Dipole strength in ${}^{89}\mathrm{Y}$ up to the neutron-separation energy

Photoexcitation of the $N=50$ nucleus ${}^{89}\mathrm{Y}$ has been performed at the bremsstrahlung facility at the superconducting electron accelerator ELBE at electron energies of $E_e^{\mathrm{kin}}=9.5$ and 13.2 MeV. About 250 levels up to the neutron-separation energy were identified. Statistical methods were applied to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. The photoabsorption cross section derived in this way up to the neutron-separation energy is combined with the photoabsorption cross section obtained from ($\gamma$, $n$) data and provides information about the extension of the giant dipole resonance toward energies below the neutron-separation energy. An enhancement of $E1$ strength has been found in the range from about 6 to 11 MeV. The experimental photoabsorption cross sections of ${}^{89}\mathrm{Y}$ and of the neighboring $N=50$ isotones ${}^{88}\mathrm{Sr}$ and ${}^{90}\mathrm{Zr}$ are compared with predictions of the quasiparticle-random-phase approximation.

Figure 1

Absolute efficiency values of the two detectors at ${127}^{°}$ determined from standard calibration sources (circles) and efficiency calculated with GEANT3 (solid line).

Figure 2

Absolute photon flux deduced from transitions in ${}^{11}\mathrm{B}$ (circles) and calculated photon flux (solid line).

Figure 3

Parts of a spectrum of photons scattered from ${}^{89}\mathrm{Y}$ combined with ${}^{11}\mathrm{B}$ at an electron energy of 9.5 MeV. This spectrum contains events of the two detectors at ${127}^{°}$.

Figure 4

Part of a spectrum of photons scattered from ${}^{89}\mathrm{Y}$ combined with ${}^{11}\mathrm{B}$ at an electron energy of 13.2 MeV. This spectrum contains events of the two detectors at ${127}^{°}$.

Figure 5

Ratios of integrated cross section $I_{s+f}$ obtained at different electron energies.

Figure 6

Detection limit in units of the reduced partial width times the branching ratio in the measurement at 13.2 MeV.

Figure 7

Experimental spectrum of ${}^{89}\mathrm{Y}$ measured at $E_{\mathrm{e}}^{\mathrm{kin}}=13.2$ MeV and $\Theta ={127}^{°}$, corrected for room background and detector response, and simulated spectrum of atomic background, multiplied with efficiency and measuring time.

Figure 8

Scattering cross sections in ${}^{89}\mathrm{Y}$, derived as ${\sigma }_{\gamma {\gamma }^{\text{'}}}=\sum _{\Delta }{I}_s/\Delta$, not corrected for branching and averaged over energy bins of $\Delta =0.2$ MeV, derived from the full intensity distribution obtained from subtraction of the atomic background from the experimental spectrum at $E_e^{\mathrm{kin}}=13.2$ MeV shown in Fig. 7 (triangles) and derived from the resolved peaks given in Table only (circles). Note that the cross sections of the peaks around 3 MeV included in “peaks $+$ cont” are influenced by feeding whereas the cross sections of the resolved peaks were deduced from the measurement at 7.0 MeV and do not include considerable feeding intensities.

Figure 9

Level densities of $J=3/2$ states as a function of excitation energy resulting from the BSFG model with the parameters taken from Ref. [34] and from the experimental levels listed in Table (EXP).

Figure 10

Simulated intensity distribution of transitions depopulating levels in a 0.1-MeV bin around 9 MeV in ${}^{89}\mathrm{Y}$. The squares depict the intensities obtained from 10 individual nuclear realizations.

Figure 11

Branching ratios of ground-state transitions obtained from the simulations of $\gamma$-ray cascades for ${}^{89}\mathrm{Y}$. The squares represent the values of 10 individual nuclear realizations.

Figure 12

Uncorrected (black circles) and corrected (red circles) photoabsorption cross sections deduced from the measurement at $E_{\mathrm{e}}^{\mathrm{kin}}$ $=$ 13.2 MeV in comparison with ($\gamma ,n$) data [38] (green squares) and ($\gamma$, $p$) calculations [43] (blue triangles).

Figure 13

Total photoabsorption cross section of ${}^{89}\mathrm{Y}$ obtained by combining the present ($\gamma$, ${\gamma }^{\text{'}}$) data, the ($\gamma$, $n$) data of Ref. [38], and the $(\gamma ,p)$ calculations of Ref. [43]. The data were averaged over 0.5-MeV bins to reduce statistical fluctuations. The black dashed line represents a Lorentz distribution with the parameters given in the text. The blue solid line is the result of the QRPA calculations discussed in Sec. 5.

Figure 14

Total photoabsorption cross section of ${}^{88}\mathrm{Sr}$ as taken from Ref. [16]. The data were averaged over 0.5-MeV bins to reduce statistical fluctuations. The black dashed line represents a Lorentz distribution with the parameters given in Ref. [16]. The blue solid line is the result of the QRPA calculations discussed in Sec. 5.

Figure 15

Total photoabsorption cross section of ${}^{90}\mathrm{Zr}$ as taken from Ref. [17]. The data were averaged over 0.5-MeV bins to reduce statistical fluctuations. The black dashed line represents a Lorentz distribution with the parameters given in Ref. [17]. The blue solid line is the result of the QRPA calculations discussed in Sec. 5.