116
A. Voinov
between s and p-wave resonances. P-wave resonances are in the range (I t ± 3/2)
and have opposite parity from the parity of the target nucleus. The fact that neutron
resonances are known in very limited energy and spin intervals indicates that level
density models are only constrained in these intervals. Both excitation energy and
spin dependences at lower and higher energies require model assumptions.
Along with neutron resonance data, data from discrete level scheme are used
to constrain model level density functions at the low excitation energy region.
Discrete level scheme is well known up to a certain excitation energy, typically,
up to 2–4 MeV depending on the mass range. However, model function in the
low-energy region might not perform well causing further uncertainties in model
parameterizations. The transition region from discrete states to continuum might be
prone to the structure effects, such as pairing and/or shell ones resulting in deviation
of model functions from smooth behavior.
The other potential issue comes from often non-trivial analysis of neutron
resonances. Missing resonances due to experimental threshold as well as misidentification of resonance spins and parities (due to difficulties in distinguishing between
s-wave and p-wave resonances) might lead to incorrect estimates of resonance
spacings. There are two major data sources on parameters of neutron resonances,
these are in Refs. [7] and [10]. The fact that for some nuclei resonance parameters
are different indicates the existence of the problem of neutron resonance evaluations.
3.2 The Oslo Method
The experimental method known as the “Oslo method” allows studying the nuclear
level density extracted from the particle-γ coincidence matrix P(E ex , E γ ), where
E ex and E γ are excitation energy and γ -ray energy, respectively [13]. Reactions
( 3 He, 3 He
γ ), ( 3 He, αγ ), (d, pγ ), or similar are used. As it is shown in Ref. [13],
such a technique allows extracting the level density function ρ(E ∗ ) oslo which
is related to the “true” level density function ρ(E ∗ ) true through the following
transformation:
ρ(E
∗ ) true = ρ(E
∗ ) oslo A exp(BE
∗ ).
(1)
Coefficients A and B need to be determined from auxiliary information, usually, the
density of levels in the discrete energy region (the first anchor point) and the density
of neutron resonances (the second anchor point) are used. Density of discrete levels
are well known up to a certain excitation energy from a level scheme [7]. The density
of neutron resonances known only in a very limited spin interval (see Sect. 3.1)
has to be converted to the total level density, integrated over all spins populated
in Oslo type experiments. This conversion relies on models because of lack of
experimental information on spin distribution in the region of neutron resonances,
as was mentioned in the previous Sect. 3.1 and will be discussed in Sect. 3.4.
A. Voinov
between s and p-wave resonances. P-wave resonances are in the range (I t ± 3/2)
and have opposite parity from the parity of the target nucleus. The fact that neutron
resonances are known in very limited energy and spin intervals indicates that level
density models are only constrained in these intervals. Both excitation energy and
spin dependences at lower and higher energies require model assumptions.
Along with neutron resonance data, data from discrete level scheme are used
to constrain model level density functions at the low excitation energy region.
Discrete level scheme is well known up to a certain excitation energy, typically,
up to 2–4 MeV depending on the mass range. However, model function in the
low-energy region might not perform well causing further uncertainties in model
parameterizations. The transition region from discrete states to continuum might be
prone to the structure effects, such as pairing and/or shell ones resulting in deviation
of model functions from smooth behavior.
The other potential issue comes from often non-trivial analysis of neutron
resonances. Missing resonances due to experimental threshold as well as misidentification of resonance spins and parities (due to difficulties in distinguishing between
s-wave and p-wave resonances) might lead to incorrect estimates of resonance
spacings. There are two major data sources on parameters of neutron resonances,
these are in Refs. [7] and [10]. The fact that for some nuclei resonance parameters
are different indicates the existence of the problem of neutron resonance evaluations.
3.2 The Oslo Method
The experimental method known as the “Oslo method” allows studying the nuclear
level density extracted from the particle-γ coincidence matrix P(E ex , E γ ), where
E ex and E γ are excitation energy and γ -ray energy, respectively [13]. Reactions
( 3 He, 3 He
γ ), ( 3 He, αγ ), (d, pγ ), or similar are used. As it is shown in Ref. [13],
such a technique allows extracting the level density function ρ(E ∗ ) oslo which
is related to the “true” level density function ρ(E ∗ ) true through the following
transformation:
ρ(E
∗ ) true = ρ(E
∗ ) oslo A exp(BE
∗ ).
(1)
Coefficients A and B need to be determined from auxiliary information, usually, the
density of levels in the discrete energy region (the first anchor point) and the density
of neutron resonances (the second anchor point) are used. Density of discrete levels
are well known up to a certain excitation energy from a level scheme [7]. The density
of neutron resonances known only in a very limited spin interval (see Sect. 3.1)
has to be converted to the total level density, integrated over all spins populated
in Oslo type experiments. This conversion relies on models because of lack of
experimental information on spin distribution in the region of neutron resonances,
as was mentioned in the previous Sect. 3.1 and will be discussed in Sect. 3.4.