3. Recent HRIBF Research - First
RIB g-factor Measurement Using Recoil-in-Vacuum Technique
(N. J. Stone, Spokesperson)
The advent of RIBs presents experimenters with new challenges as well as new opportunities. It has long been recognized that the g-factor of a nuclear ground- or excited state gives valuable evidence as to its single-particle and/or collective makeup. The usual method of studying picosecond excited-state g-factors has, in recent years, been the transient-field (TF) method. In TF experiments, beam nuclei are excited by Coulomb excitation and, as they decay by a highly anisotropic gamma emission, the angular distribution of this emission is caused to rotate by their passing through a ferromagnetic metal layer in which they are subject to Larmor precession. The observed rotation yields both magnitude and sign of the g-factor. However, the rotation angles are small, requiring high statistical accuracy in the data, and, to prevent any other perturbation of the distribution, after emerging from the ferromagnetic layer, the excited nuclei, plus the rest of the beam, are stopped in a third region of the target. With RIBs TF has problems; RIBs are orders of magnitude weaker than SIBs, so high statistical accuracy requires long runs, and stopping a RIB in the target produces large unwanted activity which can be particularly troublesome in the not uncommon case that beam impurities decay through the excited state under study.
A team from HRIBF, combined with others from Oxford, Tennessee, ANU, Brno, Maryland, Rutgers and Yale, have recently completed a successful g-factor measurement on the first 2+ state of the RIB isotope 132Te . The experiment took full advantage of the combination of the detector systems CLARION and Hyball. Thin C targets, without backing, were used so that, following Coulomb excitation, both the excited Te isotopes and the recoiling C nuclei left the target. The C nuclei were detected in Hyball, which has a series of segmented rings subtending different ranges of angle q with respect to the beam axis. The specific segment recording the event determines its reaction plane. Gamma decay of the Te 2+1 excited state at 973 keV was detected in one of the CLARION detectors, determining both the value of qg and the angle of emission f relative to the reaction plane. The un-reacted RIB activity has long lifetime and leaves the target region, so causing no unwanted background.
As the excited Te ions leave the target, their nuclear spins I are aligned perpendicular to the beam following the Coulomb excitation process. For 396-MeV beam energy incident on an 0.83 mg/cm2 C target the Te ions emerge with charge state range from ~ 32+ - 36+. For each charge state there is a range of electronic states having different total angular momentum J. I and J then precess about their mutual resultant F and this precession leads to de-alignment of the nuclear spins, producing, in turn, attenuation of the anisotropy of the angular distribution of their gamma decay. The attenuation is caused by Larmor precession over the lifetime of the excited state and is thus a measure of the g-factor of the excited state. This method of measuring g-factors, known as the Recoil-in-Vacuum method [RIV] was first developed in the 1970's, and is set to make a comeback with the advent of RIBs .
The attenuation was calibrated by short measurements on the first 2+ states in the SIB isotopes 122,126,130Te having known g-factors and lifetimes [Figs.3-1 and 3-2]. Fig.3-1 shows the unperturbed angular distribution from 130Te when it does not recoil into vacuum [the SIBs could be stopped in a second, Cu backed, target], compared with the attenuated distributions found for RIV 126,132Te using the thin C target, all having the same range of ionic charge state. Fig.3-2 shows the attenuation coefficients for the SIBs plotted as a function of the product of their excited state lifetimes and g-factors, gτ. The measured attenuations for 132Te, again with the same ionic states, are also shown in Fig.3-2. With the lifetime of 2.6(2) ps from the recent HRIBF measurement of the B(E2) of this level , these results give the g-factor as (+)0.35(5).
It is predicted that the g-factors of the 2+1 states of the Te isotopes should increase from their level of close to 0.3 at mid-shell to a sharp maximum of about 0.7 at the neutron shell closure, before falling steeply beyond the shell, possibly to negative values. The observed increase is rather less than recent calculations predict [4,5].
The combination of HRIBF detection facilities CLARION plus Hyball has proved ideally suited for RIV experiments, opening the prospect of g-factor study with RIBs of existing strength. The team aim to continue this work over the coming year, planning to measure on 134,136Te by the RIV method. We also will explore the practicability of TF measurements.
N. J. Stone et al., submitted to
G. Goldring, in Heavy Ion Collisions Vol 3, ed R. Bock (North Holland, Amsterdam, 1982) p. 484.
D. C. Radford et al., Phys.Rev.Letters 88, 222501 (2002).
J. Terasaki et al., Phys.Rev. C66, 054313 (2002).
B. A. Brown et al., arXiv:nucl-th/0411099v1 24 Nov 2004.