Neutron probe practice. 3rd edition.

This report is intended as a basic guide to the practical use of the neutron probe, its working principles and some of its applications. It is hoped that it will help the user or intending user to avoid the mistakes and frustrations commonly encountered by those new to the field. Properly used and understood the neutron probe can provide in situ measurements of soil moisture change to a precision obtainable in no other way. However, bad practice too often leads to the production of poor data containing large and often unrecognised errors. The principle of measuring soil moisture by the moderating effect of water on fast neutrons was first proposed in the late 1940s and field instruments were soon developed once the principle was shown to be practicable. The great advances in electronics since then have led to the development of highly reliable and stable systems and the dubious reputation earned for the technique by some of the early instruments is now entirely unjustified. The original use of neutron probes was mainly for the measurement of soil water storage changes for water balance purposes, but in the last few years the improvements in performance have opened up new research applications in agronomy and hydrology which are now overtaking the original uses. During recent years the principal advance has been the introduction of microprocessor controlled systems, which offer the advantage that data can be recorded in a built-in memory and either processed directly into readouts of soil water content or, perhaps more usefully, the memory can be quickly and easily unloaded into a microcomputer for providing immediate listings of processed data Although it is the "Wallingford Probe" and its successor, the "IH Neutron Probe System" which is often exemplified in this report, most of the information is applicable to any type of neutron probe.