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;Cf-252

Cf-252 is one of the most commonly used natural source in industry and laboratories. Cf-252 has a  continuous neutron spectrum up to 10 MeV that has a Maxwellian distribution, so it can be precisely described by:


<math> \frac{dN}{dE} = E^{0.5} exp(-E/T) </math> 

T= 1.3 MeV. <ref > Leo</ref>


[[File:Cf_spec.png | 90px]]


 n/g/cm^2/s
 fission fragments, double bum fission fragment spectrum

It is important to notice that 96.9% of all of Cf-252 decays are \alpha decays, and only 3.1 % occur through spontaneous fission. (considering the half life is 85.5 yeas for SF and 2.73 years for alpha decay).<ref name=Ruess08 > Reuss, P. (2008). Neutron physics. 91944 Les Ulis Cedex A, France: EDP. [http://books.google.com/books?id=2FajZzx0jy0C&dq=editions:qQFs-l23FigC&hl=en&sa=X&ei=TZCiUdCMN8m9iwLKoYGwCw&ved=0CC0Q6AEwAA]</ref>


;Th-232 and U-238


Both  U-238 and Th-232 are members of the actinides group. They are characterized by relatively high neutron fission cross sections for fast neutrons with  fission thresholds above the thermal neutron energy. The fission reaction in both elements is expected to eject 1-2 neutrons when the incident neutron energy is between 5-10 MeV as shown in Fig.9 and Fig.10.


{| border="1" cellpadding="4"
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|[[ File:ENDF_GEAN4_U238_fxsection.png |250 px]] ||[[File: Th_232_reference.png|250 px]]
|-
|Fig.9 U 238  fission cross section || Fig.10 Th 232  fission cross section <ref name="endf_Th_232_JEFF1.3">http://www.nndc.bnl.gov/exfor/servlet/E4sMakeE4 </ref>
|}



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