Spin polarization of ${}^{34}\mathrm{Al}$ fragments produced by nucleon pickup at intermediate energies

The polarization of ${}^{34}\mathrm{Al}$ fragments, produced by single neutron pickup from a ${}^9\mathrm{Be}$ target by a ${}^{36}\mathrm{S}$ projectile at 77.5 MeV/nucleon, have been observed at GANIL via the detection of resonantly destroyed β-asymmetry. The reaction-induced polarization is deduced using a tentative spin/parity assignment for the ${}^{34}\mathrm{Al}$ ground state. A positive polarization was measured near the peak of the ${}^{34}\mathrm{Al}$ yield curve. A kinematical model based on the spectator-participant model for projectile fragmentation reactions has been extended in order to take into account the features of pickup reactions, i.e., the picked-up nucleon having an average momentum equal to the Fermi momentum and aligned along the incident beam direction. The trend-line in the observed spin-orientation is very well reproduced by this model.

Figure 1

Scheme of the experimental setup. The up and down ΔE-E plastic scintillators allow the detection of the β emissions. The whole setup is positioned in a vacuum chamber between the poles of an electromagnet, not visible in this drawing. The rf-coil was not used (but present) in the measurements described in this work.

Figure 2

(Color online) Asymmetry of the β emissions as a function of the magnetic field for ${}^{34}\mathrm{Al}$ fragments implanted in Si (data from run 1).

Figure 3

(Color online) ${}^{34}\mathrm{Al}$ β-decay scheme (from Ref. 12).

Figure 4

(Color online) Schematic representation of the variables defining the spin-polarization produced via (a) projectile fragmentation or (b) pickup of a target nucleon. The axes are defined in the projectile rest-frame. The definitions of the variables are in the text.

Figure 5

Experimental yield of ${}^{34}\mathrm{Al}$ (points) as a function of the relative fragment momentum $(p_f-p_0)/p_0$ compared with the predictions of the kinematical model for pickup reactions (full line, scaled to data) and of the LISE program [7] (dashed line, scaled by 1/3). The experimental beam intensity was 1 μA. The error bars on the observed yields are included in the symbols. The rotational angle used in the calculations (full line) is ${\theta }_R=8.7°$, the grazing angle for a ${}^9\mathrm{Be}+{}^{76}\mathrm{S}$ (77.5 MeV/nucleon) reaction.

Figure 6

(Color online) Maximum estimations for the spin-polarization observed in the two runs (points and crosses, respectively) compared with the predictions of the kinematical model for pickup reactions developed here (lines). The theoretical values overestimate the experimental ones, so the calculated values are scaled by f = 0.25. The same ${\theta }_R=8.7°$ is used as for the yield calculations.