May this web site can help http://www.naa-online.net/theory/equations/

else I recommand you to get some specific book to neutrons transport for that.

Thermal neutron fluence rate determination is given in the article attached.

thank you all

Dear Benchrif,

I recommend w(E)=Eexp(-E/theta), where E represents the neutron energy and theta is equal to kT.

"Flux" is defined by previous people who have answered this question. "Fluence" is the time integral of the flux, sometimes called the irradiation. Very simple.

I have never heard the term "fluence rate", so if you want to make up a unit, the previous discussion would form a reasonable basis.

But why would you want to do so?

For any particle,

Flux = No. of particles emitted / Unit time (sec) ; So Neutron Flux = No. of neutrons     emitted /sec. and its unit is n/sec

Fluence = No. of particles emitted /  Unit area; Neutron Fluence = No. of neutrons emitted /unit area  and its unit is n/cm2

Fluence-rate = No. of particles emitted / Unit area / sec. ; Neutron Fluence-rate = No. of neutrons emitted /unit area / sec.; and its unit is n/cm2/sec.

Hence, if Q is the neutron flux from any neutron source (here,  thermal nuclear reactor), then neutron fluence-rate will be equal to Q / 4 * P i* r2, where  'Q' is the neutron flux (neutron yield) in n/sec. and 'r' is the distance in 'cm'  from the point source.

Sorry, but the definition of "Flux" posted by Sathian is incorrect.

Flux has units of particles/unit area/unit time, and is usually written as particles/cm^2/second, or particles/m^2/sec. It measures the flow of "particles" crossing a unit surface per unit time in any direction. In some fields, you can replace "particles" with "energy". It is not defined with reference to "emission", although emission can be measured in flux units if you are so inclined.

Please refer ICRU Report No.33 (1980) and ICRU Report No.60 (1998), page no. 4&5 for more clarification of my answer regarding the definition of Flux, Fluence and Fluence-rate. Obviously my answer is on the radiation dosimetric point of view. I am attaching ICRU Report No.60 (1998) along with this.