Fission fragment trasport out of U-233

U-233 Coating

The fission chamber has a fissionale material as a sensitive neutron target. The fission chambers have usually target material that make them sensitive for neutrons, them most commonly used material for detecting thermal neutron is U-235, in case of fast neutron Th-232 or U-238 represnt good target. in some cases the fission chamber may contain more than one taget to detect a wide range of emitted neutrons.<ref name= "Andriamonje"> Novel Micromegas Detector for In-Core Nuclear Reactor Neutron Flux Measurements </ref>

In this research, the neutron sensitive material used in the fission chamber is Uranium 233. U-233 has a relatively high neutron fission cross section in the energy range of 0.1-30 MeV as shown in the figure below.

The coating on a circular metal plate which is connected to the cathode. The coating of U-233 has a cirular shape of an inch in diameter and 20-40 um in thickness. The circular plate is glued to a square FR4 copper clad plate of length of 12 cm and thickness of 1 mm, the plate has a hole of 4 cm in diameter that keeps the U-233 coating exposed to the chamber gas.

Probability of a fission fragment to create a signal

U-233 is relatively sensitive but the coating thickness may stop the fission fragments. Compared to the other neutron sensitive targets, U-233 is relatively sensitive to a wide energy range of neutrons, the range may extend from 0.1 MeV to more than 30 MeV. However, coating thickness determines the if a fission fragment can escape from its surface and to cause an ionization in the gas to create a signal. The range of a fission fragment is calculated in UO3, of thickness 0.1 um and density 7.29 g/cm^3, is 12.07 +_ 0.09 um, <ref name = Hudler> J.C. hadler, Radiation Measurements Vol 43 (2008) pf S334-S336</ref> assuming the fission fragment mobility is the same as the UO3. As the thickness increases more than 0.1 um, the range will decrease according to the following equation <ref name = Hudler/> :

[math] R= R_T - \lt r\gt [/math]

[math] \lt r\gt = \frac{ln(d/R_T) + 0.5}{ 2/d (1-R_T/d) + R_T^{-1}} [/math]

Producing free electrons in the gas will concequienty create a signal. Although the fission fragment leaves the surface of U-233 coating and ionizes the gas, but minimum 200 electrons of energy around 20 eV become free through the fissin fragment's track in the U-233 coating of thickness 1-10um. <ref name = Wang> Wang Jin et al, Applied Physics Letters, Vol 82, 20 (2003) pf 3553-3555</ref>

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