Monday, December 5, 2011


The formation  is subjected  to a pulse of high-energy neutrons  (14MeV)  from  a  neutron  generator.  These pulses  are  repeated  at  a  certain  repetition  rate of 20 pulses per second. The thermal neutron population is sampled between pulses and  its rate of decay can be computed. 

Neutrons  may  interact  with  matter  in  a  variety  of modes. Because  it  is rare  that neutrons are absorbed until they lose most of their energy, each neutron often has  many  interactions  in  the  process  of  losing  its energy. This process is also called slowing down. The characteristics  of  some  of  these  interactions  can  be used  to  predict  the  formation  properties.  In  well logging,  there  are  four  major  types  of  interactions between a neutron and a  target nucleus  that can be used for  formation  evaluation:  inelastic  scattering, elastic scattering, absorption or capture of  fast and  slow or thermal neutrons. The type of interaction that is most probable  to happen  is  function of neutron energy.

The path a neutron takes as it scatter, is the change in direction at a certain distance, sometimes even toward the source  is erratic, but on average,  it gradually moves away  from  the  source.  With  each  interaction,  the neutron loses  some  of  its  kinetic  energy.  This
continues, until  it has a value  just above  thermal energy (0.025 eV). Up to this point the velocity of the neutron was so much higher than the target formation nuclei, that  the  target  nuclei  could  be  treated  as  being stationary. This  is  no  longer  the  case,  now  that  the neutron's  energy  is  just  above  thermal  energy.  The energy  of  the  thermal  neutron  is  about  equal  to  the
thermal  vibration  energy  of  the  formation  nuclei. Subsequent collisions will, on  the average, maintain the neutron's energy  in equilibrium with  the  thermal energy of  the  formation nuclei.

The neutron  energy  loss  for  any particular  collision depends upon the mass of the neutron and the mass of the  element  or  particle  being  struck,  which  is demonstrated  in picture below.
In  the  first  instance,  the  particle  being  struck  has  a larger mass  than  the neutron. A small amount of energy is transferred to the particle, but the neutron bounces back, retaining the majority of its kinetic energy. In the second  case,  the  neutron  basically  runs  over  the particle, transferring some energy to it and continues with most of  its energy. The  greatest  energy  loss  results,  when  a  neutron
collides with a particle or atom of an equal mass. This  is shown  in  the  third  instance. Here, all or nearly all of  the neutron's kinetic energy  is passed  to  the equally massed particle, which  is similar  to what happens when  two equally massed billiard balls collide head-on.