Spectroscopy of ${}^{61}\mathrm{Fe}$ via the neutron transfer reaction ${}^2\mathrm{H}({}^{60}\mathrm{Fe},p){}^{61}\mathrm{Fe}^{*}$

The direct component of the ${}^{60}\mathrm{Fe}(n,\gamma ){}^{61}\mathrm{Fe}$ cross section was investigated by populating the bound states of the ${}^{61}\mathrm{Fe}$ nucleus through the $(d,p\gamma )$ transfer reaction in inverse kinematics using a radioactive beam of ${}^{60}\mathrm{Fe}$ at $27A$ MeV. The experiment was performed at GANIL using the MUST2 array and an annular double-sided silicon strip detector for the detection of the light charged particle in coincidence with the photons measured in the EXOGAM $\gamma$-ray detectors. For the first time, the spectroscopic factors of the first $3/2^{-}$, $5/2^{-}$, $1/2^{-}$, and $9/2^{+}$ states of ${}^{61}\mathrm{Fe}$ were deduced experimentally from an adiabatic distorted wave approximation analysis of the data. The obtained results show a very good agreement with the shell-model predictions. The calculated direct component of the $(n,\gamma )$ cross section was found negligible and of about 2% of the total, indicating a dominant resonant component.

Figure 1

Experimental ${}^{61}\mathrm{Fe}$ level scheme below 2.5 MeV adapted from the work of Radulov et al. [20]. States are labeled by their excitation energy in keV and spin parity. The $\gamma$-ray decays displayed here are those for which branchings are larger than 15%. For more details see Ref. [20].

Figure 2

Schematic view of the experimental setup used to study the ${}^{60}\mathrm{Fe}(d,p\gamma ){}^{61}\mathrm{Fe}$ reaction in inverse kinematics. See text for details.

Figure 3

${}^{61}\mathrm{Fe}$ excitation energy spectra obtained with the MUST2 array (top) and the S1 detector (bottom). The blue (dark gray) histograms only rely on protons whereas the red (light gray) histograms additionally require a $\gamma$-ray coincidence with EXOGAM; each event is counted as many times as the $\gamma$ multiplicity, mainly one in the main peak structure. Laboratory angles covered by each charged-particle detector are indicated.

Figure 4

Doppler-corrected $\gamma$-ray energy spectra obtained in coincidence with protons detected in the charged-particle detectors (S1 or MUST2). The black and red shaded histograms correspond to events having an ${}^{61}\mathrm{Fe}$ excitation energy in the 0.0–1.0 MeV and 1.0–2.0 MeV ranges, respectively.

Figure 5

${}^{61}\mathrm{Fe}$ excitation energy spectrum from MUST2 events when gating on the 207 keV $\gamma$-ray transition. The red curve is a fit of the spectrum with three components (see text for details).

Figure 6

Comparison of the Doppler-corrected $\gamma$-ray energy spectra obtained in coincidence with protons detected in MUST2 (blue histogram) and S1 (red shaded histogram) for ${}^{61}\mathrm{Fe}$ excitation energy in the 0.0–1.0 MeV range.

Figure 7

Top: Fit of the ${}^{61}\mathrm{Fe}$ excitation energy spectrum (S1 detector) with the g.s. and 207, 391, and 861 keV states and a higher state centered at 1600 keV. Bottom: ${}^{61}\mathrm{Fe}$ excitation energy spectrum for MUST2 array with the contribution of the same set of ${}^{61}\mathrm{Fe}$ states. The red linear curve corresponds to the carbon background. The turquoise dash-double-dotted, pink dash-dotted, green dashed, black solid, and black dotted curves correspond to the g.s. and the 207, 391, 861, and 1600 keV states, respectively. The red thick solid curve corresponds to the sum.

Figure 8

Experimental proton angular distributions for (a) the excitation energy range corresponding to the 207 keV state and (b) the 861 keV excited state, together with zero-range ADWA calculations normalized to the data. The black solid line in (a) corresponds to a combination of calculations with $\ell =3$ for the $f_{5/2}207$ keV state (pink dashed curve) and $\ell =1$ for the ($p_{1/2}$) 391 keV (green dotted curve) and the $p_{3/2}$ gs (turquoise dash-dotted curve) states. The blue solid line in (b) corresponds to calculation with $\ell =4$ for the $g_{9/2}861$ keV state.

Figure 9

Experimental proton angular distributions for the whole excitation energy range between 2.0 and 4.0 MeV, together with zero-range ADWA calculations normalized to the data. The black solid line corresponds to an optimized combination of an $\ell =2$ for $d_{5/2}$ (red dashed curve), $\ell =3$ for $f_{5/2}$ (pink dashed-dotted curve), and $\ell =4$ for $g_{9/2}$ (blue dotted curve).

Figure 10

(a) Experimental SFs obtained in this work with their error bars. (b) Predicted SF from shell-model calculation (see text). The turquoise, green, red, pink, and blue boxes correspond to $p_{3/2}$, $p_{1/2}$, $d_{5/2}$, $f_{5/2}$, and $g_{9/2}$ orbitals, respectively. The arrow indicates the energy range that corresponds to the displayed spectroscopic factors in the second peak structure.

Figure 11

Direct capture cross section of the ${}^{60}\mathrm{Fe}(n,\gamma ){}^{61}\mathrm{Fe}$ reaction with the tedca code. The turquoise dash-dotted, pink dashed, and green dotted curves correspond to calculations of the $p_{3/2}$ ground state, $f_{5/2}$ first excited state, and $p_{1/2}$ second excited state, respectively. The black solid curve is the sum of the three contributions.

Figure 12

Three states at 200, 800, and 1600 keV are contributing with a peak yield of 30, 100, and 50 to the spectrum displayed in (a). The corresponding countings within the three chosen ranges are 30, 83, and 40. The elements of a matrix M are constructed (see text) as illustrated in (b). The inverted matrix ${\mathbf{M}}^{-1}$ is used to calculate the initial peak yields from the observed range countings.