10) eV from Ref. [1] or 5.5(10) eV, Ref. [2]. [1] R.G. Helmer and C.W. Reich, Phys. Rev. C 49, 1845 (1994). [2] Z.O. Guimaraes-Filho and O. Helene, Phys. Rev. C 71, 044303 (2005)

Why the half-life measurement of the isomer is important? To provide direct evidence for the existence of the isomer. All previous information on the isomer 229mTh have an indirect character. Thus, there exists a need to prove its existence, like by the lifetime determination. The previous attempts to determine the lifetime of the isomer had produced conflicting results. In particular, two groups attempted to determine this half-life using radiochemical and alpha-spectroscopy techniques. Browne et al., [1] concluded that the 229mTh half-life must be either shorter than 6 h or longer than 20 days, while Mitsugashira et al., [2] gives 13.9 (3.0) hrs. The half-life value would be decisive in planning of the GSI experiment. It would allow more precise and detailed theoretical considerations. In particular from the knowledge of T1/2(M1), assuming the energy of the transition, one would deduce the matrix element responsible for the mixing of states with different spin in a strong magnetic field. [1] E. Browne et al., Phys. Rev. C 64, 014311 (2001) . [2] T. Mitsugashira et al., J. Radioanal. Nucl. Chem. 255, 63 (2003).

T1/2 < 6 hrs or T1/2 > 20 days Small branch to the isomeric state of only 2% ! Production: 233U alpha decay

T1/2=13.9(3.0) hrs Production: (gamma,n) on 230Th. Problems: other reaction channels, and no firm identification of any lines as belonging to 225Ra. Solution: to confirm via different production mechanism, like via the beta-decay of 229Ac. 22 MeV 30 MeV

giving for the isomer T1/2 = 10+18-5 hrs IS386: B(M1;3/2+ - 5/2+)th = 0.025 m2N giving for the isomer T1/2 = 10+18-5 hrs E. Ruchowska et al., conditionally accepted for publication in Phys.Rev. C, on Feb 2006.

Proposed experiment (estimates for a 22-hrs irradiation): Production of the isomeric state in 229mTh via the beta decays of: 229Fr (50 s) to 229Ra (4 min) to 229Ac (1 hr) to 229gTh (7340 y) expected population of the isomeric state in 229mTh: 50% expected decay branch to the states in 225Ra: 5.2 x 10-7 expected minimum production of 229Ra: 1 x 10+6 /s expected no. of 229mTh alpha’s in a 22 hrs irradiation: 40 000 and feeding the 150 keV 3/2+[631] state in 225Ra ~20 000 losses after 22 hrs irradiation and 14 hrs cooling 80-90% Alpha detector efficiency (Si) 30-35% expected number of detected alphas to 150 keV level 1000 to all other levels 2000 Problems: strong beta activity of 229Ac 2 x 10+5 dps

lines, and possible coincidences with the gamma lines in Ge detector. The key to identify the isomeric decay is the time evolution of the alpha lines, and possible coincidences with the gamma lines in Ge detector.

Measurements will be performed in the following way: Using short (1-2 hrs) and long irradiations (22 hrs). Two beams line will be used (we propose: LA1 and LA2). For high counting rates we will use scintillator detectors for alphas (thin plastic NE111A or thin disk of BaF2); these will be placed in air. After some cooling time (active via scintillator measurements) we will place the source in vacuum for surface barrier alpha counting. Ge detectors will be used at each alpha counting station.