Feeding of Rh and Ag isomers in fast-neutron-induced reactions

Background: In ($n,n^{\prime }$) reactions on stable Ir and Au isotopes in the mass $A=190$ region, the experimentally established feeding of the isomers relative to the feeding of the corresponding ground states increases with increasing neutron energy, up to the neutron energy where the ($n,2n$) reaction channel opens up, and then decreases.

Purpose: In order to check for similar behavior in the mass $A=100$ region, the feeding of isomers and ground states in fast-neutron-induced reactions on stable isotopes in this mass region was studied. This is of especial interest for Rh which can be used as a radiochemical detector.

Methods: Excited states were studied using the ($n,n^{\prime }\gamma$), ($n,2n\gamma$), and ($n,3n\gamma )$ reactions on ${}^{103}\mathrm{Rh}$ and ${}^{109}\mathrm{Ag}$. A germanium detector array for $\gamma$-ray detection and the broad-spectrum pulsed neutron source of the Los Alamos Neutron Science Center's Weapons Neutron Research facility were used for the measurement. The energy of the incident neutrons was determined using the time-of-flight technique.

Results: Absolute partial $\gamma$-ray cross sections were measured for 57 transitions feeding isomers and ground states in ${}^{101,102,103}\mathrm{Rh}$ and ${}^{107,108,109}\mathrm{Ag}$. The feeding of the isomers was found to be very similar in the corresponding reaction channels and it is compared to the feeding determined for the ground states.

Conclusions: The opening of reaction channels at higher neutron energies removes angular momentum from the residual nucleus and reduces the population of the higher-spin isomers relative to the feeding of the lower-spin ground states. Similar behavior was observed in the mass $A=190$ region in the feeding of higher-spin isomers, but the reverse behavior was observed in ${}^{176}\mathrm{Lu}$ with a lower-spin isomer and a higher-spin ground state.

Figure 1

Partial level schemes showing the transitions of Rh isotopes relevant to the present work. All $\gamma$-ray and level energies are given in keV. The white portion of the arrows indicates the fraction of the decay that is internally converted. Data were taken from Ref. [2] in (a), from Ref. [3] in (b), and from Ref. [6] in (c).

Figure 2

Partial level schemes showing the transitions of Ag isotopes relevant to the present work. All $\gamma$-ray and level energies are given in keV. The white portion of the arrows indicates the fraction of the decay that is internally converted. Data were taken from Ref. [11] in (a), from Ref. [7] in (b), and from Ref. [10] in (c).

Figure 3

Examples of $\gamma$-ray spectra summed over all neutron energies from the (a) Ag and (b) Rh experiments. Some of the transitions from Figs. 1 and 2 are indicated. The energies of the transitions are in keV. Unmarked peaks are transitions that are not of interest for the present work; i.e., their contribution to the feeding of ground states or isomers of the ($n,n^{\prime }$), ($n,2n$), and ($n,3n$) reaction channels is already accounted for by other $\gamma$ rays, or they belong to other reaction channels.

Figure 4

Examples of absolute partial cross sections as a function of incident neutron energy for nine Rh transitions from Fig. 1. Bars along the $E_n$ axis represent the width of the neutron-energy bins in the TOF technique.

Figure 5

Examples of absolute partial cross sections as a function of incident neutron energy for nine Ag transitions from Fig. 2. Bars along the $E_n$ axis represent the width of the neutron-energy bins in the TOF technique.

Figure 6

Sums of cross sections obtained for the transitions in the level schemes in Figs. 1 and 2 prompt-feeding the 1/$2^{-}$ ground states (open squares) and $7/2^{+}$ isomers (open diamonds) of (a) ${}^{109}\mathrm{Ag}$ and (b) ${}^{103}\mathrm{Rh}$. The experimentally observed total feeding (sum of the previous two sums; solid circles) is then compared to the evaluated reaction channel cross section (solid lines) from the ENDF/B-VII.1 library (see Ref. [20], and references therein) in both cases. Legends apply to both panels. Bars along the $E_n$ axis represent the width of the neutron-energy bins in the TOF technique.

Figure 7

Sums of cross sections obtained for the transitions in the level schemes in Figs. 1 and 2 prompt-feeding the ground states (open squares) and years-long-lived isomers (open diamonds) of (a) ${}^{108}\mathrm{Ag}$ and (b) ${}^{102}\mathrm{Rh}$. The experimentally observed total feeding (sum of the previous two sums; solid circles) is then compared to the evaluated $(n,2n)$ channel cross sections (solid lines) from the ENDF/B-VII.1 library (see Ref. [20], and references therein) in both cases. Legends apply to both panels. Bars along the $E_n$ axis represent the width of the neutron-energy bins in the TOF technique.

Figure 8

Sums of cross sections obtained for the transitions in the level schemes in Figs. 1 and 2 prompt-feeding the $1/2^{-}$ ground states (open squares) of (a) ${}^{107}\mathrm{Ag}$ and (b) ${}^{101}\mathrm{Rh}$, and the isomeric $7/2^{+}$ and $9/2^{+}$ states (open diamonds) of ${}^{107}\mathrm{Ag}$ and ${}^{101}\mathrm{Rh}$, respectively. The experimentally observed total feeding (sum of the previous two sums; solid circles) is then compared to the evaluated $(n,3n)$ channel cross section (solid lines) from the ENDF/B-VII.1 library (see Ref. [20], and references therein) and from the JEFF-3.1.1 library (see Ref. [21], and references therein). Legends apply to both panels. Bars along the $E_n$ axis represent the width of the neutron-energy bins in the TOF technique.

Figure 9

Ratios of sums of cross sections obtained for the transitions prompt-feeding the $7/2^{+}$ isomers over the sums obtained for the transitions observed to feed promptly the $1/2^{-}$ ground states of ${}^{103}\mathrm{Rh}$ (open squares) in Fig. 1 and ${}^{109}\mathrm{Ag}$ (open circles) in Fig. 2. Bars along the $E_n$ axis represent the width of the neutron-energy bins in the TOF technique.

Figure 10

Ratios of sums of cross sections obtained for the tran-sitions prompt-feeding the years-long-lived isomers over the sums obtained for the transitions observed to feed promptly the ground states of ${}^{102}\mathrm{Rh}$ (open squares) in Fig. 1 and ${}^{108}\mathrm{Ag}$ (open circles) in Fig. 2. Bars along the $E_n$ axis represent the width of the neutron-energy bins in the TOF technique.

Figure 11

Ratios of sums of cross sections obtained for the transitions prompt-feeding the $7/2^{+}$ isomer of ${}^{107}\mathrm{Ag}$ and the $9/2^{+}$ isomer of ${}^{101}\mathrm{Rh}$ over the sums obtained for the transitions observed to feed promptly the $1/2^{-}$ ground states of ${}^{107}\mathrm{Ag}$ (open circles) in Fig. 2 and ${}^{101}\mathrm{Rh}$ (open squares) in Fig. 1, respectively. Bars along the $E_n$ axis represent the width of the neutron-energy bins in the TOF technique.

Figure 12

Production cross sections for various reaction channels predicted using the code ${\mathrm{CoH}}_3$ [22] in neutron-induced reactions of ${}^{103}\mathrm{Rh}$ and ${}^{109}\mathrm{Ag}$. Only channels with a maximum cross section larger than 1 mb are shown. The reaction channels are shown in the legend while the emitted particles are included in the plot. Solid lines represent neutron-only-emitting channels, dotted lines include one proton emitted, dashed lines include one deuteron emitted, and dashed-dotted lines include one $\alpha$ particle (short dashes) or one triton (long dashes) emitted. The long-dashed lines at low neutron energies represent neutron capture.

Figure 13

Upper plot: production cross sections for various reaction channels predicted using the code ${\mathrm{CoH}}_3$ [22] in neutron induced reactions of ${}^{197}\mathrm{Au}$. The reaction channels are shown in the legend while the emitted particles are included in the plot. Solid lines represent only-neutron-emitting channels and dotted lines include one proton emitted. Lower plot: Ratios of sums of cross sections obtained in GEANIE experiments described in Ref. [1] for the transitions prompt-feeding the $11/2^{-}$ isomers of ${}^{195}\mathrm{Au}$ [39] and ${}^{189}\mathrm{Ir}$ [40] over the sums obtained for the transitions observed to feed promptly the $3/2^{+}$ ground states of ${}^{195}\mathrm{Au}$ (open circles) and ${}^{189}\mathrm{Ir}$ (open squares), respectively. Specifically, the 180.1-, 261.8-, 439.5-, and 549.4-keV transitions that feed in parallel the ground state of ${}^{195}\mathrm{Au}$; the 207.1-, 387.9-, 560.3-, 628.3-, 749.5-, and 961.9-keV transitions that feed directly the $11/2^{-}$ isomer of ${}^{195}\mathrm{Au}$; the 94.3-, 113.8-, 300.5-, and 317.7-keV transitions that feed directly the ground state of ${}^{189}\mathrm{Ir}$; and the 243.5-, 364.8-, 459.5-, and 466.0-keV transitions that feed directly the $11/2^{-}$ isomer of ${}^{189}\mathrm{Ir}$, were used. Bars along the $E_n$ axis represent the width of the neutron-energy bins in the TOF technique.

Figure 14

Ratio of the sum of cross sections obtained for two transitions feeding directly the $1^{-}$ isomer over the cross section obtained for one transition observed to feed directly the $7^{-}$ ground state in ${}^{176}\mathrm{Lu}$. Specifically, the 112.9- and 310.2-keV transitions feed directly the $1^{-}$ isomer and the 184.1-keV transition feeds directly the $7^{-}$ ground state. Data are from a ${}^{\mathrm{nat}}\mathrm{Lu}$ GEANIE experiment [45]. Bars along the $E_n$ axis represent the width of the neutron-energy bins in the TOF technique.