Physics and Engineering of Radiation Detection

180 Chapter 3. Gas Filled Detectors

filled with some conventional gas. In commercially available chambers the boron is
enriched to about 20% by weight in boron-10, while in custom made chambers much
higher concentrations of boron-10 are used. The thickness of the boron coating is
kept smaller than the mean range ofα-particles. This ensures that most of theα-
particles enter the chamber volume and get detected. The detection mechanism of
slow neutrons in a cylindrical boron-lined chamber is shown in Fig.3.4.10. The boron-
lined chamber is also mostly used as a proportional counter but in high radiation
environments it can be operated in the ionization chamber region.

Anode

Cathode

Boron

α

+−

−+

−+

Li

Neutron

Figure 3.4.10: Principle of detection

of a slow neutron from a boron-lined

cylindrical chamber.

D.3 CompensatedIonChamber

Determination of slow neutron dose in a nuclear reactor is difficult due to the pres-
ence of the accompanying highγ-ray flux. A simple boron-lined ion chamber would
not work in such a situation since it can not differentiate between the two types of
particles. The flux measured from such a detector is the sum of the neutron and the
γ-ray responses and the elimination of theγ-ray background from the measurement
is impossible. The trick that is often employed in such a situation is to simulta-
neously measure the total flux and just theγ-ray flux and then subtract the latter
from the former. Such a system, consisting essentially of two separate or segmented
ionization chambers, is referred to as acompensatedion chamber.
There are several possible designs of such a system including two separate detec-
tion systems for both fluxes. However the most commonly used design consists of
a single but segmented ionization chamber for both measurements. One segment of
this detector is boron lined sensitive to both neutrons andγ-rays while the other is
an ordinary ion chamber capable of measuring only theγ-ray flux (see Fig.3.4.11).
The current measured at the central electrode in this design is the current produced
in the boron-lined segment minus theγ-ray induced current, which is proportional
to the flux of neutrons.