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F. Zeiser et al.
beams. In these cases, calculations within the statistical framework can provide
an alternative approach to obtain (n,x) cross-sections. These rely essentially on
precise measurements of nuclear level densities (NLD) and γ -ray strength functions
(γ SF) [1].
The Oslo Method [2, 3] can be used to analyze particle-γ coincidence spectra
from transfer reactions to simultaneously extract NLDs and γ SFs. In a campaign
to study actinides the method has been applied to the compound nuclei 231−233 Th,
232,233 Pa, 237−239 U, 238 Np [4–7], and 243 Pu [8] using different light-ion reactions.
The extracted γ SFs show a significant enhancement between about 2 and 4 MeV,
which is consistent with the location [9] of a low energy orbital M1 scissors
resonance (SR).
Larsen et al. [3] have shown that the population of a limited spin range may lead
to distortions of the γ SF. This has been observed in some of the previous studies
on actinides [4–8] due to the low-spin transfer using the (d,p) reaction mechanism,
where an ad hoc procedure for the correction was adopted. In this proceeding, we
focus on the first systematic analysis of the impact on the Oslo Method results for a
realistic spin-parity population for the (d,p) 240 Pu reaction.
2 Experimental Setup and Data Analysis
The (d, p) 240 Pu experiment was conducted using a 12 MeV deuteron beam at the
Oslo Cyclotron Laboratory (OCL). The 0.4 mg/cm 2 thick 239 Pu target was purified
using an anion-exchange resin column procedure [10] prior to electroplating on a
1.9 mg/cm 2 beryllium backing.
The outgoing charged particles were detected with the SiRi particle telescope [11]. SiRi consists of 64 silicon particle telescopes with a thickness of 130 μm
for the front (E) and 1550 μm for the back (E) detectors, and was placed at
backwards angles (126 ◦ to 140 ◦ ). The CACTUS array [12] measured coincident
γ rays and was composed of 26 lead collimated 5 × 5 NaI(Tl) crystals with a
total efficiency of 14.1(2)% at E γ = 1.33 MeV. Additionally, four Parallel Plate
Avalanche Counters (PPAC) [13] were used to detect fission events.
The reaction kinematics allowed for selection of (d,p) events and conversion
of the detected proton energy to the excitation energy E x of the compound
nucleus 240 Pu. Prompt γ rays were selected from a ±14 ns wide time-window
with background correction applied. The γ -ray spectra were unfolded following
the procedure of [14], using response functions [15] that were updated in 2012.
Next, an iterative subtraction technique [16] was applied to obtain the primary
γ rays P (E x , E γ ) (also called first-generation γ rays) for each E x bin from the
initial spectra, which include all γ rays of the decay cascades. Here we relied on the
assumption that the (d,p)-reaction will populate a similar spin-parity distribution
for the levels in an E x bin i as would be populated from γ -decay from a higher
F. Zeiser et al.
beams. In these cases, calculations within the statistical framework can provide
an alternative approach to obtain (n,x) cross-sections. These rely essentially on
precise measurements of nuclear level densities (NLD) and γ -ray strength functions
(γ SF) [1].
The Oslo Method [2, 3] can be used to analyze particle-γ coincidence spectra
from transfer reactions to simultaneously extract NLDs and γ SFs. In a campaign
to study actinides the method has been applied to the compound nuclei 231−233 Th,
232,233 Pa, 237−239 U, 238 Np [4–7], and 243 Pu [8] using different light-ion reactions.
The extracted γ SFs show a significant enhancement between about 2 and 4 MeV,
which is consistent with the location [9] of a low energy orbital M1 scissors
resonance (SR).
Larsen et al. [3] have shown that the population of a limited spin range may lead
to distortions of the γ SF. This has been observed in some of the previous studies
on actinides [4–8] due to the low-spin transfer using the (d,p) reaction mechanism,
where an ad hoc procedure for the correction was adopted. In this proceeding, we
focus on the first systematic analysis of the impact on the Oslo Method results for a
realistic spin-parity population for the (d,p) 240 Pu reaction.
2 Experimental Setup and Data Analysis
The (d, p) 240 Pu experiment was conducted using a 12 MeV deuteron beam at the
Oslo Cyclotron Laboratory (OCL). The 0.4 mg/cm 2 thick 239 Pu target was purified
using an anion-exchange resin column procedure [10] prior to electroplating on a
1.9 mg/cm 2 beryllium backing.
The outgoing charged particles were detected with the SiRi particle telescope [11]. SiRi consists of 64 silicon particle telescopes with a thickness of 130 μm
for the front (E) and 1550 μm for the back (E) detectors, and was placed at
backwards angles (126 ◦ to 140 ◦ ). The CACTUS array [12] measured coincident
γ rays and was composed of 26 lead collimated 5 × 5 NaI(Tl) crystals with a
total efficiency of 14.1(2)% at E γ = 1.33 MeV. Additionally, four Parallel Plate
Avalanche Counters (PPAC) [13] were used to detect fission events.
The reaction kinematics allowed for selection of (d,p) events and conversion
of the detected proton energy to the excitation energy E x of the compound
nucleus 240 Pu. Prompt γ rays were selected from a ±14 ns wide time-window
with background correction applied. The γ -ray spectra were unfolded following
the procedure of [14], using response functions [15] that were updated in 2012.
Next, an iterative subtraction technique [16] was applied to obtain the primary
γ rays P (E x , E γ ) (also called first-generation γ rays) for each E x bin from the
initial spectra, which include all γ rays of the decay cascades. Here we relied on the
assumption that the (d,p)-reaction will populate a similar spin-parity distribution
for the levels in an E x bin i as would be populated from γ -decay from a higher