TM 5-6635-386-12&P backscatter operations and permit simultaneous counting of both density and moisture. Approximately   90%   of   the   moisture   count   is   obtained from returned thermal neutrons from the closest 6 inches of soil. 1-3.  PRINCIPLES OF OPERATION General Various elements, both naturally occurring (Radium) and reactor produced (Cesium  and  Americium)  are  unstable and are slowly decaying to a more stable state.  The act of decay produces emissions of energy upon disintegration of the atoms. These   emissions   are   either   "rays"   of   electromagnetic radiation    (Gamma    Rays)    or    are    actual    particles    of material  (neutrons,  for  example).    Other  emissions  are produced  from  various  radioactive  materials;  however, we   are   concerned   with   only   the   gamma   and   neutron radiation for purposes of nuclear soil testing. Gamma radiation is emitted in several energy levels by a sealed Radium source and in a single energy level by a Cesium  source.    The  Cesium   level   is   0.66   MEV   and requires  less  shielding  than  the  multi-level  output  of  the Radium  source.    The  fixed  spectrum  emission  is  also superior for soil density determination purposes. Cesium,  a  reactor  produced  isotope  requires  a  license for use anywhere in the U.S.  and in foreign countries. Neutron emission occurs when an alpha particle emitter (Americium,    Plutonium,    or    Radium)    is    mixed    with Beryllium  powder  in  a  tightly  compressed  pellet.    The alpha particles strike the Beryllium atoms to produce fast neutrons of an average energy of 5 million electron volts. The suffix "Be" is attached to the alpha source name to denote its use as a neutron source when it is mixed with Beryllium (RaBe, AmBe, PuBe). These  emissions  are  detected  by  appropriate  detectors (Geiger    Mueller    tubes)    for    gamma    and    (Boron    Tri- fluoride-BF3 or Helium-3 H3 tubes) neutron measurements.      The   resultant   signals   are   displayed electronically as an index of soil density and moisture. Radioactivity, both gamma and neutron, may be thought of  as  being  similar  to  light  from  an  incandescent  bulb. The light rays diminish rapidly as we move away from the bulb   (by   the   inverse   square   of   the   distance   from   the lamp),   and   they   have   the   ability   to   penetrate   various materials to some degree, ranging from nearly complete penetration  (glass)  to  nearly  complete  blockage  (metal shield). Radiation obeys the same rules, although its penetration capabilities  are  generally  much  greater  than  light.    The farther we are from the source, the safer we are, and the more  absorbing  material  (shielding)  we  place  between ourselves   and   the   source,   the   safer   we   are.      It   is theoretically  impossible  to  shield  any  radioactive  source completely; however, careful tester design and appropriate choice of shielding materials can reduce the radiation to an acceptable level with negligible absorption by the user under proper operating procedures. Gamma Radiation Gamma   radiation   is   electromagnetic   "photon"   energy capable    of    penetration    of    several    inches    of    most materials.  It is essentially high energy "light ray" energy. It  is  useful  for  the  total  mass  measurement  of  heavy materials and is used to determine total density of soil. Neutron Radiation Neutron radiation consists of small, noncharged particles emitted  from  the  source  at  an  average  energy  level  of approximately 5.0 MEV.  This is known as "fast" neutron emission.        Neutron    detectors    "see"    only    slow,    or "’thermal"  neutrons.    Therefore,  the  fast  neutrons  must slow   down   or   they   will   be   ignored   by   the   detectors. Neutrons slow down by colliding with other objects much like a rifle bullet ricocheting from rock to rock. Collision   of   the   fast   neutrons   with   the   nuclei   of   large atoms  results  in  rebounding  of  the  neutrons  with  little loss   of   energy.      Collision   with   the   orbiting   electrons (approximately     1/1840th     the     weight     of     a     neutron) produces little loss of energy.  However, collision with an 1-3