Fission probabilities of ${}^{242}\mathrm{Am},{}^{243}\mathrm{Cm}$, and ${}^{244}\mathrm{Cm}$ induced by transfer reactions

We have measured the fission probabilities of ${}^{242}\mathrm{Am},{}^{243}\mathrm{Cm}$, and ${}^{244}\mathrm{Cm}$ induced by the transfer reactions ${}^{243}\mathrm{Am}({}^3\mathrm{He},{}^4\mathrm{He}),{}^{243}\mathrm{Am}({}^3\mathrm{He},t)$, and ${}^{243}\mathrm{Am}({}^3\mathrm{He},d)$, respectively. The details of the experimental procedure and a rigorous uncertainty analysis, including a correlation matrix, are presented. For ${}^{243}\mathrm{Cm}$ our data show clear structures well below the fission threshold. To our knowledge, it is the first time that these structures have been observed for this nucleus. We have compared the measured fission probabilities to calculations based on the statistical model to obtain information on the fission barriers of the produced fissioning nuclei.

Figure 1

Schematic three-dimensional representation of the setup for fission probability measurements.

Figure 2

Energy loss versus residual energy as measured in one of the Si telescopes for the ${}^3\mathrm{He}+{}^{208}\mathrm{Pb}$ reaction at 24 MeV beam energy. The ground state and first excited states of ${}^{209}\mathrm{Bi}$ and ${}^{207}\mathrm{Pb}$ are indicated by arrows.

Figure 3

Energy loss versus residual energy in one of the Si telescopes for the ${}^3\mathrm{He}+{}^{243}\mathrm{Am}$ reaction at 24 MeV. The ejectiles associated to the different Z lines are indicated.

Figure 4

Number of detected ${}^4\mathrm{He}$ (green triangles) ejectiles as a function of the excitation energy of ${}^{242}\mathrm{Am}$ after subtraction of events coming from reactions on the target backing. The ${}^4\mathrm{He}$ ejectiles stemming from reactions on target contaminants are present above 12 MeV. The blue circles represent the singles spectrum $N_{\mathrm{sing}}\left(E^{*}\right)$ after interpolation and the red squares the fission coincidence spectrum $N_{\mathrm{coinc}}\left(E^{*}\right)$ for ${}^{242}\mathrm{Am}$ (see text).

Figure 5

Energy spectrum of fission fragments detected in one solar cell in coincidence with deuterons detected in one telescope.

Figure 6

Time difference between one telescope and all the solar cells of the fission detector for the deuteron (a), triton (b), and $\alpha$ (c) transfer channels. The vertical lines indicate the selection of coincidence events. The shaded area represents the random coincidences included in this selection.

Figure 7

(a) Angular distribution of fission fragments for ${}^{243}\mathrm{Cm}$ at $E^{*}=7.5\mathrm{MeV}$. The number of detected fission events divided by the solid angle subtended by the corresponding cell is shown as a function of the average emission angle of the fragments in the center of mass of the fissioning nucleus. The full line represents the fit function according to Eq. (3) with $\frac{W\left(0^{\circ }\right)}{W\left({90}^{\circ }\right)}=1+\frac{a}{b}=1.47\pm 0.17$ and reduced chi-squared ${\chi }^2=0.96$. (b) Fission-detector efficiency as a function of $E^{*}$ of ${}^{243}\mathrm{Cm}$. The shaded area corresponds to the geometrical efficiency of our fission detector including the corresponding uncertainty. The symbols represent the effective fission efficiency including the fission-fragment angular anisotropy and kinematic effects caused by the recoil energy of ${}^{243}\mathrm{Cm}$.

Figure 8

Fission probabilities of the $244\mathrm{C}{\mathrm{m}}^{*}$ (a), $243\mathrm{C}{\mathrm{m}}^{*}$ (b), and ${}^{242}\mathrm{A}{\mathrm{m}}^{*}$ (c) as a function of excitation energy compared to the results by Gavron et al. [9] and by Back et al. [7, 24, 25]. The dashed line in the upmost left panel represents the $E^{*}$ above which the singles spectrum is significantly polluted by reactions on target contaminants. The neutron separation energy $S_n$ of each nucleus is indicated by the arrows. [(d)–(f)] Associated correlation matrices (see Sec. 3b).

Figure 9

Number of fission-fragment-triton coincidences before (red dashed line) and after (black full line) subtraction of random coincidences. The spectrum of ejectiles $N_{\mathrm{eje}}$ is represented by the dashed-dotted lines and the random-coincidences spectrum by the blue full lines.

Figure 10

Fission probability of ${}^{243}\mathrm{Cm}$ obtained with the ${}^{243}\mathrm{Am}({}^3\mathrm{He},t)$ reaction as a function of excitation energy in the proximity of the fission threshold. On the left we show the results obtained with the telescopes placed at 130° (a) and on the right with the telescopes placed at 90° (b) with respect to the beam axis. The neutron separation energy $S_n$ is indicated.

Figure 11

Calculated neutron-induced orbital angular-momentum distributions ${\sigma }_n(E_n,l)$ populated in the reaction $n+{}^{241}\mathrm{Am}$ at different incident neutron energies, $E_n$.

Figure 12

Experimental fission probabilities (full circles) compared to statistical-model calculations; see text for details.