Picosecond lifetime measurements in 109Cd and 110Cd

164 S. Harissopulos et al. / Nuclear Physics A 683 (2001) 157–181
As pointed out in [27], nuclear deorientation is expected to have a significant influence
to the intensities of γ -transitions deexciting states with spin J 4. In addition, in the
case of E2 γ -transitions (J = 2) this influence is rather strong at 0 ◦ and rather weak
at 55 ◦ . Hence, the ratio of the decay curve of a given excited state derived at a certain
angle θ to this determined at 55 ◦ , provides a reliable check for possible influence of the
nuclear deorientation effect. In case this “anisotropy ratio” is constant, the decay curve
derived at the certain angle θ needs no corrections. In the present work, this check has
been partially hindered by the fact that during the experiment the detector placed at 55 ◦ has
provided spectra with long left tails in the γ -peaks, probably due to neutron damage, and
also irregular gain shifts, which in many cases occurred in the same single spectrum. Due
to these problems anisotropy ratios have been derived via following procedure: the “less
damaged” 55 ◦ -spectra, i.e. the γ -singles spectra including relatively short left tails and no
gain shifts, have been normalized to the 260 keV γ -transition of 109 Cd. From the latter
(normalized) spectra, the intensities of the unshifted peak of the 2 + 1 → 0+ 1
γ -transition
of the 110 Cd nucleus have been derived via a very careful integration procedure. This
procedure has also been carried out for the γ -singles spectra taken at 160 ◦ . Unfortunately,
the quality of the 55 ◦ spectra did not allow to obtain anisotropy ratios for the 4 + 1 → 2+ 1
γ -transition as well. In the following, the intensities of the unshifted peak of a γ -transition
i → j obtained from spectra normalized to the 260 keV γ -transition of 109 Cd, which have
beentakenatanangleθ, will be declared as R ij (x, θ). Hereby, it has to be pointed out that
R ij (x, θ) have not been determined from difference spectra like quantities U ij (x).
The intensities R ij (x, θ = 55 ◦ ) of the 2 + 1 → 0+ 1
γ -transition of 110 Cd have been
compared with the respective R ij (x, 0 ◦ ) and R ij (x, 160 ◦ ). The corresponding anisotropy
ratios are shown in Fig. 4. According to this figure, the decay curves determined at 0 ◦
and 160 ◦ are influenced from the nuclear deorientation effect at distances x 100 µm
and x 1000 µm, respectively. In addition, by fitting a single exponential function of the
form f(x)= a · exp(−x/τ R ) + c to the 0 ◦ /55 ◦ anisotropy ratio we obtained a relaxation
time-constant τ R = 410(120) ps. It has to be emphasized that the fit function used serves
only to correct the data measured at 0 ◦ , i.e. the data points of the decay curve determined
Fig. 4. Anisotropy ratios measured in the present work for the 2 + 1 → 0+ 1 γ -transition of the 110 Cd.
The solid curve shown in (a) has been obtained by the fitting of the single exponential function
f(x)= a · exp(−x/τ R ) + c to the data, which yielded a relaxation time-constant τ R = 410(120) ps.