Event-by-Event Fission Modeling
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below the neutron separation energy S n , so that neutron evaporation continues as
long as energetically possible.
After neutron evaporation has ceased, the excited product nucleus emits photons.
First, statistical photons are emitted isotropically with an energy distribution
sampled from a black-body spectrum modulated by a giant-dipole resonance form
factor. When the nuclear excitation energy enters the regime of the tabulated decays
from the RIPL-3 compilation [1], FREYA switches to a discrete cascade which is
continued until the half-life exceeds a specified value, t max , based on the detector
response time, or until the nucleus is in its ground state. The average photon
energy and photon multiplicity depend on t max as well as the minimum photon
energy measurable in the detectors, denoted in FREYA as g min . While the photon
observables depend on both t max and g min , these quantities are not parameters but
depend on specific experimental details.
3 Recent Results on Photon Emission
The detector-dependent quantities g min and t max were added to FREYA in Ref. [2]
when the RIPL lines were introduced. We studied the effect of changing g min on the
calculated average E γ and M γ . If g min is on the order of a few hundred keV, the
total energy emitted in photons does not change much. Increasing g min to 1–2 MeV
would correspondingly reduce the measured photon energy by a similar amount.
There is a stronger dependence of M γ on g min . A 1 MeV cutoff energy reduces
the photon multiplicity by a factor of three or more, depending on c S . (A larger c S
results in the emission of more soft photons. See Ref. [2] for details.)
The effects of t max on E γ and N γ are more subtle. Increasing t max is more likely
to increase the photon multiplicity from some individual long-lived isomers but the
changes are generally on the percent level [2].
We compare our calculated prompt fission photon spectrum to the results of
Oberstedt et al. [3]. The measured high-energy slope of the photon energy spectrum
is in good agreement with the FREYA calculation, even without including the
experimental uncertainties. The peaks observed in the low-energy part of the photon
spectrum, shown in Fig. 1b, arising from the inclusion of the RIPL transitions,
also agree well with FREYA. The peaks in the data are somewhat above our
calculation, leading to a discrepancy between our calculated multiplicity and the
data using the same g min and t max . Obsertedt et al. measured M γ = 8.19 ± 0.11,
E γ = 6.92 ± 0.09 MeV, and E γ /M γ = 0.85 ± 0.02 MeV while, for the same
cutoffs, we find M γ = 6.93, E γ = 6.48 MeV, and E γ /M γ = 0.93 MeV.
295
below the neutron separation energy S n , so that neutron evaporation continues as
long as energetically possible.
After neutron evaporation has ceased, the excited product nucleus emits photons.
First, statistical photons are emitted isotropically with an energy distribution
sampled from a black-body spectrum modulated by a giant-dipole resonance form
factor. When the nuclear excitation energy enters the regime of the tabulated decays
from the RIPL-3 compilation [1], FREYA switches to a discrete cascade which is
continued until the half-life exceeds a specified value, t max , based on the detector
response time, or until the nucleus is in its ground state. The average photon
energy and photon multiplicity depend on t max as well as the minimum photon
energy measurable in the detectors, denoted in FREYA as g min . While the photon
observables depend on both t max and g min , these quantities are not parameters but
depend on specific experimental details.
3 Recent Results on Photon Emission
The detector-dependent quantities g min and t max were added to FREYA in Ref. [2]
when the RIPL lines were introduced. We studied the effect of changing g min on the
calculated average E γ and M γ . If g min is on the order of a few hundred keV, the
total energy emitted in photons does not change much. Increasing g min to 1–2 MeV
would correspondingly reduce the measured photon energy by a similar amount.
There is a stronger dependence of M γ on g min . A 1 MeV cutoff energy reduces
the photon multiplicity by a factor of three or more, depending on c S . (A larger c S
results in the emission of more soft photons. See Ref. [2] for details.)
The effects of t max on E γ and N γ are more subtle. Increasing t max is more likely
to increase the photon multiplicity from some individual long-lived isomers but the
changes are generally on the percent level [2].
We compare our calculated prompt fission photon spectrum to the results of
Oberstedt et al. [3]. The measured high-energy slope of the photon energy spectrum
is in good agreement with the FREYA calculation, even without including the
experimental uncertainties. The peaks observed in the low-energy part of the photon
spectrum, shown in Fig. 1b, arising from the inclusion of the RIPL transitions,
also agree well with FREYA. The peaks in the data are somewhat above our
calculation, leading to a discrepancy between our calculated multiplicity and the
data using the same g min and t max . Obsertedt et al. measured M γ = 8.19 ± 0.11,
E γ = 6.92 ± 0.09 MeV, and E γ /M γ = 0.85 ± 0.02 MeV while, for the same
cutoffs, we find M γ = 6.93, E γ = 6.48 MeV, and E γ /M γ = 0.93 MeV.