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With reference to Bruce's comments-
It seems the time has come to discuss Neutron probes !
I have a question regarding the sphere of influence of a Neutron probe.
If we look at how it works,
" fast neutrons are emitted froma decaying source Am241/Be .. fly ..into the
soil where they bounce around gradually slow down in process. The detector
tube of helium is responsive to weak slow neutrons... when.. sufficently
slowed down.. they can be detected.. "
" Neutrons richochet strongly from large atoms.. however dramatically slowed
down .. if ..collide with hydrogen atoms.. In most soils, the only source of
hydrogen would be water, hence the only slowing down on fast neutrons would
be due to water."
Because of the nature of its operation would not the sphere of influence
change with the following
1. THE DRYING CYCLE OF SOIL - as the soil dries, and the concentration of
hydrogens is reduced, the probability of fast neutrons travelling further
from source before colliding would be higher. Therefore sphere of influence
grows as soil dries out. And of course the reverse would happen as the soil
profile refilled.
2. SANDY SOIL compared to CLAY SOIL - If clay soil can have higher soil
moisture content then the probability of collision is higher and sphere of
influence smaller than sand.
3. DUPLEX SOIL or DRY TOP SOIL/WET SUBSOIL - If there is a change of soil
structure or soil moisture content with soil depth would not the sphere of
influence be distorted, no longer in a possible circle but odd shaped. For
example if the top 20 cm is dry sandy soil and 30 and 40 cm has perched
water table due to clay barrier at 50 cm. Then if the sensor is placed at 20
cm, would not the probiblity of collision vary between the 10-20 cm and
30-40cm and so distort the sphere of influence. The fast neutrons going into
10-20 would travel some distance before collision, some even being lost to
atomsphere and never returning while any travelling to 30 cm would reach the
required number of collisions to slow then down sooner and closer to the source.
********************************************** SURFACE
**************** 5 CM
****************** 10 CM
******************** 15 CM
********Source******** 20 CM
******************** 25 CM
************ 30 CM
****** 35 CM
** 40 CM
I was quite surprised the day the government officer came out to my office,
placed all kinds of radiation monitoring equipment on the ground around a
probe site ( clay loam ) and I placed the source at various depths. Even at
40 cm he identified loss thru to soil surface.
How reliable does this make a Neutron probe when close to soil surface ?
How does the onion shape of a wetting front under drip irrigation effect the
sphere of influence ?
I realise calibration is required for specific soil type. If such distortion
does occur, would not using one standard calibration lead to cases of under
and over calculation of irrigation requirements, especially in shallow
rooted crops (eg Lettuce, Strawberries, Potatoes etc) ?
Has any person conducted research investigating such issues ?
AND Is having a LARGE SPHERE OF INFLUENCE AND ADVANTAGE OR DISADVANTAGE ?
Regards
Peter
Peter Broomhall
Horticulural Consultant
Hortech Services Pty Ltd
a.c.n. 060 406 957
P.O. Box 370
Kallangur QLD 4503
Australia
P: +61 418 708 573
F: +61 7 3886 0389
hortech@ats.com.au