1 | The final state of radiative capture is described by either photon |
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2 | multiplicities, or photon production cross-sections, and the discrete and |
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3 | continuous contributions to the photon energy spectra, along with the angular |
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4 | distributions of the emitted photons. |
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5 | |
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6 | For the description of the photon multiplicity there are two supported data |
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7 | representations. |
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8 | It can either be tabulated as a function of the energy of |
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9 | the incoming neutron for each discrete photon as well as the eventual |
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10 | continuum contribution, or the full transition probability array is known, and |
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11 | used to determine the photon yields. If photon production cross-sections are |
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12 | used, only a tabulated form is supported. |
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13 | |
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14 | The photon energies $E_\gamma$ are associated to the multiplicities or the |
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15 | cross-sections for |
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16 | all discrete photon emissions. For the continuum contribution, the normalised |
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17 | emission probability $f$ is broken down into a weighted |
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18 | sum of normalised distributions $g$. |
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19 | $$f\left(E\rightarrow E_\gamma\right)~=~ |
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20 | \sum_{i}p_i(E)g_i(E\rightarrow E_\gamma)$$ |
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21 | The weights $p_i$ are tabulated as a function of the energy $E$ |
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22 | of the incoming neutron. |
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23 | For each neutron energy, the distributions $g$ are tabulated as a function |
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24 | of the photon energy. As in the ENDF/B-VI data formats\cite{ENDF}, several |
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25 | interpolation laws are used to minimise the amount of data, and optimise |
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26 | the descriptive power. All data are derived from evaluated data libraries. |
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27 | |
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28 | The techniques used to describe and sample the angular distributions are |
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29 | identical to the case of elastic scattering, with the difference that there |
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30 | is either a tabulation or a set of legendre coefficients for each photon |
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31 | energy and continuum distribution. |
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32 | |
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33 | As an example of the results is shown in figure\ref{capture} the energy |
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34 | distribution of the emitted photons for the radiative |
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35 | capture of 15~MeV neutrons on Uranium ($^{238}$U). |
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36 | Similar comparisons for photon yields, energy and angular distributions have |
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37 | been performed for capture on |
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38 | ${\rm^{238}U}$, |
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39 | ${\rm^{235}U}$, |
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40 | ${\rm^{23}Na}$, and |
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41 | ${\rm^{14}N}$ |
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42 | for a set of incoming neutron energies. |
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43 | In all cases investigated |
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44 | the agreement between evaluated data and Monte Carlo is very good. |
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45 | |
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46 | \begin{figure}[b!] % fig 1 |
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47 | % \centerline{\epsfig{file=hadronic/lowEnergyNeutron/neutrons/plots/cap92u238.energy.fine.epsi,height=5.5in,width=3.5in}} |
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48 | \includegraphics[angle=0,scale=0.6]{hadronic/lowEnergyNeutron/neutrons/plots/cap92u238.energy.fine.epsi} |
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49 | \vspace{10pt} |
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50 | \caption{Comparison of data and Monte Carlo for photon energy |
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51 | distributions for radiative capture of 15~MeV neutrons on Uranium ($^{238}U$). |
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52 | The points are evaluated data, the histogram is |
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53 | the Monte Carlo prediction.} |
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54 | \label{capture} |
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55 | \end{figure} |
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