Abstract Generated abstract
This study investigates nuclear resonance fluorescence from highly excited states of nickel-62, cadmium-114, and lead-208 using gamma rays produced by thermal neutron capture in iron and a self-absorption method for determining radiative widths. Measurements of scattered gamma spectra, temperature effects, and absorber dependence were used to identify elastic and inelastic transitions from 7.64 MeV states in nickel-62 and cadmium-114 and from the 7.28 MeV state in lead-208. The work reports partial radiative widths, total widths, and mean lifetimes for the observed levels, including evidence for cascade de-excitation in nickel-62 and cadmium-114 and strong retardation of their electric dipole ground-state transitions relative to Weisskopf estimates. Results for lead-208 are compared with previous measurements and support a ground-state radiative width smaller than the total width.
Full Text
Reports of the Academy of Sciences of the USSR
1967. Volume 174, No. 2
UDC 535:539.1
PHYSICS
Academician of the Academy of Sciences of the Uzbek SSR S. V. STARODUBTSEV, R. B. BEKBZHANOV, A. A. ISLAMOV
NUCLEAR RESONANCE FLUORESCENCE OF HIGHLY EXCITED STATES OF \( \mathrm{Ni}^{62} \), \( \mathrm{Cd}^{114} \), \( \mathrm{Pb}^{208} \)
Using a new method for determining the radiative widths of high-energy nuclear states \((^{1})\), we investigated the nuclear resonance fluorescence of \( \mathrm{Ni}^{62} \), \( \mathrm{Cd}^{114} \), and \( \mathrm{Pb}^{208} \), using \(\gamma\)-rays from the capture of thermal neutrons in iron.
The \(\gamma\)-rays scattered by the scatterer at an angle of \(\sim 135^\circ\) to the incident beam are recorded by an NaJ(Tl) crystal, \(d = 7\) cm and \(h = 7\) cm, connected to an FEU-52 and to a multichannel pulse analyzer. The crystal is shielded in front by a filter consisting of 1 cm of lead and 10 cm of boroparaffin. The source—scatterer—detector distance is \(\sim 4\) m. The results of all control experiments show that the resulting signals are \(\gamma\)-rays from the iron source scattered by the given scatterer.
\(\mathrm{Fe}^{57} \to \mathrm{Ni}^{62}\). The physical parameters characterizing the resonance level are the total width \(\Gamma\), the partial width for the radiative transition to the ground state \(\Gamma_{\gamma0}\), and the statistical factor \(g\). In addition, the distance \(\delta = |E_e - E_p|\) between the emission and absorption lines must be taken into account, since the resonant-scattering cross section depends on it. These parameters can be determined by measurements of the scattering and absorption cross sections at different temperatures.
The effective cross section \(\langle \sigma_{rp} \rangle\) was found to be \((0.53 \pm 0.02)\) barn for the 7.64 MeV resonance in nickel; it was calculated under the assumption of a dipole angular distribution.
Analysis of the observed temperature effect in the range 78–600°K gives the value \(\delta = (12.5 \pm 0.5)\) eV.
The existence of inelastic scattering clearly shows that, after resonant excitation of the 7.64 MeV \((1-)\) level of nickel, quanta with energies 6.47, 5.60, 5.34, 5.0, and 4.7 MeV are emitted. This occurs in competition with emission of elastically scattered quanta with energy 7.64 MeV; that is, upon de-excitation of the excited state, cascades arise and \(\gamma\)-quanta of lower energy appear, leaving the nucleus in an excited state.
Indeed, in addition to the 7.64 MeV transition to the ground state, we observe the indicated transitions with the intensity ratio
\(I_0(7.64) : I_1(6.47) : I_2(5.60) : I_3(5.34) : I_4(5.0) : I_5(4.7) \approx 1.0 : 0.2 : 0.5 : 0.2 : 0.1 : 0.5.\)
These observed transitions from the 7.64 MeV \((1-)\) level occur to the levels of \( \mathrm{Ni}^{62} \): 1.17 \((2+)\); 2.05 MeV \((2+)\); 2.30 MeV \((0+)\); 2.66 MeV \((2+)\), and 2.94 MeV \((2+)\).
Using the method of self-absorption of nuclear fluorescence \((^{1,3})\), with \((R_0 = 0.040 \pm 0.004)\) and \(n_p = 3.17 \cdot 10^{21}\ \mathrm{cm}^{-3}\), \(d = 1.5\) cm, \(\lambda^2 = 2.65 \cdot 10^{-22}\ \mathrm{cm}^2\), \(\delta = 12.5\) eV, \(\Delta_a = 7.54\) eV, \(\Delta_e = 8.2\) eV, we succeeded for the first time in determining the partial widths of all the observed \(E1\)-transitions from the 7.64 MeV \((1-)\) excited level of \( \mathrm{Ni}^{62} \). The values obtained for the partial widths of the \(\Gamma_\gamma\) transitions, presented in Table 1, make it possible to determine the total width \(\Gamma = (5.4 \pm 1.6)\) eV and the corresponding mean lifetime of the 7.64 MeV \((1-)\) level of \( \mathrm{Ni}^{62} \): \(\tau = (1.22 \pm 0.36)\cdot 10^{-16}\) sec.
The partial lifetime of the 7.64 MeV radiative transition to the ground state \((1^- \to 0^+)\) proved to be \(\tau_{\gamma}^{0} = (6.6 \pm 0.7)\cdot 10^{-16}\) sec.
The Weisskopf formula, based on the single-particle model, gives for this transition \(\tau_{\gamma\,\mathrm{single}} = 13.8 \cdot 10^{-19}\) sec. Thus, the \(E1\) transition from the 7.64 MeV \((1^-)\) level of \(\mathrm{Ni}^{62}\) turns out to be retarded by approximately 3 orders of magnitude.
Table 1
| Nucleus | Transition energy, MeV | \(I_i^\pi \to I_f^\pi\) | \(\Gamma_\gamma\), eV | Nucleus | Transition energy, MeV | \(I_i^\pi \to I_f^\pi\) | \(\Gamma_\gamma\), eV |
|---|---|---|---|---|---|---|---|
| \({}_{28}^{62}\mathrm{Ni}_{34}\) | 7.64 | \(1^- \to 0^+\) | \(1.0 \pm 0.10\) | \({}_{48}^{114}\mathrm{Cd}_{66}\) | 7.64 | \(1^- \to 0^+\) | \(0.20 \pm 0.05\) |
| \({}_{28}^{62}\mathrm{Ni}_{34}\) | 6.47 | \(1^- \to 2^+\) | \(0.33 \pm 0.11\) | \({}_{48}^{114}\mathrm{Cd}_{66}\) | 7.08 | \(1^- \to 2^+\) | \(0.05 \pm 0.01\) |
| \({}_{28}^{62}\mathrm{Ni}_{34}\) | 5.60 | \(1^- \to 2^+\) | \(1.26 \pm 0.38\) | \({}_{48}^{114}\mathrm{Cd}_{66}\) | 6.50 | \(1^- \to 0^+\) | \(0.13 \pm 0.03\) |
| \({}_{28}^{62}\mathrm{Ni}_{34}\) | 5.34 | \(1^- \to 0^+\) | \(0.60 \pm 0.18\) | \({}_{48}^{114}\mathrm{Cd}_{66}\) | 5.80 | \(1^- \to 2^+\) | \(0.18 \pm 0.04\) |
| \({}_{28}^{62}\mathrm{Ni}_{34}\) | 5.0 | \(1^- \to 2^+\) | \(0.36 \pm 0.14\) | \({}_{82}^{208}\mathrm{Pb}_{126}\) | 7.28 | \(1^- \to 0^+\) | \(0.78 \pm 0.03\) |
| \({}_{28}^{62}\mathrm{Ni}_{34}\) | 4.70 | \(1^- \to 2^+\) | \(2.15 \pm 0.64\) |
The value obtained by us for \(\Gamma_\gamma^0/\Gamma = 0.185\) by the self-absorption method is well confirmed by the result \((\Gamma_\gamma^0/\Gamma = 0.170)\) of work on elastic scattering of photons using bremsstrahlung from betatron electrons \((^2)\), but it does not confirm the ratio \(\Gamma_\gamma^0/\Gamma = 1\) assumed in \((^3)\).
\(\mathrm{Fe}^{57} \to \mathrm{Cd}^{114}\). We obtained \(\gamma\)-spectra due to nuclear resonance fluorescence in a cadmium scatterer with Sn and Cd absorbers relative to an Sn scatterer and background. The detected photons have energies 7.64; 7.08; 6.50; 5.80 MeV, i.e., when the 7.64 MeV excited state is illuminated, cascades arise and \(\gamma\)-quanta of lower energy appear, leaving the nucleus in an excited state.
For the first time using the method of self-absorption of nuclear fluorescence, we were able to determine the partial widths of all transitions from the 7.64 MeV excited level of \(\mathrm{Cd}^{114}\). The values obtained for the partial widths \(\Gamma_\gamma\) of transitions from the 7.64 MeV \((1^-)\) level of \(\mathrm{Cd}^{114}\), presented in Table 1, make it possible to determine the total width \(\Gamma = (0.56 \pm 0.13)\) eV and the corresponding mean lifetime \(\tau = (1.18 \pm 0.26)\cdot 10^{-15}\) sec of the 7.64 MeV \((1^-)\) level of \(\mathrm{Cd}^{114}\). The partial lifetime of the 7.64 MeV radiative transition to the ground state \((1^- \to 0^+)\) turns out to be \(\tau_\gamma^0 = (3.3 \pm 0.8)\cdot 10^{-15}\) sec. The Weisskopf formula, based on the single-particle model, gives \(\tau_{\gamma\,\mathrm{single}} = 9.2\cdot 10^{-19}\) sec. Thus, the \(E1\) transition with \(E_\gamma = 7.64\) MeV turns out to be retarded by approximately 4 orders of magnitude.
\(\mathrm{Fe}^{57} \to \mathrm{Pb}^{208}\). In order to test the correctness of the measurement method, we studied resonance scattering of \(\gamma\)-rays \(\mathrm{Fe}^{56}(n,\gamma)\) on \(\mathrm{Pb}^{208}\), which is the most studied.
Experiments with self-absorption (Pb and Bi absorbers of various thicknesses were used) showed that the 7.28 MeV resonance is caused by a level with width \(\Gamma_\gamma^0 = (0.78 \pm 0.03)\) eV. This is in good agreement with \((0.80 \pm 0.03)\) eV \((^3)\) and may be compared with the Doppler broadening data \((0.6 \pm 0.2)\) eV \((^2)\).
To check the validity of the relation \(\Gamma_\gamma^0/\Gamma \approx 1\) \((^3)\), we measured nuclear fluorescence at different absorber and scatterer temperatures (78; 300; 680° K), in order to detect the presence of inelastic scattering. A small resonant fluorescence was found in the energy region 4.7 and 4.1 MeV, which should have corresponded to transitions to the 2.61 MeV \((3^-)\) and 3.2 MeV \((5^-)\) states of \(\mathrm{Pb}^{208}\). However, the spin values do not allow us to consider these transitions as electric dipole transitions from the 7.28 MeV \((1^-)\) state of \(\mathrm{Pb}^{208}\) to these levels.
In our experiment \(\Gamma_\gamma^0 = 0.62\,\Gamma\), which is well confirmed by measurements of self-absorption of elastically scattered photons using the \((p,\alpha\gamma)\) reaction \((^4)\).
Institute of Nuclear Physics
Academy of Sciences of the Uzbek SSR
Received
14 XI 1966
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