Photofission neutron rate

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The goal of this calculation is to determine the portion of detected neutron singles that originate from photofission in a Th-232 target, as opposed to neutrons from gamma, 1n knockout. MCNP is used to determine the bremsstrahlung energy spectrum entering the experimental cell. This calculation cannot be done entirely in MCNP6 since it is missing cross section tables for Th-232 photofission. The neutron detectors are assumed to have 100% efficiency for neutrons above 0.5 MeV ,and 0% below 0.5 MeV.


The number of neutrons from 1n knockout and photofission are given by:

[math]N_{1n}=k \int \sigma_{1n}(E_{\gamma})*\epsilon_{1n}(E_{\gamma})*\gamma(E_{\gamma})dE_{\gamma} [/math]

[math]N_{fiss}=k \int \sigma_{fiss}(E_{\gamma})*\epsilon_{fiss}*\gamma(E_{\gamma})*\bar{\nu}(E{\gamma})dE_{\gamma} [/math]


[math]k[/math] is an arbitrary constant of proportionality which depends on target thickness, atoms/gram, units, ect.

[math]\sigma_{1n}[/math] and [math]\sigma_{fiss}[/math] are the cross-sections for 1n knockout and photofission, respectively.

[math]\epsilon_{1n}(E_{\gamma})[/math] is the fraction of [math](\gamma,1n)[/math] neutrons that are above the 0.5 MeV detection threshold.

[math]\epsilon_{fiss} [/math] is a constant equal to the fraction of [math](\gamma,fiss)[/math] neutrons above the 0.5 MeV threshold, which is equal to 0.85.

[math]\gamma(E_{\gamma})[/math] is the bremsstrahlung photon energy distribution.

[math]\bar{\nu}(E{\gamma}) [/math] is the mean photofission neutron multiplicity as a function of incident photon energy.

[math]\frac{ N_{fiss} }{N_{fiss}+N_{1n} }=0.40[/math]

So, the nuber of fissions per measurble neutron is:

[math]\frac{ N_{fiss} }{N_{fiss}+N_{1n} }=0.40[/math]

(G,1n).png FractionAbove0.5MeV.png

G,1n and G,fiss Th.png

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