In response to Cliff's question on neutron meters

[Rick Allen <AllenRic@cc.usu.edu>]

One brief response to Trevor Finch concerning his desire/attempt to find
 equations for different soil types in the form...


VSW = (Count/WaterDrumCount) * Slope + Intercept


One subtle effect on neutron probe readings can be the temperature of the
probe (detector) and of the electronics.  Some years ago, we suspected some
temperature effect on CPN gauges.  We therefore had a Ph.D. student spend
four hours in a very hot chamber (about 45 deg C) and four hours in a very
cold chamber (about -10 deg C) (one can get PhD students to do just about
anything).  The student conducted continuous shield counts.  

I do not recall now how careful the student was in keeping his body well
away from the instrument.  He was instructed to do so.  The net impact was,
however, that the shield counts changed as the probe temperature changed to
that of its surroundings, and the final shield counts were biased as a
result of the temperature.  I do not recall now which way they went.

Again, however, I do not know whether the surroundings of each chamber (they
were different) biased the base shield count in and of themselves.  As Steve
Evett has suggested, proximity of the human body or other hydrogen source
(wet soil or vegetation or PVC) will bias the shield count.

Has anyone else observed a temperature bias in the NP?   I wouldn't go to
the bank on our findings here at USU, but they do suggest some caution.

This effect would  impact the use of Trevor's VSW = (Count/WaterDrumCount) *
Slope + Intercept  approach somewhat, as the user must be careful to not
bring a "hot" or "cold" gauge into the lab for the "water drum count".

Rick Allen
Utah State University