Actinide Capture
175
3 Results for Even Uranium Isotopes
The gamma-ray spectra from neutron capture on 234,236,238 U were studied using
targets from 1 to 3 mg/cm 2 thick. For these isotopes, the continuum fission cross
section is negligible below about 0.5 MeV. DICEBOX calculations of the spectra
were made using the prescription of Kopecky and Uhl [6] for the photon strength
function: a generalized Lorentzian form (GLO) for the E1 giant dipole, and standard
Lorentzians (SLO) for the M1 “spin-flip” resonance and E2 contribution. The
parameters for the giant dipole were taken from the Empire compilation [7],
and for the spin-flip and E2 contributions from RIPL-3. In addition, a low-lying
(about 2–3 MeV) M1 component, consistent with the scissors-mode resonance, was
also included. The scissors mode was represented by 2 standard Lorentzians, at
2.15 and 2.90 MeV. The energy and width of the Lorentzians were taken from
an Oslo-method analysis [8], while the strengths were varied to provide the best
representation of the spectra. The same parameters were used for all three isotopes.
It was shown in ref. [9] that the gamma-ray spectra could not be reproduced by
the Kopecky and Uhl prescription, but additional strength at low energies, most
likely M1, was required. That analysis was updated and extended to 234,236 U in ref.
[10], with results shown below in Fig. 1. Figure 1 shows gamma-ray spectra for
several 1/2 + resonances in each isotope and for several gamma-ray multiplicities.
The calculations labeled CoH 3 were made using the generalized Lorentzian (GLO)
form for the giant dipole with parameters from ref [7]. The calculations labeled
MGLO were made using the modified generalized Lorentzian form [11] for the
counts
0
1000
2000
3000
4000
= 4
cl
M
)
γ
U(n,
234
(a)
31 eV res
77 eV res
MGLO
3
CoH
counts
0
1000
2000
3000
4000
= 3
cl
M
(b)
-Ray Energy (MeV)
γ
0
1
2
3
4
5
counts
0
500
1000
1500
= 2
cl
M
(c)
counts
0
500
1000
1500
2000
=4
cl
M
)
γ
U(n,
236
(d)
44 eV res
71 eV res
MGLO
3
CoH
counts
0
500
1000
1500
=3
cl
M
(e)
-Ray Energy (MeV)
γ
0
1
2
3
4
5
counts
0
200
400
600
=2
cl
M
(f)
counts
0
2000
4000
6000
= 4
cl
M
)
γ
U(n,
238
(g)
21 eV res
36 eV res
66 eV res
102 eV res
MGLO
3
CoH
counts
0
2000
4000
= 3
cl
M
(h)
-Ray Energy (MeV)
γ
0
1
2
3
4
5
counts
0
1000
2000
= 2
cl
M
(i)
Fig. 1 Measured γ -ray spectra for several 1/2 + resonances in 234,236,238 U(n,γ ) compared to
calculations made with photon strength-function and nuclear level-density parameters obtained
from systematics used in the CoH 3 code and using the MGLO and GLO models for the E1 strength.
The GLO calculations are labeled “CoH 3 ” in the figure, see text for more details. The resonance
energies are indicated in each panel. The y-axis counts are arbitrarily normalized
175
3 Results for Even Uranium Isotopes
The gamma-ray spectra from neutron capture on 234,236,238 U were studied using
targets from 1 to 3 mg/cm 2 thick. For these isotopes, the continuum fission cross
section is negligible below about 0.5 MeV. DICEBOX calculations of the spectra
were made using the prescription of Kopecky and Uhl [6] for the photon strength
function: a generalized Lorentzian form (GLO) for the E1 giant dipole, and standard
Lorentzians (SLO) for the M1 “spin-flip” resonance and E2 contribution. The
parameters for the giant dipole were taken from the Empire compilation [7],
and for the spin-flip and E2 contributions from RIPL-3. In addition, a low-lying
(about 2–3 MeV) M1 component, consistent with the scissors-mode resonance, was
also included. The scissors mode was represented by 2 standard Lorentzians, at
2.15 and 2.90 MeV. The energy and width of the Lorentzians were taken from
an Oslo-method analysis [8], while the strengths were varied to provide the best
representation of the spectra. The same parameters were used for all three isotopes.
It was shown in ref. [9] that the gamma-ray spectra could not be reproduced by
the Kopecky and Uhl prescription, but additional strength at low energies, most
likely M1, was required. That analysis was updated and extended to 234,236 U in ref.
[10], with results shown below in Fig. 1. Figure 1 shows gamma-ray spectra for
several 1/2 + resonances in each isotope and for several gamma-ray multiplicities.
The calculations labeled CoH 3 were made using the generalized Lorentzian (GLO)
form for the giant dipole with parameters from ref [7]. The calculations labeled
MGLO were made using the modified generalized Lorentzian form [11] for the
counts
0
1000
2000
3000
4000
= 4
cl
M
)
γ
U(n,
234
(a)
31 eV res
77 eV res
MGLO
3
CoH
counts
0
1000
2000
3000
4000
= 3
cl
M
(b)
-Ray Energy (MeV)
γ
0
1
2
3
4
5
counts
0
500
1000
1500
= 2
cl
M
(c)
counts
0
500
1000
1500
2000
=4
cl
M
)
γ
U(n,
236
(d)
44 eV res
71 eV res
MGLO
3
CoH
counts
0
500
1000
1500
=3
cl
M
(e)
-Ray Energy (MeV)
γ
0
1
2
3
4
5
counts
0
200
400
600
=2
cl
M
(f)
counts
0
2000
4000
6000
= 4
cl
M
)
γ
U(n,
238
(g)
21 eV res
36 eV res
66 eV res
102 eV res
MGLO
3
CoH
counts
0
2000
4000
= 3
cl
M
(h)
-Ray Energy (MeV)
γ
0
1
2
3
4
5
counts
0
1000
2000
= 2
cl
M
(i)
Fig. 1 Measured γ -ray spectra for several 1/2 + resonances in 234,236,238 U(n,γ ) compared to
calculations made with photon strength-function and nuclear level-density parameters obtained
from systematics used in the CoH 3 code and using the MGLO and GLO models for the E1 strength.
The GLO calculations are labeled “CoH 3 ” in the figure, see text for more details. The resonance
energies are indicated in each panel. The y-axis counts are arbitrarily normalized